ARTICLE pubs.acs.org/ac
Development of a Dual-Isotope Procedure for the Tagging and Identification of Manufactured Products: Application to Explosives ngel Rodríguez-Castrillon, Mariella Moldovan, and J. Ignacio García Alonso* Isabel Carames-Pasaron, Jose A Department of Physical and Analytical Chemistry, University of Oviedo, Julian Clavería 8, 33006-Oviedo, Spain
bS Supporting Information ABSTRACT: A novel chemical tagging approach, based on a dual-isotope procedure, is presented. The method has been applied to explosives tagging. The method is based on the addition to the explosive of two enriched isotopes of the same element, which may be already present within it, at a given molar ratio. This dual-isotope approach will give a unique fingerprint to the tagged explosive. Further, the authentication of the tagged explosive or its residues will be obtained by comparison of the ratio of molar fractions experimentally measured by inductively coupled plasma mass spectrometry (ICP-MS) with the molar fraction ratio of the tagging mixture. The novelty of this tagging method relies on working with isotope abundances and molar fraction ratios instead of the classical isotope ratios, and this fact constitutes the strong point of the described approach since the molar ratio is not affected by physical, chemical, or biochemical processes, and it is also not disturbed by environmental contamination with the natural abundance element. Furthermore, the use of molar fraction ratios overcomes the nonhomogeneous distribution of the tagging element within the explosive. As the tagging element can be present at trace or ultratrace levels, a very small amount of enriched isotopes needs to be added, denoting a low cost solution. Also, the use of enriched stable isotopes of nontoxic elements will have negligible health effects or affect the environment.
T
and suitable for detection using standard analytical instrumentation. Most chemical tags employed today do not comply with all the cited requirements. Some of them are destroyed by combustion or other processes (organic compounds, isotopically labeled compounds, DNA), and some others are easily detected (fluorescent inks) or are affected by environmental contamination (for example, rare earth elements). A field in which the tagging of products has been proposed is the case of explosives.4 Explosives are legally used in military and industrial activities but, also, illegally in terrorist and criminal actions. Forensic analysis of explosives deals with both the identification of unreacted explosives (preblast) and postblast residues.5 The identification of preblast explosive is performed to prove its possession or its intended use. On the contrary, the analysis of postblast residues can provide various levels of information to help in solving and prosecuting criminal bombing cases. Normally, the type of explosive employed in a bombing incident can be ascertained using the detection techniques cited above but, unfortunately, other important forensic information such as the identity of the manufacturer, date of manufacture, or batch number cannot be easily discovered. This information would be very useful in any criminal investigation. According to the ISO 9000:2000 guidelines,6 the traceability of a product is defined as the “ability to trace the history, application or location of manufactured and distributed products”. Therefore,
here is an increasing risk that manufactured products are not what we think they are (and, we are paying for). Nowadays, almost any goods can be counterfeited. Examples include medicines, cosmetics, fragrances, bags, clothes, foodstuff, auto parts, etc.1 One way of providing some protection against counterfeiting is by tagging the manufactured goods. Tagging approaches can be divided into overt and covert technologies. Overt tagging technologies are intended to enable the user to verify the authenticity of a product. Water marks, holograms, and laser engraving are all overt tagging technologies.2 On the contrary, covert technologies are not intended for the end user. They are invisible to the naked eye and their main purpose it to enable the manufacturer to distinguish between real and fake products. Several covert tagging technologies have already been described, and most of them are proprietary. For example, chemical compounds which are fluorescent when illuminated with ultraviolet light have been used for the covert tagging of inks. Magnetic inks, DNA coding, and isotopically labeled compounds are alternative covert tagging techniques.3 Chemical tags can be considered sophisticated covert technologies since, at low concentration levels, advanced scientific knowledge and instrumentation is required for product verification. To be used as anticounterfeit or forensic tools, chemical tags have to fulfill several requirements. Most importantly, chemical tags should not: (i) affect human health or the environment; (ii) change the properties of the product; (iii) be destroyed by physical, chemical, or biochemical processes; and (iv) be affected by adulteration, contamination, or dilution within the environment. Additionally, chemical tags should be easy to apply, cheap, r 2011 American Chemical Society
Received: July 28, 2011 Accepted: November 4, 2011 Published: November 04, 2011 121
