Filtration through nylon membranes negatively affects analysis of arsenic and phosphate by the molybdenum blue method

Filtering synthetic arsenic- or phosphate-containing solutions (1.5–47.6 μmol/L) with nylon syringe filters significantly reduced absorbances (by 6–74%) when analyzed with the colorimetric molybdenum blue method. Filtering the same solutions with cellulose acetate syringe filters yielded no significant differences as compared to unfiltered controls. The detrimental effect of nylon membranes was also observed when pure Milli-Q water was filtered and subsequently spiked with arsenic(III) or phosphate suggesting that some compound(s) eluting from the filter membranes interfere with the color formation in the assay. Consequently, we caution against using nylon filters when filtering water samples for the determination of arsenic or phosphate with the molybdenum blue method.