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Fid useful?

Gustav

We use a GC-MS-FID system in combination with a thermal desorber. I wonder what the FID is good for? One argument I have read is that the FID is probably suitable for higher concentrations. But that is not a good argument in our case, because we can repeat the sample after re-collecting with a higher split. Another argument I heard is that the FID is better at quantifying unknown substances or non-calibrated substances using a semi-quantitative evaluation. In a test with around 20 different substances with the same concentration, however, I noticed that the chromatograms of the TIC of the MS are more uniform than those of the FID. The semi-quantitative evaluation of these substances is also much better with MS. As soon as halogen compounds are involved, there is much less signal. In my opinion, the biggest disadvantage of FID in combination with MS is that the splitter makes it more complicated and makes troubleshooting more difficult. In addition, helium consumption is higher. Can anyone tell me why you might need a FID in addition to the MS?

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  • +1

    Changing the MS to FID split ratio with enough restriction to the MS would allow quite high FID signal without saturating the MS ion source and all in one injection.  Many don't have enough sample volume or the time to do more than one injection per sample.

    An FID is great if you do not need spectral data.  If you only use the MSD Total Ion Chromatogram, that data is nearly the same as an FID.  A GC with an FID is much cheaper and less complicated to run than a GC/MS.  It is better to sneak up on an unknown starting at less injection volume/higher split ratio/higher dilution and smaller peaks in the MS than with too much sample in the MS. 

    5991-8499EN_cannabis_terpenes_application.pdf (agilent.com)  "The use of both FID and MSD allows for more comprehensive data analysis, as terpenes are often in high percent values, which would saturate the MS, whereas the FID will not saturate."

    So if you are screening samples with unknown concentrations that may be very high, the GC/MS/FID is a useful configuration.

  • 0 in reply to paulsalverda

    Hello Paul, thank you for the answer. At what abundance does an MSD reach saturation? 

  • 0 in reply to Gustav

    The maximum abundance per ion on SQ instruments is approximately 8.4 × 106, To stay within the linear range of the electron multiplier we recommend staying at or below 2.0 × 106  (per ion) for SQ.

    The maximum abundance per ion on TQ instruments is approximately 1.7 × 108. To stay within the linear range of the electron multiplier we recommend staying at or below 2.0 × 107 (per ion) for TQ.

  • 0 in reply to paulsalverda

    Okay good infotmation, thanks. 

    But I wondering if there also a guideline value in the TIC? Or do you always have to look at the ions?

  • 0 in reply to Gustav

    The number of ions and size per peak is unique to each peak and the method setpoints.   I start by looking at the chromatographic peak shape. If the TIC peaks looks like shark fins, which shows column overload, there is a good chance that the detector is overloaded too. That visual screening is quickly done on most chromatograms.     And then when I see any single quad TIC peak bigger than about, oh, three to five million, I dig into the spectrum.  

    There's no reason to have gigantic peaks.  Increase the split ratio, reduce the injection volume, or reduce the concentrations until the smallest concentration TIC peak height is >3 to 5 times the average baseline noise. If the noise is low the response can be very low and still excellent. This reduces system maintenance.

    see:  Which Electron Ionization Ion Source? Part 2 of 4

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  • 0 in reply to Gustav

    The number of ions and size per peak is unique to each peak and the method setpoints.   I start by looking at the chromatographic peak shape. If the TIC peaks looks like shark fins, which shows column overload, there is a good chance that the detector is overloaded too. That visual screening is quickly done on most chromatograms.     And then when I see any single quad TIC peak bigger than about, oh, three to five million, I dig into the spectrum.  

    There's no reason to have gigantic peaks.  Increase the split ratio, reduce the injection volume, or reduce the concentrations until the smallest concentration TIC peak height is >3 to 5 times the average baseline noise. If the noise is low the response can be very low and still excellent. This reduces system maintenance.

    see:  Which Electron Ionization Ion Source? Part 2 of 4

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