Transmission Mode Desorption Electrospray Ionization (TM-DESI

Transmission mode desorption electrospray ionization (TM-DESI) coupled to an ion trap mass spectrometer capable of source collision-induced dissociati...
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Transmission Mode Desorption Electrospray Ionization (TM-DESI) for Simultaneous Analysis of Potential Inorganic and Organic Components of Radiological Dispersion Devices (RDDs) Kenyon M. Evans-Nguyen,* Amanda Quinto, Tiffanie Hargraves, Hilary Brown, Jennifer Speer, and David Glatter Department of Chemistry, Biochemistry, and Physics, The University of Tampa, 401 W. Kennedy Boulevard, Tampa, Florida 33606-1490, United States S Supporting Information *

ABSTRACT: Transmission mode desorption electrospray ionization (TM-DESI) coupled to an ion trap mass spectrometer capable of source collision-induced dissociation (CID) was used to completely analyze radiological dispersion device components. Source CID significantly enhanced the signal for metal ions by reducing adducts while eliminating chemical noise from background molecules through extensive fragmentation. Source CID spectra yielded reasonably accurate isotopic ratios for the metals studied. By switching the source CID on and off between scans, all major constituents in mixtures of simulated radionuclides and explosives were simultaneously observed. These results indicate that TMDESI/ion trap technology could be a powerful on-site tool for nuclear forensics.

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INTRODUCTION To characterize the elemental, isotopic, inorganic, and organic composition of a sample, it would typically be analyzed on several different instruments, each one specific to one type of analysis. To fully characterize a sample with mass spectrometry, it could be analyzed with inductively coupled plasma (ICP)-MS to define its elemental composition parallel to determining organic constituents using LC- or GC-MS. In developing fieldable instrumentation for forensic mass spectrometry, it would be desirable to conduct basic analysis of samples at the elemental and molecular level on one instrument. While this may involve compromise of some figures of merit relative to analysis on dedicated laboratory instruments, much useful information could be rapidly obtained on site from this type of multimode instrument. For example, the first analyses of both the organic and inorganic components of gunshot residue (GSR) was recently reported, using Raman microspectroscopy.1 In combination with chemometric methods, this approach has the potential not only to identify the presence of GSR but also to find its origin. Traditional analysis using elemental composition, determined by ICP-MS or SEM-EDX,2 is not capable of sourcing GSR. In nuclear forensics, simultaneous analysis of isotopic, elemental, and molecular composition of samples would be a powerful tool, especially for analysis of radiological dispersion devices (RDDs), or “dirty bombs”. RDDs use conventional or improvised explosives for the dispersion of radionuclides, with © 2013 American Chemical Society

the primary intention of causing mass panic and economic losses.3,4 Radionuclides used in an RDD are likely to be the most accessible, such as strontium-90, cobalt-60, and cesium137, which are used in nuclear medicine, food irradiation, and sterilization. Terrorist groups have expressed significant interest in deploying RDDs; in 1995, Chechen terrorists buried a cesium-137-based device but informed the news media of its existence rather than deploying it.3 While a traditional approach to determining the source of an RDD would be to do radiochemistry and ICP-MS to determine the isotopic and elemental composition,5 the accuracy in sourcing an RDD could be enhanced if the composition of both the organic/ inorganic explosive residue and the isotopic abundance of the radionuclides were known. We are interested in developing techniques for total analysis of forensic samples, RDDs in particular; such techniques should have the capability to analyze all aspects of a sample and be amenable to field use. Electrospray (ESI) is feasible as an ionization source for simultaneous analysis of multiple aspects of a forensic sample. ESI is capable of ionizing many types of compounds, including small molecules, proteins, polymers, inorganic ions, organometallics, and metals.6−14 The use of electrospray ionization for small molecules and proteins is ubiquitous, but far less so for Received: July 30, 2013 Accepted: November 15, 2013 Published: November 15, 2013 11826

