Hydrogen has two stable isotopes, and sulfur has four stable isotopes, and How many peaks would you observe in the mass spectrum of the positive ion of hydrogen sulfide (H2S)? Assume no decomposition of the ion into smaller fragments?
1 Answer
There would be seven peaks if you could observe them all.
Explanation:
Each sulfur isotope would give a peak.
There are three possible combinations of the hydrogen isotopes:
#"H-H (m/z = 2)"# #"H-D (m/z = 3)"# #"D-D (m/z = 4)"#
Thus, you might expect to find 4 × 3 = 12 components for the molecular ion.
However, some of these peaks overlap.
The stable isotopes of sulfur have masses 32, 33, 34, and 36.
Thus, the expected peaks are
#""^32"S": m//z = 34, 35, 36"# #""^33"S": m//z = 35, 36, 37# #""^34"S": m//z = 36, 37, 38# #""^36"S": m//z = 38, 39, 40#
If every peak were visible, you would see seven peaks.
They would have m/z = 34, 35, 36, 37, 38, 39, 40.
The relative abundances of the sulfur isotopes are
Thus, I would not expect to see peaks from
The relative abundances of
Thus, the combinations of two hydrogen atoms would give mass ratios of
We would not see peaks from
When you consider the relative abundances of all the isotopes, you get the peak ratios
I calculated the mass spectrum of the molecular ion. It looks like this: