Developing Fingerprints on Metals to Aid Knife & Gun Crime Investigations

Developing Fingerprints on Metals to Aid Knife & Gun Crime Investigations

Fingerprints are something of a staple in forensic science. For over 100 years we have used the unique details of fingerprints to identify victims and suspects, and draw connections between people and objects to place suspects at crime scenes. Fingermarks are encountered on all kinds of surfaces that can have an effect on how easy it is to visualise the mark and for how long the mark persists. As a result, the market is flooded with products for developing fingerprints, from powders to glues to chemical reagents.

Despite the options available, some surfaces, for instance metals, still prove somewhat tricky when it comes to developing prints. This is due to various factors, such as how the chemical results in the fingermark and developing reagents may react with the surface. This is obviously problematic when trying to obtain fingerprints from knives and firearms, a matter of particular importance right now worldwide. For years researchers have been examining methods of improving the detection of fingerprints on metals, including metal vapour deposition and different chemical reagents, but reliable techniques are still few and far between.

Researchers at the University of Nottingham and University of Derby in the UK are using analytical chemistry to solve this problem. Using a technique called Time-of-Flight Secondary Ion Mass Spectrometry, or ToF-SIMS, researchers have developed a way of producing images of fingerprints of various metal surfaces. ToF-SIMS utilises an ion beam which is passed along the surface of the sample, causing ions (charged chemical components) to be emitted from the sample. These are then analysed by mass spectrometry and the results used to produce a kind of map of the surface.

Researchers deposited fingermarks on various types of commonly-encountered metals, such as stainless steel and aluminium, and studied the effects of time on the ability to visualise the prints. Cyanoacrylate (or superglue) fuming, a traditional technique particularly popular when analysing metal surfaces, proved to be unreliable, with the print’s quality degrading rapidly or disappearing completely in just a matter of days. However using this new mass spectrometry-based approach, fingermarks could be visualised in samples up to 26 days old, a vast improvement on traditional methods.

The high-resolution images produced sufficient detail to not only observe ridge detail in the marks, but even the shape and position of individual sweat pores. Furthermore, and perhaps most importantly in a forensic context, the technique is non-destructive. Current methods of visualising fingerprints tend to involve adding a powder or chemical to the print, inevitably altering and potentially contaminating it. But the use of ToF-SIMS ensures the print remains intact, so further development or analysis techniques can be employed if required.

By enabling the visualisation of fingerprints that previous techniques may have failed to reveal, this method has the potential to not only aid investigators as they face the ongoing rise of knife and gun crime, but could also be applied to cold cases. However it is important to note that fingermarks deposited as part of research are not always indicative of real-world samples. In reality the fingerprints we leave behind can vary greatly in the amount of material deposited and the type of material being left behind. Traces of anything handled can be deposited in the fingermark, adding many potential variables to the real-world applicability of this kind of work. Despite this, the study demonstrates a promising new technique for the development of fingermarks on metals, which could have great implications in the investigation of violent gun and knife crimes.

 

Thandauthapani et al. Exposing latent fingermarks on problematic metal surfaces using time of flight secondary ion mass spectroscopy. Science & Justice. 2018, 58(6).

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Tracking Movements with Fingernails

Tracking Movements with Fingernails

When human remains are discovered, investigators will often turn to routine methods such as fingerprinting, DNA profiling and the use of dental records to identify the individual. But in the absence of database records for comparison, such traditional techniques may not prove all that useful, and forensic scientists must look for new ways to identify the unknown.

In recent years the use of stable isotope analysis has aided forensic investigations, particularly in establishing the geographic origin of unidentified human remains. Isotopes are different forms of an element. For example, oxygen has three naturally occurring stable isotopes: O16, O17 and O18.  These isotopes are incorporated into substances in the environment (such as water) in varying ratios. The relative abundance of isotopes can be influenced by various factors in a process known as isotopic fractionation. It has been found that isotopic ratios can be related to different regions of the world. For example, the tap water in one country may have a completely different isotopic signature in comparison to water in another country. How does this relate to the isotopes found in our bodies? Well, you are what you eat. As you consume food and water from a particular area, the atoms in our bodies express abundances similar to the food and water consumed.

