Multicommutated Anodic Stripping Voltammetry ... - Wiley Online Library

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Multicommutated Anodic Stripping Voltammetry at Tubular Bismuth Film Electrode for Lead Determination in Gunshot Residues Jose A. Rodriguez,a* Israel S. Ibarra,a Carlos A. Galan-Vidal,a Marisol Vega,b Enrique Barradob a

Universidad Autońoma del Estado de Hidalgo, Centro de Investigaciones Qumicas, Carr. Pachuca-Tulancingo Km. 4.5, C.P. 42076, Pachuca, Hidalgo, Mexico *e-mail: [email protected] b QUIANE, Universidad de Valladolid, Facultad de Ciencias, Departamento de Qumica Analtica, Paseo Prado de la Magdalena s/n, C.P. 47005, Valladolid, Spain Received: July 7, 2008 Accepted: September 19, 2008 Abstract This work describes the construction of a miniaturized flow-through electrochemical cell and its application to the determination of lead in gunshot residues (GSR) by differential-pulse anodic stripping voltammetry at bismuth films co-deposited on-line on a tubular carbon paste electrode. The developed detector was coupled to a multicommutated flow system which was designed to allow medium exchange prior to the stripping step thus reducing interferences from the analytical matrix, and to implement the standard additions method in an automatic way, using only one standard solution. For a deposition time of 60 s at 1.5 V at a flow rate of 0.5 mL min1 without oxygen removal, the detection limit of the method was 0.2 mg L1. Under these conditions the linear dynamic range was 0.3 – 10.0 mg L1 with a sampling rate of 15.0 samples h1. Repeatability of lead concentration was 3% (n ¼ 3). The method was applied to the analysis of lead in GSR on hands of shooters and concentrations up to 16.5 mg and 6.7 mg were found in palms and backs, respectively. No statistically significant differences between the results obtained by the proposed and the comparative method (F-AAS) were found, at a 95% confidence level. Keywords: Multicommutated flow analysis, Anodic stripping voltammetry, Bismuth film electrode (BFE), Lead, Gunshot residues DOI: 10.1002/elan.200804420

1. Introduction When a firearm is discharged, a cloud of vapors and particulate material, called gunshot residues (GSR), is blasted more intensely onto the regions close to the gun. These residues correspond to unburned powder, particles from the primer and case of the cartridge and also, metals from the barrel of the gun. The analysis of gunshot residues is one of the most important tests in forensic science. It has been used to estimate firing distances, to identify bullet holes and, the most important task, to determine whether or not a person has discharged a firearm. Since the work of Harrison and Gilroy [1], gunshot analysis has been based on determination of heavy metals (usually lead, barium and antimony), which originate from the primer of the cartridge. Over the years the different metal content has been qualified and quantified by many different analytical techniques including the rhodizonate test [1 – 3] and instrumental techniques such as atomic absorption spectroscopy [4], scanning electronic microscopy with energy disperse X-ray analysis (SEM-EDX) [5], capillary electrophoresis [6, 7] and anodic stripping voltammetry (ASV) at Electroanalysis 2009, 21, No. 3-5, 452 – 458

mercury film electrodes [8]. The advantage of ASVover the other techniques is the simplicity of the instrumentation, which is relatively inexpensive, and is portable as well as suitable for automation. Recently, bismuth film electrodes (BFE) were proposed as an alternative to more toxic mercury electrodes in trace metal analysis by stripping voltammetry [9 – 12]. Another advantage of BFE over mercury electrodes is that the former are less susceptible to oxygen background interferences [12]. The ability to perform stripping measurements without deoxygenation greatly simplifies the stripping protocol particularly in connection to on-line applications. Combining ASV with flow systems has significant advantages over batch techniques, such as a high level of automation, increased sampling rate, cost-effective operation and, most important, improvement of accuracy and precision and decreased risk of sample contamination due to reduced sample handling. ASV has been coupled to flow techniques such as flow injection [13, 14] and sequential injection analysis [15, 16]. An important factor to optimize in these systems is the adequate mixture of the supporting electrolyte, contained in the carrier solution, and the sample. Multicommutated flow systems allow the use of 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Lead Determination in Gunshot Residues

