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Ion Mobility Spectrometry and Its Applications in Detection of Chemical Warfare Agents

The threat of weapons of mass destruction (WMDs), such as chemical warfare agents (CWAs) and toxic industrial chemicals, is of great concern worldwide. This menace makes it necessary to detect the presence of such weapons in both military and civil environs. Toxic chemicals are used in large quantities by industry, and the information needed to synthesize them has become more accessible through the Internet. In addition to providing protection from chemical attacks, fast detection is needed to detect possible chemical leaks which may occur during accidents, disposal, or dumping. Many of these chemicals are hazardous to human health; others may be inflammable or pose environmental risks.


Toxic chemicals have great potential to inflict significant casualties, thus chemical weapons are classified as WMDs. Furthermore, CWAs are easy to disguise and are practically imperceptible, thus making them easy to use against the public. CWAs are mostly dispersed as vapors or aerosols; the more volatile the agent is, the faster it evaporates and disperses. In addition, CWAs or their degradation products may linger as soil contaminants.


Chemical warfare agents (CWA) are a constant concern for defense, first responders, and emergency medical personnel. Speedy discovery of chemical agents related to chemical warfare attacks or hazardous material incidents could prevent harmful exposure as well as assist in the successful treatment of exposure to chemical hazards.


CWAs are divided into five different categories.

i) Vesicating and blistering agents such as sulfur and nitrogen mustards cause extensive and irreversible tissue damage. Lewisite is the most infamous of the organoarsenic warfare agents.

ii) Choking agents or pulmonary intoxicants are, for example, phosgene, which does not detoxify naturally, has a cumulative effect, and may persist in sheltered areas or buildings for a long time and chlorine, which at moderate concentrations is a weak pulmonary irritant but in high concentrations is extremely lethal. Both are also indispensable industrial chemicals.

iii) Nerve agents are organophosphonates, which are further divided into three subcategories: G agents (G denotes German origin), such as Tabun (GA), Sarin (GB), and Soman (GD); V agents (mainly VX; some variants exist); and Novichok agents. Tabun was initially used as a pesticide but has been utilized for military purposes. Sarin is an extremely volatile colorless liquid that has a mild aroma of rotting fruit if impure. Soman is more poisonous than the two aforementioned and has a camphor-like odor if impure. VX is an odorless liquid with an appearance similar to motor oil. Novichoks are considered to be the most hazardous agents ever made.

iv) Blood-born agents are cyanogens such as HCN and CNCl. They are distributed by the vascular system.

v) Incapacitating agents are non-lethal CWAs. CS (tear gas) is widely used for various purposes. In addition, some nations are producing, stockpiling, and transporting large quantities of toxic industrial chemicals.


CWA detection schemes

There are many ways that CWA can be identified. For instance, Ion Mobility Spectrometry is used in mobile detectors and relies on ionized air samples and their reactive charge to a detector plate. Electrochemical sensors measures change in the electrical potential when a CWA enters the detection region.


Military forces in the world use simple colorimetric methods with the indicator tubes or papers, which have been known for a longtime. Studies on new ways of detecting CWA have been con-ducted in many research centers. Other interesting CWA monitoring types include flame photometry, thermoelectric conductivity, photoacoustic IR Spectroscopy, gas chromatography, and Raman spectroscopy.  New chemical sensors, which would be useful for such applications, are being sought. New research developments in CWA monitoring  seek to improve upon size, cost, and performance concerns within the field.


There are many CWA detection techniques. For example, common laboratory methods and instruments such as GC, LC, CE, and MS, individually or in combination, may be used. On-site screening techniques include surface acoustic wave (SAW), electrochemical, and spectrophotometric sensors. All these techniques have their pros and cons; for example, SAW sensors can be small and portable but are sensitive to moisture and may suffer from de-wetting effects that reduce responsiveness.


Spectrophotometric techniques are based on color change reactions (detection papers or detection tubes) or emission lines (flame photometric detection [FPD]). Color change experiments are easy to perform but need a relatively high amount of sample, can be time consuming, and give ambiguous results. Flame photometricdetection (FPD) can be indicated as the most important competitive detection technique. This method is used for analyzing compounds containing phosphorus and sulfur, i.e., amongst other compounds, the most important CWAs. FPD is fast and sensitive but produces false positives. Typical drawbacks to all these techniques are false positives and adsorption of the agents onto instrument surfaces. However, IMS also suffers from these disadvantages.

