Ammunition encoding by means of co-doped luminescent markers

Accepted Manuscript Ammunition encoding by means of co-doped luminescent markers

M.A.M. Lucena, A.M. Arouca, M. Talhavini, S. Alves-Júnior, I.T. Weber PII: DOI: Reference:

S0026-265X(18)30685-4 doi:10.1016/j.microc.2018.09.013 MICROC 3361

To appear in:

Microchemical Journal

Received date: Revised date: Accepted date:

5 June 2018 13 September 2018 14 September 2018

Please cite this article as: M.A.M. Lucena, A.M. Arouca, M. Talhavini, S. Alves-Júnior, I.T. Weber , Ammunition encoding by means of co-doped luminescent markers. Microc (2018), doi:10.1016/j.microc.2018.09.013

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ACCEPTED MANUSCRIPT Ammunition Encoding by Means of co-doped Luminescent Markers M.A.M. Lucena,a,b,c A.M. Arouca,b,d M. Talhavini,e S. Alves-Júniorc, I.T. Weber a,b* a

PGMTR – CCEN, Universidade Federal de Pernambuco – UFPE, 50740-540, Recife – PE, Brazil

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BSTR, Departamento de Química Fundamental – DQF, Universidade Federal de Pernambuco – UFPE, 50740-540 Recife –

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LIMA, Instituto de Química, Universidade de Brasília – UnB, 70904-970, Brasília – DF, Brazil

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PE, Brazil d

Intituto Federal de Brasília – IFB, Campus Samambaia, 72302-300, Brasília – DF, Brazil.

Instituto Nacional de Criminalística, Polícia Federal, SAIS Quadra 07 Lote 23, 70610-200 Brasília, DF, Brazil.

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Corresponding author: LIMA, Instituto de Química, Universidade de Brasília-UNB, P.O. Box 04478, 70904-970 Brasília,

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Brazil. Tel.: +55 61 3107 3898.

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E-mail addresses: [email protected], [email protected] (I.T. Weber).

ACCEPTED MANUSCRIPT Abstract: In forensics, the identification of gunshot residues (GSR) is a crucial point in firearm crime investigations. However, there is a lack of analytical methodologies to characterize the residues produced by non-toxic ammunition (NTA). To overcome this drawback, researchers have proposed the introduction of luminescent and chemical markers into ammunition. Luminescent markers, besides overcoming prob-

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lems of NTA analysis, aid to identify and collect GSR by direct visualization of luminescent residues

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under UV radiation. Furthermore, the development of new markers with unique compositions has

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opened new perspectives about encoding ammunition. In this work, we propose the use of eight codoped lanthanide metal-organic frameworks (Ln-MOF) [Y1-XLnX(BTC)] (wherein BTC = trimesic acid)

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containing discrete concentration levels of Eu3+, Tb3+, Sm3+ and/or Yb3+ ions, as chemical barcodes for

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ammunition. To assess the method efficiency, we performed blind tests in which neither the shooters and the analyst had knowledge about the marker present in each cartridge that was fired. After shots, the

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residues were collected and analyzed by scanning electron microscopy coupled with energy dispersive

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spectroscopy (SEM/EDS) and video spectral comparator (VSC) to identify the marker used. As result, all of the markers were correctly identified. In addition, correlation between the residues collected at

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different locations (as hands, firearm and shooting area) was possible. Therefore, the encoding method

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proposed proved to be a powerful analytical tool for firearm crime investigations, providing a level of

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information that cannot be achieved with the current methodologies.

Keywords: Gunshot residue, Luminescent marker, Non-toxic ammunition, Ammunition encoding process, Metal-Organic Frameworks

ACCEPTED MANUSCRIPT 1. Introduction When a firearm is fired, products from the primer detonation, from the gunpowder combustion, besides other elements from the cartridge and the weapon itself, create gunshot residues (GSR).[1–4] At the shooting, these residues disperse and can settle on any surface in the shooting area, including the shooter. Thus, the GSR identification provides powerful information that can help, for example, to iden-

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tify suspects, to confirm (or deny) a suicide hypothesis, to estimate firing distance, to identify bullet

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holes, as well as to recreate a crime scene.[1–7] That is why GSR identification is so important in foren-

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sics.

