Isotope dilution-mass spectrometry determination of blood cyanide by headspace gas chromatography

A direct and sensitive method for the determination of blood cyanide by isotope dilution was developed. The blood is placed in a headspace vial, and K13C15N is added as internal standard. Addition of phosphoric acid liberates the cyanide as HCN. The detection is accomplished by mass spectrometry after a fine mass calibration tuning. The detection limit obtained is 0.3 μmol/L. The within- and inter-run coefficients of variation are 4.4% (for a concentration of 2.5 μmol/L) and 3.9% (for a concentration of 4.7 μmol/L), respectively. The observed recovery is 98%. A round-robin exercise was carried out to compare the performance of this method with others currently in use in other clinical laboratories.