slope close to unity? confuse about using deuterium ISTD

[s2008]

Hello All,

I bring a my confusion to here and help get some clues from some people might have same experience.

The isotopic analog is labelled with sufficient number of atoms ( more than 3) of a selected isotope ( typically deuterium) is usually as internal standard (ISTD) for GC-MS analysis. The slope of correspondingly target is expected to be close to unity ( ca. 1). However, Sometimes I found the slope could range from 0.7-1.5 or even beyond this range.

The possible reasons such as :
(1) Cross-contribution between target analyte could be exclude.
(2) The condemnations or matrix interference also could be excluded.

any other parameters that could lead the slope far away the unity?

Did the slope not close 1 indicated that this isotopic ISTD is not a good candidate for target analytes thought the repeatability is good?

[James_Ball]

I am currently doing analysis of volatile organics by purge and trap GC/MS and one pair of compounds is Chlorobenzene/Chlorobenzene-d5 as target/ISTD. The response ratio for that pair is 1.225.

You can have varying response ratios for isotope pairs when doing isotope dilution analysis. There are always slight differences in response for the two compounds as I have observed. It could also be due to the fact that in the deuterated compound there may not have been 100% addition of the deuterium so that the concentration stated is not exact but maybe varies a few percent, or differences in how the compounds ionize in the source, or other reasons that are unknown. If the ratio is consistent and reproducible then I would think your analysis will be good.

[lmh]

You really need the input from a good organic chemist, but in some circumstances the slope may quite reasonably vary from 1. For it to be 1, you assume the internal standard is chemically identical to the analyte, just heavier, so it ionises and fragments identically. But deuterium does affect chemistry; being twice as heavy as hydrogen, it does affect how a molecule moves, and the rates of some reactions. Carbon 13 is less prone to the problem because the fractional mass change is so much smaller, 13/12.