Chromatography Forum

Hi there, thanks in advance for any help or advice!

I'm using GC MS for quantitation. Originally I used selective ion monitoring mode to quantify analytes of interest, which I have commerical standards of. So I have been able to make a calibration curve for each and quantify those using specific ions.

However, I would now like to look at all the compounds in the samples and see % composition and change. But I don't have standards for each, so I can't make calibration curves.

I ran the GC MS in full scan mode and SIM mode so I did try using the TIC to look at each compound as a percentage of the total area. However, a lot of the compounds don't separate well and peaks merge, so I can't really use the TIC.

Is there any way to use the extracted ion chromatograms of analytes and get a % composition without calibration curves of each? I did add an internal standard to each sample as well.

Thank you, I hope this made sense!

Not without standards. TICs is short for tentatively identified compounds and they're called that for a reason.

Not without standards. TICs is short for tentatively identified compounds and they're called that for a reason.

???

I've never known TIC to be anything other than "Total ion chromatogram" or some variant thereof-basically the chromatogram of everything hitting the detector. I've done quantitation based on TIC areas(whether with a calibration curve or IS) for as long as I've been doing GC-MS.

To the OP:I'd want to see your full method, but there should be a valid way to quantitate based on an extracted chromatogram provided that you build a calibration curve based on it and/or an IS. I have done this before to tease out difficult to separate compounds, although generally I was only quantifying one of them. Of course this assumes the co-eluting compounds each have a signature ion that you can use.

Not without standards. TICs is short for tentatively identified compounds and they're called that for a reason.

???

I've never known TIC to be anything other than "Total ion chromatogram" or some variant thereof-basically the chromatogram of everything hitting the detector. I've done quantitation based on TIC areas(whether with a calibration curve or IS) for as long as I've been doing GC-MS.

To the OP:I'd want to see your full method, but there should be a valid way to quantitate based on an extracted chromatogram provided that you build a calibration curve based on it and/or an IS. I have done this before to tease out difficult to separate compounds, although generally I was only quantifying one of them. Of course this assumes the co-eluting compounds each have a signature ion that you can use.

Hi, yes thank you, by TIC I meant total ion chromatogram.

I'm looking at sterols and I have a calibration curve for one sterol, the most abundant in my data, made using a standard (and an internal standard). But there are other structurally similar sterols (isomers or one double bond different) which I don't have standards for.

And those which co elute do have different identifying ions to each other, so I can get nice extracted ion chromatograms. I worked out some values with the TIC and internal standard, like with an FID, but I think it's misleading due to the coelution.

I can't collect more data on the same instrument so I'm hoping I can figure out a way with what I have.

For the isomers, which have the same major ions as the sterol I originally quantified, could I use the same calibration curve to quantify using the EIC? But then what about the ones with different ions?

Thank you!