G. A. Eiceman

A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds

Ion mobility spectrometry has become the most successful and widely used technology for the detection of trace levels of nitro-organic explosives on handbags and carry on-luggage in airports throughout the US. The low detection limits are provided by the efficient ionization process, namely, atmospheric pressure chemical ionization (APCI) reactions in negative polarity. An additional level of confidence in a measurement is imparted by characterization of ions for mobilities in weak electric fields of a drift tube at ambient pressure. Findings from over 30 years of investigations into IMS response to these explosives have been collected and assessed to allow a comprehensive view of the APCI reactions characteristic of nitro-organic explosives. Also, the drift tube conditions needed to obtain particular mobility spectra have been summarized. During the past decade, improvements have occurred in IMS on the understanding of reagent gas chemistries, the influence of temperature on ion stability, and sampling methods. In addition, commercial instruments have been refined to provide fast and reliable measurements for on-site detection of explosives. The gas phase ion chemistry of most explosives is mediated by the fragile CONO(2) bonds or the acidity of protons. Thus, M(-) or M.Cl(-) species are found with only a few explosives and loss of NO(2), NO(3) and proton abstraction reactions are common and complicating pathways. However, once ions are formed, they appear to have stabilities on time scales equal to or longer than ion drift times from 5-20 ms. As such, peak shapes in IMS are suitable for high selectivity and sensitivity.

Ion Mobility Spectrometry in Analytical Chemistry

Abstract Recent literature relevant to the role of ion mobility spectrometry (IMS) in analytical chemistry is discussed. Included are sections dealing with instrumentation, spectral collection techniques, the theory of ion mobility in gases, and the dynamics of atmospheric pressure ionization. The pros and cons of radioactive ionization, photoionization, laser ionization, surface ionization, and electrofied spray ionization are considered. Analytical applications are separated into the use of IMS as a stand-alone spectrometer, and the use of IMS as a detector following gas, liquid, and supercritical fluid chromatography.

Ion Mobility Spectrometry: Principles and Applications

Abstract General principles are reviewed for ion mobility spectrometry including new methods for ion separation through field dependent mobilities in strong electric fields with high frequency asymmetric waveform. Additionally, recent advances in the instrumentation for the characterization of ion mobilities in air at ambient pressure are described and critically reviewed. Advances in instrumentation, understanding of principles of measurements by IMS, and the development of hyphenated technologies have resulted in an increase in the number of applications in recent years.

Ion mobility spectrometry: arriving on site and moving beyond a low profile.

Trends in the development of ion mobility spectrometry are reviewed. A discussion of general principles of operation includes an overview and examines drift tube technology and ionization and response. A discussion of the scope of applications examines detection and characterization of single substances and complex mixtures. The technology that permitted the development of hand-held instruments is reviewed. A discussion of barriers to hand-held ion mobility spectrometers includes gas-phase chemistry, ion characterization and separation, sample inlets, bacterial assays by ion mobility spectrometry, databases and artificial intelligence, and ion sources. Directions for future applications and research are examined.

A MEMS radio-frequency ion mobility spectrometer for chemical vapor detection

A first-of-a-kind micro-electro-mechanical systems (MEMS) radio-frequency ion mobility spectrometer (rf-IMS) with a miniature drift tube of total volume 0.6 cm 3 has been fabricated and tested. The spectrometer has detection limits in the parts per billion (ppb) and the ability to identify chemicals such as isomers of xylene not resolved in conventional time-of-flight ion mobility spectrometry. Spectrometer operation with a miniature 10.6 eV (l ¼ 116:5 nm) UV photodischarge lamp and a 1 mCi radioactive ionization source has been demonstrated. The resultant spectra with both these ionization sources are similar, with several additional peaks evident for the radioactive source. The effect of varying the carrier gas flow rate on the resultant spectra has been investigated and optimal flow conditions are found at flow rates between 2 and 3 l/min. The rf-IMS has been interfaced to a mass spectrometer (MS) and rf-IMS spectral peaks have been confirmed. The rf-IMS/MS configuration illustrates another use for the rf-IMS as a pre-filter for atmospheric pressure chemical ionization (APCI) mass spectrometry applications. # 2001 Elsevier Science B.V. All rights reserved.

