Mass spectrometry of flames

A quadrupole mass spectrometric analyser for use in flame ionization is described, together with its application to the ions produced by traces of sodium, added as chloride, in premixed hydrogen-air flames. Negative ions (Cl¯ and OH¯) appear to be of subsidiary importance in the main ionization processes, which rather proceed through direct production of electrons. Nevertheless, it is established that the appearance of Cl¯ ions is consistent with dissociative attachment HCl+e¯ → Cl¯+H, rather than by three-body attachment Cl+e¯+M → Cl¯+ M. Hydroxyl ions probably are formed in the same way. The ratios of the amounts of A+ (alkali) and A+. H2O (alkali monohydrate) ions have been measured in the cases of lithium, sodium and potassium. Equilibrium constants for the reaction A+.H2O ⇌ A+ + H2O are obtained and heats of monohydration of 44, 28 and 17 kcal/mole deduced respectively for A ≡ Li, Na, K. Small amounts of H3O+ appear in the reaction zone because of traces of hydrocarbon impurities in the flame gas supplies. These are severely reduced by alkali addition, apparently by an exchange reaction A+H3O+ → A++H2O+H, with a rate constant of 1.1 x 10-8 cm3/s at 2000 °K, rather than by recombination with electrons produced by direct ionization of alkali H3O++e- → H2O+H. In contrast with the steady production of A+ ions in the burnt gases by reactions of the type A+M → A++e¯+M, there is an anomalously high rate of production in the reaction zone. This appears difficult to explain without introduction of processes involving ‘hot’ electrons.