small volumes of sample and carrier solutions (less than 10.0 mL), thus improving the reproducibility of mixing conditions and leading to highly reproducible mass transport between the electrode surface and the flowing solution. Multicommutated systems belong to a new generation of flow techniques. They can be considered as an analytical network which involves the actuation of n active devices (or n operations with a single device) on a single sample [17]. Other advantage of multicommutated flow systems is the implementation of easily controlled on-line standard addition methods without sample handling which decreases risk of sample contamination. In this work, an automated flow method for lead determination in GSR samples by differential-pulse anodic stripping voltammetry at tubular bismuth-film electrodes has been developed. The multicommutated flow system, used in combination with the tubular electrochemical cell, ensures perfect mixing of sample and reagents and guarantees that the hydrodynamic flow conditions are not disturbed. In addition, the proposed method allows the continuous regeneration of the BFE, which is newly plated in each analytical cycle onto a graphite substrate, thus preventing the passivation of the detector and further improving the reproducibility of the results. The influence of several electrochemical and flow variables was evaluated, and optimal working conditions were selected. Validation of the results obtained with this procedure was performed on real GSR samples by comparison with FAAS. The deoxygenation step is avoided in the proposed system, which results in shorter analysis times.

2. Experimental 2.1. Reagents All solutions were prepared by dissolving the respective analytical grade reagent in deionized water with a resistivity not less than 18.0 MW cm provided by a Milli-Q system (Millipore). 0.1 mol L1 acetate buffer of pH 4.5 was prepared by dissolving appropriate amounts of acetic acid and sodium acetate; this solution was employed to dilute all reagents. A 10 mg L1 Bi(III) solution was prepared by dilution of 1.0 g L1 standard solution of Bi(III) (as nitrate) with the acetate buffer solution and used to plate the bismuth films, and simultaneously as supporting electrolyte and carrier solution. Working standard solutions of Pb(II) were prepared weekly from a 1.0 g L1 stock solution (PE Pure, Atomic spectroscopy standard), by dilution with the acetate buffer solution. Working solutions (1.0 – 10.0 mg L1) of Triton X-100, hexadecyl-trimethylammonium bromide and sodium dodecyl sulfate (all from Aldrich) were prepared by dissolution of adequate amounts of the pure compounds in acetate buffer solution. Working standard solutions (1.0 – 10.0 mg L1) of the interfering ions studied, Ba(II), Cu(II), Sb(II), were prepared by dilution of the standard stock solutions (1.0 g L1 PE Pure, Atomic spectroscopy standard) with acetate buffer. 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

2.2. GSR Sampling and Sample Preparation GSR samples were obtained by the swabbing technique [8]. While wearing gloves, a cotton tipped swab is soaked into the HNO3 solution (5.0% v/v) and then a certain part of the hand is swabbed for GSR collection: the right palm, right back, left palm, or left back. The cotton tip is then cut from the stem with scissors and placed in a 2.0 mL safe-locked polypropylene tube (Eppendorf). The collection continues for each section of the hand. The lead contained in the sample was recuperated by sonication, during ten minutes, of the cotton tip into 1.0 mL of 0.1 mol L1 acetate buffer solution at pH 4.5. A volume of 0.5 ml of the sample extract was diluted to 2.0 mL with the same acetate buffer.

2.3. Apparatus In the developed multicommutated flow system, schematized in Figure 1, the solutions and samples were aspirated through an automatic syringe (Sy) with 10 mL of total capacity (MicroBu 2030, Crison). The direction of solutions and samples inside the manifold was controlled by threeway solenoid valves, V1, V2 and V3 (N-Research, Cadwell, NJ, USA). The instrumental devices were controlled using Autoanalysis 5.0 software. All tubing connecting the different components of the flow system was made of 0.8 mm i.d. Omnifit PTFE. Anodic stripping voltammetric measurements in the differential-pulse mode were made using an Autolab PGSTAT 30 potensiostat/galvanostat (Eco Chemie, Switzerland); data were acquired using GPES software (v 4.6). Unless otherwise stated, the following differential-pulse parameters were applied: pulse amplitude, 50 mV; step potential, 5 mV; interval time, 0.8 s and modulation time, 0.05 s.

