Mass Spectrometric Analysis of Solids

Abstract

tant unanswered question concerns the processes in the rf spark discharge plasma which led to the observed molecular ions. There are three possible explanations for the observation of ions with masses greater than the molecular weight of the carboxylate under investigation. The first model is the evaporation of the carboxylate as a polymer, (MA)", where M is again the metal and A the carboxyl-ate, followed by ionization and very rapid dissociation to the M2A+ species and subsequent rearrangements and fragmentations. This model is consistent with the behavior of inorganic compounds in high-temperature mass spectrometers equipped with a Knudsen cell. However, the heavy amount of sputtering in the ion source region of the mass spectrometer suggests that the immediate region of spark discharge is a moderate-to-high-pressure source of ions. Thus, the possibility that an important source of the observed molecular ions is ion-molecule reactions within the rf discharge must be considered as a second model. A third model postulates the formation of observed species by solid-state processes followed by evaporation and ion-ization. To gain some insight into these possibilities, a study of the spark source mass spectra of selected mixtures was undertaken. Mixtures containing approximately equimolar amounts of sodium butyrate and either copper metal, CuCl2, or KC1 were subjected to sparking. Sodium butyrate was chosen because it gives an intense M2A+ ion and the spark source mass spectra of copper(II) butyrate and potassium butyrate were known. The CuCh was chosen as the second component since Cu(II) has a great tendency to form chelate complexes. The use of copper metal then should allow the distinction between processes occurring within the electrode and processes occurring in the plasma phase involving Cu1+ or Cu2+ ions. This assumption requires that metal ions in the plasma phase have the same electronic configurations regardless of their different chemical sources and is probably valid when consideration is given the nearly equal sensitivity of the spark source mass spectrometer to the various elements, regardless of chemical origins (38). Finally, KC1 was chosen as the second component of the mixture because potassium does not show a great tendency to form chelate complexes in solution chemistry. Data were recorded from exposed plates for the metal-containing molecular ions prominent in the mass spectrum of each pure carboxylate. The ions chosen were MO + , M2O"1", MA+, M2A+, and NaMA+, where M is either copper or potassium and A is the butyrate group. The Na2C4H702+ (M2A+) peak from sodium butyrate at m/e 133 was used to compare relative exposures. These data are summarized in Table IV. None of the predicted "cross product" ions are formed in the cases of the Cu metal-sodium butyrate and KC1-sodium butyrate mixtures. These results are direct evidence that ion-molecule reactions were not an important source of molecular ions in these cases. However, in the case of the CuCl2-sodium butyrate mixture, there is significant formation of NaCuC4H702+ and the possibility of some formation of CuC4H702+. (Unfortunately, Cu20+ interferes with these peaks. However, CU2O"1" is not observed in the copper powder mixture.) A possible conclusion follows that the CuCl2 and sodium butyrate react in the solid phase, evaporate as a molecular complex which during or after ionization undergoes decomposition to the observed NaCuC4H7C>2+ ion. This interpretation is supported by the observation that significant amounts of NaCl+ are formed, which would require the postulation of either an ion-molecule reaction between sodium and chlorine or the same molecular complex postulated for the CuNaC4H702 +. Since no peaks were observed at m/e 78, 80 in the KC1 mixture, the occurrence of the required ion-molecule reaction seems unlikely. ESCA spectra of seven different lead-oxygen species are examined including adsorbed oxygen on lead, two crystal forms of PbO, PbsO-i, PbOz, and two forms of adsorbed water on PbOa. The spectrum of Pbs04 exhibits the behavior to two PbO molecules and one Pb02 molecule although specific characterization of the crystal structure of PbO in Pba04 could not be made. The binding energies of the Pb 4f electrons in PbOa are found to be lower than those of the Pb 4f electrons in PbO. The apparent reversal is rationalized in terms of a relaxation effect. Two kinds of adsorbed water are also found on Pb02-These are explained in terms of a surface adsorbed water species and a water or hydroxyl species which occupies lattice vacancies near the surface. These spectra were then used to characterize electrochemically generated Pb02. It is proposed that this technique may prove valuable in characterizing these electrochemically produced materials. The technique of X-ray photoelectron spectroscopy (XPS or ESCA) has found extensive recent application in structural characterization of a variety of inorganic and organic compounds by appropriate correlation of measured electron binding energies of the various atoms in