Particle analysis method

A satisfactory test for the identification of FDR on a suspected firer has been sought by forensic scientists for many years. A satisfactory test would be one that is simple, reliable, fast, inexpensive, and conclusive. Until recently efforts have concentrated mainly on the detection of inorganic components of FDR and encompass qualitative and quantitative methods, culminating in the particle analysis method, which is the most informative method currently available. A brief outline of the most important developments follows. 

A more definite method was sought resulting in the particle analysis method, which is claimed to conclusively identify FDR particles.139144... 

These three effects are utilized in the particle analysis method as the surface of the sample is traversed (scanned) by the electron beam. The secondary electrons are used to view the sample, the backscattered electrons are used to identify likely FDR particles, and the X-rays are used to provide details of the elemental composition of the particles. 

Despite all the considerable improvements, the particle analysis method remains a lengthy and costly procedure. These disadvantages have renewed interest in the possibility of detecting the organic components of FDR, either as a primary method or as a screening technique. 

The development of the particle analysis method for FDR detection and identification involved consideration of how the particles are formed and of their physical and chemical nature.170,171... 

From a single firing of three types of. 38 special caliber ammunition the results shown in Table 17.1, for promptly collected residue, were obtained using the particle analysis method.170... 

At the start of 1978 the particle analysis method183 replaced the flameless atomic absorption bulk elemental method184 as the firearm residue detection method in the NIFSL. Since then the particle analysis method has been substantially improved by the use of a sample concentration/cleanup procedure,185 the addition of a backscattered electron detector, and the development of an automated residue detection system.186 187 Despite these improvements the technique remains costly and labor intensive. Certain aspects of the system required further work, in particular, the particle classification scheme discharge particles from mercury fulminate-primed ammunition and discharge particles from new primer types (Sintox). 

The detection and identification of the organic constituents in FDR has the potential to be used either as a screening technique or, much more likely, as a complementary technique to the particle analysis method. The particle analysis method has proved very satisfactory and has been well tried and tested in casework and court. The objective is to devise an efficient system for organic firearm residue detection that is entirely compatible with the particle analysis method. As a suspect may need to be examined for both firearm and explosive residue the method must also be compatible with organic explosive residue detection techniques. 

It would be desirable to examine environmental and occupational particles containing one or more of the elements strontium, titanium, and zinc and to compare them with discharge particles, in order to establish whether or not it is possible to differentiate the FDR particles. If not, the particle analysis method may still be a useful tool to provide supporting evidence, particularly if organic propellant constituents are also detected on the suspect. 

The analytical methods currently used by this laboratory are chromatography (GC/TEA HPLC/PDME) for explosive residues and the particle analysis method (SEM/EDX) for FDR. The latter method involves the detection and identification of individual FDR particles therefore any sampling technique must be nondestructive. 

The search for a simple, reliable, fast, inexpensive, and conclusive test for FDR has yet to be realized. There is no doubt that the particles analysis method is the most desirable and informative method to date, but it is more suited to a moderate FDR caseload. In Northern Ireland we had a heavy FDR caseload and multisuspect cases were common. In one incident we had 38 suspects, with three or four upper outer garments each, to be examined. For a heavy FDR caseload the fast, inexpensive FAAS bulk elemental system is attractive, particularly considering the downgrading of the uniqueness of the particles detected coupled with the lengthy and costly procedures of the particle analysis method. The properties of FDR plus the lengthy and costly procedures of the particle analysis method, the limitations of a positive result... 

It is interesting to note that the automobile features often in the interpretation of FDR results. In the Price method231 the hands of suspects were examined for the presence of lead in particulate form. This method was discredited when it was discovered that automobile exhausts eject numerous lead particles into the environment from leaded petrol. When bulk elemental analysis methods were developed it was shown that lead could originate from leaded petrol and the car battery terminals. Antimony was also present in battery terminals and barium was present in motor oil. Finally, in the particle analysis method it has been claimed that lead, antimony, and barium unique FDR particles can originate from automobile brake linings.)... 

Kasdan HL, Chapoulaud E, Dougherty WM, Halby S, Tindel JR. Comparison of automated urine particle analysis methods a new flow imaging system versus flow cytometry. Clin Chem 2002 48 A5. 

Most particle-analysis methods (SEM, SIMS and FT-TIMS) require the removal of particles from the swipe substrate and deposition on a flat surface. In the case of FT-TIMS, the particles are removed by ultrasonic treatment in a suitable suspension medium such as ethanol or siloxane. The suspension is then mixed with collodion and dried as a thin layer on the Lexan plastic for irradiation. After irradiation, this collodion layer can be peeled off to allow chemical etching of the fission tracks in the plastic. The collodion layer can also be replaced on the Lexan after etching with a slight offset so that the particles and tracks are visible under a Kght microscope at magnification 250-500. Replacement of the collodion layer is not necessary if a comparator microscope is available that allows viewing two objects (i.e., the collodion layer and the Lexan with tracks) simultaneously. 

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