Metrohm 791VA User Manual

791 VA Detector

Manual

8.791.1013

Metrohm AG
CH-9101 Herisau
Switzerland
Phone [phone]
Fax [phone]
[e-mail]
www.metrohm.com

791 VA Detector

Manual

8.791.1013 01.2006 chs

Teachware
Metrohm AG
CH-9101 Herisau
[e-mail]

This documentation is protected by copyright. All rights reserved.

Although all the information given in this documentation has been
checked with great care, errors cannot be entirely excluded. Should you
notice any mistakes please send us your comments using the address
given above.

Documentation in additional languages can be found on

http://products.metrohm.com under Literature/Technical documenta-

tion.

Table of contents

Table of contents

1 Introduction ........................................................................................... 1

1.1 Instrument description .............................................................................1

1.2 Parts and controls.....................................................................................2

1.3 Information on the Instructions for Use ................................................4

1.3.1 Organization ..................................................................................4

1.3.2 Notation and pictograms ..............................................................5

1.4 Safety notes................................................................................................6

1.4.1 Electrical safety..............................................................................6

1.4.2 General precautionary rules..........................................................6

2 Installation.............................................................................................. 7

2.1 Setting up the instrument.........................................................................7

2.1.1 Packaging......................................................................................7

2.1.2 Check ............................................................................................7

2.1.3 Location.........................................................................................7

2.1.4 Arrangement of the instruments....................................................7

2.2 Mains connection ......................................................................................8

2.2.1 Setting the mains voltage.............................................................. 8

2.2.2 Fuses ............................................................................................. 9

2.2.3 Mains cable and mains connection .............................................. 9

2.2.4 Switching the instrument on/off.....................................................9

2.3 656 Electrochemical detector................................................................10

2.3.1 Installation and startup ................................................................10

2.3.2 Connection to 791 VA Detector...................................................10

2.4 6.5303.030 ELCD cell for IC...................................................................11

2.4.1 Assembling the ELCD cell...........................................................11

2.4.2 Connection to 791 VA Detector...................................................13

2.5 Analog output connections ...................................................................14

791 VA Detector

3 Operation...............................................................................................15

3.1 Electrochemical detection .....................................................................15

3.1.1 Classes of substances which can be determined...................... 15

3.1.2 Influence on detector signal........................................................16

3.1.3 Selection of polarization voltage .................................................17

3.1.4 Procedure for unknown substances ...........................................17

3.1.5 Practical information about ELCD...............................................18

3.2 Electrodes.................................................................................................20

3.2.1 Working electrodes .....................................................................20

3.2.2 Reference electrode ....................................................................22

3.2.3 Auxiliary electrode .......................................................................22

3.3 Operating element functions.................................................................23

3.4 Startup.......................................................................................................26

3.4.1 Preparing the ELCD cell..............................................................26

3.4.2 Zero balancing and equilibration phase .....................................27

3.4.3 Measuring procedure..................................................................29

I

Table of contents

4 Maintenance – Malfunctions.......................................... 31

4.1 Maintenance and servicing ...................................................................31

4.2 Shutdown..................................................................................................31

4.3 Malfunctions and their rectification .....................................................32

4.4 Instrument test with the dummy cell.................................................... 34

4.4.1 Check of "meas" and "damp" commutators and

of current amplifier offset ............................................................34

4.4.2 Check output voltage.................................................................. 35

4.4.3 Check of current compensation .................................................36

4.4.4 Check of current and voltage overload ...................................... 38

4.4.5 Check sensitivity knob

4.4.6 Check of noise from current amplifier......................................... 40

8 ............................................................. 39

5 Appendix................................................................................................. 41

5.1 Technical data..........................................................................................41

5.2 Standard equipment ...............................................................................43

5.2.1 2.791.0010 VA Detector for HPLC .............................................. 43

5.2.2 2.791.0020 VA Detector for IC ....................................................44

5.3 Optional accessories.............................................................................. 45

5.4 Warranty and conformity .......................................................................46

5.4.1 Warranty ......................................................................................46

5.4.2 EU Declaration of conformity ...................................................... 47

5.4.3 Certificate of conformity and system validation.......................... 48

5.5 Index..........................................................................................................49

List of figures

Fig. 1: Front of the 791 VA Detector ............................................................................. 2

: Rear of the 791 VA Detector.............................................................................. 3

Fig. 2

Fig. 3

: Setting the mains voltage.................................................................................. 9

Fig. 4

: Connection 656 – 791 .....................................................................................10

Fig. 5

: ELCD cell assembly ........................................................................................12

Fig. 6

: Connection of ELCD cell at detector block..................................................... 13

Fig. 7

: Connection of ELCD cell at 791 VA Detector.................................................. 13

Fig. 8

: Selection of polarization voltage .....................................................................17

Fig. 9

: Circuit of 6.2813.020 Dummy Cell................................................................... 34

791 VA Detector

II

1.1 Instrument description

1 Introduction

1.1 Instrument description

The 791 VA Detector allows the use of electrochemical/amperometric
detection in IC and HPLC; various working electrodes are available de­pending on the application. It can be used in series with another detec­tor (e.g. 732 Conductivity detector) or simply as a «stand alone» detec­tor.

The self-ventilating measuring cell functions according to the time­proved wall-jet principle with three-electrode technology. A silver/silver
chloride system is used as the reference electrode and a solid gold pin
as the auxiliary electrode. The carbon paste electrode and the silver
electrode have proved to be the best working electrodes; but other
electrode materials such as glassy carbon, gold, platinum and impreg­nated graphite are available for special applications.

The 791 VA Detector is available in the two following versions:

• 2.791.0010 VA Detector for HPLC

The measuring cell is built into the 656 Electro­ chemical detector.

• 2.791.0020 VA Detector for IC

The measuring cell belongs to the accessories and
is built into the 733 IC Separation Center.

791 VA Detector

1

1 Introduction

1.2 Parts and controls

13

2 4567

VA Detector791

I

comp

0.02 nA

+

~

+

overload meas

015

U

pol

10 mV

I

0.01

comp

x5damp

.

I

nAx50

50

10

0,5

0,1

5

1

Met rohm

I

(U = 1 V)

out

µ

0,1

0,5

1

5

100

10

A

4

11 89

Fig. 1

Mains pilot lamp

1

Lit up when instrument switched

: Front of the 791 VA Detector

10

7 Commutator

Changing the polarity of I

on

2 Overload display 8 Adjusting knob

Setting the current measuring sensi­tivity

3 Commutator

Changeover Stand-by/Measure

4 Commutator

Switching on/off damping

5 Commutator

Switching on/off multiplication of

by a factor of 5

I

comp

9 Potentiometer

Setting the absolute compensation
current I

comp

10 Potentiometer

Setting the relative compensation
current I

comp

11 Digital switch

Setting the polarization voltage

comp

6 Commutator

Switching on/off multiplication of

by a factor of 50

I

comp

791 VA Detector

2

1.2 Parts and controls

12

151413

Type 1.791.0010

f = 50-60 Hz Made by Metrohm Herisau Switzerland
S = 5 VA

Fuse

100-240 V: 1A(T)

WARNING - Fire Hazard -

For continued protection replace only
with the same type and rating of fuse

pilot
voltage

Nr.

16 17 18 19

AE RE WE shield

output 0...1 V

com 0...10 mV

Fuse holder

12

changing the fuses, see section 2.2

26 20

25

Fig. 2

: Rear of the 791 VA Detector

20 Selector

Selection of polarity for analog
output

13 Mains voltage indicator 21 Selector

Selection of full-scale deflection for
analog output

14 Earthing socket 22 Analog output (live)

15 Connection AE

23 Analog output (common)

Connection of auxiliary electrode

16 Serial number 24 Pilot voltage

Input for external voltage input for
potentiostat control

17 Connection RE

Connection of reference electrode

25 Mains switch

To switch instrument on/off:
I = ON 0 = OFF

21222324

18 Connection WE

26 Mains connection plug

Connection of working electrode

19 Connection for protective screen

791 VA Detector

Mains connection, see section 2.2

3

1 Introduction

1.3 Information on the Instructions for Use

Please read through these Instructions for Use carefully before you put
the 791 VA Detector into operation. The Instructions for Use contain
information and warnings to which the user must pay attention in order
to assure safe operation of the instrument.

1.3.1 Organization

These 8.791.1013 Instructions for Use for the 791 VA Detector pro­vide a comprehensive overview of the installation, startup procedure,
operation, fault rectification and technical specifications of this instru­ment. The Instructions for Use are organized as follows:

Section 1 Introduction

General description of instrument, parts and controls
and safety notes

Section 2 Installation

Installation of accessories, connection to IC system

Section 3 Operation

Electrodes, operating element functions,
startup

Section 4 Maintenance – Faults

Maintenance, fault rectification

Section 6 Appendix

Technical data, standard equipment, options, warranty,
declarations of conformity, index

To find the required information on the instruments you will find it an
advantage to use either the Table of contents or the Index at the
back.

791 VA Detector

4

1.3 Information on the Instructions for Use

1.3.2 Notation and pictograms

The following notations and pictograms (symbols) are used in these In­structions for Use:

15 Part or control of 791

Hazard

This symbol draws attention to a
possible danger to life or of injury if
the associated directions are not
followed correctly.

Warning
This symbol draws attention to
possible damage to instruments or
instrument parts if the associated
directions are not followed cor­rectly.

Caution
This symbol marks important
information. First read the associ­ated directions before you con­tinue.

