Page 1
Rosemount Gas Analysis
Frequently Asked Questions (FAQs)
GC Webinar Series
March 2019
Gas Chromatograph's Response Factors
and Why They Are Important
Emerson hosted the webinar "Gas Chromatograph’s Response Factors". These are the questions asked by the
audience members along with the answers provided by our subject matter experts.
■ How much does a variation of the carrier pressure
affect GC’s Response Factor (RF)?
The variation in carrier pressure does not directly impact Response
Factors. It can indirectly impact Response Factors. If the pressure
is increased, ow will increase and the components will elute
out faster. The Retention Time (RT) will shift in - all else being
constant. The shifting of when components elute may cause
components to be affected by valve setting (valve timing) and/
or inhibits time events. The peak area of certain components thus
can be impacted. If the peak area of a component is impacted,
then the Response Factor of that component will be impacted as
Response Factor = peak area/calibration concentration.
■ What can cause Retention Time (RT) to shift
in a day and night cycle?
Retention Time (RT) is impacted by order of elution, pressure,
and ow of the carrier gas and time events. It can also be impacted
by restrictions/contamination as they impact ow and pressure.
When one thinks of changes happening from day to night, one
thinks of things affected by temperature variation. An increase
in temperature would increase the ow, thus RT would move in
slightly. A decrease in temperature would cause a decrease in ow,
thus the RT would move out slightly. These changes in RT should
be very small. If there is enough change in temperature to create
noticeable changes in ow and RT, one would be concerned about
hydrocarbon dropout. As the temperature drops, it is possible
to drop below the hydrocarbon dew point (HCDP) of the sample
if the sample line is not heated. The heavier hydrocarbons would
drop out rst. As the hydrocarbons drop out, the composition
of the sample being analyzed would change. This should be
noticeable in the analysis report.
■ What effect does Response Factor (RF) deviation
have on Response Time (RT)? For example, one of
the webinar slides showed a RF deviation of 0.26
and a RT deviation of 0?
As shown on the webinar slide, there may not be any affect.
Response Factor (RF) and its deviation is impacted by
concentration variation of the component. The calibration
gas/known standard certicate lists a tolerance. It is possible
for the GC to be operating properly but have a slight variation
of the concentration. Retention Time (RT) is impacted by order
of elution, pressure and ow of the carrier gas, and time events.
It can also be impacted by restrictions/contamination as they
impact ow and pressure.
■ Sometimes when there are rains or storms,
the Response Factors can change due to
changes in atmospheric pressure. Is there any
recommendation to avoid this effect?
No. The sample loop is vented to atmospheric pressure to ensure
a consistent sample size. If the GC did not vent to atmosphere,
then the sample pressure would determine the amount of sample
in the sample loop. Sample pressure is more likely to vary within
a given time period than the atmosphere's pressure. There would
also be variation in pressure between the sample pressure
and the calibration gas pressure if the sample loop was not vented
to atmosphere.
■ Will a bad or almost empty calibration bottle
negatively impact Response Factors and Response
Factor Order?
Yes. If what is in the calibration cylinder does not match what is
on the certication, then the Response Factors will be inaccurate
and likely in the wrong order.
Page 2
Frequently Asked Questions (FAQs)Rosemount Gas Analysis
■ In case of liquid calibration, would the vaporizer
condition affect Response Factor or not?
If the vaporizer is working correctly, then it should not affect the
Response Factors of the components. Potential errors would be if
the liquid fractionated before it hit the vaporizer or if the amount
of the liquid entering the vaporizer was too large and not all of it
is vaporized.
■ Can you miss a Response Factor shift slowly
over time if calibrating daily, causing miss
measurement, let’s say 1% a day?
Yes. The GC makes slight adjustments to the Retention Times and
Response Factors during calibration. As long as the shifts are less
than the limits, the Response Factors and Retention Times will
be updated. This is why Emerson recommends doing a validation
every 30, 60, 90 days. Please check our webinar recording on
Validation that goes over this in detail. You are looking at the
Response Factors and Retention Times of each component
for shifts by comparing the current calibration report with the
validation from 30 days ago, 60 days ago, and 90 days ago, or by
comparing the last 3 calibration reports.
■ How are the Ideal Response Factors calculated
for a calibration gas?
There is Response Factors (RF), Relative Response Factors (RRF)
and Ideal Relative Response Factors. Relative Response Factors
are calculated by taking the Response Factor of component A
and dividing it by the Response Factor of component B. One may
divide the other components in the stream by component B as
well as use another component. The components of the Response
Factors must follow the same path, use the same sample loop and
the same detector. The Response Factors must be calculated from
a known standard or calibration gas. Relative Response Factors
should not change. A change in a Relative Response Factor means
there is an error in the system. An Ideal Relative Response Factor is
based on many years of experience with specic applications such
as C6+ and C9+.
■ How is Relative Response Factor (RRF) and Ideal
Relative Response Factor calculated?
The RRF (relative response factor) is calculated by taking the
Response Factor of Component A divided by the Response Factor
B. Component A and Component B must ow through the same
sample loop, ow path and detector. Relative Response Factors
should not change. The Ideal Response Factor is developed
over years.
■ Where do you find the Relative Response Factor
(RRF) and the Ideal Response Factor?
The Ideal Relative Response Factor check is only available on the
Rosemount 370XA model. The ideal ratio is based on many years
of testing of C6+ gas chromatographs. A user would not be able
to see the Ideal Ratio nor the allowable variation.
■ Do we have the option of graphing the Response
Factor like it was indicated on the webinar slide
using MON software?
