Mobility based devices at ambient pressure between electrospray ionization source and a mass spectrometer

Novel gas-phase ion mobility based devices as well as those that are based on the existing Drift-time Ion Mobility (IMS) and Differential Mobility Spectrometry (DMS) techniques are evaluated for their use as gas-phase ion mobility based filters between electrospray ionization (ESI) source and a quadrupole mass spectrometer (MS). Signal/Noise improvements up to 102-fold are achieved with DMS filtering and enhancement of analyte signals from the chemical background allows detecting the peptide ions in a mixture, which are not detectable without DMS pre-filtering due to high levels of background noise in the mass spectra. A dual-shutter IMS drift tube, constructed with resistive glass tubes that provides uniform electric field inside the drift tube and simplifies its construction, is coupled to MS for the first time and shows the separations of protonated caffeine ions from its cluster ions with different mobilities prior to the introduction to MS. Furthermore, two novel Low Mobility Pass Filters (LMPF) with planar and cylindrical designs are studied for their potential to filter ions of lower mobilities from the system by improving the signal-to-noise ratios in the mass spectra. Planar LMPF is constructed from existing DMS drift tube and operates with low amplitude symmetric high frequency waveform in contrast to more complex electronics of DMS where the waveforms with much higher amplitudes and with asymmetric shape are applied. The cylindrical design of LMPF, which offers very simple alternative design for continuous ion filtering, is constructed with two cylindrical conductive tubes separated with gas-tight insulator tube made from teflon. One of the tubes is connected to ground potential and connected to the MS, while the other tube is applied a negative DC voltage to deflect the ions that are produced by ESI operating in the positive mode. This filter also demonstrates a low mobility cut-off by removing solvent ions, and thus, simplifying the ion population in the ESI plume for the measurement with mass spectrometry.;Moreover, issues that are related to the operation of IMS drift tubes and have not been addressed throughout the history of IMS, such as the impact properties of an ion shutter to the performance of IMS and the failure of utilizing IMS as stand-alone detectors for liquid chromatographs (LC) at high flow rates are studied. For the studies with ion shutters the mutual effects of electric fields of the ion shutter and the drift tube and their influence to the sensitivity of IMS drift tubes are examined. It is found that the transfer efficiency of ions through the ion shutter is dependent on their mobilities suggesting that the sensitivity of mobility drift tubes equipped with traditional ion shutters is governed by the individual ion mobilities. These experimental findings are supported and elucidated with a computer-modeling software.;An IMS drift tube that operates at ambient pressure and equipped with desolvation region and a liquid drain interface to accept the ESI generated aerosol-ions from LC with high flow rate effluent is evaluated to understand the reasons why IMS drift tubes at ambient pressure have failed as stand-alone detectors for LC. Several instrumental parameters, such as the temperature of the drift gas, the composition of aqueous-organic solvent mixture, and the liquid flow rate are studied for their effect to the performance of the drift tube. These studies demonstrate that IMS drift tubes must be maintained at high temperatures to avoid ion-neutral clustering processes that may complicate the data to interpret. Moreover, an increase of water concentration in aqueous buffer-methanol solvent mixture is found to deteriorate the performance of the IMS drift tubes and are attributed as another reason for the failure of IMS to be used as a detector for LC.