Detection mass bias in atmospheric pressure ionization mass spectrometry.

A previously uncharacterized source of detection mass bias is shown to be associated with atmospheric pressure ionization mass spectrometry (APIMS), and is attributed to a mass dependence in the sampling of ions from the supersonic free jet expansion of gas emerging from the ion source. The halide ions Cl (-), Br(-), and I(-) are shown to be transported from the ion source aperture to a quadrupole mass filter with efficiencies that increase linearly with increasing mass of the ion. While the polyatomic anions SF 6 (-) and C7F 14 (-) are detected with even greater efficiencies than would be expected for monatomic anions of the same mass, this additional sensitivity to the polyatomic anions is thought to be related to ion loss processes occurring within the ion source. The experimental conditions under which these mass bias effects can be minimized or enhanced in APIMS are described.

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.