Rapid, reliable field detection of hazardous materials is a top priority in both private and government security markets. Simultaneously, mass spectrometers (MSs) and gas chromatographs (GCs) have undergone democratization through miniaturization, simplification, enhanced user-friendliness, and applicability to specific tasks. Thus GC-MS systems, mostly using electron ionization, are the most common portable MS systems for explosives detection in the field.
“But chromatography adds time and complexity,” says Brian Musselman, PhD, CEO of IonSense (Saugus, MA). IonSense has teamed with Waters, a leader in both LC and LC-MS, to produce ambient ionization-capable mass detectors which, according to Musselman, are “highly reliable, and about as small as you can get with high performance—that is, the ability to obtain the mass of an ion in seconds.”
Rather than relying on GC to separate explosive residue components, IonSense uses its direct analysis in real time (DART) ambient ionization source to generate ions from samples with little or no sample preparation. DART generates ions from almost any type of sample—gas, liquid, or solid—which makes it ideal for screening or in some cases confirming the presence of explosives in either unexploded or detonated form. The ionization method also works for drugs, toxic chemicals, chemical warfare agents, inks and dyes, pesticides, and food adulterants.
IonSense has recently introduced a thermal desorption unit to its DART source, which permits sample introduction through the same types of swabs transportation security personnel already use for screening luggage, plastics, metals, packaging, and paper currency for explosives residues. DART ionizes compounds by combining thermal desorption to vaporize the sample, and Penning ionization to generate an ionizing gas region that surrounds sample molecules. For samples collected through swabbing, analytes flow directly into the DART gas stream, where they ionize, typically by addition of a proton to the intact molecule. Protonated species enter the atmospheric inlet of the MS, where they are analyzed in seconds.
Explosion analysis almost always includes unexploded materials in the residue. “The major difference between detonated and undetonated materials is the quantity of unexploded material,” Musselman says, since “nothing burns completely.”
SIMPLIFYING DETECTION AND ANALYSIS
Where a GC separation might take 15 to 30 minutes, DART ionization takes a few seconds, and provides much more information—the mass peaks defined by the GC trace, plus additional compounds. For explosives, DART shows protonated and deprotonated ions, acceler ants, initiators, and mass variants of each depending on the DART method. IonSense has developed a compan ion data analysis program that utilizes a reverse library search algorithm to determine potential identity for each major chemical in the sample. The program, PIMISA, is unique to IonSense DART ionizers. It works by process ing mass spectra collected by tweaking the inlet voltage of the mass detector to generate fragment peaks that are diagnostic for specific compounds of interest. With PIMISA, the company’s DART-QDa system “simplifies analysis, reduces complexity, and gives confirmatory answers,” according to Musselman.
A strength of DART for these forensic appli cations is that it presents a gentler ionization method that is more like LC-MS ionization, thus greater opportunity to uncover more low-concentration components than conven tional GC-MS. Analysis might provide a de tailed match between residues found at an ex plosion site and items in a suspect’s vehicle or home, or between two sites. Where convention al GC and GC-MS analyses may confirm the identity of explosives found in two locations, DART-MS on-site can provide instant feedback regarding ingredients and impurities, which is particularly useful for evidence gathering.
Airports, a venue where explosives are of tremen dous concern, use ion mobility instrumentation to screen for bombs. Agents typically introduce samples via swabs. A positive reading triggers further investigation, usually a physical search. The compact DART-QDa with the thermal desorber has the potential to provide a rapid means of confirming the presence of dangerous chemicals from samples detected during routine screening with non-MS-based threat detection devices such as NIR, Raman, and IMS.
The DART-QDa instrument was not designed to be a portable system, according to Musselman. Its footprint, the fact that it does not require solvents for sample prep and separation, and its capability to use nitrogen as the ionizing gas make it more suitable to a mobile laboratory environment.
Due to limited sample quantity and potential contributions from the surrounding environment, post-blast analysis is more difficult than the detection of unexploded materials. “Therein lies the main challenge of the forensic analysis of explosives,” says Adam B. Hall, PhD, director of the Mass Spectrometry Facility at Northeast ern University (Boston, MA). Post-blast condi tions add complexity to the analysis as well.
Having knowledge of the scene, collecting materials for comparison, and utilizing standards improve the likelihood of successful detection or identification of unexploded materials.
SCREENING OR CONFIRMATION?
Energetic compounds tend to be of low molecular weight, below 500 amu, which makes them suitable for GC or LC anal ysis and subsequent MS detection through chemical ionization, electrospray ionization, and electron impact. MS methods often include gas or liquid chromatography.
