Every compound will absorb or transmit light over a certain range of wavelengths, and so that this can be used to measure the amount of a known chemical substance.

ICP-MS is the most sensitive way of determining multiple elements in liquids. Solutions are atomised and ionised in a high energy plasma and the resulting ions measured by mass spectrometry. ICP-OES uses the light emitted from the plasma to determine the elements present.

Atomic absorption spectroscopy (AAS) measures the absorption of light by gaseous free metallic ions. The technique is used to determine the total concentrations of individual elements in a sample.

Electrochemistry: the study of chemical changes of particles caused by the movement of electrons from one element to another. Is your pure water adequate?

Gas chromatography is one of the most sensitive and commonly used analytical techniques for the separation and determination of various compounds.

Liquid chromatography (LC) is used to separate mixtures of species in solution. They can then be detected by sensitive techniques including mass spectrometry and UV spectrophotometry. High Performance Liquid Chromatography (HPLC) and Ion Chromatography (IC) are the most common LC methods used.