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traceability refers: (i) to the ability of a manufacturer to trace a product through its processing procedures; and, (ii) to also have the ability to retrace a product back to the manufacturer. In most cases, product traceability is monitored by the use of a part or lot number. Very few analytical techniques have been described for the tagging of explosives for traceability purposes. The marking of the explosives themselves or suitable chemicals contained within them has been proposed to further identify the origin of explosives in forensic investigations. The different technologies proposed for the marking of explosives include particulate, chemical, isotopic, and biological taggants.5 According to Husakova et al.,7 such “markers” should possess a high vapor pressure to survive the detonation and allow the identification of the explosive employed. Initially, multilayered plastic chips containing fluorescent dyes and magnetic layers were proposed.8 Other tagging technologies include a combination of rare earth elements added to the explosive.9 Phosphorescent10 and luminescent11 compounds have also been proposed. Most of the tagging technologies in use are proprietary and no complete information about their fundamentals can be obtained. Isotopic tags have been proposed in several patents for the tagging of different products including explosives. For any given element, the relative abundance of its naturally occurring isotopes is almost constant in nature giving rise to the so-called natural isotopic abundance pattern. Obviously, the addition of one enriched stable isotope of an element, which may be already present in the explosive at natural isotopic abundance, will provide a unique isotopic signature to the tagged product. As a result, and for the purpose of traceability, the comparison of isotope ratios in the unidentified and in the tagged products with a known isotopic composition will be sufficient. The addition of a single enriched stable isotope and the measurement of the resulting isotope ratios have been proposed, for example, for the tagging of a range of products including explosives.12,13 A similar isotopic procedure was proposed by Lin et al.14 for the tagging of inks. Unfortunately, these single isotopic tags are particularly affected by contamination by the same element of natural isotopic abundance; are easily tampered with; and, are not suitable for coding. In this paper, we present a novel dual-isotope tagging technique which fulfills all the above cited requirements, and also allows coding of the tagged product. The proposed method is based on the addition to the product of two enriched isotopes of the same element at a given molar ratio. To demonstrate the applicability of this dual-isotope tagging procedure, the tagging of black powder has been investigated.
Table 1. Isotopic Abundances, Expressed in Atom Percent, of the Natural and Enriched Tin Used for Tagging the Black Powder isotope
Sn nat
Sn117
Sn119
112
0.97
0.01
0.00
114 115
0.66 0.34
0.01 0.01
0.00 0.00
116
14.54
1.68
0.01
117
7.63
89.85
0.11
118
24.21
7.18
14.33
119
8.62
0.73
82.40
120
32.65
0.51
3.13
122
4.61
0.02
0.01
124
5.78
0.01
0.01
with a planar membrane sampling device (MSD) (Global Fia, Fox Island, WA, U.S.A.). The separating membrane was a 38 μm microporous PP/PE/PP trilayer membrane from Celgard (Charlotte, NC, U.S.A.). Reagents and Materials. All reagents used were of analytical grade. Ultrapure water was obtained from a Milli-Q system (Millipore, Molsheim, France). Tin certified standard solution (1000 mg L1) was purchased from Merck (Darmstadt, Germany). Hydrochloric acid (Merck) was purified by sub-boiling distillation prior their use for sample preparations and spikes digestions. Generation of the tin hydride was achieved using 1% w/w sodium borohydride (Merck) stabilized with 0.64 M sodium hydroxide (Merck). Enriched 117Sn (89.8%) and 119Sn (82.4%) were supplied in metallic form by Isoflex (San Francisco, CA, U.S.A.). The solid tracers were dissolved in HCl and made up to volume with MilliQ water. Their isotope composition was determined by continuous nebulization ICP-MS using a natural abundance tin solution for mass bias correction.15 Table 1 shows the isotope composition of both isotopically enriched tracers in comparison with natural abundance tin. These enriched Sn solutions contained about 1 mg of metal per gram of solution and were employed to prepare mixed dual-isotope solutions suitable for labeling. For measurement purposes only isotopes 117, 118, 119, 120, 122, and 124 were considered. An aluminum, potassium perchlorate, and sulfur black powder (BP) mixture has been selected for the tagging experiments. Black powder was provided by Pirotecnia Pablo (Cangas del Narcea, Asturias, Spain). Procedures. Measurement of Tin Isotope Ratios by Hydride Generation ICP-MS. To generate tin hydride, the sample, dissolved in 0.64 M HCl, was continuously mixed, using a T piece, with the 1% w/w NaBH4 solution in 0.64 M sodium hydroxide. Polyvinyl chloride (PVC) tubing of 1.02 mm i.d. and a fourchannel peristaltic pump were employed. Then, the mixture reached a PTFE (polytetrafluoroethylene) reactor, and the volatile tin hydride was separated from the liquid solution within the help of a planar membrane sampling device (MSD). Within the MSD the liquid and the carrier gas (0.1 L/min Ar) circulated in opposite directions through the two inner channels. Finally, the liquid was pumped to waste meanwhile the tin hydride was transported to the ICP-MS. An additional argon makeup flow (1.2 L min1) was employed for maximum sensitivity in the ICPMS. The hydride generation system used is illustrated in Figure 1, and the chemical and ICP-MS conditions are summarized in Table 2.