Comment
This symbol marks additional
information and tips.

791 VA Detector

5

1 Introduction

1.4 Safety notes

1.4.1 Electrical safety

While electrical safety in the handling of the 791 VA Detector is assured
in the context of the specifications IEC 1010-1 (protection class 1, de­gree of protection IP40), the following points should be noted:

• Mains connection

Setting of the mains voltage, checking the mains fuse and the
mains connection must be effected in accordance with the instruc-

tions in section 2.2.

• Opening the 791 VA Detector

If the 791 VA Detector is connected to the power supply, the instru­ment must not be opened nor must parts be removed from it, other­wise there is a danger of coming into contact with components which
are live. Hence, always disconnect the instrument from all voltage
sources before you open it and ensure that the mains cable is
disconnected from mains connection 26 !

• Protection against static charges

Electronic components are sensitive to static charging and can be
destroyed by discharges. Before you touch any of the components
inside the 791 VA Detector, you should earth yourself and any tools
you are using by touching an earthed object (e.g. housing of the
instrument or a radiator) to eliminate any static charges which exist.

1.4.2 General precautionary rules

• Handling of solvents

Check all lines periodically for possible leaks. Follow the relevant
instructions regarding the handling of flammable and/or toxic solvents
and their disposal.

791 VA Detector

6

2.1 Setting up the instrument

2 Installation

2.1 Setting up the instrument

2.1.1 Packaging

The 791 VA Detector is supplied together with the separately packed
accessories in special packagings containing shock-absorbing foam
linings designed to provide excellent protection. The instrument itself is
packed in an evacuated polyethylene bag to prevent the ingress of
dust. Please store all these special packagings as only they assure
transport of the instrument free from damage.

2.1.2 Check

After receipt, immediately check whether the shipment is complete and
has arrived without damage (compare with delivery note and list of
accessories in section 5.2). In the case of transport damage, see
instructions in section 5.4.1 "Warranty".

2.1.3 Location

Position the instrument in the laboratory at a location convenient for
operation, free from vibrations and protected against a corrosive
atmosphere and contamination by chemicals.

2.1.4 Arrangement of the instruments

The 791 VA Detector can be placed on the 732 IC Detector or the 733
IC Separation Center.

791 VA Detector

7

2 Installation

2.2 Mains connection

Follow the instructions below for connecting to the power supply. If
the instrument is operated with a mains voltage set wrongly and/or
wrong mains fuse, there is a danger of fire!

2.2.1 Setting the mains voltage

Before switching on the 791 VA Detector for the first time, check that the
mains voltage set on the instrument (see Fig. 3) matches the local
mains voltage. If this is not
on the instrument as follows:

the case, you must reset the mains voltage

1 Disconnect mains cable

Disconnect mains cable from mains connection plug 26 of the
791 VA Detector.

2 Remove fuse holder

Using a screwdriver, loosen fuse holder 12 and take out com­pletely.

3 Change mains voltage

Completely remove voltage selection insert 13 by hand, rotate it
through 180° and reinsert it. The required mains voltage (115 or
230 V) must now be visible from the front.

4 Check fuses

Carefully take both fuses out of fuse holder 12 and check their
specifications:

100…240 V 1 A (slow-blow) Metrohm-No. U.600.0016

5 Insert fuses

Change both fuses if necessary and reinsert in fuse holder 12.

6 Install fuse holder

Push fuse holder 12 back into the opening of 791 VA Detector
by hand until it clicks into place properly.

791 VA Detector

8

2.2 Mains connection

100 – 120 V220 – 240 V

13

12

230

115

12 Fuse holder

13 Voltage selection

insert with display
of voltage

25

26

Fig. 3

: Setting the mains voltage

2.2.2 Fuses

Two fuses 1 A/slow-blow for 100…240 V are installed in the fuse holder
12 of the 791 VA Detector as standard.

Ensure that the instrument is never put into operation with fuses of
another type, otherwise there is danger of fire!

For checking or changing fuses, proceed as described in section 2.2.1.

2.2.3 Mains cable and mains connection

Mains cable

The instrument is supplied with one of three mains cables

25 Mains switch

26 Mains connection

plug

• 6.2122.020 with plug SEV 12 (Switzerland, …)

• 6.2122.040 with plug CEE(7), VII (Germany, …)

• 6.2133.070 with plug NEMA 5-15 (USA, …)

which are three-cored and fitted with a plug with an earthing pin. If a dif­ferent plug has to be fitted, the yellow/green lead (IEC standard) must
be connected to protective earth (protection class 1).

Any break in the earthing inside or outside the instrument can make it
a hazard!

Mains connection

Plug the mains cable into mains connection plug 26 of the 791 VA De­tector (see Fig. 3).

2.2.4 Switching the instrument on/off

The 791 VA Detector is switched on and off using mains switch 25.
When the instrument is switched on lamp 1 lights up.

791 VA Detector

9

2 Installation

2.3 656 Electrochemical detector

2.3.1 Installation and startup

Inserting the detector cell in shield housing, fitting the electrodes and
startup of the 656 Electrochemical detector (order number 2.656.0020)
are described in detail in the 656 Instructions for use.

2.3.2 Connection to 791 VA Detector

The electrodes used in the detector cell are each connected to the cor­responding connection of the 791 VA Detector with a 6.2120.020 Elec­trode cable as shown in Fig. 4. In addition the two earthing sockets are
connected with a 6.2106.020 Cable.

6.2120.0 20 6.2106.0 20

656

AE WE RE

Fig. 4

AE RE WE

: Connection 656 – 791

791

791 VA Detector

10

2.4 6.5303.030 ELCD cell for IC

2.4 6.5303.030 ELCD cell for IC

2.4.1 Assembling the ELCD cell

The reference electrode and working electrode must be inserted in the

6.5303.030 ELCD Cell supplied with the 2.791.0020 VA Detector. The
cell is then screwed into the detector block built into the 733 IC Separa­tion Center. Proceed as follows (see Fig. 5 and Fig. 6):

1 Insert reference electrode

Reference electrode 39 (6.0727.000, details see section 3.2.2)
included in the supply is already filled with c(KCl) = 3 mol/L and
protected with a cap. It is assembled as follows (see Fig. 5):

• Screw off the cap of electrolyte storage vessel 41 of reference

electrode 39. Normally PTFE gasket 31 remains in the cap
and can be left there.

• Insert one of the supplied PTFE gaskets 31 into the corre-

sponding opening of cell body 32 and screw reference elec­trode 39 into this opening.

2 Insert working electrode

Working electrode 28 is not supplied with the 791 VA Detector
and must be ordered separately. Details about the working
electrodes and information about pretreatment (e.g. polishing)
can be found in section 3.2.1. A working electrode is assembled
as follows (see Fig. 5):

• Insert working electrode 28 in screw nipple 29.

• Push one of the supplied PTFE gaskets 31 sufficiently far

along the working electrode from below.

• Carefully insert working electrode 28 into the corresponding

opening of cell body 32 from above and press it in until it
clearly clicks into position.

• Push screw nipple 29 downwards and screw it in.

3 Connect ELCD cell to the detector block

The ELCD cell is screwed onto the 1.733.0X10 Detector block
mounted inside the 733 Separation Center. Proceed as follows
(see Fig. 5 and Fig. 6):

• Screw cell body 32 onto detector block 42 with the help of

the two red knurled screws 43.

• Push one of the supplied PTFE gaskets 31 over screw nipple

33 and screw this into the corresponding opening of cell
body 32.

• Place a screw nipple 36 and a PTFE gasket 35 on the outlet

end of the outlet capillary 37 permanently connected to de­tector block 42.

• Insert outlet capillary 37 with screw nipple 36 in place into

screw nipple 33 and press in outlet capillary 37 until it
reaches the stop.

791 VA Detector

11

2 Installation

• Screw nipple 36 onto screw nipple 33 to fix outlet capillary 37

is position.

4 Discharge

• Pull outlet capillary 38 which is already mounted on cell body

32 through one of the openings in the rear panel of the 733 IC

Separation Center so that it emerges outside the instrument.

• Insert outlet capillary 38 in a sufficiently large waste container

and fix it in position.

4139 3140

2731

28

29

30

31

35

36

38

: ELCD cell assembly

Fig. 5

27

ELCD cell (6.5303.030) 35 PTFE gasket (6.2704.020)

28 Working electrode 36 Screw nipple (4.420.2580)

29 Screw nipple (6.2730.000) 37 PEEK outlet capillary from

1.733.0X10 Detector block

32

31

33

34

35

36

37

30 Sealing ring (E.301.0028) 38 PTFE outlet capillary (6.1803.000)

31 PTFE gasket (6.2704.010) 39 Reference electrode (6.0727.000)

32 Cell body 40 Ag/AgCl Reference system

33 Screw nipple (6.2731.030) 41 Electrolyte vessel

34 Sealing ring (E.301.0004)

791 VA Detector

12

2.4 6.5303.030 ELCD cell for IC

42

37

43

28

32

39

43

38

: Connection of ELCD cell at detector block

Fig. 6

Working electrode 39 Reference electrode

28

32 Cell body 42 Detector block (1.733.0X10)

37 PEEK outlet capillary from

1.733.0X10 Detector block

38 PTFE outlet capillary

(6.1803.000)

2.4.2 Connection to 791 VA Detector

The ELCD cell electrodes are each connected to the corresponding
connection of the 791 VA Detector with a 6.2120.020 Electrode cable as
shown in Fig. 7. The 791 VA Detector must be switched off and in the
"stand-by" position when this is taking place.

