Improved analysis of explosives samples with electrospray ionization-high resolution ion mobility spectrometry (ESI-HRIMS)

Abstract

A novel high resolution ion mobility spectrometer (HRIMS) equipped with an electrospray ionization (ESI) source was used to analyze trace levels of several energetic materials. The ESI-HRIMS system allowed rapid analysis of explosive samples that were difficult to detect using conventional IMS systems while providing resolving powers, R, greater than 60 and good sensitivity toward the troublesome homemade explosives (HME). This research demonstrated analysis of trace levels of explosives in both positive and negative ion modes, including thermally labile explosives such as TATP and PETN that are detected as their intact molecular ions. Ion mobility spectra and reduced mobility values (K_o) of common explosives were reported in good agreement with previously published values. The high performance IMS system provided a rapid, effective analytical tool that was a suitable upgrade for current explosive sample screening and routine analysis applications.

Introduction

Ion mobility spectrometers (IMSs) have become common tools for detecting trace levels of explosives in a variety of homeland security and force protection detection scenarios [1], [2], [3], [4], [5]. IMS-based detection systems are among the most widely used chemical detection methods at airports, high security buildings, customs, and other security check points. IMS characterizes substances based on their velocity as gas phase ions moving in an electric field. Swarms of these ions will achieve a constant velocity, $v$, as they move through a voltage gradient at ambient pressure proportional to the electric field, E, in the IMS, where $v$ is related to the proportionality constant known as the mobility, K, for a given substance (see Eq. (1)). Differences in the mobilities give different drift velocities, thus different drift times, separating the various chemical components. The mobility is fundamentally related to a combination of the ion's size/shape, charge, q, reduced mass, μ, for collisions with the neutral inert drift gas and their interaction potential, which consequently affects their collision cross-section, Ω_d [1], [6]. Compared to other spectrometric chemical analysis technologies, e.g., mass spectrometry, IMS is a relatively low resolution technique. However, IMS does benefit from very high sensitivity, good scalability, low power consumption, and ambient pressure operation. Many currently deployed IMS systems utilize thermal desorption of a sample from a swab in conjunction with a radioactive ⁶³Ni beta ionization source [1], [2], [3], [4]. While this combination is effective for detecting particular energetic materials, thermally labile materials suffer from degradation in the desorber and are not detected effectively [3], [4]. Although a radioactive ionization source provides reliable field operational performance, the combination of thermal desorption followed by ionization by a radioactive source is not a viable approach for all analytes of interest.

Electrospray ionization-ion mobility spectrometry (ESI-IMS) was first demonstrated by Shumate and Hill [7]. Numerous researchers have since demonstrated the potential of using ESI-IMS for the analysis of a variety of semi-volatile and non-volatile compounds, including environmental contaminants [8], [9], [10], illicit drugs [11], [12], [13], [14], [15], [16], [17], pharmaceuticals [18], [19], [20], [21], [22], chemical warfare agents [23], [24], [25], explosives [26], [27], [28], [29] and biological molecules [30], [31], [32], [33]. The bulk of the research has been performed on custom-built instruments in academic or government research labs. Until recently, commercially available IMS instruments with an ESI source have been unavailable. One newer commercial instrument utilizes ESI with traveling-wave ion mobility spectrometry (TW-IMS) [34], [35]. In this method, a series of symmetric potential waves continuously pass through a drift tube to propel ions with a velocity, $v$, determined by their mobility, K, shown in Eq. (1) below for a typical drift time IMS with field, E [1], [6], [35]:

$$K=\frac{v}{E}\propto \frac{q}{{\mu }^{1/2}{\Omega }_{\text{d}}}$$

Current applications of TW-IMS include the analysis of complex biological and macrocyclic structures [31], [36], [37], [38], [39].

Moreover, a traditional drift time IMS with an ESI source is also now available commercially. The previous difficulties in producing an instrument of this type with ESI has been partially attributed to the complexity of introducing liquid phase samples into a high temperature drift tube under ambient pressure conditions, then managing the voltage requirements associated with the source and IMS drift tube [40], [41]. In this work, results are presented using the first commercially available drift time ESI-IMS system with high resolution drift tube to provide better analysis of explosive samples in a rapid fashion. A variety of dissolved military/commercial explosive materials have been screened; specifically, Primasheet, Detasheet, PETN, Semtex A, Semtex H, HMX, C4, and TNT. In addition, homemade explosives (HME) ammonium nitrate (AN), urea nitrate (UN) and triacetone triperoxide (TATP) have been successfully detected with the ESI-HRIMS, which is more amenable to reliable detection of thermally labile analytes like PETN and TATP as intact molecular ions [42], [43]. Detection of these threats is limited in current explosives trace detection (ETD) systems employing thermal desorption sample introduction where the compounds decompose at the high temperatures inside the desorber, in contrast to the “softer” ionization via ESI for sample introduction. These ETDs capabilities can be further hindered by the presence of other components in the sample that have similar mobilities as the target, such as other explosives, binder materials or impurities [3], [4]. The higher resolution of the HRIMS vs. current ETDs can resolve all of these components into individual peaks and use this additional information to better confirm the identity of the material. In addition, preliminary data are presented that indicate that this high resolution system can generate quantitative or semi-quantitative information about explosives composition for routine explosives sample analysis.