’ EXPERIMENTAL SECTION Instrumentation. An Agilent 7500ce (Tokyo, Japan) inductively coupled plasma mass spectrometer (ICP-MS) was used in this work. The ICP-MS instrument was operating at a RF power of 1500 W. A Meinhard nebulizer and a Scott double-pass quartz spray chamber cooled down to 2 °C were employed for the sample introduction by continuous nebulization. The torch position and ion lens voltage settings were optimized daily for optimum sensitivity with a 10 ng g1 Li, Co, Y, Tl, Ce solution in HNO3 1% (w/w). Alternatively, tin measurements in the samples have been carried out also by continuous hydride generation. The gaseous stannane was continuously generated by NaBH4 reduction of tin in the acidified sample solution (0.64 M HCl), and separated 122
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Sn and 119Sn) will be added to the target product. Each enriched isotope will possess a certain isotope distribution of its stable isotopes which will be known when preparing the mixture (see Table 1). This mixture of the enriched isotopes will have a molar ratio, NSn119/NSn117 which will need to be measured experimentally by ICP-MS. The measurement of the molar ratio in the tagging mixture will be performed in the same way as in the sample as explained below. The absolute amount of the two enriched isotopes added, NSn117 and NSn119, does not need to be known. Also, the product may contain an unknown and variable amount of the same element of natural isotope abundance, NSn‑nat so, the total amount of the element in the tagged product, Ns, will be:
Figure 1. Scheme of the hydride generation system.
Table 2. Operating Conditions of the Hydride Generation ICP-MS Unit chemical conditions HCl 0.64M 1% w/w NaBH4/0.64 M NaOH
Ns ¼ NSn-nat þ NSn117 þ NSn119
ICP-MS Agilent 7500ce parameters
Please note that NSn‑nat is unknown and both NSn117 and NSn119 can be unknown. Only NSn119/NSn117 is known. The mass balance in eq 1 can be performed also for all and each of the isotopes of the element in the tagged product. For example, for a given isotope i the mass balance will be
1.2 L 3 min1
make-up gas flow
0.1 L 3 min1
carrier gas flow nebulizer pump
0.4 rps
QP focus
0V
Oct bias
6 V
QP bias
8 V
i i i Nsi ¼ NSn -nat þ NSn117 þ NSn119
2s per isotope
peaks per point
3
replicates
5
Ns Ais ¼ NSn-nat AiSn-nat þ NSn117 AiSn117 þ NSn119 AiSn119
ð3Þ
where Ais, AiSn‑nat, AiSn117, and AiSn119 are the isotope abundances for isotope i in each isotopic pattern (Table 1). By dividing eq 3 by eq 1, we obtain
Product Labeling and Identification. A given mixture of, at least, two enriched isotopes of the same element (in this case, 117 Sn and 119Sn) are added to the target product. The isotopic composition of the element in the tagged product is then accurately measured by ICP-MS. For this purpose, all isotope ratios, R, relative to the most abundant isotope of the natural abundance element are measured, corrected for mass bias15 and the isotope abundances calculated using: Ai ¼
ð2Þ
Equation 2 can be expressed as a lineal combination of total amounts and isotope abundances as
ICP-MS Agilent 7500ce data acquisition integration time
ð1Þ
Ais ¼ xSn-nat AiSn-nat þ xSn117 AiSn117 þ xSn119 AiSn119 ð4Þ where xSn‑nat, xSn117, and xSn119 are the molar fractions of each isotopic pattern contributing to the observed final isotopic pattern in the product. If the element used as tag contains n stable isotopes (n g 3), we can define one equation of isotope abundances and molar fractions (eq 4) for each isotope. For tin, this set of equations can be expressed, in matrix notation, as
Ri n
∑ Ri
i¼1
2
2 3 A117 A117 s Sn-nat 6 118 7 6 118 6 As 7 6 ASn-nat 6 119 7 6 119 6 As 7 6 A -nat 6 120 7 ¼ 6 Sn 6 As 7 6 A120 6 6 Sn-nat 7 6 A122 7 6 A122 4 s 5 4 Sn-nat 124 As A124 Sn-nat
From these data, the molar fractions xSn‑nat, xSn117, and xSn119 (Sn-nat = natural abundance Sn, Sn117 = tracer enriched in 117 Sn, and Sn119 = tracer enriched in 119Sn) and their uncertainties, are determined by multiple linear regression using the data from Table 1 as reference. The traceability of the product will be based on the comparison of the ratio of molar fractions experimentally measured, xSn119/xSn117, with the molar ratios prepared during the course of the tagging process, NSn119/ NSn117. The xSn119/xSn117 ratio will be independent from the absolute value of the molar fraction of the natural element in the product, xSn‑nat, and so, will not be affected by the concentration of natural element in the product being that constant or variable. Obviously, by changing the original NSn119/NSn117 molar ratio different tags can be prepared using the same isotopes. Also, by changing the isotope compositions of the enriched isotopes different tags can be prepared.