(6.0727.000)

43 Knurled screw (V.900.4006)

791 VA Detector

6.2120.020

RE

Fig. 7

6.2120.020

6.2120.020

WE

AE

AE

AE RE WE

: Connection of ELCD cell at 791 VA Detector

791

13

2 Installation

2.5 Analog output connections

The 791 VA Detector has two analog output sockets: 22 (live) and 23
(common) for connecting a chart recorder of a different data recording
instrument. The full-scale deflection range for the analog output (1 V or
10 mV) can be set with switch 21; the polarity can be changed with
switch 20 (see section 3.3).

The 6.2115.010 Cable can be used for connecting recorders with ba­nana plug connections; the 6.2115.060 Cable can be used for connect­ing data recording instruments with terminals.

791 VA Detector

14

3.1 Electrochemical detection

3 Operation

3.1 Electrochemical detection

3.1.1 Classes of substances which can be determined

A precondition for the use of electrochemical detection is that the sub­stances to be determined are electrochemically active on the particular
working electrode used, i.e. they can easily be oxidized or reduced. The
electrical current produced by this reaction is proportional to the con­centration of the substance throughout a wide range. It is measured,
amplified and recorded as a function of time by the 791 VA Detector in
the form of a chromatogram.

The following table provides an overview of the classes of substances
and ions which can be detected by oxidation or reduction. The ap­proximate polarization voltage obtained when an Ag/AgCl/c(KCl) =
3 mol/L reference electrode is used is given; this depends to a large ex­tent on the working electrode and eluents used (see section 3.2). The
limits of detection which can be achieved are in the lower ppb range.

Structural formula Classes/substances Polarization voltage

R

OH

R

NH

2

5

3

N

H

Aromatic hydroxy compounds

Antioxidants
Catecholes
Flavones
Halogenated phenols
Hydroxybiphenyls
Hydroxycumarines
Methoxyphenols
Oestrogens
Phenols
Tocopherols

Aromatic amines

Anilines
Benzidines
Sulfonamides

Indols

Indolyl-3-compounds
5-Hydroxy-indols

+800 … +1200 mV

+ 800 mV
+ 1000 mV
+ 1200 mV
+ 800 mV
+ 1000 mV
+ 800 mV
+ 1000 mV
+ 1200 mV
+ 800 mV

+ 1000 mV
+ 600 mV
+ 1200 mV

+ 1000 mV
+ 800 mV

791 VA Detector

S

N

H

Phenothiazines + 1000 mV

15

3 Operation

Structural formula Classes/substances Polarization voltage

RSH

O

O

R NO

R

R

Mercaptans + 800 mV

Miscellaneous

Ascorbic acid
Carotenes
Purine derivatives
Vitamin A

Quinones

Vitamin K

1

+ 800 mV
+ 800 mV
+ 1000 mV
+ 1000 mV

– 700 mV

2

Nitro compounds

Nitrophenol
Nitroglycerin
Isosorbidedinitrate

Inorganic anions

Nitrite
Sulfide
Thiosulfate
Chlorite
Bromide
Iodide
Cyanide
Thiosulfate
Thiocyanate

– 700 mV
– 800 mV
– 800 mV

+ 1100 mV
+ 0 mV
+ 1100 mV
+ 1100 mV
+ 150 mV
+ 150 mV
+ 0 mV
+ 1100 mV
+ 1200 mV

3.1.2 Influence on detector signal

The sensitivity of detection, i.e. the height of the detector signal and the
background current in the electrochemical detection depend on the fol­lowing parameters:

• polarization voltage

• temperature (approx. 1.5 %/°C)

• flow rate of eluent

• surface of electrodes and interior of cell

• Eluent composition

(pH value, type and concentration of conductive salt,
type and concentration of organic components, etc.)

Each change of one of these parameters will change the background
current and the detector signal. Therefore, every parameter change
needs a stabilization time to elapse; this is normally a few minutes for

791 VA Detector

16

3.1 Electrochemical detection

current ranges > 5 nA. Please pay particular consideration to this on
changing the eluent or the sample solution: the "same" eluent may have
a different background current because eluents are never exactly the
same (ionic strength, pH value, etc.!).

3.1.3 Selection of polarization voltage

The hydrodynamic voltammograms show the voltage limit from the
point where a substance is oxidized or reduced. This voltage limit is
generally higher than the voltage predicted by thermodynamics be­cause of inhibitions of the electrode reaction. Increasing voltage values
give rise to increasing currents up to a maximum value, the diffusion­controlled limiting current.

In Fig. 8 for example, analytes a, b, and c are detected at voltage U
whereas at voltage U

, component a is detected selectively. Therefore

1

the polarization voltage to be set has to be optimized for sensitivity on
one hand and selectivity on the other.

The polarization voltage to be set also depends on the type and condi­tion of the working electrode (see section 3.5) as well as on the eluent.
The latter could be illustrated by Fig. 8, saying that the curves are from
one single analyte taken in 3 eluents with different pH values.

I

Diffusion­controlled
limiting
current

abc

U

1

Fig. 8

: Selection of polarization voltage

U

2

Background
current

U

pol

;

2

3.1.4 Procedure for unknown substances

The hydrodynamic current/voltage curve of an analyte with unknown
electrochemical characteristics is recorded as follows:

• Short circuit the system, i.e. connect the injector directly (without

separating column) to the electrochemical detector.

• Set current range of 791 VA Detector to 100 … 500 nA.

• Set polarization voltage of 791 VA Detector to +1200 mV.

• Inject analyte (20 … 100 ng).

791 VA Detector

17

3 Operation

If no signal is observed, change the pH value of the eluent (a proto­nated or deprotonated substance may be oxidized more easily). Gen­erally, a higher polarization voltage has to be applied with lower pH val­ues (approx. 50 … 100 mV per pH unit).

st

• 1

experiment with eluent of pH ≈ 6

nd

• 2

experiment with eluent of pH ≈ 3

If there is still no signal, the analyte is "inactive", i.e. not suitable for elec­trochemical detection.

As soon as a signal is observed, decrease the polarization voltage in
steps of 100 mV: a hydrodynamic current/voltage curve results.

For detection, select the lowest polarization voltage U
good signal is still observed.

3.1.5 Practical information about ELCD

In principle the electrochemical detector can be connected to all HPLC
and IC systems. In order to carry out work successfully, particularly in
trace-level quantities, all components such as pump, dampening sys­tem, injector, separating column, capillaries, couplings, etc. must be in
perfect condition. In electrochemical detection it is possible that inter­ferences may occur which, for example, are not observed with UV or
conductivity detection. These can often be traced back to contaminants
from the water supply system (e.g. Fe(II) ions), contaminants from the
separating column (e.g. organic residues) or variations in pressure
caused by leaking connections.

Particular attention must be given to the following points:

Cleanliness

The general demands placed on trace analysis work also apply here.
Each possible source of corrosion of steel components must be
avoided as Fe(II) ions may interfere. The eluent should also not contain
any complexing agents.

at which a

pol

Pulsation

The noise of the detector signal depends on the pressure variation in­side the cell and on the back-pressure (max. 1 bar), which is deter­mined by the length of outlet capillary 38 (i.d. 0.3 mm). The use of a
pulsation dampener (e.g. Metrohm 6.2620.150 Pulsation dampener) for
dampening such pulsations has proved itself in practice and is also
adequate if only single-piston pumps are used.

Eluent

Most of the solvents normally used in HPLC and IC can be used to pre­pare eluents. The conductivity which is necessary for detection is
achieved by the addition of a conductive salt. Concentrations of 1…10
g/L are normally adequate for this purpose, this results in a conductivity
of approx. 1…10 mS/cm. Sulfates, nitrates, phosphates, acetic acid,

791 VA Detector

18

3.1 Electrochemical detection

sulfuric acid, perchloric acid, lithium perchlorate, etc. can be used for
this. Chlorides and hydroxycarboxylic acids should not be used; some
reagents which form ion pairs can be used.

Solvent gradients can be used in less sensitive ranges (current range >
100 nA). The eluent is degassed as usual by vacuum or passing a
stream of helium through it.

Stationary phase

In principle all reversed-phase and ion exchanger materials can be
used. New columns must be purged well and for a sufficiently long time
for any contaminants which may be present to be washed out. This
"equlibration phase" of a column may take more than 24 hours in the
most sensitive areas.

Sample solution

Whenever possible, the eluent used should also be used as the solvent
for the sample in order to minimize interference owing to changing sur­roundings (as small a front peak as possible). Very small amounts of
the active substance should be used, in general < 100 ng.

791 VA Detector

19

3 Operation

3.2 Electrodes

3.2.1 Working electrodes

The quality of a working electrode WE can be assessed in terms of the
following criteria:

• Background current (after adequate running-in time):

this should be small and constant

• Interference level ("noise"):

this should be low

• Response characteristics:

after switching on, the low background current should be
quickly reached

It should nevertheless be borne in mind that increases in both back­ground current and noise levels are often due to other causes than a
poor-quality working electrode (for example, contamination of the HPLC
system, malfunction of the pulsation dampener, etc.). Generally speak­ing, both background current and noise levels tend to increase with
higher polarization voltages, and are usually greater in eluents based
on mixed solvents than in those composed of purely aqueous solutions.

Glassy carbon electrode GCE

Ordering designation 6.0805.010

Sensor material Glassy carbon, GC

Chemical resistance good resistance to acetonitrile, methanol, wa-

ter, etc.

