Melted Paraffin Wax as an Innovative Liquid and ... - ACS Publications

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Melted Paraffin Wax as an Innovative Liquid and Solid Extractant for Elemental Analysis by Laser-Induced Breakdown Spectroscopy Rodrigo Papai,† Roseli Hiromi Sato,† Lidiane Cristina Nunes,‡ Francisco José Krug,‡ and Ivanise Gaubeur*,† †

CCNH, Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, UFABC, Avenida dos Estados, 5001, Bloco B, 09210-580, Santo André, São Paulo Brazil ‡ CENA, Centro de Engenharia Nuclear na Agricultura, Universidade de São Paulo, Avenida Centenário 303, 13416-000, Piracicaba, São Paulo Brazil S Supporting Information *

ABSTRACT: This work proposes a new development in the use of melted paraffin wax as a new extractant in a procedure designed to aggregate the advantages of liquid phase extraction (extract homogeneity, fast, and efficient transfer, low cost and simplicity) to solid phase extraction. As proof of concept, copper(II) in aqueous samples was converted into a hydrophobic complex of copper(II) diethyldithiocarbamate and subsequently extracted into paraffin wax. Parameters which affect the complexation and extraction (pH, DDTC, and Triton X-100 concentration, vortex agitation time and complexation time) were optimized in a univariate way. The combination of the extraction proposed procedure with laser-induced breakdown spectroscopy allowed the precise copper determination (coefficient of variation = 3.1%, n = 10) and enhanced detectability because of the concentration factor of 18 times. A calibration curve was obtained with a linear range of 0.50−10.00 mg L−1 (R2 = 0.9990, n = 7), LOD = 0.12 mg L−1, and LOQ = 0.38 mg L−1 under optimized conditions. An extraction procedure efficiency of 94% was obtained. The accuracy of the method was confirmed through the analysis of a reference material of human blood serum, by the spike and recovery trials with seawater, tap water, mineral water, and alcoholic beverages and by comparing with those results obtained by graphite furnace atomic absorption spectrometry.

A

and versatility, the low amount of sample mass ablated by LIBS analysis can result in low detectability and precision, sometimes seen as inappropriate for quantitative applications.6,15 Numerous methods have been developed to perform elemental analysis using LIBS in solid samples,16,17 but a few methods propose the use of this technique in liquid samples.18−23 The direct analysis of liquid samples by LIBS presents some challenges, including low analytical signal intensity, since part of laser energy is used to vaporize the solvent24 and splashing occurrence on the liquid surface;25 both phenomena can affect the results’ representativeness.26 To avoid the drawbacks of LIBS in liquid analysis, the transfer of the liquid sample to solid substrate has been a widely used strategy.17,27,28 Some studies describe liquid sample freezing18,29 or evaporation,25,30−33 analyte precipitation,34 extraction in solid phase,6,35−42 analyte electrodeposition,43−46 and liquid phase microextraction (LPME) combined with organic extract evaporation on the solid surface.47−51

iming at practicality, while reducing reagent consumption and residue generation, a general trend to reduce sample quantity and method development time has been observed.1,2 New methods described as simple, fast, sensitive, multielement, of low cost and with reduced sample consumption have achieved great interest from the scientific community.3 However, small sample quantities result in both low analytical signals and a decrease of the methods’ detectability, including a lack of both representativeness and homogeneity.4,5 Some procedures, applied prior to the detection step, can lead to analyte separation and preconcentration, thus improving detectability and selectivity, and also allow the elimination interference by the matrix. The analytical methods, which allow a multielement analysis with a simple sample preparation step, are currently exploited by using laser-induced breakdown spectroscopy (LIBS) for solid, liquid, and gas samples analysis.6−9 LIBS uses pulsed laser energy (typically tens to hundreds of millijoules per pulse) for ablation and generation of plasma as excitation source which vaporizes a small amount of the sample (10−300 μg).10 Spectra emitted are used for the identification and quantification of excited species from the sample.7,11−14 Despite its simplicity © 2017 American Chemical Society

Received: September 23, 2016 Accepted: February 3, 2017 Published: February 3, 2017 2807

