Direct Real-Time Detection of RDX Vapors Under ... - ACS Publications

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Direct Real-Time Detection of RDX Vapors Under Ambient Conditions Robert G. Ewing,* David A. Atkinson, and Brian H. Clowers Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, United States S Supporting Information *

ABSTRACT: The results in this manuscript represent a demonstration of RDX vapor detection in real time at ambient temperature without sample preconcentration. The detection of vapors from the low volatility explosive compound RDX was achieved through selective atmospheric pressure chemical ionization using nitrate reactant ions (NO 3 − ) and NO3−·HNO3 adducts generated in an electrical discharge source. The RDX vapors were ionized in a reaction region, which provided a variable (up to several seconds) reaction time. The reaction times were controlled either by flow in an atmospheric flow tube (AFT) or by an electric field in an atmospheric drift tube (ADT). Both AFT and ADT were interfaced to a quadrupole mass spectrometer for ion detection and identification. Recorded signals were observed for RDX concentrations below 25 ppq using selected ion monitoring (SIM) of the RDX-nitrate adduct at m/z 284.

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eliminate the issues related to either the use of canines or the sampling of particles, it becomes obvious that a vapor-based, instrumental technique would be the optimal approach to chemical-based explosives detection. Challenges with Low Vapor Pressures Requiring Subparts Per Trillion (subppt) Detection Levels. The vapor detection of explosives requires significant sensitivity improvements in current technologies for practical use. To illustrate the magnitude of this challenge, the saturated equilibrium concentration of RDX is approximately 5 parts per trillionvolume (pptv) at 25 °C.18 This value represents the highest concentration that could be available for vapor detection. However, because of the “sticky” nature of RDX molecules, the actual levels available for detection are expected to be much less due to losses related to surface adhesion and dilution in the surrounding environment.19−21 Previous efforts geared toward the vapor detection of lowvolatility explosives have explored the use of a variety of instrumental platforms, including cavity ring-down spectroscopy,22 amplifying fluorescence polymers,23,24 chemically sensitive microcantilevers,25,26 a surface acoustic wave (SAW) device with chemoselective polymer coating,27 and surface-enhanced Raman spectroscopy (SERS) with nanoparticle cluster arrays.28 Given that explosives detection is a fairly well-covered research area, there are numerous reviews of the efforts undertaken to advance the state of the art.4,29−34 Very few, if any, of the methods described were demonstrated as capable of reaching the sensitivity and response time needed for direct, real-time

he detection of explosives is a critical security technology need, for applications such as aviation security, warfighter protection in theater, and broader counter-terrorism activities.1−3 Although the long pursued goal has been to “sniff” for explosives vapor, much the way trained canines do, the exceedingly low vapor pressures of most military explosives preclude this approach.4−8 The direct vapor detection approach has worked well for the more volatile explosives, such as nitroglycerin and ethylene glycol dinitrate (EGDN), which was demonstrated decades ago using ion mobility spectrometry.9,10 Moving toward in-the-field vapor detection of common military explosives, such as RDX and PETN, has been much more difficult. In order to maintain a functional security capability despite the inability to directly analyze vapors, the explosives detection community focused instead on particle-based detection of low-vapor pressure explosives.2,5 Screening for explosive particles via residue collection typically involves either the use of a small strip of material that is used to manually swipe the surface,11 or pulses of air are used to move particles off a surface and into a detection system for analysis.12−14 Although these particle-sampling methods can be effective, there are issues with reproducibility and variability of yield because of substrate or explosive form, as well as issues with perceived invasiveness.15,16 A capability for direct vapor detection of low-volatility explosives would negate the issues related to sampling (and desorbing) particles for detection. The issues with sampling particles often lead to the use of canines for detection, especially for large items where size (such as vehicles or cargo) makes particle sampling impractical. However, a variety of factors affect the performance of canines used for detection, including handler influence, recurring costs, selectivity, interferences, environmental conditions, and training. A full evaluation of these issues is covered elsewhere.17 To © 2012 American Chemical Society

