GC/MS vs GC/FID(as Toluene Equivalents)

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

11 posts Page 1 of 1
Dear friends in the world,
I have questions regarding measurement method difference between GC/FID and GC/MS.
For the quantitation of ambient VOCs, we use GC/FID type. GC/MS also has qequipped with thermal desorption type like GC/FID system.
Our GC/FID sytem has preconcentration type(Tenax TA, 20mg) + GC/FID sytem to monitor VOCs in ambient air. Calibration ranges are 5~20ppbv(5, 20, 50ppbv)using diluter with 10ppm standard gas cylinder.
When I compare the individual concentration(e.q. Toluene, m,p-Xylene) between GC/FID and GC/MS, toluene concentrations are very similar each other.
But m,p-xylene concentrations are too much different. Usually, GC/MS concentration has 1.6times(n=5) higher than GC/FID type.
Would anybody tell me this difference or recommend something?
Best regards.
From Lee
Jaehwan Lee wrote:
... But m,p-xylene concentrations are too much different. Usually, GC/MS concentration has 1.6times(n=5) higher than GC/FID type.
...

Which method gives results closer to true concentrations ?
Does your column resolve p-xylene from m-xylene ?
How do you quantitate GC/MS signals ?
Knowledge gained during method validation may help you understand reasons of differences.
Thanks.
GC/FID concentration is more closer than GC/MS.
In this case, we tested accuracy at 10ppb concentration.
GC/FID method is dilution adsorption from 10ppm stdnadard cylinder gas.
GC/MS method is adsorption from liquid standard using adsorption tube.
Quantitation is based on Toluene equivalents for toluene, and m,p-xylene.
Unfortunately, our column(HP-5) can't resolve m,p-xylene.
Best regards.
Lee
These two methods have different detectors and different adsorption.
IMHO you should determine desorption coefficients for both methods and correct results with them.
If you do not resolve p- and m-xylene then you determine them as a sum. May be p- and m-xylenes have slightly different responses on FID and the composition of mixture in air is different then this in calibration mixture.
I have read this post several times and am still somewhat confused. First, I am going to invoke Rod's law (sometimes called Peter's law; you can find it here on ChromForum). You have left out a lot of information, which makes it really hard to help you.

There are multiple reasons that you would get different calculated results between GC-MS and GC-FID, but by far the most common one is that you are saturating your detector on the MS. Xylenes are just about the most sensitive compounds there are on MS; their response factors are huge. FID doesn't have that problem. The responses really are similar across a wide range of compounds, plus FID is quite linear. Despite what the instrument companies would like you to believe, MS is not all that linear. Since you don't see it with toluene (single compound) and you do see it with m,p-xylenes (two compounds eluting in the same peak) I would suspect that you are working near the top of your effective calibration range for those compounds. Try dropping your EM voltage.

The other thing that jumps out to me is that you do have to correct the calibration curve for the fact that you are eluting both compounds together. Have you done that on both systems?
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
Hi, Mark,
Thanks for your reply.
Okay, I will let you know our test procedure in detail.

We have cryogenic trap(Tenax TA 20mg) + GC/FID system to check VOCs concentrations in the our customer field site.
Our GC/FID online monitoring system has pressurized standard cylinder inside of instrument to make calibration curve automatically. Our calibration range was 0, 5ppbv, 20ppbv, 50ppbv. For example, if I want to make 10ppbv level sample, N2 flow was 1000mL, and our cylinder standard gas flow was 1mL.
Target concentration was made in the mixing chamber.

In order to compare the results between cryogenic GC/FID and thermal desorption GC/MS for 10ppbv mixing chamber sample, sampling was conducted as follows;
One sample was passed through cryogenic Tenax TA trap for 15min at the rate of 0.1L/min(Total volume was 1.5L) and other sample was collected using air sampler at the rate of 0.1L/min for 120min(to analyze sample using GC/MS) individually. GC/FID online system run time was about 40min. GC/FID system data was expressed as average for 3 time analyses (3 cyles for 120min). GC/MS offline data was expressed as one data for 2hrs sampling period. In this case, thermal desorption GC/MS sampling adsorbent is Tenax GR type(200mg packed in the stainless steel tube to desorb using high temp helium carrier gas).

Finally, when we compare Toluene concentration between GC/FID and GC/MS, concentration showed very similar result for each. The ratio(MS/GC-FID) was about 1.1. However, m,p-xylene ratio(MS/GC-FID) was about 1.7. m,p-xylene's concentration was also calculated using Toluene equivalents.
Our customer also use Toluene equivalents to quantitate Toluene and m,p-xylene to make comparison.

If possible, would you let me know the reason of difference and some advises?
If I use individual standard for m,p-xylene instead of Toluene equivalents, concentration would be same?

FYI,
Our crogenic GC/FID online system's column was HP-5(30m).
GC/MS column may be 60m length, and......
I don't know detail information due to limited information from our customer.

Best regards.
From Lee
Hi, Mark,
Thanks for your reply.
Okay, I will let you know our test procedure in detail.

We have cryogenic trap(Tenax TA 20mg) + GC/FID system to check VOCs concentrations in the our customer field site.
Our GC/FID online monitoring system has pressurized standard cylinder inside of instrument to make calibration curve automatically. Our calibration range was 0, 5ppbv, 20ppbv, 50ppbv. For example, if I want to make 10ppbv level sample, N2 flow was 1000mL, and our cylinder standard gas flow was 1mL.
Target concentration was made in the mixing chamber.