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Conventional ammunition primers are composed of Sb, Ba and Pb salts, and after shots, spheroidal particles containing these elements are produced.[2–4,8] Scanning electron microscopy coupled with

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energy dispersive spectroscopy (SEM/EDS) is the analytical technique selected to unequivocally char-

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acterize the GSR.[8] However, with the development of lead-free ammunition (or non-toxic ammunition, NTA), this scenario with well-established protocols changed since the conventional techniques

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proved inefficient in characterizing this type of residues.[9] Given this, the development of new analyti-

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cal tools for GSR identification has become essential.

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Some researchers have proposed the introduction of chemical or luminescent markers into ammunition. Even before the NTA development, the use of a series of chemical elements in certain propor-

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tions as way to identify and trace ammunition origin had been proposed.[10] More recently, the introduction of gadolinium as a chemical marker in NTA in several European countries has been reported.[11] However, despite uncommon, the Gd3+ does not enjoy of the luminescent properties presented by other lanthanide ions used in luminescent markers (as Eu3+, Tb3+ Dy3+ and Sm3+), so it can be used only as a chemical taggant (not a luminescent one).[12–17] The great advantage of luminescent markers compared to the only-chemical markers is that they allow visual identification of the luminescent residues (LGSR) directly in loco, using only an UV lamp.

ACCEPTED MANUSCRIPT This feature simplifies the analysis of the suspect and the crime scene, ensures a more efficient collection of the residues and, consequently, its lab analysis.[16,18] Furthermore, since lanthanide ions are uncommon, these materials can act as both luminescent and chemical markers.[12–20] The simplicity of the proposed method has increased interest in luminescent markers, stimulating the development of new materials aiming to create a method to encode ammunition.[14,15,17,21]

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Basically, this proposal involves the development of markers with unique chemical compositions (ob-

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tained by introducing small amounts of different lanthanides into a stable matrix) to act as a sort of am-

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munition barcode. In turn, these codes could mark ammunition batches and help to trace the ammuni-

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tion’s origin. Using up to 10 elements and 3 discrete concentration levels (e.g., 3, 6 or 9 %) in a matrix, for example, more than 1 million unique codes could be created,[10] and this number becomes even

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greater considering different matrices. In the case of the luminescent markers, however, it is important that the introduction of these new elements not affect the luminescence, the thermal and chemical stabil-

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ity and, consequently, the efficiency as luminescent markers for GSR identification.

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Among the materials tested as luminescent markers for GSR,[13–24] lanthanide metal-organic

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frameworks (Ln-MOFs) are noteworthy for having high thermal and chemical stability, besides high luminescence, producing highly luminescent GSR.[13,14,16–18,21,22,24] Within MOF-based markers,

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the [Eu(BTC)] (wherein BTC = trimesic acid),[22] isostructural to the MIL-78,[25] is very promising

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because, in addition to its high performance as a luminescent marker, it is non-toxic.[22] The present work proposes a method for encoding ammunition using luminescent and isostructural co-doped MOFs [Y1-XLnX(BTC)] (Ln = Eu3+, Tb3+, Sm3+ and/or Yb3+) with unique compositions. Thus, besides overcoming the challenge brought by the lead-free ammunition, it is expected to offer a method able to provide a level of information that cannot be achieved with the methodologies currently used in GSR analysis. It is important to highlight that these co-doped markers can be used in

ACCEPTED MANUSCRIPT other applications beyond the proposal here, for example, as security features in documents[26] or in high added-value products, among others.

