An explosion leaves quite some traces behind. Not only the obvious traces like damage on surroundings but also traces one cannot see with the naked eye. These small traces are particles from the explosive which fly around after detonation. One does not need to be directly in contact with explosives to become contaminated. People on a certain distance from the detonation come in contact with explosive residues as well, this could be either directly or via vapour.
Not only military explosives, also explosives from industrial sources and improvised explosive devices (IEDs) are used in terrorist attacks. [1] Identification of the used explosive is important because this could lead to the bomb maker, but is also crucial to prevent attacks in the future.
Making a bomb isn't hard, recipes are readily available on the internet, so everyone can do it. Luckily isn't every bomb the same, there are various bomb types and different ingredients could be used. The majority of explosive materials contain a fraction of one of the following unstable compounds: TNT (2,4,6- trinitrotoluene), RDX (cyclotrimethylenetrinitramine), PETN (pentaerythritol tetranitrate), NG (nitroglycerin) or C-4 (composition-4) a plastic explosive.
Since the last ten years more and more interest and effort is put into studying and developing detection techniques for explosive residues. This rising interest could be ascribed to the numerous terrorist attacks in short time. Well known examples are the "nine-eleven" disaster from 2001 in New York, Bali nightclub bombing in 2002, 2004 the train bombing in Madrid and the subway and bus bombings in London in the year 2005. [2]
In the ideal world there is one cheap detection method (machine) with a high success rate and a low false alarm rate, which detects all kind of traces. This is obviously not the case in reality. There are various traces and detection surfaces which require each a different detection technique. Distinction could be made in traces found on and in human beings. Detection of traces on humans meaning: on their clothes, on hair and on their hands, this results in explosive particles in fingerprints. Explosive traces In humans are expected in blood and urine. Different detection techniques for these undergrounds are the main concern in this literature paper.
Additionally detection techniques for explosives in general, as well as sampling techniques for certain traces found on surfaces will be described.
Sampling methods Explosives
Not every trace could be sampled the same way, and not every trace is in the same state when detected.
Explosive traces can be found as:
vapours
dissolved in or forming small fluid droplets (aerosols)
attached to small inert particles in air (microparticles)
crystal fragments or crystals of the explosive called microfragments
clusters of these microfragments or microparticles [3]
Sampling of a trace is a challenge itself. Knowledge about properties of explosives and adsorption effects are necessary to choose the optimal sampling and transport method.
Depended on the detector used later, sampling is done with different methods.
Samples could be acquired by using different extraction methods such as supercritical fluid extraction (SFE), solid-liquid extraction (SLE) and solid phase extraction (SPE).david moore Swipes are used to sample surfaces. All extraction techniques for the different occurrence of samples will be briefly discussed below.
Liquids
SBSE, stir-bar sorptive extraction, a variant on solid phase extraction (SPE) was studied by Lokhnauth and Snow. SBSE has already been used in a broad range of samples but Lokhnauth and Snow were the first who applied this technique for the analysis of trace explosives.
They wanted to create a method to determine explosive particles in aqueous samples using the extraction and pre-concentration possibilities of SBSE in combination with the speed, sensitivity and portability of ion mobility spectrometry, IMS. Difference between SBSE and normal SPE is the extraction device, this device exists out of a magnetic stir bar covered with a thin glass sheet. The outer layer of the device is made of polydimethylsiloxane, PDMS, this part is responsible for partition between the analyte and the detector. A lower water volume/coating volume ratio gives an increase in recovery of analytes and therefore an improvement of sensitivity. [4]
Vapours
Vapour trapping devices or pre-concentrators are used to capture and concentrate explosive vapours before introducing them into the detection system to increase sensitivity of the system. [5]
One general disadvantage of pre-concentrators is that it will increase the overall detection time.
Various different vapour traps are used: volume traps, surface traps and solide surfaces.
Every vapour trap has its own advantages and disadvantages. Volume traps are efficient but difficult to desorb quickly. Membrane filters are surface traps, they are also very efficient but need a large sampling pump due to the required pore size. This results in a high pressure drop at flow rates. Explosive particles are released by increasing the membranes temperature. It's difficult to quickly heat a membrane, this is also the problem with volume traps. Pre-concentrators which do not have this time consuming heating problem are based on fullerenes. Due to their low thermal mass fullerenes don't have this problem. Fullerenes are fully built out of carbon molecules and can easily be used as a coating on clean metal surfaces. This layer than acts as adsorber and collector for organic vapours. Disadvantage of fullerenes is the low efficiency of 40% for ethylene glycol dinitrate, EGDN. Another coating that could be used is the Tenax GC polymer. This polymer has an collection efficiency of 100% for EGDN. Nevertheless thermal degradation and loss of trapping efficiency occurs after repeated heating cycles under ambient air conditions are major drawbacks for Tenax GC. reference ref Forensic&environmental detection of explosives jehuda yinon..
