Chemical ionization is a soft ionization technique used in mass spectrometry to produce ions from molecules. In this process, a reagent gas, such as methane or isobutane, is ionized to create reactive ions. These ions then interact with the sample molecules, leading to the formation of ions that are often less fragmented than those produced by other methods, like electron impact ionization. This technique is particularly useful for analyzing large biomolecules and complex mixtures. By providing a gentler approach to ionization, chemical ionization helps preserve the integrity of the sample, allowing for more accurate mass measurements and structural information about the molecules being studied.