A117 Sn117 A118 Sn117 A119 Sn117 A120 Sn117 A122 Sn117 A124 Sn117
2 3 3 A117 e117 Sn119 6 118 7 7 2 3 A118 6e 7 Sn119 7 x 6 119 7 7 Sn nat 7 A119 e 7 6 7 Sn119 7 6x 7 þ 6 6 120 7 Sn117 4 5 6 7 7 A120 Sn119 7 6e 7 xSn119 122 7 122 7 6 ASn119 5 4e 5 A124 e124 Sn119
ð5Þ As n = 6, the system of equations is over determined, and we can include an error vector in eq 5. The values of the unknown xSn‑nat, xSn117, and xSn119 and their uncertainties are then obtained by multiple linear regression (e.g., using the function LINEST in Excel) where the matrix of isotope abundances is that given in Table 1 (excluding masses 112, 114, 115, and 116). The measured xSn119/xSn117 molar fraction ratio in the sample should be identical to the measured xSn119/xSn117 molar fraction ratio in the tags and independent of the value of xSn‑nat obtained. Please note that xSn119/xSn117 = NSn119/NSn117 (the molar fraction ratio is equal to the molar ratio).
’ RESULTS AND DISCUSSION Theoretical Background. A given mixture of, at least, two enriched isotopes of the same element (in this case isotopes 123
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isotopically enriched tin added was approximately equal to the original tin content in the samples. The black powder was tagged at three different molar ratios in order to simulate tagging and codification for different production batches. The employed NSn119/NSn117 tagging mixtures are indicated in Table 3, and were measured both by continuous nebulization and hydride generation on the pure spike mixture. As can be observed, good agreement was observed for the values obtained by both measurement techniques. For the real samples only hydride generation was employed in order to avoid introducing large amounts of aluminum into the plasma which generated occlusion problems in the sampler and skimmer cones. Three aliquots of 20 g of black powder were mixed with 16 mL of each tags prepared in acetone, at a concentration level of 250 μg L1. After homogenization the acetone solvent was evaporated to dryness, and ten portions of the differently tagged black powders were analyzed. For each analysis, ca. 0.1 g of powder was mixed with 20 mL of 2% (v/v) HCl. After 20 min in an ultrasonic bath, the solutions were filtered, and the resulting filtrate was directly analyzed by ICP-MS using hydride generation. Additionally, and in order to confirm the persistence of the dualisotope tag after detonation, three cartridges of 2 g each of the three tagged black powders were exploded. The remaining residues for each blasted cartridge were collected and subjected to the same soft acid extraction and sample preparation than the corresponding original black powder. For security reasons, all blasting experiments were carried out under the assistance of the company Pirotecnia Pablo (Cangas del Narcea, Spain). Figure 3 shows the found xSn119/xSn117 molar fraction ratios, and their uncertainties, as a function of the normalized xSn‑nat present in the analyzed samples. Average numerical data are presented in Table 3 for the different samples before and after detonation. The horizontal lines in the figure indicate the original xSn119/xSn117 molar fraction ratios of the tags whereas the circles and triangles correspond to the xSn119/xSn117 measured molar fraction ratios within the tagged black powder and in the postblast residues, respectively. On the basis of the results presented in this figure, it can be concluded that all samples were unambiguously identified after detonation. As can be observed in Table 3, the ratio of molar fractions between the tagging isotopes Sn119 and Sn117 in the three tagged black powders were identical to those in the prepared mixtures and, last but not least, the added isotope signature was maintained after detonation. It is worth stressing that the variable content of natural abundance tin in the samples (xSn‑nat), especially in the postblast residues, did not influence positive identification of the tagged black powders. Here it should be pointed out that an ideal tagging approach must be able to cope with the heterogeneity of the product since covert chemical tagging usually resorts to a naturally present impurity in order not to modify the properties of the product. Of course, impurities over a nonhomogeneous solid product will likely be randomly distributed. Figure 3 also compares the effect of the variability of the concentration of the natural abundance element when using the traditional (single isotope ratio) and the proposed (molar fraction ratio) approaches for one of the tagged black powder (mixture A). For the studied samples, the xSn‑nat variation in the preblast samples denotes the nonhomogeneous distribution of the natural tin in the black powder, meanwhile the increased xnat values in the postblast samples indicate further contamination of the sample with tin of natural isotope abundance. As the xSn‑nat
Figure 2. Effect of dilution with tin of natural isotope abundance on the molar fraction ratios.
