Chromatography Forum

Hi,
I have been using GC-MS for identification and quantification of organic acids in human urine. I have used C13 labelled Myristic acid as an internal standard and it elutes at 28.5 min while the total run time was 38 min. I have run a few organic acid standards for which the results were fine. Then i have run a sample of urine and citric acid is found to co-elute with Internal satndard which generally leads to increase in peak area and hence not useful for quantification. Is there a way to resolve this issue because there are a lot urinary metabolites which elute almost till 33 min and the retention times are very close. Thanks in advance for your inputs.

Hello

As long as your IS has different ions (main/target and qualifiers) than other compounds, co-elution is not a problem. That is one of Mass Spectrometry advantage.

Regards

Tomasz Kubowicz

Hi Tomasz Kubowicz,
Thanks for the reply. If i am not wrong, if we use selective ion monitoring, it should be able to detect specific analytes. will it be able to give specific peak areas too. Thanks

The approach should work whether full scan or SIM. You just need to be sure the ion you use for the IS has nothing in common with the coeluter. Also, if the coleluter concentration is much higher than the IS, you may see the IS response suppressed.

If you quantitate using extracted ion response instead of total ion response you should be able to separate the co-eluting compounds if they have unique fragmentation patterns.