MASS SPECTROMETRY RESOURCE

Electron Ionisation (often incorrectly called Electron Impact) and Chemical Ionisation are generally considered to be the 'classical' methods of analyte ionisation. Both techniques are still routinely used today for the analysis of low-mass, volatile, thermally stable organic compounds, especially when coupled with gas chromatography (GC-MS). Both techniques follow the same basic setup and source design (in fact it is quite common for ion sources to be dual EI and CI).

Electron Ionisation (EI)

Firstly the analyte must be vaporised. This is usually achieved by heating the probe tip containing a droplet of the analyte in solution. If the sample is thermally unstable, this will often be the first cause of sample fragmentation. Once in the gas-phase, the analyte passes into an EI chamber (see fig 1.) where it interacts with a homogeneous beam of electrons typically at 70 electron volts energy. The electron beam is produced by a filament (rhenium or tungsten wire) and steered across the source chamber to the electron trap. A fixed magnet is placed, with opposite poles slightly off-axis, across the chamber to create a spiral in the electron beam. This is to increase the chance of interactions between the beam and the analyte gas. There are no actual collisions between analyte molecules and electrons, ionisation is caused by electron ejection from the analyte or by analyte decomposition. Scheme 1 shows some of the processes that can occur during the EI process.

Consider the analyte molecule AB. The first two process that might occur are the direct result of energy transfer from the electron beam to the analyte, causing primary fragmentation and the second main cause of fragment ions in the spectrum. The third process is electron ejection from the analyte to create the energised radical ion. This can then either lose energy through 'ion cooling' and stabilise (accounting for the radical molecular ion in the spectrum) or lose energy through secondary fragmenting - the third cause of fragment ions in the mass spectrum. These high levels of fragmentation in EI spectra often result in the the technique being termed a 'hard' method of ionisation. The harsh conditions required to volatilise some types of analyte and the high levels of residual energy possessed by the ions after ionisation cause the high levels of fragment ions observed in the mass spectrum.

Fig 1. Schematic side-view of an EI source.

Scheme 1. Some of the ion formation reactions that can occur in EI.

Chemical Ionisation (CI)

Chemical ionisation is a lower energy alternative to EI for volatile analytes. In CI, there is a reagent gas (user ammonia or methane) in the ion chamber. Scheme 2 shows ion formation in CI using methane as the reagent gas. In equation (a), methane is ionised by an electron beam in the same way as with EI. Equation (b) shows the ionised reagent gas reacting with un-ionised reagent gas to form the carbocation (protonated methane). This step requires the CI reagent gas to be at a critical pressure - too low a pressure, and no ionisation of the analyte can take place. Equation (c) shows proton transfer from the carbocation to the analyte (AB) to form the protonated analyte molecule (ABH⁺). If the pressure of the reagent gas is too high, then the side reactions (d) and (e) can also occur, leading to formation of the analyte adduct ion - this is seen as an M_AB+29 m/z peak in the spectrum (i.e. occurring 28 m/z higher than the ABH⁺).

Scheme 2: Some of the ion formation reactions that can occur during methane CI.


In CI, ionisation is due to proton transfer and is therefore a much lower energy process. This results in less residual energy being possessed by the protonated molecules so that fragmentation is greatly reduced. However, CI still requires volatilisation of the analyte, so thermal degradation of the analyte can still lead to fragment ions being observed. However, CI is generally considered a much 'softer' ionisation method than EI, and until the development of desorption methods, was the only way to analyse most small, biologically important molecules (sugars, amino acids, lipids etc.).