Lead Isotope Ratio Determination for the Forensic Analysis of Military

A study focusing on the ability to utilize isotopic analysis of Pb used in small arms projectiles from varying geo- graphic origin was undertaken usin...
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Anal. Chem. 2003, 75, 5022-5029

Lead Isotope Ratio Determination for the Forensic Analysis of Military Small Arms Projectiles Gavin A. Buttigieg,*,† Mark E. Baker,‡ Joaquin Ruiz,‡ and M. Bonner Denton†,‡

Department of Chemistry and Department of Geosciences, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721-0041

A study focusing on the ability to utilize isotopic analysis of Pb used in small arms projectiles from varying geographic origin was undertaken using the GV Instruments Isoprobe multicollector inductively coupled plasma mass spectrometer. With the exception of two classes of ammunition, these samples were able to be distinguished from one another. Also, following rigorous normalization and mass bias correction, these data were compared to data collected by geochemists in localities corresponding to projectile manufacture. This comparison was successful when projectiles in certain economically isolated countries were analyzed. Pb isotopic analysis with MC-ICPMS is shown to be useful as a tool for distinguishing between ammunition of various manufacturing origins. Evidence gathered at a crime scene involving the use of a firearm may include the recovery of bullets, shell casings, powder or primer residues, ammunition, and weapons. Each of these items or materials can yield vital clues, ideally leading to the arrest and conviction of guilty parties. Striations on a projectile fired through a particular gun barrel can help investigators determine whether the bullet recovered at the scene came from a particular weapon.1 However, the recovered bullet may be fragmented or severely deformed, the barrel may not contain distinct markings, or a firearm may not be recovered making the association with any particular weapon difficult if not impossible. Similarly, tool marks on the primer from the impact of the firing pin or markings on the cartridge casing from extractor and ejector markings can also be used to tie a particular weapon to the crime. This is only possible if casings are retrieved from the crime scene, which, depending on the firearm, may or may not be likely. In cases where physical methods of identification have turned up empty handed, it is necessary to utilize other information resulting from the use of a firearm in the commission of a crime. Since the small arms cartridge is such a chemically complex unit, useful information is often gleaned from chemical analysis resultant from firearm discharge. Investigators have the ability of linking a suspect to the firing of a weapon, by detection of gunshot residue (GSR), which currently only determines whether a firearm has been discharged by a given individual; little qualitative information is routinely gleaned from such tests.1 Preliminary studies have been * Corresponding author: (e-mail) [email protected]. † Department of Chemistry. ‡ Department of Geosciences. (1) Meng, H. H.; Caddy, B. J. Forensic Sci. 1997, 42, 553-570.

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able to link unknown primers to given manufacturers; this may lead to techniques where more information is extracted from collected GSR.2,3 The usefulness of GSR will always be limited by the inability to detect it at distances greater than a few yards.1,4,5 While each of these techniques individually may not provide the definitive answers often obtained from other types of forensic evidence, such as is the case with fingerprints and DNA analysis, collectively they may provide enough useful information to aid an investigation. Forensic scientists have determined that it is possible to link a given projectile to other bullets manufactured at a similar period of time by detecting elements present in Pb at low concentrations such as Ag, As, Bi, Cd, Cu, Sb, and Sn.6-8 These analyses have led to this technique gaining general acceptance within the forensic community and to its use as a tool in many prosecutions by providing the ability to link a single projectile (or projectile fragment) found at a crime scene to ammunition manufactured during a similar period of time. However, a research study led by the former Federal Bureau of Investigation laboratory chief metallurgist, William Tobin, has sparked a rather large controversy and has led to a National Academies of Science review of the validity of conclusions drawn as a result of elemental analysis of bullet Pb.9,10 In their study, Tobin et al. question the choice of elements analyzed in order to draw conclusions as to the “source” of Pb used in small arms projectiles. They argued that the elements commonly chosen for such analysis do not contain an adequate ability to distinguish between batches of Pb manufactured at various times due to deliberate control of those elements and fractionation of elements during Pb primary and secondary smelting as well as elemental fractionation during the bullet manufacturing process such as the extrusion of wire from Pb billet.9 One question left in the mind of the reader after reviewing the study conducted by Tobin et al. is, would an alternate analysis that does not depend on the measurement of elements controlled by the Pb smelters be useful for distinguishing between Pb (2) Buttigieg, G. A.; Denton, M. B. Unpublished, 2003. (3) Haag, L. Assoc. Firearms Toolmark Exam. 2001, 33, 326-331. (4) Krishnan, S. S. J. Can. Soc. Forensic Sci. 1973, 6, 55-77. (5) Lichtenberg, W. Forensic Sci. Rev. 1990, 2, 38-62. (6) Koons, R. Crime Lab. Dig. 1988, 15, 33-38. (7) Koons, R. Spectroscopy 1993, 8, 16-21. (8) Andrasko, J.; Kopp, I.; Abrink, A.; Skiold, T. J. Forensic Sci. 1993, 38, 11611171. (9) Randich, E.; Duerfeldt, W.; McLendon, W.; Tobin, W. Forensic Sci. Int. 2002, 127, 174-191. (10) Mejia, R., Sample, I. New Sci. Internet Publ., 17 April, 2002. http:// www.newscientist.com/news/news.jsp?id)ns99992172. 10.1021/ac0301346 CCC: $25.00