This provides the basis for using isotope analysis to trace materials back to a certain geographic region. It has already been demonstrated that the isotopic analysis of bones, teeth and other bodily tissues can allow for individuals to be traced to particular locations, typically through the analysis of oxygen, hydrogen and sulphur isotopes. However last year, researchers at the University of Utah took a different approach, this time focusing on fingernails.

As with bones and teeth, the isotopic content of our fingernails will be affected by factors such as the food and water we consume. As fingernails are estimated to grow at a rate of 3-4mm per month, they are a prime target for studying isotopic patterns in an individual over a shorter timespan (less than six months as oppose to years). This is by no means the first study of isotope abundances in fingernails, but previous research has typically focussed on single timepoints rather than tracking the same individuals over time. As global travel becomes more commonplace, it is increasingly likely that human remains could have originated from any part of the world. Therefore, we need to understand how travel can cause changes in isotope abundances within the body.

This study aimed to establish whether fingernail isotope ratios were different in a group of local people in comparison to non-locals who had recently moved to the area (in this case Salt Lake City in the United States). Over a period of a year, fingernail clippings were collected at multiple timepoints from a group of volunteers, about half of which were local residents and the rest individuals who had recently arrived from various locations across the US and the world. The fingernail clippings were cleaned (to remove surface components and contaminants that could interfere with the analysis) and subjected to analysis by isotope ratio mass spectrometry (IRMS). IRMS is a particular type of mass spectrometry that allows us to measure the isotopic abundance of certain elements typically hydrogen, carbon, nitrogen, and oxygen. You can read more about IRMS here.

The isotope values of samples from residents were used to construct a baseline of expected values for the area, with isotope values from non-residents’ samples being compared with these. Initially, samples from non-residents showed a wide range of isotopic values, which is to be expected given they had only recently moved to the area. Some residents did fall within the expected range of locals, but these participants had moved from relatively nearby locations, which could explain the similar relative isotopic abundances. However after about 3 months, the fingernail isotopic patterns shifted until the non-residents could no longer be distinguished from the residents. This indicates that although the relative abundance of isotopes in our fingernails can shed some light on geographical movement, it can only provide information relating to the past few months. Inevitably there will always be a certain amount of error associated with such analyses, with variation from the likes of short-term travel and random dietary changes being impossible to account for.

 

Mancuso, C. J, Ehleringer, J. R. Resident and Nonresident Fingernail Isotopes Reveal Diet and Travel Patterns. Journal of Forensic Sciences. 2019, 64(1).

 

Interview with Forensic Taphonomist Professor Shari Forbes

What is your current job role and what does this entail?

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Forensic taphonomist Professor Shari Forbes.

I am a Canada 150 Research Chair in Forensic Thanatology and the Director of the Secure Site for Research in Thanatology (SSRT). The SSRT represents the first human taphonomy facility in Canada and is the only place in this distinct climate where we can study the process of human decomposition through body donation. My role is to lead and conduct research at this facility, specifically in the field of forensic thanatology and decomposition chemistry. This role also involves engaging collaboratively with our external partners who can benefit from the research and training we conduct at the facility, notably police, forensic services, search and rescue teams, military, human rights organisations, and anyone involved in death investigations.

What initially attracted you to your particular field of research?

I have always had a passion for science and knew that I wanted to pursue a career in a scientific field where I could clearly see the impact of my work. When I was in high school, I enjoyed reading crime novels and probably understood what forensic science entailed better than most people (this was before the advent of CSI, Bones, NCIS, etc.!). My love of science combined with my interest in criminal investigations naturally led to pursuing a career in this field. At the time, there was only one university in Australia that offered a forensic science degree so the decision of where and what to study was relatively easy. Although chemistry wasn’t my strongest subject at school, I enjoyed the degree because it applied chemistry to forensic science and in this way, I could understand how my skills would help police investigations.

Can you tell us about the research you’re currently involved in?

