A sensitive, direct colorimetric assay of serum zinc using nitro-PAPS and microwell plates

A direct colorimetric assay of zinc in 0.02-ml of serum using a new, water-soluble reagent, 2-(5-nitro-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol disodium salt dihydrate (nitro-PAPS epsilon = 14.5 X 10(4) l/mol per cm at 574 nm), is presented. Use of microwell plates allowed a reduction in the sizes of samples and reagents without affecting the sensitivity or the precision. Zinc and interfering metal ions in serum are bound as cyanide complexes; the zinc ions are preferentially demasked by use of the chloral hydrate, permitting a colorimetric reaction with the chromogen. Within-run and between-run coefficients of variation (CV) were in the ranges of 1.6-2.3% and 1.8-5.2%. This method correlated well with atomic absorption spectrometry.