Mass Spectrum

A mass spectrum is the two-dimensional representation of ion abundance versus m/z. This implies that both an ion’s m/z and its abundance are detected. Ion abundance is reflected by the signal area, more simply by signal height. The most intensive peak of a mass spectrum is termed the base peak and the intensity of the others is represented as relative to the base peak. By normalization of the base peak intensity to 100 % the appearance of a mass spectrum becomes independent of the absolute amount of sample. Thereby, mass spectra can be compared even when they were generated from different amounts of sample and/or on different instruments.

Often, mass spectra are simply displayed as histograms whereas profile spectra are employed when peak shape also plays role. A list of m/z and intensity values is useful for more detailed analysis of a spectrum.

The signal at highest m/z within a spectrum normally reflects the molecular ion and the corresponding peak is generally termed molecular ion peak. All other signals must therefore represent fragment ions thereof. Some fragment ions are directly formed by dissociation of the molecular ion while others, mainly low-mass ions, often result from multi-step processes.

The electron ionization mass spectrum of ascorbinic acid, the substance that already served to illustrate the dimensions of molecular size and mass, is shown below.