2. Materials and Methods

(wherein Ln = Eu3+, Sm3+, Tb3+ and/or Yb3+, BTC = trimesic acid, and X = 0,05 – 0,2) was

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xLnx(BTC)]

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2.1 Synthesis and characterization of the MOFs [Y1-xLnx(BTC)]. A set of eight MOFs [Y1-

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synthetized by a microwave-assisted hydrothermal method. For this, the oxides Y2O3 (Alfa Aesar, 99,99%), Eu2O3 (Sigma Aldrich, 99,5%), Sm2O3 (Sigma Aldrich, 99,9%) and Yb2O3 (Sigma Aldrich,

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99,99%) were used to prepare their respective nitrates. The TbCl3·(H2O)6 (Alfa Aesar, 99,9%) and the

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ligand BTC (Sigma Aldrich, 95%) were used without any treatment. For synthesis, 0,35 mmol of the metal, 0,35 mmol of the ligand and 5 mL of deionized water

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were added to a 10-mL quartz reactor. Reaction was performed in a CEM Corporation microwave (Dis-

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cover Proteomics®) with maximum power of 100 W, for 20 minutes, 150°C and autogenous pressure.

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The powder obtained was washed with deionized water and ketone, dried at 75°C for 12 h, and characterized by X-Ray Diffraction (XRD, Bruker D8 Advanced), Photoluminescence Spectroscopy (PLS,

as-synthetized

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The

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Spectrofluorometer Fluorolog Horiba Jobin Yvon FL3-22) and SEM/EDS (FEI Quanta 200 FEG).

[Y0,90Eu0,05Sm0,05(BTC)]

MOF

(M3),

were

[Y0,95Eu0,05(BTC)]

[Y0,80Eu0,10Sm0,10(BTC)]

(M1), (M4),

[Y0,90Eu0,10(BTC)] [Y0,95Tb0,05(BTC)]

(M2), (M5),

[Y0,90Tb0,10(BTC)] (M6), [Y0,85Yb0,10Tb0,05(BTC)] (M7) and [Y0,80Yb0,10Tb0,05Eu0,05(BTC)] (M8). 2.2 Test of encoding ammunition (blind tests). To evaluate the potential of these MOFs as luminescent markers and to perform the test of encoding ammunition, the MOFs were individually added to the gunpowder (5 wt%) of 9mm non-toxic ammunition produced by CBC®, as previously described in ref [15]. Three ammunition cartridges were prepared containing each marker. To keep the

ACCEPTED MANUSCRIPT composition unknown, each set of three ammunition received a code chosen by a volunteer, and this information was kept in a sealed envelope until the end of the experiments. Then, a shooter was instructed to enter in the shooting range alone, choose one of the sets of ammunition, indicate its code on a form and perform the shots. This form was also kept in a sealed envelope. After the shots, three analysts (to avoid bias) entered the shooting range with an UV lamp (254 nm) and collected the LGSR on

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the floor, shooter’s hands and firearm, using stubs covered with double-sided adhesive conductive car-

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bon tape. The stubs were identified with the number of the shot and the collection place (e.g., shot 1 –

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hands, firearm or floor). The same procedure was followed by eight different shooters and by the analysts. All shots were performed at the indoor shooting range in the ballistics service of the Brazilian

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Federal Police using different 9mm pistols. Each pistol was used just once to avoid contamination; and

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they had not been previously cleaned to provide a more realistic scenario. Once collected, the residues were analyzed by Video Spectral Comparator (Foster & Freeman

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VSC® 6000/HS) under UV-C radiation (254 nm) to acquire images and emission spectra and by

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SEM/EDS (FEI Quanta 200 FEG) to analyze the chemical composition. In the SEM analysis, a manual scanning was performed, where particles with morphology similar to the pure markers were searched

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and analyzed. When the particle had no rare earth element, the EDS acquisition was stopped, and anoth-

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er particle was searched and analyzed. The same procedure was repeated until 5 particles with rare earth elements were found and analyzed. An average of 5 spectra by stub was acquired to estimate the metal

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content. Based in the results, the analysts tried to identify the markers. The analysts’ conclusions were recorded and kept in another sealed envelope. At the end of the experiments, the 17 envelopes were opened (in the presence of 5 persons) to confront the shooters' choices, the conclusion of the analysts and the coding system of the volunteer.

ACCEPTED MANUSCRIPT 3. Result and Discussions 3.1 MOFs characterization. It is known that water molecules at the lanthanide coordination sphere can quench the luminescence of a material[27] and reduce its thermal stability, consequently limiting its use as luminescent marker for GSR. For this reason, it is important to develop markers without coordinated water molecules, such as the MIL-78 phase. The XRD of the as-synthetized MOFs

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(Figure A.1) showed that eight crystalline 3D-frameworks isostructural to the MIL-78[25] were quickly

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obtained by a MW-assisted hydrothermal method, using different lanthanides.

F2 transition of Eu3+ (618 nm), while for samples M5, M6 and M7, the 5D4-7F5 transition of Tb3+ (545

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7

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The excitation spectra of samples M1, M2, M3, M4 and M8 were acquired monitoring the 5D0-

nm) was monitored (Figure 1). For all samples, an intense and broadband centered at 302 nm related to

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the π-π* transition of the ligand, as well as narrow and less intense peaks related to the f-f transitions of

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lanthanides were observed. This means that the main excitation mechanism occurs through the ligand, by an indirect process called antenna effect.[28] Since the lanthanides are inefficient in absorb

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energy,[29] the antenna effect is an important mechanism to improve the luminescent properties of a

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material.

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Figure 1 - Excitation spectra of the markers. Spectra of samples doped/co-doped with Eu3+ (M1, M2, M3, M4 and M8) were

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acquired monitoring the 5D0-7F2 transition (618 nm). Spectra of samples doped/co-doped with Tb3+ (M5, M6 and M7) were

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acquired monitoring the 5D4-7F5 transition (545 nm).

The emission spectra of the as-synthesized MOFs, obtained by exciting samples at 302 nm, are

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shown in Figure 2. For samples doped with Eu3+ (Figure 2.a), the transitions 5D0-7FJ (J = 0 - 4) from this

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ion were observed,[30,31] while in samples co-doped with Sm3+ (Figure 2.b), both transitions from Eu3+ (5D0-7FJ) and Sm3+ (4G5/6-6H5/2 and 4G5/6-6H9/2) were observed.[32] For samples doped with Tb3+ or co-

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doped with Tb3+ and Yb3+ (Figure 2.c), the Tb3+ transitions 5D4-7FJ (J = 6 – 2) were perceived,[30] while for the sample co-doped with Tb3+, Yb3+ and Eu3+ (Figure 2.d), both Eu3+ and Tb3+ transitions were present.[30] These transitions are responsible for the intense red-orange or green emission colors observed in the samples (Figure A.2) and were used to characterize the LGSR by VSC. It is important to highlight that the concentration quenching (that occurs when emitter centers are too close to transfer energy between each other)[33] was avoided by using small amounts of lantha-

ACCEPTED MANUSCRIPT nides. This feature, together with the absence of coordinated water, made all samples highly lumines-

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cent, thus promising for use as luminescent markers in ammunition.

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Figure 2 - Emission spectra of the MOFs containing a) Eu3+; b) Eu3+ and Sm3+; c) Tb3+ or Tb3+ and Yb3+; and d) Yb3+, Tb3+ and Eu3+. Transitions shown in red, green and turquoise colors referring to the Eu3+, Tb3+ and Sm3+ transitions, respectively.

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Emission spectra were acquired exciting samples at 302 nm.

It is known that a high concentration of lanthanides can quenches the luminescence of a material. This effect, called concentration quenching, occurs when ions of the same species are close enough, generally less than 5 Å, to transfer energy to each other (i.e., from an excited to a non-excited Ln3+).[33] As result, energy is lost in a non-radiative way. In the MOF MIL-78, the shortest calculated distance between two Ln3+ ions is 3.96 Å, which allows the transfer of energy. So, in these markers, the concen-

ACCEPTED MANUSCRIPT tration quenching can be avoided by using small amounts of lanthanides. To evaluate the effect of the concentrations of lanthanides in these MOFs, other spectroscopic parameters were measured, such as the lifetimes of the excited states, radiative and non-radiative decay rates, as well as the luminescence quantum efficiencies. The radiative and non-radiative decay rates and the luminescence quantum efficiencies were calculated only for samples doped with Eu3+, since this ion has a hypersensitive transition

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(5D0-7F1) and a purely magnetic dipole transition (5D0-7F2) that can be used as parameter. Unfortunately,

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the same procedure cannot be performed with the Tb3+ transitions.

