I would also like to thank the undergraduate and graduate members of the Hammer Research Group for their assistance in the construction of the LIBS instrument, and for training me in the Igor program. RYAN ALEXANDER GREGG: Development of a laser-induced degradation spectrometer for lead-free gunshot residue analysis. Research has shown that Laser-Induced Breakdown Spectroscopy (LIBS) can be used as an effective method of screening for the common elemental components in gunshot residue.
Barium (Ba), lead (Pb), and antimony (Sb) form the primer cap of most ammunition involved in forensic analysis of gunshot residue. This article discusses the development of the Ole Miss Department of Chemistry and Biochemistry's first LIBS instrument for forensic analysis of lead-free use of Gd as a tracer element for identification purposes. Samples using the lead-based and lead-free bullets were analyzed by the LIBS system consisting of an Ocean Optics USB2000 CCD camera with a Continuum 1064 nm Nd:YAG laser as the excitation source.
Spectra of a lead-free and lead-free gunshot residue sample were also obtained, but were found to be too noisy to identify emission lines due to lack of time. Further discussion in this thesis describes how this problem can be fixed so that this project can continue in the future.
Overview of Gunshot Residue
- Mechanism of Formation and Composition
- Methods of Analysis
- Forensic Applications
- References
As the pressure continues to build in the chamber, the bullet will be ejected from the case toward the bottom of the barrel. Colorimetric tests have been developed for the analysis of various components of GSR since the early 1930s. Mass analysis of GSR consists of analyzing the entire concentration of the elemental components that make up GSR instead of individual particles.
In addition to the X-ray spectrum collected from the sample, the electron beam from the SEM produces a magnified image of the sample that far surpasses any image. The most common places to find GSR are on the hands, face, clothing, and other objects in the firearm's discharge environment. The most common areas for hand sampling are the back and inside of the palm of each hand5.
As the distance from the firearm to the firing increases, there will also be a significant increase in the size of the area where GSR will be deposited. This phenomenon is due to the fact that the GSR plume expands out of the barrel in the x-, y-, and z-plane directions. Identifying the pattern of GSR distribution after firearm discharges can be done using many of the colormetric tests discussed earlier in this chapter.
Other particles containing only some of the particles in the "unique to GSR" classification are placed in .
Laser Induced Breakdown Spectrometry
- Theory and Instrumental Design
- Advantages and Disadvantages
- Experimental Applications…
- References
Plasma formation by laser light excitation is the most important theoretical part of LIBS instrumentation. The expansion of the plasma, however, is carried out at supersonic speeds in the form of an expanding shock wave2. Exciting the sample to the plasma state causes the individual elements to occupy higher energy states than the initial ground state.
This figure shows the emission wavelengths of Gd, one of the elements discussed in Chapter 3. The first phenomenon is plasma shielding, a phenomenon that affects the amount of laser pulse to which the sample is exposed. As mentioned earlier, the plasma cloud will expand simultaneously as the laser pulse continues to hit the rest of the sample.
The result of this is that the laser light will be absorbed by the lawn of excited atoms, ions and free electrons and will not reach the core of the sample to cause further ablation2. The second phenomenon is the Stark effect, a phenomenon that occurs due to the presence of the free electron field that is formed after exposure to the laser pulse. The result is a peak broadening effect found primarily in the initial nanoseconds of exposure to the pulse, specifically the time of initial ablation of the sample material.
Although this lens can help optimize the experimental design by collecting more of the emitted light from the sample, it is not the only method of collecting light from the sample. Due to the fact that there are very small wavelength differences between the emitted radiation of the many different elements possible in the analyte, it is very important to have a high resolution spectrophotometer for this method. Interpretation of the spectra involves identifying the most abundant peaks on the spectrum according to their wavelength.
Accuracy is directly determined by the homogeneity of the sample and thus the accuracy is found. The ablation zone in LIBS is small and thus the plasma plume contains a small amount of analyte that emits low amounts of light. One of the best examples of the application of LIBS in the field has been the many scientists who have done feasibility studies of using LIBS for elemental analysis on the Mars rover.
In this paper, Dockery outlines the application of LIBS to the detection of GSR in the hands of the shooter. All the authors have done their studies on bullets that have lead-based primers.
Analysis of Lead-free GSR Using LIBS
- Introduction
- Experimental
- Results and Discussion…
- Conclusions
- References
The sample box was closed during sampling to prevent contamination from the external room lights and to ensure analyst safety from the IR laser light. A fiber optic cable was placed at a 90° angle to the sample, with the collecting end of the optic. The collection end of the optics was placed as close to the sample as possible to ensure maximum collection of the emitted light.
The caliber change between the guns played no role in changing the deposition of the GSR. After the subjects fired their guns, a sample was taken by a gloved analyst by pressing the adhesive tape surface onto the skin of the hands. The sampling sites, including the first knuckle of the first finger, the web between the first finger and the thumb, and the first knuckle of the thumb, were those described as the highest areas of GSR deposition by Schwoeble and Exline8.
Standard spectra were also obtained for some of the elemental components of the GSR samples, including a spectrum of Ba (FischerScientific Standard), Pb (FischerScientific Standard), Zn (FischerScientific Standard), and Gd (SigmaAldrich Standard). Also 20 laser pulses were taken for each of the standards and the spectra from each standard were analyzed. There is a clear difference in the signal-to-noise ratio of the spectra obtained from the standards and samples.
With the exception of Gd(I) 453.781 nm, all these characteristic emission lines for Gd are in the UV region and are therefore outside the range of the detector (>350 nm). Both the lead-based and lead-free GSR samples showed strong background continuum emission that significantly reduced the quality of the spectra overall. The source of this continuous emission is the Bremsstrahlung radiation associated with the initial ablation of the sample material and the formation of the plasma.
This Bremsstrahlung radiation is exacerbated in the samples more than the standards due to the high density of the electron field of multiple elements instead of a single element, causing the continuum emission to be much higher in the sample spectra than the standard spectra10. The issue of the continuum emission from the Bremsstrahlung radiation was addressed in the LIBS literature by using a detector that has the ability to control the timing of data collection with very high precision. Because the continuum emission is generated at the moment of ablation and the start of plasma generation, a delay before data collection can ensure that the short lifetime of the Bremsstrahlung radiation is avoided overall.
Using these parameters with the detector used in this thesis is not possible because the timing control function of the Ocean Optics spectrometer can only delay data collection in millisecond units, not microseconds. A data collection delay of milliseconds would miss all analytically useful emission lines, while no data collection delay results in saturation of the detector with the radiation continuum and a spectrum devoid of analytically relevant emission lines.
