The quality control program must give special emphasis to blank control whenever a blank correction is significant. Environmental control can range from simple good housekeeping practices to conducting all operations in an ultraclean room. Most of the sources of the blank are variable, and it is variability that determines the uncertainty of the blank correction and, often, the lower limit of trace concentration that can be determined with reliability. The analytical blank results from contamination arising from four principal sources, namely: the environment the analysis is performed in; the reagents used in the analysis; the apparatus used; and the analyst performing the analysis. To improve both the accuracy and lower limit of trace determinations, it is imperative to control the variability of the analytical blank. The only practical way to accomplish this is to reduce the size of the blank itself by controlling the sources of the blank.