Simple stability-indicating assay for histamine solutions

A chemical stability-indicating assay for the routine analysis of histamine in isotonic-phosphate buffer solutions was developed and evaluated. A quantitative colorimetric assay was developed by adapting the Pauly reaction specific for the imidazole group for the determination of histamine in buffer solutions. Stock solutions of histamine diphosphate 1 mg/mL were diluted with isotonic phosphate buffer to produce standards for an imidazole-group assay and for the USP assay method based on the Folin reaction of free amino groups. The specificity of the assays was evaluated by subjecting samples to ultraviolet irradiation and heat for various time periods and then analyzing for histamine content. Both of the colorimetric assays provided reproducible linear calibration curves passing through the origin for histamine diphosphate concentrations. For the Pauly-reaction assay, the average coefficients of variation between and within runs were 0.5% and 0.1%, respectively. The assay had recoveries of 100.6% and 100.8% at 8 and 25 micrograms/mL of histamine diphosphate concentrations, respectively, and a sensitivity of 3 micrograms/mL. Based on the results of the Pauly-reaction assay, irradiation caused decreases in the apparent concentrations as large as 69.2% depending on the duration of the irradiation and distance from the light source. When the histamine diphosphate solutions were stored in the dark at 60 degrees C for five days, a decrease of only 6.3% occurred. Based on the results of the Folin-reaction assay, irradiation caused increases in the apparent concentrations as large as 50.6%. The Pauly colorimetric assay based on the imidazole group appears to be more appropriate for histamine solutions than the amino-group assay based on the Folin reaction.