IChO 2003 · Theory problem 12

Mass spectrum of the SbHCl+ ion

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Natural antimony consists of the following 2 stable isotopes: \(\ce{^{121}Sb}\), \(\ce{^{123}Sb}\). Natural chlorine consists of the following 2 stable isotopes: \(\ce{^{35}Cl}\), \(\ce{^{37}Cl}\). Natural hydrogen consists of the following 2 stable isotopes: \(\ce{^{1}H}\), \(\ce{^{2}H}\). How many peaks are expected in a low resolution mass spectrum for the ionic fragment \(\ce{SbHCl+}\)?

(a) 4 ( )
(b) 5 ( )
(c) 6 ( )
(d) 7 ( )
(e) 8 ( )
(f) 9 ( )

Official English text of the problem.

Answers

  • 12. (c) 6 peaks, at nominal \(m/z = 157,\ 158,\ 159,\ 160,\ 161,\ 162\).

Solution

12. The fragment is the chloridohydridoantimony(1+) ion, \(\ce{SbHCl+}\), represented by SMILES [H][Sb+]Cl.

Each element has two isotopes, giving \[ 2\times2\times2=8 \] possible isotopic combinations. In a low resolution mass spectrum, combinations with the same nominal mass contribute to the same peak.

For this singly charged ion, the nominal \(m/z\) is obtained by adding the three isotope mass numbers. The eight combinations give:

Antimony isotope Chlorine isotope Nominal \(m/z\) with \(\ce{^{1}H}\) Nominal \(m/z\) with \(\ce{^{2}H}\)
\(\ce{^{121}Sb}\) \(\ce{^{35}Cl}\) 157 158
\(\ce{^{121}Sb}\) \(\ce{^{37}Cl}\) 159 160
\(\ce{^{123}Sb}\) \(\ce{^{35}Cl}\) 159 160
\(\ce{^{123}Sb}\) \(\ce{^{37}Cl}\) 161 162

The tabulated mass-to-charge ratios are in \(\mathrm{Th}\), where \(1\,\mathrm{Th}=1\,\mathrm{u}/e\).

Replacing \(\ce{^{121}Sb}\) with \(\ce{^{123}Sb}\) or replacing \(\ce{^{35}Cl}\) with \(\ce{^{37}Cl}\) increases the nominal mass by the same amount, \(2\,\mathrm{u}\). Consequently, two combinations overlap at 159 and another two overlap at 160. Their small exact-mass differences are unresolved at low resolution.

Thus there are six distinct peaks, so the correct choice is (c). Isotope abundances determine their relative intensities.

The answers were checked against the official answers published by the organisers.