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substitute “Jim Shockey’s Gold”, 7 mg of the powder was placed in 7 mL of water. Most of the powder was insoluble and settled at the bottom of the sample container; 1 μL of the supernatant was spotted onto polypropylene mesh. Simulated ANFO was made as a solid mixture of 95% ammonium nitrate salt and 5% diesel fuel, a common composition for improvised ANFO. In the ANFO−cobalt mixture, 8 mg of 300 mesh metallic cobalt was mixed thoroughly into 12 g of ANFO. Spotted samples of the ANFO mixtures were prepared by covering ∼150 mg of the solid mixture in a 1/1 mixture of acetone and water. The use of this solvent was meant to extract many of the components of the fuel along with the analytes to see if this complex matrix interfered with analysis. In the experiments evaluating swabbing of various surfaces, 50 μL of the solution of interest was placed on the surface and allowed to dry. The wetted swabs were held using tweezers and repeatedly brushed over the area with the dried sample. For the simulated RDD experiment, 5 mg of solid cesium chloride was distributed onto a piece of electrical tape, which was then attached to a perchlorate-based firecracker explosive. The firecracker was detonated on a cinder block in a loosely closed plastic container. After detonation, polypropylene mesh swabs, soaked with methanol, were used to swab various locations in the plastic container. Mass Spectrometry. A Thermo Fisher Scientific LTQ XL linear ion trap mass spectrometer (Thermo Fisher Scientific Inc., Waltham, MA) was used in all experiments. The ion transfer capillary was held at 200 °C, and the capillary voltage was 5 kV. Source collision-induced dissociation (CID) was performed by applying a supplementary DC voltage (100 V) in the region of the mass spectrometer after the skimmer; this feature is built into the LTQ XL and referred to as “source fragmentation” in the software. In experiments where metals and organic compounds were analyzed simultaneously, multiple scan events were programmed into an instrumental method to rapidly cycle between scanning with source CID in positive mode for metal analysis and without source CID in negative mode for explosive analysis. TM-DESI Ionization Conditions. Initially, a TM-DESI source built in-house on the basis of the preliminary designs of the Cooks laboratory16 was used. Briefly, the DESI source consisted of a T junction (Swagelok, Solon, OH) connected to a 1/16 in. steel tubing gas inlet, a fused silica capillary (150 μm outer diameter, 50 μm inner diameter, Polymicro Technologies, Phoenix, AZ) solvent inlet, and stainless steel capillary (1/16 in. outer diameter, 250 μm inner diameter) gas/solvent outlet. The fused silica capillary was fed through the junction and the stainless steel capillary such that it provided the inner solvent stream, with nitrogen flowing through the junction and forming a gas sheath in the stainless steel capillary. Voltage was applied to the solvent stream by connecting the high-voltage output of the LTQ XL instrument to a platinum electrode in a microjunction (Upchurch Scientific, Oak Harbor, WA) connected to the silica capillary used for spray solvent transport. The conductive junction was housed in an interlocked chamber for safety purposes. The DESI spray source was mounted on a Probot (Dionex, Sunnyvale, CA) modified to function as an X-Y-Z translational stage arm for positioning. In later experiments, a modified commercial OmniSpray Ion Source (Prosolia Inc., Indianapolis, IN) was used. The Omni-Spray emitter and associated translational stage were removed from the rest of the source and mounted independently on an optical post. The post was mounted to

metal analysis. While ESI is capable of metal analysis, it is 2−3 orders of magnitude less sensitive than ICP ionization and the highly acidic aqueous solutions used to digest metals are problematic for maintaining a stable, consistent spray.9 ESI is still used for speciation analysis of metal ligand complexes and for studying the gas-phase ion properties of metals, but it is not generally thought of as an elemental analysis technique. A variant of ESI, desorption electrospray ionization (DESI), is a promising technique for forensic applications; it is a rapid, surface-based technique requiring little or no sample preparation.15,16 Additionally, DESI functions well with portable instrumentation.17,18 Transmission mode DESI (TM-DESI) simplifies the sampling geometry and reduces the technique to holding a mesh swab in front of the spray emitter. In TM-DESI, the electrospray emitter is in line with the mass spectrometer inlet. A sample, deposited on a mesh material, is placed between the inlet and mass spectrometer so that desorbed sample passes through the mesh into the mass spectrometer.15,19 Similar DESI methods have been used for speciation studies,20,21 but DESI has not been used for elemental analysis. The results reported here demonstrate that TM-DESI coupled to an ion trap mass spectrometer is amenable to simultaneous analysis of soluble metal salts, inorganic ions, and organic compounds in forensic samples. While our interest is primarily in RDDs, these methods have a broad range of applications. This technique could be particularly useful for rapid field screening to prioritize sample collection efforts and provide preliminary investigative data. Collected samples could then be analyzed with more traditional approaches in the laboratory to produce data suitable for admission in a court of law.

