Ion mobility spectrometry (IMS) is capable of separating ionic species at atmosphere pressure and promises to have applications in detecting chemical warfare agents, explosives, and drugs. Miniaturization of such instruments can potentially provide "shirt pocket" devices that deliver information about ambient chemical species. We have been studying fundamental advantages and technical barriers in developing miniature ion mobility spectrometry. Our current miniature IMS demonstrates a feasible resolution and high sensitivity in separating ion mobility spectra produced by photoionization of NO, O2, and methyl-iodide. Broadening induced by Coulomb repulsion was determined to have a major effect on the resolution of the miniature device.
Miniaturization and Separation
Figure 1 shows the configuration and setup of the miniature IMS, which consists of a 1.7 mm diameter drift channel, 35 mm in length. Ultraviolet laser radiation (266 nm) enters the ionization region through a quartz window, intercepted the sample gas, and exits through another quartz window. To ionize NO molecules, a two-photon process is needed. The free electrons generated by photoionization were captured by NO, O2 and CH3I to form negative ions. Ions were separated in the drift region according to their mobilities. Figure 2 shows ion mobility spectra of negative ions detected by the miniature IMS under laser radiation of NO, NO + impurity, and NO + impurity + CH3I mixtures, respectively.
Factors that determine resolution include: (1) initial ion pulse width, (2) broadening by Coulomb repulsion between ions in both the ionization and drift regions, (3) spatial broadening by diffusion of the ion packet, and (4) ion-molecule reactions in the drift region.
It is noted that the Coulomb contribution to the resolution depends on the total number of ions initially generated and thus should vary with the energy of the ionization laser pulse. We measured the resolution as a function of laser energy. The resolution was found to be lower at higher laser pulse energy. This effect was significantly larger at the lower bias voltage. These measurements suggest that Coulomb repulsion effect on resolution needs to be considered in developing miniature ion mobility spectrometry.
Future and Acknowledgement
To improve resolution, replacement of laser ionization with a corona discharge source may be helpful. Future devices may be possible that employs the manufacturing advantages of photolithography and micromachining. Microfabricated IMS devices could find application as process and field chemical sensors where conditions are well characterized. These miniature devices may also be relevant to more complex analysis scenarios when coupled to microfabricated gas or liquid phase chemical separation devices. This research was sponsored by the US DOE, Office of Research and Development.