When the number of isotopes measured is not equal to the total number of isotopes of the element, as it is the case here, the sum of all molar fractions is larger than 1. However, the ratio of molar fractions xSn119/xSn117 is independent of the number of isotopes measured. For tin only isotopes 117, 118, 119, 120, 122, and 124 were measured and the observed molar fraction for a pure natural abundance tin solution was xSn‑nat = 1.2. So, all values of xSn‑nat were normalized by dividing them by 1.2. Comparison of Isotope Ratios and Molar Fraction Ratios. The key idea behind this approach is to work with isotope abundances and molar fraction ratios instead of the classical approach using only isotope ratios.1214 To demonstrate this behavior, three tagging mixtures containing NSn119/NSn117 molar ratios approximately 0.3, 1, and 3 were prepared. Then, serial dilutions of the tags with natural abundance tin were made while maintaining a constant total tin concentration. The tin isotope abundances at masses 117, 118, 119, 120, 122, and 124 for all these solutions were measured by the quadrupole ICP-MS instrument. Equation 5 was applied, and the obtained results are illustrated in Figure 2. As can be observed, the molar fraction ratios xSn119/ xSn117 obtained for all three tagging mixtures were truly independent from the increasing amount of natural abundance tin. Only at high xSn‑nat values the measurement uncertainties for the molar fraction ratios xSn119/xSn117 increased, as it is observed in the graph. So, it is demonstrated that this procedure is able to produce tagging mixtures which are not affected by variable contamination with the same element of natural isotope composition. However, when the tagging mixture is diluted to a very high degree with the natural abundance element more precise isotope ratio measurements will be required. So, the quadrupole ICP-MS instrument used here will need to be substituted, for example, with a multicollector ICP-MS instrument. Tagging of Black Powder. For the tagging of explosives it is important to ensure that (i) the tag will survive detonation and (ii) that it will not be affected by the likely contamination that could take place during and after explosion. To demonstrate the applicability of this dual-isotope tagging procedure for the labeling of explosives, the tagging of black powder with 117Sn and 119Sn was evaluated both before and after detonation. Tin was found as impurity at trace level within the black powder, and therefore it was selected as tagging element. The amount of 124
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Table 3. Molar Ratios, NSn119/NSn117, Obtained for the Enriched Mixtures A, B, and C, and Molar Fraction Ratio xSn119/xSn117 Found in the Tagged Black Powder Samplesa parameter
mixture A
mixture B
mixture C
continuous nebulization
2.656 ( 0.015
0.8576 ( 0.0024
0.2864 ( 0.0005
hydride generation
2.645 ( 0.013
0.8566 ( 0.0016
0.2863 ( 0.0005
preblast black powder (n = 10)
2.638 ( 0.023
0.8584 ( 0.0041
0.2872 ( 0.0011
postblast residues (n = 3)
2.671 ( 0.030
0.8625 ( 0.0115
0.2932 ( 0.0085
molar ratio NSn119/NSn117
molar fraction ratio xSn119/xSn117
a
Uncertainty is given as one standard deviation.
to be added to the product will be very small, representing a lowcost solution. In the example described in the manuscript, the amount of the two enriched isotopes added was very low (200 ng of enriched Sn per gram of black powder). The cost of the spikes was ∼7 U.S. $/mg, so we can calculate that the price of tagging 1 kg of black powder would be ∼1.4 U.S. $. Obviously, this amount can be reduced if less amount of tagging material is used. Additionally, the small amount of added dual-isotope tag will represent a negligible contribution to the isotope abundances of the tagging element in nature. Finally, a wide range of enriched stable isotopes could be selected for dual isotope tagging. These stable isotopes can be selected specific for each product to have a negligible effect on human health and on the environment.
