Determination of Total Cyanide in Soil by Isotope Dilution GC/MS Following Pentafluorobenzyl Derivatization

The high toxicity of cyanide, along with its widespread industrial use, has fuelled interest in the development of analytical methods for its determination in complex matrices. In this study, we propose a novel approach for the measurement of total cyanide in soil samples based on single-step derivatization with pentafluorobenzyl bromide (FBn-Br) followed by quantitation with gas chromatography mass spectrometry in negative chemical ionization mode. The reaction between CN and FBn-Br resulted in the identification of several derivatives such as FBn-CN, (FBn)(FPh)CH-CN, and (FBn)(FPh)C-CN. The relative proportion between such compounds was dependent on experimental conditions. When a 100 μL aliquot of an alkaline-aqueous extract was reacted with 700 μL of 1.3% FBn-Br in acetone, the tri-alkylated derivative was the most abundant. In such conditions a detection limit of 0.5 ng/g of CN was attained. Soil samples were initially spiked with an alkaline solution of KCN internal standard and suspended in 7.5% aqueous NaOH. Determination of total cyanide was achieved by digestion of the alkaline extract with HPO to produce HCN which was then trapped in 0.1% NaOH in a sealed double vial system, followed by reaction with FBn-Br. Isotope dilution calibration was chosen for quantitation, and the validity of the novel method was demonstrated by analysis of soil Certified Reference Materials (CRMs) and by spike recovery tests.