Direct analysis of barium, calcium, potassium, and manganese concentrations in tobacco by laser-induced breakdown spectroscopy

Microchemical Journal 126 (2016) 545–550

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Direct analysis of barium, calcium, potassium, and manganese concentrations in tobacco by laser-induced breakdown spectroscopy Daniel Menezes Silvestre a, Flavio de Oliveira Leme a, Cassiana Seimi Nomura a, Angerson Nogueira do Nascimento b,⁎ a b

Instituto de Química, Universidade de São Paulo, São Paulo, SP, 05508900, Brazil Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, Diadema, SP, 09972-270, Brazil

a r t i c l e

i n f o

Article history: Received 13 November 2015 Received in revised form 21 January 2016 Accepted 21 January 2016 Available online 29 January 2016 Keywords: Cigarette Tobacco LIBS ICP OES

a b s t r a c t This work deals with method development for the direct measurement of Ba, Ca, K, and Mn concentrations in tobacco by laser-induced breakdown spectroscopy (LIBS). Cigarette and tobacco samples from different regions of Brazil were freeze-dried and cryogenically ground for 10 min. Pellets containing 0.5 g of the ground material were prepared by applying 10 T cm−2 pressure for 5 min in a cylindrical die set. Accurate determination of concentrations was achieved via comparison with calibration data. Calibration standards were prepared from a selected milled tobacco sample either diluted with high-purity cellulose or spiked with increasing amounts of standard solutions of the analyte, followed by freeze-drying for 48 h and cryogenic mill homogenization. After acid decomposition, inductively coupled plasma optical emission spectroscopy (ICP OES) was used to determine the mass fraction of each analyte in the calibration standards. Using the proposed method, various tobacco samples were analyzed by LIBS; the results showed a concentration range of 70–120, 25,000–30,000, 30,000–40,000, and 120–250 μg g−1 for Ba, Ca, K, and Mn, respectively. The accuracy of the method was checked by the analysis of certified reference materials and by comparison of the results with obtained by ICP OES after acid decomposition. The results of Student's t-test showed 95% confidence level, indicating that the proposed LIBS method can be used for the direct analysis of tobacco. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Tobacco contains harmful substances, classified as either organic molecules or toxic elements. These toxic substances can be present in the native soil or result from pesticides used; in both cases, incorporation of such toxic substances into tobacco leaves may occur, resulting in exposure of the smoker to carcinogenic effects and large amounts of toxic elements such as As, Cd, Cr, Pb, and Ni [1]. In addition to carcinogenic effects, kidney, nervous system, and cardiovascular problems may occur, as well as alteration of neural development due to smoke inhalation [2–5]. The neurotoxic effects induced by Mn in humans, for example, mainly emerge subsequent to inhalation exposure, where the metal can enter the brain tissue through the olfactory pathways. Inhaled Mn at exceedingly high levels may lead to manganism, characterized by motor and postural signs consistent with those inherent to Parkinson's disease [6]. The toxicity of barium compounds depends on specific species, but lethal doses in humans usually range from 1 to 30 g [7]. Barium is a dermal chemical irritant and may cause dermal lesions. When this element is ingested orally or inhaled can cause tachycardia, hypertension and benign granulomatous pneumoconiosis [8]. Therefore, monitoring of ⁎ Corresponding author. Fax: + 55 11 3319 3472. E-mail address: [email protected] (A.N. do Nascimento).

http://dx.doi.org/10.1016/j.microc.2016.01.015 0026-265X/© 2016 Elsevier B.V. All rights reserved.

elemental concentrations in tobacco samples has been pursued continuously for a clearer understanding of the harmful effects of these substances in the human body. Furthermore, the ratio of Mg/Ca presents the wrapper and the tobacco leaves may indicate the origin of cigarette, helping in the identification of smuggled products and also a way to establish a fingerprint to identify illegal cigarettes [9]. Current methods for determining elemental concentrations include spectroscopic techniques such as flame atomic absorption spectrometry (F AAS), inductively coupled plasma-optical emission spectrometry (ICP OES), and inductively coupled plasma-mass spectrometry (ICP-MS). These techniques, however, most of time require the use of different sample introduction systems, such as nebulization systems. Consequently, it is necessary to convert a solid sample in an aqueous solution previously to the sample introduction into the equipment. Several procedures exist for the solubilization of solid samples in aqueous or organic medium; among them, acid digestion assisted by microwave irradiation or extraction of analytes using a proper solvent can be highlighted [10–13]. These sample preparation processes allow the use of aqueous solutions in the calibration step but require significant time and high reagent consumption, in addition to generating acid waste. Therefore, an alternative would be the direct analysis of solid samples, which may be made by suspension or direct sampling of a pulverized sample with controlled particle size, which has some advantages compared to conventional procedures:

