Ion Distribution Profiling in an Ion Mobility Spectrometer by Laser-Induced Fluorescence

Measuring the ion distribution pattern in a drift tube under atmospheric pressure is very useful for studies of ion motion and design of ion mobility spectrometers (IMS); however, no mature method is available for conducting such measurements at present. We propose a simple and low-cost technique for profiling the two-dimensional ion distribution in any cross section of a drift tube. Similar to particle-image velocimetry, we first send sample ions with fluorescence properties into the drift tube and use a receiving plate to collect and accumulate them. Then, the receiving plate is illuminated by exciting light, and the ion distribution appears as a fluorescence image. In this study, Rhodamine 6G was selected as a typical fluorescence-tracer particle. Electrospray ionization (ESI) was chosen as an ionization source to keep the fluorophore undamaged. A plasma-cleaned coverslip was placed at the detection position as a receiving plate. When a layer of ions was collected, the slide was placed under the exciting light with a wavelength of 473 nm. A camera with a 490 nm high-pass light filter was used to capture the fluorescence image representing the ion distribution. The measured-ion detection efficiency of the method was 156 ion/dN, which is equivalent to the level of IonCCD. In addition, we studied the ion-passing characteristics of a Bradbury-Nielsen (BN) ion shutter and the ion-focusing effect in the drift tube using this method. The two-dimensional ion-distribution images behind the ion shutter and the images of the focused ion spot were first observed experimentally. Further theoretical analysis yielded the same conclusions as the experimental results, proving the feasibility of this method and producing a deeper understanding of ion motion in the IMS. This method has promising prospective application to the design, debugging, and optimization of IMS instruments and hyphenated systems.