Various research groups are working on the improvement of vapour concentrating methods. Cooks et al. avoid analyte loss and time waste by double sided MIMS. With this technique the same side of absorbing membrane substance is also exposed to the air to collect explosive particles and then immediately presented to a mass spectrometer.
Surfaces
Explosive particles can be present on the surface of a packed explosive device, when packing isn't done carefully or surroundings have contaminated the package.
Methods used for sampling a surface are: taking a swipe of the surface, a reagent that is sprayed on the surface, material on surface is volatilized or the material is directly extracted from the surface by soaking the surface with a solvent. [6]
In general trace collection devices couldbe passive or active systems. A passive collection apparatus stops and holds vapours, particles and aerosols. Most passive collectors have filter/selective adsorber combinations.
When a swipe is taken with a cotton glove, the glove needs to be vacuumed to collect particles from this glove into a collector or pre-concentrator. The sample is deposited into the analyzer section of the detector by heating the metal or fiber filter. [7] The temperature must be high enough to vapourize the captured explosives, but not too high to reduce possibility of fragmentation of the explosives. artikel for.inves. of explosives (kopieen)
Active collectors need external energy input to be effective. Active accumulation is done by airborne explosive samples. A capture device is used which yields explosives from large volumes of air, and deposit these explosive particles in a small liquid or dry aggregate.
Desorption electrospray ionization (DESI) and desorption atmospheric pressure chemical ionization (DAPCI) are both sensitive and selective methods that are used prior to mass spectrometry analysis of surface materials. Both methods will be discussed more in detail later.
After a sample is collected it needs to be stored until being analyzed. The sample needs to be transported to the laboratory where analysis is done, that's why a sample requires to be packed correctly. Packing is also necessary to counteract contamination from other collected samples. When package isn't done carefully the evidential value is lost and the sample is useless. Once the samples arrives in the laboratory, separation from the sample matrix and other materials that possibly interfere with the sample is crucial. [8] Therefore a separation method prior to identification is useful. Chromatographic methods are one of the most important tools for separation. Kolla examined the promise of different chromatographic techniques to the discovery of explosive traces. He concluded that the combination of chromatographic techniques with selective detection techniques is the most important facet in trace determination. [9]
Most common analysis/detection techniques used for explosive trace detection
Various techniques are available to detect trace amounts of explosives. Every instrument has different capabilities, features and prizes. A comparison between different trace detection techniques was made by Paul Jankowski, see appendix I.
Mass spectrometry coupled to chromatography is often used together due to their quantitative and qualitative aspects.
Widely used detection techniques and combinations will be discussed in this section.
Ion mobility spectrometry, IMS
The ion mobility is the size, mass, charge and shape of an ion. With IMS the sample is inserted in the vapour phase and than being ionized. The ions travel into a so-called drift tube, were at the end the velocity is measured. On the basis of the measured velocities other ion mobility parameters are determined.
Different ionization sources are granted for IMS use, for instance a radioactive source like Nickel, 63Ni, electrospray ionization, corona discharge, atmospheric pressure photonisation or an alkali-bead emissive source. [10] Mostly the same ionization techniques are used in Mass spectrometry.
Fetterolf and Clark used IMS to detect trace explosives in 1993 and concluded it is a sensitive and specific detection apparatus with detection as low as 200 pg for common explosives. [11]
A critical review about the use of IMS to detect explosives and explosive related compounds was done by Ewing and colleagues. [12]
As Fetterolf mentioned already IMS with their low detection limits and without the need for sample preparation seemed a promising technique. Nevertheless complications arose with vapour concentration and irregular responses. These problems are nowadays solved and IMS is used as an in-field analyzer. The purpose of the review was to sum up the reactions responsible for detection and explain research done with APCI reactions in IMS.
When using corona discharge, any negative substance will immediately be quenched and discharged. Khayamian et al. succeeded to develop a method were negative corona discharge is able to analyze TNT, PETN, and RDX. [13]
In 2004, a new method was created, the desorption electrospray ionization method, DESI.
A pneumatically-assisted electrospray is sprayed onto a possible contaminated surface.
