Mass Spectrum - X-axis: m/z (mass-to-charge Ratio)

X-axis: m/z (mass-to-charge Ratio)

The x-axis of a mass spectrum represents a relationship between the mass of a given ion and the number of elementary charges that it carries. This is written as the IUPAC standard m/z to denote the quantity formed by dividing the mass of an ion by the unified atomic mass unit and by its charge number (positive absolute value). This has been referred to as a mass-to-charge ratio, although in some ways it does not fit this description. The IUPAC Gold Book gives an example: "for the ion C₇H₇2+, m/z equals 45.5".

Since a mass spectrum x-axis represents a relationship between the ion mass and the number of elementary charges that a given ion carries it contains mass information that may be extracted by a knowledgeable mass spectrometrist. Once this is done many mass spectrometrists use dalton (Da) as the unit of mass in order to avoid the clumsy "atomic mass units".