Quality control in water analyses

The author discusses the definitions and principles associated with the practice of quality control in water analyses and makes some recommendations for their application. The differences between the definitions of accuracy in U.S.A. and U.K. are illustrated by the EPA approach and that of the Water Research Centre (WRC). The former has defined accuracy as the difference between an average value and the true value when the latter is known or assumed, the WRC bases its definition on individual analytical measurements and uses a bull's eye target analogy for describing analytical error. The performance characteristics used by WRC are tabulated and the different forms of bias (calibration, blank correction and interference) are discussed. Other concepts considered are control charts, method valuation, detection limits and inter-laboratory control.