Mass Spectrometry

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What is a mass spectrometer?

A mass spectrometer measures the masses of atoms and molecules

How does it work?

Produces positive ions that are deflected by a magnetic field according to their mass-to-charge ratio (known as their m/z ratio)

Calculates the relative abundance of each ion

Displays abundance as percentage

Positive ions could be:

Positively charged atoms

Positively charged molecules

Positively charged fragments of molecules

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Diatomic molecules

Diatomic molecule: Contains two atoms covalently bonded together

The diagram below shows the mass spectrum of chlorine, which exists as diatomic molecules \(Cl_2\)

There are two peaks corresponding to the isotopic masses 35 and 37. These peaks have abundances approximately in a 3:1 ratio.

This allows us to calculate the relative atomic mass of chlorine:

\[ \text{Relative atomic mass = }\frac{ (3\times 35) + (1\times 37)}{4} = 35.5\]

This also allows us to calculate the abundance of each chlorine isotope.

Cl-35 has 75% abundance, while Cl-37 has 25% abundance i.e. The chance of selecting one Cl-35 atom is 0.75, while the chance of selecting one Cl-37 atom is 0.25

There are also peaks corresponding to the masses 70,72 and 74. These are due to the formation of diatomic molecules that combine different isotopes (i.e. both of the chlorine atoms in the molecule do not have the same mass)

These peaks have an approximate abundance ratio of 9:6:1.

The peak at 70 is due to the combination of two Cl-35 atoms. The chance of selecting one Cl-35 atom is 0.75, so the chance of selecting two is \(0.75 \times 0.75= 0.5625\)

The peak at 72 is due to the combination of one Cl-35 atom with one Cl-37 atom. The chance of this is \(0.75 \times 0.25= 0.375\)

The peak at 74 is due to the combination of two Cl-37 atoms. The chance of this is \(0.25 \times 0.25= 0.0625\)

The ratio of these abundancies , 0.5625:0.375:0.0625, is the same as 9:6:1.

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Polyatomic molecules

Molecular ion peak: The peak with the highest m/z ratio, the M peak. This peak is helpful in figuring out the relative molecular mass of a compound using a mass spectrum.

Mass spectra of organic compounds with large molecular masses often show an M+1 peak. This is due to the small percentage of carbon-13 present. This peak should not be used to calculate the relative molecular mass of the compound.