Agilent Technologies SpectrAA User Manual
Practical Use of SpectrAA Series for
Multielement Analysis
Author
Brian D. Frary
Application Note
Atomic Absorption
Introduction
The SpectrAA system is designed for automatic multi-element analysis in flame furnace and hydride modes, up to 12 elements in 67 samples (45 in furnace mode) with
minimum operator supervision. When operating in the flame mode an operator
should be near at hand but can be carrying out other tasks.
When operating in the furnace mode the system can be left unattended as there are
no flammable gases and no exposed ignition source hence it can be left to operate
overnight.
A scenario is described for analysis by flame and shows how even a very complex
analytical program is easily handled by the SpectrAA.
The components of the system are:
SpectrAA-30 or SpectrAA-40 Atomic Absorption
Spectrophotometer
PSC-56 Programmable Sample Changer
GTA-96 Graphite Tube Atomizer
VGA-76 Vapor Generator Accessory
DS-15 Data Station
The above units except for the DS-15, are only fitted with an on/off power switch.
Programming and control of each unit in the system is through the DS-15 keyboard
but no computer programming knowledge is required by the operator. In fact the
system is designed for one finger operation using the “fill in the form” (F.I.T.F)
method developed by Agilent Technologies.
2
required (from 0.1% to 99.9%) and set a default time (1 to 500
seconds) for the system to achieve this. If the system is
unable to achieve the precision within the default time specified, measurement will stop and the precision achieved
printed out with the result. (The smaller the absorbance
signal the longer the measurement time required to obtain a
given precision).
This facility is extremely useful when high precision is required
but the element concentration (and thus absorbance) is
expected to vary widely. The operator can set the default time
for the lowest expected absorbance knowing that higher
absorbances will be read in a shorter time with no loss of
precision or waste of analysis time.
The concentrations of lead are expected to be low thus the
air-acetylene burner will be used to obtain higher sensitivity.
(NOTE When high sensitivity is not required a burner change
can be avoided by using a nitrous oxide-acetylene burner for
an air-acetylene flame. This eliminates programming a
“Pause” to change burners). PROMT mode could be used for
measurement but in this case the mean of three replicate
readings of 7-second integration is sufficient for the analytical
requirements.
Chromium forms ferro-chrome compounds in the air-acetylene flame and will therefore be determined using the nitrous
oxide-acetylene flame. This flame will also be used to determine molybdenum and silicon. All three elements will be
determined using five second integration and three replicates.
A delay of five seconds is programmed when using continuous aspiration so that each solution will be aspirated for five
seconds before any readings are taken. This ensures that the
previous solution is washed out of the nebulizer/burner
system. Cross contamination is prevented and the determinations will be more accurate. In addition a RINSE RATE and
RINSE TIME may be programmed if required (where sample
concentrations cover a wide range).
RECALIBRATION RATE and/or RESLOPE RATE are selected
on previous experience with the samples and elements.
Typical values could be reslope 10, recalibration 20.
Up to five standards may be used for each element calibration. 5 mixed standards will be loaded into the sampler
carousel, but the number used will vary from element to
element (see Table 1).
Versatility and ease of use has been achieved in the system
design. The user friendly programming is designed to lead the
operator through the correct procedures.
At power up the DS-15 screen displays 3 modes of operation.
The operator selects one mode using the soft keys labelled:
DEVELOP METHOD
MODIFY METHOD
AUTOMATIC RUN
DEVELOP METHOD used by a chemist to set up and store
methods for the use of other staff.
MODIFY METHOD used in a similar manner.
AUTOMATIC RUN used for routine automatic analysis.
Flame Analytical Procedure
The following scenario shows how aII the facilities of the
system may be used to perform a complex analytical program.
A laboratory has the following samples for analysis:
• 20 steel samples for the determination of chromium,
copper, lead, manganese, molybdenum, nickel and silicon.
• 10 water samples for the determination of iron, sodium
and arsenic.
• 10 slag samples for the determination of calcium, iron
and silicon.
Steel Samples
Analytical method development has shown that most of the
elements in the steel samples can be determined by normal
calibration using mixed standards containing iron. The
SpectrAA- will be calibrated in the concentration mode using
the integration measurement mode for these elements.
Chromium and nickel concentrations must be determined
accurately over a narrow range and bracketing standards calibration will be used for these determinations. High precision
of measurement is also required thus PROMT mode will be
used.
PROMT is Precision Optimised Measurement Time. This
allows the operator to specify the precision of measurement
3
Water Samples
EPA methods specify standard additions calibration for water
analysis. Iron will be determined by this method using the airacetylene flame burning on a nitrous oxide-acetylene burner.
Continuous aspiration would normally be used for water samples but in this case the program “Fe in Slag” will be used
which uses the micro-sampling technique. Thus two different
sample types can be run on one program, the recalibration
rate being defined by the number of water samples.
Arsenic will also be determined by standard additions calibration but using hydride generation. Sodium will be determined
by EMISSION using normal calibration.
Two addition standards will be prepared from sample 1 and
used to produce the calibration graph. The other 9 samples
will be measured against this calibration because the matrices will be very similar.
Slag Samples
As the matrices will be complex and variable all elements will
be determined using STANDARD ADDITIONS calibration. The
dissolved solids concentration will be very high therefore continuous aspiration will not be used. Instead, the MICRO-SAMPLING feature of the PSC-56 will be used and measurements
made in peak height mode.
In micro-sampling the sampling probe dips into the solution
for a programmed time. During this time a certain volume of
liquid is drawn into the flame, the longer the time the greater
the volume (for a one second dip about 350 µL). An integration “window” of eight seconds is set on the system. This
allows enough time for the solution to travel through the nebulizer/burner system into the optical path to be measured. A
volume of 200 µL or more will give the same signal as continuous aspiration, 100 µL will give about 80% and 50 µL about
50% of the continuous aspiration signal. Micro-sampling is
also useful when only small volumes of sample are available
[1,2,3,4].
A RINSE between each sample and a SAMPLE FLUSH will
also be programmed. Using sample flush the probe dips into a
solution for half a second, the slug of liquid is drawn through
the capillary “flushing out” any traces of the previous solution
aspirated: no measurement is made on this solution. After
about 10 seconds the probe dips into the solution for the
specified dip time and the signal produced is measured.
Iron will be determined using the air-acetylene flame, silicon
and calcium using the nitrous oxide-acetylene flame, both
flames being run on the nitrous oxide-acetylene burner.
Typical system parameters for some elements are shown in
Figures 1 to 6.
Table 1.
Although RECAL will not be used for program 1, it is included here as it is part of the stored program.
AA = Air-Acetylene
NA = Nitrous Oxide-Acetyleen
Loading...