Charged microdroplets from this spray interact with the analyte and secondary ions are produced. The main difference and advantage of DESI over other ionization sources is that when coupled to MS is sensitive, specific, has short analysis times (around 5 seconds), does not need sample preparation and is able to detect explosives in situ on many surfaces (paper, plastic, glass, leather and skin). Detection limits are up to the sub-nanogram for most explosives and sub-picogram for TNT. [14] , [15]
Mass spectrometry, MS
Mass spectrometry is the method of choice when looking to selectivity and sensitivity characteristics. The advantage of some MS methods is that sample preparation isn't necessary. This cuts back in the analysis time, which is also preferred choosing a detection method.david moore artikel
Mass spectrometry is an analytical instrument to determine what the mass of a molecule, atom or fragments of molecules is. [16]
A mass spectrometer mainly consists of an ionization source, an analyzer and a detector.
In general the first step in MS is to ionize the sample, ions are accelerated by an electric or magnetic field and separated conform their mass-to-charge ratio, m/z.
In this first step a sample can go directly into the ionization source or prior to this it can undergo some type of chromatography.
Numerous ionization techniques are available and the technique to choose depends on the analyte. When the analysis is in the gaseous state, gas-phase methods like Electron Impact, EI, and chemical ionization, CI are possible. Desorption methods, such as matrix-assisted laser desorption ionization, MALDI, and Fast Atom Bombardment, FAB are available. Spray methods used to ionize molecules are Electrospray Ionization, ESI and Atmospheric Pressure Chemical Ionization, APCI.
The ionization method used depends on the type and state of a sample. Most common methods used for biological samples are MALDI and ESI, samples are then ionized from their liquid or solid state. [17]
Essentially there are two different methods for separation and analysis: methods based on time separation like the time-of-flight mass spectrometer, and methods based on geometric separation for instance the magnetic mass analyzer, quadrupole mass analyzer and the ion trap.
Not only the ionization source can vary widely, also the mass analyzer options are divers. Most common analyzers are the double focusing Magnetic Sector, quadrupole mass filter, quadrupole ion trap, linear time-of-flight analyzer, reflectron time-of-flight-analyzer and the Fourier Transform Ion Cyclotron Resonance mass analyzer, the FT-ICR-MS.
The detector response (usually intensity or relative abundance) versus the m/z ratio is given in a mass spectrum.
When using laser based ionization methods these are often combined with mass spectrometry. These techniques have excellent properties for chemical analysis to overcome the longer detection time which explosives with low vapour pressure require due to pre-concentration. MS methods are selective and sensitive enough that pre-concentration isn't necessary any more.
Time-of-flight mass spectrometer, TOF-MS [18]
Separation is based on the kinetic energy of ions and their difference in mass to charge ratio, m/z. Separation is done on the basis that high mass ions travel slower than ions with lower masses. Ions travel over a distance due to an applied voltage on the backplate. This voltage discharge the ions out of the ionization source.
In a perfect environment all ions would have the same kinetic energy, ½mv2. M in this formula is equal to the mass of the ion and v to it's velocity. When ions have similar kinetic energy, lighter iones travel faster to the detector than heavier ones.
Although ions have the same mass a difference in kinetic energy is possible due to a voltage difference. This is achieved because ions are not always ionized at the same distance of the backplate. When ionized closer to the backplate ions will have a higher voltage difference and in such a way more kinetic energy than when ionized further away.
To overcome this problem a reflectron is built in. A reflectron is a series of hollow rings held at increasingly positive potential, terminated by a grid whose potential is more positive than the accelerating potential on the backplate of the source. Ions entering this reflectron will be slowed down, stopped and reflected back to the backplate. The degree of slowing down depends on the speed the ion had when entering the reflectron.
Due to this reflectron all ions with the same mass will reach the grid (in front of the detector) in the same time, irrespective of their initial kinetic energy.
Positive aspects from the TOF-MS are the accuracy of about 0.001 and high acquisition rate of 100spectra per second or higher. It also has a high resolving power of 1000-25000.
One downside of this method is the operating pressure of 10-9 bar, this is lower than for instance the transmission quadrupole and magnetic sector instruments which have an operating pressure of 10-12 bar.
TOF-MS is often used in combination with laser ionization of explosives or explosive related compounds.
REMPI, resonance enhanced multiphoton ionization, uses TOF-MS to detect ions, Marchall and colleagues were the first who used this technique. A drawback was that REMPI wasn't able to differentiate between TNT and background nitro compounds due to arising fragmentations of the compounds coupled with NO+. This problem was overcome by the group of Ledingham, they used a multiphoton ionization (MPI). MPI ionizes ultrafast, and before fragmentation is done the parent component is ionized. This group also used this technique at 800 nm in combination with nanosecond laser desorption of solid samples at 266nm. [19]
Studies represented that this ultrafast MPI TOF-MS is also possible without laser desorption, achieving the same results.