DOI: 10.1021/acs.analchem.6b03766 Anal. Chem. 2017, 89, 2807−2815

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Analytical Chemistry

All buffer solutions used to evaluate pH optimization are presented in Supporting Information. Buffer solution comprising 1.5 mol L−1 C6H8O7 (citric acid) and 4.33 mol L−1 NaOH was used after pH optimization at 5.0. When necessary, the pH of the buffer solutions was adjusted by adding 2 mol L−1 nitric acid or 5 mol L−1 sodium hydroxide. Paraffin wax with different melting temperature ranges (53−57, 58−62, and ≥65 °C), purchased from Sigma-Aldrich (ASTM D 87) and paraffin wax typically used for manufacturing candles (purchased in a local market) were evaluated. General Instrumentation. An analytical balance (MSA 255P-1CE-DA, Cubis, Sartorius) was used. The UV−vis measurements were made using a spectrophotometer with diode array detector (Cary 8453, Agilent). The pH values were measured with a pH meter Metrohm Model 914 equipped with a combined glass electrode. For temperature control, when necessary, a hot plate was used (CS-MAG HS 7, IKA-Werke GmbH & Co. KG). For the dispersion of melted paraffin wax, a vortex agitator (AP56, Phoenix Luferco) was used. LIBS Instrumentation. Experiments were carried out with a Q-switched Nd:YAG laser (Brilliant, Quantel, France) at 1064 nm, generating 5 ns pulses up to (365 ± 3) mJ, in a 6 mm diameter beam with M2 factor 3.95, the deprotonated form (DDTC−) is predominant in solution and the complexation reaction is favorable.60 When the pH is higher than 7.0 hydrolysis of Cu(II) is deemed to occur, therefore resulting in a decrease in the analytical signal.62 From pH above 11.0, the formation of a precipitate of [Cu(DDTC)2] was observed, which resulted in a heterogeneous distribution in the melted 2810


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Figure 3. Effects of (a) number of pulses, (b) laser fluence, (c) delay time, and (d) integration time gate in analytical signal and the signal-to-noise ratio obtained by LIBS. Experimental conditions: 60 J cm−2, averaged spectra from 10 sites per solid extract. Error bars correspond to ±1 standard deviation of 3 solid extracts 10.00 mg L−1 Cu.

properties.65 LIBS operational parameters, such as number of accumulated laser pulses, fluence, delay time, and integration time gate were defined by univariate optimization by using a solid extract corresponding to 10.00 mg L−1 Cu in solution and the obtained results are shown in Figure 3a−d. Because of higher ablation, the integrated area increased with the number of pulses. Number of pulses greater than 50 was not possible, since the depth of the sample was exceeded. For the effects of the laser fluence, two and 3-fold improvements in emission signal intensities and signal-to-noise ratios, respectively, were observed when fluence was increased from 35 (160 mJ per pulse) to 60 J cm−2 (275 mJ per pulse). This positive relationship between emission intensity and fluence was related to improvements in the ablation efficiency and atomization/excitation processes within the laser-induced plasma.56 Delay time was chosen at 2.0 μs, because at 1.5 μs continuous radiation emitted by the plasma was observed, reducing the signal-tonoise ratio. There was no statistically significant difference at 95% probability confidence level in the analytical signal at different integration time gates, but a small improvement in the signal-to-noise ratio was obtained at 10.0 μs. Higher signal-tonoise ratios and emission intensities for Cu I 324.755 were obtained after 2.0 μs delay, 10 μs integration time gate, 50 accumulated laser pulses per site and 60 J cm−2 (Figure 3a−d). These parameters were selected for further experiments. Table 1 shows a summary of all the chemical and instrumental parameters evaluated, range evaluated, and the best conditions selected for developing the proposed method. Analytical Features, Extraction Efficiency, and Homogeneity. The LIBS calibration curve, fragment of LIBS spectra

was not required. Even observing a quick separation between the extractant and the aqueous phase, 20 s were set prior to this separation, which also favored the elimination of air bubbles in the extract. The time influence on complex formation was evaluated immediately after mixing the reagents (t = 0 min) and after different complexing times, up to 60 min. At room temperature, the complex formation is dependent on the time, reaching a maximum at around 45 min (Figure S2). However, the results obtained using the extraction procedure, in which solution is heated at 85 °C for 120 s, did not rely on the time of complexation. It is believed that the heating step, before adding the melted paraffin wax, favored the kinetics of the complexation reaction. On the basis of the results, it was decided to carry out the extraction immediately after mixing the solutions (t = 0 min), which resulted in an increase in the analytical frequency. Regarding the 4 types of paraffin wax no significant differences were observed in the extraction efficiency, suggesting that all of them could be used as extractant. However, by using paraffin wax with mp ≥ 65 °C, a solidification process was noted during the vortex agitation step which required an additional heating step before removing the extract to an acrylic support. Paraffin wax with mp 53−57 °C was selected for the extractant. Figure S3 presents the integrated area versus pH, DDTC:Cu(II) ratio, Triton X-100 concentration, vortex agitation time, time complexation, and different types of paraffin wax. Instrumental LIBS Optimization. It has been demonstrated that the analytical performance of LIBS depends on the operational parameters, laser characteristics, and sample 2811


