Mass spectrum

A mass spectrum is an intensity vs. m/z (mass-to-charge ratio) plot representing a chemical analysis. Hence, the mass spectrum of a sample is a pattern representing the distribution of ions by mass (more correctly: mass-to-charge ratio) in a sample. It is a histogram usually acquired using an instrument called a mass spectrometer. Not all mass spectra of a given substance are the same. For example, some mass spectrometers break the analyte molecules into fragments; others observe the intact molecular masses with little fragmentation. A mass spectrum can represent many different types of information based on the type of mass spectrometer and the specific experiment applied; however, all plots of intensity vs. mass-to-charge are referred to as mass spectra. Common fragmentation processes for organic molecules are the McLafferty rearrangement and alpha cleavage. Straight chain alkanes and alkyl groups produce a typical series of peaks: 29 (CH₃CH₂⁺), 43 (CH₃CH₂CH₂⁺), 57 (CH₃CH₂CH₂CH₂⁺), 71 (CH₃CH₂CH₂CH₂CH₂⁺) etc.

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 mass spectrometrist.

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