Ionization of nitromethane in the flame ionization detector

Abstract

. Can. J. Chem. 59, 1819Chem. 59, (1981. The mass spectra observed on sampling the unburnt gases of a flame ionization detector to which nitromethane has been added are due mostly to proton transfer reactions of NH4+ and H30+. These are formed in the reaction zone from the primary ion HCO+, and pass through the unburnt gases to the sampling orifice. The ions which are detected may be attributed to pyrolysis (CH4NO+ and NO+) or hydrogenation products (CH2NH2+ and CH3NH2+) of nitromethane. The small amount of NO+ observed in the unburnt gases is attributed either to a primary ionization mechanism which produces NO+ from excited nitrogen atoms or molecules in the reaction zone, or to a reaction of H30+ with HNO,, a pyrolysis product of nitromethane. Observations of CH2NH2+ in the flame and in double resonance experiments performed in an ion cyclotron resonance mass spectrometer give limits on the proton affinity of methanimine (CH2NH) of 847.6-862.3 kJ mol-I. Chem. 59, 1819Chem. 59, (1981. Les spectres de masse d'echantillons de gaz qui n'ont pas brule dans un detecteur par ionisation a flamme auxquels on a ajoute du nitromethane sont dus principalement aux reactions de transfert de proton du NH4+ et du H30+. Ceux-ci se forment dans'la zone de reaction a partir de I'ion primaire HCO+ et sont vChicules, par les gazqui n'ont pas brule, dans I'orifice d'echantillonnage. Les ions qui sont dktectes peuvent etre attribues a la pyrolyse (CH4NO+ et NO+) ou aux produits d'hydrogenation (CH2NH2+ et CH3NH2+) du nitromethane. On attribue les faibles quantitks de NO+ observees dans les gaz qui n'ont pas brulk soit un mecanisme d'ionisation primaire qui produit du NO+ a partir des atomes d'azote excites ou des molCcules presentes dans la zone de reaction, soit a une reaction de H30+ avec HNO,, un produit de pyrolyse du nitromethane. Les ions CH2NH2+ observes dans la flamme ou dans des experiences de double resonance realisees dans un spectromktre de masse it resonance d'ion dans un cyclotron donnent des limites de l'affinite de la methanimine (CH2NH) pour le proton de 847,6-862,3 kJ mol-I.[Traduit par le journal] Introduction Studies of ions in flames have tended to concentrate on premixed flames of hydrocarbons (1-6), although some work has been done on ammonia and cyanogen flames (7,8). The flame ionization detector (FID) operates with a wide range of organic compounds and uses a small, conical, diffusion flame, in contrast to the large, flat, premixed flames employed in most of the recent studies. Investigations of the FID type of flame (9, 10) have shown the ionization mechanism is similar to that suggested for premixed flames (3), namely the reactions: