Hydrogen sulfide is a deadly gas for which no point-of- care device for measuring biological samples exists. Current methods of measuring sulfide include high performance liquid chromatography and gas chromatography/ mass spectrometry, but these methods require non-portable equipment. Another technique for measuring sulfide is Ellman's test; although it has a high level of sensitivity and can be measured in a spectrophotometer, it has a low dynamic range, as precipitates can form at high concentrations. The results presented here use the vitamin B12 precursor, cobinamide, which goes through a spectral shift upon binding of sulfide, to measure sulfide levels. Here it is shown that cobinamide binds to sulfide with a binding affinity of 5.27 x 10⁸ M⁻¹. The limit of detection and limit of quantification of this assay are calculated to be 1.88 [mu]M and 7.21 [mu]M, respectively. Although the cobinamide assay is not as sensitive as Ellman's test, the cobinamide assay is simpler and these limits are still sufficient since sulfide poisoning victims have blood concentrations of sulfide in the micromolar range. To simulate biological samples, Conway diffusion cells have been used. In these cells, the smallest percentage of recovery of hydrogen sulfide is 95.9%, which suggests that measurement in biological samples is possibl