Mullen et al. used TOF-MS in combination with Single Photon Laser Ionization, SPI, to detect explosives and explosive related compounds. SPI TOF-MS was already used to make a distinction between alkanes, alkenes and aromatic compounds in complex matrixes. The outcome was promising for Mullen et al. to investigate whether SPI TOF-MS could also be used to determine the limits of detection (LOD) of nitrobenzene, NB, and 2,2-dinitrotoluene (2,4-DNT) in the gaseous phase.refereren mullen et al(14)
Audrey Martin and co-workers used two reflectron TOF mass analyzers combined to single-particle aerosol mass spectrometry, also called SPAM, to identify high explosives. The two reflectron TOF mass analyzers are responsible for the generation of both positive and negative ions. Micrometer sized particles of TNT, PETN, RDX, composition, Semtex 1A and Semtex 1H were identified by their parent peak or adduct peak in case of RDX. Identification is possible from a particle of about 1 pg. This method is sensitive, specific, reliable and reagent-free.
Nowadays IMS is used at airports as a detection system. Advantages to use SPAM over IMS is the fact that operating conditions such as laser fluence don't need to be changed and it's not restricted to threshold settings. IMS is not capable to screen a large group of explosive compounds due to the necessity to optimize parameters. The principles of IMS was explained earlier.
SPAM is also capable to detect biological and chemical agents, that's why it is a multifunctional point detector to observe a variety of threats at a scene. [20]
Quadrupole Ion Trap mass spectrometry
The quadrupole ion trap is used as a chromatography detector. Molecules which exit the chromatography column enter the cavity of the analyzer through a heated transfer line and undergo ionization. This cavity is formed by two end caps and a ring electrode. Ionization could also take place by injecting a reagent gas into the cavity. The ring electrode has a constant-frequency radio-frequency voltage, this is necessary that ions stably travel around the cavity. By changing the amplitude of the radio-frequency voltage ions will be send out the stable trajectory and leave the cavity by one of the end caps and become captured and detected in the electron multiplier.
Quadrupole ion trap has a resolving power of 1000-4000, it's mass-to-charge accuracy m/z is 0.1, with a maximum of about 6000.
This is an advantage over other mass analyzers were only a small part of the ions arrive at the detector.
During the 7th international symposium on analysis and detection of explosives in Edinburgh Scotland a lot of methods were presented which are based on the use of ion trap MS. Jehuda Yinon and coworkers used LC/MS to characterize and identification of the origin of explosives. The origin is important to know because not every manufacturer will make the same explosive material with the similar byproducts, additives, organic impurities and has the same purification of the natural product. LC with atmospheric pressure chemical ionization, APCI, in the negative mode, coupled to ion trap MS could characterized TNT samples and byproducts. [21] More details about APCI are discussed under the heading APCI.
On this same symposium Yasuaki Takada and coworkers represented their findings about a new detection system based on a new ACPI source, the APCI, Ion-trap MS with Counter flow introduction, CFI.
The APCI with Counter flow introduction, meaning the gaseous sample which goes into the ion source travels in the opposite direction of the ion flow produced by the ion source. This new APCI source improves ionization efficiency. [22]
Chromatography coupled to mass spectrometry [23]
When chromatography is coupled to MS both quantitative and qualitative information is provided. MS need high vacuum conditions to have nothing to do with collision between ions during separation. Chromatography however needs high pressure during separation. When combining both techniques the problem arises that a large excess of matter needs to be removed between both apparatus. With gas chromatography, GC, this problem is solved by the invention of narrow capillary columns. When choosing liquid chromatography, LC, this isn't simply solved due to the enormous gas formation when liquid vapourizes. This gas must be discharged before ion separation in MS takes place. Usage of nonvolatile mobile phases needs to be avoided when coupling LC to MS.
Extra resources are necessary to introduce a liquid out of the chromatograph into the MS, for instance pneumatically assisted electrospray, also known as ion spray, and atmospheric pressure chemical ionization, APCI.
Electrospray, ion spray [24]
In electrospray an electric field takes care of the formation of aerosols and charged particles out of liquids. Liquids that come out of a chromatography column go directly into a nebulizer capillary coaxial along with a nitrogen gas flow, N2. Inside the nebulizer the voltage can be switched in such a way that ions can flow from 0 V to -3500 V (for positive ions) or vice versa (for negative ions).