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EXPERIMENTAL METHODS Materials. All metal salts and solvents used as well as the polyurethane foam wipes were obtained from Fisher Scientific (Hampton, NH). The stable isotopes of all metals of interest were used as analogues to the radionuclide (i.e., 133Cs in place of 137Cs), for safety purposes. Metal salt solutions were made in deionized water, while high-purity Optima grade water and methanol (Fisher Scientific) were used in the 1/1 water/ methanol spray solution mixture. The cyclotrimethylenetrinitramine (RDX) explosive standard was obtained from Accustandard (New Haven, CT) as 1 mg/mL solutions in methanol/acetonitrile. Gunpowders and gunpowder substitutes were purchased from Cabela’s Inc. (Sidney, NE). The polypropylene mesh used was obtained from Small Parts Inc./Amazon Supply (Logansport, IN). The mesh sizing was 200−250 μm strands with 300−400 μm openings; this mesh was chosen on the basis of published optimized conditions.22 The mesh was prepared as previously described23 by cutting it into 5 × 10 mm pieces, cleaning it with a water/methanol/ acetone mixture, and allowing it to dry before use. Polyurethane foam was cleaned in the same manner as the polypropylene mesh prior to use. Sample Preparation. For most experiments, samples were prepared from solution by spotting 1 μL of solution onto a sample mesh using a pipet. In experiments where the motion stage was used to introduce mesh substrates into the ion source, 2 μL of solution was spotted to yield a larger spot size. The concentrations of the solutions were adjusted to deposit specific amounts of sample on the mesh substrates. Solid sample compounds were dissolved (or partially dissolved) prior to spotting. For analysis of the black powder 11827

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surface and surface contaminants. The maximum available supplemental voltage (100 V) was used to induce fragmentation, reducing or eliminating many of the ions from organic species present in the background. The elimination of unwanted background ions is the motivation for using source CID rather than performing CID in the ion trap. Organic contaminants that are eliminated in the source do not interfere with the injection and trapping of analyte ions. Source CID significantly improved the signal for all metals analyzed; therefore, it was used in all further analysis of metal salts. Ionization of Metal Salts. Soluble salts of several metals were analyzed using the optimized source CID method. To characterize the mass spectra of the compounds with the source CID method, 400 ng of each was deposited onto the mesh surfaces. Table 1 summarizes the resulting TM-DESI mass

an isolated optical breadboard (Newport Corporation, Franklin, MA) to raise the height of the spray emitter to the inlet of the mass spectrometer. The performances of the source built in house and the modified Omni-Spray source were indistinguishable; however, the Omni-Spray source was more compact and convenient. TM-DESI was performed under optimal conditions described by Chipuk and Brodbelt.22,23 The DESI spray source was directly aligned with the inlet (0° angle) of the mass spectrometer at a distance of 12 mm from the inlet. The mesh was held 2 mm in front of the spray capillary and 10 mm in front of the inlet of the mass spectrometer. In studies of the analytical performance of TM-DESI with the mixture of oxidizers and simulant radionuclides, a motion stage (IonSense, Inc., Saugus, MA) was used to control the geometry and the movement of the mesh and scan multiple spotted samples in a controlled fashion. This motion stage is typically used with the IonSense DART-SVP ion source, but was modified to work with the TM-DESI format by mounting it on raised pedestals anchored to the optical breadboard. Multiple spotted mesh samples were loaded onto the motion stage and scanned through the DESI-sprayer/mass spectrometer inlet in transmission mode. Samples were scanned at 0.3 mm/s.

Table 1. Summary of Metal Salt TM-DESI Spectra compd magnesium chloride aluminum nitrate potassium chloride calcium chloride chromium chloride

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RESULTS AND DISCUSSION Source CID Optimization. The use of source CID significantly enhanced the signal-to-noise ratio for all metal salts tested. Accelerating the ions in a region of the ion optics at a higher pressure facilitates CID with the residual atmospheric gases, resulting in fragmentation and charge neutralization reactions prior to the ions entering the trap.6−10 Example spectra are shown in Figure 1; no signal is apparent for strontium nitrate without using source CID. The cesium

nickel nitrate cobalt chloride copper sulfate strontium nitrate sodium iodide cesium chloride barium nitrate lead nitrate

peaks observed

base peak

Mg+, MgOH+, MgCl+, [MgOH + xH2O]+, [MgOH + yH2O + CH3OH]+, [MgOH + H2O + zCH3OH]+; x = 1−3, y = 1, 2, z = 1, 2 Al+ K+, [2K + Cl]+