Received: October 11, 2012 Accepted: December 7, 2012 Published: December 7, 2012 389

dx.doi.org/10.1021/ac302828g | Anal. Chem. 2013, 85, 389−397

Analytical Chemistry

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related to PTR-MS, SIFT-MS, and APCI-MS to achieve quantitative, real-time measurements of explosive vapor, using selective reactant-ion chemistry under ambient conditions with no preconcentration. SIFT/MS provides parts per billion (ppb) levels of detection for analyte vapors.37 From known reaction times, analyte concentrations can be determined in the absence of a calibration curve if the reaction rate constant is known. This technique is frequently used to evaluate and measure ionization kinetics. This concept is routinely employed for PTR-MS measurements, often made under subambient pressure conditions and has proven useful as an instrument for atmospheric monitoring of volatile organic compounds. The reaction region allows ionization times of approximately 100 μs, which translates to sensitivities in the low per trillionvolume range.38,39 However, as its name implies, PTR-MS does not function explicitly in the negative mode, and thus its demonstrated applicability to explosives and other electronegative compounds has been limited.40,41 Further, the few examples of explosive vapor detection approaches using PTRMS have only been demonstrated using conditions above ambient temperatures and in many cases using preconcentration.40,41 APCI-MS is a very sensitive technique for compounds that form positive and negative ions. This technique is attractive for applied explosives detection as a result of increased sensitivity from the increased number of collisions occurring at atmospheric pressure. However, these have not been able to detect the concentration levels needed for explosives vapor detection. The common theme from these technologies is to increase the probability of collisions between a reactant ion and a limited number of analyte ions in a comparatively large number of inert gas molecules. This is achieved in APCI-MS by operating at atmospheric pressure and by the introduction of a “reaction region” in SIFT- and PTR-MS. The following equation describes the relationship between concentration, reaction time, and ion signal.

vapor detection of compounds such as RDX and PETN. It should be noted that a number of efforts documented in some of these references have demonstrated sensitive vapor detection of TNT. However, the vapor pressure of TNT is high enough that it does not approach the significant challenge of detecting compounds that are orders of magnitude less volatile, such as RDX and PETN. Therefore, TNT-specific detection methods are not considered in comparison to the work in this manuscript. Current State of the Art in Explosives Vapor Detection. Security applications that would use a vapordetection device for low volatility explosives typically have aggressive performance requirements, such as fast response (real time) and ultrahigh sensitivity (ability to detect subequilibrium concentrations of the explosive vapor signature), and there is little precedent for this level of performance. There are a limited number of previously documented methods for low-volatility explosive compounds at subparts per trillionvolume levels, with reasonable analysis time (seconds). These are described below. All other reported methods for ultrasensitive detection require prohibitively long detection times due to slow detector response, long instrument integration time, or preconcentration of analyte from the vapor phase. A detection method using secondary electrospray ionization (SESI) as an ionization source interfaced with different mass spectrometers was reported.35 This method did not demonstrate vapor detection from an explosive mass source term specifically but suggested the capability by using a calibrated vapor concentration based on an electrospray system that vaporized known masses into a dilution gas stream. From these measurements, the authors demonstrated detection of 0.2 ppt for PETN using an API-5000. The MS/MS capabilities of the API-5000 were used to reduce noise and allow for detection at subparts per trillionvolume levels, although at the cost of ion current (factor of 6 to 7). A method using a counter flow APCI mass spectrometry (ion trap) demonstrated low-level detection of TNT and RDX.36 This work claimed detection of TNT at 0.3 ppt and also claimed vapor detection of RDX from C-4, although no detection level estimate was provided. This method monitored the m/z 46 ion as a NO2− daughter ion, originating from the m/z 268 RDX-nitrite adduct parent ion. While significant improvements in sensitivity are required, any gains in this domain without concern for selectivity are counterproductive, as universally increasing sensitivity effectively raises the chemical noise and offsets improvements. Thus, to successfully attain real-time, direct (no preconcentration) detection of explosives vapor at ambient environmental conditions (no heat or energy input to enrich vapor concentrations), real and meaningful increases in selectivity are required along with required increases in sensitivity. A number of current mass spectrometry techniques demonstrate impressive levels of sensitivity for trace vapor analysis. These include selected ion flow tube-mass spectrometry (SIFT-MS), proton transfer reaction-mass spectrometry (PTR-MS), atmospheric-pressure chemical ionization-mass spectrometry (APCI-MS), and gas chromatography/mass spectrometry (GC/MS) with preconcentration. However, despite their merits, these techniques have been unable to demonstrate the routine ability to achieve subparts per trillion (subppt) detection limits in real-time. In this work, we demonstrate an analytical approach that builds upon concepts

[A−] = [R−]0 [A]kt

(1)

In eq 1, k is the reaction rate constant (approximately 10−9 cm3 molecule−1 s−1), t the reaction time in seconds, [A] the concentration of the analyte, [A−] the concentration of the analyte ions (measured signal), and [R−]0 the initial reactant ion concentration (measured signal). Because k is a constant, increasing the reaction time will consequently increase the number of analyte ions available for detection. Provided that the losses resulting from secondary reactions (if any) or diffusion are minimized, increases in reaction time can result in lower observed detection limits. If the extended reaction time also applies to other chemical species that may be considered chemical noise, the signal-to-noise ratio (SNR) of the ultimate measurement remains the same, and detection limits have not increased. For this reason, increased selectivity is needed as well. To increase selectivity, the chemical ionization reagent selected must preferentially interact (charge transfer is not the sole mechanism for ion production) with the analyte of interest to produce the desired increases in SNR. In the case of the present work, NO3− created from a discharge source at atmospheric pressure was selected as the primary reactant ion. NO3− has a comparatively high electron affinity and as a result does not readily transfer charge to most species. However, nitroamines and nitrate esters form stable adducts with NO3− ions. By using the nitrate ions, background chemical noise can 390



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be reduced and provide enhanced SNRs for the adduct ions of interest. The research presented here describes a technique that uses APCI to produce a selective reactant ion, NO3−, and introduces these ions into a reaction region at atmospheric pressure, allowing for reaction times of milliseconds to several seconds (103−105 times greater than PTR-MS), followed by mass spectrometry detection. Two systems described herein have been developed to provide adjustable reaction times in this region. One system, atmospheric flow tube-mass spectrometry (AFT-MS), relies on bulk gas flow to carry the ions through the reaction region. The second system, atmospheric drift tube mass spectrometry (ADT-MS), uses an electric field to control ion transit time. The selective ionization and increased reaction times in both allow for subparts per trillionvolume levels of detection for RDX vapor.

Figure 1. Diagram of the atmospheric flow tube-mass spectrometer (AFT-MS). Two different instrumental configurations are used to introduce the explosive vapor: (A) allows for independently controlled sample flow and dilution gas air-flow rates by forcing air down the tube and (B) allows for the “sniffing” of vapors from various objects through the use of suction from a sampling pump.

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EXPERIMENTAL SECTION Ionization Source. A dielectric barrier discharge ionization source was the primary ionization source used in these studies because of its ease of operation and comparative stability. A point-to-plane corona discharge ionization source and a photoemission ambient pressure ionization (PAPI)42 source were also shown to produce the nitrate reactant ions and demonstrated the ability for vapor detection of RDX. The dielectric barrier discharge ionization source was similar to a distributed ion plasma source (DPIS) described by Waltman et al.43 and consisted of a miniature neon light bulb (5 mm in diameter and 1 cm in height, Eiko A3C, Shawnee, Kansas) with one of the inner leads connected to one end of a RF power supply, which produced a 42 kHz waveform with a 2.5 kV p-p voltage using approximately 60 mA of current. The other lead from the power supply was connected to a stainless steel wire mesh wrapped around the outside of the bulb. The electronics were designed so that the source could be floated above ground potential, but for these studies, the source was referenced to ground potential. The ionization of air at atmospheric pressure (both purified and room air) with reaction times greater than a few milliseconds produced nitrate ions as the predominant reactant ions (m/z 62 and 125), as described previously.44 Reaction Region. The primary goal in designing the reaction region was to attain a system capable of providing approximately 1011 to 1012 collisions for an ion prior to entering the mass spectrometer. To achieve this number of collisions, the reaction region was operated at or near ambient pressures, and ions were allowed residence times of hundreds of milliseconds to several seconds. The ions were conveyed from the ionization source to the inlet of the mass spectrometer by two mechanisms, bulk flow or an electric-field gradient. Bulk flow was achieved with an AFT-MS instrument and the electricfield gradient with an ADT-MS instrument. No heat was applied to either system, and both the gas and the device were held at room temperature, approximately 22 °C. Figure 1 (panels A and B) are diagrams of the AFT with ion movement driven by forced air and suction, respectively. The reaction region consisted of a 1 in. outer diameter (o.d.) copper tube [2.36 cm inner diameter (i.d.)] that was about 71 cm long. The exit of the 71 cm tube was inserted, by means of a 1 in. quick-connect coupling flange (Kurt J. Lesker Co., Pittsburgh, Pennsylvania), into a cylindrical aluminum housing approximately 15 cm i.d. (the same one used to house the ADT described below). This aluminum housing was fastened onto the front end of the mass spectrometer where the commercial

