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Abstract
To improve the quantitative analysis accuracy of an aqueous solution using surface-enhanced laser-induced breakdown spectroscopy (SENLIBS), the filter paper was used as a transmission medium by placing it onto the surface of a metallic substrate to make the microdroplet spreading more uniform in a fixed region of the substrate surface. The trace elements (Cu, Pb, Cd, and Cr) in an aqueous solution were detected successfully using this method. The results showed that the sample preparation repeatability of SENLIBS was noticeably improved with the aid of filter paper. Moreover, the limit of detection (LoD) values was similar to those without filter paper. Furthermore, the R2 values were improved from 0.6192~0.9321 to 0.9481~0.9766, the RMSECV values were decreased from 0.53~1.95 μg/mL to 0.33~1.06 μg/mL, and the average relative error (ARE) values were decreased from 8.96~22.31% to 4.28~14.37% with the aid of filter paper. This demonstrated that the use of filter paper could improve the quantitative analysis accuracy of SENLIBS by increasing the sample preparation repeatability.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-induced breakdown spectroscopy (LIBS), as an atomic emission spectrometry analytical technique, has been applied in waste-water monitoring [1]. Compared to conventional techniques for analyzing heavy metal elements in aqueous solutions, such as atomic absorption spectrometry (AAS) [2,3], atomic fluorescence spectroscopy (AFS) [4], and inductively coupled plasma-atomic emission spectrometry (ICP-AES) [5], its main advantages are minimal sample preparation, micro-volume sample analysis, in situ, online, and stand-off analysis capabilities [6–10].
However, the detection sensitivity for determine trace elements in aqueous solutions using LIBS is still too low to satisfy industrial requirements. It suffers from several problems, such as water splashing, surface ripples, and intensity extinction, which cause a weaker spectral intensity and shorter plasma lifetime. Several methods have been proposed to improve sensitivity when analyzing aqueous solutions, such as changing the samples from a static liquid to dynamic liquid (e.g., liquid flow, liquid droplet, liquid aerosol, and liquid jet) [11–13], magnetic confinement [14], double-pulse LIBS (DP-LIBS) [15], LIBS combined with laser-induced fluorescence (LIBS-LIF) [16], nanoparticle-enhanced LIBS (NELIBS) [17], liquid-to-solid phase transition [18,19].
Among these methods, the most promising approach is liquid-to-solid phase transition due to the use of cost-effective single-pulse LIBS equipment. It just requires simple sample pretreatment such as freezing liquid into ice by liquid nitrogen [20], absorbing heavy metals using adsorbent materials [21], enriching the heavy metals on the electrode by electrodeposition [22], and drying liquid samples on a nonabsorbent solid surface [23,24]. Among them, the last method has been reported as surface-enhanced LIBS (SENLIBS), which dried the liquid sample as a solid layer or deposited liquid sample as gel-like layer on a nonabsorbent substrate surface, and then analyzed by LIBS. Compared with other liquid-to-solid phase transition methods, SENLIBS has unique advantages, such as a smaller sample for analysis, a simple preparation process, reuse of the metallic surface by polishing. SENLIBS has been proven to be a versatile analytical technique for water [19,25–31], oli [32–34], biological fluid samples [35], pharmaceutical and multimineral formulation sample [36], powders [36,37], food sample [36,38,39], and textiles [40] etc. However, the drawback of this method is poor sample preparation repeatability. The main reason is that the liquid diffusion region and uniformity are seriously affected by the roughness of the metallic surface, liquid type, and liquid concentration [41–45]. As a result, the concentration distribution of the analytes elements on the substrate surface after being dried is inhomogeneous, which limited the quantitative analysis accuracy of liquid samples using SENLIBS.
