If nothing else helps, check out the manual if you've got an isotope ratio MS as part of your GCMS installation.

I have ordinary gcms its not designed for ISOTOPE analysis

Hi Prabodh,

you make a better chance on a helpful answer if you describe your problem a bit more extensively. What substances do you want to measure, which isotopes, which instrument do you have. Maybe there is someone out there who has found a way.

In general, GC is (a bit) isotope-sensitive, but that's what you see on a GC-irMS machine. It's hard or impossible to deconvolute the signal you get into isotopes' contributions if you don't know the ratios.

HI Dr. Neubert, I am trying to authenticate essential oils components like Thymol, methyl salicylate (nonchiral) through ordinary GCMS (GCMSQP 2010 Ultra Shimadzu). I have done SIM analysis got some clue of adulterated components but still having problem on confirmation and quantitation. First of all I analyse the component on Scan mode and after then I run the same sample on SIM mode. I use peak intensity generated of isotopes from SIM analysis and compare that with authentic synthetic sample. I am finding authentic synthetic sample has always higher M+1 isotopic ratio than the authentic natural compounds. So far as I used this paper https://www.jstage.jst.go.jp/article/bbb/67/3/67_3_599/_pdf as the reference paper for my work. May be I am using wrong approach to selecting the peak. Do I need to select MS from top of the peak or has to maintain same retention time whenever I have compare two MS? Thanks

Prabodh

Dear Prabodh, the short answer to your question is NO. Scanning MS instruments cannot detect, let alone quantify subtle variations in stable isotopic composition of organic molecules at natural abundance level. There 2 reasons for this. (1) Even in SIM mode information is lost since the MS peak jumps between masses M+, (M+1)+ and (M+2)+. On other words isotopologues are detected neither simultaneously nor continuously. (2) Conventional MS systems are equipped with a single detector, an EM, which is a cascading detector not an ion counter. As a consequence, repeated MS measurements of the same masses in the same compound are not accurate enough for isotope ratio measurement at natural abundance level. The RSD of MS measurements is 10% (5% at best), which is not good enough to accurate measure difference in natural abundance at a level of 0.001 atom% or less. Conventional scanning MS systems can only be used to detect isotopic enrichment levels in excess on 0.5 atom% above natural abundance (on a good day and with multiple enrichment this can be squeezed down to 0.2 atom% excess). For 13C this means a conventional MS can only reliably detect 13C enrichment (13C labelling) if the 13C abundance exceeds 1.58 atom% (if we say for simplicity's sake the typical natural abundance level of 13C is 1.08 atom%). For a detailed explanation see Forensic Sci. Int.; 174, 259-261 (2008).

The paper by Satake et al you are quoting is based on a method developed using13C labelled compounds (labelled = enriched) and extrapolating from that using a multivariate dataset and chemometric analysis of these datasets. However, this paper did not report specificity and sensitivity of this methods as determined by analysing a sample set of unknown history (i.e. a set comprised of authentic and non-authentic essential oils to determine the false positive and false negative rate of their method). So, not surprisingly authentication of essential oils (or any food or flavour compound) exploiting the subtle differences of isotopic composition at natural abundance level is still done by isotope ratio mass spectrometry. For example, the difference in 13C composition of natural vanillin and synthetic vanillin is approx. 0.011 atom% (= 1.0893 atom% - 1.0783 atom%). Detection this kind of difference is only possible using an Isotope Ratio Mass Spectrometer (IRMS). For your application you would need an IRMS coupled to a GC (as pointed out already by Dr Neubert). Given the minute differences in 13C isotopic composition of authentic essential oils than can vary between the same oil producing plant due to different growing conditions (e.g. different geographic location), one needs to exploit their biochemical (biogenetic) relationships of terpenoids and create a multi-component "fingerprint" by analysing several compounds that are biosynthetically linked. An example of this can be found in an "old" paper of mine: J. Chromatogr. A., 842, 351-371 (1999). However, in the meantime detection of counterfeit or misrepresented essential oils is no longer achievable by analysing of 13C abundance alone. One also needs to determine 2H abundance to achieve an acceptable level of discriminatory power. I hope you find this explanation helpful.

Hi Prabodh,

You choose a very complicated way to solve a very easy problem.

So far as I understand, you want to identify and quantify some compounds in an essential oil. You have the synthetic compounds so you have the reference mass spectra. As W Meier says low resolution mass spectra is not accurate enough to take into consideration the relative abundances of the isotopes of carbon. Moreover this information is not needed for your problem. You just have to identify each compound on the basis of their retention time on the TIC and of their mass spectra compared with your reference compounds. Then you will have to choose the base ion (i.e. 135 for thymol and 120 for salicylic acid methyl ester) to create your SIM sequence and quantify with.

HI Dr. Lemee, You mean Peak intensity of base peak varies from adulterated vs natural compounds? I have problem in wintergreen, people mix synthetic methyl salicylate with natural methyl salicylate or same thing happens with other essential oils. So, I would like to solve my problem just by ordinary GCMS.

ok, I did not understand that you were trying to differentiate natural and synthetic compounds. In this case GCMS is not sufficient. You need to measure delta 13C with IRMS.

Thank you Dr. Lemee, Do you agree that there is always same isotopic ratio of same origins compounds. i.e. ratio of M and M+1 peak or M+2 peak. I think synthetic has higher ratio of M+1/M so far as I observed. That gives me the hint of adulteration so far from my preliminary studies. I would like to extend that upto quantification level. And isotopic ratio of natural compounds also varies from place to place. I dont have access to IRMS thats my problem and I have heard its a big budget equipment not commonly available.

Dear Prabodh, I appreciate your problem of not having access to an IRMS system. However, this is not a good enough reason to use a method that will not answer your research question with sufficient precision, let alone sufficient accuracy or power of discrimination. If you do not have access to an IRMS you need to find somebody who has and is happy to collaborate with you. In the US, the best address for IRMS applied to questions of (food) authenticity would Prof J Ehleringer's lab at the University of Utah. As an alternative to collaboration, you could contact Lesley Chesson at Isoforensics (http://stableisotopes.com/) in Salt Lake CIty to find out how much they would charge for analysing your samples.