source would normally reside. This housing contained a 1/8 in. port to which the house vacuum could be connected. The 71 cm tube was inserted so that the end of the tube was approximately 0.5 cm away from the mass spectrometer orifice. The inlet of the 71 cm tube was connected to a 1 in. tee. The ionization source was positioned in the center of the tee, providing an overall reaction region of approximately 74 cm. The electrical leads for the ionization source were extended through a section of 3/8 in. ceramic tubing held in place with reducing unions and a Teflon ferrule entering the top of the tee. A 15 cm section of 1 in. o.d. copper tubing was connected to the left end of the tee. For the forced-air design, the 15 cm tube was connected by reducing unions to a 1/4 in. tee that enabled the introduction of a sample gas and a makeup gas in varying proportions. For the suction design, the 15 cm tube was left open for the introduction of sample vapors, and suction with controlled flow was applied to the aluminum housing through the 1/8 in. port. Figure 2 is a diagram of the ADT. This apparatus consisted of stacked, nickel-coated, copper rings with an approximate 5 cm i.d. connected by a series of resistors (500 kOhm, 0.1%/°C, Caddock Electronics, Roseburg, Oregon) to establish an electric field capable of moving ions from the ion source to the mass spectrometer inlet. The length of the drift tube was

Figure 2. The atmospheric drift tube-mass spectrometer (ADT-MS) consisted of stacked metal rings (approximately 2 in. i.d.) connected by a series of resistors that were used to establish an electric field. Ions traversed the ADT from the source to the mass spectrometer under the influence of this weak electric field. The length was approximately 14 cm, with an electric field that was varied from 1200 to 50 V (equals 86 V/cm to 3.6 V/cm). Lower voltages produced slower ion transit times. 391



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analyte ions available for detection. If ionization is not selective, then sensitivity to all compounds should increase. Typical APCI reaction times range from microseconds to a few milliseconds. Increasing the reaction times from milliseconds to seconds allows the chemical ionization process to approach thermodynamically favored product ions. To demonstrate the results of two diverse reaction times, ions formed from a pointto-plane corona discharge in unpurified room air are displayed in Figure 3 for negative ions. The bottom spectrum was

approximately 14 cm and was contained in an aluminum chamber fastened to the front end of the mass spectrometer. The applied voltage varied from 1200 VDC to 50 VDC, producing electric fields ranging from 86 V/cm to 3.6 V/cm with lower voltages resulting in slower ion-drift velocities. Ion residence times (related to reaction time) will be based upon ion mobility and be on the order of tens of milliseconds to a few seconds. When using the ADT as the mechanism by which ion transit times were controlled, the ionization source was inserted approximately 4 cm into the front of the ADT, providing a reaction region of 10 cm. Mass Spectrometer. An API-5000 (AB-Sciex, Framingham, Massachusetts) triple-quadrupole mass spectrometer was used to detect the ion species generated in the AFT or ADT configurations. Minimal modifications to the instrument included removing both the commercially supplied ionization source and the interface plate providing the plenum gas and attaching either the AFT or the ADT, depending on the desired experimental design. At no time was the API heater used. Unless specified otherwise, all spectra acquired under this effort were obtained in the negative mode for m/z ratios ranging from 10 to 500. In an attempt to minimize atmospheric pressure interface dissociation events, interface voltages were minimized. With the use of the vendor-defined nomenclature, the following potentials were used: OR, −2 V; DP, −7; interface heater, off. Chemicals and Materials. A 1 mg/mL RDX solution obtained from Accustandard (New Haven, CT) in a 50:50 mix of methanol:acetonitrile was used. Clean air was provided by compressed lab air and purified by a Parker Domnick Hunter pure air generator (model CO2RP140, Gateshead, U.K.). Moisture content of approximately 40 ppm was produced using this system as measured by a GE Panametrics Moisture Image Series I (Billerica, MA). The flow of gas used for sample introduction was controlled using a Cole-Parmer mass-flow controller (model 32907-69, Vernon Hills, IL) and a Humonics Inc. flow meter (Veri-Flow 500, Rancho Cordova, CA). Sample Introduction and Vapor Generation. RDX vapors were generated at ambient temperature and pressure (normally about 750 mmHg for Richland, Washington and 22 °C in the laboratory). Of the two methods used to produce RDX vapor, the first involved placing a small residue of RDX on a glass microscope slide and the other involved coating quartz wool with RDX and placing it inside a 1/4 in. stainless steel tube. For the first approach, 5 μL of RDX solution was placed on a glass microscope slide cover and allowed to evaporate, leaving approximately 5 μg of residue on the surface of the slide. For the second method, 25, 50, and 100 μL of the RDX solution were added separately to 25, 50, and 100 mg of quartz wool, respectively, and each of these were placed inside three separate 1/4 in. stainless steel tubes with lengths of 4.0, 7.5, and 15 cm, respectively. For the approach employing RDXcoated quartz wool, the intent was to provide sufficient surface area to allow the RDX vapor to reach saturated equilibrium prior to being exposed to the ionization source. A similar RDX response was observed from the mass spectrometer for all three tubes, suggesting that a longer tube with more material did not provide a greater signal and indicating that all three tubes generated vapor concentrations that approached saturated levels at room temperature.