Voltage range (–800) … 0 … +1200 mV (pH < 7)

Regeneration

Sprinkle a little aluminium oxide powder (Al
on the polishing cloth (ordering number of both these accesso­ries: 6.2802.000) and moisten with distilled water. Hold the elec­trode vertically and polish with a circular motion for about
20…60 s, exerting only light pressure. Rinse with distilled water
and clean with dry cloth. A glassy carbon electrode should always
be polished for a short while immediately before being fitted to
the detector cell.

, grain size 0.3 µm)

2O3

Ultra-Trace graphite electrode

Ordering designations 6.1204.100 Ultra-Trace graphite tip

+ 6.2103.110 Contact pin

Sensor material Graphite (turnover is three times larger than in

GC-electrode; does not need tamping)

Chemical resistance good resistance to acetonitrile, methanol, wa-

ter, etc.

Voltage range –800 … 0 … +1200 mV (pH < 7)

Regeneration

With 6.2827.000 Trimming tool (see electrode data sheet).

791 VA Detector

20

3.2 Electrodes

Carbon paste electrode CPE

Ordering designation 6.0807.000

Sensor material paste composed of spectroscopic grade car-

bon powder and paraffin

Chemical resistance good resistance to aqueous solutions; limited

resistance only to water/acetonitrile and wa­ter/methanol mixtures

Voltage range (–200) … 0 … +1200 mV (pH < 7)

Regeneration

The sensor surface of the CPE is more vulnerable to both me­chanical damage and chemical attack than that of the GCE, it is
more easily regenerated: often it suffices merely to rub the tip of
the electrode backwards and forwards a few times against the
surface of a sheet of ordinary, smooth paper, or alternatively the
upper layer of carbon paste can be renewed (see below) and
then polished in this manner.

Filling

Carefully remove the old carbon paste from the approximately

2.5 mm deep cavity with a suitable sharp instrument. Take care
not to damage the platinum contact wire at the bottom of the cav­ity. Using a small spatula, fill bit by bit with the fresh 6.2801.020
carbon paste and tamp well down using the 6.2826.000 stopper.
The cavity should be filled over the brim with paste. Now rub off
the excess paste by continuous circular motion against a sheet of
paper with a hard, smooth, non-porous surface; the circular mo­tion plus the light pressure will polish the surface of the paste,
which must be smooth and shiny when the operation is finished.
There should be no cracks in the peripheral zone where the paste
is up against the plastic material of the electrode shaft.

Metal electrodes

Ordering designations 6.1204.120 Pt tip

6.1204.130 Ag tip

6.1204.140 Au tip
+ 6.2103.110 Contact pin

Chemical resistance good resistance to acetonitrile, methanol, wa-

ter, etc.

Voltage range Pt: (–1200) … 0 … +200 mV (pH < 7)

Ag: (–800) … 0 … +400 mV (pH > 7)
Au: (–400) … 0 … +1200 mV (pH < 7)

Regeneration

Sprinkle a little aluminium oxide powder (Al
on the polishing cloth (ordering number of both these accesso­ries: 6.2802.000) and moisten with distilled water. Hold the elec­trode vertically and polish with a circular motion for about
20…60 s, exerting only light pressure. Rinse with distilled water
and clean with dry cloth. A metal electrode should always be pol­ished for a short while immediately before being fitted to the de­tector cell.

, grain size 0.3 µm)

2O3

791 VA Detector

21

3 Operation

Amalgamated electrode

"Home-made" amalgamated electrodes are prepared as follows:

Base material 6.1204.140 Au tip

Preparation Put a few drops of Hg on the front side of the

electrode and leave it for a few minutes. Knock
off excess Hg and rinse electrode with distilled
water.

Chemical resistance good resistance to acetonitrile, methanol, water
Voltage range –1200 … 0 mV

Regeneration polish electrode with aluminium oxide and re-

amalgamate

3.2.2 Reference electrode

The reference system of the 6.0727.000 Reference electrode (RE) is
Ag/AgCl/c(KCl) = 3 mol /L (see Fig. 5). The electrolyte vessel 41 can be
refilled. The KCl solution has to be renewed every 1…2 months.

Ordering designations for KCl solutions: 6.2308.020 (250 mL)

3.2.3 Auxiliary electrode

A gold electrode is built-in in the detector cell as an auxiliary electrode.

6.2313.000 (1 L)

791 VA Detector

22

3.3 Operating element functions

3.3 Operating element functions

Mains switch

The 791 VA Detector is switched on and off using mains switch
25 on the rear of the instrument (see Fig. 2):

I Instrument switched on
0 Instrument switched off

~

After the instrument has been switched on, the mains pilot lamp 1
lights up to show that the instrument is ready for use.

Relative compensation current

rel. is set using the

comp

I

comp

The relative compensation current I
potentiometer 10 (see Fig. 1). The value set is related to the
setting of the adjusting knob 8 (current I).

.

I0.01

Setting range –500 (red) … +500 (white) × 0.01 x I

Resolution 0.01 x I per scale division

Absolute compensation current

The absolute compensation current I
potentiometer 9 (see Fig. 1). The value set is not related to the
setting of the adjusting knob 8 (current I).

abs. is set using the

comp

I

comp

0.02 nA

Setting range

direct 0 … 1000 × 0.02 nA = 20 nA
with commutator 5 5 × 20 nA = 100 nA
with commutator 6 50 × 20 nA = 1000 nA

Resolution

direct 0.02 nA per scale division
with commutator 5 0.10 nA per scale division
with commutator 6 1.00 nA per scale division

The polarity of the absolute compensation current is set sepa­rately with commutator 7.

Changeover Stand-by/Measure

meas

For this changeover commutator 3 (see Fig. 1) is used.

Button depressed

Measuring position, i.e. measuring cell connected.

Button not depressed

Stand-by position, i.e. measuring cell disconnected.

791 VA Detector

23

3 Operation

Damping

damp

The damping is switched on and off using commutator 4 (see
Fig. 1).

Button depressed

Signal damped for all ranges set on adjusting knob 8 in or­der to suppress interference (spurious signals).

Button not depressed

No signal damping (exception: the most sensitive range,

0.1 nA, is always damped to some extent).

x50

x5

Multiplication of I

For this changeover commutator 5 (see Fig. 1) is used.

Button depressed

abs. by a factor of 5

comp

Multiplication of the absolute compensation current by a
factor of 5.

Button not depressed

No multiplication of the absolute compensation current.

Multiplication of I

abs. by a factor of 50

comp

For this changeover commutator 6 (see Fig. 1) is used.

Button depressed

Multiplication of the absolute compensation current by a
factor of 50.

Button not depressed

No multiplication of the absolute compensation current.

+

Polarity of I

For this changeover commutator 7 (see Fig. 1) is used.

Button depressed

comp

abs.

All current values set on the potentiometer 9 have a positive
sign.

Button not depressed

All current values set on the potentiometer 9 have a nega­tive sign.

791 VA Detector

24

3.3 Operating element functions

overload

Overload display

If the electronic current measuring circuit is overloaded, the red
light 2 is lit up (see Fig. 1).

Remedies

Select a less sensitive current range with adjusting knob 8
or compensate current with potentiometer 9 and/or 10 (for
further measures see section 4.3).

Polarization voltage

is set using the digital switch 11 (see

pol

+

015

U

10 mV

The detection voltage U
Fig. 1).

Setting range –1990 … 0 … +1990 mV

pol

Resolution 10 mV

Current measuring sensitivity

nA

50

10

5

1

0,5 100

0,1

(U

out

I

= 1 V)

µ

A

The sensitivity of current measurement is set using adjusting

0,1

knob 8 (see Fig. 1). The set range applies to the full-scale

0,5

deflection of the analog signal at sockets 22 and 23 (1 V or

1

5

10 mV depending on the setting of switch 21).

4

10

Setting ranges 0.1, 0.5, 1, 5, 10, 50 nA

0.1, 0.5, 1, 5, 100, 10'000 µA

Polarity of analog output

The polarity (+ or –) of the analog output signal is set using
switch 20 (see Fig. 2).

Full-scale deflection of analog output

0...1 V

0...10 mV

The full-scale deflection (1 V or 10 mV) of the analog output
signal is set using switch 21 (see Fig. 2).

791 VA Detector

25

3 Operation

3.4 Startup

3.4.1 Preparing the ELCD cell

The ELCD cell must be correctly connected to the HPLC or IC system
before it is filled (see section 2.3 and section 2.4). Then proceed as fol­lows:

1 Fill reference electrode (only if necessary)

Reference electrode 39 (6.0727.000) is supplied in a filled
condition. However, the KCl electrolyte solution must be re­placed every 1 to 2 months. Proceed as follows:

• Screw Ag/AgCl reference system 40 out of electrolyte vessel

41 (see Fig. 5).

• Fill electrolyte vessel 41 to the brim with c(KCl) = 3 mol/L.

• Screw Ag/AgCl reference system 40 back onto electrolyte

vessel 41. Take care that no air bubbles are found in the elec­trolyte vessel (to check: hold reference electrode against the
light).

2 Connect reference electrode

• Insert reference electrode 39 into cell body 32 (see Fig. 5).

• Connect reference electrode 39 to connection 17 of the 791

VA Detector with 6.2120.020 Cable (see Fig. 7).

3 Connect auxiliary electrode

• Connect the auxiliary electrode in cell body 32 to connection

15 of the 791 VA Detector 791 with 6.2120.020 Cable (see

Fig. 7).

