INTRALABORATORY WATER QUALITY CONTROL : CONSTRUCTION OF CONTROL CHARTS

Abstract

A particular water quality study or any organised water quality monitoring programme involves the collection, analysis and interpetation of analytical data, which leads to a decision making approach. A laboratory is expected to be able to specify the quality of its data in quantitative terms. This requires the existance of some degree of quality assurance. Quality assurance is a set of operating principles, that if strictly followed during sample collection and analysis, will produce data of defensible quality. One of the essential component for assining accurate and reliable result is analytical quality control (AQC), which refers to routine application of procedures for controlling the measurement process. The performance of a measurement system can be demonstrated by determination of accuracy and presision. The data generated are plotted as a control chart in a manner that indicates wheter the measurement system is in a state of statistical control. On a control chart, the warning limits represent: the level within which values are expected to be 95% of the time and the control limits represent the 99% confidence level. An excercise has been undertaken to develop a few control charts, for the Environmental Engineering Laboratory, University of Roorkee, Roorkee. Eleven water quality parameters were selected for this purpose. The experimental analysis has been carried out on samples of known strength to access both precision and accuracy, and to justify the experimental method adopted. A sample prepared by mixing standard solution has also been analysed by similar manner to check the interference of different solutes and ionic strength on the estimation. Two samples collected from the 1-lindon river were analysed by two different analysts to see the variation in results. On the basis of above study and control charts, it is concluded that the analytical facilities and capability of the laboratory are satisfactory. The control chart can help to provide confidence to the analyst in analysing other samples.