Analyzing nonvolatile residue using aerosol formation and measurement

COMPENDIUM: Achieving contamination control in industries using such process liquids as ultrapure water requires a convenient method to measure the tiny fraction of material that is left over after the liquid evaporates. In the past, this nonvolatile residue (NVR) has been determined from the change in weight of a container in which the liquid to be tested was evaporated by heating. Although this process was slow, it was adequate for determining parts-per-million concentrations, but it lacks the sensitivity required to measure the lower, parts-per-billion, levels of contamination that are of current interest. A new approach, which has been explored in various ways and commercialized, involves the production of spray droplets that are caused to dry, with the dried particles then analyzed using highly sensitive aerosol analyzers. This article describes the new technology, discusses various equipment options, and compares various approaches on the basis of physics and chemistry.