Formaldehyde contamination of the DNPH reagent is frequently encountered due to its
widespread occurrence in the environment. The DNPH reagent in Procedure 2 must be purified by
multiple recrystallizations in HPLC-grade acetonitrile. Recrystallization is accomplished, at 40 -
60EC, by slow evaporation of the solvent to maximize crystal size. The purified DNPH crystals are
stored under HPLC-grade acetonitrile. Impurity levels of carbonyl compounds in the DNPH are
determined prior to sample analysis and should be less than 25 mg/L. Refer to Appendix A for the
recrystallization procedure.