Determination of methemoglobin and total hemoglobin in toxicological studies by derivative spectrophotometry.

A method for the determination of methemoglobin in the presence of other hemoglobin subforms (i.e., oxy-, deoxy-, and carboxyhemoglobin) by use of derivative spectrophotometry is proposed. The method, which uses the first-derivative of the spectrum at 645 nm, is straightforward and expeditious, so it is of special interest to forensic toxicology laboratories as it allows the simultaneous determination of the methemoglobin saturation percentage and the hemoglobin concentration. This facilitates interpretation of the results and provides a better understanding of the significance of methemoglobin saturation in specific cases. Based on an analysis of interferences, the presence of other hemoglobin subforms or of endogenous components of plasma does not detract in any way from the performance of the method.