Molybdate reactive high molecular weight (MW > 5000) phosphorus (RHMW-P) in solution was separated from low molecular weight material (PO4-P) by Sephadex gel (G25–150). PO4-P is as close to orthophosphate as has been possible to achieve with molybdenum blue technology. Chromatograms of near-surface waters of 32 lakes showed that RHMW-P was rarely the dominant component of dissolved reactive phosphorus (DRP), and so cannot be the major influence causing discrepant estimates of orthophosphate concentrations derived from molybdenum blue and radiotracer technology. The low molecular weight material (PO4-P) may still contain chemically reactive organic phosphorus which could lead to overestimation of orthophosphate. Recent literature points to discrepancies in radiotracer estimates too, so further effort is required to provide reliable assessment of orthophosphate in lake water. Estimates of orthophosphate flux from solution to particulate material based on 32PO4-derived turnover times are likely to be in error irrespective of the source of orthophosphate estimate.
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White et al. (1981) studied this question.