2.4. Construction of the Tubular Detector The working and auxiliary electrodes were prepared from graphite-paraffin (carbon paste) pellets obtained by dissolving 0.25 g of paraffin wax in 10.0 mL of warm n-hexane (40 8C) in a beaker placed in a water-bath, and by adding 4.75 g of graphite powder (with stirring). After complete evaporation of the organic solvent, 0.20 g of the dry paraffined graphite powder, was pressed with a 10.0 mm diameter pellet press at 19 000 kg cm2 for 5 min. Disks 10.0 mm in diameter and 1.2 mm thick were obtained. Electrical contact was made through a cable (Fig. 2.a) attached by solder to a small rectangular silver plate (1.0 3.0 mm) which was glued to a square-shaped (5.0 6.0 mm) fragment of the carbon paste pellet (Fig. 2.b) using a conductive, silver-based, epoxy resin. Two identical electrically wired carbon paste fragments were then placed in a hollow Perspex holder (1.0 1.0 4.0 cm) (Fig. 2.c) filled with a nonconductive epoxy resin. The distance between the electrodes was about 3.0 mm. The final electrochemical cell

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Electroanalysis 2009, 21, No. 3-5, 452 – 458

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Fig. 1. Multicommutated flow system for Pb(II) determination: S, sample; SS, standard solution of Pb(II); CS, carrier solution (10 mg L1 of Bi(III) in 0.1 mol L1; acetate buffer solution, pH 4.5); D, detector; Sy, 10.0 mL syringe; W, Waste; V1, V2, V3, three-way solenoid valves; R1, R2, reaction coils (60 cm). Sequence of a complete analytical cycle representing the solenoid valves positions on and off.

was then left at 25 8C for one week. After hardening, a channel 0.8 mm in diameter was drilled perpendicular to the electrodes through the centre of the Perspex holder. A reference electrode (Fig. 2.d) was made by sealing a 0.5 mm diameter silver wire in a polyethylene pipette tip. Approximately 4 cm of silver wire were attached to a shielded electrical wire with solder. Silver/silver chloride reference electrodes were prepared by anodizing the silver wire into a 3.0 mol L1 KCl solution for 2.0 min at 0.10 V. A salt bridge was prepared by mixing 0.5 g of granular agar and 2.5 g of NaCl dissolved in 10 mL of water. The solution was boiled and a second pipette tip immersed into the boiling solution for 1.0 min while negative pressure was applied, thus drawing the agar solution into the tip of the reference electrode. This tip was immediately immersed in room temperature water to gel the agar. The reference electrode was completed by filling the second tip with a 3.0 mol L1 KCl solution and inserting the first tip containing the Ag/ AgCl wire. The surface of the tubular cell was daily moistened with double distilled water and polished using a cotton thread soaked in alumina. It was then thoroughly rinsed with deionized water.

Fig. 2. Schematic representation of the tubular electrochemical detector: a) electric shielded cable; b) carbon paste electrode (paraffin:graphite, 5:95); c) Perspex holder (1.0 cm 1.0 cm 4.0 cm); d) reference electrode. Electroanalysis 2009, 21, No. 3-5, 452 – 458