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Lifetimes measurements were acquired at room temperature using the wavelengths of maximum

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excitation (302 nm) and emission (618 nm for samples M1, M2, M3, M4 and M8, and 545 nm for sam-

(Eq. 1)

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ples M5, M6 and M7). The radiative decay rates (ARAD) were calculated by equation 1: [34–36]

(Eq. 2)

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cording to equation 2: [34–36]

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Wherein A0J is the coefficient of spontaneous emission of each 5D0-7FJ transition, calculated ac-

In equation 2, the magnetic dipole transition 5D0-7F1 is used as reference, since this transition is

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practically insensitive to the chemical environment around the Eu3+ ions. In this equation, S0J and S01 are the areas under the curves of the transitions 5D0-7FJ and 5D0-7F1, respectively, and σ0J and σ01 are their respective energy barycenters. The A01 coefficients were calculated by equation 3 (the refractive index n was assumed to be equal to 1.5): [34–36]

(Eq. 3)

ACCEPTED MANUSCRIPT The total decay rate (ATOT) is the inverse of the lifetime (τ) and was used to calculate the nonradiative decays rates (ANRAD) by equation 4. The luminescence quantum efficiencies were calculated by equation 5: [34–36] (Eq. 4)

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(Eq. 5)

Lifetimes values ranging from 1.36 to 2.77 ms were observed (Table I), with longer lifetimes be-

than

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presented

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other

Ln-BTC

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ing reached by samples with lower concentrations of lanthanides. Furthermore, the lifetimes were longer MOFs

in

literature

(as

[Eu(BTC)(H2O)6],[34]

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[Eu(BTC)(H2O)][37] and [Eu(BTC)][22], with values of 0.23, 0.84 and 0.93 ms, respectively). The

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longer lifetimes could be assigned to a reduction of channels that quench the luminescence, such as coordinated water molecules and the concentration quenching effect. This statement is in agreement with

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the lower rates of non-radiative decays and the higher luminescence quantum efficiencies calculated for

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the samples doped/co-doped with Eu3+ (Table A.I). As a comparison, the luminescence quantum efficiency (η) of the MOF containing 5% of Eu3+ (sample M1) was 54%, while for other Ln-BTC MOFs in

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literature this value was much lower, as η = 19% for the MOF containing 100% of Eu3+ (MOF

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[Eu(BTC)])[22], and η = 12% for the MOF containing 100% of Eu3+ and six molecules of coordinated water (MOF [Eu(BTC)(H2O)6])[34]. It is noteworthy that there are no significant changes in the lifetimes and in the luminescence quantum efficiencies of samples doped with 5 or 10% of Eu3+ (M1 and M2), which indicates a limitation in the sensitivity of the steady-state emission technique to dopant concentration.

ACCEPTED MANUSCRIPT Table I – Luminescent lifetimes (ms) of the MOFs. Measurements were acquired at room temperature, using the wavelengths of maximum excitation (302 nm for all samples) and emission (618 nm for samples M1, M2, M3, M4 and M8, and 545 nm for samples M5, M6 and M7). Lifetime (ms)

M1

2.77

M2

2.72

M3

2.37

M4

1.71

M5

2.45

M6

2.42

M7

2.34

M8

1.36

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Sample

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3.2 Performance as luminescent markers for GSR. After the shots with marked ammunition, luminescent residues were visualized on the shooters’ hands, firearms and firing range (Figure 3) under

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UV-C radiation (254 nm), showing that the co-doped MOFs presented sufficient thermal stability to

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support the high temperature and pressure of the shots. In addition, the visual observation of luminescent residues made it easier to collect GSR samples and simplified the reconstruction of the simulated

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crime scene, helping, for example, to estimate the shooting position and the bullet trajectory. It is im-

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portant to note that, since UV radiation is harmful to the skin, the exposure time should be as short as possible. So, in the proposed method, the shooter's inspection with UV light to visualize LGSR can be done in just a few seconds.

