External Quality Assessment of Absorbance Measurements on Spectral and Spectral-Line Photometers

Abstract

Summary: From 1984From -1987 quality control surveys on photometric measurements were carried out in 600-800 laboratories. The participants measured the photometric absorbance of the control samples at 4 wavelengths of the mercury spectrum: 334.1 nm, 365.4 nm, 404.7 nm and 546.1 nm.The medians of the results were without exception lower than the target values, but only very few of them deviated more than 1%. The dispersion of the values did not follow a normal distribution. Two thirds of the values were concentrated within a very small ränge, while about 10% lay outside the 2-to 3-fold ränge. It was found that longer wavelengths resulted in a smaller dispersion of readings than shorter ones. Furthermore, precision showed a significant dependency on the absorbance readings of the samples, on the one band, and on the different photometers, on the other. this error is not to be expected and remains un-noticed Photometers are among the most important measur-until it becomes quite large. Regulär measurement of ing Instruments for carrying out quantitative deter-control samples with a defined absorbance enables minations in clinical chemistry. Means for the quality the early recognition of .photometric error. Such concontrol of photometers äs well äs the results of inter-trol specimens were sent out by the Department for laboratory studies were described in a number of External Quality Control of the Deutsche Gesellschaft publications (e.g. I.e. (1-4)). The objects of the in-für Klinische Chemie to the laboratories that particterlaboratory studies, however, were mainly spectro-ipated in the quality control surveys for photometers photometers. A comparison of the readings of a larger in the years [1984][1985][1986][1987]. number ofspectraHine photometers on the one band, ^ ^^ ^ of ^ consisted of mea . and spectrophotometers, on the other, has iiot yet surements ^ in ^,^ laboratories with differeen reported. ent equipment. In view of these various origins of the A carefully maintained photoiiieter should make only data, not every one of the phenomena observed in a slight contribution to the total error of the analytical this study could be interpreted in detail. It was very results. However, since the error of photometric meas-clear, however, that the majority of measurements urement is usüally relatively small, any increase in showed only slight errors. In contrast, a considerable