2.5. Multicommutated DPASV System (MC-DPASV) The analytical cycle begun with the insertion of the carrier solution (10.0 mg L1 Bi(III) in 0.1 mol L1 acetate buffer ) at a flow rate of 0.5 mL min1 during 60.0 s. The sample was introduced by using a binary sampling strategy consisting on the intercalation of multiple small sample segments of 10.0 mL (V1 in position on and V2 in position on, see Fig. 1) with multiple small segments (10.0 mL) of carrier solution (V2 in position off). The total aspirated volume in each cycle was therefore 250.0 mL of sample and 250.0 mL of carrier solution. The binary sampling strategy creates multiple reaction interfaces which contribute to a faster reaction zone homogenization that increases the precision and the sampling rate. The strategy used to insert standard additions was also based on binary sampling, which consisted in the alternate insertion of sample (V1 in position on and V2 in position on), carrier solution (V2 in position off) and standard solution (V1 in position off and V2 in position on) plugs (see Fig. 1). To assess the perfect mixing of sample/ standard and carrier segments, reaction coils of 60 cm long were used (R1 and R2). The mixed solution was propelled to the detector where Bi(III) and Pb(II) were co-deposited by applying a potential of 1.5 V for 60.0 s to the working tubular carbon paste electrode. After the deposition step, 2.0 mL of carrier solution (V2 in position off) were aspirated through the electrochemical cell at a flow rate of 30.0 mL min1 (exchange media). Then the flow was stopped and an anodic scan applied from 1.0 to 0.0 V in the differentialpulse mode using the pulse parameters detailed above. An anodic peak corresponding to the re-oxidation of deposited lead was obtained at 0.46 V. In order to remove the film and to regenerate the electrode surface prior to the next analytical cycle, a conditioning potential of 2.0 V was applied for 5 s while the carrier solution was flowing at 0.5 mL min1. The application of positive potentials greater than þ 1.2 V (vs. Ag/AgCl) to carbon paste electrodes results in increased electron transfer rates presumably via formation of oxygen-containing surface states which promote the complete removal of old films from the electrode [18]. The effect of the conditioning potential (0.6 and 2.0 V, vs. Ag/AgCl) was evaluated through the signal obtained analyzing a 5.0 mg L1 Pb(II) solution in experimental


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conditions described above. A continuous decrement in the signal height was obtained when a 0.6 V (vs. Ag/AgCl) conditioning potential was applied. The repeatability of the signal, expressed as %RSD (n ¼ 3), was 9.3 and 2.5% when 0.6 and 2.0 V conditioning potentials were applied. According to the results, electrode conditioning at highly anodic potential prevented the passivation of the electrode and improved the sensitivity and repeatability of the measurements.

3. Results and Discussion 3.1. Optimization of Experimental Conditions Pb(II) concentration in gunshot residues has been reported in the range from 3.0 to 16.0 mg per sample [19]. Considering the sample preparation procedure employed (lead extracted with 2 mL acetate buffer and then extracts four-fold diluted with the same acetate buffer solution), lead concentrations below 10 mg L1 should be expected in most GSR samples. Therefore, a standard solution of 5.0 mg L1 Pb(II) was employed throughout the optimization experiments. To maximize the deposition of bismuth onto the solid graphite substrate, the co-deposition potential was selected as negative as possible but avoiding hydrogen evolution. Therefore, the deposition potential was set invariably at 1.5 V (vs. Ag/AgCl). The parameters of the differentialpulse modulation applied during anodic stripping have been detailed in Experimental. In stripping voltammetric analysis at BFEs, four main steps should be studied: bismuth film formation, analyte preconcentration, analyte anodic stripping and regeneration of the electrode surface. In order to optimize the bismuth film formation and the co-deposition of lead, the effect of the supporting electrolyte on the Pb(II) stripping signal was studied. A variety of supporting electrolytes were assayed, including 1.0 mol L1 hydrochloric acid [20] and 1.0 mol L1 acetate buffer at pH 4.5 [16]; the highest peak current was obtained in hydrochloric acid, but the repeatability of the signal, expressed as %RSD, was poor (13.6% for HCl vs. 2.7% for acetate buffer). According to the results, the use of low pH values of the carrier/electrolyte imparts a compromise situation between an increase of stripping signal current and hydrogen evolution background. Acetate buffer solution at pH 4.5 was therefore selected for subsequent analyses. The effect of the concentration of acetate buffer solution as supporting electrolyte was also investigated at 0.1, 0.5, and 1.0 mol L1. The stripping peak current of lead and its repeatability was similar at the three concentrations assayed (see Table 1). Homogeneous mixing of sample and carrier solutions is an important parameter in flow techniques coupled to ASV. In flow injection analysis (FIA) the use of confluence points preceding the detector is a common practice to adjust the pH for sample deposition step [21]. Sequential injection analysis (SIA) applies other strategies to improve diffusion processes such as the use of concentrated 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Table 1. Influence of buffer concentration (pH 4.5) on the stripping peak current (mean and %RSD, n ¼ 3) of a solution containing 5.0 mg L1 of Pb( II ). Conditions: flow rate, 0.5 mL min1, deposition potential, 1.5 V; deposition time, 60 s. Acetate buffer concentration (mol L1)

