Negative ion drift velocities in mixtures of methane and sulphur hexafluoride

Abstract

The velocities of negative ions drifting in mixtures of methane and sulphur hexafluoride have been measured. Measurements were made as a function of gas composition, reduced electric field strength between 3 and 185 Td, and total gas pressure in the range 100-600 Torr at 300K. The data indicate that SF6- (SF6) ions are the dominant ion in all of the gas mixtures studied. Extrapolation of the data using Blanc's Law was used to determine the velocity of sulphur hexafluoride ions drifting in pure methane.

Ion-ion recombination in mixtures of methane and sulphur hexafluoride

“…In calculating the ion drift velocities of the negative ions we have used the data of Cornell and Littlewood (1986), who suggested that the SF;(SF6) ion is dominant at these pressures. For the positive ions we have used the results of Fleming and Rees (1969).…”

“…The drift velocity of negative ions in mixtures of methane and sulphur hexafluoride have been measured by Cornell and Littlewood (1986). Those for the positive ions in the pure gases can be calculated from the Langevin polarisation limit of the reduced zero field mobility…”

“…For the gas mixtures the mobility can be interpolated from the values in pure methane and pure sulphur hexafluoride using Blanc's Law (Blanc 1908, Mason andHahn 1972). Cornell and Littlewood (1986) obtained values for the drift velocities of the negative ions which were approximately 20 per cent less than those calculated from the corresponding polarisation limit mobilities. It is likely that the drift velocities obtained from the polarisation limits of the mobilities of the positive ions are similarly overestimated, in which case the ion-ion recombination rates obtained from equations ( 8) and ( 10) will need to be revised.…”

The effect of drift gas on the separation of active pharmaceutical ingredients and impurities by ion mobility–mass spectrometry

“…However, in some cases such as complex mixture analysis, improved selectivity and resolution is required. Three methods are commonly used to increase the selectivity of drift tube IM separations: (1) the use of higher electric fields and pressures, combined with longer drift tubes, has been used to achieve resolutions up to ≈200 [7][8][9][10][11][12], (2) the use of shift reagents [13][14][15][16][17], defined as chemicals which complex or react selectively with an analyte to form a gas-phase ion of decreased mobility and increased drift time, or (3) changes to the drift gas composition or temperature [18][19][20][21][22][23][24].…”

“…The effect of drift gas mixtures on ion mobility has been studied using atomic ions [19,[33][34][35], but there have been few reports of the use of binary drift gas mixtures in the analysis of molecular species to enhance ion mobility separations. The mobilities of SF 6 clusters in mixtures of SF 6 and methane have been reported using atmospheric pressure IM [19] and a 10% methane in argon drift gas mixture has been used to study the mobilities of low molecular weight aromatic compounds at atmospheric pressure using IM-MS [29].…”