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Analytical Chemistry Table 1. Chemical and Instrumental Conditions Evaluated and Optimized

chemical parameters

LIBS instrumental parameters

variablea

range investigated

optimum condition

pH [DDTC]:Cu ratio Triton X-100 agitation vortex time complexation time paraffin wax type pulses number fluence delay time integration time gate

1.0−12.0 0:1−150:1 0−1.0 g L−1 0−120 s 0−60 min 53−65 °C 15−50 35−60 J cm−2 1.5−3.0 μs 3.0−10.0 μs

5.0 100:1 0.25 g L−1 75 s 0 min 53−57 °C 50 60 J cm−2 2.0 μs 10.0 μs

Table 2. Analytical Characteristics of the Paraffin-LIBS Method for the Copper Determination parameters

value

linear range (mg L−1) coefficient of determination (R2)a sensitivity (L mg−1)a detection limit (mg L−1) quantification limit (mg L−1) reproducibility (%)b

0.50−10.00 0.9990 1443 ± 21 0.12 0.38 3.1

a

Number of calibration points = 7. bEstimated from measurements carried out in 10 solid extracts with 5.00 mg L−1 Cu (10 sites per solid extract and 50 pulses per site).

per site). This result (3.5%) is in agreement with the reproducibility obtained from standard way (3.1%, Table 2) of 10 replicates of 5.00 mg L−1 Cu a given concentration, in this case (1 replicate = 10 sampling sites in one solid extract, 50 pulses per site). A trending decrease in CV with increasing copper concentration was observed. Although, the site-to-site precision is a good indication of the homogeneity of the copper distribution in the solid extract, the degree of homogeneity, represented by the homogeneity constant (He),5,68 was also determined by applying the equation He = CV(%) × √m, where m is the ablated mass in mg, and the CV used was the highest site-to-site measurements (8.6%). Because of the solid extract stability, even over long storage periods, it was possible to estimate the ablated mass (583 ± 19 μg) by weighing the solid extracts before and after LIBS analysis with 50 laser pulses at 60 J cm−2 per site. Resulting homogeneity constant was 6.6, which is an additional indication that the material may be regarded as sufficiently homogeneous for analysis. For this attribution the boundary condition is He < 10.5,68 The extraction efficiency was evaluated from the mass of Cu(II) remaining in the aqueous phase after the extraction procedure determined by GF AAS. The copper mass transferred to the extractant phase was estimated by the difference between the initial mass of copper in the solution (mi) and the mass found in the solution by GF AAS (mf). The recovery of analyte was used as an indication of the efficiency of the extraction procedure, R(%) = 100 × (mi − mf)/mi. The value of R = 94.1 ± 0.2% was estimated graphically by linear regression of Cu(II) extract into paraffin wax versus the initial copper mass, in Figure S4. The enrichment factor E ≅ 18 was estimated using R value and aqueous and organic phase volume. The surface morphology of the solid extracts containing different copper concentrations was analyzed by atomic force microscopy (AFM). Table 3 show the mean roughness values69

a

The variables are arranged in the table in the order in which they have been optimized from top to bottom. Once the best condition of a variable is set, it was fixed and kept constant for the next study.

for Cu I 324.755 nm and solid extracts for different Cu concentrations are shown in Figure 4. Linear responses ranging

Figure 4. Copper calibration curve from solid extracts standards analyzed by LIBS. Conditions: 60 J cm−2, averaged spectra from 10 sites, 50 pulses per site. Error bars correspond to ±1 standard deviation of 3 standard solid extracts.

from 0.50 to 10.00 mg L−1 were observed, being described by the equation I = 1443(±21) × C − 59(±107), R2 = 0.9990, where I is emission intensity and C is Cu(II) concentration solution in milligrams per liter. The detection and quantification limits (LOD and LOQ) were estimated according to the IUPAC recommendations at a 99% confidence level.66,67 LOD = 3σB/s and LOQ = 10σB/s, where σB is the standard deviation of 10 replicates of method blank (1 replicate = 10 sampling sites in one method blank and 50 pulses per site) and s is the slope of the calibration curve. Table 2 summarizes the analytical features of the proposed method. Precision of the developed method was assessed by repeatability and by reproducibility. Coefficient of variation to all solid extracts obtained in this work in site-to-site measurements (repeatability) varied from 0.7% to 8.6% (n = 1 solid extract, 10 sites, 50 pulses per site) and the extract-to-extract measurements (reproducibility) ranged from 0.8% to 18.5% with an average value of 3.5% (n = 3 solid extracts, 10 sites per extract, 50 pulses