© 2003 American Chemical Society Published on Web 08/30/2003

contained in projectiles of various batches of ammunition. One such technique is Pb isotopic analysis. The isotopic composition of Pb deposits found in nature varies substantially due to the decay of radioactive isotopes of uranium and thorium.11 The abundance of uranogenic and thorogenic Pb is well studied and routinely used by geochemists as a technique for determining the age of geological materials and for ore prospecting. While the concentration of 204Pb is fixed by the original amount of Pb in the ore, 206Pb, 207Pb, and 208Pb concentrations will vary as a function of the initial concentration and subsequent radioactive decay of 238U, 235U, and 232Th, respectively.11 Due to the variation of the isotopic composition of Pb depending on ore source, it is reasonable to expect some variation to be present in Pb bullet alloys from various manufacturers or countries of origin. Several techniques have previously been investigated for use in examining Pb isotope ratios of Pb alloys found in small arms projectiles.8,12-15 Thermal ionization mass spectrometry (TIMS) was utilized by Andrasko to examine Pb smears and bullet fragments recovered from a homicide, demonstrating the applicability of Pb isotope ratios to an actual field investigation.8 The investigation illustrated the ability of TIMS to provide highprecision isotope ratios of Pb, a necessity for discerning bullet samples. Andrasko examined more than 90 bullets of the same caliber and was capable of observing clear differences and similarities in the Pb isotope ratios of the samples. While TIMS is capable of providing high-precision Pb isotope ratio measurements, it requires that the Pb be separated from other elements prior to analysis, due to varying mass bias effects generated during ionization of Pb in varying matrixes. The investigator achieved this presample cleanup by electroanalytical deposition of the previously dissolved Pb onto an anode as PbO2. Redissolution of the purified Pb was then performed prior to analysis. Andrasko stated the analyses were extremely taxing to the laboratory resources and would be difficult to implement on a routine basis. Additionally, to maximize the accuracy and precision possible with TIMS, it is necessary to implement extremely precise multiple spike techniques that are tedious and time-consuming to perform; however, Andrasko did not mention the use of these techniques in his study.8,16 Inductively coupled plasma-quadrupole mass spectrometry has also been utilized for the determination of Pb isotope ratios in bullets. The advantage of ICPMS is its ability to perform an array of analytical measurements, including isotope ratio analysis, without the need for extensive sample pretreatment other than sample dissolution. Dufosse and Touron utilized a quadrupole ICPMS as a comprehensive technique in order to discern the suspected shooter in a hunting death.14 They used a combination of trace chemical analysis of the bullet Pb, as well as antimony percentage and Pb isotope ratios, to determine the likely shooter. The repeatability of the ratio measurements by this technique was reported to be better than 1%, with the accuracy within 2% of a (11) Doe, B. R. Lead isotopes; Springer-Verlag: New York, 1970. (12) Guinn, V. P.; Purcell, M. A. J. Radioanal. Chem. 1977, 39, 85-91. (13) Haney, M. A.; Gallagher, J. F. J. Forensic Sci. 1975, 20, 484-500. (14) Dufosse, T.; Touron, P. Forensic Sci. Int. 1998, 91, 197-206. (15) Stupian, G. W.; Ives, N. A.; Marquez, N.; Morgan, B. A. J. Forensic Sci. 2001, 46, 1342-1351. (16) Woodhead, J. J. Anal. At. Spectrosc. 2002, 17, 1381-1385.

Pb standard reference material, which is more than 1 order of magnitude worse than is possible using TIMS.14 Stupian used a Cameca 5f secondary ion mass spectrometer with an O2+ ion beam as an ionization source to achieve accuracy of ∼2% and precision of 0.5%.15 The aim of his research was to determine whether ammunition found on two suspects was similar to that found at crime scenes. He concluded that the ammunition found in the possession of the suspects was dissimilar from ammunition recovered from the crime scenes. Stupian stated in his conclusion that this analysis is best suited to comparisons of two volumes of ammunition, to ensure valid statistical certainty.15 The multicollector arrangement of TIMS instrumentation allows for the simultaneous measurement of the isotopes of interest, resulting in high-precision ratio measurements, yet as noted by Andrasko and others, time-consuming sample preparation requirements make its use somewhat impractical for routine forensic applications.8,16 On the other hand, the use of a quadrupole ICPMS circumvents the need for such extensive sample preparation, thus allowing for high sample throughput, yet suffers from relatively poor precision, since individual isotopes are measured sequentially rather than simultaneously.14 Pb isotope ratio determinations with MC-ICPMS allow for simultaneous measurement of each of the relevant isotopes, utilizing the advantages of TIMS, while also capitalizing on the advantages of quadrupole ICPMS by not requiring a need for isolation of the Pb from other elements in the sample prior to analysis. The study described in this document establishes the possibility of using MC-ICPMS for the analysis of Pb extracted from military small arms projectiles. Samples studied were collected from a far broader geographic distribution than had been previously analyzed, from 10 manufacturers and a total of 8 countries.8,14,15 The fact that these samples originated from such varied geographic locations should allow us to determine whether isotopic differences between samples from different locations are great enough to be useful for distinguishing between samples of different geographic origin. Additionally, it should be clear whether Pb used in the manufacture of projectiles retains any isotopic character native to the country of manufacture, a comparison that has not been previously attempted. Since we intend to utilize previously published data in the geological literature for comparison of regional isotopic character, it is also necessary that the data collected in this study be treated with the same level of rigor as is common in geological analysis, a practice that has not been implemented in previous studies involving isotopic analysis of projectile Pb. Toward this end, we have investigated and utilized very accurate and precise methods for mass bias correction of data collected using the GV Instruments Isoprobe MC-ICPMS. EXPERIMENTAL SECTION Caution! Live ammunition can detonate if not handled correctly. One should avoid exposing ammunition to excessive heat or force and should only disassemble live ammunition with the correct tools, those being a kinetic bullet remover or a press-mounted collet-type bullet remover. Additionally, Pb is toxic and has been identified as a carcinogen; protective clothing and proper ventilation should be used when undertaking the procedure described herein. Sample Collection. American Eagle (from the United States) caliber 5.56 × 45 mm NATO ammunition (11 samples) was taken Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

