Detection of systematic errors by the analysis of precision

The extension of accuracy in trace element analysis to the ppB level cannot be based directly on the study of Standard Reference Materials because most of these are certified only at the ppM levels. Control of accuracy at such low levels may be achieved by an analysis of precision which serves to detect unknown sources of variation in replicate measurements. This approach was used to locate errors in connection with the determination of arsenic in human serum by neutron activation analysis. Successive applications of the analysis of precision led to the detection of two unknown sources of variation. In addition, the analysis of precision indicated that the current evaluation of small photopeak areas was not entirely unbiased. The apprehension of these errors led to changes in the methodology which in turn yielded both better precision and improved accuracy at the ppB level of concentration