A new spectrophotometric method for the toxicological diagnosis of cyanide poisoning.

A spectrophotometric method for the determination of hydrogen cyanide in biological fluids based on the release of cyanide ion by the addition of a strong acid and its subsequent specific reaction with hydroxocobalamin to give cyanocobalamin is proposed. The release of cyanide ion is accelerated by aeration with a stream of an inert gas (nitrogen) that carries it into the hydroxocobalamin solution. Although the in vitro reaction develops to completion within 20 min, reproducible quantitation in biological media takes 45 min. The cyanocobalamin formed is quantitated by second-derivative visible spectrophotometry from the absorbance difference between 333 and 361 nm, the measured signal being proportional to the cyanide ion concentration in the sample.