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reduced pretreatment of the sample, minimizing the risk of contamination due to the use of reduced quantities of reagents or low exposure to the environment, losses of analyte is avoided, low consumption or no use of toxic/corrosive reagents (HF, for instance), minimizing waste generation which contribute to the green chemistry and the possibility of analyzing a small quantity of sample [14–21]. In this regard, laserinduced breakdown spectroscopy (LIBS) holds great promise for the direct analysis of elemental concentration in solid samples. LIBS utilizes a laser pulse of sufficient energy directed at the sample to heat, vaporize, and excite a small portion of the species present in the sample into a plasma that reaches temperatures up to 20,000 K. Spectroscopic interrogation of the vaporized sample permits multielemental analysis of a solid sample in a short time [22]. Despite these advantages, there is a lack of methods using standard calibration curves to determine the elemental concentration in complex sample matrices by LIBS. This lack of methods primarily results from the pronounced effect of the matrix, which interferes in the interaction between the laser and the sample. In addition, there are few certified reference materials (CRMs) available with reference values for the sample size typically used in LIBS analysis, disfavoring the possibility of method accuracy evaluation [23]. These disadvantages have opened new frontiers for scientific research targeted toward strategy identification allowing for quantification free of a matrix-dependent process [24]. A possible solution is the use of calibration-free laser induced breakdown spectroscopy (CF-LIBS) for the quantitative analysis of tobacco samples [25]. On the other hand, CF-LIBS is often only semiquantitative due to effects like radiation self-absorption and the inaccuracy of parameters such as transition probability, electron number density, and plasma temperature used in the mathematical model [26–27]. Considering these difficulties, the aim of the present work is the proposal of a method for the direct analysis of Ba, Ca, K, and Mn in tobacco samples by LIBS using solid standard calibration.

torch diameter was 2.0 mm. Operating instrument parameters were as follows: power, 1350 W; auxiliary gas flow rate, 0.2 L min−1; nebulizer gas flow rate, 0.45 L min− 1; coolant gas flow rate, 15 L min− 1; and pump rate, 2.5 mL min−1. The analytical wavelengths (nm) for each element were as follows: Ba (II) (233.527), Ca (II) (318.128), K (I) (769.896), and Mn (II) (257.61). Tobacco samples were dried in a freeze dryer (Thermo Fisher Scientific, USA) and ground in a cryogenic mill (Marconi MA 775, Marconi, Brazil). Samples were pelletized using an automatic hydraulic press (XPress 3635, Spex SamplePrep, UK). Acid decompositions were performed in a high-pressure microwave oven (4 Speedwave, Berghof, Germany).

2. Experimental

2.3. Sample preparation

2.1. Instrumentation

The filter and external paper were removed from approximately 20 cigarettes, resulting in around 5.0 g of tobacco for each cigarette brand. Tobacco samples were freeze-dried for 48 h and ground cryogenically for 10 min. Analyte concentration was subsequently determined by ICP OES. Acid decomposition was carried out in a closed-vessel microwave oven using 0.2 g of sample and diluted oxidant mixture (2.0 mL nitric acid, 1.0 mL hydrogen peroxide, and 3.0 mL deionized water). The heating program was executed in three steps (temperature, °C, ramp (min), hold (min)): 1 (140, 5, 1); 2 (180, 4, 5); and 3 (200, 4, 10) using 725 W of power and a maximum pressure of 50 bar. After heating, the sample was diluted with deionized water to a final volume of 10.0 mL. The same procedure was adopted to the CRMs to check the method accuracy. For LIBS analysis, sample pellets were prepared by transferring 0.5 g of the ground sample to a 13 mm die set and applying 10 ton cm− 2 (metric tons) for 5 min. The pellets were approximately 3 mm thick, with 13 mm diameter. LIBS calibration was performed with a selected milled tobacco sample either diluted with high purity cellulose in the proportion of 10, 20, 30, and 50% (w w−1), or spiked with increasing amounts of analyte. Spiking was achieved by adding 3.5 g of milled tobacco to 20.0 mL of successive dilutions of stock solutions containing increasing amounts of Ba, Ca, K, and Mn. After shaking for 2 min, the mixture was frozen using liquid N2 and freeze-dried for 48 h. The dried samples were homogenized in a cryogenic mill for 6 min and pelletized prior to LIBS analysis. The mass fraction references of each standard were obtained by ICP OES after acid decomposition.