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from four boxes within a single sealed case, dated 1999. Sellier and Bellot (from the Czech Republic) caliber 5.56 × 45 mm NATO ammunition was collected from seven boxes within a single sealed case; two of the boxes were marked “663” while five were marked “584”; there is no indication as to what these markings signify. Russian “Wolf” caliber 7.62 × 39 mm ammunition was collected from six separate boxes contained in a single sealed case dated 2000. South African caliber 7.62 × 51 mm NATO ammunition was taken from a single box within an unsealed case; headstamps indicate a manufacture date of 1970. Winchester (USA) caliber 5.56 × 45 mm NATO ammunition was collected from four separate boxes within a single case dated 1999. Lake City, U.S. manufactured caliber 5.56 × 45 mm NATO ammunition was taken from three boxes, from an unsealed case, dated 1990. Korean (7.62 × 51 mm NATO), Chinese (9 × 19 mm NATO), and Egyptian (7.62 × 51 mm NATO) ammunition were taken from separate boxes, from unsealed cases, dated 1980, 1991, and 1975, respectively (five samples each). Romanian caliber 7.92 × 57 mm ammunition was collected from five separate boxes within a single hermetically sealed case, dated 1974. Sample Preparation. The ammunition was disassembled using a kinetic bullet remover (Dillon Precision, Scottsdale, AZ, Part 17999). After reviewing the previous methods of Pb extraction (cutting with a razor and drilling with a lathe), and in light of the fact that maintaining the integrity of the sample was not an issue, it was determined that the most efficient and clean method of Pb extraction would result from heating the projectile. The projectiles were subsequently heated with a propane torch past the melting point of Pb (this procedure should be conducted in a hood), after which the molten core was poured into a beaker of 18-MΩ purified water. Caution! Do not attempt melting Pb from a projectile that is suspected of being a tracer or an incendiary! Approximately 10 mg of the recovered Pb was weighed in to a clean, dry, 24-mL Teflon container (Savillex Corp., Minnetonka, MN, Part 0275R-SB). To each Teflon container was added ∼10 mL of distilled 16 M nitric acid. The containers were capped and heated on a hot plate; although complete dissolution was achieved within 90 min, samples remained on the hot plate until analysis, typically 48 h. Prior to analysis, 1 mL of the sample solution was combined with 140 µL of 1000 ppm thallium standard and diluted to 125 mL with 18-MΩ purified water. Thallium was utilized as an internal standard in order to model mass discrimination behavior. Instrumental Conditions. Analysis was performed on the GV Instruments Isoprobe MC-ICPMS. Details of the instrumental geometry of this system have been published.17,18 Samples were introduced by free aspiration with a Teflon low-flow concentric nebulizer into a water-cooled cyclonic spray chamber. Instrumental parameters are given in Table 1. All isotopes were measured on separate Faraday detectors. Data Collection. All analyses were conducted in static mode. Data were collected in three blocks of 30-10-s integrations; each block was initialized with a 30-s background correction. Between each projectile sample, NIST certified standard NBS 981 was run to monitor instrument stability. Instrumental washout was achieved (17) Rehkaemper, M.; Halliday, A. N. Int. J. Mass Spectrom. Ion Processes 1998, 181, 123-133. (18) Rehkaemper, M.; Mezger, K. J. J. Anal. At. Spectrosc. 2000, 15, 14511460.

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Table 1. Instrumental Parameters of the GV Instruments Isoprobe rf power accelerating voltage nebulizer gas coolant gas intermediate gas Ar collision gas

