Nitro PAPS is a highly sensitive colorimetric reagent for detecting Fe (II), forming water-soluble complexes at pH 3.0-8.0 (λ max=582 nm, ε=107000). This reagent is suitable for detecting Fe (II) in serum and can also be used to detect micromolar levels of Cu, Zn, Ni, and Co. Thioacetic acid and SDS complexes can be used to mask Cu and Zn in serum. Nitro PAPS can detect Zn in serum by using CN - as a reagent to mask Fe and Cu.