Introduction to mass spectrometry

The introduction booklet provides an overview of mass spectrometry and support for interpreting mass spectra. The accompanying worksheet gives learners a chance to apply their learning to solve a crime.

What is mass spectrometry?

This is a very powerful analytical tool that can provide information on both molecular mass and molecular structure.

How does mass spectrometry work?

In a mass spectrometer a stream of positively charged ions is produced along with an associated magnetic field and their deflection in a controlled external magnetic field is studied in detail.

It is important that the atoms or the molecules of the substance being investigated are free to move so if the sample is not a gas it must first be vaporised.

Next, the sample must be ionised. This is achieved by bombarding the sample with high energy electrons from an electron gun. These knock off an electron to produce a positive ion. For example, consider a helium atom He(g) + e^- -> He⁺(g) + 2e^-

The high energy electron bombardment may also cause molecules to be broken into many different fragments. For example, methane molecules CH₄ can be fragmented to produce CH₃⁺ CH₂ ⁺ CH⁺ and C⁺ 

The positive ions are then accelerated by an electric field and focused into a fine beam by passing through a series of slits with increasing negative potential. It is important that the ions can move freely through the apparatus without colliding with air molecules so the system has all the air removed to create a vacuum. The beam of fast moving positive ions is deflected by a strong external magnetic field. The magnitude of deflection depends upon two factors:

• the mass (m) of the ion – the lighter it is the more it will be deflected
• the charge (z) on the ion – ions with 2⁺ charges are deflected more than 1⁺.

These two factors are combined into the mass to charge ratio (m/z). When m/z is small the deflection is large.

Finally ions which make it right through the machine are detected electronically. As the positive ions arrive at the detector they pick up electrons to become neutral. This movement of electrons is detected, amplified and recorded. The external magnetic field involved in deflection can be adjusted so that ions with different m/z ratios can be detected. A printout of intensity vs m/z ratio is produced.