1350 W -6000 V 0.85 L min-1 14 L min-1 1 L min-1 1.3 mL min-1

by aspiration of 18-MΩ purified water followed by 2% HNO3 until no 208Pb was detected. Data Reduction and Analysis. Mass bias occurs due to varying transport efficiencies from the point of sample introduction to the point of ion detection.17-24 The cause of this anomaly has been attributed to space charge effects resulting from the concentrated ion beam entering the mass spectrometer, mainly at the skimmer cone and extraction lens region.23,24 In addition, heavier ions are preferentially transported through the inlet region as well as the mass spectrometer resulting in a mass spectrum slightly skewed toward the higher mass isotopes, at ∼1% per amu in the mass range of Pb.23,24 It has also been noted that day-today accuracy of the MC-ICPMS is variable and must also be accounted for.17,18,25,26 In order for results collected on this instrument to be comparable to published geological data (collected on TIMS instruments), the employment of one of several mass bias corrections is necessary. The currently accepted mass bias correction techniques have been discussed in great detail elsewhere and therefore will not be covered here.18 Empirical Tl normalization of NBS 981 data (to correct for dayto-day instrumental drift) followed by power law, exponential law, and exponential law with β corrected for Pb mass bias techniques were examined to determine which offered the greatest accuracy and precision improvements.16-18,25,26 Accuracy was determined by comparison of our corrected data with that of Galer’s triple spike TIMS data;27 these results are presented in Table 2. The exponential law proved the most effective for minimizing error between Galer’s published NBS 981 data and NBS 981 standards analyzed throughout this study, while the exponential law with β corrected for Pb shows a slight advantage in data precision. Due to the maximized accuracy of measurements of NBS 981 following implementation of both the empirical Tl normalization and the exponential law, these techniques were utilized for mass discrimination correction of all data collected during this study. Selection of these data reduction techniques is in agreement with previous studies.17,18 Since 202Hg was not present at detectable levels, no correction 204 of Pb for 204Hg interference was implemented. RESULTS Instrumental Precision and Accuracy. Student’s t-test was performed on the NBS 981 data, and if any sample was determined (19) Begley, I. S.; Sharp, B. L. J. Anal. At. Spectrosc. 1997, 12, 395-402. (20) Praphairaksit, N.; Houk, R. S. Anal. Chem. 2000, 72, 4435-4440. (21) Heuzen, A.; Hoekstra, T.; Wingerden, B. J. J. Anal. At. Spectrosc. 1989, 4, 483-489. (22) Ignacio, J.; Alonso, G. Anal. Chim. Acta 1995, 312, 57-78. (23) Allen, L. A.; Leach, J. J.; Houl, R. S. Anal. Chem. 1997, 69, 2384-2391. (24) Xie, Q.; Kerrich, R. J. Anal. At. Spectrosc. 2002, 17, 69-74. (25) White, W. M.; Albarede, F.; Telouk, P. Chem. Geol. 1999, 167, 257-270. (26) Marechal, C. N.; Telouk, P.; Albarede, F. Chem. Geol. 1999, 156, 251273. (27) Galer, S. G. Chem. Geol. 1999, 157, 255-274.

Figure 1. Mean values for selected groups of small arms projectile samples. Error bars represent plus and minus two standard deviations for the sample set.

Table 2. Error (ppm) between Measured Standard Reference Material NBS 981 and NBS 981 As Reported by Galer for Each of the Three Mass Bias Correction Techniques Explored, Following Empirical Thallium Correction corr tech

Pb ratio

mean

Galer

error (ppm)

exp exp exp exp exp power power power power power exp β exp β exp β exp β exp β

208Pb/206Pb

2.16 71 0.91464 36.7263 15.4962 16.9424 2.16771 0.91465 36.7295 15.4978 16.9439 2.16771 0.91464 36.7263 15.4963 16.9424

2.16771 0.91475 36.7219 15.4963 16.9405 2.16771 0.91475 36.7219 15.4963 16.9405 2.16771 0.91475 36.7219 15.4963 16.9405

0.00 -118.1 119.1 -4.164 112.4 0.00 -107.1 208.1 95.97 201.6 0.00 -117.7 120.8 -2.903 113.2

207Pb/206Pb 208pb/204Pb 207Pb/204Pb 206Pb/204Pb 208Pb/206Pb 207Pb/206Pb 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb 208Pb/204Pb 207Pb/206Pb 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb

an outlier, it and the two adjacent projectile Pb samples were discarded. All results, corrected for mass bias errors by the exponential law, following an empirical accuracy correction may be viewed in Table 3. As is presented in Table 2, accuracy varied from 0 to 120 ppm (versus Galer’s data) and internal precision (data not shown) was measured on the order of 100-200 ppm per sample (which is comparable to previously published data17,18), while the precision (percent relative standard deviation (% RSD)) of daily NBS 981 readings were on the order of 0.02%. Since these latter measurements represent, by far, the greatest uncertainty in daily measurements, RSD (2σ) of NBS 981 per data collection session was chosen to represent the error present in all measurements taken during that session. Projectile Dispersion. When viewing the tabulated results, it is apparent that isotopic data from some classes of ammunition

exhibit a relatively large amount of scatter. Most notably, those classes are Korean and Lake City (USA), which exhibit class precision of ∼1 order of magnitude poorer than other classes analyzed (Table 3). Chinese and Egyptian projectile data exhibit slightly larger scatter, with relative standard deviation (2σ) approximately double those of other classes. Figure 1 displays data collected from all samples retained in the study (excluding Lake City and Korean projectiles) using ratios of 207Pb/206Pb and 208Pb/206Pb. These ratios were chosen to maximize variation between sample classes; this phenomenon has been noted by Doucelance and Manhes.28 Using 206Pb ratios is not the norm in geological references since the nonradiogenic 204Pb provides a nonvarying denominator, useful for the construction of growth curves, which is not necessary in the current line of research. As is apparent when viewing Figure 1, most ammunition classes are easily distinguishable from one another when 208Pb/ 206Pb and 207Pb/206Pb measurements are compared. However, data collected from Sellier and Bellot samples (Czech republic) and Winchester (USA) samples show some level of overlap, as do data collected from Egyptian and Russian (Wolf) samples. To determine whether these classes may be distinguished from one another using Pb isotopic analysis, it was necessary to conduct a statistical comparison of the overlapping classes. The results of a t-test comparison of means concluded that Egyptian and Russian projectile Pb was significantly different when isotopic measurements 207Pb/206Pb, 207Pb/204Pb, and 206Pb/204Pb were compared. The separation between Egyptian and Russian classes is presented graphically in Supporting Information (SI 1) and in tabular form in Supporting Information (SI 2). Additionally, a statistical comparison of isotopic data collected from projectiles manufactured by Winchester (USA) and Sellier and Bellot (Czech Republic) concluded that they are significantly different from one (28) Doucelance, R.; Manhes, G. Chem. Geol. 2001, 176, 361-377.

