Amanda M. D.
de Jesus
ab,
Miguel Ángel
Aguirre
b,
Montserrat
Hidalgo
b,
Antonio
Canals
b and
Edenir R.
Pereira-Filho
*a
aDepartment of Chemistry, Federal University of São Carlos, Rodovia Washington Luiz, km 235, Caixa Postal 676, São Carlos, CEP 13565-905, São Paulo, Brazil. E-mail: erpf@ufscar.br
bDepartment of Analytical Chemistry and Food Science and University Institute of Materials, Faculty of Science, University of Alicante, Alicante E-03080, Spain
First published on 25th June 2014
Laser-induced breakdown spectroscopy (LIBS) is a promising analytical technique with well-known advantages and limitations. However, despite its growing popularity, this technique has been applied mainly to solid samples and there have been a smaller number of studies devoted to liquid samples. This lack of studies is mainly due to experimental difficulties in the analysis of liquid matrices. Sensitivity can be improved and matrix effects minimized in the LIBS analysis of aqueous samples by using a dispersive liquid–liquid microextraction (DLLME) procedure followed by drying the extract on a suitable surface prior to laser irradiation. The combination of DLLME-LIBS is fast, easy to use, and inexpensive. The small volume of the final extract is sufficient for LIBS analysis, and the procedure generates little waste. It is likely that this combination could be automated during future work. The limits of detection (LOD) and quantification (LOQ) achieved using the proposed method were 30 and 70 μg L−1 for Mo and 5 and 20 μg L−1 for V, respectively. Using this method, we analyzed samples of pharmaceutical, multimineral formulation, soil, mineral water and a reference material NCS ZC 85005 (Beef Liver). In the latter, the concentration of V was below the LOQ, and the recovery of Mo was 103%.
The LIBS technique has been successfully used for the determination of elements in different types of samples. These include biological materials,2,3 metal alloys,4,5 polymers,6,7 soil and minerals,8,9 and geological samples,10 among others.11,12 LIBS is applied mainly to solid samples, primarily because the samples can be analyzed directly without further preparation if standards are available.
The determination of V and Mo is generally difficult. This is especially true in the case of aqueous samples. The most common experimental difficulties when using LIBS are the formation of plasma and the generation of bubbles that affect the characteristics of subsequent plasmas.13,14 These drawbacks result in poor sensitivity and reproducibility in aqueous samples.13–16
One practical way to circumvent the limitations of LIBS with aqueous samples is to dry the sample on a suitable surface. We present the use of a microextraction technique followed by the evaporation of the organic phase as one reliable example. Liquid–liquid extraction has been widely used to eliminate interference and increase the sensitivity of analytical procedures. There has been an increase in the use of miniaturized liquid–liquid extraction since the year 2000. Among these techniques is dispersive liquid–liquid microextraction (DLLME), which is in accordance with the principles of green chemistry: it is a simple, fast and inexpensive procedure.17
The use of a single drop of DLLME solvent dried on an aluminum surface combines the benefits of preconcentration by microextraction with the advantages of LIBS, such as multi-element determination. The goal of this study was to combine the DLLME technique with LIBS in the determination of V and Mo.
Analytical reference solutions were prepared by diluting stock standard solutions containing 1000 mg L−1 of V and Mo High-Purity Mono Element Standard Solutions (Charleston, USA) with ultrapure water.
The solution of chelating agent 8-hydroxyquinoline (8-HQ) (Vetec, Rio de Janeiro, RJ, Brazil) was prepared daily by dissolving the appropriate amounts of 8-HQ in 10 mL of ethanol and storing these solutions in brown glass flasks. Nitric acid 65% (w/w), H2O2 30% (w/w) and HClO4 65% (w/w) (Merck, Darmstadt, Germany) were used for microwave sample preparation.
A delay system consisting of two pulse generators (delay generator/digital pulse, Model DG 535, Stanford Research Systems, Inc. and 1 Hz to 50 MHz pulse generator, model PM-5715, Philips) was used for synchronizing the firing of the laser and data acquisition. An LG laptop (Intel Core 2, 1.00 GB of RAM and Windows Vista) equipped with AvaSoft© complete software (v. 7.6.1., Avantes) was used for data acquisition.
