The measurement of high proton affinities for a variety of metallic compounds: a new approach by flame-ion mass spectrometry

A small amount (less than or equal to 10(-6) mol fraction) of four alkaline earth metals, tin and yttrium were introduced into five, premixed, fuel-ri ch, H-2-O-2-N-2 flames at atmospheric pressure in the temperature range 182 0-2400 K. Aqueous salt solutions of the metals were sprayed into the premix ed flame gas as an aerosol using an atomizer technique. Ions in a flame wer e observed by sampling flame gas through a nozzle into a mass spectrometer. The concentrations of the major neutral metallic species present in the fl ame were calculated from thermodynamic data currently available. The princi pal metallic ions observed were AOH(+) (A = Mg, Ca, Sr, Ba, Sn) and A(OH)(2 )(+) (A = Y), formed initially by proton transfer to AO and OAOH from H3O+, a natural flame ion. Except for Mg, the ions were also produced by chemi-i onization processes. By adjusting the concentration(s) of the salt solution in the atomizer, it was found that a pair of ions could be brought into eq uilibrium within the time scale of the flame; the pairs included H3O+ with a metal ion or two metallic ions. Because water is a major product of combu stion, a very large difference in proton affinity PA(0)(AO) - PA(0)(H2O) le ss than or equal to 490 kJ mol(-1) (117 kcal mol(-1)) could be attempted fo r the proton transfer equilibrium. Using PA(0)(H2O) = 691.0 kJ mol(-1) (165 .2 kcal mol(-1)) as a reference base to anchor the proton affinity scale, i on ratio measurements led to proton affinity PA(0) values of 766, 912, 1004 , 1184, 1201, and 1222 kJ mol(-1) (183, 218, 240, 283, 287, and 292 kcal mo l(-1)) corrected to 298 K for OYOH, SnO, MgO, CaO, SrO, and BaO, respective ly; of these, only the value for OYOH has not been reported previously. If it is assumed that the neutral thermodynamic data are correct (although som e appear to be in error), the uncertainties in the PA results reported here are +/- 21 kJ mol(-1) (5 kcal mol(-1)). The realization that these equilib ria can be achieved in flames provides a new approach to consolidate and bu ild the high end of the proton affinity ladder, primarily of metallic speci es which are not accessible at lower temperatures. (Int J Mass Spectrom 181 (1998) 181-199) (C) 1998 Elsevier Science B.V.