GC-FID Low signal or normal range?

Hi all, 

I have a 6890A GC with a FID. We recently acquired the instrument and I have been running standards up to test methods. I have been getting signals that seem suspiciously low. 

I am injecting Matreya Bacterial CP FAME mix (C11-C20 methyl esters) which are TMS derivatized. Stock concentration is 10 ug/uL total which is about .38 ug/uL for each of the 26 constituents in equal concentration. I injected 2 uL at a 1:50 split. This yields nice peaks, but the signal barely registers. The analyte peaks were right around 100pA in area and about 30 pA in height. The highest peak has an area of just over 200 pA and height of 50 pA (which I think is two analytes that will need to be resolved but that's a separate issue). Meanwhile, the solvent peak has an area of about 60,000 pA. I am not trying to get a MLD and am in the range of what my sample concentration will be. 

The baseline sits right around 10 pA and is very consistent so there is no issue with noise. Analyte peaks are thus only about 3X the baseline.

I am still somewhat new to the GC world but I would have expected these peaks to be quite a bit higher than that. From what I have seen elsewhere this seems to be on the higher end of injection amounts and splits?

So my question is what is a normal signal range for this amount of carbon? Are peaks this small expected? What concentration do y'all normally inject and what range of signals do you get? 

I am not sure if this is normal or there is an issue with the instrument that needs to be addressed. 

Other details of instrument settings: 

Inlet temp of 200C. 30m DB-5 column. Temperature program starts at 50, ramps to 150 at 20 C/min, ramp to 320 at 4 C/min. Detector temperature is 340 C. See screenshot below for other parameters. 

Below is also a screenshot of the peaks - without the solvent peak - for your reference. Again I know I will need to adjust parameters to resolve some peaks but that is to come. 

Thanks in advance for your help and any recommendations on what to try running are greatly appreciated. 

  • Hi DeepSeaGinger, 

    I don't think this looks bad at all.  But sometimes it helps to run through the math :)  Your estimated concentration in vial is 0.38ug/uL, or 380ng/uL for each peak and you are injecting 2uL, so you are introducing approx 760ng of each compound into the inlet.  Once the sample is injected, you are also splitting that injection 50:1, which means that the majority of your injection is going out the split vent with ~15ng making it onto the column.  The detector can only see the mass that gets on to the column.   If you would like to see larger peaks, adjust your split ratio.  Maybe try decreasing the ratio to 20:1 which equates to just under 40ng on column.  You can play around with different split ratios until you have the response you are looking for and the peak shape stays symmetrical.   

    The solvent is usually the bulk of the injection, so that peak should be easily distinguishable by a significantly higher signal, provided it has CH bonds.  

    Let us know how it turns out!  


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