FID and MSD, why different results?

[karlssa2]

We have two methods for determination of fatty alcohols (decanol, dodecanol, heptadecanol). The first one is for GC-MSD and the other based on GC-FID. The GCs are both Agilent 7890, and all chromatography settings are identical. The only thing that differs is the detection techniques and the internal standard. With MS system we use dekafluoro-biphenyl and with the FID (tetradecanol). The peak areas are used for quantifications. Specific mass fragments are used for quantification on the MSD. The peaks are pure, no interfering compounds.


But, why do we get 25% higher results with the GC-FID? The comparisons are made twice, so I exlude that we have prepared our solutions improperly.

[Peter Apps]

Probably inlet discrimination, but if that is generating a consistent bias then there must also be a problem with your calibrations.

Peter

[Consumer Products Guy]

Hmm - we use BSTFA derivatization and external standard quantitation for fatty alcohols, over three decades now, works fine.

We see stearyl alcohol in solid antiperspirant formulations. We see cetyl alcohol in lotions and hair conditioners.

[RogerBardsley]

What does the MS data look like if you use total ion or mass range to quantitate verses specific mass fragments?

[Peter Apps]

We have two methods for determination of fatty alcohols (decanol, dodecanol, heptadecanol). The first one is for GC-MSD and the other based on GC-FID. The GCs are both Agilent 7890, and all chromatography settings are identical. The only thing that differs is the detection techniques and the internal standard. With MS system we use dekafluoro-biphenyl and with the FID (tetradecanol). The peak areas are used for quantifications. Specific mass fragments are used for quantification on the MSD. The peaks are pure, no interfering compounds.


But, why do we get 25% higher results with the GC-FID? The comparisons are made twice, so I exlude that we have prepared our solutions improperly.

Because tetradecanol is a better internal standard than decafluorobiphenyl when the analytes are alcohols.

Peter

[mckrause]

Peter as usual is spot on but being a bit cryptic. One of the primary guidelines to selecting adequate internal standards is that they should chromatograph similarly to your compounds of interest and should, if possible, be similar functionally to your compounds of interest. This is not quite as big an issue for FID as it is for MS, since the range of RRFs is not as wide (more consistent ionization). There is absolutely no reason that you shouldn't use tetradecanol as your IS for your MS work as well as for the FID; there are myriad reasons that you shouldn't use decafluorobiphenyl as an IS. This is issue number 1.

Issue 2 is that the effective working range of a MS is significantly less than the effective working range of a FID. It is crucial for good MS work that you are working in a concentration range that provides consistent ionization; ie., that you not overload the MS source. I would hazard a guess that you are injecting too much of your analyze. While nice big peaks are wonderful they are not necessarily reproducible from a fragmentation standpoint in MS. You stated that you are using extracted ion techniques for quantification. I would suggest that you need to try injecting less mass, use tetradecanol as your IS and see if the data doesn't match up better.

[Peter Apps]

Peter as usual is spot on but being a bit cryptic.

We get cryptic questions (most of the necessary details omitted) - I give cryptic answers.

Peter