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Table 3. Pb Isotope Ratios of Projectiles Following Empirical TI Correction and Exponential Mass Bias Correctiona RPb208/206 RPb207/206 RPb208/204 USA (Am. Eagle) 9/17/01 AE 113 AE 121 AE 142 error 9/18/01 AE 111 AE 112 AE 133 AE141 AE 122 AE 143 AE 123 AE 132 mean 2σ % 2σ error Czech Repub 11/30/01 Sb 663 21 Sb 663 11 Sb584 32 Sb584 33 Sb 584 41 Sb 584 31 Sb 584 51 mean 2σ % 2σ error China USA (LC) 11/30/01 LC 1 LC 2 LC 3 mean 2σ % 2σ error Russia 2/1/02 Wolf_1 Wolf_3 Wolf_4 Wolf_5 Wolf_6 mean 2σ % 2σ error S. Africa 2/1/02 SA_1 SA_2 SA_3 mean 2σ % 2σ error

RPb207/204

RPb206/204

2.01456 2.01321 2.01326 0.00023 2.01074 2.01160 2.00905 2.00970 2.01168 2.01255 2.01198 2.01216 2.01186 0.00320 0.15882 0.0019

0.81842 0.81778 0.81780 0.00005 0.81729 0.81745 0.81699 0.81711 0.81746 0.81764 0.81752 0.81760 0.81755 0.00077 0.09402 0.0004

38.6612 38.6726 38.6712 0.0072 38.6682 38.6266 38.5425 38.5782 38.6400 38.6580 38.6561 38.6365 38.6374 0.0832 0.2153 0.0551

15.7062 15.7091 15.7085 0.00223 15.7171 15.6967 15.6735 15.6851 15.7017 15.7055 15.7069 15.6992 .15.7009 0.0244 0.1556 0.0160

2.09583 2.09582 2.09581 2.09298 2.09300 2.09292 2.09296 2.09419 0.00306 0.14607 0.0003

0.85506 0.85505 0.85507 0.85457 0.85453 0.85448 0.85456 0.85476 0.00057 0.06644 0.0001

38.32048 38.32151 38.31165 38.29229 38.28536 38.29149 38.28679 38.30137 0.03188 0.08325 0.0214

15.63406 15.63438 15.63068 15.63485 15.63109 15.63327 15.63256 15.63299 0.00324 0.02071 0.0083

USA (Win.) 19.1909 2/1/02 Win._1 19.2094 Win._2 19.2083 Win._3 0.00176 Win._4 19.2308 mean 19.2020 2σ 19.1844 % 2σ 19.1960 error 19.2078 Egypt 19.2084 6/10/02 Egypt1 19.2129 Egypt2 19.2015 Egypt3 19.2048 Egypt4 0.0245 Egypt5 0.1276 mean 0.0105 2σ % 2σ 18.28414 error 18.28471 Korea 18.28008 6/10/02 Korl 18.29561 Kor2 18.29213 Kor3 18.29575 Kor4 18.29315 Kor5 18.28937 mean 0.01257 2σ 0.06874 % 2σ 0.0082 error

2.06202 2.05918 2.02692 2.04937 0.03900 1.90302 0.0003

0.83678 0.83566 0.82378 0.83207 0.01441 1.73162 0.0001

38.54875 38.55553 38.60038 38.56822 0.05611 0.14548 0.0214

15.64333 15.64661 15.68797 15.65931 0.04976 0.31776 0.0083

18.69462 18.72376 19.04390 18.82076 0.38759 2.05936 0.0082

2.10853 2.10459 2.10636 2.10857 2.10579 2.10677 0.00349 0.16576 0.0021

0.86238 0.86151 0.86143 0.86241 0.86229 0.86200 0.00098 0.11414 0.0005

38.2073 38.1389 38.2070 38.2010 38.1474 38.1803 0.0683 0.1789 0.0719

15.6267 15.6121 15.6253 15.6243 15.6208 15.6218 0.0118 0.0753 0.0229

18.1204 18.1217 18.1389 18.1171 18.1155 18.1227 0.0188 0.1036 0.0165

2.13518 2.13042 2.13018 2.13192 0.00564 0.26444 0.0021

0.88344 0.88000 0.87921 0.88088 0.00450 0.51040 0.0005

37.8427 37.8620 37.9412 37.8820 0.1044 0.2756 0.0719

15.6575 15.6394 15.6598 15.6523 0.0224 0.1429 0.0229

17.7234 17.7721 17.8113 17.7689 0.0880 0.4952 0.0165

6/10/02 China1 China2 China3 China4 China5 mean 2σ % 2σ error Romania 6/10/02 Rom1 Rom2 Rom3 Rom4 Rom5 mean 2σ %2 error