Graphene is an incredible manmade compound whose versatility is becoming more and more apparent. With a thickness of only one atom, graphene is 200 times stronger than steel, yet one square meter of graphene weighs only 0.77 milligrams. Its thermoelectrical properties have been leveraged as a radiation detection application, and it also has a great many defense applications, including bulletproof vests  and chemical warfare detection.


Nanoelectromechanical (NEMS) mass resonators are systems which can be used to identify gas molecules in the air and to determine the mass of cells, biomolecules and gas molecules. When further weight is applied onto a surface, the NEMS sensor can report on changes to the resonance frequency of the NEMS resonator. The utilization of graphene sheets within a NEMS sensor could reduce the overall weight of the sensor. Researchers at the Engineering Department at the University of Rijeka in Croatia have investigated the employment of graphene sheets to form the NEMS resonator, specifically to detect the existence of chemical warfare agents


Catching Chemicals

Suppose you have a liquid sample which you suspect contains minute traces of CWA. How do you determine what is contained with any amount of confidence? Dr. D. K. Dubey and associates from the Defense Research and Development Establishment in India seeks to solve this problem. “Our research is an advancement in the state-of-the-art analytical techniques needed to verify the agents used in the field,” says Dr. Dubey, “to save humankind from the menace of chemical warfare agents.”


By using iron oxide nanoparticles which are covered with poly methacyrlic acid-co-ehtylene glycol dimethacrylate, researchers take advantage of the resulting “sticky” substance which encourages attachment of the CWA particles. As Dr. Dubey states: “Extracting hydrophobic chemicals, like these agents, from a hydrophobic background, like organic liquids, is a tough challenge to achieve analytically. But efficient and sensitive analytical methods are pivotal in the early detection and identification of toxic agents, so we wanted to take on this challenge. Our new method allows the efficient identification of chemical warfare agents in organic liquids; we hope it will be helpful for the international community involved in verifying and preventing the use of these devastating chemicals.”


Ion Mobility Spectrometry

The possibility of using more complex devices, e.g. miniature mass spectrometers for CWA detection is also being investigated. When fast detection of chemical warfare agents in the field is required, the ion mobility spectrometer may be the only suitable option. IMS involves both ionization of the sample and analysis of the ions formed at ambient temperature and at ambient or reduced pressure.


Detection of highly toxic substances for military purposes was the area that needed very sensitive and relatively selective instruments allowing on-site analysis. Detectors using the IMS technique met appropriate metrological requirements and, in addition, they were small, light and not very complex. It was possible to build portable analyzers as well as on-board devices designed for installation in military vehicles.


Ion mobility spectrometry (IMS) is an analytical technique based on studying the ions movement in gases. This short and simple definition refers to a large group of methods characterized by various principles of detection and different constructions of instruments. Like flame ionization detectors (FID) and electron capture de-tectors (ECD), ion mobility spectrometers can be included to the group of ionization detectors.


The analysis process in IMS consists of two important stages. The first one is the creation of ions containing analyte molecules or their fragments. Analyte ionization is characterized by selectivity resulting from different effectiveness of ionization of various substances. Ionization occurs in the ionic reactor,which can be treated as the receptor part of an IMSD. The next stage is the separation of ions, which occurs in the transducer part of the detector, which is called the drift region or ion separator. The separation of ions is a process that distinguishes IMS from simple ionization methods, such as FID and ECD.


In IMS, gas phase ions are created by ionizing neutral molecules using photon, corona, flame, ESI, or radioactive ionization. Most of the instruments use radioactive ionization sources like 63Ni or 241Am because they are simple, convenient, and stable. The ions produced are separated by their different velocities through a drift gas in an electric field. IMS separation is based on ion mobility, with the relationship between ion velocity and electric field depending on the ion’s weight, charge, and shape.  As opposed to simple ionization detectors, IMSDs are selective, i.e. they make possible to distinguishthe sample components.


Analyte ionization methods in ionic reactors of various IMSD types are similar, while the principle of operation and the design of the transducer parts vary considerably. Ions produced in the reactor region are transferred through the detector towards the collector where the output signalis ultimately generated. This ion movement takes place in a stream of flowing gas (advection) and in an electric field (drift). In various types of detectors the advection or drift mechanism of transport predominates.


Having good performance in wide detection range, fast response, high sensitivity and database renewability, Ion Mobility Spectrometer has been widely used for quality checkout, toxic gas monitoring, chemical warfare agents’ detection, drug and explosive goods inspection. It
has become an important tool for airport and train station staff, security personnel and narcotics squad.


