Undesirable inorganic ions dissolved in feed water can be removed by ion exchange. Anions, cations, and divalent cations are normally analyzed as part of an ion scan. The trace metals, Fe, Cu, Ni, Zn, and Co, are frequently analyzed in purified water as well. Although online resistivity readings over time indicate the change in ion concentrations, it will not specify the ion(s) and it does not detect many trace metals such as iron or nickel.

Ions and trace metals are quantified to PPT levels using the technique of ion chromatography. In addition to the ones named above, some of the items measurable by ion chromatography include fluoride (Fl-), sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), Bromide (Br-), chloride (Cl-), nitrate (NO3-), sulfate (SO42-), phosphate (PO43-) and lithium (Li+ ).

The presence of ions can be an indication of poor performing exchange resins, or failures from system equipment (pumps, valves or degradation of metal components). Their presence in purified water can cause defects on products and interfere with plant operations.