Chapter 16, Problem 16.35P

Interpretation Introduction

Interpretation:

Whether the compound that produced the spectrum shown in the problem contains Cl, Br, or S is to be determined.

Concept introduction:

In a mass spectrometer, the molecules of the compound are bombarded with high energy electrons. This generally results in one electron from the molecule being knocked out. The resulting ion is a radical ion with an odd number of electrons, called the molecular ion. Since the mass of an electron is negligible, the mass of the molecule and the molecular ion are essentially the same.

The molecular ion is a high energy species and breaks up into a number of fragments, one of which carries the charge. Another source of fragmentation is the bombarding electrons themselves. These electrons often have energies sufficient to not just knock out an electron but also enough to break bonds.

The only species detected in the mass spectrometer are the charged species and their separation is based on the $\text{m/z}$ ratio, where m is the mass (amu) and z the charge.

The mass spectrum shows the distribution of the relative abundance of various ion fragments as a function of their $\text{m/z}$ ratio. In most cases, the ions generated have a +1 charge, which means the mass m of each ion fragment is the same as its $\text{m/z}$ value.

This means the $\text{m/z}$ value for the molecular ion (${\text{M}}^{+}$) peak corresponds to the molecular mass. Since carbon (as well as other elements) naturally occurs as a mixture of isotopes, the molecular ion often gives rise to two peaks. Naturally occurring carbon consists of two isotopes, ${}^{\text{12}}{\text{C and }}^{\text{13}}\text{C}$. The molecular ion peak, therefore, will show up as a pair of peaks at ${\text{M}}^{+}$ and $\text{M+1}$. Common heteroatoms such as Cl, Br, and S have two naturally occurring isotopes. Chlorine has two isotopes with mass numbers 35 and 37, with relative abundances in the approximate ratio 3:1. Bromine has two isotopes with mass numbers 79 and 81, with nearly equal abundance. Sulfur has three isotopes of mass number 32, 33, and 34. The abundance of ${}^{33}\text{S}$ is very low, so only ${}^{32}\text{S}$ and ${}^{34}\text{S}$ are likely to show up in a mass spectrum. Their relative abundance ratio is 22.5:1.