Separation science and hyphenated techniques

Liquid chromatography coupled to mass spectrometry (LC-MS) is now probably the most common used technique in mass spectrometry. It is comprised of the coupling of a liquid chromatography (LC) system and a mass spectrometer (MS) to provide a single integrated system referred to as a ‘hyphenated technique’. LC-MS provides a number of advantages over either technique in isolation.

1. Measurement of time-resolved m/z values for multiple compounds in a sample.

2. An automatable approach to introducing multiple samples into a mass spectrometer.

3. A way to analyse complex mixtures by MS by reducing matrix effects and ion-suppression.

4. An analytical approach capable of separating and identifying structural isomers.  

5. A method to combine relative and absolute quantitation with mass spectral data.

There are many types of LC-MS because no single LC approach provides a universal analytical system. Many are selective for different types of chemical compounds (functional groups and physiochemical properties). For example, reversed-phase-liquid chromatography (RP-LC) partitions analyte molecules between a polar eluent and non-polar alkyl chains bonded directly to the surface of silica particles that make up the stationary phase in a column. The degree of retention depends on the relative hydrophobicity of the analyte. Low and medium polar compounds are well retained and resolved by RP-LC-MS but highly polar and ionic compounds do not have sufficient hydrophobicity and are not generally well characterised. Highly polar compounds elute simultaneously in the ‘void volume’ without being retained at all (see IC-MS for an alternative appropriate for analysis of these types of molecules).