I was told that it's best to develop my GC detection and quantification method with the single quad mass spec and then switch the method to FID to generate the calibration curves and do quantitation because it's more reliable. We do have the split flow capabilities so we might be able to do both I guess. Do you guys agree that FID is better for quantitation? I have read many papers that have done quantitation with mass spec and not sure why they wants me to switch to FID.
Hello Mahsa Lotfi Marchoubeh,
You are absolutely right from my point of view to have doubts about the switch to FID. With the MS, you could quantify without any problem. The only point worth noting is the linear range, which is much smaller with the usual Qudrupol MS than it is with an FID. If your samples are in a narrow expected range, the MS should not cause any problems. You can also run the MS in selective mode to improve quantitation if impurities or spurious peaks occur. This would not be possible with an FID. We often use our GC-MS for quantitative work. However, we do not use an FID in the system. GC-FID is a different instrument for us.
Many greetings
Joachim Horst
GC-FID / GC-MS: Two different detectors with different characteristics and operating modes = different selectivity. You have provided no information about the specific sample types your application is for, so no useful answer to your question can be provided. FID is excellent for many samples types and coupled to properly developed methods and standards, can provide excellent, reliable quant results (most recommend it for these reasons and it is a great starting place). MSD (you did not say which type of MS detector you are using), is it an EI type (MSD) ?. Two different detectors are often a great idea to use with GC, but the method must be optimized for both of them to be useful. In general, FID is less expensive and far easier to use (esp for a new user like yourself). MS in general requires a great deal more training than FID (though library databases may help with qualitative ID's) has a number of issues when quantification is desired (peak sampling #, variations in settings used will change the results obtained etc). How applicable MS is for quant depends on the MS type, the sample type (which you did not provide) and exact method used. So it may or may not make sense for you to run your samples using FID this time.
*Please seek the assistance of a professional chromatographer who is familiar with the instrumentation, software and applications you are running for more information (ask your employer to hire one or bring one in on a consulting basis to provide training. It will be money well spent).
William Letter Thanks for the response. As always, I benefit from your wisdom shared on this platform. The MSD we have is an EI single quad one from Agilent. We would like to use it for air samples desorbed from sorbent tubes for concentrations of VOCs and PAHs in the ppt ranges. We are also going to be looking for total hydrocarbon count which I realize we need to use FID for. I am generally familiar with setting MSD parameters such as sampling rate. However, I have not worked with the FID. I do understand that if I split my flow to go into both the FID and MSD it has some advantages. We do not have the resources to hire consultants and professionals and we are a research lab so we have "method development and problem-solving" objectives in addition to obtaining good data. I appreciate your answer and comments.
- I would not recommend splitting flow. Run separate, optimized methods for each application. Keep things simple.
- GC-MSD (EI) is by far the easiest MS detector variant to use and maintain. PAH's in air are easy in SIM mode. Great multi-purpose detector, but it can easily output incorrect data just like the rest of them.
- If you have the resources to purchased a GC-MSD system, than you do have the resources to hire professionally trained users and/or hire professional consultants to provide on-site professional training.
Employers wrongly think they are saving money hiring people who do not have the needed experience and will just figure it out. Learning "on-the-job will take care of it" is the biggest mistake and waste of money for employers. It does not work that way in the real world (I am one of the people they call for help, but only after spending years and millions of dollars not achieving their goals. The answer is clear). Then you have the 'cheap' companies that say, "send them to one of the free or low-cost classes at Pittcon (or similar)", LOL. You can not learn these techniques from a day long sales class. The sale people want the employers to think that so they spend all their money on the hardware and leave nothing for training...
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