Mg35Cl+/ [MgOH + H2O]+ chemical background K+

Ca+, CaOH+, CaCl+, [CaOH + H2O]+

CaOH+

Cr+, CrO+, [CH + CH3OH]+, [CrO + H2O]+, [Cr + CH3OH + H2O]+, [CrO + 2H2O]+, [Cr + CH3OH + H2O]+ Ni+, NiO+, [Ni + H2O]+, [Ni + 2H2O]+ [Ni + CH3OH + H2O]+, [Ni + 2CH3OH]+ Co+, CoO+, CoOH+, [CoOH + H2O]+, [CoOH + 2H2O]+ Cu+, [Cu + CH3OH + H2O]+, [Cu + 2CH3OH]+ Sr+, SrOH+, [Sr + CH3OH]+, [SrOH + H2O]+, [SrNO3 + H2O]+, [SrNO3 + CH3OH]+, [SrNO3 + 2H2O]+, [SrNO3 + H2O + CH3OH]+ − I , [2I + Na]− Cs+

Cr+

Ba2+, BaOH+, [Ba + CH3OH]+, BaNO3+ Pb+, PbOH+, [Pb + NO]+, [PbOH + H2O]+, PbNO3+

Ba2+ Pb+

Ni+ Co+ Cu+ SrOH+ I− Cs+

spectra, and all of the mass spectra are shown in the Supporting Information. Peak assignments were deduced on the basis of published spectra for conventional ESI or source CID-ESI of similar compounds9,10 as well as isotopic ratios, where available. Several previous studies of metal salts using electrospray were designed to study the gas-phase ion chemistry of multiply charged metal ions.6−8,12 In order to maintain multiply charged ions in the gas phase and avoid charge reduction reactions, extensively hydrated adducts are desirable. As associated waters are lost in the source or in CID processes, charge reduction becomes more likely. While adducts may be desirable in fundamental studies of gas-phase ions, adducts are not desirable for analytical purposes; they dilute the signal by distributing the metal ions over several peaks, complicate spectra, and are more likely to result in overlapping interferences. For most of the compounds studied, the M+ or MOH+ ion was the base peak in the spectrum, with minimal contribution from adducts. High levels of background chemical noise were an issue in analysis of several species, particularly low-mass metals. The base peak for magnesium chloride was a convolution of Mg35Cl+ and [MgOH + H2O]+. These peaks are isobaric and unresolved in this

Figure 1. TM-DESI mass spectra of 400 ng of Sr(NO3)2 (upper two spectra) and CsCl (lower two spectra), with and without source CID.

data showed signal split between the Cs+ ion and the methanol/water adducts. Source CID enhanced the signal through fragmentation of adducts, combining the signal for the three separate peaks into the Cs+ ion peak. The signal amplification has been previously observed in studies of gasphase metal ions and has often been referred to as “harsh” electrospray ionization.8,9 An additional benefit to source fragmentation is the reduction of chemical noise from organic compounds. Ambient ionization techniques such as DESI often yield significant chemical noise through interactions with the 11828

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analysis. On the basis of the skewed isotopic ratio between the Mg35Cl+ and Mg37Cl+ peaks in the spectrum, it is apparent that the [MgOH + H2O]+ ion is present in the spectrum. While this is not an issue for simply detecting the presence of magnesium chloride, it complicates any attempt at isotopic analysis. Aluminum nitrate yielded a weak signal, with the largest peak from the aluminum, Al+, smaller than the largest peak in the background. The compounds with masses greater than aluminum yielded strong signals for the base peak corresponding to the metal. Chromium, nickel, cobalt, copper, and strontium have a significant number of adducts present, but at low levels. The only nonmetal atomic ion studied, iodide, yielded an intense signal in negative mode, with source CID. TM-DESI ionization of insoluble solid forms (e.g., metallic lead and cobalt, strontium titanate) of these compounds yielded no signal. Several different DESI methods were attempted to promote ionization, but none were harsh enough to yield gas-phase ions from the more refractory forms of the elements studied. Isotopic Analysis. The potential to use TM-DESI for elemental isotopic analysis was evaluated using naturally occurring isotopic ratios. The ratio of the peak heights of the less abundant isotope(s) to the most abundant isotope for the compounds with more than one isotope is shown in Table 2.

Figure 2. Comparison of actual (a) and theoretical (b) isotopic distributions of copper (left) and chromium (right) in source CID TM-DESI mass spectra.

