Mass-to-charge ratio (m/z): the coordinate used by mass spectrometry to ‘identify’ molecules

The mass-to-charge ratio (m/z) is the ratio of an ion’s mass to its charge and is the most fundamental unit on the x-axis in mass spectrometry. The core function of a mass spectrometer is to separate the ions produced by the ionisation of a sample according to their m/z values and then detect them one by one, ultimately producing a mass spectrum with m/z on the x-axis and the relative abundance of ions on the y-axis. For single-charge ions, the m/z value is approximately equal to the mass of the molecule (or fragment); therefore, the first step in interpreting a mass spectrum is to determine the m/z of each peak, and then, by combining this with the charge number and isotopic peak shape, to infer which molecule it corresponds to.

Why use ‘mass-to-charge ratio’ rather than simply ‘mass’? This is because mass spectrometry separates ions based on the interaction of electric and magnetic fields with charged particles, and this interaction depends on both mass and charge. An ion with two charges has an m/z that is only half that of a single-charge ion, and will therefore appear further to the left in the spectrum. Only by understanding this can one correctly interpret multi-charge peaks, addition peaks and fragmentation peaks, and avoid mistaking different charged forms of the same substance for several distinct substances.

In mass spectrometry imaging, m/z acts as the ‘coordinate’ for localising molecules. The instrument acquires mass spectra point by point across the sample; by extracting the intensity of a specific m/z value at all pixel points, a spatial distribution image of that molecule is obtained. The example provided on the Neo Source INSTRUMENT website is very intuitive: in the imaging of fresh tobacco leaves, m/z 104.11 is mainly distributed at the leaf base, whilst m/z 516.49 is mainly distributed along the leaf veins; in a study on Atractylodes macrocephala conducted in collaboration with the China Academy of Chinese Medical Sciences, m/z 183.0807 and m/z 301.2164 were both strongly correlated with cinnabar-coloured spots, leading to the inference that the accumulation of specific polyynes and sesquiterpenes was the cause of the spots.

For m/z to be accurately ‘identified’, sufficiently high mass resolution and mass accuracy are required. Both the New Ceyuan imaging source and the ProC series utilise dual-field reflection time-of-flight (TOF) mass analysers, which provide both high resolution and high accuracy in m/z determination. This enables the separation of molecules with only slight differences in mass, thereby reducing errors in substance identification. MSI DPI and MSI LDPI can be retrofitted to mass spectrometers from Agilent, AB SCIEX and Thermo.