Table 3. Roughness of Paraffin Solid Extractsa

a

extract

Ra (nm)

method blank 0.75 mg L−1 Cu 2.50 mg L−1 Cu 5.00 mg L−1 Cu 7.50 mg L−1 Cu

100 66 61 53 35

Results from atomic force microscopy images.

(Ra) calculated through AFM images, shown in Figure S5. A trend of Ra decreasing was observed with the gradual addition of the copper complex. 2812


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be seen in Table 4, recoveries ranging between 92% and 106%, 92% and 117%, 92% and 130%, and 80% and 95% were obtained for samples containing mineral water, tap water, seawater, and cachaça, respectively. On the basis of the results achieved, it is possible to conclude that the proposed method can be used to quantify Cu(II) in all studied samples without any significant interference from the matrix. The slightly lower recovery values obtained for the cachaça sample can be related to some matrix interference affecting the extraction of the analyte. It was observed that during the extraction procedure paraffin wax extracted other compounds present in the cachaça, giving the solid extracts a odor, very typical of such beverages. The proposed method was also used for another cachaça sample and the result was subsequently compared with that obtained by GF AAS. Table 5 shows the values and according

Figure 5. Solid extracts used for calibration curve before and after analysis by LIBS. From left to right, it is a method blank to a concentration of 10.00 mg L−1 Cu.

Figure 5 shows a photograph of solid extracts, before and after the LIBS analysis used to obtain the calibration curve. Accuracy Evaluation and Applicability of the Method to Real Sample Analysis. To verify the accuracy of the proposed method, an analysis was carried out of reference material of human blood serum containing Ca, Cu, Fe, Li, Mg, P, Zn, Na, K, and S as major components at concentrations ranging from 1 to 3000 mg L−1 and Al, Cr, Co, Au, Mn, Hg, Ni, and Se, as minor components, at concentrations ranging from 0.7 to 540.0 μg L−1. Moreover, there are other 50 element indicative values, ranging from 0.7 ng L−1 to 740 μg L−1. There was no significant difference between the reference value (1.69 ± 0.08) mg L−1 Cu obtained by inductively coupled plasma-sector field mass spectrometry (ICP-SFMS)70 and the one determined by LIBS (1.77 ± 0.08) mg L−1 Cu at a 95% confidence level. From the results achieved, the possible matrix effects influencing the extraction procedure, such as competitive chelate formation with concomitant metals, or preferential diffusion of such concomitant metals chelates into the extractant phase, were not observed in the proposed method. It is also worth noting that the standard deviation obtained is very close to that obtained by more established methods such as ICP-SFMS. Although LIBS is known for its low precision of analysis, this method provides a very significant improvement in the reproducibility with a CV of only 4.5%. In the extraction procedure applied to the reference material, the precipitation of a white solid in the heating step was observed, very similar to protein denaturation. During the vortex agitation, the white solid was broken, partially solubilizing in the melted paraffin wax and allowing the slightly turbid aqueous phase after separation. This behavior did not interfere with results, allowing copper quantification. Fragments of LIBS spectra obtained after extraction showed Fe and Zn emission lines, indicating that these elements were also extracted into paraffin wax. Spiked and recovery trials were done in four different samples: mineral water, tap water, seawater and cachaça. As can

Table 5. Copper Determination in Cachaça by LIBS and GF AAS sample

LIBS (mg L−1)

cachaça (Ipê-Roxo)b

1.09 ± 0.06

a

GF AASa (mg L−1)

concordancec (%)

1.10 ± 0.04

99.1

b

Sample diluted 1:20 for analysis. Aged in a barrel made from Handroanthus avellanedae for 3 years. cConcordance is defined by (LIBS value × 100)/(GF AAS value).

to a statistical t test performed at a 95% confidence level, no significant differences were found between the two methods.

