Article pubs.acs.org/ac
Simple, Fast Matrix Conversion and Membrane Separation Method for Ultrasensitive Metal Detection in Aqueous Samples by LaserInduced Breakdown Spectroscopy Xu Wang,†,‡ Yin Wei,†,‡ Qingyu Lin,‡ Ji Zhang,‡,§ and Yixiang Duan*,‡ †
Analytical & Testing Center, ‡Research Center of Analytical Instrumentation, Key Laboratory of Bio-resource and Eco-environment, Ministry of Education, College of Life Sciences, and §College of Chemistry, Sichuan University, Chengdu, Sichuan 610065, P.R. China ABSTRACT: A fast, low cost, and sensitive sample pretreatment method was specially developed for laser-induced breakdown spectroscopy (LIBS) based on metal precipitation and membrane separation for simultaneous elemental analysis in liquid samples. The metal elements were reacted with the chelating reagent 2,4,6-trimercapto-1,3,5-triazine (TMT) in the first step and separated by mixed cellulose ester microfiltration membrane subsequently. A specific membrane supporter with smaller aperture than the commercially available needle filter was designed and assembled in the presented research to increase the sensitivity. As a result, the detection limits of Cu, Ag, Mn, and Cr obtained in this research were 2.59 ng·mL−1, 0.957 ng·mL−1, 0.958 ng· mL−1, and 1.29 ng·mL−1 respectively, which were greatly improved from direct liquid analysis by LIBS. Satisfactory linearity, reproducibility, and accuracy were also obtained in the concentration range of 10−120 ng·mL−1 for all four elements tested at the optimized experimental conditions. The analytical figures of merits of the proposed method are at an advanced level and are comparable to those reported among other non-LIBS research. In addition, the separation mechanism in this research was initially explored and further varied to be based on metal precipitates adsorption by membrane fibers, rather than a regular sizedependent obstruction.
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research concerning matrix conversion based on different mechanisms has been reported.11−14 Evaporation of the sample solvent can accomplish matrix conversion and analyte preconcentration simultaneously. Filter paper,12 amorphous graphite,13 and CaO11 were also used as the solid substrate for LIBS analysis. Pasquini and co-workers employed the ring oven technique to preconcentrate Na, Fe, and Cu from a fuel ethanol sample into a very small area on filter paper; the limits of detection (LODs) for Na, Fe, and Cu obtained from a 0.6 mL ethanol sample were 0.7 μg·mL−1, 0.4 μg·mL−1, and 0.3 μg· mL−1 respectively.14 As described above, the sample volume should be small, and the heating apparatus was required to ensure the method practicability. If the volatility of the liquid sample, water for instance, is weak, the evaporation time could be even longer. To overcome the problems mentioned above, some pretreatment methods developed for other instrumental analysis techniques were adapted for LIBS, and good results have been obtained. De Jesus applied dispersive liquid−liquid microextraction (DLLME) with LIBS to avoid the sample solvent evaporation procedure, and 5 ng·mL−1 of V was
aser-induced breakdown spectroscopy (LIBS) technique was initially introduced in the 1960s,1,2 and, then, the hardware system became more and more powerful and portable as a result of fast development of laser source and electronic technology. With the available portable3 or even handheld4 apparatus, the LIBS technique shows great potential for field deployable and in situ analysis. Although LIBS achieved successful qualitative5 and quantitative analysis, or even trace level elemental detection6 for samples in solid phase, it failed to provide a satisfying performance in direct liquid sample analysis. In bulk liquid analysis, emission lights generated from LIBS plasma are usually quenched so that the spectral intensity also decreased7 significantly. If the laser breakdown occurred near the liquid surface, splashed water droplets might even contaminate the optical receiver of the LIBS instrument.8 Changing the sample configuration into liquid jet9 or microdroplets10 can reduce the negative impacts mentioned above to some extent; however, since solvent vaporization consumes part of the laser pulse energy, the energy used for elements excitation was considerably reduced and so was the detection capability. Besides changing the sample configuration, because solid samples are more suitable for LIBS analysis, transferring the analytes from the liquid phase to solid phase is the most popular strategy for liquid sample pretreatment,11 and much © 2015 American Chemical Society
Received: January 19, 2015 Accepted: May 12, 2015 Published: May 12, 2015 5577
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This filtration membrane has the advantages such as low cost (the purchase price in China is only 0.5% of the 3 M Empore Extraction Disks used by Goode), soft texture that it can be cut or manipulated easily unlike the Al2O3 membrane Shi et al. used, and no metal contamination for a clean spectral background. Several common water pollutant elements were selected as examples to be tested by the proposed method. According to the experimental results, besides the univalent and divalent metal ions, the trivalent ion Cr3+ can also be detected by the presented method. The experimental conditions were optimized, and the mechanism related to the membrane separation procedure was investigated. According to the final experimental results, excellent linearity in a certain linear range and very low detection limits were obtained in short analytical time.
detected;15 but after the extraction, the extract solvent with analytes still cannot be analyzed directly by LIBS, and another 5 min was necessary to evaporate the solvent. Solid phase extraction or solid phase microextraction (SPE/ SPME) was also employed as a sample pretreatment for LIBS as an alternative method.16,17 In SPE/SPME, solid phase substrates with or without chemical activity were usually merged in the liquid samples, and the analytes can be adsorbed by the substrates. Owing to that the adsorbents of SPE/SPME can be directly analyzed by LIBS without the elution procedure, SPE/SPME were widely used in LIBS related research. Various materials such as wood,18,19 bamboo charcoal,20 nanographite,21 ion exchange membranes,22,23 modified plasticized PVC membrane,24 and nanochannel porous membrane25 were used as the adsorbents in SPE/SPME-LIBS elemental analysis for liquid samples. Among the research reported in the literature, membrane separation really overshadowed other branches of SPE methods in consideration of not only the sensitivity but also the method feasibility. Recently, plasticized PVC membrane containing a complex reagent was synthesized and employed as the SPE adsorbent in a selective SPE-LIBS analysis for Cu2+ in water. An LOD of 15 μg·L−1 was obtained by simply immersing the membrane in the liquid samples, but the extraction time was as long as 50 min, meaning the problem of time-consuming.24 Shi et al. developed a pretreatment method based on 3D nanochannel porous Al2O3 membranes. Sensitive detection for Cu, Ag, Pb, and Cr was accomplished by simply transferring a 100 μL sample onto the membrane surface and evaporating the sample solvent; the LOD obtained for Cr was as low as 81 ng·mL−1.25 However, both research failed to utilize the greatest advantage of the membrane separation technique, which is the full interaction between the membrane and large volume of liquid sample in a short time by passing the liquid through the membrane. