ANALYSIS OF THIOCYANATES AND ISOTHIOCYANATES BY AMMONIA CHEMICAL-IONIZATION GAS-CHROMATOGRAPHY MASS-SPECTROMETRY AND GAS-CHROMATOGRAPHY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Under NH3-chemical ionization (CI) conditions alkyl thiocyanates give mass spectra which show only the adduct ions (M + NH4)+ (base peak) an d (M + NH, .NH3)+. Allyl thiocyanate and aromatic thiocyanates show fr agmentation similar to the corresponding isothiocyanates but still pro duce (M + NH4)+ as base peak and (M + NH4 . NH3)+ as a prominent ion. These properties allow the thiocyanates to be easily distinguished fro m isothiocyanates whose NH3-CI mass spectra indicate considerable frag mentation but little, or no, adduct ion formation. The isothiocyanates are further characterized by relatively abundant M+ and (M + H)+ ions in spectra of the C1-C5 alkyl isomers and the ion at m/z 115 as base peak for the longer-chain alkyl isothiocyanates. Thiocyanates and isot hiocyanates can also be differentiated on the basis of their gas-phase Fourier transform (FT) IR spectra. The spectra of isothiocyanates are dominated by a very intense absorption band at ca. 2060 cm-1 (-NCS) s imilar to that seen in liquid film spectra. Thiocyanates, in contrast, show only weak absorption at ca. 2165 cm-1. Due to their weak interac tion with infrared, aromatic thiocyanates are difficult to detect by t his technique. Allyl thiocyanate is a special case. Under normal GC-Fr -IR conditions it quickly isomerizes to allyl isothiocyanate. Allyl th iocyanate was only detected in admixture with allyl isothiocyanate, wi th light-pipe temperatures below 100-degrees-C.