To address the above problems, several methods have been proposed, such as forming small pockets to confine droplets (Method 1), spreading a water droplet on the laser-patterned silicon wafer substrate (Method 2), spreading a water droplet on the laser-pretreated metallic substrate (Method 3), and using large laser beams spot to cover the entire droplet area for plasma formation (Method 4). For example, Metzinger A. et al. reported that the relative standard deviation (RSD) of signal values was about 10% by Method 1 [35]. Bae D. et al. improved the RSD of 625 single-shot K I line intensities from 1300% to 59% by Method 2 [19]. Niu S. et al. improved the RSD of 300 single-shot Cr I line intensities from 145% to 50% by Method 3 [40]. Aras N. et al. improved the RSD of signal-to-noise (SNR) values from 28% to 16% by Method 4 [28]. However, the best RSD values are still excess 10%, which will still affect the quantitative analysis accuracy seriously.
Filter paper is potentially a useful transmission medium to overcome the above problems due to its characteristics: hydrophilicity of cellulose, high permeability, and high porousness [46–48]. By placing the filter paper onto the metallic substrate surface, the liquid droplet may penetrate easily and spread evenly onto the metallic surface. Up to now, few report has focused on the application of filter paper as a transmission medium, especially in SENLIBS analysis.
In this work, the sample preparation repeatability, the quantitative sensitivity, and the accuracy for heavy metal elements determination in aqueous solution using SENLIBS with filter paper as a transmission medium were investigated.
2. Experimental
2.1 Sample preparation
Stock solution (500 μg/mL of CrCl3, CdCl2, CuCl2, and Pb(NO3)2) was prepared by dissolving the given amounts of each corresponding analytical reagent in distilled water. Eight standard aqueous solutions of Cu, Pb, Cd, and Cr elements were prepared by diluting the stock solution with deionized water. The concentrations of each element in the standard solutions were in the range of 3-10 μg/mL.
Filter paper (201, Hangzhou wohua Filter Paper Co. Ltd. Hangzhou, China) was chosen as the transmission medium between the standard solution and the metallic substrate, and was cut into several 6-mm diameter round sheets. A zinc target (Zn: 99.993%-99.995%, Kurt J. Lesker Company) that contains no Cr, Cd, Cu, and Pb elements was used as a metal substrate. It was cleaned using 1200-grit silicon carbide (SiC) abrasive paper and then washed with ethyl alcohol three times to remove surface impurities. Each aqueous solution was prepared using the following procedures (see Fig. 1(a)): (1) Each 6 mm filter paper was placed on the surface of Zn-metal substrate; (2) the Zn-metal substrate with filter paper was kept at 70°C by a constant temperature heating plate; (3) microdroplets of 40 μL standard aqueous solution were manually deposited on the filter paper by using a micropipette; (4) these microdroplets were dried by the constant temperature heating plate for about 4 minutes, and then the filter paper was taken off; (5) finally, a solid prepared layer on the surface of a Zn-metal substrate that contained analyte elements (Cu, Pb, Cr, and Cd) was prepared.
Except for the first and fourth step, both the sample preparation procedures and the sample preparation conditions for SENLIBS without filter paper were same to those with filter paper.
2.2 LIBS instrument
The experimental setup for LIBS is schematically illustrated in Fig. 1(b). A Q-switched Nd:YAG laser (Quantel Brilliant B, maximum energy: 400 mJ/pulse, wavelength: 532 nm, pulse width: 5 ns) was used to ablate the samples. The laser beam was reflected by a refector mirror and then vertically focused onto a target sample by a plano-convex lens (f = 100 mm). The focal point of the lens was placed at 4 mm below the target surface. To provide a fresh surface for each laser ablation, the target was mounted onto a 2D motorized translation stage at a speed of 5 mm/s. The plasma emission was collected by an optical fiber (50 μm × 200 cm) through a light collector (f = 200 mm), and the other end of the fiber was coupled into an echelle spectrometer (Andor Tech., Mechelle 5000) with a wavelength range of 200-950 nm. An intensified charge-coupled device (ICCD) (Andor Tech., iStar 334T) camera triggered by the laser was used for spectral acquisition. The relative spectral resolution of the system was λ/Δλ = 5000.
For elemental analysis, the higher signal-to-noise ratio (SNR) is, the lower LoD would be achieved. To obtain the highest SNR, the laser energy, gate delay time, and gate width were optimized. As the results, the optimal laser energy was 40 mJ, the gate delay and gate width were set to 2 μs, respectively.