’ ASSOCIATED CONTENT
bS Figure 3. Molar fraction ratios xSn119/xSn117 found in the tagged black powder (black, gray and white circles) and in the postblast residues (corresponding triangles). The horizontal lines represent the xSn119/ xSn117 molar fraction ratios in the tags added to the black powder, left axis. Measured 119Sn/117Sn isotope ratios for mixture A before and after detonation (gray squares and triangles), right axis.
molar fraction changed in the samples so did the 119Sn/117Sn isotope ratios measured in the tagged product (right axis in Figure 3). Isotope ratios close to 1.5 were measured in the detonation residues while values of the order of 2.1 were measured before detonation. So, the classical approach would be unable to identify the tagged sample unambiguously. Here it must be mentioned that double isotope ratio plots could have been employed. In this case, theoretical dilution curves will need to be computed.
Supporting Information. A list of the terms used, with their associated definitions, is provided as supplemental material (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +34 985103484. Fax: +34 985103125. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by the MICINN (project CTQ2009-12814) and the FICYT (project PCTI-IB09-089). We thank Pirotecnia Pablo (Cangas del Narcea, Spain) for the supply of the black powder sample. ’ REFERENCES (1) Anti-counterfeit technologies for the protection of medicines. IMPACT-WHO report, 2008. (2) Lehtonen, M.; Staake, T.; Michahelles, F.; Fleisch., E. Workshop on RFID Security—RFIDSec 2006, 2006 (3) Bernardin, C.; Emmerecht, E.; Delaigl, J.-F.; De Coninck, J.; Cornelis., J. Belgian Science Policy—Report Project PA-11, 2006 (4) Taggants in Explosives, Congress of the United States, Office of Technology Assessment, 1980. (5) National Research Council, Containing the Threat from Illegal Bombings, 1998. (6) ISO 2000. Quality management systems—Fundamentals and vocabulary. European Standard (EN ISO 9000:2000, Point 3.5.4). Committee for Standardisation, Brussels, Belgium. (7) Husakova, L.; Sramkova, J.; Stankova, J.; Nemec, P.; Vecera, M.; Krejcova, A.; Stand, M.; Akstein, Z. Forensic Sci. Int. 2008, 178, 146–152.
’ CONCLUSIONS In conclusion, dual-isotope chemical tags constitute a novel and unique fingerprint approach with undeniable advantages over standard procedures. The result is secure and difficult to be copied, as the original isotope compositions used (Table 1) will not be disclosed. By controlling the selected tagging element, tagging isotopes and the molar fraction of the tagging mixture, a wide range of tagging labels can be generated. For tagging-based solutions the cost per tag is an important factor and will limit the products with which they can be used. In the proposed dualisotope technology the tagging element will be present at trace or ultratrace levels, and therefore the amount of enriched isotopes 125
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(8) Schaerer, J. The Marking of Explosives for the Purpose of Their Identification in Switzerland. Ad Hoc Group of Specialists on the Detection of Explosives; ICAO: Montreal, Canada, 1994. (9) Anderson II, D. K.; Gonzalez, M. E.; Valenti, N. P. Tagging chemical compositions. U.S. Patent US5,677,187, 1997. (10) Frederick, M. R.; Robert, C. M. Phosphor identification method, particularly adapted for use with explosives, for providing a distinctive information label. U.S. Patent US4,013,490, 1997. (11) Osvath, P.; Brown, M.; Murray, C.; Charles, J. Explosive tagging. Intl. Patent WO2008138044A1, 2008. (12) Welle, R. P. Isotopic taggant method and composition. Intl. Patent WO9743751, 1997. (13) Welle, R. P. Fragmented taggant coding system and method with application to ammunition tagging. U.S. Patent US7,112,445, 2006. (14) Lin, L.; Helfrick, J.; Vasudevan, S.; Bell, L. A.; Wisnosky, M. G. Methods for tagging and authenticating inks by using compositions. U.S. Patent US2007111314, 2007. (15) Rodríguez-Castrillon, J. A.; Moldovan, M.; Ruiz Encinar, J.; García Alonso, J. I. J. Anal. At. Spectrom. 2008, 23, 318–324.
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