Influence of the Capillary Design and Conditions upon the Transmission Efficiency of a Portable Ion Trap Mass Spectrometer

The capillary is a widely used atmospheric pressure interface in mass spectrometers that may be heated to enhance the desolvation of ions. In this study, the ion transmission efficiency of a portable ion trap mass spectrometer was investigated using capillaries with different inner diameters and temperatures, different vacuum pressures, and different ion optics designs. Due to the incomplete desolvation and the space charge effect inside the miniature ion funnel, the increase of ion intensities with the inner diameter of the capillary leveled off when the inner diameter of the capillary was larger than 0.37 mm. With the increase of capillary temperature, the ion intensities first increase due to better desolvation and then decrease due to greater diffusion loss. The optimal temperature of the capillary increases with the inner diameter of the capillary and the mass-to-charge ratio of ion. In addition, the pressures of different vacuum stages also exerted remarkable effects on the sensitivity of the portable mass spectrometer. From the perspective of instrument miniaturization, optimal transmission efficiency can be obtained when the first-stage vacuum pressure is maintained from 4 to 6 mbar. For a capillary with an inner diameter of 0.37 mm, the background of mass spectrum can be significantly reduced by replacing the pinhole behind the ion funnel with an off-axis skimmer.