3. Results and discussion
3.1 Sample preparation repeatability
When a liquid droplet spreads on a solid surface, the maximum spreading diameter is affected by the droplet properties (e.g., liquid density, surface tension, and viscosity) and the contact angle (e.g., the roughness and wettability of the impacted solid substrate) [41–45]. Consequently, liquid spreading region and uniformity are different with different liquid samples. Fortunately, the filter paper could be used to solve these problems. Figure 2 shows the distribution images of different aqueous solutions dried on the surface of a Zn-metal substrate without (a) and with (b) the aid of filter paper. Each sample with the same content was prepared for three times. For example, the prepared layers inside the dotted line and the solid line were prepared with a concentration of 10 and 60 μg/mL, respectively. Comparing Fig. 2(a) and 2(b), it can be seen that the distribution of the droplet was more homogeneous with the aid of filter paper than that without filter paper. That is to say, not only the spreading regions of droplets were different, but also the distributions in the droplets were inhomogeneous without the aid of filter paper. Therefore, the filter paper is very helpful to obtain the homogeneous distribution layer and control the spreading region.
Fig. 2 Images of the different aqueous solutions prepared on a Zn-metal substrate without (a) and with (b) the aid of filter paper.
To further verify the uniformity of the droplets diffusion on the metal surface, the point-to-point (PTP) method was used [as shown in Fig. 3(a)]. For PTP method, the spectral intensities were obtained by point analysis of the prepared layer formed by a single droplet diffusion. Each spectral intensity was acquired for single-shot. The smaller spectral intensities RSD of the analytical element, the more uniform the liquid diffusion. To verify the consistency of spreading regions for different droplets on the metal surface, the surface-to-surface (STS) method was used [as shown in Fig. 3(b)]. For STS method, the spectral intensities were obtained by accumulation of concentric analysis of the prepared layer formed by whole droplet diffusion. Each spectral intensity was accumulated for 90 shots. The smaller RSD of the spectral intensities is, the better the consistency of the diffusion range of the liquid is. If both of RSDs of spectral intensities by the above method were smaller, the better sample preparation repeatability will be obtained.
Fig. 3 Point-to-point (PTP) method (a) and surface-to-surface (STS) method (b) for verify the effectiveness of the filter paper.
To further verify the effectiveness of the filter paper, the intensities of the Cd I 508.58 nm were measured by PTP and STS method. Figure 4 shows the intensities of Cd I 508.58 nm changing with the number of laser shot and sample without (dotted line) and with (solid line) the aid of filter paper. As shown in Fig. 4(a), the relative standard deviation (RSD) of the Cd I line intensities was reduced from 36.56% to 19.14% with the aid of filter paper by PTP method. This indicated that the Cd element in the liquid droplet was evenly spread on the Zn-metal substrate surface with the aid of filter paper. Moreover, the RSD of the Cd I line intensity was reduced from 12.05% to 8.04% with the aid of filter paper by PTP method [as shown in Fig. 4(b)]. This indicated that the spreading regions for different droplets on the metal surface were same with the aid of filter paper. Furthermore, the average RSD (ARSD) values for analytical elements lines (Cu, Pb, Cd, and Cr) were reduced from 32.60% and 10.44% to 22.03% and 7.48% by PTP and STS method with the aid of filter paper, respectively [as shown in Fig. 5]. As a result, the ARSD values by both PTP and STS method were improved more than 28.00%. This indicated that the sample preparation repeatability of SENLIBS was noticeably improved with the aid of filter paper.
Fig. 4 The analytical element (Cd) intensity obtained without (blue triangle) and with (red circles) the filter paper (FP) by PTP and STS method.
Fig. 5 The RSD values for analytical elements lines obtained by PTP (a) and STS (b) method (blue column) and with (red column) the aid of filter paper (FP).
3.2 Calibration curves and limits of detection
As well known, filter paper has certain adsorption capacity. The content of analytical elements on metal substrate surface will be reduced when taking off filter paper, which may decrease the thickness of prepared layer and sensitivity of SENLIBS. For SENLIBS, the thickness of prepared layer is reduced, the ablation amount of the substate will be increased under the same experimental condtions. To reduce the effect of different thickness of prepared layer, the substrate element Zn was chosen as the reference element for quantitative analysis.
