Organic Mass Spectrometry: Principle, introduction and ionization methods

Organic mass spectrometry is a chemical analysis technique that can, to a first approximation, be compared to a laboratory balance on a molecular scale. Its principle is based above all on the measurement of the mass-to-charge ratio (m/z) of an ion, with the aim of identifying or quantifying a chemical entity in its pure state, or more broadly, a set of molecules present in complex mixtures. In mass spectrometry, structural identification is possible if m/z ratio measurement accuracies are sufficient to enable us to propose the elemental compositions of the ions detected. Structural characterization can be clarified by interpreting ion fragmentation spectra, which are characteristic of the molecules introduced into the spectrometer. Quantitative analysis can be achieved by coupling mass spectrometers with techniques for separating the constituents of a complex mixture, such as gas or liquid chromatography, or electrophoretic techniques like capillary electrophoresis. It is based on the existence of a relationship between the intensity of detection of a signal received at the mass spectrometer output and the quantity of ionic species produced in the ionization source. Mass spectrometry can also be used to determine the thermochemical properties of gaseous ionic species, or to study molecular interactions in the condensed or gaseous phase.

Mass spectrometers are analytical devices widely used in chemistry, biology, geology, pharmaceutical and environmental sciences. They operate by combining an "ionization" source with an "analyzer". The source forms gaseous ions from a solid, liquid or vaporized sample; the analyzer then sorts them according to their m/z ratio.

There are many different types of mass spectrometer, differing in the nature of their sources and analyzers. Each type of mass spectrometer therefore has its own specific characteristics, which need to be taken into account when mass spectrometry analyses are implemented in a given field of application. Detecting one or more target molecules in a biological, environmental or agri-food matrix may call for the use of a mass spectrometer different from that required to characterize biopolymers (proteins, sugars, lipids, etc.) for biology or medicine. The nature of the molecules to be ionized (size, polarity, thermal fragility...) will guide the choice of ionization method. The analytical application sought (determination of target molecules, resolution of chemical structure, characterization of materials, etc.) will also influence the development of the analysis method and the choice of mass spectrometer to be installed in the laboratory.