Ion formation of N-Methyl carbamate pesticides in thermospray mass spectrometry: The effects of additives to the liquid chromatographic eluent and of the vaporizer temperature

Abstract

The effects of three additives-ammonium acetate, ammonium formate, and nicotinic acid-to the liquid chromatographic (LC) eluent and of the vaporizer temperature on the ion formation of N-methyl carbamate pesticides in thermospray (TSP) mass spectrometry was investigated by using filament- or discharge-assisted ionization. Nineteen carbamates and 12 of their known environmental degradation products were used as model compounds. The additives cause a strong reduction in the abundance of the characteristic fragment ions [M + H - CH3NCO](+) and [M - H - CH3NCO](-) for some of the carbamates. The addition of nicotinic acid reduces the quasimolecular ion intensity and, in most cases, produces nicotinic acid adduct ions. The addition of ammonium acetate or ammonium formate increases the intensity of the quasimolecular ion and in most cases produces a base peak for the ammonium adduct ion. The combination of a suppression of fragmentation and an enhancement of quasimolecular ion formation produces an overall gain in sensitivity. As to more specific effects, the addition of the ammonium salts reduces the intensity of M(-•) with the chlorinated carbamate barban and suppresses the formation of "odd" adduct ions in the TSP mass spectra of most other carbamates. Monitoring the intensity of the fragment and the quasimolecular ion signal as a function of the probe stem temperature, and the related probe tip temperature, proved to be an easy method to study the thermal degradation of the carbamates. This monitoring procedure showed that methiocarb and its sulfone already suffer from thermal degradation at a stem temperature of 90°C and that these compounds will therefore present problems in quantitation with LC/TSP mass spectrometry.

BIOTRANSFORMATION OF A GABAARECEPTOR PARTIAL AGONIST IN SPRAGUE-DAWLEY RATS AND CYNOMOLGUS MONKEYS: IDENTIFICATION OF TWO UNIQUEN-CARBAMOYL METABOLITES

“…Fragment ion m/z 239, which is postulated to be an isocyanate, is unique to M7 compared with 1, and it is believed to form by sequential dehydration and anilide cleavage of [M7 ϩ H] ϩ . Thermal degradation of carbamic acids and carbamates to isocyanates is well known (Fishbein and Zielinski, 1969;Honing et al, 1994;Liska and Slobodnik, 1996;Sanda et al, 1997). a The single proton at 7.50 ppm integrated to 0.3H due to slow relaxation and short relaxation delay.…”

Comparative study of different thermospray interfaces with carbamate pesticides: Influence of the ion source geometry

“…For an unambiguous identification of carbamates in environmental samples, several additional fragment ions are required for proof of presence. Several methods have been reported to increase the number of fragment ions ; for example, use of different ionization modes [1,6], postcolumn addition of adduct reagents [4,19] or increased operating temperatures of the TSP in terface …”

“…For an unambiguous identification of carbamates in environmental samples, several additional fragment ions are required for proof of presence. Several methods have been reported to increase the number of fragment ions ; for example, use of different ionization modes [1,6], postcolumn addition of adduct reagents [4,19] or increased operating temperatures of the TSP in terface to enhance thermal fragmentation [5]. An increase of either the ion source or vaporizer temperature typically causes the dissociation of adduct ions, such as molecular ammonium adduct ions [ [1,5,18].…”

“…Interlaboratory validations have shown that this interface can be used for the quantification of a variety of pesticides [12,13]. Man y experimental parameters, however, such as repeller voltage [14,15], interface temperature [1,5], and the composition of the mobile phase [16,17], require adjustments to obtain the necessary limits of detection and reproducibility, Mass spectrometric investigations of carbamate pesticides via different types of TSP interfaces have been reported widely [1][2][3][4][5][6][7][18][19][20][21][22][23][24][25][26]. These studies, however, indicate that solitary protonated molecules and /or molecular ammonium adduct ions are formed by carbamates that provide no structural information.…”

“…C urrently thermospray (TSP) has been used extensively as a liquid chromatography-mass spectrometry (LC-MS) interface for the determination of a wide range of chemical classes; for example, pesticides [1][2][3][4][5][6][7], azo dyes [8], drugs [9], and toxins [10,11]. Interlaboratory validations have shown that this interface can be used for the quantification of a variety of pesticides [12,13].…”