A method for the analysis of phosphate and calcium in small samples of plasma by atomic absorption spectrophotometry.

Inorganic phosphate is determined by a molybdenummethodby using atomic absorption spectrophotometry, on the basis of the observations of Berenblum & Chain (1938) and Zaugg & Knox (1966, 1967). The method permits analysis for calcium in the same sample and can detect 0.1 ,ug of P. The procedure given below is designed for automatic diluters and dispensers and uses 0.2ml of plasma. It is suitable for phosphate concentrations between 0.5 and 20mg of P/lOOml and calcium concentrations between 1 and 20mg of Ca/lOOml. Analyses have a mean coefficient ofvariation of 1%. Since only one-quarter of the sample is needed after deproteinization, the sample volume could be reduced at the cost of a small loss in convenience and precision. Fig. 1 shows the results of testing by the method of additions. Successive increments of phosphate or of calcium added to samples of plasma increased the observed concentrations by the calculated amount, without mutual interference. Principle. Excess of molybdate and a buffer are added to a deproteinized sample, followed by an immiscible solvent. In the optimum pH range (1.7-2.1) phosphomolybdic acid is quantitatively extracted. Free molybdic acid is then complexed by adding citrate. Phosphate is determined by spraying the solvent layer for molybdenum, and calcium by spraying the aqueous phase after removal of residual organic solvent. Conditions investigated. Studies of the optimum conditions for themethod are summarized as follows. (a) Solvent. Extraction of phosphomolybdic acid from water requires an immiscible solvent that