A Miniature Ion Mobility Spectrometer for Explosives Detection Andrew Goodin, William F. Siems, Christina L. Crawford, Prabha Dwivedi, and Herbert H. Hill, Jr. Department of Chemistry, Washington State University, Pullman WA CONCLUSIONS & FUTURE STUDIES EXPERIMENTAL METHODS Purpose & Method: To develop and characterize an energy efficient miniature ion mobility spectrometer (IMS) for explosives detection. Results: A wide range of explosives have been detected and confirmed with reduced mobility values (Ko) from the literature. Characterization including determination of the empirical resolving power parameters α, β, and γ was also performed. The authors would like to thank the WSU College of Sciences and the Homeland Security Advance Research Projects Agency for their support of this project. ACKNOWLEDGEMENTS OVERVIEW RESULTS The miniature IMS demonstrated the ability to selectively separate and detect a number of explosives even though the size and power were significantly reduced. As expected, miniaturizing the IMS reduced it resolving power. A detailed evaluation of the resolving power of this mini-IMS has identified areas where resolving power can be improved in future designs. This size, shape and resolving power make this Mini-IMS ideal for interfacing with orthogonal separation devices such as miniaturized differential ion mobility spectrometers and mass spectrometers. The solid explosive samples were heated in vials with vapor being introduced through the sample inlet of the IMS. The first gate opened at times ranging from µs, while the second gate acted as an aperture grid (Faraday shield). The IMS cell had an internal temperature of 415 K. Data shown is an average of 8 spectra. Res. Power Ideal: Meas. Res. Pwr. Figure 1: Operating the IMS at low power will allow for eventual portable applications. Figure 10: Resolving Power Equation. Where t d is drift time and ω is the peak width at half height. INTRODUCTION We have developed and tested a miniature ion mobility spectrometer that is a third of the size and runs at one-tenth the power of a conventional IMS. In addition to decreased size and power, novel instrumental concepts in the construction of this sensor included design and construction of a Bradbury-Nielsen ion gate, a double gate design for mobility selective aaaaaaa operation, and “plug-and-play” electrodes for ease of instrument construction and optimization. The miniature IMS was designed to be coupled to additional instrumentation such as a high-field asymmetric waveform ion mobility spectrometer (FAIMS) or a mass spectrometer. Figure 2: IMS electrodes 8.7 ± ± Figure 9: IMS Spectra of Explosives illustrating unique arrival times. Ion Current (nA) Drift Time (ms) 1- Siems, W. F.; Wu, C.; Tarver, E. E.; Hill, H. H., Jr. Anal. Chem. 1994, 66, Figure 3: Peak width as a function of α, β, and γ (Left). 1 Reduced Mobility Equation (Right). The width of a peak ( ω ) is a function of temperature (T), drift time (t d ), voltage (V), gate pulse (t g ) and three empirical parameters ( α, β, and γ ) which are related to diffusion, repulsion, and other sources of non-ideality, respectively. The reduced mobility of an ion (K o ) is a function of temperature (T), pressure (P), length of the drift tube (L), mobility (K), drift time (t d ) and electric field (V/L). This value is constant for a particular ion in a particular drift gas, and thus allows identification of analytes by IMS. Figure 6: The miniaturized IMS with macor insulation box. Figure 7: The Bradbury-Nielsen gate allows for a “cleaner” gating of ions and an increase in resolution. Figure 5: IMS spectrum with TNT (K o = 1.46) as calculated from the equation in Fig. 4. Literature: (K o = 1.45). 2 Measured (Neg.): Measured (Pos.): Blank PETN 2,4-DNT TNT RDX Tetryl 2- Garofolo, F.; Marziali, F.; Migliozzi, V.; Stama, A. Rapid Commun Mass Sp. 1996, 11, Figure 8: Negative Mode α and β plots obtained from the left eqn. in Fig. 3 account for the resolving power calculated in Fig. 11 showing that the design (possibly the grounding of the outer shell) causes higher simulated diffusion and repulsion, limiting the maximum resolution from ideal. +mode - modeIdeal 1 α (V/K) β γ (s 2 ) Ion Mobility Spectrometry (IMS) is a gas phase separation technique in which ions are separated in an electric field by their size and shape. Because of its ease of operation, high sensitivity, and rapid analysis time, IMS is currently the method of choice for the detection of explosives. Ion Mobility Spectrometers are employed at airport checkpoints, border stations, tourist attractions, and military installations for the rapid detection of threats from the presence of explosive materials. THEORY Figure 4: Schematic of IMS system showing separation of gas phase ions.