For the 370XA and 700XA models, there is the option for natural
gas applications. It is found by logging onto MON2020, and
clicking on Logs/Reports on the Menu bar. It is called Molecular
Wt. vs. Response Factor. The factory will plot the Response
Factors using this feature before we ship a 370XA and 700XA gas
chromatographs.
■ What is the linearity of the calibration curve?
Specication 2198-16 goes into great details on linearity check
and delity plots. The delity plot is based on the Response
Factors from the calibration gas using peak area measurement.
The Response Factors are plotted against the molecular weight
of the components. For hydrocarbons, if all is in order,
the resultant plot will form a straight line. The slope of the
plot should remain constant. For Rosemount XA Series gas
chromatographs, the delity plot is available in MON2020
software for standard natural gas applications. It can be found
under Logs/Reports, Molecular Wt vs. Response Factor.
■ If your Response Factor is high for N2, where would
you start investigating the leak at? Is there
a common place where a leak might come from?
Air most often happens to get into the GC system when the
calibration cylinder is changed. Emerson recommends purging
the line several times to remove the air before calibrating the GC.
If the calibration cylinder was not changed nor repair made to the
valves or regulators, the external ttings on the sample handling
plate and at the regulators are next places to check.
■ I have a C6+ application where Neo Pentane
Response Factor is always a little higher than
I pentane. This happens on all our calibration
cylinders. Any thoughts as to why?
Neo-C5 and i-C5 have very close boiling points. What you are
seeing is dependent on amounts of n-C4, neo-C5, and i-C5 in the
calibration gas. It is caused by separation from n-C4 to neo-C5
which is somewhat less than complete.
Page 2
Page 3
Rosemount Gas AnalysisFrequently Asked Questions (FAQs)
■ Why couldn't you use Helium as the carrier gas
for this specific application? Assuming this is for
the analysis of Ar, N
calibration gas for Ar and N
Helium could be used. By default, Emerson selected H
as the carrier for such as applications because it tends to be less
expensive and more readily available than Helium.
and H2 and H2 is shown as the
2
.
2
2
■ Should the sample pressure and the calibration
pressure be the same?
Not necessarily. More critical is the pressures for the calibration
and the unknown sample are in the range of 15–30 psig. The x
sample loop is equalized to atmospheric pressure in the rst
5–10 seconds of the analysis cycle. If pressure is too little, there
may not be enough pressure to clear the sample loop. If the
pressure is too high, then venting the sample loop to atmosphere
pressure may not be fully completed and the pressure of the
sample loop is not set to atmospheric pressure.
■ What is the maximum sample gas pressure?
For most of the Emerson gas chromatographs, the sample
pressure is between 15–30 psig. Consult the product's manual
for specic requirements.
■ On the Final Calibration Report, you have a ratio
split after the C6+. What does that stand for?
The ratio split stands for the amount of C6s, C7s and C8s you
believe are in your unknown sample. There are 4 standards within
the Rosemount XA GC series. You can also custom congure the
splits to match your requirements.
■ How to know when is the time to calibrate
an installed GC? Will the GC tell me when
to calibrate?
For most applications, one can set an auto-calibration.
The GC and the GC manufacturer don’t dictate the time frame
for a GC calibration. One's calibration requirements are set by
one's customer, tariffs and specications one needs to comply
to regulatory requirements, company's operating procedures, etc.
Once one has determined the calibration frequency, the GC can
be set to calibrate itself at that frequency.
■ What are the benefits of auto-calibration?
The biggest value of auto-calibration is that the GC does it on its
own without intervention from the technician. One can set the
frequency, number of runs, number of runs to average, and the
alarms one wants to receive if calibration fails per dened limits.
■ Can a Gas Chromatograph tell me the water
content? (usually very small for pipeline/natural
gas quality)
Water content can be measured with the Rosemount 700XA,
1500XA and Model 500 GCs. Rosemount reports the water value
using the Relative Response Factor for Methanol. Most of our water
measurements are at 50–1500 ppm with a TCD.
■ If I have several different heavier hydrocarbons, am
I correct that the C6+ category will not necessarily
be enough info for me to accurately estimate the
Hydrocarbon Dew Point (HCDP)?
Emerson will only supply a hydrocarbon dew point when C6s,
C7s, C8s and C9+ are measured. When assigning ratios for C6+
components and assuming nothing heavier than C8s, the error
rate on the hydrocarbon dew point calculation can be as high
as 40%.
■ Do you plan to go over Force Calibrations
and when it is necessary to perform them?
The presentation on Response Factors does not cover Force
Calibration. Our upcoming webinar on June 12
calibration of a GC. There is also a webinar recording on the
Validation of your GC that discusses Force Calibration. In general,
you should never do Force Calibration unless you know why you
are doing it.
th
will review
Page 3
Page 4
Rosemount Gas Analysis
Frequently Asked Questions (FAQs)
GC Webinar Series
March 2019
Global Headquarters
Emerson Automation Solutions
10241 West Little York, Suite 200
Houston, Texas 77040
USA
Toll Free + 866 422 3683
+ 713 396 8880
+ 713 466 8175
GC.CSC@Emerson.com
www.Emerson.com/RosemountGasAnalysis
Standard Terms and Conditions of Sale can be found at
www.Emerson.com/en-us/pages/Terms-of-Use.aspx
The Emerson logo is a trademark and service mark of Emerson Electric Co.
All other marks are the property of their respective owners.
©2019 Emerson. All rights reserved.
NGC-FAQ-Webinar-Series-GC-Response-Factors
Consider it Solved.
Emerson Automation Solutions supports you with innovative
technologies and expertise to address your toughest challenges.
For more information, visit Emerson.com/RosemountGasAnalysis.
Loading...