was the base peak, but there were obvious interfering peaks in the isotope pattern of this ion. However, the isotopic analysis of the PbOH+ ion was accurate. Isotopic data from TM-DESI with an ion trap mass analyzer is not meant to be analogous to stable isotope ratio mass spectrometry (IRMS), where very high isotopic precision and accuracy can be used to determine the potential source of a material. Most mass analyzers, including ion traps, and most ionization methods, including electrospray, are not suitable for determining δC or δN ratios to a high enough degree of accuracy.24 While the excellent isotopic precision of IRMS is desirable, the instrumentation is not fieldable and is not capable of providing information about intact organic compounds. Ion traps and DESI are both amenable to the development of portable instruments.17 The accuracy achieved in these experiments is suitable for radiochemical measurements, where isotope ratios are expected to vary more widely. This method is suitable for determining the extent of enrichment of a 60Co source, for example, which could be used to help find the source or age of the radioactive material. Isotope information of this kind would be critical to determining the origin of an RDD. Simultaneous Determination of Simulant Radionuclides and Common Oxidizers. To evaluate the capabilities of TM-DESI for combined isotopic and explosive analysis, a mixture of common oxidizers used in explosive materials (ammonium nitrate, sodium chlorate, and sodium perchlorate) and stable isotope salts of potential RDD radionuclides (60Co, 90 Sr, and 137Cs) was analyzed. To more carefully control the method of sample introduction so that analytical performance could be evaluated, a motion stage was used to scan the mesh substrates through the region between the DESI sprayer and the inlet of the mass spectrometer. The mass spectrometer was cycled between positive mode with source CID and negative mode without source CID using an automated instrumental method to optimize signals for both the explosives and the metals. As shown in Figure 3, all oxidizers and simulant nuclides were present in the TM-DESI spectrum of the mixture. The spectra for the metals in the presence of the oxidizers are more complex than those for the metals alone. The unassigned peaks present in the spectrum are not present in spectra for the mesh background and are a result of adducts formed in the presence of the oxidizers and/or reactions

Table 2. Isotopic Ratios isotope ratio 44

40

CaOH/ CaOH Cr/52Cr 50 Cr/52Cr 54 Cr/52Cr 60 Ni/58Ni 62 Ni/58Ni 65 Cu/63Cu 86 SrOH/88SrOH 87 SrOH/88SrOH 134 Ba/138Ba 135 Ba/138Ba 136 Ba/138Ba 137 Ba/138Ba 206 PbOH/208PbOH 207 PbOH/208PbOH 53

exptl

actual

0.027 0.022 0.076 0.113 0.049 0.052 0.028 0.028 0.381 0.385 0.141 0.139 0.417 0.445 0.143 0.119 0.092 0.085 0.042 0.034 0.087 0.092 0.106 0.109 0.151 0.156 0.486 0.459 0.438 0.421 Average Error 10%

error, % 26 33 5 2 1 1 6 20 9 25 5 3% 3% 6% 4%

For each compound, except lead, the base peak in the spectrum was used for the isotopic analysis. The actual ratios were calculated from spectra simulated by the Xcalibur software, and errors were determined by comparison of the experimental and actual ratios. Figure 2 shows the actual and theoretical isotopic distributions of copper and chromium as examples. Although the average error for all of the isotope ratios was 10%, most of the error in the individual isotopic ratios was well below that value. The higher error of the outliers ( 44 CaOH/ 40 CaOH, 53 Cr/ 52 Cr, 86 SrOH/ 88 SrOH, and 134 Ba/138Ba) was attributed to isobaric interfering species from the high chemical background associated with ambient ionization techniques. There was obvious isobaric interference in the spectra for lead, magnesium, and potassium. The lowmass background was too intense to get useful isotopic ratios for magnesium and potassium. In the case of lead, the Pb+ ion 11829

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Figure 3. TM-DESI spectrum of a mixture of 1 μg of SrCl2, 1 μg of CoCl2, 200 ng of CsCl, 50 ng of NaClO4, 50 ng of NaClO3, and 200 ng of NH4NO3 on a polypropylene mesh, taken switching from negative mode (top) to positive mode with source CID (bottom).

the mesh is in the sample region, unless the sprayer is specifically focused on the actual sample spot. Because the sample is spotted in the center of the mesh, there is total ion signal corresponding to chemical noise, for both sides of the mesh adjacent to the sample spot, while there is no sample signal for these regions. The fact that the sample signal returns to the baseline after the sprayer has passed the sample spot and that the final blank mesh has no signal for the sample indicates that there was no carryover. When scanning across the samples, the presence of the signal was primarily dependent on the position of the sample relative to the TM-DESI source. In several experiments, the DESI spray focus was maintained directly on the sample spot to test the persistence of the signal over time. The length of time over which the signal decayed varied significantly, typically from 2 to 10 s, dependent on the amount of sample deposited on the mesh. The relative amount of material present on the mesh substrates was studied by integrating the “peaks” formed when the meshes were scanned through the TM-DESI interface at a constant rate. Several different concentrations of the common oxidizer−nuclide mixture were run, with multiple replicates for each concentration. The data could not be used for quantitation on the basis of absolute peak area because of unacceptable uncertainty in the signal between runs; however, the relative peak areas were consistent. For all of the runs, the relative amounts of nitrate, chlorate, and perchlorate spotted on the mesh were constantthey were each diluted equally. The data correspond with the constant ratio of the oxidizers, even at different absolute concentrations. Over all of the different absolute concentrations, the ratio of chlorate to nitrate signal was constant at 0.46 ± 0.03 (7% relative uncertainty) and the ratio of perchlorate to nitrate signal was 0.91 ± 0.15 (16% relative uncertainty). The information obtained from TM-DESI about the relative amounts of oxidizers in the sample is reliable enough to assist in sourcing the sample. Different formulations of explosives will have different ratios of oxidizers, and these ratios can be effectively determined with this method.