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ACCEPTED MANUSCRIPT

xLnx(BTC)].

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Figure 3 – LGSR on the shooters’ hands, firearms and firing range after shots with 9mm NTA marked with MOFs [Y 1-

As luminescent particles are uncommon in the environment, their presence on a suspect is a strong indicative of LGSR, enabling it to be used as a screening procedure. After visual observation, the LGSR presence can be confirmed analyzing the residues by techniques that detect lanthanide ions (as

ACCEPTED MANUSCRIPT SEM/EDS, ICP-MS, ICP-OES, NAA, PLS or VSC) or even organic compounds characteristic of GSR (as FTIR or Raman spectroscopies).

3.3 Test of encoding ammunition (blind tests). In this work, the collected residues, identified only by the shot number and the place of collection, were analyzed by VSC and SEM/EDS. In emission

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spectra (Figure 4), the main transitions of the Eu3+ (5D0-7FJ, wherein J = 0 - 3), Tb3+ (5D4-7FJ, wherein J = 6 - 3) and Sm3+ (4G5/2-6H9/2) ions were observed.[30,32] Thus, only in the VSC analysis, it was possi-

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ble to classify the residues in 4 different categories: containing I) only Eu 3+ (shots 1 and 3); II) Eu3+ and

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Sm3+ (shots 2 and 5); III) Tb3+ (only Tb3+ or Tb3+ and Yb3+) (shots 6, 7 and 8); or IV) Eu3+, Tb3+ and

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Yb3+ (shot 4). The Sm3+ transitions, despite the low intensity, could be distinguished from Eu3+ones. This result is even more interesting because VSC is already used in forensic labs, making it fea-

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sible to use in real situations. Furthermore, the analysis is very simple, fast, non-destructive and does not

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require sample preparation (i.e., the measurements are carried out directly on the SEM stubs). And despite the lower resolution compared to specific spectrophotometers, lanthanides could be identified and

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LGSRs grouped in four different categories, as mentioned, helping to determine the origin of the am-

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munition.

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Figure 4 – Emission spectra obtained by VSC (254 nm) of the LGSR collected at different locations.

In SEM/EDS analysis, particles with morphology similar to those of pure markers were searched

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and analyzed. EDS spectra were acquired only from those that presented at least one of the characteris-

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tic elements of the markers. With this, each stub was analyzed in about 20 minutes, which is a breakthrough in this field. The time of analysis is a relevant parameter considering forensic issues and that’s

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why it was carefully considered in this work. As result, from the total of 112 particles analyzed in all stubs, only 14 could not be identified: 5 particles presented only Y or Yb, 2 presented Y and Yb and 7 others were clearly contaminated (once shots were performed on the same day and place, it is not surprising the contamination by particles dispersed in the environment). These 14 particles did not impede the correct identification of the marker and were not used by the analyst to construct the graphs in Figure 5, which present the average content of lanthanides in the residues. It is important to notice that, besides the rare earths used in this work, no

ACCEPTED MANUSCRIPT other rare earth element was detected, pointing the low risk of environmental or occupational contamination by these elements. Lastly, the analyst tried to identify the marker present in each shot comparing the content of lanthanides in the residues (Figure 5) with those in the pure markers (Table A.II). With

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this, matches were made and the marker used in each shot was determined, as shown in Table II.

Figure 5 – Average contents and standard deviations of lanthanides in LGSR collected at different locations, after shots with ammunition marked with the MOF [Y1-xLnx(BTC)]. Table II – Identification of the markers used in each shot based on emission spectra and EDS analysis (analysts’ conclusions).

