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2. Compositional Analysis
Pages 12-25

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From page 12...
... PREVIOUS INSTRUMENTAL METHODS Histoncally, a number of instrumental methods have been used for the determination of elements in lead, including atomic absorption spectrometry (AAS) ,~ neutron activation analysis (NAA)
From page 13...
... However, the added precipitation step increases overall sample preparation time and lowers the precision and accuracy of the element measurements. INDUCTIVELY COUPLED PLASMAOPTICAL EMISSION SPECTROPHOTOMETRY The analytical characteristics of ICP-OES make it a useful technique for metal determinations.
From page 14...
... A Instrumental Methods of Analysis, Seventh Ed.; Wadsworth Publishing: Belmont, CA, 1988.
From page 15...
... ICPOES is a technique that combines good quantitative multielement capability, wide linear dynamic ranges, good sensitivity, limited spectral and chemical interferences, low detection limits, and speed and ease of data handling and reporting with widespread (multiple-vendor) instrument availability and reasonable cost.
From page 16...
... The published precision data, precision data from crime-scene and suspect bullet samples, and other, newer precision data more reflective of the current FBI CABL procedure should be included in the written protocol. The protocol should also describe how precision differs in the low, middle, and high ranges of each element's measurable concentrations.
From page 17...
... The calibration and control samples are also divided into three sub-samples randomly labeled "a," "b," and "c." The FBI's "Calibration and Control of Analytical Procedure" section lacks much of the information that is normally present in well-documented analytical protocols throughout the chemical industry. For example, standard FBI practice states that "a" calibration standards, "a" control samples, and all "a" series bullet lead sub-samples are run first, then the "b" series, and then the "c" series.
From page 18...
... Most analytical laboratories use a formal control chart system. Such a system defines an average value of the measured variable, warning limits (means + 2SD)
From page 19...
... But publication of the protocol and the research and data that support it in peer-reviewed journals or at a minimum publication of the protocol in other public venues would offer an opportunity for review and validation of the protocol. Publication options for the protocol include such a limited venue as Forensic Science Communications (on the FBI Web site)
From page 20...
... Te in bullet lead has been quantified using ICP-MS.2i However, Te, Se, and other elements that might be considered occur at ppm or sub-ppm levels, at or near the detection limit of the analytical technique. The precision of the measurement decreases quickly as measurements are taken near the detection limits of the instrument.
From page 21...
... Measurement of Lead Isotopic Compositions The relative amounts of lead isotopes (206Pb, 207Pb, and 208Pb) in different geographic regions can differ from 17% to 36%.22 The reason for the variation in lead isotopic composition is the radioactive decay of thorium and uranium to lead by the following paths.23 238U ~ 206Pb 235U ~ 207Pb 232Th ~ 208Pb If sufficient precision and mass resolution is available, ICP-MS may be able to distinguish the origins of lead on the basis of isotopic ratios.
From page 22...
... The MC-ICP-MS instrument allows for the simultaneous measurement of the relevant lead isotopes, with the advantages of TIMS and the advantages of ICP-MS because it does not require the isolation of lead from other elements before analysis. The results showed that the MC-ICP-MS instrument had precision and accuracy that were about ten times better than those in a similar study of quadrupole ICP-MS.29 Differences were observed with bullets obtained from economically isolated regions of the world, such as the former Soviet Union and South Afnca.
From page 23...
... FINDINGS AND RECOMMENDATIONS Finding: The current analytical technology used by the FBI inductively coupled plasma-optical emission spectroscopy (ICP-OES) is appropriate and is currently the best available technology for the application.
From page 24...
... Recommendation: The FBI should continue to measure the seven elements As, Sb, Sn, Cu. Bi, Ag, and Cd as stated in the current analytical protocol.
From page 25...
... COMPOSITIONAL ANALYSIS 25 Recommendation: The FBI should evaluate the potential gain from the use of high-performance inductively coupled plasma-optical emission spectroscopy because improvement in analytical precision may provide better discrimination.


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