Figure 3. Negative ions generated from corona discharge in room air. The bottom spectrum represents the corona discharge occurring at the pinhole to the mass spectrometer. The top figure is with the corona discharge occurring just inside a metal tube of a 91 cm long AFT reaction region with approximately 10 L/min of suction.

obtained with the corona wire located approximately 1 cm away from the mass spectrometer orifice with a voltage of −3200 V applied to the wire with a 4 MΩ resistor inline. The top spectrum represents a similar corona discharge, occurring just inside the 1 in. outer diameter tube but approximately 91 cm away from the spectrometer orifice with a sampling flow of about 10 L/min, which provided an ion residence time of approximately 2.4 s. The total ion signal observed for all ionized species was significantly greater when the ionization source was located next to the mass spectrometer orifice. However, given the importance of SNR when the focus is on a target analyte or class of compounds, ions that do not originate from the target species (i.e., RDX) serve only to increase the levels of confounding chemical species and reduced SNR. Further, this figure illustrates that placing the ionization source directly in front of the MS inlet, which is still common practice as it provides consistent and high signal levels, may not always provide the greatest SNR. Figure 3 highlights the relationship between the concentration and relative charge affinity of a species to the ionization time. If sufficient reaction time is provided, the range of observed species will be limited to only those capable of forming stable adducts or ones that possess the highest electron affinities. The observed species in Figure 3 for the ion source located at the MS interface is similar to those commonly observed in APCI-MS, namely O2−(H2O)n and CO3−(H2O)n for the negative ions. However, with longer reaction times in the same air space (unpurified room air), the spectrum changes dramatically. The negative-ion spectrum is quite interesting, since the increased reaction time leads to a

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RESULTS AND DISCUSSION Ionization Selectivity. As outlined in the introduction, increased reaction times provide for increased numbers of 392



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very simple spectra dominated by nitrate species (NO3−, NO3−·H2O, and NO3−·HNO3). The nitrate ion has a high electron affinity and does not ionize many compounds, thus producing the low observed background. The nitrate ion, however, does form highly selective adducts with certain explosive compounds, like RDX. A similar experiment was conducted to observe positive ions using the same apparatus but with +2800 V applied to the corona wire. The same unpurified room air used for the negative ionization, at a flow rate of about 10 L/min, was also used here. The observed positive ions with the corona source located at the MS interface are similar to those commonly observed in APCI/MS, namely H+(H2O)n. However, with longer reaction times in the same air space (unpurified room air), the spectrum changed dramatically. The predominant lowmass positive ions were observed as peaks at m/z 74 and 118. The even masses indicated pronated species that contain an odd number of nitrogen atoms, likely amines with relatively high proton affinities. Further, there was a significant number of unidentified ions observed above m/z 250, which appear as a result of the longer reaction times. The number of ions observed in the positive spectrum was an indication of the variety of species present in the room air and demonstrates the selectivity achieved with nitrate reactant ions observed in Figure 3. RDX Vapor Detection. In an effort to control the ion residence time and RDX vapor concentration, the AFT-MS with the instrumental configuration shown in Figure 1A was used to generate the spectra in Figure 4. RDX vapor generation