RPb208/206

RPb207/206

RPb208/204 RPb207/204 RPb206/204

2.09131 2.09190 2.09147 2.09366 2.09209 0.00216 0.10340 0.0021

0.85658 0.85672 0.85662 0.85997 0.85747 0.00333 0.38817 0.0005

38.2003 38.2146 38.2040 38.0866 38.1764 0.1203 0.3152 0.0719

15.6464 15.6506 15.6475 15.6439 15.6471 0.0055 0.0350 0.0229

18.2662 18.2679 18.2666 18.1914 18.2480 0.0756 0.4141 0.0165

2.10926 2.11645 2.10669 2.11455 2.11511 2.11241 0.00842 0.39842 0.0011

0.86646 0.86563 0.7786382 0.86508 0.86564 0.86532 0.00195 0.22492 0.0002

37.9872 38.1574 38.0641 38.1447 38.1328 38.0973 0.1426 0.3744 0.0342

15.6047 15.6064 15.6077 15.6053 15.6063 15.6061 0.0023 0.0147 0.0108

18.0098 18.0290 18.0682 18.0392 18.0287 18.0350 0.0428 0.2373 0.0089

2.04393 2.04477 2.22558 2.10740 2.04441 2.09322 0.15773 7.53525 0.0011

0.83570 0.83626 0.96008 0.87529 0.83585 0.86863 0.10777 12.40675 0.0002

38.3109 38.3113 35.7150 37.4951 38.3435 37.6351 2.2632 6.0136 0.0342

15.6640 15.6684 15.4069 15.5733 15.6765 15.5978 0.2294 1.4708 0.0108

18.7437 18.7362 16.0475 17.7921 18.7553 18.0150 2.3494 13.0417 0.0089

2.09040 2.08183 2.09047 2.08310 2.08119 2.08540 0.00930 0.44608 0.0011

0.85107 0.84739 0.85109 0.84786 0.84701 0.84888 0.00405 0.47677 0.0002

38.5941 38.6429 38.6021 38.6295 38.6164 38.6170 0.0397 0.1027 0.0342

15.7129 15.7293 15.7159 15.7230 15.7163 15.7195 0.0132 0.0841 0.0108

18.4626 18.5620 18.4657 18.5443 18.5550 18.5179 0.0990 0.5346 0.0089

2.08290 2.08307 2.08284 2.08293 2.08275 2.08290 0.00023 0.01125 0.0011

0.84169 0.84176 0.84169 0.84157 0.84152 0.84165 0.00020 0.02353 0.0002

38.7931 38.7736 38.7798 38.7848 38.7790 38.7821 0.0146 0.0377 0.0342

15.6762 15.6682 15.6713 15.6703 15.6683 15.6708 0.0065 0.0416 0.0108

18.6245 18.6137 18.6187 18.6204 18.6191 18.6193 0.0078 0.0417 0.0089

a Error is reported as twice the standard deviation of NBS-981 readings taken on the corresponding day of analysis. 2σ and % 2σ values are calculated for the ammunition class.

another when isotopic measurements 207Pb/206Pb, 208Pb/204Pb, and 207Pb/204Pb are compared. The plot of 207Pb/204Pb versus 208Pb/ 204Pb presented in Supporting Information (SI 3) illustrates the separation of these two sample classes. These data are presented in tabular form in Supporting Information (SI 2). Romanian Projectiles. Viewing data by country of manufacture, data are clearly distributed, with the aforementioned exceptions, in well-defined sample clusters. The most tightly grouped of those clusters is formed by data collected from Romanian projectile Pb. The relative standard deviation (2σ) for 208Pb/206Pb measurements of Romanian samples was 0.011 25%, the smallest 5026

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of the study. Measurements 206Pb/204Pb, 207Pb/204Pb, and 208Pb/ 204Pb collected from Romanian samples are indistinguishable from data collected in the Romanian Persani mountain range and also in agreement with data collected in the northwest region of Romania (Baia Mare and Baia Borsa), which is the largest Pb ore deposit in the country (see Figure 2).29-31 This ammunition was recently (March 2002) imported into the United States in unopened (security tabs still intact) 780 round wooden cases, with (29) Downes, H. Lithos 1996, 35, 65-81. (30) Marcoux, E.; Grancea, L.; Lupulescu, M.; Milesi, J. P. Miner. Deposita 2002, 37, 173-184.

Figure 2. Selected groups of small arms projectile samples plotted with data collected from geological analysis of various Pb mining regions. The linear function connects Kazakstani and Siberian Pb; both Russian and Egyptian projectiles lie between these two sources. The ellipse defines the cluster of Romanian Pb from data collected at various Romanian mines and Romanian projectiles analyzed during this study.

hermetically sealed cans. This ammunition also exhibits physical characteristics that are unique for ammunition recently imported in this caliber such as a lacquered steel case and a mild steel core. Russian Projectiles. Russian ammunition manufactured under the brand name Wolf was actually Russian military ammunition manufactured at the Tula arsenal ammunition manufacturing plant (one of five largely self-sufficient ammunition factories in operation in Russia), Tula is located just outside Moscow. This ammunition was commercially packaged for sale in the United States. Analysis of Wolf ammunition yielded very reproducible results, with relative standard deviations (2σ) of between 0.01 and 0.02% across all measurements. A significant percentage of Russian Pb is imported from Kazakhstan, so to determine a possible source for projectile samples, it is necessary to also compare Pb mined from this exSoviet satellite state.32,33 207Pb/206Pb and 208Pb/206Pb measurements of Russian ammunition were found to be in good accord with Pb detected in aerosol particles around the city of Moscow,33 as well as Pb used in various Russian products (Figure 3).34 In both studies by Mukai, it was determined that the Pb isotope values were due to a combination of Kazakhstani and Siberian Pb (Siberia is the largest Pb-producing region in Russia).33,34 This is apparent when one views the data presented in Figure 2, where data collected from projectiles manufactured in Russia fall between data collected from Kazakhstani and Siberian Pb.35,36