ACCEPTED MANUSCRIPT Stub (place of collection) Firearm

Floor

Hands

marker

1

M1

M1

M1

M1

2

M3

M3

M3

M3

3

-

M2

-

M2

4

M8

M8

M8

M8

5

M4

M4

M4

M4

6

M5

M5

M5

M5

7

M7

M7

M7

M7

8

M6

M6

-

M6

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Shot

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Identified

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A comparison of forms containing the analysts’ conclusions with those containing shooters’

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choices and the code system used, revealed that all markers were correctly identified. In addition, correlation among the residues collected at different places (floor, hands and firearm) was possible in 87% of

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the stubs. This result suggests that it is possible to establish connections between a shooter, a firearm, a

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shooting area, as well as a victim, based on the ammunition barcode. This kind of information, which is not possible with the current methodologies of GSR identification, can be very helpful in reconstructing

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a crime, especially when multiple firearms and ammunition are involved.

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Finally, based on the price of analytical reagents and considering a lab scale, 1g of the marker [Y0,95Eu0,05(BTC)] costs U$1.36, and it is enough to mark 62 9mm ammunition cartridges (so cost per unit is about U$ 0,02). Compared to other markers previously reported, which contain 100% of Eu3+ or Tb3+, for example, this represents a cost reduction of up to 82%, since yttrium oxide is considerably less expensive than lanthanide oxides such as Eu2O3 and Tb2O3. Obviously, this value could be optimized and reduced for an industrial-scale operation. Besides, since the MOF [Eu(BTC)] is non-toxic[22] and

ACCEPTED MANUSCRIPT the lanthanide ions are chemically similar, it is expected that the markers [Y1-xLnx(BTC)] are also nontoxic.

4. Conclusions

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In this work, the development of an ammunition encoding system using luminescent co-doped

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MOFs with unique chemical compositions is proposed. To assess the feasibility of this proposal, a set of

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co-doped MOFs [Y1-xLnx(BTC)] containing different proportions of Eu3+, Sm3+, Tb3+ and Yb3+ were synthesized and used as luminescent markers in ammunition. After shots, luminescent residues were

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visualized under UV-C radiation, easily collected and analyzed by VSC and SEM/EDS, without any

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sample preparation. Based only on VSC results, the residues were classified into 4 distinct groups based on the lanthanide emission spectra. By SEM/EDS, the marker used in each ammunition cartridge was

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identified based on its lanthanide contents, enabling identification of the ammunition used for all shots.

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Furthermore, connections between a shooter, a firearm and a specific shooting area were possible based on the residues composition. Finally, the techniques used (VSC and SEM/EDS) are both available and

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commonly used in forensic labs. Additionally, all measurements could be performed in a few minutes,

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making this analytical approach feasible. Thus, the proposed ammunition encoding process provided information that is not acquired with the currently methodologies, proving be a powerful analytical tool

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for the investigation of crimes when firearms are involved. Appendix A. Supplementary Data X-ray powder patterns, 1931 CIE chromaticity diagram and coordinates, images of the pure markers under UV-C radiation, radiative and non-radiative decay rates, luminescence quantum efficiencies, and content of lanthanides in pure markers are available in Appendix A.

ACCEPTED MANUSCRIPT Acknowledgment We thank federal forensic experts Eduardo Makoto Sato and Ronei Maia Salvatori from the National Institute of Criminalistics – Brazilian Federal Police (NIC/BFP) for research collaboration. The English text of this paper has been revised by Sidney Pratt, Canadian, MAT (The Johns Hopkins Uni-

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versity), RSAdip - TESL (Cambridge University).

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Funding: This work was supported by CAPES (Núcleo de Estudos em Química Forense –

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights



Luminescent markers were added to non-toxic ammunition (NTA) and provided an easy and reliable way to identify gunshot residues (GSR). Encoding ammunition was achieved using different markers with unique compositions.



A set of eight co-doped lanthanide metal-organic frameworks was used as chemical barcodes

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Blind tests were performed and correlation between the residues collected at different locations

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(as hands, firearm and shooting area) was possible.

All analysis were performed using facilities compatible with forensic routine (SEM/EDS and

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video spectral comparator).

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for ammunition.