thus all surfaces, supplied gas and the ionization source were maintained at room temperature. An example of RDX vapor detection is shown in Figure 4 with background air (bottom) and RDX vapor (top). The RDX vapor was diluted to 8% of saturated levels with a sample flow of 320 mL/min and a dilution flow of 3680 mL/min, providing a total gas flow of 4 L/min. The RDX peak is observed as the nitrate adduct at m/z 284. In order to display both the nitrate reactant ions at m/z 62 and 125, as well as the RDX-nitrate adduct, the intensity was scaled by a factor of 100 above m/z 200. The distance in the reaction region from the ionization source to the mass spectrometer’s inlet is approximately 74 cm. With an i.d. of 2.36 cm, the volume of the cylinder is 324 cm3. At a flow rate of 4 L/min, the residence time for ions in the reaction region is approximately 4.9 s. Since ionization is occurring at atmospheric pressure, the third-order clustering rate constant corresponds to an effective second-order rate constant, assumed here to be the collision rate constant, kc. An estimated value was used here, kc = 2 × 10−9 cm3 molecule−1 s−1. From Figure 4, the [NO3−]0 was 5.73 × 105 counts per second (cps) and the [RDX·NO3−] was 6.95 × 103 cps. The RDX concentration can be calculated from eq 1, with t = 4.9 s, k = 2 × 10−9 cm3 molecule−1 cm−1, and [RDX·NO3−]/[NO3−]0 = 0.012. Thus, [RDX] = 1.2 × 106 molecules/cm3. With approximately 2.46 × 1019 molecules/cm3 at 298 K and 1 atm, this values corresponds to a concentration of [RDX] = 0.050 pptv. This value is approximately an order of magnitude lower than the concentration predicted by an 8% dilution of the saturated equilibrium vapor pressure (5 pptv), which is about 0.4 pptv. The discrepancy in these values is reasonable since the signal calculated value is lower than the value predicted using the vapor pressure and the lower level likely results from losses of RDX to the walls. Inconsistencies between the measured and predicted values could also result from the original RDX vapor source not being at equilibrium or errors in the estimation of the collision rate constant, kc. Reaction Time vs Sensitivity. Another method for controlling the ion residence time in the reaction region is with an electric-field gradient. An ADT was configured as shown in Figure 2. This is similar to an ion mobility spectrometer, except there is no shutter grid and the sample gas is introduced at the back end to provide analyte vapors throughout the drift region. Ions generated at the ionization source move toward the inlet of the mass spectrometer while interacting with the sample gas containing low levels of RDX molecules. Lower electric fields result in longer ion residence times in the reaction region. This provides a higher probability of a reactant ion colliding with an RDX molecule, however, longer residence times also result in a loss of total ion signal through diffusion. Both results are displayed in Figure 5 for RDX vapor in the ADT-MS, with varying electric fields. RDX vapor was introduced as shown in the figure by passing 1.5 L/ min air through the 4 cm long, 1/4 in. diameter tube containing RDX on quartz wool. The intent was that RDX concentration within the reaction region would be close to saturated levels of approximately 5 pptv. The dielectric barrier discharge ionization source was placed approximately 4 cm into the drift region. The voltages applied across the 14 cm drift tube were 1200, 600, 300, 150, 100, and 50 V, resulting in electric fields ranging from 86 to 3.6 V/cm. The ion drift velocity is a product of the ion mobility (K) and electric field (E). With a fixed drift length, the drift velocity is directly proportional to V, and thus ion

Figure 4. Mass spectra of background air (bottom) and RDX vapor (top) using AFT at 4 L/min total flow. The top spectrum was observed for a saturated RDX vapor flow rate of 320 mL/min in air at room temperature diluted with 3680 mL/min air for a final concentration that is approximately 8% of saturation or 0.4 pptv.

was achieved by depositing 25 μg of RDX onto 25 mg of quartz wool and placing inside a 4-cm long 1/4-in.-diameter stainless steel tube. By flowing a metered stream of purified air through the tube, saturated RDX vapor could be introduced into the AFT. In addition to the sample vapor introduction, a secondary stream of purified air was supplied to provide a dilution gas for the RDX vapor to achieve subequilibrium concentrations. Here and throughout all subsequent experiments no heat was applied 393



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Figure 5. Signal intensities of the reactant ion (m/z 62) and the ratio of RDX signal (m/z 284) to reactant ion signal with varying electric fields in the ADT-MS. A decrease in voltage results in an increase in the ion residence time in the reaction region, leading to an increase in the relative RDX signal combined with an overall decrease in total signal intensity.

Table 1. Calculated RDX Vapor Concentration Using Data from Figure 5 V 1200 600 300 150 100 50

1/V 0.00083 0.0017 0.0033 0.0067 0.01 0.02

E Field (V/cm) 86 43 21 11 7.1 3.6

drift velocity (cm/s) 197 99 49 25 16 8.2

time (s) 0.05 0.1 0.2 0.41 0.61 1.22

ratio [A−]/[R−]0

density (molecules/cm3)