A novel micromachined high-field asymmetric waveform-ion mobility spectrometer

Abstract The fabrication and characterization of a novel micromachined high-field asymmetric waveform-ion mobility spectrometer (FA-IMS) is described. The spectrometer has a 3×1×0.2 cm3 rectangular drift tube and a planar electrode configuration. The planar configuration permits simple construction using microfabrication technology where electrodes and insulating regions are made with deposited metal films on glass substrates. The spectrometer is characterized using organic vapors (including acetone, benzene, and toluene) at ambient pressure and with air as the drift gas. Ions are created in air at ambient pressure using photo-ionization with a 10.6 eV photo discharge lamp (λ=116.5 nm). The micromachined FA-IMS exhibited behavior consistent with conventional FA-IMS designs where compensation voltage was effective in discriminating between ion species in high-field radio-frequency (RF) regimes. Excellent resolution of benzene and acetone ions in mixtures illustrates an advantage of the FA-IMS over low-field ion mobility spectrometry. Detection of toluene at concentrations as low as 100 ppb has been demonstrated. Improvements in detection limits, by as much as 100×, are anticipated with improved ionization source designs. The ability to transport both positive and negative ions simultaneously through the FA-IMS drift tube is demonstrated here for the first time. Ion intensity is found to be proportional to sample concentration, although clusters of sample ions and neutrals at high concentrations illustrate the need for a drift region which is kept free of sample neutrals. Micromachining promises cost, size, and power reductions enabling both laboratory and field instruments.

Ion-mobility spectrometry as a fast monitor of chemical composition

Ion-mobility spectrometry (IMS) has been known best for screening explosives at airports, detecting chemical agents for the military, and monitoring stack gas emissions in industry. These have been supplemented by applications for determining the accumulation of impurities in gas switches for hydroelectric generating stations and for measuring the levels of volatile organic compounds in the life-supporting atmosphere of the International Space Station. An enlarged scope of applications for this fast method has become possible through advances in miniaturization of drift tubes and new methods for characterizing ions. Also, ion sources of broad importance, such as electrospray ionization, have been added to drift tubes at ambient pressure. Finally, the ion chemistry, which long made IMS a sensitive and selective sensor but also frustrated attempts to rationalize response, has been explored and clarified. New methods for introducing structural content into mobility spectra mean that mobility spectrometers can be general analytical measurement devices and can provide rapid analysis of samples spanning the range of vapor pressures and chemical classes. This article presents selected recent developments in IMS with emphasis on speed of measurement or on uses for which no other method has been convenient or economical.

Recent Developments in Ion Mobility Spectrometry

Abstract The general principles and technical implementations of traditional time-of-flight ion mobility spectrometers and analyzers with field-dependent mobilities were reviewed in our last article in this journal. Recent advances in instrumentation and new applications since 2006 are highlighted in this review. In addition to traditional applications as military chemical-agent detectors, ion mobility techniques have become popular for different purposes. Though ion mobility spectrometry was solely used as vapor sensor in the past decades, further developments in ionization techniques (especially electrospray ionization) now permit its routine use for the analysis of liquid samples. The coupling of ion mobility spectrometry with selective sample preparation techniques such as molecular-imprinted polymers, coupling with chromatographic techniques, the use of dopants, and application of selective ionization sources has led to an expanded number of applications in industrial and environmental analysis with complex sample matrices due to an improved selectivity in comparison with traditional stand-alone spectrometers. Furthermore, new developments in hyphenated techniques, especially ion mobility–mass spectrometry couplings, has resulted in an increased number of new applications for the analysis of biomolecules and pharmaceutical samples and in clinical diagnostics.

Chemical standards in ion mobility spectrometry

Positive ion mobility spectra for three compounds (2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine), dimethyl methylphosphonate (DMMP) and 2,6-di-t-butyl pyridine (2,6-DtBP)) have been studied in air at ambient pressure over the temperature range 37–250 ◦ C with (H2O)nH + as the reactant ion. All three compounds yield a protonated molecule but only 2,4dimethylpyridine and dimethyl methylphosphonate produced proton-bound dimers. The reduced mobilities (K0) of protonated molecules for 2,4-dimethylpyridine and DMMP increase significantly with increasing temperature over the whole temperature range indicating changes in ion composition or interactions; however, K0 for the protonated molecule of 2,6-di-t-butyl pyridine was almost invariant with temperature. The K0 values for the proton-bound dimers of 2,4-dimethylpyridine and DMMP also showed little dependence on temperature, but could be obtained only over an experimentally smaller and lower temperature range and at elevated concentrations. Chemical standards will be helpful as mobility spectra from laboratories worldwide are compared with increased precision and 2,6-di-t-butyl pyridine may be a suitable compound for use in standardizing reduced mobilities. The effect of thermal expansion of the drift tube length on the calculation of reduced mobilities is emphasized.

Advances in Ion Mobility Spectrometry: 1980–1990

Abstract Ion mobility spectrometry (IMS) was first introduced in the late 1960s as an instrumental technique for detecting organic compounds at trace concentrations in air. Despite certain at tractive features of IMS in environmental mon itoring and laboratory studies, the growth of IMS from 1970 to 1980 exhibited some disappointing trends as suggested in Figure 1. Interest in IMS declined generally after 1976 by what may be ascribed to a broad disenchantment from unmet expectations and misunderstanding of response characteristics. A new cycle of interest in IMS began 1980 resulting in advances in all aspects of IMS. Additionally, small rugged IMS units suited for operation in hostile environments became available in fulfillment of the purposes originally suggested for IMS. This has occurred through unpublicized developmental programs within military establishments of the U.S. and the U.K.