In order to compare the results obtained, an ICP OES spectrometer (Perkin Elmer, model Optima 4300DV, Norwalk, CT, USA) with dual view capacity but that was operated in the axially viewed plasma mode (radiofrequency power of 1400 W) was used.
A beef liver certified reference material (NCS ZC 85005) was also used. A sample mass of 100 mg was weighed and MW-digested using 10 mL of HNO3 65% (w/w). The digestion program was configured as follows: 20 min at 180 °C (in the first 10 min the temperature was increased from room temperature up to 180 °C). In all cases the microwave power was 1000 W.
The two variables, the pH value and the volume of the extractant solvent, showed a significant effect on the Plackett–Burman experiment. Microsoft Excel was used in these calculations.
Therefore, a central composite design (CCD) was performed to optimize these two variables. Here the variables were investigated at five levels and the coded values ranged from to and Microsoft Excel was also used. Table 1 shows the values established in the CCD to investigate the behavior of pH and extractant solvent (SE) volume and the predictive ability of the emission signals obtained for V and Mo. While carrying out the CCD, 12 additional experiments were performed with the V and Mo concentrations fixed again at 500 μg L−1. Four experiments were performed at the central point (variables coded in 0, see experiments 9–12 in Table 1) to calculate the sum of the squares for the pure error and to evaluate the significance of the coefficient models proposed for V and Mo.
The regression models (only the significant coefficients) proposed for V and Mo are presented as eqn (1) and (2), respectively:
V (emission intensity) = 38328 − 12886pH − 10985(pH2) | (1) |
Mo (emission intensity) = 15823 − 4125pH − 3270SE − 3527(pH2) − 3614(SE2) | (2) |
In the case of V, only the linear and quadratic coefficients for pH presented significant values at a confidence level of 95%. In this case, any extractant solvent volume between the evaluated range (32 and 88 μL) can be used. For Mo both linear and quadratic coefficients of pH and extractant solvent volume were significant. Fig. 2 shows the overlapped contour plots for the models obtained for V and Mo. As observed for V (see vertical lines), high signals are obtained when the pH is in the range of 3.0 to 3.8, but the signal is indifferent to the extraction solvent volume in the evaluated range (32–88 μL). For Mo, an optimal condition exists when the pH value lies between 3.0 and 3.8 and the extraction solvent volume is between 48 and 56 μL (see ellipses). For this reason, a compromise condition is necessary to determine both analytes in the same microextraction procedure. Observing the practical operational conditions, a pH of 3.6 and an extraction volume of 50 μL were chosen as optimal conditions for both the variables studied and both the analytes. The other final optimized conditions for the DLLME procedure were: a concentration of 8-HQ of 0.1(%) w/v, a vortex time of 2 (min), a centrifugation time of 8 (min) and a centrifugation speed of 4000 (rpm).
Fig. 2 Contour plots overlapped for the regression models proposed for V (vertical lines) and Mo (ellipses). The star shows the optimal conditions. |
As mentioned in the experimental section (section 2.5), after the microextraction procedure, a droplet of the organic layer with a volume of 10 μL was dried on an aluminum plate (see details in Fig. 1) and then subsequently analyzed by LIBS.
Fig. 3 shows some emission signals obtained for V (Fig. 3a) and Mo (Fig. 3b) when 10 μL aqueous standard solutions were analyzed by only LIBS (40 mg L−1), i.e. without the prior DLLME procedure and by DLLME-LIBS (100 μg L−1). As can be observed when 100 μg L−1 of V and Mo was determined combining DLLME-LIBS it was possible to obtain analytical signals in the same order of magnitude when 40 mg L−1 was determined using only LIBS. The combined method of DLLME-LIBS was linear from 20 to 750 μg L−1 for V and from 70 to 750 μg L−1 for Mo.