between the oxidizers and the strontium and cobalt. However, the peaks assigned to cobalt, strontium (SrOH+), and cesium based on the previously discussed spectra for the metal salts alone were present in all samples of the oxidizer−nuclide mixture tested. Controlled sample introduction through the use of the motion stage yielded reproducible data within each run, as shown in Figure 4. Although only the nitrate and total ion

Figure 4. Representative total ion signal and nitrate ion signal over time as polypropylene mesh substrates spotted with 1 μg of SrCl2, 1 μg of CoCl2, 200 ng of CsCl, 50 ng of NaClO4, 50 ng of NaClO3, and 200 ng of NH4NO3 are scanned through the TM-DESI interface by a motion stage at a constant rate of 0.3 mm/s.

signals are shown, these are representative of all of the oxidizers and the metals in the mixture. There is no signal at all in the regions between the mesh samples because the metal supports of the motion stage block the DESI sprayer completely between samples. The signal for the nitrate ion occurs over a time range narrower than that for the total ion signal. A comparison of these signals demonstrates that there is no nitrate signal while 11830

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Figure 5. TM-DESI spectrum of 50 ng of CsCl and an aqueous extract of a black powder substitute (Jim Shockey’s Gold) on a polypropylene mesh, taken switching from negative mode (top) to positive mode with source CID (bottom).

powder or fuel oil. The nitrate, chlorate, and perchlorate ions in simple solutions are readily detected using this method, as described in the previous section. To test the performance in more complex matrices and more closely mimic real-world conditions, probable explosive formulations were combined with radionuclide analogues and tested. Black powder compressed in containers is the basis for many domestic IEDs.25 To simulate this type of IED used in an RDD, CsCl was combined with a black powder substitute (Jim Shockey’s Gold26) and analyzed with TM-DESI. The black powder studied uses ascorbic acid as a substitute for the traditional carbon/sulfur fuel mixture, with nitrate and perchlorate as the oxidizers.26 The cesium is apparent in the positive mode source CID spectrum (Figure 5). There are several other significant peaks in this spectrum that are not in the background and are not chemical noise from the ionization of ambient species. However, these peaks could not be assigned with high confidence. Given that the black powder is a complex mixture of ionic and organic species, these peaks are likely clusters and/or adducts formed from black powder components. The negative ion spectrum contains all of the major components of the black powder. The ascorbic acid appears intact in the spectrum as [M − H]−, [M + Cl]−, and [M + NO3]−. Ascorbic acid levels vary between manufacturers, and traditional black powders use no ascorbic acid; therefore, ascorbic acid is an important analyte in tracing the source of black powder used in an IED. In addition to TM-DESI analysis of the black powder substitute−cesium mixture, conventional (angled) mode DESI was also used for this sample. Angled-mode DESI yielded a reduced signal for the same amount of material deposited, relative to TM-DESI. However, angled-mode DESI can be used for direct analysis of objects without swabbing or extraction and deposition. More conventional explosives, such as high explosives used in military and construction/demolition applications, could also be used in an RDD. The ability of TM-DESI to simultaneously detect both a conventional high explosive and a radionuclide