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CONCLUSIONS When using melted paraffin wax as an extractant, it was possible to add some of the advantages of liquid−liquid extraction (extract homogeneity, short-time extraction, low cost, simplicity in operation) to the solid-phase extraction. The procedure developed in this study can be seen to be simple, quick and practical in terms of execution characteristics that minimize sample preparation errors. Notable is the high efficiency (∼94%) of transfer of the analyte from the aqueous to the organic phase and the fast obtaining of a solid phase of high homogeneity ready and adequate to be used in methods employing microsampling and/or more adequate to solid analysis, for example, laser ablation inductively coupled plasmamass spectrometry (LA-ICP-MS), LIBS, solid sampling graphite furnace atomic absorption spectrometry (SS GF AAS), X-ray fluorescence (XRF), and energy-dispersive spectroscopy (EDS).

Table 4. Determination and Recovery of Copper in Seven Different Levels of Intentional Contamination in Real Samples mineral water added (mg L−1) 0.00 0.50 0.75 1.25 2.50 5.00 7.50 10.00

found valuea

seawaterc

tap water found valuea

recovery (%)

b

b 0.53 0.69 1.15 2.40 4.68 7.28 9.70

± ± ± ± ± ± ±

0.07 0.05 0.09 0.20 0.11 0.25 0.27

106 92 92 96 94 97 97

± ± ± ± ± ± ±

14 6 7 8 2 3 3

found valuea

recovery (%)

cachaçad

b 0.55 0.88 1.15 2.30 4.67 6.97 9.90

± ± ± ± ± ± ±

0.05 0.03 0.06 0.16 0.10 0.20 0.56

110 117 92 92 93 93 99

± ± ± ± ± ± ±

10 5 5 6 2 3 6

found valuea

recovery (%)

recovery (%)

b 0.54 0.98 1.42 3.25 4.80 6.89 9.63

± ± ± ± ± ± ±

0.09 0.18 0.10 0.15 0.25 0.23 0.42

108 130 113 130 96 92 96

± ± ± ± ± ± ±

18 24 8 6 5 3 4

0.47 0.67 1.10 2.31 4.75 6.03 8.19

± ± ± ± ± ± ±

0.06 0.13 0.05 0.17 0.32 0.50 0.43

94 90 88 92 95 80 82

± ± ± ± ± ± ±

12 17 4 7 6 7 4

Value ± experimental standard deviation, n = 3. bNot detected. cSample collected in Santos, State of São Paulo, Brazil, and filtered through a cellulose membrane of 0.45 μm prior to analysis. dSample of cachaça aged in an oak cask for 12 years. a

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The combination of this proposed procedure, by using melted paraffin wax as an extractant followed by LIBS, makes it possible, in addition to improving detectability and elimination of matrix interference, to largely improve the precision level of results. This procedure requires heating of the solution, which favors the reaction kinetics between Cu(II) and DDTC; however, this means it is not possible to use the proposed method to determine volatile chemical elements. This work inaugurates a range of procedures that combine the two extraction methods widely used in the preparation of samples (solid phase and liquid phase extractions), opening up possibilities of research toward the search for new chemical compounds that present this ability to join both methods. It is believed that the greatest legacy of this work is to provide an extraction procedure that has great potential for the preservation of constituents of liquid samples, immobilizing the analytes in a stable solid phase until the appropriate moment of analysis. Paraffin wax can also be used as a solid reference material, which is scarce, after metals immobilizing. The method was also successfully used in quite distinct samples (seawater, alcoholic beverage, and human blood serum), proving a high level of effectiveness in separating the analyte from the matrix. The procedure certainly can be easily applied to the extraction of a wide range of organic and inorganic compounds, covering many possibilities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b03766. GF AAS heating program, extraction procedure details, LIBS spectrum paraffin wax (method blank), time-scan of complex formation in presence and absence of surfactant, complexation and extraction variables optimization, linear regression used to calculate extraction efficiency, AFM images, and supplementary experimental section (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +55 11 4996-0187. Fax: +55 11 4996-0187. ORCID

Ivanise Gaubeur: 0000-0002-0352-5289 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS FAPESP (São Paulo Foundation Research 2011/19730-3 and 2016/12095-4). CAPES (Coordination of Superior Level Staff Improvement) for granting a postgraduate scholarship. We thank André Sarto Polo (PhD) for allowing the use of a UV− vis spectrophotometer, Andressa Vidal Muller (Postgraduate student) for assisting in the preparation of some figures, and Cassiana Seimi Nomura (PhD) and Daniel Menezes Silvestre (Postgraduate student) for making it possible to complete the initial training in LIBS.

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REFERENCES

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