Use of ion clustering equilibria in analysis by atmospheric pressure electron capture mass spectrometry

The role of ion-bound cluster formation in negative ion mass spectrometry

RATIONALE

The ionization mechanisms operative in negative ion atmospheric pressure mass spectrometry are far from being properly understood. In an excess of oxygen superoxide (O(2)(-)) is generally the primary charge-carrying species that is generated. However, subsequent reactions leading to the finally detected ion signals remain obscure.

METHODS

Since adiabatic expansion induced cluster growth and collision-induced dissociation (CID) processes rendered a representative sampling of ion distributions present in the source difficult, a custom-built thermally sampling time-of-flight mass spectrometer was used for the investigations. Using atmospheric pressure laser ionization of toluene as the reagent gas, high yields of thermal electrons were observed, but only negligible amounts of by-products. Ab initio calculations for individual ion/molecule reaction pathways were performed.

RESULTS

Electron capture by molecular oxygen resulted in the formation of subsequent superoxide water clusters as well as distinct analyte-adduct ions. By adjusting the extent of CID within the ion optical stages of the mass spectrometer, the cluster distribution changes to smaller cluster sizes and the analyte signals strongly shifted towards M(-) or [M-H](-). The observed superoxide water cluster distribution was close to thermal. The theoretical results confirmed the experimental findings.

CONCLUSIONS

In negative atmospheric pressure mass spectrometry the water concentration in the ion source (determining the ionization efficiency) and the CID energy provided through electrical fields (determining the ion distribution) are primary, critical parameters for the observed overall ionization mechanism and efficiency.

Cluster-assisted decomposition reactions of the molecular anions of SF6 and C 7F 14

The cluster ions formed by the attachment of dimethylsulfoxide (DMSO) and methanol to the molecular negative ions of C7F14 and SF6 have been studied by a pulsed e-beam high pressure mass spectrometer (PHPMS) and by an atmospheric pressure ionization mass spectrometer (APIMS). The free energy change (ΔG°) for the clustering equilibria reaction, M(-)+S⇌M(-)S, at 35°C are found to be -7.7 and -7.2 kcal/mol for S = DMSO and M(-)=C7F 14 (-) and SF 6 (-) respectively, and -6.4 and -4.5 kcal/mol for S = methanol and M(-)=C7F 14 (-) and SF 6 (-) respectively. While the cluster ions formed by DMSO are found to be stable against side reactions, those formed by methanol undergo decomposition processes in which the central core ion is fragmented. At 35 °C, the rate law for the decomposition of the SF 6 (-) (CH30H)1 ion is second-order, involving the M(-)(CH30H)1 cluster ion and another methanol molecule. While the C7F 14 (-) (CH30H)1 ion also decomposes through this second-order process, a competing unimolecular mechanism is also operative at 35°C. With increases in the PHPMS ion source temperature to 150°C, the unimolecular decomposition process becomes progressively dominant for both of the M(-)(CH30H)1 cluster ions of C7F14 and SF6. Methanol cluster ions of the type M(-)S2 are not observed under any of the conditions examined here. When methanol or water partial pressures of a few torr or higher are present in the buffer gas of the APIMS ion source, the decomposition reactions are very fast and only the fragment ions produced by these reactions are observed in the electron-capture (EC)-APIMS spectra of C7F14 and SF6 . Also, in the methanol-containing APIMS ion source, the course of the SF 6 (-) decomposition reaction is altered so that fragment ions of the type F(-)(S)n dominate the EC-APIMS spectrum of SF6 at all ion source temperatures. For C7F14, fragment ions of the type F-(S)n become dominant at lower ion source temperatures. These previously unknown reactions are expected to be important in the analysis of perfluorinated compounds by mass spectrometric methods that utilize ionization by electron capture or negative chemical ionization. The nature of the fragment ions produced in these cluster-assisted reactions may also provide a new source of information concerning the structures of the molecular negative ions of SF6 and C7F14.