My research focuses on the chemical processes of soft tissue decomposition and the by-products released into the environment. This can include compounds released into air, water, soil, textiles, or anything surrounding the body. The majority of my research at the moment focuses on the release of volatile organic compounds (VOCs) into the air to better understand the composition of decomposition odour. Although this is not pleasant work, it is very important to understand the key compounds used by cadaver-detection dogs for locating human remains. If we can identify the key VOCs and determine when they are present, we can enhance the training and success of cadaver-detection dogs in complex environments such as mass disasters.

You were heavily involved in the establishment of the Australian Facility for Taphonomic Experimental Research. What were some of the greatest challenges in this and how has the facility since developed?

It took approximately 3.5 years to establish AFTER from the day we started planning it to the day it opened in January 2016. I have since realised this is not that long compared to some of the other facilities that are currently operating but there were challenges and hurdles that we faced along the way. In Australia, establishing a human taphonomy facility essentially requires three things: 1) an organisation willing to lead and support it; 2) a body donation program; and 3) accessible land that can be used for taphonomic research. We were fortunate that the University of Technology Sydney (UTS) had these three things and we also had the financial and in-kind support of all of our partners including academic institutions, police services and forensic laboratories. Once we had this support and made the decision to proceed, we still needed to seek approval from our local council to use the land for this purpose; apply for funding to build the facility; and apply to have the facility licenced to hold human remains for the purposes of taphonomic research and training. Thankfully, everyone we engaged with was highly supportive of the facility and willing to work with us to ensure we followed all legislation and regulations. We also ensured we had a strong communication plan to raise awareness with the general public about the benefits of these facilities and how important the research is to assist in the resolution of death investigations.

AnnaZhu_UTSScience_1750

The Australian Facility for Taphonomic Experimental Research

Since opening, we have been amazed by the general interest in AFTER and the number of people wanting to donate their body. We have also increased our partnerships to benefit more police and forensic services as well as others services such as the cemetery industry. We are currently planning to provide more training opportunities, particularly relating to disaster victim recovery and identification, and to establish more AFTER facilities across Australia to better represent the diverse climates experienced across the country.

You recently left the University of Technology Sydney to relocate to Canada. How will your role and research be changing as you make this move?

I was honored to be the Director of AFTER and it was a difficult decision to leave Australia. However, I recognise the importance of these facilities and the need to establish them in other countries so when I was asked to open Canada’s first human taphonomy facility, it was an opportunity I could not turn down. My experience in Australia has already assisted greatly in establishing the facility in Quebec and we will certainly be able to open the facility much more rapidly as a result. Like in Australia, we hope it acts as a template for future facilities across Canada since this country also has very diverse climates. In reality, neither my role nor my research will change significantly. The greatest change will be the climate and its impact on the process of decomposition!

Finally, do you have any advice for young scientists eager to pursue a career in your field of work?

It sounds like a cliché, but I always encourage students to pursue a career in a field they are passionate about. If you had told me 20 years ago that I would being leading not one, but two ‘body farms’ I would never have believed it (especially after just reading Patricia Cornwell’s novel that gave these facilities that name!). But I knew I was passionate about studying a science that was deeply applied and had a clear impact on society. I had no idea where it would lead me or even if I would get a job in the field, but without that passion, I would not have been motivated to do any of the things I have done; namely: complete my degree, continue on with a PhD, do research in decomposition chemistry, and ultimately become an academic so that I could continue my passion of conducting forensic taphonomy research. So if you are going to do something for the next fifty years, make sure it is something you love doing!

Find out more on the Secure Site for Research in Thanatology website.

 

This is Part 17 of our series of interviews with forensic professionals. If you’re a forensic scientist (academic or industry) or a crime scene investigator and would like to be part of this series of interviews, get in touch by emailing locardslabblog[at]gmail.com.

Drug Detection at Your Fingertips: Illicit Drugs in Fingerprint Sweat

Drug Detection at Your Fingertips: Illicit Drugs in Fingerprint Sweat

Researchers have developed a new tool for the rapid detection of a number of illicit drugs using only the sweat of an individual’s fingerprint.

Typically, the procedure to test for drugs in human beings necessitates the collection of blood or urine and laboratory-based analysis by gas or liquid chromatography with mass spectrometry. Unfortunately these standard methods are somewhat invasive, require potentially time-consuming laboratory-based analysis, and use complex pieces of analytical instrumentation requiring a trained operator to use. They are inevitably unsuitable for rapid, in-situ screening of potential drug users.