subparts per trillionvolume

0.0015 0.003 0.0075 0.012 0.015 0.053

× × × × × ×

0.60 0.60 0.75 0.60 0.50 0.88

1.50 1.50 1.80 1.50 1.20 2.20

07

10 1007 1007 1007 1007 1007

pptv. Using this approach, the vapor concentration for each data point from Figure 5 is calculated in Table 1. The data from Figure 5 were all collected under the same vapor conditions, and the concentration is expected to be the same, as the flows remained constant; only the ion residence time was adjusted by changing the electric field. With the exception of the last data point, which had the highest level of uncertainty, the calculated concentration of RDX vapor was about 0.6 pptv. This value is about 1 order of magnitude lower than the literature value for saturated RDX vapor and is similar to the calculations from Figure 4. The lower value is likely because of losses of RDX to the walls, prior to the gas entering the reaction region. Observed Detection Limits Using AFT-MS. An attempt to measure detection limits was based on the assumption that saturated levels of RDX vapor, reported in the literature to be about 5 pptv, were generated from the 4 cm, 1/4 in. o.d. tube with RDX residue on quartz wool. With the use of this configuration shown in Figure 1A, a controlled flow rate of air was passed through the tube containing RDX and mixed with a second flow of clean air in a 1/4 in. tee. The total flow entering the reaction region was kept close to a constant level of 5 L/ min. Data were collected using selective ion monitoring of m/z 62 (the reactant ions) and 284 (RDX-nitrate adduct), with a 200 ms dwell time for each ion. Data from the selected ion monitoring (SIM) of 284 are displayed in Figure 6. For this

residence time is inversely proportional to voltage. To represent ion residence time, the x axis is displayed as the inverse of the applied voltage. As seen in Figure 5, as the voltage decreases (an increase in 1/V), the ion residence time increases and the total ion signal decreases and approaches the noise floor at about 0.02 V−1. However, the increased ion residence time results in an overall increase in the relative RDX response as shown by an increase in the analyte/reactant ion ratio. Longer residence times will provide better sensitivities, however, it comes at an overall loss of ion signal as observed by the error bars on the RDX/reactant ion ratio response at 0.02 V−1. A reduced mobility value (K0) of 2.1 cm2 V−1 s−1 was measured for the nitrate ion in our laboratory with an IMS at 30 °C. The mobility of the nitrate ion at 760 Torr and 298 K is calculated to be 2.3 cm2 V−1 s−1. At 50 V, and thus an electric field of 3.6 V/cm, the ion drift velocity (vd) is 8.2 cm/s. At this drift velocity with a drift length from source to MS inlet of 10 cm, the ion residence time in the reaction region is 1.2 s. At this residence time, the analyte/reactant ion ratio from Figure 5 is 0.053. From eq 1 with t = 1.2 s, a ratio [A−]/[R−]0 = 0.053, and an estimated rate constant of 2 × 10−9 cm3 molecule−1 s−1, the calculated analyte concentration is 2.2 × 107 molecules/cm3. With approximately 2.46 × 1019 molecules/cm3 at 298 K and 1 atm, this value corresponds to a concentration of [RDX] = 0.90 394



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Figure 6. Selected ion monitoring of m/z 284 (RDX peak) with a 200 ms dwell time of various concentrations of RDX in the AFT with a 71 cm reaction region. Each letter represents a different flow ratio of “saturated” RDX vapor to total flow. The total flow was approximately 5 L/min. The flow rates of saturated RDX vapor used for dilution experiments were 0, 25, 50, 100, 200, 500, and 50 mL/min for A−G, respectively. The data presented represent an 11-point moving average.

Table 2. Calculated Concentrations Based on Data from Figure 6 A B C D E F G

gas flow sample/total (mL/min)

[RDX]dilution (ppqv)

[RDX]signal (ppqv)

[RDX]adjusted (ppqv)

ratio [RDX]dilution to [RDX]adjusted

0/5000 25/5025 50/5050 100/5100 200/5000 500/5000 50/5000

0 25 50 98 200 500 50

3.9 4.8 6.2 9.7 20 55 5.9

0 0.9 2.3 5.8 16 51 2.0

− 28 22 17 12 10 25

calculations above were used, indicated a concentration of [RDX] = 3.9 ppqv. This value represents the baseline value that results from either chemical or electrical noise using SIM at m/ z 284, or it could also be related traces of RDX in the system. The subtraction of this baseline value from the calculated value of RDX at 4.8 ppqv provides an adjusted value of 0.9 ppqv. Calculations of the RDX concentration in units of ppqv from the data in Figure 6 is reported in Table 2, which includes by columns: [RDX]dilution calculated from the flow ratio assuming 5 pptv saturated RDX vapor, [RDX]signal calculated based on the analyte/nitrate signal intensities using eq 1, [RDX]adjusted obtained by the subtraction of the background signal of m/z 284 from the [RDX]signal, and the ratio of [RDX]dilution to [RDX]adjusted. The ratio shows that the concentration calculated based on flow dilution ranges from 10 to 28 times higher than that of the adjusted calculations based on the signal intensity. The largest differences between [RDX]signal and [RDX]adjusted occur at the lowest sample flow. One possible explanation is that with the turbulence in mixing the sample and dilution flows, a significant portion of the RDX collides and sticks to the walls. Upon the basis of the mixing arrangement within the tee, it seems likely that more sample loss to the walls could occur at the lower sample flows. This could be investigated by using other flow arrangements.