4 Prepare working electrode

• Prepare and polish working electrode 28 according to section

3.2.1.

• Insert working electrode 28 into cell body 32 (see Fig. 5).

• Do not

at this point.

5 Start pump

• Switch on delivery drive of HPLC or IC pump.

connect working electrode 28 to the 791 VA Detector

791 VA Detector

26

3.4 Startup

3.4.2 Zero balancing and equilibration phase

Before the first measurement is made the working electrode must be al­lowed to equilibrate and the chart recorder or data recording instrument
must be zero-balanced. Proceed as follows:

1 Chart recorder settings

If a chart recorder is used to record the analog signals then
proceed as follows (the working electrode must not
nected to the 791 VA Detector):

• Switch on 791 VA Detector.

• Set commutator 3 on the 791 VA Detector to "Stand-by".

• Set the required polarity (+ or –) with switch 20; use switch

21 to set the required voltage range for the analog output on
the 791 VA Detector (1 V or 10 mV).

• Set potentiometer 9 on the 791 VA Detector to 0.

• Use knob 8 on the 791 VA Detector to set the current sensitiv-
ity to 0.5 µA.

• Switch on recorder.

• Set the voltage measuring range on the recorder to the same

setting as on the 791 VA Detector (1 V or 10 mV).

• Use potentiometer 10 on the 791 VA Detector to adjust the
zero point of the recorder.

be con-

2 Settings with a data recording program

If a PC recording and evaluation program is used for recording
the analog signals then proceed as follows (the working elec­trode must not

be connected to the 791 VA Detector):

• Switch on 791 VA Detector.

• Set commutator 3 on the 791 VA Detector to "Stand-by".

• Set the required polarity (+ or –) with switch 20; use switch

21 to set the required voltage range for the analog output on
the 791 VA Detector (1 V or 10 mV).

• Set potentiometers 9 and 10 on the 791 VA Detector to 0.

• Use knob 8 on the 791 VA Detector to set the current sensitiv-
ity to 0.5 µA.

• Switch on the data recording program on the PC and start

recording data.

• If required, use potentiometer 10 on the 791 VA Detector to
adjust the zero point (so that the mV display on the PC shows

0).

791 VA Detector

27

3 Operation

3 Overload test

For this test the working electrode must not be connected to the
791 VA Detector:

• Set the polarization voltage on the 791 VA Detector to the

required value for the following determinations with digital
switch 11.

• Set commutator 3 on the 791 VA Detector to "meas". Over-

load display 2 must light up briefly and then go out again.

• If overload display 2 remains lit up for a longer period of time

this indicates that there is a faulty electrical contact between
the reference electrode and the auxiliary electrode. Eliminate
the cause (e.g. air in the system or in the reference electrode,
blocked diaphragm, faulty cable connection) and repeat the
test.

• If the overload test is OK, set commutator 3 on the 791 VA

Detector back to "Stand-by".

4 Equilibrating the working electrode

• Connect working electrode 28 to connection 18 of the 791 VA

Detector with the 6.2120.020 Cable.

• Set commutator 3 on the 791 VA Detector to "meas".

• Observe equilibration curve: the curve should flatten out

noticeably within a relative short time (10…20 min), otherwise
the working electrode must be repolished or refilled.

• In current measuring ranges > 5 nA a constant base current

is usually achieved with one hour. At higher sensitivities the
working electrode should be equilibrated overnight.

5 Zero balancing the base current

(only for recorders)

• Set the current measuring sensitivity on the 791 VA Detector

with knob 8 to the required value.

• Use potentiometer 9 on the 791 VA Detector to adjust the

base current to the zero point of the recorder. If the setting
range of potentiometer 9 is not sufficient then it can be ex­tended by using commutator 5 (× 5) or 6 (× 50).

791 VA Detector

28

3.4 Startup

3.4.3 Measuring procedure

In order to be able to carry out measurements it is essential that the
working electrode has achieved a stable base current (see section

3.4.2). If the value of the base current regains its original value after the
determination then as many determinations as required can be carried
out. Attention should be given to the following points:

1 Carrying out the determination

• Inject the sample solution into the HPLC or IC system.

• Record and evaluate the current curve.

2 Allow ELCD system to run overnight

• If the ELCD system is to be used again on the following day
then the 791 VA Detector should not be switched off over­night. The working electrode is constantly under the polariza­tion current and is quickly ready for new measurements.

• The HPLC or IC pump is set to a minimal flow rate and runs
overnight. This means that the base current is stabilized after
only a few minutes.

3 Switching off the ELCD system

• Set commutator 3 on the 791 VA Detector to "Stand-by".

• Unplug working electrode 28.

• Switch off the 791 VA Detector.

791 VA Detector

29

3 Operation

791 VA Detector

30

4.1 Maintenance and servicing

4 Maintenance – Malfunctions

4.1 Maintenance and servicing

Care

The 791 VA Detector requires proper care and attention. Excessive con­tamination of the instrument could possibly lead to malfunctions and a
shorter service life of the inherently rugged mechanical and electronic
parts.

Spilled chemicals and solvents should be wiped up immediately. It is
especially important to protect the plug connections at the rear of the
instrument (particular the mains plug) against contamination.

Although constructional measures have been designed to virtually
eliminate such a situation, should corrosive media penetrate the
interior of the instrument the mains plug of the 791 VA Detector must
be immediately disconnected to prevent extensive damage to the
instrument electronics. Inform Metrohm service if your instrument have
been damaged in such a way.

The instrument must not be opened by untrained personnel. Please
comply with the safety notes in section 1.4.1.

Maintenance by Metrohm service

Maintenance of the 791 VA Detector is best done as part of an annual
service performed by specialists from the Metrohm company. If work is
frequently performed with caustic and corrosive chemicals, it may be
necessary to shorten the interval between servicing.

The Metrohm service department is always willing to offer expert advice
on the maintenance and servicing of all Metrohm instruments.

4.2 Shutdown

If the ELCD cell is shut down for a considerable length of time, the en­tire HPLC or IC system (without
rinsed free from salt with methanol/water (1:4) to avoid crystallization
of eluent salts with the corresponding subsequent damage.

column and suppressor) must be

791 VA Detector

31

4 Maintenance – Malfunctions

4.3 Malfunctions and their rectification

Malfunction Cause Rectification

No signal • VA Detector is in "stand-by"

mode.

• VA Detector switched off or

disconnected from mains
voltage.

• Recorder switched off or

disconnected from mains
voltage.

• Recorder disconnected from

VA Detector.

• Electrodes disconnected.

• Broken interconnecting

cables.

VA Detector
shows "overload"

• Current range set exceeded

by too high a concentration of
substance.

• The VA Detector is either

incorrectly balanced or not
balanced at all.

• Reference electrode discon-

nected.

• Auxiliary electrode discon-

nected.

• Polarization voltage badly

adjusted.

• Press "meas" button on VA

Detector.

• Connect VA Detector and

switch on.

• Connect recorder and switch

on.

• Connect recorder to the VA

Detector.

• Connect electrodes.

• Use new interconnecting

cables.

• Wait or set a higher current

range at the VA Detector.

• Balance the recording instru-

ment and VA Detector correctly.

• Connect reference electrode.

• Connect auxiliary electrode.

• Adjust polarization voltage

correctly.

Excessively high
background current

Baseline unstable

• Polarization voltage too high.

• Eluent contaminated.

• Eluent contaminated by Fe(II)

ions from rusty steel parts
(traces only suffice!)

• Highly retarded or enriched

components from the separat­ing column.

• Pressure fluctuations due to

poor seals, blocked filters or
capillaries, defective pump,
etc.

• Eluent contaminated by Fe(II)

ions from rusty steel parts
(traces only suffice!).

• Defective reference electrode.

• Polarization voltage should

generally not be set higher than
1200 mV.

• Prepare fresh eluent using only

the purest (AR grad) chemicals.

• Check entire HPLC or IC system

for rust; check resistance of all
materials to the reagents in use.

• Rinse the column well with

eluent or regenerate it, use new
column if necessary.

• Check HPLC or IC system for

these faults.

• Check entire HPLC or IC system

for rust; check resistance of all
materials to the reagents in use.

• Replace reference electrode.

791 VA Detector

32

4.3 Malfunctions and their rectification

Malfunction Cause Rectification

Noise level too high

Spikes on baseline

• Inadequate pulsation

damping.

• Electrostatic screening

defective.

• Background current too high.

• Eluent outlet tubing under

electrostatic influence.

• Defective working electrode.

• Defective 791 VA Detector.

• Air in detector cell.

• Air entering detector cell.

• If a CPE working electrode is

in use, the carbon paste is not
in good condition.

• Use pulsation dampener.

• Check screening, if necessary

connect steel column and/or its
outlet to socket 14 of the VA
Detector by means of a cable.

• Check screening, if necessary

connect steel column and/or its
outlet to socket 14 of the VA
Detector by means of a cable.

• Immerse eluent outlet tubing

completely in spent solution in
waste recipient or renew PTFE
tubing if necessary.

• Replace working electrode.

• Check 791 VA Detector using

the dummy cell (see section

4.1.4)

• Unscrew working electrode and

deaerate detector cell.

• Degas eluent or subject it to

appropriate treatment.

• Refill CPE and ensure paste is

well tamped down.

Isolated spikes on
baseline

Substance turnover
too low or non­existent

Substance turnover
falls off

Front peak too high • Environment altered by

• Electrostatic discharges from

operating personnel on the
apparatus.

• Mains interference.

• Electromagnetic interference

from environment.

