Chromatography Forum

Hello everybody,
I'm doing a quantification using, for the same sample, both GC-FID and a GC-MS analysis. Knowing the differences between the two detectors, I can't understand how it can find totally different total concentrations for the same sample, calibration, method and column. In particular, the concentrations at FID are clearly higher. I don't really undestand how I can measure a total concentration measured for FID of 70 ppm and for MS in under the LOD.
Thank you for your attention

One thing to remember is that the MS will filter out many co-eluting compounds that the FID may not.

One thing to remember is that the MS will filter out many co-eluting compounds that the FID may not.

Thank you so much for your reply.
But, considering the total area, shouldn't I have two very similar values, if both detectors have been calibrated with the same standards? Because the problem is that the total areas, in addition to the areas of the single peaks, are completely different (Tot area MS <FID) and I won't explain it. Furthermore, the chromatograms are also totally different.

Try quantifying your MS results using the TIC (total ion chromatogram) while scanning over the total mass range expected.

Try quantifying your MS results using the TIC (total ion chromatogram) while scanning over the total mass range expected.

Thak you for you suggestion. I did this: I indetified and quantified all the picks looking the TIC signal. Can be only a problem of LOQ?

Try quantifying your MS results using the TIC (total ion chromatogram) while scanning over the total mass range expected.

Thak you for you suggestion. I did this: I indetified and quantified all the picks looking the TIC signal. Can be only a problem of LOQ?

What type of analytes are you looking for?

Hydrocarbons will respond better on the FID and have lower LOQ than they will on MS, but Chlorinated compounds could respond better on the MS than the FID. How much difference in LOQ between the detectors will depend on each analyte you are looking for.

What is the analyte? 70 ppm should be plenty to see in scan mode on a SQ MS.

Would be helpful to see the method parameters.

Was the known assay of your analyte which was used to make up your 70 ppm solution determined by area percent on FID or by weight using a high purity standard supplier? How was the standard material assayed by the supplier? What is the mass unit range of the analyte?

If you want more than guesses and speculation you need to tell us exactly what you are doing, and how you are doing it.

Peter

Thank you so much for your replies. My STDs are gas diuted manually with gastights. The GC-MS analysis was conducted with a preconcentration step. Instead, the GC-FID analysis was conducted with a direct iniection of standard into the column. I want to see, in my actually condition, if the response of quantification obtained with FID can be closed to MS quantification. Actually, I reviesed my calibration curve (I go to lower concentration) and data: I observed an improvement of the correlation, but the data are slighlty different, mostly at lower concentrations. My analytes are hydrocarbons.

With a good calibration you should get an agreement in "Result" or quantified concentration, but the "Response" in area counts will be different simply because it is two different detection techniques.

Hydrocarbons fragment quite a bit on MS so unless you are looking at total area of all masses the response will be lower than for an FID. Honestly even with using total ion chromatogram areas they will most likely be less sensitive than the FID for hydrocarbons.

Thank you for all the replies!