The N2 flow and electric field are responsible for the formation of fine aerosols or charged particles. Gaseous ions arise because these were already available in the mobile phase on the column. Charged particles which come about often are protonated bases, and ionized acids. Other charged particles arise from combinations between the analyte and stable ions present in solution.
Positive ions from aerosols are pulled by the glass capillary which enters a negative potential of about -4500V inside the MS, the same happens for negative ions but the other way around. A vacuum pump is coupled to the apparatus to cut back the pressure to ï¾3 mbar.
General advantages of this technique are that it's a easy to operate sensitive technique and is able to detect high mass compounds. The parent ion is fully detected and detection ranges lay in the low femtomole to zeptomole range, with use of nanospray. Another advantage is that this technique could be connected to HPLC, to increase selectivity.
Disadvantages of this technique are that no fragmentation takes place, and a polar sample is always required which is soluble in polar solvent. The sensitivity to salts provides numerous peaks in the spectra which could hinder the analysis. [25]
Atmospheric pressure chemical ionization, APCI [26]
In stead of an electric field APCI uses heat together with a coaxial N2 flow to create fine aerosol mist. Ions are created from gas-phase reactions between ions and molecules. When high voltage is given to a metal needle in the path of the aerosol an electric corona appears around this needle. Electrons are now brought into the aerosol; both positive and negative ions are formed.
Co-workers of the Bureau of Alcohol, Tobacco and Firearms in Washington were researching the possibilities of atmospheric pressure ionization (API) methods and modern GC/MS methods.
The positive points of this technique are definitely the insensitivity to salts, and it's ability to handle with high flow rates. Detection of the parent ion, the possibility to use normal phase solvents and connection to HPLC are also plus points. The drawbacks of this technique are that volatile and thermal samples are needed. herhalen referentie advantages
High liquid Pressure Chromatography, HPLC
HPLC is a type of chromatography where solvent is pumped at high pressure through the column, with the stationary phase. The analyte is separated on the basis of the speed the different molecules go through the column. The smaller the particles in the stationary phase the denser the column is, and the slower analyte flows through the column by the large resistance of the stationary phase. Different types of columns could be used to separate the sample. [27]
Chemiluminescence 4, [28]
Chemiluminescence is the process where energy is released in the form of light due to a chemical reaction. In other words Chemiluminescence (CL) is the generation of electromagnetic radiation in the form of light due to the production of light. This happens during the transition of molecules from an excited state to their ground state energy level.
CL can take place in the gas, liquid and solid state. An example of Chemiluminescence is during pyrolysis of nitro or nitrate groups at high temperatures into NO or NOx. Explosives containing these groups will undergo the reaction given in figure 1 and light is emitted.
Advantages of CL are the high sensitivity, wide detection range and simple and inexpensive technique. The fact that it does not require a radioactive source is also a huge advantage because this reduces time and paperwork during transport of the system. Disadvantages of this technique are the limitation to nitrogen-containing explosives and the selectivity. Nevertheless as mentioned earlier most explosives contain nitrogen. The selectivity problem could be fixed when coupling CL to a separation technique.
An important method for detecting trace levels of explosives is the thermal energy analyzer, TEA, a gas phase CL detector. Since 1989, GC-TEA has been adopted by the Forensic Explosives Laboratory in Kent (England) as its principle technique for explosive trace analysis.
There is a general procedure for collection of organic explosive residues from surfaces using cotton swabs or vacuum into a filter. Pre-treatment such as solid-phase extraction (SPE) and supercritical fluid extraction (SFE) are often required.
Identification of an explosive trace is based on comparing retention times of explosives in a standard solution. The right GC conditions need to be explored to analyze all explosives.
The standard procedure for GC-TEA is sampling with cotton wool swabs or vacuum onto a filter with two different sampling solvents; a mixture of ethanol and water in equal volumes and methyl-tert-butyl ether. A standard TEA solution (generally containing 11 high common explosives) at low concentration is analyzed before and after the sample. Three different types of columns for GC are used, and in the end conformation is done by GC with MS when possible.
Sensitivity of the GC-TEA depends on the analyzed explosive but could be as high as a few pictograms. Normally the detection rate of TEA in combination with HPLC or GC is at low nanogram range, but this could be improved by using silica capillary column GC.
This change in column could also improve the selectivity due to the possibility that vacuum extracts from clothing could be analyzed without sample clean-up. Better peak shape could be obtained by changing the standard amplifier and noise filtration system.