experiment, the RDX vapor source was removed and the end of the 1/4 in. tee was capped momentarily, each time the flow through the RDX tube was adjusted. Once the desired flow rate was achieved, it was connected back to the 1/4 in. tee for approximately 1 min. The flow-rate ratios for RDX and clean air in milliliters per minute are as follows 0/5000, 25/5000, 50/ 5000, 100/5000, 200/4800, 500/4500, and 50/4950, and are indicated in Figure 6 as A−G, respectively. RDX vapor is detectable at the lowest RDX concentration added at 25 mL/ min into a 5025 mL/min total flow. The average signal intensity was 179 cps, with a standard deviation of 9.4 cps, whereas the background signal with no sample was 144 cps, with a standard deviation of 6.0 cps. Assuming saturation of RDX vapor at about 5 pptv, this dilution would equate to a concentration of about 25 ppqv. The actual concentrations are likely lower, resulting from losses to the walls prior to the vapor entering the reaction region. The calculated concentration based on the signal intensities and reaction time using eq 1 suggests a lower concentration. With an ion residence time (from the flow of 5025 L/min) of about t = 3.9 s, an estimated kc = 2 × 10−9 cm3 molecule−1 s−1, an average RDX adduct signal of 179 cps, and a nitrate signal of 1.94 × 105, the [RDX] is 1.2 × 105 molecules/cm3 or 4.8 ppqv at 298 K and 1 atm. The background signal from the SIM of 284 when no RDX vapor was added provided a value of 144 cps, which, when the same 395



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Vapor Analysis of Explosives Residue. Using the system described in Figure 1B for sampling room air, vapor emissions from RDX residue were investigated. A glass microscope slide was spiked with a solution containing RDX, and the solvent was allowed to evaporate, leaving a residue of approximately 5 μg RDX on the glass slide. With an applied sampling suction of about 8 L/min, the SIM of m/z 284 was monitored. When the glass slide was placed in front of the inlet, a rise in the signal intensity was observable within 2 to 3 s. The response was similar to that shown in Figure 6. When the sample was removed from the inlet, the signal rapidly returned to the baseline after 2 to 3 s, which is equal to the flight time down the tube. This same glass slide has remained on a laboratory bench at ambient conditions for demonstration purposes for a period of about 1 year. After a years’ time, a sample taken from the glass slide still produces similar signals when introduced to the inlet. Data recorded recently exhibited a background level of about 20 cps for a SIM of m/z 284, and when the slide was brought in front of the inlet, a response of about 160 cps was observed. This level is similar in intensity to that of label D in Figure 6.

Article

ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded in part by Laboratory Directed Research and Development funding at the Pacific Northwest National Laboratory. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute.

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REFERENCES

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SUMMARY Provided here is a demonstration of ppqv RDX vapor detection at ambient temperatures without preconcentration within 1 to 2 s. This was accomplished through significant increases in ionization reaction time along with the selective ionization provided by the nitrate reactant ion. The reaction time was controlled both by flow and by low electric fields, both of which exhibited similar supporting results. Increased reaction time yields higher relative responses, however, too long of a time results in the loss of ion signal, likely through diffusional losses. Thus, there are limits and an implied optimal balance for the ionization time. The findings here also provide direct evidence of true vapor and not particle detection. For example, a sample slide provided similar signal responses over a period of one year. Further, if particulates were being detected, one would expect to see spikes in signal instead of a consistent response, as was observed. In addition, the samples were introduced upstream and away from the ionization source, thus eliminating the possibility of surface ionization mechanisms. Calculated concentrations, based on dilution of saturated vapor, correlated reasonably well with concentrations that were calculated from a reaction rate constant, reaction times, and the ratio of the analyte ion to the reactant ion signal intensities. These estimates based on ion signals and estimated k values were typically ∼1 order of magnitude below the values based on dilution ratios and the literature value of saturated vapor pressure of RDX at 25 °C. The lower levels calculated are likely because of losses of vapor to the walls but could also be attributed to nonsaturated levels, errors in estimating kc, or in the reported equilibrium pressure of RDX in the literature. Although these values do not agree exactly, the values calculated here from the ion signals provide support for the equilibrium vapor pressure of 5 pptv reported in the literature. The detection limits of RDX vapor, based on the literature value and dilution ratios, compared to the calculated values, based on the ion signal ratios and an estimated collision rate constant, range from 25 to 1 ppqv without preconcentration. At these very low vapor levels, confirmation by means of other analytical methods is difficult and will require further investigation. 396



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