Water analyses are done by several methods. The most common types of measurements are gravimetric (weighing), electrochemical (using meters with electrodes) and optical (including visual). Instrumental methods are becoming increasingly popular, and instrumentation is getting "smarter" and easier to use with the inclusion of microprocessors. In the simplest case, a sample may just be placed in an instrument and a result read directly on a display. More often some physical separation technique or chemical procedure is needed before a measurement is made, in order to remove interferences and transform the analyte-- the target of the analysis-- into a form which can be detected by the instrument.

Since even raw sewage is generally more than 99.9% water, most environmental analyses are measuring very low concentrations of materials. The results of these measurements are usually expressed in the units "milligrams per liter," abbreviated as mg/L. Since a milligram is one thousandth of a gram, and a liter of water weighs about a thousand grams, a mg/L is approximately equal to one part per million by weight.

A part per million ("ppm") is only one ten thousandth of one percent. For toxic metals and organic compounds of industrial origin, measurements are now routinely made in the part per billion (microgram per liter) range or even lower. At such low levels, sensitive equipment and careful technique are clearly necessary for accurate results. Avoiding contamination of the sample and using methods which prevent interferences from other substances in the water are crucial requirements for successful analyses.