A J200 Tandem LIBS system (Applied Spectra, CA, USA) with a Nd:YAG laser operating at 266 nm (laser beam diameter of 4 mm, 25 mJ per pulse, and pulse duration of 6 ns) was used. The LIBS system contained a high resolution spectrometer CCD 6 channels (Aurora, Applied Spectra, USA) with a spectral range from 186.132 to 1044.359 nm. Analyses were performed using the instrumental parameters showed in the Table 1 and the C(I) (247.856 nm) emission line was used as the internal standard. An iCAP 6300 Duo ICP optical emission spectrometer (Thermo Fisher Scientific, Cambridge, England) equipped with axially and radially viewed plasma was used throughout for plasma analysis. The spectrometer had a simultaneous charge injection device (CID) detector (166.25 to 847.00 nm detection range), and argon was used to purge the Echelle polychromator. The sample introduction system was composed of a cyclonic spray chamber and a Meinhard nebulizer. The injector tube of the

Table 1 LIBS instrumental parameters adopted in analysis. Instrumental parameter

Adopted

Energy per pulse (mJ) Spot size (μm) Repetition rate (Hz) Accumulated laser pulses Delay time (μs) Integration time (ms) Emission lines(nm)

25 50 10 100 0.25 1.050 Ba(II) 493.409 Ca(I) 422.673 K(I) 766.490 Mn(II) 259.373 C(I) 247.856

2.2. Reagents and samples Aqueous solutions were prepared using high-purity water (18 MΩ cm) obtained from a Milli-Q purification system (Millipore, Bedford, MA, USA). For ICP OES calibration, solutions with 20% nitric acid (v v−1) were prepared by serial dilutions of stock solutions containing 1000 mg L−1 of Ba (BaCl2), Ca (CaCl2), K (KCl), and Mn (MnCl2) (Merck, Germany). Acid decomposition was conducted using a mixture of nitric acid (65% w w−1) and hydrogen peroxide (30% w w−1) (Merck, Germany). Ten cigarette and tobacco samples from different brands (C1 to C10) were purchased in a local market of São Paulo, Brazil. Cellulose powder (3642 Cellulose Binder, Spex NJ, USA) was used to prepare calibration standards by either dilution of a selected milled tobacco sample or spiking with increasing amounts of Ba, Ca, K and Mn. The accuracy of the elemental determination method by ICP OES was evaluated by analyzing three certified reference materials (CRMs) from NIST (National Institute of Standards and Technology, Gaithersburg, Maryland, USA): apple leaves (SRM 1515), pine needles (SRM 1575a), and tomato leaves (SRM 15713a).

2.4. ICP OES optimization To evaluate the robustness of the method, the emission line intensity ratio of Mg (II) (280.2 nm) and Mg (I) (285.2 nm) was checked with a

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6

6

K I 766.489 nm

6

3.6x10

K I 769.896 nm

Intensity (a.u.)

Ca II 396.847 nm

2,0x10

B

Ca II 393.366 nm

6

2.7x10

6

1.8x10

5

6

1,2x10

5

8,0x10

5

9.0x10

4,0x10

0.0 0 50 ns 100 ns 150 ns 200 ns 250 ns 300 ns 392

394

396

398

0,0 0 50 ns 100 ns 150 ns 200 ns 250 ns 300 ns 762

400

764

Wavelength (nm)

766

768

770

772

Wavelength (nm) 5

5

4.0x10

C

D

Ba II 493.407 nm 5

1.5x10

Mn II 259.373 nm

5

2.4x10

5

1.6x10

4

Mn II 260.568 nm

Intensity (a.u.)

3.2x10

4

9.0x10

4

6.0x10

4

3.0x10

8.0x10

0.0 0 ns 50 ns 100 ns 150 ns 200 ns 250 ns 300 ns

0.0 0 ns 50 ns 100 ns 150 ns 200 ns 250 ns 300 ns 491

492

493

494

495

496

Wavelength (nm)

5

1.2x10

Intensity (a.u.)

390

6

1,6x10

Intensity (a.u.)

4.5x10

A

258

259

260

261

262

Wavelength (nm)

Fig. 1. – Temporal evolution of the plasma spectrum induced in a cigarette pellet. Signal data accumulated from 100 laser shots. Fragments of spectrum showing the decay of the lines (A) Ca (II) 393.366 nm and Ca (II) 396.847 nm, (B) K (I) 766.489 nm and K (I) 769.896 nm, (C) Ba (II) 493.407 nm, and (D) Mn (II) 259.373 nm and Mn (II) 260.568 nm.

standard solution containing 1 mg L− 1 of Mg. The radiofrequency power was varied from 750 to 1350 W, while the other parameters of the spectrometer were kept constant.