Drawbacks of Ion Mobility

IMS as an analytical technique has several specific weaknesses. The primary drawbacks are its low resolving power, limited selectivity, and the experimental nature of the technique. But for some intrinsic technology reasons, IMS instrument has always been perplexed in false alarm or missing alert, troubled in humidity interference. So it is difficult to distinguish ions which have very similar mobility times with a low resolution IMS tool.


Matrix effects such as humidity, temperature, and the composition of the sample may influence the detector’s response. Ideally, IMS should be used in environments with controlled temperature, low humidity, and controlled amounts of dopants, thus requiring delicate engineering and parameter optimization for in-field use. The separation of ions is highly dependent on ion mobilities in a drift gas under the influence of an electric field, which may be affected by altering the polarizability and mass of the drift gas, thus changing the chemistry of ion−neutral molecule interactions. Selectivity could be improved by adjusting the electric field strength and drift tube pressure or temperature; other ions may also be examined. Nonetheless, high collision rates at atmospheric pressures could be advantageous in IMS. Based on the thermodynamic equilibrium between the ions and the neutral molecules, IMS is able to selectively ionize whole classes of compounds with collectively unique thermodynamic properties such as high proton affinity.


Even though IMS has the above-mentioned drawbacks, numerous advantages tip the balance in its favor. The main advantages of IMS are instrumental simplicity, small size, light weight, portability, reliability, ease of operation, real time monitoring capability, fast response, short analysis time, low power consumption, low operating cost, and high sensitivity, specifically for persistent CWAs.


In addition, IMS encounters many problems such as no response, system halting, blurred screen and so on in extreme environment. The phenomenon has been troubling users for a long time. As to application in Chemical Warfare Agents (CWAs) detection in war circumstance, the environment would be more severe.  Therefore, it is the time to develop an improved ion mobility spectrometry to meet the needs and it is of great significance.


Illuminating Ionization

Motivated by the need for small and highly sensitive CWA detectors to warn military personnel of a potential threat, a researcher from the Combat Capabilities Development Command Chemical Biological Center at Aberdeen Proving Ground teamed up with a researcher from STC at the U.S. Army CCDC Chemical Biological Center to investigate machine learning based spectral interpretation. Their approach uses data from an ion-mobility spectrometer (IMS) and a photoionization detector with electrochemical sensors.


They observed an enhanced alarm performance in chemical detectors by utilizing an innovative data fusion technique combined with a random forest classification method. Experiments were performed using two M4A1 Lightweight Chemical Detectors and one Multirae Pro PID detector, all of which were exposed to both bis(2-chloroethyl)sulfide 99.7% (HD) and Methyl Salicylate, 99+% (MeS). The Receiver Operating Charcteristic (ROC) curve is shown in Figure  giving the true positive rate and false positive rate for HD, MeS and None, after the machine learning algorithm was applied. Results indicated a 98% true positive rate for HD and no false positives for the prediction of None and MeS


ROC Curve showing predicted True Positive Rate vs False Positive Rate for labeling alarm None, HD or MeS


FLIR will team with Purdue University to develop ion mobility and two-dimensional mass spectrometry (2D MS/MS) for a modular chemical-detection system.

Officials of the Defense Threat Reduction Agency’s Joint Science and Technology Office (DTRA JSTO) in Arlington, Va., have announced in March 2021,  a contract potentially worth $8 million to FLIR Systems for chemical-detection technologies based on ion mobility spectrometry (IMS) and mass spectrometry (MS) technology. FLIR will team with Purdue University in West Lafayette, Ind. to develop ion mobility design and two-dimensional mass spectrometry (2D MS/MS) into a modular chemical-detection system.


It will be lightweight enough for one person to carry that can screen downrange areas for the presence of harmful chemicals. The system will function as a sensor payload for unmanned vehicles, and as an embedded real-time monitor for chemical releases. The program’s goal is to provide warfighters with small, fast, interconnected chemical detection and identification tools for a wide range of scenarios. Experts from FLIR and Purdue will work together to develop chemical-detection sensors that collect large amounts of data from one sample and remove the need for pre-recorded data libraries of known threats.


Advanced algorithms will enable analysts to identify and classify never-before-seen threats with modular front-ends and options for communication protocols and power inputs for a broad range of detection missions. The three-year effort will result in a mature integrated IMS/MS prototype ready to move into military programs of record, experts say. FLIR and Purdue will do the work in West Lafayette, Ind.


There are many constraints in CWA detection which make it a challenging field, including high sensitivity, high specificity, low false alarm rate and rapid detection. Innovators in a variety of fields including physics, chemistry, and materials science continue to develop strategies to combat these obstacles. CWA detection, like many problems in engineering, requires innovation and mathematical rigor.


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