# Behavior of Natural Abundance and Mass Spec Ratios

The presence of sulfur in a mass spec is 96% and 4%. I believe this to be because the natural occurrence of sulfur isotopes are within these proportions.

However, chlorine behaves in a 66%/33% ratio in mass spec, but the most abundance isotopes of chlorine are 75%/25%. Why is this so?

• See also chemguide mentioning 3:1 atomic ions and 9:6:1 molecular ions, based on webbook.nist.gov Sep 3 '20 at 13:40
• What sort of compounds are you using ? Sep 3 '20 at 14:03
• Or in more general terms: ratio (66:33 relative to each other) vs percent (75% and 25% relative to their sum). Sep 3 '20 at 15:29
• Side note: isotopic distributions may vary both along time and place. This is why IUPAC's atomic weight for Li seems to be less and less precise today and compared to other elements. Isotopic fingerprints are used in geology not only to tell age, but origin of the samples (example) e.g.for the analysis of gemstones as well as in forensics (example). Sep 3 '20 at 21:35

You are probably mixing natural abundance (NA) and relative abundance (RA). In mass spectrometry RA is a more valuable parameter as it can be directly obtained as the $$y$$-coordinate of a plotted mass spectra: the most abundant ion (isotope) corresponds to the base peak, which is always $$100\%.$$ In other words, RA reflect isotope ratio, not NA. For the isotopes of the elements RAs can easily be derived from NAs via normalization; however, the problem of finding RAs of the various isotopic molecular species is a bit less trivial [1].
$$\newcommand{\d}[2]{#1.&\hspace{-1em}#2} \begin{array}{lllrlrlr} \hline Z & \text{Isotope} & & &\text{Mass}/\pu{u} & \text{NA}&\hspace{-1em}/\% & \text{RA}&\hspace{-1em}/\%\\ \hline 16 & \ce{^{32}S} & \ce{[E]} & \d{31}{9720711744(14)} & \d{94}{99(26)} & \d{100}{000} &\hspace{-1em} \\ & \ce{^{33}S} & \ce{[E + 1]} & \d{32}{9714589098(15)} & \d{0}{75(2)} & \d{0}{789} \\ & \ce{^{34}S} & \ce{[E + 2]} & \d{33}{96786700(5)} & \d{4}{25(24)} & \d{4}{433} \\ \hline 17 & \ce{^{35}Cl} & \ce{[E]} & \d{34}{96885268(4)} & \d{75}{76(10)} & \d{100}{000} \\ & \ce{^{37}Cl} & \ce{[E + 2]} & \d{36}{96590260(6)} & \d{24}{24(10)} & \d{32}{399} \\ \hline \end{array}$$