Researchers at the University of East Anglia and Intelligent Fingerprinting Ltd (a spin-out company from the university) have been working on a method of conducting simple, rapid drug analysis using sweat from a person’s finger. The technique has been developed to detect four classes of drugs – cannabis, cocaine, amphetamines and opiates, with cannabis being detected based on the presence of Δ9-tetrahydrocannabinol (THC), cocaine on the presence of benzoylecgonine, and opiates via the detection of morphine.

The finger of an individual is firmly pressed onto the Drug Screening Cartridge. This is then filled with a buffer solution before insertion into the reader for analysis. Capable of detecting drugs down to the picogram level, the system is a fluorescence-based lateral flow competition assay containing four drug-bovine serum albumin conjugate lines on a nitrocellulose test strip.  In short, when a sample is introduced to the test strip, fluorescently-tagged antibodies pass over the conjugate lines. As these antibodies are specific to each drug class of interest, if that drug is present they will bind to the drug. At the end of the test, a fluorescence signal is measured. If none of the four drug classes were present, a maximum fluorescence signal will be obtained. However if any drugs were present to bind with the antibodies, there will be a decrease in the fluorescence signal proportional to the drug concentration. Within about 10 minutes, the device then gives a simple pass/fail response, requiring no specialist knowledge or excessive training to operate and interpret the results.

Furthermore, the technique has also been demonstrated to be effective when applied to the deceased. Researchers worked with a number of UK-based coroners to obtain fingerprint sweat samples from 75 deceased individuals. The most common drug detected was opiates, which is a logical finding considering the number of terminally ill patients who are prescribed morphine during palliative care.

In order to compare the new technique with those typically employed in the detection of drugs in human beings, analysis of blood samples was conducted by LC-MS-MS. The results between the two methods correlated well, with the accuracy between DSC of fingerprints and LC-MS-MS of blood being 88-97%, depending on the drug. This demonstrates the effectiveness of the method and its ability to stand up to existing techniques, though there are inevitably some shortcomings. Authors of the study have stated that there are known accuracy issues with lateral flow measurement devices, thus this new technology should be used as a presumptive screening method prior to confirmation by mass spectrometry. Furthermore, the range of target drugs is clearly currently limited, though future development could no doubt enable other classes of drug to be included.

Full details of the findings can be found in the Journal of Analytical Toxicology.

 

References

Hudson, T. Stuchinskaya, S. Ramma, J. Patel, C. Sievers, S. Goetz, S. Hines, E. Menzies and D. A. Russell, J. Anal. Toxicol., 2018, 6–10.

Forensic Magazine. Fingerprint Drug Screen Test Works on the Living and Deceased. [Available online] https://www.forensicmag.com/news/2018/10/fingerprint-drug-screen-test-works-living-and-deceased

 

Tracking Illicit Drugs with Strontium Isotope Analysis

Tracking Illicit Drugs with Strontium Isotope Analysis

The manufacture and distribution of illicit drugs such as heroin is a primary focus of many major law enforcement organisations worldwide, including the Drug Enforcement Agency (DEA) in the United States and the National Crime Agency (NCA) in the United Kingdom. Unfortunately, as drug shipments pass hands between dealers and cross borders so rapidly, it can be difficult if not impossible to trace a batch of drugs back to an initial manufacturer. As a result of this, the chances of locating and arresting the manufacturers of illicit drugs can be slim.

To a forensic drugs analyst, a whole range of characteristics can be examined and used to classify and compare different batches of the same drug, including physical appearance, packaging, and chemical composition. To an extent, heroin chemical signatures are already beneficial in comparing different batches of the drug in attempts to establish links and possible sources of the narcotics. This may be based on agents or adulterants a product has been cut with, and the relative concentrations of those substances. The manufacturing process itself can vary in terms of chemicals and apparatus used and the skills of the manufacturer, resulting in further characteristic differences in the chemical profile. However these differences may not be distinct enough to be valuable and are certainly not able to pinpoint the country from which a batch originated. Though there is still no reliable method of tracing an illicit drug back to a particular location, ongoing research is aiming to change this.