• Polarization voltage too low.

• Active surface of working

electrode blocked by sub­stances.

• Active surface of working

electrode blocked by sub­stances.

• Quantity of substance too

great.

sample solution.

• Eluent contaminated by Fe(II)

ions from rusty steel parts
(traces only suffice!).

• Such discharges cannot be

completely avoided, but are not
usually too troublesome at
normal (i.e. not too dry) atmos­pheric humidity.

• Connect all instruments to the

power supply via a suitable filter
(e.g. Metrohm 615 Mains Dis­tributor).

• Change "environment".

• Increase polarization voltage.

• Polish working electrode or

renew paste in CPE.

• Polish working electrode or

renew paste in CPE.

• The sample quantity should be

less than 100 ng per injection to
avoid blocking of the electrode
surface.

• Whenever possible, use eluent

as solvent for the sample solu­tion.

• Check entire HPLC or IC system

for rust; check resistance of all
materials to the reagents in use.

791 VA Detector

33

4 Maintenance – Malfunctions

4.4 Instrument test with the dummy cell

The correct functioning of the 791 VA Detector can be checked with the
included 6.2813.020 Dummy cell. Several simple checks can be carried
out with this extremely simplified electrical simulation of a detector cell.

Fig. 9 shows the construction of the dummy cell. The checks described
below can be carried out with the recording instrument which is con­nected (chart recorder, integrator, PC data recording program) or, for
precise measurements, with a voltmeter which is connected to analog
outputs 22 and 23. The dummy cell is connected to the corresponding
cable instead of the electrode. Each test can be carried out separately.
The settings of any operating elements which are not mentioned are ir­relevant.

AE

10 M

Ω

1%

Ω

10

1 µF

RE

Fig. 9

: Circuit of 6.2813.020 Dummy Cell

WE

4.4.1 Check of "meas" and "damp" commutators and of current amplifier offset

1 Set digital switch 11 U

to +1000 mV.

pol

+

100

2 Set adjusting knob 8 to 0.1 µA.

3 Set potentiometer 9 I

791 VA Detector

34

abs. to 0.

comp

50

10

5

1

0,5 100

0,1

0.02 nA

I

comp

0,1

0,5

1

5

4

10

4.4 Instrument test with the dummy cell

4 Set all commutators 3 … 7 to "off" position

(non-depressed position).

5 Set selector 20 on the rear panel to +.

6 Set selector 21 on the rear panel to 0…10 mV.

7 Bring recorder of voltmeter to 0 using potenti-

ometer 10 I

rel.. The value shown on the

comp

knob scale must not vary from 0 by more than
120 scale divisions.

8 Depress commutator 3 "meas": The recorder or

voltmeter should read –10 mV.

9 Depress commutator 4 "damp" and wait until

recorder of voltmeter have attained a stable
position.

0...1 V

0...10 mV

I

comp

meas

damp

.

I0.01

10 Set commutator 3 "meas" to "off" position: The

time constant τ to be measured, i.e. the time
which has to elapse until 63.2 % of full-scale
deflection is reached, should have a value of
app. 3 s (consider τ of recorder or voltmeter).

4.4.2 Check output voltage

1 Set digital switch 11 U

2 Set adjusting knob 8 to 0.1 µA.

to 0 mV.

pol

meas

+

000

50

10

5

1

0,5 100

0,1

0,1

0,5

1

5

4

10

791 VA Detector

3 Set commutators 4 … 7 to "off" position (non-

depressed position).

35

4 Maintenance – Malfunctions

4 Set selector 20 on the rear panel to –.

5 Set selector 21 on the rear panel to 0…1 V.

6 Set recorder sensitivity to 1000 mV.

7 Set commutator 3 "meas" to "on" position

(depressed position).

8 Set potentiometer 9 I

abs. to 0.

comp

9 Bring recorder of voltmeter to 0 using potenti-

ometer 10 I

10 Alter the settings of digital switch 11 U

comp

rel..

pol

in

steps of 100 mV and 10 mV: The settings
shown on the digital switch should be repro­duced on the recorder or voltmeter within a
tolerance of <12.5 mV and <1.25 mV respec­tively.

0...1 V

0...10 mV

meas

I

comp

0.02 nA

+

I

comp

.

I0.01

140

11 Set digital switch 11 U

to – and alter the

pol

settings in steps of 100 mV and 10 mV: The
settings shown on the digital switch should be
reproduced on the recorder or voltmeter within
a tolerance of <12.5 mV and <1.25 mV respec­tively.

4.4.3 Check of current compensation

1 Set digital switch 11 U

2 Set adjusting knob 8 to 0.1 µA.

to 0 mV.

pol

250

+

000

50

10

5

1

0,5 100

0,1

0,1

0,5

1

5

4

10

791 VA Detector

36

4.4 Instrument test with the dummy cell

3 Set selector 20 on the rear panel to –.

4 Set selector 21 on the rear panel to 0…1 V.

5 Set commutator 3 "meas" to "on" position

(depressed position).

6 Set potentiometer 9 I

abs. to 0.

comp

7 Bring recorder of voltmeter to 0 using potenti-

ometer 10 I

8 If potentiometer 10 I

comp

rel..

rel. is now moved

comp

counterclockwise by 100 divisions (= 1 turn)
then this should correspond to 1000 mV on the
recording instrument or voltmeter.

0...10 mV

0.02 nA

0...1 V

meas

I

comp

I

comp

I

comp

.

I0.01

.

I0.01

9 If potentiometer 9 I

abs. is now moved

comp

clockwise by 100 divisions (= 1 turn) then this
should correspond to the following values on
the recording instrument or voltmeter:

direct: +20 mV

×5 button 5 depressed: +100 mV
×50 button 6 depressed: +1000 mV

10 Set commutator 7 (+) to "on" position

(depressed position).

11 Similarly check the functioning of potentiometer

10 I

rel. according to items 7…9.

comp

I

comp

0.02 nA

I

comp

+

.

I0.01

791 VA Detector

37

4 Maintenance – Malfunctions

4.4.4 Check of current and voltage overload

1 Set digital switch 11 U

to +10 mV or

pol

–10 mV.

2 Set adjusting knob 8 to 50 nA.

3 Set selector 20 on the rear panel to –.

4 Set selector 21 on the rear panel to 0…1 V.

5 Set potentiometer 9 I

abs. to 0.

comp

+

001

50

10

5

1

0,5 100

0,1

0...1 V

0...10 mV

I

comp

0.02 nA

0,1

0,5

1

5

4

10

6 Set commutators 3 … 7 to "off" position (non-

depressed position).

7 Bring recorder of voltmeter to 0 using potenti-

ometer 10 I

comp

rel..

8 Set commutator 6 "×50" to "on" position

(depressed position).

9 Turn potentiometer 9 I

abs. clockwise until

comp

overload display 2 lights up: The voltage at the
recorder output should be app. 10.5…12 V.
Without digital voltmeter, read the value on the
scale of potentiometer 9: The value should be
between 500 and 620 scale divisions.

10 Reset potentiometer 9 I

abs. to 0: The

comp

overload display 2 extinguishes.

I

comp

.

I0.01

x50

I

comp

0.02 nA

0.02 nA

791 VA Detector

38

I

comp

4.4 Instrument test with the dummy cell

11 Set commutator 3 "meas" to "on" position

(depressed position).

12 Unplug dummy cell at "AE": the overload

display 2 lights up.

4.4.5 Check sensitivity knob 8

1 Unplug dummy cell.

2 Set adjusting knob 8 to 50 nA.

3 Set commutators 3 … 7 to "off" position (non-

depressed position).

meas

50

10

5

1

0,5 100

0,1

0,1

0,5

1

5

4

10

4 Set potentiometer 9 I

abs. to 0.

comp

5 Set selector 21 on the rear panel to 0…1 V.

6 Bring recorder of voltmeter to 0 using potenti-

ometer 10 I

rel. (set line recorder first to mid-

comp

scale).

7 Turn adjusting knob 8 through all positions and

check the deviation from zero after a waiting
time of app. 5 s. Deviation should be < ±1 V.

I

comp

0.02 nA

0...1 V

0...10 mV

I

comp

.

50

10

5

1

0,5 100

0,1

I0.01

0,1

0,5

1

5

4

10

791 VA Detector

39

4 Maintenance – Malfunctions

4.4.6 Check of noise from current amplifier

1 Connect dummy cell.

2 Set digital switch 11 U

to 0 V.

pol

3 Set adjusting knob 8 to 1 nA.

4 Set commutator 3 "meas" to "on" position

(depressed position).

5 Set commutators 4 … 7 to "odd" position

(non-depressed position).

6 Set selector 21 on the rear panel to 0…1 V.

+

000

50

10

5

1

0,5 100

0,1

meas

0...1 V

0...10 mV

0,1

0,5

1

5

4

10

7 Allow the recording instrument to record for a

longer period of time (first adjust chart recorder
to middle of the scale). Typical deflections
caused by noise are about 10 mVpp or less (the
656 housing must be earthed).

