New strategies for the determination of phenylurea pesticides by gas chromatography with hot splitless inlet systems

Direct gas chromatographic methods to analyse phenylurea pesticides are dis couraged by the thermal instability of these compounds, that in conventiona l hot splitless inlet systems leads to extensive and irreproducible formati on of isocyanates and amines. However a careful control of the operating co nditions, like the inlet temperature, the pressure and the presence of suit able chemical additives las acetic acid, tow-molecular-mass amines, organic anhydrides) can either: (i) minimise the thermal decomposition enabling th e direct GC-MS analysis of phenylureas, or (ii) lead to reproducible conver sion to isocyanates. Experimental design was employed to study the effect o f the experimental variables on the thermal transformation of phenylurea pe sticides in splitless inlet system. Two strategies were alternatively optim ised: (i) the minimisation of degradation reactions to increase the signal of phenylureas, (ii) the maximisation of the degradation to isocyanates tha t are in turn determined. The maximal yields in isocyanate were obtained wi th high inlet temperatures, low carrier flows in the injection phase and th e presence of acetic anhydride. By contrast, the use of relatively low inle t temperatures, high carrier flows during the injection and the presence of an amine maximise the response of the parent compounds. (C) 2001 Elsevier Science B.V. All rights reserved.