Assessment of the Accuracy of Spectrophotometric Measurements and Methodologies

Modern day spectrophotometers are used to perform both relative and absolute transmittance or absorbance measurements. In order to discuss these two distinct functions, this paper is divided essentially into two parts. The first part discusses the instrumental parameters that must be verified in order to assure that the measurement is accurate and outlines the several types of National Bureau of Standards (NBS) standard reference materials (SRM) that are currently available, or in process, for performing these verifications. The second part of the paper focuses on analytical method accuracy and describes a number of spectrophotometric procedures that have been widely used at NBS with the belief that they produce accurate results. In instances where sufficient data exist, the accuracy of these procedures is reexamined by comparison of the spectrophotometric values with those obtained by the accepted definitive isotope dilutionmass spectometric method. The paper concludes with some projections on the possible future use of ultraviolet-visible absorption spectrophotometry as a practical, multielement technique.