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Chemical ionization mass spectra of dexamethasone and betamethasone have been obtained via liquid chromatography mass spectrometry using a moving belt interface. These results have confirmed a concentration-dependent competition between thermal degradation and vaporization. Caution should be exercised when dealing with molecules subject to thermal instability since sample introduction via the mechanical transport system does not guarantee evaporation without thermal decomposition.

Under ammonia and methane chemical ionization conditions, using a liquid chromatographic moving belt interface for sample introduction, molecular weight information has been observed for two representative N-phenykarbamates, propham (isopropyl N-phenylcarbamate) and chlorpropham [isopropyl N-(3 -chlorophenyl)carbamate]. Displacement reactions can occur in which the side-chain is expelled as a neutral olefin via a hydrogen rearrangement. The temperature dependence of these spectra has been studied and the relative concentration of [MH 1' ions increases with decreasing temperature. INTRODUCTIONAnalysis of carbamate pesticides by gas chromatography has been seriously hampered by their reported thermal lability.17' Attempts at analytical separation and identification of N-phenylcarbamates have employed derivatization3 or liquid chromatogra hy4 to avoid thermal decomposition to isocyanates.' More recently, liquid chromatography/mass s ectrometry (LC/MS) has been successfully emp1oyed7,'in the quantitative analyses of carbamates and ureas. While the use of the moving belt interface employed in these previous studies has avoided thermal decomposition via flash vaporization from a polyimide belt, a full explanation of the resultant fragmentation processes was not forthcoming. As a first preliminary step to developing an analytical protocol for residue work, the characterization of such compounds is commonplace in ascertaining the appropriate temperature, reagent gas and ions to be monitored. In the course of characterizing two representative carbamates, propham (1) and chloropropham (2) by LC/MS, certain spectral elements emerged which required detailed study and comment. To assist in this structural investigation, the corresponding N-methylated chlorpropham (3) was prepared and also studied. In this paper, we now report an SE2 reaction as responsible for the major fragment ion in these two compounds via a hydrogen rearrangement mechanism. t Author to whom correspondence should be addressed. EXPERIMENTALAll spectra were obtained on a Finnigan Model 3300 quadrupole mass spectrometer equipped with a chemical ionization (CI) source, INCOS data system and Finnigan liquid chromatographic moving belt interface. Kapton belts were used with a variable flash vaporizer temperature, a CI source temperature of 180°C and a source pressure of 0.8 Torr (adjusted to maximize the intensity of the CH: or the NH: signals). Compounds were spotted on the moving belt using 2 ~1 of standard solutions made up at the concentration level of 1 mg m1-I.The N-methyl derivative of chlorpropham was prepared by alkylation with iodomethane and sodium hydride. The isolated product was estimated by mass spectrometry to be greater than 95% pure and was used without further purification. Figure 1 illustrates the mass spectra obtained for propham (1) using CH4, C2H4 and NH3 as reagent gases. At 220 "C a protonated molecular ion at rn/z 180 is observed with a low relative abundance. The major fragment ion at m / z 138 represents a loss of 42 u from [MH]+. Whi...