The Development of Atomic Absorption Spectroscopy Method 

Forensic Science is the application of Physical Sciences to the criminal and civil laws that are enforced by police agencies in the criminal justice system. This particular branch of science is used to unlock the story that is contained within the evidence collected at a crime scene. Forensic laboratories have varying abilities to perform forensic examinations. This discrepancy is based on issues such as funding, size and equipment. Smaller laboratories are unable to perform certain analyses due to lack of instrumentation (Saferstein). This widespread lack of a universal method to analyze and identify components has led to the loss of funds in many local laboratories. The development of a scientifically uniform method for analysis of evidence through the use of available instrumentation will indeed increase productivity. A tool that has been recently increasingly used is the Atomic Absorption Spectroscopy Method to analyze the problematic piece of evidence known as GSR (Tobin).

Gunshot residue – also known as GSR – is released from the discharge of a firearm.

Gunshot Residue if generated from the interior interactions of the firearm, cartridge case, bullet, base primer and propellant. Primer mixture is responsible for the inorganic matter and the propellant supplement the organic matter. Within the forensic science community, there is a need for analyst to make association between the firearm and the suspect (Tobin). This associations begins with the confirmatory analysis of the GSR and either matching or disproving it with the evidence provided. The primary components of the Gunshot Residue primer include lead styphnate (explosive), antimony sulfide (fuel), and barium nitrate (oxidizer) (Tobin). These components generate large amount of heat and pressure which then ignites the organic propellant and create higher temperatures and pressures, nearing 3.600°C and 40,000 psi.

The metals and organic components are then vaporized due to the high heat and pressure additives that they anneal into spheroid shaped particles (Basu). The newly formed organic particles in conjunction with the inorganic particles are both characteristics forming the GSR. The inorganic components are most commonly tested for positive characteristics of GSR. There are many factors associated with GSR that make it increasingly difficult to associate residue with a specific firearm or suspect.

The introduction of lead-free ammunition has presented a challenge for GSR analysis for many forensic laboratories. The usual characters such as lead, barium and antimony as the staples are no longer suitable for cases involving lead free ammunition. This is mostly due to the lack of heavy metals. Figure 2 demonstrates the difference between OGSR and IGSR in side-by-side evaluation. As previously states the organic components of GSR known as OGSR, have been the components of which to identify GSR through smokeless powder propellant. The analysis of OGSR is indeed beneficial due to the amount of smokeless powder present within ammunition. The amount of smokeless powder is significantly larger than the amount of primer mixture present. This may potentially lead to a greater amount of residue arising due to the propellant when compared to that from the primer mixture.

Gunshot propellant come in a variety of numerical bases. Single and double based powders are most common propellants in ammunition, triple based powders are less common due to the large caliber, rockets and military usage (Maitre). Additives to the powders such as flash inhibitors, stabilizers and plasticizers allow an improved stability, workability and control the burning rate. Methyl-centralite and ethylcentralite are specifically restricted to the production of smokeless powder and are considered the main components of the propellant. The nitrated-derivates of Diphenylamine (DPA) such as N-nitrosodiphenylamine and others are considered characteristics of GSR (Maitre).

One of the main problems with analysis of Organic Gunshot Residue is although its abundance in ration of bullet, it is indeed considered a trace elemental evidence. Law enforcement investigators test clothing and skin for GSR to determine a persons culpability if they were near the firing of the gun. Gunshot residue can travel between 3-5 feet from the gun. Since this is not considered a far distance the analysis of trace particles which can be detected as far as many meters away (Mukesh). This practice is used often within forensic examinations of a crime scene. Table 1, displays the trace elements measurements and examples within everyday examples. A laboratory can spend a great deal of funding to operate the trace realm analytical instruments. The level of expertise among the trace analyst must be incredibly high.

