quantifying using the TIC

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Hello,
I'm a new forum user so please forgive me if my question is already answered here. I checked but could not see this topic.
I have been taught to IDENTIFY analyte peaks using the EIC (extracted ion chromatogram) but to QUANTIFY them by integrating the peaks using the TIC (total ion chromatogram).
This is fine when the peaks are well above the signal/noise ratio of 3 and are definite peaks, not noise. However there are occasions where a clear peak is observed using the EIC, but when using the TIC the peak is in the noise and thererfore not quantifiable.
Normally we would not quantify if the peak was in the noise, it would be listed as 'not detected'. However, would it actually be valid to integrate a peak in the noise using the TIC if it is very definitely a 'proper' peak when seen in the EIC?
I am not sure what regulating agency you would report to, but for EPA and most other groups the quantitation is done using a single mass as you do with the EIC. If you are using TIC and there is another compound there with different masses but same retention time, how would you separate the two? EIC would give more reliable results in the case of any background interference, which is why you see a peak in EIC but not in TIC, so your detection limit can be much lower using EIC.

Most GC/MS software is set up to do quantitation using single mass extraction and using other chosen masses as qualifiers to confirm positive results.

Some laboratories still use TIC simply because they converted and old method from FID to MS and the analyst was used to how the FID data was handled. It is valid, but it is not using the MS instrument to its fullest potential :)
The past is there to guide us into the future, not to dwell in.
Thanks for your reply. I don't need to report to any agency, luckily, because I'm a student at a university. I had a little prior experience of GCMS in a previous job (England) working for an essential oils company, and there I did use the EIC to quantify.
I agree with you that by using the TIC there is a possibility of quantifying the wrong analyte.
As I'm being taught I guess I'd better stick to their way, using the TIC. Perhaps this is what is done in this field of study (don't want to identify the research field except to say that it's environmental, and samples are very complex natural matrices).
I see many papers state they use extracted masses to quantify, but a few have used the TIC. This field of study seems a lot less uniform in its approach compared to my own discipline (analytical chem). I'm realising that changing disciplines can often invite a clash of approaches to analysis.
Many thanks for your insight.
to be honest, complex mixtures are precisely where I'd most want to use EIC rather than TIC. The good news is that if you're ever in doubt, you can quantify both ways and check the data are consistent.
Of course if you're collecting data with a SIM method then your TIC isn't really a TIC, and things might not be quite so bad. If you're collecting data with a scan method then obviously SIM+EIC will give a vast improvement in sensitivity.
JaneLouise wrote:
Thanks for your reply. I don't need to report to any agency, luckily, because I'm a student at a university. I had a little prior experience of GCMS in a previous job (England) working for an essential oils company, and there I did use the EIC to quantify.
I agree with you that by using the TIC there is a possibility of quantifying the wrong analyte.
As I'm being taught I guess I'd better stick to their way, using the TIC. Perhaps this is what is done in this field of study (don't want to identify the research field except to say that it's environmental, and samples are very complex natural matrices).
I see many papers state they use extracted masses to quantify, but a few have used the TIC. This field of study seems a lot less uniform in its approach compared to my own discipline (analytical chem). I'm realising that changing disciplines can often invite a clash of approaches to analysis.
Many thanks for your insight.


I work mostly in environmental and all of our methods here quantify using EIC. You can take a quick look at the US EPA methods 8270 and method 8260 for an idea of how quatitation and qualification are handled for GC/MS samples.
The past is there to guide us into the future, not to dwell in.
Thank you for your replies and advice. I will check both EIC and TIC. I am running in Scan mode most of the time, but using SIM if I am trying to quantify analytes in very low abundance.
I appreciate your input. Cheers!
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