HPLC could also be coupled to TEA in stead of GC, although it is not often used. In HPLC it's not the difference in vapour pressure but the difference in polarity, size and shape which determine the selectivity for the column and the difference in retention time. The minimum detectable amount for explosives was 4-5 pg that was injected onto the column.
An other chromatographic technique that could be coupled to TEA is supercritical fluid chromatography, SFC. This technique is not applied for detection of explosives but there was an investigation which described this SFC-TEA coupling and the application to detect trace amount of explosives. Thermally unstable or non-volatile compounds could be detected with this technique, these could not be detected by HPLC or GC.
Detection of explosive traces on humans
Hair
Hair has the capacity to absorb chemical compounds from its environment, but also from someone's blood. Due to this potential hair analysis is competent as a forensic technique.
In cases of drug abuse hair analysis is readily being used. The protein structure of hair makes it possible to trace which components were in the blood at the time new hair is made in the follicle. Drugs and drug metabolites could be identified in the new made hair; this could be evidence of a persons drug use. Human hair grows monthly on average one centimeter. With this in mind the three centimeters closest to the head correspond to the previous three months. A pattern of a person's drug use could be shown by cutting hair into fragments. Analyzing these fragments could establish whether a person is a constant drug user and if the drug use reduces or increases over the last months. [29]
Since 1980 investigators of the RARDE, Royal Armament Research and Development Establishment, proposed hair could be used to indicate exposure to explosives. [30] , [31] Now a second field in the forensic world is interested in using hair analysis during their research, but in a different way. Hair is now used as a carrier for explosives. Hair could become in contact with explosive particles directly or indirect via interaction with explosive vapour or transfer from contaminated hands.
Most research about detection of explosives in hair is done by Jimmie C. Oxley, James L. Smith and coworkers. 2,3,4 They studied the capability to detect common military explosives in hair, with other words the sorption of explosives to hair in the vapour phase. [32] , [33]
Hair strands were weighed, put into an aluminum foil weighing boat and placed into the jars with solid explosives on the bottom. After the exposure time was completed, acetonitrile was added to the hair, the samples were sonicated for 20 minutes, and shake overnight for extraction.
With use of GC-ECD or HPLC (in case of EGDN) a quantification of five different explosives ( TNT, RDX, PETN, EGDN, TATP) sorbed to hair was made. During these experimental conditions vapour pressure seemed an important criterion for sorption of explosives to hair. A straight relationship between the degree of sorption and the available vapour of the particular explosive is the result. Even after washing or being in an explosive free environment detectable amounts of explosives were found. Persistency of explosives to hair is of course related to vapour pressure and water solubility, the explosives with the lowest vapour pressure are lost first. When washing hair, explosives with the highest water solubility will eliminate first. Another factor affecting sorption is hair colour; red and black hair absorb explosive vapour the best.
GC-ECD is a quantitative and extremely sensitive method, but very time and labor consuming. Sample processing of hair into solvent takes about three hours, chromatography itself takes about 30 minutes to complete. reference accumulation of explosivesinhair In the forensic field time is a critical point, the sooner quantification is done, the better. A follow-up study by Oxley and coworkers included more explosives, nitroglycerin (NG), diacetone diperoxide (DADP) and 2,4-dinitrotoluene (DNT) and examined more factors affecting sorption. Limitations on sorption are time dependent, there could be made a difference in initial rapid or slow long-term sorption. Hair colour was already examined in their earlier study but now variation within one hair colour and age are tested. In contrast to earlier findings, sorption of explosives is not only related to hair colour, variation within one group varies widely, concluding that the uptake of explosives is an individualistic element. No difference in uptake is seen between different explosives, if one explosive is sorbed well to hair others will do this with the same strength. Age, race and gender are no contributing factors in degree of sorption. reference accumu part2
After these laboratory tests Oxley and coworkers started a new research to determine whether their laboratory results could be translated into the field by pre- and post- blast hair sampling of individuals involved in explosive disposal.ref hair as forensic evidence of explosive handling
PETN, TNT and RDX are tested and three contamination modes are tested, namely condensation of explosive vapour, deposition of airborne explosive particles and cross-contamination of explosive particles to hair via hands or clothing. Explosive particles from hair were collected via brushing with a comb fitted with cheesecloth and made wet with methanol. Analysis is done by GC-ECD as in the earlier lab tests. It could be concluded that explosives can successfully be detected on hair of people exposed to these high energy particles. In the morning after the experiments explosives are still detected, a finding that proves explosives are persistent overnight. Between contamination modes no difference was seen.