Fig. 3 Emission signals of VII (310.23 nm) (a) and MoI (379.83 nm) (b) using LIBS and DLLME-LIBS methodologies. |
A comparison of the figures of merit obtained with the proposed method (DLLME-LIBS) and using only LIBS analysis is shown in Table 2. By using two standard calibration curves with microextraction (DLLME-LIBS) and without microextraction (LIBS), it was possible to estimate the preconcentration factors as 12-fold for V and 9-fold for Mo.
Parameters | VII (310.23 nm) | MoI (379.83 nm) | ||
---|---|---|---|---|
LIBS | DLLME-LIBS | LIBS | DLLME-LIBS | |
a n = 10. b Sensitivity DLLME-LIBS/Sensitivity LIBS. c LOD LIBS/LOD DLLME-LIBS. | ||||
Linear range (number of calibration points = 5) | 0.2 to 40 mg L−1 | 20 to 750 μg L−1 | 0.5 to 40 mg L−1 | 70 to 750 μg L−1 |
Correlation coefficient (number of calibration points = 5) | 0.995 | 0.994 | 0.966 | 0.966 |
Sensitivity (counts L mg−1) | 7575 | 82901 | 1407 | 9810 |
LOD (μg kg−1) | 60 | 5 | 300 | 30 |
LOQ (μg kg−1) | 200 | 20 | 500 | 70 |
Blank signal (mean ± standard deviation) | 145 ± 24 | 158 ± 39 | 387 ± 213 | 245 ± 73 |
Repeatabilitya (500 μg L−1) (RSD %) | — | 6 | — | 9 |
Relative sensitivityb | 11 | 7 | ||
Relative LODc | 12 | 9 |
All the digested samples (pharmaceutical, multimineral formulation and soil), including the reference material (food), were analyzed using only the proposed DLLME-LIBS procedure in order to prove experimentally the feasibility of this combination.
Table 3 shows the results obtained for the pharmaceutical, multimineral formulation and soil samples. These results were compared with those obtained from ICP OES analysis. Using these ICP OES results as reference values, the recovery obtained using the DLLME-LIBS methodology ranges from 92 to 104%. As observed from this table, pharmaceutical (vanadium chelate) and multimineral formulation samples were tested. The first has been suggested for the treatment of diabetes, and the second is a multimineral and multivitamin supplement. The V concentration in the chelate was high (3352 mg kg−1), whereas a much lower concentration was found in the multivitamin sample (9.9 mg kg−1). Only Mo was observed in the multimineral at a concentration of 13.2 mg kg−1. In the case of the soil sample, only V was detected with a concentration of 12.0 mg kg−1.
Samples | Analyte concentration (mg kg−1) | |||
---|---|---|---|---|
ICP OES | DLLME-LIBS (recovery, %) | |||
V | Mo | V | Mo | |
Pharmaceutical (vanadium chelate) | 3210 ± 92 | <LOD | 3352 ± 748 (104) | <LOD |
Multimineral formulation | 10.7 ± 2.4 | 13.7 ± 2.7 | 9.9 ± 2.7 (92) | 13.2 ± 4.9 (96) |
Soil | 12.3 ± 3.0 | <LOQ | 12.0 ± 5.0 (97) | <LOQ |
The analysis of solid samples by digestion + microextraction + LIBS has been made to demonstrate experimentally the feasibility of this combination. In addition, the solid sample digestion makes feasible the comparison with aqueous calibration standards.
The trueness of the proposed procedure was evaluated from the analysis of a certified reference material (CRM), NCS ZC 85005 (Beef Liver). Vanadium and Mo certified values are 0.267 (reference value) and 3.97 ± 0.28 mg kg−1, respectively. The V concentration found was below the LOQ of the proposed method and the Mo recovery was 103%.
The sensitivity obtained with DLLME-LIBS is approximately 11 and 7 times greater for V and Mo, respectively, than that obtained without DLLME, and the LOD is approximately 12 and 9 times lower for V and Mo, respectively.
This study presents a new step forward in the applicability of LIBS to the analysis of liquid samples. Obviously, further work is required and this is under investigation in our laboratories.
This journal is © The Royal Society of Chemistry 2014 |