Peak height (mA )

0.1 0.5 1.0

16.9 (2.1) 16.7 (1.6) 17.6 (3.1)

carrier solutions (> 1.0 mol L1) and/or the use of alternate segments [16]. The on-line implementation of binary sampling principle based on smaller volumes in the multicommutated flow system proposed here produces a faster diffusion of the sample into the carrier solutions. At this point, even at the lower carrier concentration tested, the mixing of sample and supporting electrolyte was complete; therefore a concentration of 0.1 mol L1 acetate buffer was selected. The bismuth film was generated on the surface of the tubular carbon paste electrode by on-line in situ plating. The stripping peak current is affected by the film thickness which depends on the concentration of the Bi(III) in the carrier solution. The effect of the concentration of Bi(III) on the stripping signal was evaluated in the concentration range from 0.1 to 20.0 mg L1. The stripping current of lead increased sharply with increasing Bi(III) concentration up to 10.0 mg L1, and then remained nearly constant (Fig. 3). Therefore, an optimal Bi(III) concentration of 10.0 mg L1 was chosen. The flow rate plays an important role on the cathodic deposition of Bi and Pb on the tubular carbon paste electrode, thus affecting the sensitivity of the analytical methodology. Therefore, the influence of the flow rate on the stripping peak current of lead was investigated in the

Fig. 3. Effect of Bi(III) concentration in the carrier solution on the stripping current of a solution containing 5.0 mg L1 of Pb(II). Conditions: flow rate, 0.5 mL min1; deposition potential, 1.5 V; deposition time, 60 s.



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Table 2. Influence of flow rate and deposition time on the stripping peak current (mean and %RSD, n ¼ 3) of a solution containing 5.0 mg L1 of Pb( II ). Conditions: sample volume, 0.25 mL, carrier solution, Bi( III ), 10 mg L1 in acetate buffer solution (0.1 mol L1, pH 4.5), deposition potential, 1.5 V. Flow rate (mL min1)

Deposition time (s)

Peak current (mA )

%RSD

0.5 1.0 1.5 2.0 2.5

60.0 30.0 20.0 15.0 12.0

23.8 15.6 13.5 7.4 4.5

1.7 2.9 1.7 1.9 5.3

Table 3. Optimized experimental conditions of MC-DPASV system for determination of Pb( II ) in GSR samples. Parameter

Optimized value

Supporting electrolyte

0.1 mol L1 acetate buffer (pH 4.5) 10.0 mg L1

Concentration of Bi( III ) in supporting electrolyte Flow rate during deposition Deposition potential Deposition time

0.5 mL min1 1.5 V mV 60 s

range of 0.5 to 2.5 mL min1. Since the available amount of diluted GSR extracts is limited to 2 mL, a constant sample volume of 0.25 mL was injected in all cases. Therefore, lower flow rates imply the use of longer deposition times. The results (see Table 2) indicate that decreasing flow rate causes a significant increase in the sensitivity of the measurements. This increase is correlated with the longer contact time between the electrode and the metal ions contained in the flowing solution associated with longer deposition times. In the case of small volume samples, like GSR extracts, multicommutated flow systems are an alternative to minimize the sample consumption. Since a flow rate of 0.5 mL min1 yielded higher anodic peak currents while consuming less sample volume, it was chosen as the optimal flow rate value. The deposition time needed to analyze 0.25 mL of sample extract in these conditions is 60.0 s and this time was selected for further experiments. The optimum conditions established for this procedure are summarized in Table 3.