791 VA Detector

40

5.1 Technical data

5 Appendix

5.1 Technical data

Measuring instrument

Measuring technique Direct current amperometry

Polarization voltage U

Current measurement Measurement: direct, not integrated

Damping Switchable

Compensation current I

absolute

Range: –1990 … 0 … +1990 mV

pol

Resolution: 10 mV
Deviation: ± 1 % ± 1 mV

Sensitivity: 0.1, 0.5, 1, 5, 10, 50 nA

0.1, 0.5, 1, 5, 100, 10'000 μA
Deviation: ± 15 % for 0.1 nA … 5 nA
± 1.5 % for 10 nA … 0.1 μA
± 1.0 % for 0.5 … 100 μA
± 4.0 % for 10'000 μA
Time constants: ≈ 5.0 s for < 0.1 nA
≈ 0.6 s for 0.1 ... 0.5 nA
≈ 0.3 s for 0.5 ... 1 nA
≈ 0.1 s for 1 ... 5 nA

Time constant: 1.7 s

comp

Basic range: –20 … 0 … +20 nA
Resolution: 0.02 nA
Multiplication: 5 ×, 50 ×

relative

Potentiostat Input resistance: 1012 Ω (typ. value)

Analog output

Signal U = f ( I )

Polarity Switchable

Max. load < 5 mA

Input resistance at
connected instrument

Range: –5.00 … 0 … +5.00· I
Resolution: 0.01· I

Input current: < 5 pA (typ. value)
Zero-point drift: 20 μV/K (typ. value)
Output voltage: ± 10 V
Output current: ≤ 10 mA

1 V or 10 mV DC for the full current range I

> 2 kΩ

791 VA Detector

41

5 Appendix

Mains connection

Voltage 115 V: 100...120 V ± 10 %

230 V: 220...240 V ± 10 %
Switchable with voltage selector insert in the fuse
holder (see section 2.2.1)

Frequency 50...60 Hz

Power consumption 5 VA

Fuse 5 mm dia., 20 mm length

100…240 V: 1 A (slow-blow)

Safety specifications

Construction/testing According to IEC 1010 / EN 61010 / UL 3101-1,

protection class 1, degree of protection IP40

Safety directions The Instructions for Use include information and

warnings which must be heeded by the user to
assure safe operation of the instrument.

Electromagnetic compatibility (EMC)

Emitted interference Standards met:

EN55011 (class B), EN55022 (class B),
EN 50081-1/2

Immunity to interference Standards met:

IEC801-2/IEC1000-4-2 (class 3), IEC801-4/
IEC1000-4-4 (class 3), IEC1000-3/EN61000-3,
IEC1000-4-11/EN61000-4-11

Ambient temperature

Nominal operating range +5…+40°C

(at 20…80 % atmospheric humidity)

Storage, transport –40…+70°C

Housing

Material of cover Polyurethane rigid foam (PUR) with fire protection

for fire class UL94VO, CFC-free

Material of base Steel, enameled

Dimensions

Width 260 mm

Height 129 mm

Depth 366 mm

Weight 3.6 kg (with accessories)

791 VA Detector

42

5.2 Standard equipment

59

5.2 Standard equipment

Subject to changes !
All dimensions are given in mm.

5.2.1 2.791.0010 VA Detector for HPLC

The 2.791.0010 VA Detector includes the following parts:

Quant. Order No. Description

1 6.2105.030 Cable

Connection cable 791 VA Detector
(analog output) – recorder

1 6.2122.0X0 Mains cable

to customer's specifications:

Cable socket Cable connector

Type IEC 320/C 13 Type SEV 12 (CH…)............................... 6.2122.020

Type IEC 320/C 13 Type CEE (7), VII (D…) .......................... 6.2122.040

Type CEE (22), V Type NEMA 5-15 (USA…)...................... 6.2122.070

1 6.2813.020 Dummy Cell

Electrical simulation of the ELCD cell for
testing and checking functions

1 8.791.1013 Instruction for Use (English)

for 791 VA Detector

1.2 m

791 VA Detector

43

5 Appendix

67

∅

∅

59

5.2.2 2.791.0020 VA Detector for IC

The 2.791.0020 VA Detector includes the following parts:

Quant. Order No. Description

1 6.0727.000 Reference electrode

Mini reference electrode for ELCD
cell with Ag/AgCl/c(KCl) = 3 mol/L
reference system;
including cap

3 6.2120.020 Electrode cable

Electrode cable with mini socket
for connection pins 2 mm and mini
plug type F

1 6.2122.0X0 Mains cable

to customer's specifications:

Cable socket Cable connector

Type IEC 320/C 13 Type SEV 12 (CH…) ...............................6.2122.020

Type IEC 320/C 13 Type CEE (7), VII (D…) ...........................6.2122.040

Type CEE (22), V Type NEMA 5-15 (USA…).......................6.2122.070

1 6.2617.000 Tool for PTFE gasket

To remove 6.2704.010 and 6.2704.020
PTFE gaskets

∅ 18

83

2 m

4

67

4 6.2704.010 PTFE gasket

Gasket for reference electrode,
working electrode and eluent inlet

3 6.2704.020 PTFE gasket

Gasket for eluent inlet

1 6.2813.020 Dummy Cell

Electrical simulation of the ELCD cell for
testing and checking functions

1 6.5303.030 ELCD cell

1 8.791.1013 Instruction for Use (English)

for 791 VA Detector

4.3

∅ 12

6

4.5

791 VA Detector

44

5.3 Optional accessories

∅

∅

M3

∅

5.3 Optional accessories

Order No. Description

6.0805.010 Mini Glassy Carbon electrode
Diameter of active zone: 2.8 mm.

6.0807.000 Mini Carbon paste electrode
Diameter of active zone: 3 mm.

6.1204.XXX Electrode tips for working electrode

Together with 6.2103.110 Contact pin forms the working
electrode. The following electrode tips are available:

Order No. Disk material Shaft material

6.1204.100 Ultra Trace Graphite PVC

6.1204.120 Pt PEEK

6.1204.130 Ag PEEK

6.1204.140 Au PEEK

Disk diameter: 2.0 +0 / –0.05 mm

6.2103.110 Contact pin

Together with 6.1204.1X0 Electrode tip forms the
working electrode.

7

45

7

45

52.5

21.3

7

6.2801.020 Carbon paste

Carbon paste for 6.0807.000 Mini carbon
paste electrode

6.2826.000 Filling tool

Filling tool for 6.0807.000 Mini carbon paste
electrode

20

83

791 VA Detector

45

5 Appendix

5.4 Warranty and conformity

5.4.1 Warranty

The warranty on our products is limited to defects that are traceable to
material, construction or manufacturing error which occur within 12
months from the day of delivery. In this case, the defects will be recti­fied in our workshops free of charge. Transport costs are to be paid by
the customer.

For day and night operation, the warranty is limited to 6 months.

Glass breakage in the case of electrodes or other parts is not covered
by the warranty. Checks which are not a result of material or manufac­turing faults are also charged during the warranty period. For parts of
outside manufacture insofar as these constitute an appreciable part of
our instrument, the warranty stipulations of the manufacturer in question
apply.

With the regard to the guarantee of accuracy, the technical specifica­tions in the instruction manual are authoritative.

Concerning defects in material, construction or design as well as the
absence of guaranteed features, the orderer has no rights or claims ex­cept those mentioned above.

If damage of the packaging is evident on receipt of a consignment or if
the goods show signs of transport damage after unpacking, the carrier
must be informed immediately and a written damage report demanded.
lack of an official damage report releases Metrohm from any liability to
pay compensation.

If any instruments and parts have to be returned, the original packaging
should be used if at all possible. This applies above all to instruments,
electrodes, burette cylinders and PTFE pistons. Before embedment in
wood shavings or similar material, the parts must be packed in a dust­proof package (for instruments, use of a plastic bag is imperative). If
open assemblies are enclosed in the scope of delivery that are sensi­tive to electromagnetic voltages (e.g. data interfaces etc.) these must
be returned in the associated original protective packaging (e.g. con­ductive protective bag). (Exception: assemblies with built-in voltage
source belong in a non-conductive protective packaging). For damage
which arises as a result of non-compliance with these instructions, no
warranty responsibility whatsoever will be accepted by Metrohm.

791 VA Detector

46

5.4 Warranty and conformity

5.4.2 EU Declaration of conformity

EU Declaration of Conformity

The METROHM AG company, Herisau, Switzerland hereby certifies, that the in­strument:

791 VA Detector

meets the requirements of EC Directives 89/336/EEC and 73/23/EEC.

Source of the specifications:

EN 50081-1/2 Electromagnetic compatibility, basic specification;

Emitted Interference

EN 61010 Safety requirements for electrical laboratory measurement

and control equipment

Description of the instrument:

High-sensitivity current measuring instrument for electrochemical detection

in HPLC and IC.

Herisau, September 11, 1998

Dr. J. Frank Ch. Buchmann

Development Manager Production and
Quality Assurance Manager

791 VA Detector

47

5 Appendix

5.4.3 Certificate of conformity and system validation

Certificate of Conformity and System Validation

This is to certify the conformity to the standard specifications for electrical appli­ances and accessories, as well as to the standard specifications for security and
to system validation issued by the manufacturing company.

Name of commodity: 791 VA Detector

Name of manufacturer: Metrohm Ltd., Herisau, Switzerland

Principal technical information: Voltages: 100…120, 220…240 V
Frequency: 50…60 Hz

This Metrohm instrument has been built and has undergone final type testing
according to the standards:

IEC801-2/IEC1000-4-2 (class 3), IEC801-4/IEC1000-4-4 (class 3),
IEC1000-3/EN 61000-3, IEC1000-4-11/EN 61000-4-11, EN55011 (class B),
EN55022 (class B), EN50081-1/2
— Electromagnetic compatibility

IEC1010, EN61010, UL3101-1 — Security specifications

The technical specifications are documented in the instruction manual.