The relative signals for the simulated radionuclides were also consistent. The ratio for cobalt to cesium signal was 0.52 ± 0.04 (8% relative uncertainty), and the ratio for cobalt to cesium signal was 0.65 ± 0.05 (8% relative uncertainty). Sourcing information for radionuclides can be obtained on the basis of isotope ratios, for isotopes such as 60Co, which are produced by enrichment of the stable isotope (59Co). A consistent ratio between different elements could also help with sourcing materials, because the ratio between the radionuclide and its decay products could be used as a signature for the origin of the material. In addition to the relative amounts of isotopes and decay products, it is important to have some measure of the absolute amount of radionuclide present. To provide an estimate of the absolute amount of material, cesium was used as a pseudointernal standard. As shown in Figure S-14 (Supporting Information), a calibration curve was constructed by spotting varying concentrations of the oxidizer−radionuclide mixture, without cesium, and then spotting a constant amount of cesium on top of the mixture. The plots have reasonable linearity, but the data should only be considered semiquantitative. These data could help determine the areas covered with the highest amounts of radionuclide, both for safety/disaster response and for sample collection purposes. It is likely that the precision of the absolute radionuclide quantitation would be improved significantly by using isotopic internal standards in a standard addition-type experiment. For example, in an analysis of a 60Co-based RDD, initial data would yield the level of enrichment on the basis of the 60Co/59Co peak ratio. In subsequent analyses, the sample could be spiked with additional 59Co, which would act as a stable isotope internal standard. Explosive Mixtures Spiked with Simulant Radionuclides. To evaluate the capabilities of TM-DESI for combined isotopic and explosive analysis, mixtures meant to simulate possible RDD formulations were analyzed. The explosives most likely to be used in a real-world IED would be ammonium nitrate, potassium chlorate, and/or potassium perchlorate in combination with a fuel source such as aluminum 11831

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The spectra for these mixtures, again, demonstrate the flexibility of DESI coupled to an ion trap capable of source CID for parallel elemental and molecular analysis of soluble species. Direct Analysis of Swabs. An appealing aspect of TMDESI in forensic analysis is the use of the mesh substrate directly as a swab. Swabbing surfaces of interest is a common practice in forensic and homeland security workflows, and these swabs can be directly exposed to the TM-DESI source for analysis. Initial experiments focused on depositing a mixture of dissolved nitrate, chlorate, perchlorate, and cesium salts on various surfaces. These surfaces were then swabbed with polypropylene mesh or polyurethane foam. Polyurethane foam was chosen for testing, in addition to polypropylene mesh, because its use as a swab for transmission-mode direct analysis in real time (DART) analysis has been well-characterized.27,28 Swabs had to be wetted prior to use to be effective. Methanol was preferable to water for wetting the swabsthe water did not uniformly wet the hydrophobic polypropylene and the signal for methanol-wetted polyurethane was more intense than for the water-wetted substrate. While spectra of analytes could be obtained for samples spotted and dried on polyurethane foam mesh, stretching the foam while it was in the TM-DESI interface was necessary to improve the porosity. Polypropylene mesh swabs of the oxidizer−cesium mixture deposited on glass (Supporting Information, Figure S-17) and on the adhesive side of a piece of electrical tape (Supporting Information, Figure S-18) readily yielded spectra with peaks for all components in the sample at high signal-to-noise ratios. However, the signal overall for polyurethane mesh was reduced relative to that for polypropylene mesh (Supporting Information, Figure S-18). The swabbing of glass substrates with oxidizer and cesium residue using the polyurethane was repeated in triplicate. Only one of the swabs yielded any signal for nitrate, chlorate, perchlorate, or cesium, and the signal-tonoise ratio for this swab was well below the signal for polypropylene swabs. Attempts to swab soil spiked with the oxidizer−cesium mixture and obtain a signal were unsuccessful. After initial direct swabbing attempts yielded no signal, simple extractions of spiked dirt were performed by coating the dirt in a thin layer of excess water or a methanol/water mixture. Spotting these extracts onto the polypropylene mesh also did not result in detection of the compounds. Notably, the excess soil was blown off the surface before it was put into the TM-DESI source using a can of compressed air duster. This step is necessary to avoid introducing solid particles into the ion optics or ion trap, which would significantly degrade the performance of the instrument. It is likely that the material of interest was absorbed into dirt particles and then blown away prior to TM-DESI analysis. To approximate postdetonation residue for an RDD using CsCl with a powder-based IED as a dispersant, 5 mg of CsCl was spread on electrical tape and attached to a perchlorate/ aluminum powder firecracker (Figure 7a). A perchlorate oxidizer with aluminum fuel is a common formulation in domestic IEDs, and the firecracker is a good approximation of a larger-scale device. The cesium-laced explosive was placed on a porous concrete surface inside a plastic container and detonated (Figure 7b). Numerous areas on the plastic and concrete surfaces were swabbed with methanol-soaked polypropylene mesh swabs. Analysis indicated the presence of both perchlorate and cesium. Angled-mode DESI was used to directly analyze the concrete surface on which the device was

