An Accurate Method for the Determination of Inorganic Phosphate in Serum

Abstract

Cali's approach to accuracy is reviewed, and it is concluded that the main factor limiting its extension to analytes other than elements is the lack of suitable definitive methods of analysis. The work published on calcium, and work known to be under way on other elements, is based on the use of stable isotope dilution-mass spectrometry (I.D.M.S.) as the definitive technique. This paper outlines the theoretical considerations involved in extending I.D.M.S. to the determination of molecules; in particular, exact equations for the calibration graph obtained by I.D.M.S. are developed, and special cases of practical significance are defined. It is concluded that in theory there is no reason why accurate results should not be realised. The determination of isotope ratios is central to the technique of I.D.M.S. The means available for such measurements are discussed, and it is concluded that adequate precision can be achieved with standard equipment. To illustrate the application of I.D.M.S. to molecules the steps involved in an assay for inorganic phosphate are defined, and the protocol thus evolved is applied to specimens from the Wellcome Group Quality Control Programme. Although good correlation was found between I.D.M.S. results and the participants' mean results, there was a substantial bias. It is argued that this is a result of the difficulty of defining “inorganic phosphate” as a chemical entity. Some requirements for future work in this field are enumerated.