3.2. Analytical Properties of the Procedure The calibration plot for Pb(II) obtained in the optimized experimental conditions is shown in Figure 4. Three replicate measurements of each standard solution were made and the mean values were used for calculations. A lineal dependence of the stripping current with the injected concentration of lead was found in the concentration range Electroanalysis 2009, 21, No. 3-5, 452 – 458

0.3 – 10.0 mg L1 (1.2 – 40.0 mg sample1) with a practical limit of detection of 0.2 mg L1 (0.8 mg sample1) for a deposition time of 60 s (Fig. 4). Table 4 shows the regression parameters of the standard curve and other validation parameters. The instrumental limit of detection was calculated according to the IUPAC criteria [22], i.e., 3.29 times the value of se/b1, where se is the square root of the residual variance of the standard curve and b1 is the slope. The intermediate precision of the procedure, expressed as the relative standard deviation for six determinations made on different days on a standard solution of 5.0 mg L1 lead in acetate buffer, was 2.6%. The repeatability of the stripping peak of lead was below 5% RSD for all standard solutions and samples. Under optimal conditions, 15 analytical cycles per hour can be carried out. Lead in GSR extracts was quantified by the standard additions method using two additions and two replicated measurements after each standard addition. In these conditions, the total analysis time per sample was 24 min. The effect of the sample constituents (Cu(II), Sb(III), Ba(II)) present in the GSR extracts was studied. Solutions containing 5.0 mg L1 of Pb(II) and the potentially interfering element at higher concentrations (maximum interfering element/lead rate assayed, 10 : 1) were analyzed. The added element was considered to interfere when it caused a variation in the stripping signal greater than or equal to 5% compared to the response obtained in its absence. The results showed that, at the concentrations in which they were present in the samples tested (interfering element/lead rate 0.5 : 1), none of the ions interfered in the determination of Pb(II). For Cu(II), the addition of 5.0 mg L1 to the Pb(II) solution (Cu/Pb ratio 1 : 1) does not cause any change, and the maximum addition corresponding to 50.0 mg L1 (Cu/Pb ratio 10 : 1) causes an increase of only 7.0% of lead stripping current. The same strategy was used for Sb(III) and Ba(II): the maximum addition (10 : 1) causes a decrease in the stripping signal of only 2.0 and 1.5%, respectively. However, such high concentration of these elements cannot be found in any GSR. The interference of surface active substances on the stripping current of Pb(II) was investigated since surfactants are ingredients in soaps and therefore residues of these compounds are expected in the hands of the suspect. Moreover, surfactants can adsorb on the surface of the electrode thus hindering the co-deposition and subsequent stripping of bismuth and lead. Triton X-100, hexadecyltrimethylammonium bromide and sodium dodecyl sulfate were used to investigate the effect of nonionic, cationic and anionic surfactants, respectively. Studies were performed within the 0.0 – 10.0 mg L1 range for these potentially interfering species. Calibration curves of model solutions containing the interfering species and 5.0 mg L1 Pb(II) showed no significant interference for anionic surfactants and a larger influence for nonionic and cationic surfactants. However, the medium exchange prior the stripping step minimizes the interfering effect in Pb(II) determination.



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Fig. 4. DP-voltammograms of Pb(II) standards, obtained in acetate buffer solution (0.1 mol L1, pH 4.5).

3.3. Analytical Applications The proposed method was used to determine Pb(II) in 32 GSR samples, corresponding to one shot performed by eight volunteers, using a 0.22 caliber handgun. After each shot the palm and back of each hand were swabbed. Table 5 shows the results obtained. In order to validate the results, the concentration of lead in the GSR extracts was also determined by Flame Atomic Absorption Spectrometry (F-AAS). The totality of the results (both hands and both parts) obtained by both methods were compared using a paired t-test. The samples with a Pb(II) concentration < LD were discarded. The values of calculated t (0.71) were then compared to a critical value for 26 degrees of freedom at the 95% confidence level (t ¼ 2.06). No significant differences were seen between the results obtained with each method. Significant differences were found in lead concentration measured by MC-DPASV in GSR samples collected from backs and palms (calculated t, 5.27; critical t value for 10

degrees of freedom and a 95% confidence level, 2.23; samples with values below LD discarded), thus indicating that deposition of GSR occurs mainly in palms. The repeatability data obtained with real samples was calculated by making repeated measurements (n ¼ 2) on the samples collected, RSD values were below 5% in all cases. These data demonstrate that repeatability of the analytical measurement does not increase much with respect to the values obtained with standard solutions. The medium exchange used in the stripping step minimizes interferences from the analytical matrix improving the repeatability.

