MS instruments consist of three modules: an ion source, which can convert gas phase
sample molecules into ions (or, in the case of ESI, move ions that exist in solution into the
gas phase); a mass analyzer, which sorts the ions by their masses by applying
electromagnetic fields; and a detector, which measures the value of an indicator quantity
and thus provides data for calculating the abundances of each ion present. The technique
has both qualitative and quantitative uses. These include identifying unknown compounds,
determining the isotopic composition of elements in a molecule, and determining the
structure of a compound by observing its fragmentation. Other uses include quantifying the
amount of a compound in a sample or studying the fundamentals of gas phase ion
chemistry (the chemistry of ions and neutrals in a vacuum). MS is now in very common use
in analytical laboratories that study physical, chemical, or biological properties of a great
variety of compounds.