Hall, who has co-authored articles on DART with Brian Musselman, believes that the bomb detection technology currently used in airports is antiquated. “MS-based platforms would be a big step up.” DART is in place at some federal forensics facilities and some state laboratories but is not yet mainstream in municipal crime labs, airport security, or border protection.
“Forensics tends to be a late adopter of new technology in comparison with other analytical areas.”
The benefit of DART is speed. The drawback is that, lacking a chromatographic front end, spectra tend to be more complex for some sample types.
“I’ve read different reports on whether DART is more suitable for screening or confirmatory anal yses,” Hall says. “It often depends on what kind of mass spectrometer you’re utilizing. If you can do fragmentation and/or high-resolution mass determi nations, you’ll have greater confirmatory ability. On the other hand, more traditional GC and LC front ends give you retention time, which can be evaluated against known standards.”
Several companies sell briefcase-sized MS systems, but despite their being at the lower end of price (and capability), cost remains an issue for some security applications. Smaller units tend to be simpler quad rupoles or ion traps.
Despite the less-than-stellar evaluation of IMS by MS proponents, the technology is far from outdated. Tom Chand, sales manager at Real Sensors (San ta Ana, CA), maintains that IMS is alive and well.
He mentions the technology pedigree of detectors (General Electric), and technologic improvements implemented over the years that have generated up to 40,000 deployments of IMS systems worldwide. Real Sensors manufactures gas permeation tubes for IMS instrumentation used in airports. IMS operates through the separation of gaseous ions based on their mobilities in an electrical field.
“Early implementations required two different instruments for detecting narcotics and explosives,” says Chand. The issue was that IMS systems could only employ short-lived permeation tubes contain ing either positively or negatively charged dopant materials. “Permeation tubes were also short-lived and required constant replacement,” adds Chand.
“Plus, the IMS detector used radioactive materials, which made sales difficult in some countries.” New er models by Smiths Detection (Newark, CA) can carry long-lived tubes of both dopant types and use non-radioactive detectors.
DOWN TO ELEMENTALS
Isotope ratio mass spectrometry (IRMS), an un derutilized forensic method, is based on the natural distribution of common stable isotopes in different parts of the world or from different sources. Thus the 13C/12C ratio of olive oil sourced in Turkey will differ from the ratio of olive oil from Tuscany. Similarly, isotope ratios can provide an elemental signature for the origins of bomb-related material.
In a 2014 paper, scientists from IsoForensics (Salt Lake City, UT), which specializes in IRMS, analyzed carbon and nitrogen isotope ratios of PETN (pentaerythritol tetranitrate), an extremely powerful plastic explo sive. They demonstrated that IRMS could uniquely identify PETN in ways HPLC could not. Limits of precision for measuring single samples were 0.3% for carbon and 0.4% for nitrogen.
“This establishes a baseline discrim ination power that is not on the level of modern DNA analysis for individ ualization, but is certainly better than chemical abundance analysis for ex plosives,” says John Howa, a chemist at IsoForensics. Analyzing stable isotope ratios for hydrogen and oxygen could improve our ability to discriminate sources of explosives.
According to Howa, a probabili ty-based source-to-source compari son, similar to how DNA is used for individualization, requires not only the ability to discriminate two sam ples from different sources, but also an evaluation of whether samples did, or did not, originate from the same source. “This requires an understand ing of background variation and the proper selection of control samples. A large effort to collect and analyze samples of known origin is necessary to provide a probabilistic value, such as a likelihood ratio, for evaluation of explosive evidence.”
Moreover, predicting geographic origin through isotope ratios is less straightfor ward for explosives than, say, for foods.
The raw materials and fillers used to produce explosives often originate from many locations. “A better understanding of these networks, as well as a pro cess-based model for relating the isotope ratios of an explosive to their precursors, could further increase the reliability of the technique for predicting the origin of explosives,” Howa adds.
IRMS of individual explosive components, such as the military high ex- plosive RDX (Royal Demolition eXplosive), may provide a link between two detonations occurring at different locations—provided the original chemical signatures are unmodified during or after detonation. This requires purifying the chemical component to ensure that the measured isotope ratio is associated with the explosive and not other materials asso ciated with the bomb(s) or surrounding materials. How isotopic signa tures change during detonation has not been well studied, although some groups have investigated isotope ratios in post-explosion soot.
In fact, carbon and hydrogen isotope ratios of non-explosive bomb com ponents (e.g., plasticizers and binders) may be used to discriminate unex ploded explosives samples when analysis of the actual explosive cannot.
“Isotope ratio analysis of explosives is but a small part of explosives and post-blast forensics,” Howa says. “An interesting use of isotope ratios occurs during reclamation of land contaminated by explosive residues. Related to possible uses of isotope ratios for bomb forensics are isotope ratio surveys of adhesives (tape), plastic, metals, or any material used to make a bomb."
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