Czech Projectiles. The results for the analysis of Sellier and Bellot 5.56 × 45 mm ammunition were reproducible; however, the results indicate a split between the third and fourth samples (it was determined not to render these two clusters statistically different from one another). It is clear that this variation is not due to instrumental bias since NBS 981 values were stable. As was mentioned by Koons, Stupian et al., and in the Tobin paper, often projectiles from the same box and case contain Pb produced at a different time and of different geological origins, which may be the case here.6,9,15 The Czech Republic, according to the U.S. Geological survey of 2001, mines no Pb (due to poor yield regional Pb deposits) and has not done so in the past 10 years.37 Instead, the Czechs rely solely on recycled scrap Pb from both domestic and foreign sources. For this reason, it was not expected that Czech Pb would exhibit strong regional characteristics, which has proven to be the case.38,39 Chinese Projectiles. The Chinese are very cryptic with their arms manufacturing plant designations. The only information as to the locale of manufacture was “LY” stamped on the base of the ammunition; the date of manufacture was 1991. No information on a plant associated with LY was found. Due to China’s size, and varied geology throughout its provinces, Pb isotopic character has been observed to vary drastically from province to province. Results from the analysis of Chinese Pb were most similar to Pb mined in the region of Southern China.40,41 Chinese samples

(31) Cook, N. J.; Chiaradia, M. Proc. Soc. Geol. Appl. Miner. Deposits, Rotterdam, 1997, 813-816. (32) Levine, R. M. U.S. Geol. Surv. 2001, 1-17. (33) Mukai, H.; Machida, T.; Tanaka, A.; Vera, Y. P.; Uematsu, M. Atmos. Environ. 2001, 35, 2783-2793. (34) Mukai, H.; Tanaka, A.; Fujii, T. J. Geophys. Res. 1994, 99, 3717-3726. (35) Wooden, J.; Czamanske, G.; Bouse, R.; Linkhachev, A.; Kunilov, V.; Lyul’ko, V. Econ. Geol. 1992, 87, 1153-1165.

(36) Brown, J. Econ. Geol. 1962, 57, 673-720. (37) Steblez, R. M. U.S. Geol. Surv. Rep. 2001, K1-K10. (38) Santholzer, V. Chemie 1947, 3, 18-19. (39) Moeller, P.; Dulski, P.; Gerstenberger, H. Appl. Geochem. 1998, 13, 975994. (40) Bing-Quan, Z. J. Geochem. Explor. 1995, 55, 171-181. (41) Bing-Quan, Z.; Yu-Wei, C.; Jian-Hua, P. Appl. Geochem. 1999, 16, 409417.

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Figure 3. Selected groups of small arms projectile samples plotted with data collected from geological analysis of various Pb mining regions of Russia. The ellipse defines the cluster of Russian Pb from data collected at various Russian mines, Pb in Russian products, and Russian and Egyptian projectiles analyzed during this study.

showed sample-to-sample dispersion of approximately double that of Russian Pb. South African Projectiles. The oldest Pb measured in this study was found in South African ammunition. Our measurements indicate that South African projectile Pb exhibits conspicuously low levels of radiogenic Pb. Unfortunately, of the references available, the majority offered ratios that were distinguishable to those observed during the current investigation, yet all references exhibited, to varying degrees, the characteristically low levels of radiogenic Pb.42-48 Selected samples from Reid’s 1997 study are similar to the isotopic signatures of Pb extracted from South African ammunition (Figure 2).43 Egyptian Projectiles. Data collected from Egyptian samples displayed reproducibility similar to data collected from Chinese samples. No information regarding the year of manufacture of the ammunition was available. A comparison of these data to published Pb isotopic data for this region was not conclusive. Data collected from Egyptian projectiles did cluster near Russian projectile data and Russian-mined Pb (Figure 3) and also fell between Siberian and Kazakhstani Pb (Figure 2).33-36,49-51 The Russians did supply the Egyptians with arms and ordnance during Soviet times; (42) Vlastelic, I.; Abouchami, W.; Galer, S. J. G.; Hofmann, A. W. Geochim. Cosmochim. Acta 2001, 65, 4303-4319. (43) Reid, D. L.; Welke, H. J.; Moore, J. M. Econ. Geol. 1997, 92, 248-258. (44) Manton, W. I.; Tatsumoto, M. Earth Planet. Sci. Lett. 1971, 10, 217-226. (45) Sumner, D. Y.; Bowring, S. A. Precambrian Res. 1996, 79, 25-35. (46) Foelling, P. G.; Zartman, R. E.; Frimmel, H. E. Chem. Geol. 2000, 171, 97-122. (47) Bau, M.; Romer, R. L.; Luders, V.; Beukes, N. J. Earth Planet. Sci. Lett. 1999, 174, 43-57. (48) Snodgrass, R. A. J. S. Afr. Inst.f Min. Metall. 1986, 86, 97-111. (49) Gillespie, J. G. Precambrian Res. 1983, 20, 63-77. (50) Stacey, J. R.; Doe, B. R.; Roberts, R.; Deleveaux, M. H.; Gramlich, J. W. Contrib. Mineral. Petrol. 1980, 74, 175-188. (51) Zimmer, M.; Kroener, A.; Jochum, K. P.; Reischmann, T.; Todt, W. Chem. Geol. 1994, 123, 29-51.

