Particle Analysis Method

This method employs a scanning electron microscope equipped with elemental analysis capability (SEM/EDX) and combines details of the morphology and elemental composition of individual FDR particles. A particle classification scheme was developed and is based on the elemental composition and morphology of individual FDR particles and is used to classify particles as one of the following: (a) of nonfirearm origin, (b) consistent with originating from the discharge of a firearm, or (c) definitely from the discharge of a firearm. The ability to identify FDR particles uniquely and to distinguish them from environmental sources of lead, antimony, and barium eliminates the threshold problem inherent in bulk elemental analysis.

In the SEM a beam of electrons is accelerated from a hot tungsten filament down a column by a high anode potential (up to 50 kV), and by the use of three electromagnets (electromagnetic lenses) it is focused on the surface of the sample (specimen) which is mounted on a metal stub. The primary electron beam interacts with the elements at the surface of the sample causing, among other things, secondary electrons, backscattered electrons, and characteristic X-rays to be emitted from the sample surface. Figure 16.3 illustrates the different interactions.

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.

Scanning is achieved by the use of scanning (raster) coils which cause the primary beam to be electromagnetically deflected across a given area of the sample surface. The raster pattern of the beam is synchronized with the scanning pattern of the cathode ray tube.

Electron beam

Characteristic X-rays y

Backscattered electrons

Cathodoluminescence

Auger electrons

Secondary electrons

Secondary electrons

Specimen current

Transmitted electrons

Figure 16.3 Electronic interaction with sample surface.

The low energy secondary electrons (less than 50 eV) ejected from the sample surface are attracted to a scintillator on the end of a perspex light guide, the other end of which is in contact with the window of a photomul-tiplier tube. The amplified signal is displayed on a cathode ray tube which records the image. The secondary electrons are attracted to the scintillator by a positively charged cage surrounding the end of the light guide. The number of electrons reaching the scintillator is dependent on (a) the topography of the sample surface since this will influence whether or not a particular area on the sample surface is visible to the primary beam and detector and (b) the elemental nature of the sample surface as this will affect the energy of the secondary electrons and consequently their susceptibility to the cage potential. The topography is the more important of the two factors.

Backscattered electrons are those electrons that have undergone single or multiple scattering events, and escape back through the surface of the sample with energies greater than 50 eV. Backscattered electrons travel in straight lines and because of their higher energies they are not attracted to the secondary electron detector. Backscattering increases as the atomic number of the sample increases and there is such a strong correlation with atomic number that the relationship forms the basis for a contrast mode in SEM.

FDR particles contain elements with high atomic number (heavy metals) and this fact is used to aid the search for FDR particles located among many other particles with similar morphology but from non-firearm-related sources. The backscattered electron image is displayed on a separate screen. All particles containing heavy metals show stronger emission and appear as bright areas on the screen; only the bright particles are potential FDR and consequently only these particles need to be analyzed. Without the aid of a backscattered image, all particles with similar morphology to FDR would need to be analyzed.

Characteristic X-ray emission is one process by which an atom may stabilize itself following ionization by the electron beam. When an electron from an inner atomic shell has been dislodged, an electron from an outer shell will replace it. The difference in energy between the initial and final state may be emitted as X-radiation. The various shells of an atom have discrete amounts of energy. It follows that their energy difference, emitted as X-radiation, is also a discrete quantity and is characteristic of the atom from which it was released. X-ray spectroscopy in the SEM, as used for FDR work, involves the identification of radiation of specific energies using a special detector, and can identify elements heavier than sodium. Detection of lighter elements is not possible using a conventional detector due to the inefficiency of X-ray generation for the lighter elements. Figure 16.4 illustrates the basic components of a SEM suitable for FDR work.

Samples that do not conduct electricity and heat (insulators) cause problems in the SEM unless they can be made conductive by some means.

Back Scattered Electrons

(2) Backscattered electrons

(3) X-rays-energy dispersive spectrometer with printer.

Figure 16.4 Basic components of a scanning electron microscopic suitable for FDR detection.

(2) Backscattered electrons

(3) X-rays-energy dispersive spectrometer with printer.

Figure 16.4 Basic components of a scanning electron microscopic suitable for FDR detection.

Problems include charging and overhearing of the sample. Coating samples with a thin layer of conductive material helps to overcome the problems.

Samples for FDR examination are taken from a suspect's skin and/or clothing surfaces by a nondestructive sampling technique and prepared for examination in the SEM. FDR samples are coated with a very thin layer of carbon using either a vacuum coater or sputter coater before introduction to the SEM. The examination involves searching for FDR particles among many other particles from occupational and environmental sources, and is a labor-intensive and time-consuming task. FDR particles are recognized by a combination of morphology and elemental composition, and are classified as FDR using a scheme developed for the purpose. The classification system will be discussed later.

Particle analysis is the most informative method to date for the identification of FDR particles. It does, however, suffer from several major disadvantages including high cost of instrumentation and lengthy and tedious procedures requiring specialized staff. Since its introduction serious attempts have been made to solve the time problem. These include the use of backscattered electron images, automation of the search procedure, and sample manipulation to pre-concentrate the sample prior to SEM examination.145-151

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.

Detection of Organics in Firearm Discharge Residue

Chromatographic techniques are the main methods that have been used to separate, detect, and identify organic components of FDR.152-162 Other methods considered include molecular luminescence,163 infrared spectroscopy,164 Raman spectroscopy,165 electron spin response spectrometry,166 microchemical crystal tests,167,168 ultraviolet spectroscopy/nuclear magnetic resonance/polarography.169

Many of the organic constituents of FDR are explosive or explosive-related compounds and much of the work already done on the detection of explosive residues can be extended to include FDR. Explosives and their residues are usually analyzed using chromatographic techniques. Chromatography is the general name given to the methods by which two or more compounds in a mixture physically separate by distributing themselves between two phases: (a) a stationary phase, which can be a solid or a liquid supported on a solid, and (b) a mobile phase, either a gas or a liquid which flows continuously around the stationary phase. The separation of individual components results primarily from differences in their affinity for the stationary phase.

Of all the methods investigated for organics in FDR it would appear that high-performance liquid chromatography (HPLC) and gas chromatography with a mass spectrometer detector (GC/MS) are currently the most promising.

Was this article helpful?

+2 0

Post a comment