One method of studying the history and even origin of a sample is to use isotopic composition. Isotopes are different forms of elements that are incorporated into substances in the environment in varying ratios and abundances, influenced by a number of factors that can alter these ratios. These processes can be described as isotopic fractionation. Interestingly, isotopic ratios can be characteristic to different regions of the world, enabling certain materials to be traced back to the geographic region based on the ratios of particular isotopes contained within that material. With this in mind, they have often been used to trace unidentified human remains to a particular location or study the origin of food products. Focusing on isotopes allows for heroin samples to be studied and compared based on regional characteristics as oppose to the variation caused by the production process.

For the first time, researchers at Florida International University have utilised strontium isotope ratio analysis to determine the provenance of illicit heroin samples. 186 unadulterated, undiluted heroin samples of known origin were obtained from a number of geographic regions including Southeast Asia, Southwest Asia, South America, and Mexico. Of a particularly challenging nature is South American heroin and SA-like Mexican heroin, which can be extremely difficult to differentiate based on their chemical compositions alone. Heroin samples were dissolved via a microwave-assisted acid digestion method before being subjected to a technique known as a multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS). This instrument utilises an inductively coupled plasma ion source to ionise target analytes, which are then separated and analysed by the mass spectrometer. The use of MC-ICP-MS allows for the strontium concentration of particular samples to be determined. The strontium isotope ratio (87Sr/86Sr) value of each individual sample was then compared with the overall mean values of ratios from different regions in order to establish the likely origin of that particular heroin sample. Samples from the same geographic region would be expected to exhibit a similar isotope ratio.

icpms

Multi-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) Source: www.thermofisher.com

The results demonstrated the possibility of differentiating between heroin of different geographic origin. South American and Mexican heroin samples were correctly classified 82% and 77% of the time respectively. South East and South West Asian heroin samples were somewhat more difficult to differentiate due to more of an overlap between strontium isotope ratio values. SE Asian samples were correctly classified 63% of the time and SW Asian samples only 56% of the time. It is not clear whether this elemental strontium is endogenous or the result of external contamination, but either way it is sufficiently characteristic to be associated with a particular geographic location.

The strontium isotope composition of heroin can be affected by a number of factors, including the soil in which components are grown and groundwater in the area, which can result in region-specific isotope compositions. The use of strontium isotope ratio analysis has presented promising results in the origin determination of illicit heroin. Although a larger scale study incorporating samples of a more worldwide origin would be ideal, initial results suggest that this technique could allow for an unknown illicit drug sample to be traced back to a country of origin, aiding criminal intelligence agencies in the war against drugs.

 

Debord, J., Pourmand, A., Jantzi, S., Panicker, S. & Almirall, J. Profiling of Heroin and Assignment of Provenance by 87Sr/86Sr Isotope Ratio Analysis. Inorg Chim Acta. In press (2017).

Instant Insect Identification to Aid Forensic Entomology Investigations

Instant Insect Identification to Aid Forensic Entomology Investigations

During the investigation of a suspicious death, entomological (that is, insect-related) evidence may be able to provide vital clues as to when the victim died. Determining time since death, or post-mortem interval, can be one of the most important aspects of such an investigation, so it comes as no surprise that a great deal of research has been directed towards improving these estimations.

Insects can play a huge role in estimating time since death. Various types of species of insect will often visit the scene of a death in a relatively predictive manner, either to feed on the decomposing remains (known as necrophagous insects), to prey on other insects present, or to find a suitable place to lay their eggs. Blow flies, a group which includes common flies such as the bluebottle and the greenbottle, are often of particular interest. Forensic entomologists will typically study the insects, eggs and larvae present at a death scene, utilising the type of bugs found and their stage of development to track back to the likely time at which they arrived, thus when the victim may have died. However in order to accurately do this, entomologists must often collect insect specimens for closer inspection and even to rear to adulthood in order to determine the exact species, which is evidently a time-consuming process requiring a high level of expert knowledge.

