Molybdenum assay (phosphorus vs inorganic phosphate)?

Recently, I have been getting blood tests done for phosphorus. I see some laboratories report phosphorus and others report inorganic phosphate. The normal range for phosphorus seems to be about 0.1 mmol/l higher than that of inorganic phosphate. I expected that, because I expect that there are some phosphorus-containing molecules that are not H$\_2$PO$\_4$.


My expectation is also that the laboratories are doing something similar to the Molybdenum-method to test the blood serum. But does the Molybdenum-method provide estimates for inorganic phosphate or phosphorus? And how do the laboratories that report inorganic phosphate perform their assay differently than those that report phosphorus?




Direct molybdenum method detects inorganic phosphorus/phosphates(1). To determine total phosphorus, a sample gets first mineralised, forming phosphates, e.g. at high temperature by peroxodisulphate oxidation(2).




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(1) I have used it interchangeably depending on if phosphate content is expressed as phosphorus or phosphate. It is advantageous to express phosphates as inorganic phosphorus, if compared with organic or total phosphorus.


It is still possible there is inorganic P in other form than phosphate, not reacting directly. But in context of biology, it is unlikely.


It is common practice that concentration of some analytes may be expressed in terms of equivalent components that may not be present in the sample at all. Recalculation is trivial.




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(2) This is done during total phosphorus analysis in waste water, where samples are heated with peroxodisulphate solution in pressurised sealed vials. Samples with colloid parts usually become clear when oxidation is done.


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