4. Conclusions The utility of BFE plated onto carbon paste as working electrodes for on-line anodic stripping voltammetry, with medium exchange, has been investigated using a multicommutated flow system coupled to a tubular electro-

Table 4. Regression parameters of the calibration plots of mean peak current (in mA, n ¼ 3) vs. Pb( II ) concentration (in mg L1). Parameter

Value

Square root of residual variance, se Number of standards, N Intercept confidence interval, b0 t s(b0) (mA ) [a] Slope confidence interval, b1 t s(b1) (mA L mg1) [a] Linear range (mg L1) Instrumental limit of detection (mg L1) Sampling rate (samples h1)

0.32 6 0.18 0.90 4.84 0.17 0.3 – 10.0 0.2 15.0

[a] tcrit(0.05;4) ¼ 2.78 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim



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Table 5. Contents of Pb( II ) (mean and %RSD, n ¼ 2) in GSR samples as determined by the proposed method and F-AAS.

5. Acknowledgements

Shooter Hand Pb (mg sample1)

The authors thank the CONACyT, Mexico (Project CB2006-61310) and the Consejera de Educacioń, Junta de Castilla y Leoń, Spain (Project VA029A07) for financial support.

1 2 3 4 5 6 7 8

Left Right Left Right Left Right Left Right Left Right Left Right Left Right Left Right

MC-DPASV FASS Back Palm

Back

5.1 (1.6) < L.D. 6.1 (0.6) 3.1 (0.7) 2.2 (2.1) 3.1 (0.4) 5.6 (0.1) < LD 3.5 (1.3) 5.8 (1.1) 2.7 (1.4) 2.0 (0.5) 0.9 (1.6) 1.6 (1.3) 1.3 (3.2) 1.9 (3.8)

5.3 0.5 6.7 2.7 1.3 3.5 5.9 0.2 3.8 6.2 2.8 2.1 0.7 1.7 1.3 2.0

8.7 (1.0) 1.9 (2.2) 13.9 (0.5) 7.1 (1.7) 5.2 (0.4) 3.8 (1.2) 16.2 (0.4) 5.4 (1.2) 4.7 (0.5) 11.4 (0.3) 7.9 (0.2) 4.8 (0.5) 7.0 (1.4) < LD < LD < LD

Palm

(1.7) 8.9 (4.0) (3.5) 2.4 (3.9) (1.5) 14.1 (4.9) (3.9) 6.9 (1.5) (4.6) 5.1 (4.6) (3.8) 3.2 (3.5) (4.0) 16.5 (1.7) (5.8) 5.3 (3.5) (3.3) 4.6 (1.7) (0.6) 11.5 (2.1) (2.3) 7.9 (2.0) (8.1) 4.7 (0.6) (7.6) 7.0 (5.8) (2.2) 0.4 (8.2) (2.0) 0.5 (8.0) (3.9) 0.6 (7.6)

6. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

chemical cell. The methodology proposed was applied to lead determination in gunshot residues. The binary sampling principle is a useful tool to minimize the reagents and sample consumption. The linear response range (0.8 – 40 mg sample1) achieved with the system makes it suitable for lead measurement in gunshot residues with usual concentrations. Samples with lead concentrations out of that range can also be easily measured by using smaller/larger sample aliquots, or shorter/longer deposition times. The proposed multicommutated flow voltammetric system is an alternative for cost-effective and highly automated determination of GSR samples, which handling must be reduced as much as possible in order to preserve their traceability and reduce the risk of alteration due to their vulnerability.

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]


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