For the first time, researchers at the University of Albany have applied a technique called direct analysis in real time with high resolution mass spectrometry, or DART-HRMS for short, to the analysis of blow fly eggs. Published in the latest issue of the journal Analytical Chemistry, the technique has demonstrated the possibility of almost instantly differentiating between different fly species based on the amino acid profiles of the eggs.

DART-MS, developed in 2005 by Dr Chip Cody of JEOL, is an ambient ionisation mass spectrometry technique that allows for samples to be directly analysed without any time-consuming sample preparation steps, and perhaps most importantly without destroying the sample. The sample is simply presented in its native state between the ion source and the inlet of the mass spectrometer, enabling compounds present in the sample to be ionised and drawn into the instrument for analysis and identification.

dartms

Sampling interface of DART-MS. Source: Wikimedia Commons

During this investigation, researchers used pieces of pork liver to attract a number of different blow fly species before transporting them to the laboratory. The flies were reared until they lay new eggs, which would be the focus of the analysis. The study utilised specimens of a number of species, including Calliphora vicinia, Lucilia coeruleiviridis, Lucilia sericata, Phormia regina, along with specimens from the Phoridae and Sarcophagidae families. Even to the eye of an expert, the eggs of these specimens are often indistinguishable. The eggs were simply placed in an ethanol solution and the mixtures directly subjected to DART-HRMS analysis.

The technique focused on the analysis and identification of amino acids in the eggs, essentially enabling researchers to produce a chemical fingerprint unique to eggs of a particular species. Examination of the mass spectra showed that the different species exhibited a unique chemical fingerprint, and by using multivariate analysis it was possible to better visualise the similarities and differences between amino acids detected in the eggs of different species.

Unsurprisingly, many amino acids were common to multiple species. For instance, alanine, isoleucine and proline were detected in four of the species, whereas valine was detected in all but one of the egg samples. However some compounds were unique to particular species, and it is these unique amino acids that will prove to be most beneficial in differentiating between the eggs of different species. For instance, glutamine and tryptophan were only present in the eggs belonging to P. regina. Interestingly, the research also demonstrated the ability to distinguish between families as well as species, with some compounds only detected in the eggs of specific families.

By using this particular technique, almost instantaneous identification could be achieved. Of course this research has included only a very limited number of species, thus a much bigger investigation would be necessary before the technique would really be beneficial to a legal investigation. Not only would further species need to be included, but another potential development would be the production of a chemical profile database against which unknown insect samples could be compared. Developed further, the use of DART-MS could save investigators a lot of time in the identification of insects of forensic interest.

 

References

Cody, R. B., Laramée, J. A. & Durst, H. D. Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient Conditions. Anal. Chem. 77, 2297–2302 (2005).

Giffen, J. E., Rosati, J. Y., Longo, C. M. & Musah, R. A. Species Identification of Necrophagous Insect Eggs Based on Amino Acid Profile Differences Revealed by Direct Analysis in Real Time-High Resolution Mass Spectrometry. Anal. Chem. (2017) In Press

 

Keeping the Skies Safe with Analytical Chemistry

Ever since events such as 9/11, the Lockerbie bombing and the (fortunately) failed shoe bomber, the stringency of airport security has been ever increasing. Anyone who has passed through an airport has no doubt witnessed the occasional swabbing of luggage or electronic items. The staff will take a quick swab of the item, stick it into a mysterious machine and usually send the passenger on their way with little explanation of what has just occurred.

But what exactly are they testing for in this scenario, and just what is the instrument they’re using?

As one might expect, the biggest target of this security step is explosive substances as a counter-terrorism measure, in addition to illicit narcotics in an attempt to crack down on drug trafficking. In an airport setting, the analytical testing technique of choice is ion mobility spectrometry.

Ion mobility spectrometry (IMS) is an analytical technique used to identify chemical compounds based on the differences in the movement of ions under an electric field. The concept for the technique was established in the early twentieth century, however it was not until the 1970s that the instrumentation was actually properly developed. There are currently tens of thousands of IMS devices deployed around the world. Not only are they utilised in airports for drug and explosives screening, but also by the military for the detection of chemical warfare agents and in industrial settings to monitor air quality. The range of applications is potentially vast, but the principles of operation are the same.

