Gravimetric analysis describes the methods for finding the chemical constituent (analyte) in a solution (or other compound) through analytical chemistry. These methods use the mass of a solid to determine the makeup of the compounds, so the analyte either needs to be filtered, thermally precipitated, or vaporized into a solid for analysis. The methods used to process analytes vary based on the current state of the matter. Advanced training in the special care required to minimize inaccurate analysis is most often received in post-graduate chemistry degree programs. The same principles apply in pharmacology to some degree; the drug industry uses many of the same terminology although the definitions are slightly different.
Dissolved materials are often treated with reagents to allow the materials to concentrate before washing and filtering. In most cases, the precipitate material in a solution does not separate as a pure material. The precipitate that contains foreign constituents mixed with the analyte being analyzed is called coprecipitation. This extra material can be avoided with better diluted solutions. If coprecipitation has already occurred, additional chemical reactions can be applied using reagents typically having an ion in common with the primary precipitate in order to reduce the analyte down to the precipitate being sought.
A frequent example given on filtering is the measurement of solids (like salt, for example) suspended in water. After being poured through a filter, the remaining substance can be dried and measured. It is important to note that gravimetric analysis is about measuring properties of analytes, not actually detecting the analyte itself. It is those measured properties of analytes that allow identification of the chemical constituents.
The drug industry uses thermal gravimetric analysis to detect drug impurities in many cases where more expensive spectrophotometric equipment is unavailable or cost-prohibitive (such as third world countries). This differs from volumetric (titrimetric) analysis which is a related discipline of chemical analysis. In most cases gravimetric analysis refers to chemistry and not pharmacology.