The matrix effects on the sensitivity and selectivity were studied by scanning the emission lines of the standard solution containing 10 mg L− 1 of each analyte as well as the digested sample. Results

Fig. 2. – Integrated area as a function of spot size (■), fluence ( ) and signal background ratio (○) for Ba, Ca, K, and Mn.

Recovery (%)

84 99 97 94 52.7 ± 1.7 49,753 ± 1964 26,141 ± 988 230 ± 19 101 101 85 110

63* 50,500 ± 900 27,000 ± 500 246 ± 8

Found (μg g−1)

LIBS measurements were carried out at 30 different sites on pellet surfaces in order to minimize drawbacks related to sample microheterogeneity. The average of each 10 spectra was considered a replicate. For background (BG) correction, the BG average of regions surrounding each the selected emission lines was calculated and subtracted from the maximum intensity of each selected emission line. The LOD was estimated according to International Union of Pure and Applied Chemistry (IUPAC) recommendations [29]. 3. Results and discussion

6.0 ± 0.2 2500 ± 100 4170 ± 70 488 ± 12* 6.1 ± 0.3 2528 ± 236 3538 ± 314 539 ± 65 93 101 93 101 49 ± 2 15,260 ± 100 16,100 ± 200 54 ± 3 45.5 ± 1.2 15,439 ± 801 14,961 ± 953 54.6 ± 4.4 0.19 5.73 188.62 1.07 0.02 0.57 18.86 0.11 0-25 0-300 0-200 0-25 Ba (II) 233.527 Ca (II) 318.128 K (I) 769.896 Mn (II) 257.61

*non-certified values

Certified (μg g−1)

2.5. LIBS analyses

0.9997 0.9938 0.9855 0.9988

Pine Needles SRM 1575a

Found (μg g−1) Found (μg g−1)

LOQ (μg g−1) LOD (μg g−1) Linear Range (mg L−1)

r2

obtained by this evaluation were used to choose the analytical emission line and the signal-to-background ratio (SBR) for each element. After this, the background correction was manually selected for each emission line in quantitative measurements. The limits of detection (LOD) and limits of quantification (LOQ) were calculated using a previously reported methodology [28].

3.1. LIBS optimization

Emission Line (nm)

Table 2 Evaluation of accuracy using SRM (1515, 1575a e 1573a) for ICP OES analysis.

Recovery (%) Apple Leaves SRM 1515

Certified (μg g−1)

Recovery (%)

Certified (μg g−1)

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Tomato Leaves SRM 1573a

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To obtain the best analytical performance, LIBS analysis requires proper optimization of instrument parameters such as delay time and laser energy. For this purpose, the effects of delay time, spot size, laser energy, and number of pulses on obtained signals were checked by monitoring the atomic/ionic emission and SBR of the analytes. In general, LIBS measurements are delayed because in early stage is characterized by continuum emission from microplasma due bremsstrahlung processes, which involves collisions of electrons with ions and atoms and recombination of electrons with ions and can affect the detection of weaker line emissions. The time lag between plasma formation and the start of plasma light observation is named delay time [22]. The influence of delay time was evaluated from 0 to 300 ns after the laser pulse with background emission continuously decreasing at longer delay times (Fig. 1). An intense contribution of the continuum emission was observed, mainly for barium and manganese, when the delay time was shorter than 50 ns. After 100 ns, the continuum emission quickly decreased for all analytes due to the expansion and concurrent cooling of the plasma. However, after 250 ns, the SBR also decreased significantly. Therefore, a delay time of 250 ns was selected for further analysis of the analytes as a compromise to maintain a high SBR for the selected wavelengths. The spot size is linked with the lens-to-sample distance (LTSD). The LTSD affects the laser energy delivered on the sample surface, and consequently, the amount of mass removal by ablation and emission line intensities [30]. The effect of spot size on the analyte emission signals was evaluated from 35 to 120 μm, and the integrated area of emission signal and SBR were monitored (Fig. 2). When smaller spot sizes are used, smaller amounts of material are ablated. This condition is not adequate in non-homogeneous samples. However, the net fluence is higher, which could favor the analyte excitation process, thereby improving signal intensity. Therefore, a spot size of 50 μm was chosen by considering the analyte emission intensity and coefficient of variation (CV). The energy per pulse was monitored from 5 to 25 mJ pulse−1, with the higher energy chosen because it presented the highest analyte emission intensities. The number of laser pulses in each sample spot was evaluated, with the analyte emission intensities increasing linearly when the laser pulse was increased from 5 to 75 pulses. Above 100 laser pulses, emission intensities decreased and deviations in the measurement increased for the standards. It is possible that the increased ablation expected for higher numbers of laser pulses results in the formation of deep craters, making it difficult for emissive radiation to reach the spectrometer [31].