As you may have witnessed, a small swab is rubbed over the surface to be tested, typically a piece of luggage or an electronic device such as a laptop, before being inserted into the ion mobility spectrometer. As the sample needs to be introduced in its gaseous form, the swab may be subjected to heating in order to thermally desorb analytes from the swab and allow them to be transported into the instrument for analysis. In order to manipulate the analytes entering the instrument, they must first be converted into ions, their charged form. Ionisation is typically achieved using a radioactive source, such as 63Ni (nickel-63) or 241Am (Americium-241), which first form reactant ion species from the carrier gas (usually air), which then leads to the ionisation of the sample material. These newly-formed ions will then enter a region under an electric field and drift towards a series of electrodes. The ions will pass through the drift region at different speeds depending on the shape and size of the ion clusters and strike the electrodes, the signals being amplified and detected. Depending on the instrument and needs of the analysis, either positive or negative ions will be produced (in some cases both simultaneously).

ims

IMS schematic. Source: Smiths Detection (www.smithsdetection.com)

The IMS utilised in airports will typically hold a database of known explosive and narcotic substances against which to compare samples. There will be a certain threshold, typically based on peak intensity, that must be reached before a positive identification will be indicated, and if there is a “match”, the operator will be alerted to a potential identification.

In comparison to other analytical tools available, ion mobility spectrometers are far from being the best. For instance mass spectrometry, an alternative technique for the analysis and identification of chemical compounds, can offer greater sensitivity, higher resolution, improved accuracy and better identification. So why use IMS? It essentially comes down to cost and ease of use. The simple design and ability to operate at atmospheric pressure means the instruments can be fairly small in size, some even being hand-held and so rendering them completely portable. They have low power consumption, so can simply be powered by a few AA batteries. The ease of use of the IMS means anyone can be trained to use the instrument, thus technical or scientific expertise is not required.

But what is perhaps most important for use in an airport setting with potentially thousands of passengers each hour, is the ability to conduct analyses quickly, and this is something that the IMS can offer. Many commercial ion mobility-based instruments can provide results in a matter of seconds. For instance, the IONSCAN by Barringer (now owned by Smiths Detection) boasts the ability to detect over 40 explosives and narcotics in just 8 seconds.

In a security setting there are three primary types of IMS that may be encountered. The smallest of the devices are handheld and sample by drawing in analytes present in the atmosphere. These may be used to analyse potential hazards relating to unattended baggage, for example. The second type, which is perhaps the most commonly encountered IMS in airports, is a benchtop instrument which requires introduction of the sample via some type of swab. And finally, some airport security units may utilise a larger, human-sized IMS portal. This setup uses airflow to dislodge particles of explosives or drugs from clothing or the passenger’s body and analyse them.

Unsurprisingly, the instruments are not infallible, and false positive or negative results are a possibility. Some ions will have the same drift time so may be indistinguishable from known explosives or drugs, triggering an alarm. In actual fact this response may simply have been caused by a cosmetic or pharmaceutical product that happens to produce a response similar to a known narcotic. On the contrary, dirt, oil and other contaminants may mask the presence of substances of interest, thus causing no alert despite the presence of a drug or explosive.

Furthermore, the IMS is somewhat limited in that it can only identify the presence of a compound contained within its database. So whereas it may be able to detect common explosives such as RDX, TNT and PETN, and frequently encountered narcotics such as cocaine, heroin and cannabis, it would not necessarily alert to the presence of an unknown compound (unless it was very similar in chemical structure to something in the database).

Fortunately research in the field of analytical chemistry is constantly ongoing, aiming to improve instrumentation and analytical techniques to resolve these issues and ultimately produce more reliable and robust security measures.

 

References

G. Ewing et al. A critical review of ion mobility spectrometry for the detection of explosive and explosive related compounds. Talanta. 54 (2001) 515-529.

Homeland Security Science & Technology. IMS-Based Trace Explosives Detectors for First Responders. [online] Available: https://www.dhs.gov/sites/default/files/publications/IMSTraceExploDetect-SUM_0506-508.pdf

Smiths Detection. Ion Mobility Spectrometry (IMS). [online] Available: https://www.smithsdetection.com/index.php?option=com_k2&view=item&layout=item&id=40&Itemid=638