A rapid detection apparatus is the ion mobility spectrometer, IMS. With detection times less than 10 seconds it could be an improved instrument to work with in the forensic field.3
IMS is a known apparatus since 1970, but under the name plasma chromatography, and grow to an analytical tool to detect drugs, environmental pollution and explosives. Hardly any applications of IMS analysis involve human hair, but some are known in drug analyses. [34] Jimmie Oxley and coworkers from the Chemistry department of the University of Rhode Island in Kingston tested four explosives (TNT, NG, EGDN and TATP) on contaminated hair.2 Three different sampling techniques were tested: direct hair input into the vapour desorption unit, swabs from hair and acetonitrile extracts of hair placed into the desorber. These diverse techniques are chosen because of the fact that IMS instruments are known to have limited sample introduction. Nevertheless all common military explosives (TNT, NG and EGDN) were promptly detected with all three sampling techniques by IMS in the E-mode, explosives mode. TATP was harder to detect in the E-mode and required higher amounts of explosives on hair, but was detected at low amounts during the N-mode for narcotics.
Hair analysis is proven to be non-invasive technique and therefore an important tool in forensics. When comparing both techniques for detection of explosives on hair, IMS seems a more promising technique than GC-ECD because of the shorter analysis time and detection limits in the nanogram to pictogram range.
Fingerprints
The most important purpose of fingerprints is identification of the person who left them. This is realized by comparing friction skin ridges from a suspects fingerprint and a print found on the crime scene. Therefore the ridge pattern needs to be intact after the print is detected. Identification is not the only goal a fingerprint can have. Illicit substances can also be traced back in the deposit of a fingerprint, for example particles of explosives and illicit drugs. [35] So not only detection but also analysis techniques need to be nondestructive. In our case identifying explosives in fingerprints needs to be nondestructive.
Explosive residue particles weren't the first extrinsic material that was analyzed in finger deposits. A few non-invasive techniques to analyze finger deposits and foreign materials herein are already known. [36]
Williams et al. analyzed fingerprint residues with help of infrared micro spectroscopy. Raman spectroscopy was used to detect drugs of abuse in latent and cyanoacrylate-fumed fingerprints by Day et al.
Extrinsic trace residual elements in fingerprints were analyzed by Grant et al. to identify individuals handling with key materials. Ricci et al. used infrared spectroscopic imaging to collect chemical images of latent finger marks. Plastic explosive particles were analyzed with Raman microscopy by Cheng et al. They gained Raman spectra and Raman band images of these plastic explosives. Most nondestructive analysis techniques used till now are Raman spectroscopy and infrared microscopy.
Contrary Mou and collegues combined the destructive Fourier transform infrared spectroscopy (FTIR) with the Attenuated Total Reflection (ATR) microscope, to create a new non-destructive technique.8
FTIR was earlier used successfully to detect explosives, but had the disadvantage to be destructive and needs sample preparation before detection is possible. Samples cannot be detected in the solid state, prior to detection they need to be vapourized.
With this new technique the ATR microscope is first used to locate particles in the fingerprint. FTIR is than used to measure the spectra of these particles.
When using FTIR in combination with ATR samples could be used immediately without additional preparation. Another benefit of this technique is the possibility to observe a specific position in the sample, due to switching between visual and measurement mode.
Clothing
A well-known saying in forensics is Locard's principle: "Every contact leaves a trace." Consequently explosive particles can be found on peoples clothing. Due to the research by Crowson and co-workers it could be concluded that explosive particles in background levels of public places is scarce. That's why it is very unlikely that a random person becomes contaminated with a significant amount of explosives in public areas. An explosive particle on someone's clothing is therefore a strong evidence this person was in contact with an explosive device. [37] ,refereren naar yinon counterterrorist
Ali et al. showed that confocal Raman microscopy is a direct analysis method with numerous advantages over other known direct analysis methods. Various studies have yet used Raman spectroscopy to detect and identify explosive particles, however Ali et al. were the first who utilized this technique for in-situ identification of explosive particles on clothing. [38] PETN, TNT and ammonium nitrate were identified on a range of different textiles, both natural and synthetic fibres. Dyed and fluorescent textiles were also covered in the testing series but Raman spectra could be obtained without any problems.
Prior to use Raman spectra, identification with optical microscopy of the supposed explosive is necessary. Raman spectra were collected using a near-infrared diode laser of 785 nm and a 50x objective lens. Spectra could be made from explosive particles up to 5 nm and roughly 180 pg, no sample preparation is necessary. This technique is only applicable when suspicion of presence of explosive particles on the examined clothing piece is already there. Nevertheless Raman microscopy is a reliable fast, molecular specific and when applied with confocal microscopy a non-destructive technique. nogmaals ali in situ refereren
A follow-up study by Ali and co-workers proved that Raman spectroscopy is also applicable to some precursors (hexamethylenetetraamine HMTA and pentaerythritol) of the previous investigated explosives.