was illustared using a mixture of RDX and cesium. Both components were apparent in spectra of the mixture (Supporting Information, Figure S-15). Several of the radionuclides that could potentially be used in an RDD are common in water-soluble salt forms, such as the 137 Cs and 89Sr chloride salts used in nuclear medicine. However, 90 Co occurs most frequently in insoluble, metallic form. Strontium-90 can be in several chemical forms, as it is a fission product usually obtained from nuclear waste streams. As previously noted, insoluble solids were not suitable for direct analysis with TM-DESI. However, TM-DESI is very tolerant of complex sample matrices and can be used with solubilized or digested solid samples. Previous work using electrospray for elemental analysis of digested solids was hindered by the need to use harsh acidic solutions as an electrospray solvent.9 Because the sample is deposited on the surface for TM-DESI, this limitation is eliminated. To test the capabilities of TMDESI in characterizing IEDs contaminated with radionuclides in insoluble chemical forms, an ammonium nitrate−fuel oil (ANFO) formulation was spiked with 300 mesh metallic cobalt particles. In the resulting spectrum, the nitrate ion was clearly observed, even in the presence of the potentially interfering diesel fuel from the ANFO (Figure 6). No signal was observed

Figure 6. TM-DESI spectra of an ANFO simulant spiked with metallic cobalt particles and briefly extracted with solvent taken in negative mode (top) and exposed to 3 M nitric acid taken in positive mode with source CID (bottom).

for the cobalt when it was directly analyzed with TM-DESI, as expected. However, when the cobalt-spiked ANFO was briefly covered in 3 M nitric acid, and the acid extract was directly spotted onto a mesh and analyzed, the cobalt signal was clearly observed. Note that the base peak in the spotted nitric acid extract is unassigned. A blank sample run where plain polypropylene mesh was exposed to the 3 M nitric acid under the same conditions had no significant peaks, indicating that the cobalt peak is from the ANFO−Co mixture and that the peak at m/z 65 is the product of ANFO exposure to nitric acid. The nitric acid matrix did not interfere with the analysis, as it would in an ESI experiment, and the cobalt was effectively detected with simple, minimal sample preparation that could be carried out in the field. 11832

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directly deposited on mesh substrates, digested, and then deposited or swabbed from a variety of solid surfaces. This method is of particular interest for forensic samples where both the elemental/isotopic information and molecular information must be obtained. While more traditional elemental analysis mass spectrometers (e.g., IRMS and ICP-MS) cannot be readily incorporated into portable instruments, ion traps with DESI ion sources are promising for fieldable forensic analysis. Expanding the capabilities of ambient ionization is a step toward “universal” mass spectrometers used for forensic and military applications in rapid, on-site determination of a broad spectrum of analytes. Of particular interest, valuable investigative information could be rapidly determined from debris after an explosion on the basis of the information-rich and comprehensive data obtained. Further, field implementation of this method would allow prioritization of samples for definitive analysis with established laboratory methods. Current efforts are focused on simplified ionization of refractory materials by DESI in concert with laser ablation in a multimode source.

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ASSOCIATED CONTENT

S Supporting Information *

TM-DESI mass spectra of salts, using CID (Figures S-1−S-13), calibration curves for cobalt and strontium, using cesium as an internal standard (Figure S-14), spectrum for a mixture of the explosive RDX and cesium (Figure S-15), and spectra of oxidizer−cesium residue swabbed from surfaces (Figures S-16− S-18). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*K.M.E.: e-mail, [email protected]; tel, 813-257-1702. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Defense Threat Reduction Agency, Basic Research Award No. HDTRA1-11-1-0012 from the Defense Threat Reduction Agency Basic Research Program in Nuclear Forensics (at Draper Laboratory) and The University of Tampa David Delo Research Professor Grant. K.M.E. thanks Graham Cooks, Ayanna Jackson, and Abraham Badu-Tawiah for training in the construction and use of DESI sources. The authors thank Robert Cotter for the loan of camera imaging equipment and the Probot. This article is dedicated to the memory of Robert Cotter, an outstanding mentor and role model.

Figure 7. Detonation of a simulated RDD (a) in a closed container (b) followed by swabbing areas in the container postdetonation (c) and analyzing the swab with TM-DESI (d) while switching from positive mode with source CID (e) to negative mode (f).

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detonated, but no signal for either perchlorate or cesium was obtained. The postdetonation debris TM-DESI analysis was simple, rapid, and effective; the area was swabbed, the swab was held in front of the DESI emitter, and the immediate spectra yielded real-time results. Overall, swabbing solid surfaces such as glass, electrical tape, plastic, and even porous concrete was highly effective for detection of RDD-relevant species.

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CONCLUSIONS These data indicate that TM-DESI coupled to an ion trap with source CID capabilities can simultaneously analyze metal salts, metals, inorganic ions, and organic compounds, with reasonable isotopic accuracy. Analysis was effective when samples were 11833

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