Metrohm Ltd. is holder of the SQS-certificate of the quality system ISO 9001 for
quality assurance in design/development, production, installation and servicing.

Herisau, September 11, 1998

Dr. J. Frank Ch. Buchmann

Development Manager Production and
Quality Assurance Manager

791 VA Detector

48

5.5 Index

5.5 Index

A

Absolute compensation current........... 23

Adjusting knob 8

Figure...................................................2

Operation.......................................... 25

Test ................................................... 39

Ag electrode ......................................... 21

Amalgamated electrode....................... 22

Ambient temperature............................ 42

Analog output 22

Connection ....................................... 14

Figure...................................................3

Technical data .................................. 41

Analog output 23

Connection ....................................... 14

Figure...................................................3

Technical data .................................. 41

Appendix............................................... 41

Arrangement of the instruments..............7

Au electrode .................................... 21,22

Auxiliary electrode

Connect to 791................................. 26

Notes ................................................ 22

B

Back pressure ...................................... 18

Background current ........................ 20,32

Base current ......................................... 29

Baseline unstable ................................. 32

C

Cable (6.2105.030)............................... 43

Cable (6.2106.020)............................... 10

Cable (6.2115.010)............................... 14

Cable (6.2115.060)............................... 14

Cable (6.2120.020)..................... 10,26,44

Carbon paste........................................ 45

Carbon paste electrode .................. 21,45

Care ...................................................... 31

Caution ....................................................5

Cell body 32

Assembly .......................................... 11

Figure........................................... 12,13

Certificate of conformity

and system validation ...................... 48

Changeover Stand-by/Measure........... 23

Chart recorder ................................. 14,27

Check ......................................................7

Classes of substances ......................... 15

Classes of substances

which can be determined................. 15

Cleanliness ........................................... 18

Comment.................................................5

Commutator 3

Figure...................................................2

Operation.......................................... 23

Test ................................................... 35

Commutator 4

Figure...................................................2

Operation.......................................... 24

Test ................................................... 35

Commutator 5

Figure ..................................................2

Operation ..........................................24

Commutator 6

Figure ..................................................2

Operation ..........................................24

Commutator 7

Figure ..................................................2

Operation ..........................................24

Compensation current ..........................41

Conformity.............................................47

Connection 15 "AE"

Connecting the

auxiliary electrode .............................13

Figure ..................................................3

Connection 17 "RE"

Connecting the

reference electrode ......................13,26

Figure ..................................................3

Connection 18 "WE"

Connecting the working electrode.... 13

Connection of working electrode......28

Figure ..................................................3

Connection 19

Figure ..................................................3

Connection 656 – 791...........................10

Contact pin (6.2103.110) ......................45

Current amplifier offset.......................... 34

Current compensation ..........................36

Current measurement ........................... 41

Current measuring sensitivity................25

D

Damping...........................................24,41

Data recording instrument ....................27

Declaration of conformity ......................47

Degree of protection ..........................6,42

Detector block 42

ELCD cell assembly ..........................11

Figure ................................................13

Detector signal ...................................... 16

Diffusion-controlled limiting current ...... 17

Digital switch 11

Figure ..................................................2

Dimensions ...........................................42

Disposal...................................................6

Dummy cell .................................34,43,44

E

Earthing ................................................6,9

Earthing socket 14

Figure ..................................................3

ELCD cell 27

Assembly...........................................11

Connection to VA Detector 791 ........13

Figure ................................................12

Ordering designation ........................44

Startup...............................................26

Electrical safety .......................................6

Electrochemical detection.....................15

Electrochemical Detector 656

Connection to VA Detector 791 ........10

Installation ......................................... 10

Electrode cable (6.2120.020) ...............10

Electrodes ............................................. 20

Electrolyte vessel 41

Assembly...........................................11

Figure ................................................ 12

Filling reference electrode ................26

Electromagnetic compatibility ..............42

Eluent ....................................................18

EMC.......................................................42

Emitted interference..............................42

Equilibration phase ............................... 27

EU Declaration of conformity................ 47

F

Filling tool .............................................. 45

Front ........................................................ 2

Front peak ............................................. 33

Full-scale deflection .............................. 25

Fuse holder 12

Change fuses...................................... 8

Figure ............................................... 3,9

Fuses............................................... 8,9,42

G

General precautionary rules ...................6

Glassy Carbon electrode .................20,45

Gold electrode ......................................45

H

Handling of solvents ...............................6

Hazard..................................................... 5

Housing.................................................42

I

Immunity to interference .......................42

Information on the

Instructions for Use............................. 4

Installation ............................................... 7

Instructions for Use (8.791.1013) ...........4

Instrument description ............................ 1

Instrument test ......................................34

Interference level................................... 20

Introduction ............................................. 1

K

Knurled screw 43

ELCD cell assembly.......................... 11

Figure ................................................ 13

L

Leaks.......................................................6

Location .................................................. 7

791 VA Detector

49

5 Appendix

M

Mains cable................................... 9,43,44

Mains connection.................................. 42

Procedure ........................................... 8

Safety notes ........................................6

Mains connection plug 26

Figure ............................................... 3,9

Mains connection................................ 9

Mains frequency ................................... 42

Mains pilot lamp 1

Figure .................................................. 2

Ready for use display ....................9,23

Mains switch 25

Figure ............................................... 3,9

Operation ..........................................23

Switching the instrument on/off .......... 9

Mains voltage........................................42

Setting ................................................. 8

Mains voltage indicator 13

Figure .................................................. 3

Maintenance .........................................31

Malfunctions.......................................... 32

Measuring procedure ...........................29

Measuring technique ............................ 41

Metal electrodes ..............................21,45

Metrohm service ...................................31

Multiplication ......................................... 24

N

No signal ............................................... 32

Noise ........................................... 18,20,40

Noise level............................................. 33

Notation................................................... 5

O

Opening the 791 VA Detector................. 6

Operating element functions ................23

Operation ..............................................15

Optional accessories ............................ 45

Organization............................................ 4

Outlet capillary 37

Assembly ..........................................11

Figure ........................................... 12,13

Outlet capillary 38

Assembly ..........................................12

Figure ........................................... 12,13

Output voltage ......................................35

Overload........................................... 32,38

Overload display 2

Figure .................................................. 2

Operation ..........................................25

Overload test......................................... 28

P

Packaging ............................................... 7

Parts and controls...................................2

Pictograms.............................................. 5

Pilot voltage 24

Figure .................................................. 3

Platinum electrode ................................ 45

Polarity.........................................24,25,41

Polarization voltage................ 15,17,25,41

Potentiometer 9

Figure.................................................. 2

Operation .......................................... 23

Potentiometer 10

Figure.................................................. 2

Operation .......................................... 23

Potentiostat ........................................... 41

Power consumption.............................. 42

Practical information about ELCD ........ 18

Precautionary rules ................................. 6

Protection class ..............................6,9,42

Protective earth....................................... 9

Pt electrode........................................... 21

PTFE gasket 31

Assembly ..........................................11

Figure................................................ 12

Ordering designation........................ 44

PTFE gasket 35

Assembly ..........................................11

Figure................................................ 12

Ordering designation........................ 44

Pulsation ............................................... 18

Pulsation dampener ............................. 18

R

Rear......................................................... 3

Reference electrode 39

Assembly ..........................................11

Connect to 791 ................................. 26

Figure........................................... 12,13

Filling................................................. 26

Notes................................................. 22

Ordering designation........................ 44

Reference system 40

Figure................................................ 12

Filling reference electrode ................ 26

Relative compensation current............. 23

Response characteristics ..................... 20

Rinsing the IC system........................... 31

S

Safety directions ...................................42

Safety notes ............................................ 6

Safety specifications............................. 42

Sample solution .................................... 19

Screw nipple 29

Assembly ..........................................11

Figure................................................ 12

Screw nipple 33

Assembly ..........................................11

Figure................................................ 12

Screw nipple 36

Assembly ..........................................11

Figure................................................ 12

Sealing ring 30

Figure................................................ 12

Sealing ring 34

Figure................................................ 12

Selection of polarization voltage .......... 17

Selector 20

Figure.................................................. 3

Selector 21

Figure.................................................. 3

Serial number 16

Figure.................................................. 3

Service .................................................. 31

Servicing ............................................... 31

Setting the mains voltage....................... 9

Setting up the instrument ....................... 7

Settings

Mains voltage ..................................... 8

Shutdown.............................................. 31

Silver electrode ..................................... 45

Spikes ................................................... 33

Spilled chemicals ................................. 31

Standard equipment............................. 43

Startup .................................................. 26

Static charges......................................... 6

Stationary phase................................... 19

Storage ................................................. 42

Substance turnover .............................. 33

Switch 20

Operation.......................................... 25

Switchover ........................................ 14

Switch 21

Operation.......................................... 25

Switchover ........................................ 14

Switching on/off the instrument......... 9,23

T

Technical data ...................................... 41

Tool (6.2617.000) ................................. 44

Transport ................................................ 7

Transport damage................................ 46

U

Ultra Trace Graphite electrode........ 20,45

V

Voltage selection insert 13

Change fuses ..................................... 8

Figure.................................................. 9

W

Warning................................................... 5

Warranty................................................ 46

Waste container.................................... 12

Weight................................................... 42

Working electrode 28

Assembly .......................................... 11

Connect to 791 ................................. 26

Equilibrate......................................... 28

Figure........................................... 12,13

Overview ........................................... 20

Z

Zero balancing...................................... 27

791 VA Detector

50