Other trace detection systems that can deal with detection of explosive particles on clothing are systems used at airports and governmental buildings. These trace detection methods are usually ion mobility spectrometry (IMS) based techniques.
Detection of traces in humans
Blood and urine
Explosive particles could be detected in water and soil. Several studies showed that these compounds are toxic at low concentrations. (ref extraction determination TALANTA [39] ) Methods to measure energetic compounds and their biotransformation products are necessary for a better understanding of the exact effect these compounds have to the environment and organisms. HPLC and GC are used to determine energetic compounds in water, soil and plant material. (ref extraction TALANTA)
Tissues and fluids contain proteins and lipids, these complex and large compounds hinder separation and make analyses of energetic compounds hard. (ref extraction TALANTA)
Zang et al. tested a fast and sensitive method to determine certain explosive compounds and biotransformation products in blood with help of gas chromatography coupled to electron capture detection (GC/ECD).
Blood sample treatment and extraction for GC/ECD takes about 3-4 hours. This method is applicable for TNX, DNX, TNT, MNX and RDX. GC/ECD is not possible for HMX, due to its low volatility elution, analysis isn't possible in an acceptable time without making use of thermal degradation. Precision and accuracy results are a little bit higher when concentration of explosive compounds increase. High recovery, precision and accuracy are possible within a concentration range of 1- 1250 ng/ml. Zang et al. also showed that injection port temperature is very essential for this method.
One earlier study in detecting RDX in human plasma is from Ã-zhan and co-workers. They used High Pressure liquid chromatography with a reversed-phase C18 column and UV-DAD using Tox-clean RC cartridge for solid phase extraction. [40]
Advantages of GC/ECD used by Zang over Ã-zhan's method are the lower detection limits and better chromatographic resolution. In case of RDX, HPLC is used more often due to the difficulty to quantify with GC.
Jehuda Yinon enumerates in his book "Toxicity & Metabolism of Explosives" research groups who worked to the analysis and detection of certain explosives in blood and plasma.
A lot of research is done for analysis of NG and metabolites with use of GC or GC coupled to ECD. GC is also coupled to MS to detect NG and metabolite particles in blood and plasma. Other techniques than GC are also used to detect trace amounts NG and metabolites in blood, for instance HPLC coupled to TEA or other methods.
In appendix X an table with an overview of these results are shown for NG and its metabolites.
NG was also analyzed together with PETN with use of a colorimeter method by Yagoda. This method wasn't limited to blood but could also be used for detection of both explosives and their metabolites in urine.
TLC radio-chromatography assay was used to determine 14C radioactive PETN in blood, 77.8% of these radioactive labeled compounds could be recovered. The same method was used by King and Fung. These researchers tried the same but improved the technique by creating an easier extraction procedure and obtained a recovery percentage higher than 90%.
Researchers Yu and Goff already did research to NG and metabolites, with use of HPLC-TEA as can be seen in appendix XX, they also used this method successfully for PETN and metabolites with detection limits of 0.1 ng.
HPLC-TEA was also used by Fine et al. who analyzed RDX in blood with detection limit of 100 pg/ml plasma.
HPLC is also used in combination with UV detection around 240 nm to detect RDX in biological samples.
Gas chromatography was used by Schneider and colleagues to extract RDX from different bodily substances like blood, feces, urine and tissues.
The same technique was used by Litchfield to detect and differentiate between EGDN and EGMN in blood.
TNT was just like NG extensively tested. A lot of research groups used chemical reactions to do colour tests, for instance the Webster test often used for urine samples. Later some drawbacks of this test were not noticed like false positives and negatives in very severe cases, also the colours were unstable which led to false conclusions.
The exact order of steps per method used by different research groups are extensively described in the aforementioned book by Jehuda Yinon: Toxicity & Metabolism of Explosives.
Conclusion
There are various detection methods for trace explosives. Depending on the state of the sample a detection method can be chosen. Some methods require pretreatment of a sample, before introduced into the analyzer.
Knowledge about explosive materials is required for choosing the right extraction technique to obtain the sample from a certain surface, and later for picking the right analysis method and optimal usage.
Techniques used often are Ion mobility spectrometry, Mass Spectrometry coupled to chromatographic methods and chemiluminescence.
