Mass Spectrometry - AIMS Lab

Common Ionization Methods for Mass Spectrometry

 

If mass spectrometry is to be useful to the researcher the first task is to produce ion(s) in the gas phase; further, some of the ion(s) should indicate the molecular mass. The mass analyzer can then determine the mass to charge (m/z) ratio of the ion(s) and a detector can record their presence. The mass spectrometrist's motto is "no results without a charge".

In Electron Ionization (EI) and Chemical Ionization (CI) the molecule must be vapourized (in vacuum) prior to ionization. These techniques work well for volatile, thermally robust molecules; if the molecules are involatile or prone to thermal decomposition another ionization technique should be employed.

Advantages for EI are that the spectrum contains structural information (albeit sometimes difficult to interpret!) and the data is library searchable. If the molecular ion is weak or absent CI may be attempted to minimize ion fragmentation through formation of (generally) protonated molecules in the positive ion mode and (generally) deprotonated molecules in the negative mode.

If a thermally sensitive and/or involatile molecule can be ionized in solution then Electrospray Ionization (ESI) may be employed. ESI produces ions from neutrals via proton transfer (or abstraction) in the liquid phase, removal of the solvent and transmission of the ions into a mass spectrometer. The solvents employed are usually methanol/water or acetonitrile/water; the former generally gives better ionization. The technique is very gentle and produces minimal fragmentation which means enhanced molecular weight, but reduced structural information. A further consideration is that the molecule must be soluble (~10uM or so) in the solvent and capable of proton transfer reactions. Compounds which are already ions and soluble may be measured. ESI may also produce multiply charged ions: this is advantageous for proteins and peptides but usually detrimental for the analysis of polymeric species due to the multitude of mers and the overlap of their multiply charged envelopes.

Hydrophobic and polymeric molecules are often analyzed by Matrix Assisted Laser Desorption Ionization (MALDI) wherein the analyte is co-crystallized with a matrix molecule and the sample ionized by initiating (generally) proton transfer reactions between the analyte and matrix using laser irradiation. Usually, only singly charged ions are observed making the spectrum less complicated, but requiring mass spectrometers with a large mass range. Time of Flight (ToF) mass analyzers are commonly employed.