In order to compare the results between cryogenic GC/FID and thermal desorption GC/MS for 10ppbv mixing chamber sample, sampling was conducted as follows;
One sample was passed through cryogenic Tenax TA trap for 15min at the rate of 0.1L/min(Total volume was 1.5L) and other sample was collected using air sampler at the rate of 0.1L/min for 120min(to analyze sample using GC/MS) individually. GC/FID online system run time was about 40min. GC/FID system data was expressed as average for 3 time analyses (3 cyles for 120min). GC/MS offline data was expressed as one data for 2hrs sampling period. In this case, thermal desorption GC/MS sampling adsorbent is Tenax GR type(200mg packed in the stainless steel tube to desorb using high temp helium carrier gas).

Finally, when we compare Toluene concentration between GC/FID and GC/MS, concentration showed very similar result for each. The ratio(MS/GC-FID) was about 1.1. However, m,p-xylene ratio(MS/GC-FID) was about 1.7.

If possible, would you let me know the reason of difference and some advises?

FYI,
Our crogenic GC/FID online system's column was HP-5(30m).
GC/MS column may be 60m length, and......
I don't know detail information due to limited information from our customer.

Best regards.
From Lee
Your air sample volume for GC-MS is nearly ten times as big as for GC-FID, and you may be getting breakthrough form the trap for GC-MS.

Peter
Peter Apps
OK, I've read this now a couple of times so I want to make sure I understand this.

You're running 120 ng of m,p-xylenes onto a Tenax GR trap, then desorbing to the GC-MS. Meanwhile, you are cryogenically trapping m,p-xylenes for the GC-FID analysis onto a Tenax TA trap, and you are running 15 ng of m,p-xylenes onto that trap. Peter's point is valid - you are using radically different sample sizes (not to mention traps) and trying to compare the results, which is questionable.

I think there are a couple of possibilities.

One, I still haven't gotten a response to my initial question about the standard. All of the standards that I have used have m-xylene and p-xylene concentrations listed, so for m,p-xylenes you have to add the two together. Are you certain that the standards being used are the same? Can't even begin to tell you how many times this particular issue has bit me on the butt. If you have one standard that has 10 ppm of m,p-xylenes and the other standard has 10 ppm of m-xylene and 10 ppm of p-xylene then your true ratio would be 0.85, which is pretty good for different methods.

Secondly, if you are desorbing the entire 120 ng of m,p-xylenes into your MS you are way, way beyond the linear working range of your system. I am at a bit of a loss as to why you'd need 10x more sample for xylenes for your GC-MS than for your FID; GC-MS sensitivity for xylenes is exquisite and significantly better than for FID. You didn't give us any details about your calibration, but given that this is probably an environmental lab you are most likely trying to apply a linear curve to the world (sorry, James, but it is true). MS is not inherently linear except over short calibration ranges, so I would take a hard look at my calibration.

One of the problems in comparing disparate methods is that there are variables in each method that have to be carefully controlled. You have radically different methods here; you really need to very carefully control all of the variables. For starts, I'd definitely run a sample of their calibration gas through your MS system and compare it to yours. Wouldn't be the first time 2 "certified" standards popped up with different concentrations.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
OK, I've read this now a couple of times so I want to make sure I understand this.

You're running 120 ng of m,p-xylenes onto a Tenax GR trap, then desorbing to the GC-MS. Meanwhile, you are cryogenically trapping m,p-xylenes for the GC-FID analysis onto a Tenax TA trap, and you are running 15 ng of m,p-xylenes onto that trap. Peter's point is valid - you are using radically different sample sizes (not to mention traps) and trying to compare the results, which is questionable.

I think there are a couple of possibilities.

One, I still haven't gotten a response to my initial question about the standard. All of the standards that I have used have m-xylene and p-xylene concentrations listed, so for m,p-xylenes you have to add the two together. Are you certain that the standards being used are the same? Can't even begin to tell you how many times this particular issue has bit me on the butt. If you have one standard that has 10 ppm of m,p-xylenes and the other standard has 10 ppm of m-xylene and 10 ppm of p-xylene then your true ratio would be 0.85, which is pretty good for different methods.

Secondly, if you are desorbing the entire 120 ng of m,p-xylenes into your MS you are way, way beyond the linear working range of your system. I am at a bit of a loss as to why you'd need 10x more sample for xylenes for your GC-MS than for your FID; GC-MS sensitivity for xylenes is exquisite and significantly better than for FID. You didn't give us any details about your calibration, but given that this is probably an environmental lab you are most likely trying to apply a linear curve to the world (sorry, James, but it is true). MS is not inherently linear except over short calibration ranges, so I would take a hard look at my calibration.

One of the problems in comparing disparate methods is that there are variables in each method that have to be carefully controlled. You have radically different methods here; you really need to very carefully control all of the variables. For starts, I'd definitely run a sample of their calibration gas through your MS system and compare it to yours. Wouldn't be the first time 2 "certified" standards popped up with different concentrations.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
Sorry for the double post - the computer operator is a moron.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
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