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Table 3 Figures of merits: linear range, correlation coefficient (r2), slope, LOD, and LOQ for LIBS analysis. Emission Line (nm)

Ba (II) 493.409 Ca (I) 422.673 K (I) 766.49 Mn (II) 259.373

Linear Range (μg g−1)

85-280 9000-20,000 16,000-60,000 100-500

Without Internal Standard

With Internal Standard (C(I) 247.856 nm)

Slope

Intercept

r2

Slope

Intercept

r2

LOD (μg g−1)

LOQ (μg g−1)

604.4 86.5 132.4 235.9

70,379.6 999,366.3 5,168,023.9 19,095.8

0.6944 0.7288 0.7329 0.9506

1.1 × 10−3 1.4 × 10−4 2.5 × 10−4 3.2 × 10−4

−5.2 × 10−3 4.7 × 10−2 1.3 2.9 × 10−4

0.9818 0.9785 0.9859 0.9940

25 488 197 32

84 1610 651 105

3.2. Analytical figures of merit and elemental analysis Analysis of materials by ICP OES after acid digestion was performed to obtain the mass fraction reference values. The method was applied to 3 CRMs after acid digestion, and no significant differences were observed for Ba, Ca, K, and Mn in the t-test at a confidence level of 95%. The results and figures of methods are presented in the Table 2. However, it is important to mention that by adding the equivalent amounts of the acid in the analytical reference solutions allowed the best recovery of elements. This additional step in preparation of solutions was due to the fact that an excess of acid may interfere in the sample injection rate, change in distribution of aerosol droplet size and in the plasma excitation conditions. These factors are influenced by the high viscosity of digested solutions, surface tension and high consumption of energy during the process of formation of the species in the plasma [32–33]. Considering this analytical strategy, the results obtained in the recovery test showed a range of 84–110% for all target elements, demonstrating no interference from the matrix in the proposed method. LIBS figures of merit were evaluated by linear range, correlation coefficient (r2), and LOD and LOQ in μg g−1 (Table 3). The LOD of LIBS method for Ba, Ca, K, and Mn was 25 μg g−1, 488 μg g−1, 197 μg g−1, and 32 μg g−1, respectively. Despite the LOD and LOQ obtained by LIBS for all elements is larger than observed for ICP OES, LIBS method can be considered an interesting alternative since is allows a reduction in the sample pretreatment step, it reduces the consumption of reagents contributing to the green chemistry and helps in the development of new analytical procedures. The linear range for Ba (85–280 μg g−1) and Mn (100–500 μg g− 1) was smaller than that obtained for Ca (9000–20,000 μg g−1) and K (16,000–60,000 μg g−1) because these elements showed a weak relation between emission intensity and high

concentrations of calibration standard. The linear coefficient correlation (r2) for Ba, Ca, and K was less than 0.75 when the analysis was performed without an internal standard, due to the ablation process of these analytes. Considering that all samples and the standard had high carbon concentrations, carbon was chosen as an internal standard in this work, using the 247.856 nm carbon emission line. Using carbon as an internal standard increased the coefficient correlation to approximately 0.99 for all elements, indicating that the carbon emission line was effective as an internal standard in the correction of ablation processes for analytes. The results of LIBS analysis using the optimized instrument parameters described above showed good agreement with the results obtained by ICP OES (Fig. 3), with the t-test yielding a confidence level of 95%. Agreement between ICP OES and the proposed analytical strategy demonstrates that our method can be accurately applied to determine the concentration of major elements in tobacco samples. This is in agreement with other studies that determined the concentrations of toxic elements in tobacco samples by LIBS [2,5,8].

4. Conclusions A calibration strategy was developed using the carbon emission line as an internal standard for determining elemental concentrations in tobacco samples by LIBS. The accuracy of the method was checked by the analysis of CRMs and by comparison with the results obtained in ICP OES after acid decomposition. The results showed 95% confidence level when Student's t-test was applied, indicating the proposed strategy could be recommended for tobacco direct analysis using synthetic solid standards by LIBS.

Fig. 3. The elemental concentrations for 10 cigarette samples (C1 – C10) determined by LIBS and ICP OES.

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