FID vs MS - different results?

[skunked_once]

Samples: Fatty Acid Methyl Esters (FAME) from oilseed

GC: Agilent 5890 with FID and Agilent 5890 with MS

Column: DB-23, 30 m x 0.25 mm, 0.2 u film

I am measuring the fatty acid composition (by area %) of oilseeds and am getting different results from the two instruments from injections of the same sample on the same day. Is there some difference in the way the two detectors measure the peak components that would result in the following differences?

Palmitic acid: FID = 4.0% MS = 4.9%
Stearic acid: FID + 6.4% MS = 9.3%
Oleic acid: FID = 87% MS = 81%
Linoleic acid: FID = 2.6% MS = 4.6%

[dblux_]

FAME analysis with FID is not very accurate until FID correction factors are utilized for calibration.

Unfortunately I have no experience with FAME determined on MSD.


If you have certified FAME standard you may check which detector is more accurate without correction factors - only area %.
I'm curious about such comparison.

[rb6banjo]

The detection mechanisms are completely different. I'd be surprised if you would get the same answer, even for molecules as similar as these.

You should calibrate your instruments for the analytes and measure the absolute concentrations of them in your mixture. Comparison of the concentrations is the best way to evaluate. Area% is a "back-of-the-envelope" sort of calibration (I don't even like to call it that) method.

[Don_Hilton]

The mass spectrometer counts ions generated in the ion source - and not all of them. Ionization is actually a low yield process and varies somewhat from one type of molecule to another. The number of ions counted depends on the mass range collected.

The FID counts ions generated from the carbon in the molecules (CHO+, if I recall the proposed ion correctly) and gives a response *approximately* proportional to the number of carbon atoms passing through the detector. And thare are some cases to keep in mind: CS2 is a fantastic solvent for use with the FID -- no signal from the solvent. And one day I made up a crude oil sample in CS2, which is also good for disolving some of the more difficult parts of the crude, and injected the sample into my GC/MS. I stood wondering why the huge solvent tail from CS2 - then... (face palm) Oh yeah, CS2 still ionizes in the MS...

So on a good day the trace on the GC-FID and GC-MS will match pretty well. And sometimes - still on a good day, the traces won't match and you can see stuff with the MS that were hidden from you on the FID. And on a bad day, match or no match, neither instrument tells you what you want to know.

[Peter Apps]

The results from MS in TIC mode and FID are often surprisingly close considering how different the two are in how they generate and detect ions. If you are just taking % area then the threshold settings on the MS can mean that you see fewer (or more) of the smaller peaks, or that some of the minor ions (which would otherwise contribute to the TI current for that peak) are not registered.

Bear in mind also that analyses that cover a wide range of molecular weights are very sensitive to inlet discrimination - and seemingly minor differences in gas flow, pressure, split and liner geometry can have surprisingly large impacts on discrimination.

Peter

[skunked_once]

Thanks for all of your replies, suggestions, and insights. I suspected that the two detectors could provide different results and now I have a better understanding of how that can be possible. It's Friday night, I'm still at the lab, and I'm bailing water with a sieve trying to finish up the sample sets from the last two weeks.

[lgchrom]

FAME analysis with FID is not very accurate until FID correction factors are utilized for calibration.

Unfortunately I have no experience with FAME determined on MSD.


If you have certified FAME standard you may check which detector is more accurate without correction factors - only area %.
I'm curious about such comparison.

I have compared Area percent to concentration percentages in the standard mix with GC-FID and the results are surprisingly close. Probably close to the uncertainty that this method inherently has. At least for the common 37 FAMEs of the food industry.

Why would you say that it's not very accurate? What correction factors would you use for the FAMEs?

[dblux_]

... I have compared Area percent to concentration percentages in the standard mix with GC-FID and the results are surprisingly close. Probably close to the uncertainty that this method inherently has. At least for the common 37 FAMEs of the food industry.

Why would you say that it's not very accurate? What correction factors would you use for the FAMEs?

Standard ISO 5508 (Animal and vegetable fats and oils - Analysis by GC of methyl esters od fatty acids) demands to report results with accuracy 0.1%.
Focus your attention on C4:0 and other FAME with less than 8 carbons. Are they close to certified values ?

Corrections factors are well described in ISO 5508 (in AOCS methods as well).

[lgchrom]

... I have compared Area percent to concentration percentages in the standard mix with GC-FID and the results are surprisingly close. Probably close to the uncertainty that this method inherently has. At least for the common 37 FAMEs of the food industry.

Why would you say that it's not very accurate? What correction factors would you use for the FAMEs?

Standard ISO 5508 (Animal and vegetable fats and oils - Analysis by GC of methyl esters od fatty acids) demands to report results with accuracy 0.1%.
Focus your attention on C4:0 and other FAME with less than 8 carbons. Are they close to certified values ?

Corrections factors are well described in ISO 5508 (in AOCS methods as well).

Thanks for the information. Low carbon FAME's are not as close as the other ones. You are right. But I usually focus on C14 and up. Most of the analyses are performed on vegetable oils etc.

That said 0.1% accuracy is usually satisfied for most FAMEs.

Thanks again for your input.

It would be interesting to hear more things about the MSD FAME analysis.