Article Determination of calcium to phosphate elemental ratio in nat...

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The classical (reference) methods are compleximetric titration of the Ca and gravimetric measurement of the P. Modern methods are ICP.

Inductive couple plasma is the most accurate method.

Determination of calcium, magnesium and phosphorus in human stones by inductively coupled plasma atomic-emission spectroscopy

Michael A. E. Wandt, M. A. Bruno Pougnet and Allen L. Rodgers

Abstract

A method for the determination of calcium, magnesium and phosphorus in human stones is described. Calculi are analysed by inductively coupled plasma atomic-emission spectroscopy after digestion with nitric-perchloric acid mixtures. The over-all precision of the analytical procedure is better than 2%(relative standard deviation). Data for 41 South African stones analysed so far are presented. Their use in the quantitative assessment of crystalline phases in connection with other techniques, such as X-ray powder diffraction, is discussed and the application of the method in the routine analysis of human stones is emphasised.

https://pubs.rsc.org/en/content/articlelanding/1984/an/an9840901071/unauth#!divAbstract

The standard wet methods are a molybdovanadate precipitation or colorimetric method for phosphate and a titrimetric method for calcium. These are both in the AOAC standard methods publication. These methods are also provided in the Association of Fertilizer and Phosphate Methods Manual. ICP-OES will provide incorrect results unless the method includes an ionization buffer and internal standard. The best internal standard has been scandium or beryllium. There is an AOAC article on the method. The principal author was James Bartos. Without the use of the internal standard and buffer, your phosphate results will vary with the calcium concentration. If you are analyzing many samples it is worth setting the method up. If you only looking for few results, send them out to an outside lab for analysis. The molydovanadate method is very reliable, and requires a visible spectrophotometer.