Quantification with deuterated ISTD

Mike Lange , May 16 at 16:00
M
Mike Lange
Dear all,
at the moment i am quantifying phosphatidylcholine molecular species by the means of a deuterated ISTD (i.e. d7-PC(15:0/18:1), Isotopic Purity >99%) in an LC-MS experiment.
When it comes to absolute quantification I am using the formula: c(LipidX) = AUC(Lipidx)/AUC(ISTD) * c (ISTD) [AUC = Area under curve].
Due to isotopic overlap of saturated and unsaturated PC species I am only quantifying adduct mass [M] but I am not taking into account [M+1], [M+2], [M+3],..... In order to make up for that i am using an approach identical to [Han and Gross (2001), Anal. Chem., 295, 88-100] in which I use AUC of [M] and calculate the [M+1] and [M+2] theoretical intensity based on the number of carbons and then add up AUC of [M], [M+1] and [M+2]. This works perfectly fine for native lipids.

Now comes the problem. Since I am using a deuterated ISTD I do not only have to take into account the [M+1] and [M+2] due to 13C abundance BUT also [M-1], [M-2],... for the isotopic enrichment (e.g. "d5","d4", "d3",...) whereas each [M-x] also has a 13C abundance profile that needs to be taken into account during the calculation.
I have tried using a binomial expansion for the deuterium enrichment (Ref. 1)and combine it with a binomial expansion for the 13C Abundance profile. Unfortunately this does not reflect the experimental values.

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So now my question is do you know about any publications that have investigated such a problem and found an equation to solve that?
Kind Regards
Mike
Ref. 1 https://www.isotope.com/corporate-overview/newsletters.cfm?nid=The%20Standard%20%E2%80%93%20April%202011&aid=Understanding%20Isotopic%20Enrichment
Gerhard Liebisch
Dear Mike,
If I understand this correctly you are analyzing the monoisotopic peak and perform a type I correction of the isotope profiles, which is a common practice for non-labelled species. For the labelled standard you aim to perform a similar correction which is a very complicated endeavor.
I would suggest a more practical way i.e. analysis of the isotopic pattern of your labeled standard and calculation of a correction factor based on these data. This experimentally determined isotopic profile makes you also independent from the manufacture´s analysis.
In general, we would like to remind you of the importance of coelution of analyte and internal standard for quantification - please see:
https://lipidomics-standards-initiative.org/guidelines/lipid-species-quantification
Best,
Gerhard