Inorganic mass spectrometry was unapproachable and im¬practical because it was too expensive and volatile to be bothered with. Inorganic matter evaluation is now more affordable. There are many other trace analysis techniques today that function well for example low limit carbon and sulfur determinators, oxygen, nitrogen, and hydrogen determinators, nuclear methods, and isotope dilution mass spec¬trometry (Mukesh). It is also possible to isolate inclusions and phases from an alloy and characterize them in the same manner as the alloy matrix. One example of this includes the use of Atomic Absorption Spectroscopy. This last subject is closely related to trace work since tramp elements are often associated with compounds and metallic phases in an alloy (Mukesh).

Organic and Inorganic Gunshot residue is detectable on the skin after firing as few as one gunshot. There are several analytical techniques that have been successfully utilized for the detection of IGSR such as gas chromatography, micellar electrokinetic capillary electrophoresis, desorption electrospray ionization-mass spectrometry and liquid chromatography with mass spectrometry (Mainstre). The analytic and instrumental aim is limited to the information provided by the experts on OGSR and IGSR traces and judicial parties involved in the investigative process. This unfortunate disadvantage requires forensic scientist to inquire about a better understanding of IGSR and OGSR traces beyond the simple question of analytic detection. This is also limited due instrumentation deficit and laboratory funding.

Addition training and guidance to the potential value of IGSR analysis is necessary (Dalby). An important component of evaluation of inorganic component of GSR is the time component associate. The majority of IGSR is said to be lost within the first three hours after initial firing. This evaluation was confirmed doing scanning electron microscopy coupled with energy observation. This leads to the need of an instrument in which is both incredible sensitive and time sensitive. Such a technique that comes to mind is Atomic absorption spectroscopy.

Atomic absorption spectroscopy analytical technique to measure the concentration of elements using the absorption of optical radiation by free atoms in the gaseous state. Within analytical chemistry and instrumental analysis, it is used to determine the concentration of a particular element within a sample. This is extremely important in the analysis of Gunshot residue to measure those key components within the inorganic layer of GSR. Atomic absorption spectroscopy also known as AAS, can detect a wide variety of elements form solution or directly from solid samples using electrothermal vaporization. The technique is extremely sensitive, being able to detect a positive sample as small as parts per billion of a gram within a sample. The method of analysis was discovered by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff in the second half of the 19th century (Science History Institute). During the 1950s, the modern form of Atomic Absorption spectroscopy was developed.

The Atomic Absorption spectrometer makes use of analysis and recording the wavelengths of light specifically emitted by an element. AAS is described as an instrumental method of analysis in the sense of it being rapid, accurate, sensitive. AAS was a more systematic approach to analyze specific elements also found in GSR due to its enhanced reproducibility. In order analyze the sample for its atomic constituents, it must be atomized. The atomizers most commonly use nowadays are flames and electrothermal atomizers (Science History Institute). The atoms are then irradiated by optical radiation, whether element specific line of radiation or continuum radiation. The radiation then passes through a monochromator to separate the element-radiation from another radiation which is then finally measured by a detector.

The method of Atomic Absorption Spectroscopy has served the forensic community for over 40 years and continues to work effectively for as gunshot powder residue analysis. The advantage of atomic spectroscopy over these other methods lies in the practical considerations of accessibility and cost as much as in sampling requirements and detection limits. Another pro to this method includes the specificity, reliability, and sensitivity of the instrumentation. The AAS method of analysis are rapid and replicable, which aid in the pros and use of the method. The instrument is able to analyze the presence of antimony, barium and lead in GSR specifically. The method of collection is swab with 5% Nitric Acid to collect residue. This method of collection increases sterility and increases recovery of the sample collected. The use of Atomic Absorption Spectroscopy, although one of the first groundbreaking techniques is becoming outdated. The use of a new method to replace it is on the rise to increase the specificity as well as other components of collecting GSR.

Throughout this paper the types of GSR as well as the particular components that are present and easily transferrable will be explained in great detail. The statistics of false convictions or faulty evidence due to only components being present in analysis will also be elaborated on. This paper will determine the pros and con of the Atomic Absorption Spectroscopy method in regard to analysis and confirmatory testing. Lastly analyzing newer methods of GSR, both organic and inorganic, testing will be explained in thorough detail, itemizing the pros and cons between the two.