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however, no conclusion as to the origin of Pb contained within these projectiles can be drawn from this limited study. Projectiles Manufactured in the United States. Recycled Pb in the United States is a major contributor (67%) to Pb used in manufacturing, and the United States only imports a small percentage of Pb; therefore, it is safe to assume that Pb used for bullet manufacture would be largely composed of recycled Pb, of predominantly U.S. origin.52 Despite the fact that Pb mined in the United States exhibits varying uranogenic and thorogenic character, one cannot conclusively localize the source of U.S. Pb.11,53-60 Data collected from Pb extracted from ammunition manufactured in the United States were well separated, with Pb samples from American Eagle manufactured projectiles exhibiting relatively high levels of radiogenic Pb, specifically 206Pb, compared to both published data of U.S. Pb samples and projectile samples within the current study. When compared to previous data collected from the largest ore-producing mines in the United States, these data correlate most closely with Pb mined in the Western region of the United States; however, as previously mentioned, no conclusion can be drawn from this similarity.49,57,59,60 Both Lake City arsenal-produced ammunition and Winchester ammunition display much less unique Pb isotope ratios. (52) Smith, G. U.S. Geol. Surv. 2002, 94-95. (53) Barnes, H. L. Geochemistry of hydrothermal ore deposits; J. Wiley & Sons: New York, 1979; Chapter 1. (54) Hemming, S. R.; McLennan S. M.; Hanson, G. N. Geochim. Cosmochim. Acta 1994, 58, 4455-4464. (55) Stacey, J. S.; Moore, W. J.; Rubright, R. D. Earth Planet. Sci. Lett. 1967, 2, 489-499. (56) Alienkoff, J. N.; Walter, M.; Kunk, M. J.; Hearn, P. P. Geology 1993, 21, 73-76. (57) Doe, B. R.; Stacey, J. S. Econ. Geol. 1974, 69, 757-776. (58) Rye, D. M.; Doe, B. R.; Deleveaux, M. H. Econ. Geol. 1974, 69, 814-822. (59) Zartman, R. E. Econ. Geol. 1974, 69, 792-805. (60) Stacey, J. S.; Zartman, R. E.; Nkomo, I. T. Econ. Geol. 1968, 63, 796-814.

CONCLUSION The GV Instruments Isoprobe MC-ICPMS provided accuracy and precision comparable to TIMS results, with a 1 order of magnitude improvement over similar analysis performed using quadrupole ICPMS.14 Additionally, analyses conducted using MCICPMS required less laborious sample preparation than TIMS by removing the need for Pb purification. Moreover, the Tl spike mass bias correction implemented during this study required fewer analyses and significantly less time than the multispike mass bias correction necessary to maximize the accuracy of TIMS analysis. These results favor MC-ICPMS over TIMS for routine high-resolution isotopic analysis of large numbers of samples. Of the methods investigated, the most effective normalization and mass bias correction technique for the GV Instruments Isoprobe MC-ICPMS was the empirical 205Tl/203Tl correction followed by the exponential mass bias correction. The results of this study indicate that if one intends to compare whether two “pools” of small arms projectiles exhibit similar isotopic character, it is necessary to first determine the amount of data scatter within each sample “pool”. This was deemed necessary due to the analysis of Korean and Lake City projectiles, where sample-to-sample variation was too great for any conclusions to be drawn from the comparison of these samples to other groups of projectiles. However, data scatter within a group has also been found in the majority of classes studied to be small, on the order of 0.01-0.5% RSD (2σ), therefore providing the ability to distinguish between pools of small arms projectiles utilizing Pb isotopic analysis. As a result of the findings of this study, we must agree with Stupian et al. in stating that this technique is most useful when a comparison between two classes of multiple samples is being made. Additionally, an attempt to determine whether a single sample is a member of a group of samples will not yield a conclusion that has a great level of statistical certainty. Since these analyses were conducted with the same rigorous treatment of instrumental results that is utilized in geochemical analysis, it was possible to compare these analyses to those conducted by the geochemical community. In doing so, it has been illustrated that projectile samples from economically isolated regions of the world, such as the former Soviet Union and surrounding select former satellite states as well as South Africa

to a certain degree, retain significant regional Pb isotopic character. In countries where a large amount of Pb is imported and recycled, no regional comparisons were deemed possible. However, this does not negate the utility of using Pb isotopic analysis to distinguish between projectiles from countries where significant recycling occurs. In light of the ability of Pb isotopic analysis to provide additional independent variables for distinguishing between projectile samples, and since the need to increase the number of variables to increase the distance between classes of Pb samples is of the utmost importance, future studies should combine Pb isotopic analysis with trace elemental analysis. The selection of trace elements should be made after investigating those elements of greatest use to the geological community when determining ore genesis, while avoiding elements that are strictly controlled or undergo significant fractionation during Pb processing. Additionally, since large amounts of ammunition from the former Soviet Union and surrounding satellite states (such as from Albania, many competing plants in Russia and Bosnia Herzegovina) are now being imported into the United States, and in light of the promising results of this limited study, a large-scale study into the isotopic signatures of imported projectiles and Pb mined in the respective regions is warranted. ACKNOWLEDGMENT The assistance of Dr. John Chesley, of the University of Arizona Geochemistry Department, for his guidance in implementation of mass bias corrections is gratefully acknowledged. The instrumental assistance and support provided by Liam Murray, Alan Kirk, Darren Hutchinson, Brian Beer, Azim Kara, Peter Kerr, Cedric Marsh, Simon Meffan-Maine, Brian Moss, Harry Seed, and Patrick Turner, all of GV instruments, is also gratefully acknowledged. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review April 2, 2003. Accepted July 24, 2003. AC0301346

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