Figure 6 shows the calibration curves of analytical elements lines (Cu I 324.75 nm, Pb I 405.78 nm, Cd I 508.58 nm, and Cr I 520.84 nm) analyzed by SENLIBS without (blue lines) and with (red lines) the aid of filter paper. The coefficient of determination R2 values of the curves for analytical elements were improved from 0.4490~0.9637 to 0.9676~0.9896, which means that the calibration curve for SENLIBS with the aid of filter paper has a good linearity for liquid sample analysis. Table 1 lists the slope of calibration curve (S) and LoD values of analytical elements by SENLIBS without and with the aid of filter paper. As shown in Table 1, both the S and LoDs for SENIBS with filter paper are similar to those without filter paper. The results demonstrate that the sensitivity of SENLIBS was not obviously affected by the filter paper.
Fig. 6 Calibration curves of analytical elements (Cu, Pb, Cd, and Cr) in the standard samples analyzed by SENLIBS without (blue lines) and with (red lines) the aid of filter paper (FP).
Table 1. The slope of calibration curve (S) and LoD values of analytical elements by SENLIBS without and with the aid of filter paper.
3.3 Accuracy improvement
The R2, relative error (RE), and root-mean-square error of cross-validation (RMSECV) were used to evaluate the accuracy [49,50]. Each spectral intensity was obtained by STS method, which was accumulated for 90 shots and repeated 10 times. Figure 7 shows the correlation curves of the prepared concentration versus predicted concentration for analytical elements (Cu, Pb, Cd, and Cr) analyzed by SENLIBS without (blue triangles) and with (red circles) the aid of filter paper, respectively. As shown in Fig. 7, the R2 values of analytical elements was improved from 0.6192~0.9321 to 0.9481~0.9766 with the aid of filter paper. Therefore, the average 1-R2 value was improved about 84.07%. Figure 8 shows the average RE (ARE) (a) and RMSECV (b) values of the prepared concentration for analytical elements (Cu, Pb, Cd, and Cr) analyzed by SENLIBS without (blue column) and with (red column) the aid of filter paper, respectively. The ARE values were decreased from 8.96~22.31% to 4.28~14.37% with the aid of filter paper. The average ARE value was improved about 53.48%. The RMSECV values were decreased from 0.53~1.95 μg/mL to 0.33~1.06 μg/mL with the aid of filter paper. The average RMSECV value was improved about 52.42%. These results revealed that the proposed sample preparation method, with the aid of filter paper, can effectively improve the accuracy of the SENLIBS.
Fig. 7 Correlation curves of the prepared concentration versus predicted concentration for analytical elements (Cu, Pb, Cd, and Cr) analyzed by SENLIBS without (blue triangles) and with (red circles) the aid of filter paper (FP). The linear curve is a plot of y = x (black solid line).
Fig. 8 ARE (a) and RMSECV (b) values of prepared concentration for analytical elements analyzed by SENLIBS without (blue column) and with (red column) the aid of filter paper (FP).
4. Conclusions
A new sample preparation method for analyzing aqueous solution in SENLIBS with the aid of filter paper was proposed, by which the filter paper was used as a transmission medium to spread the microdroplet onto the metallic substrate evenly in a fixed region. By this method, the sample preparation repeatability was improved more than 28.00%. For determining the analytical element (Cu, Pb, Cd, and Cd), the sensitivity of SENLIBS with filter paper was similar to those without filter paper, and the accuracy of SENLIBS could be improved more than 52.00% with the aid of filter paper. Accordingly, this proposed method can be used as a feasible pretreatment method for detecting trace heavy metal elements in aqueous samples using LIBS analysis.
Funding
National Instrumentation Program of China (No. 2011YQ160017); National Natural Science Foundation of China (No. 61805002 & 61475001); Anhui Provincial Key Research and Development Program (No. 1804a0802193); Anhui University Natural Science Research Project (No. KJ2018A0308 & KJ2018A0312); Innovation Funds of Anhui Normal University (No. 2018XJJ104).
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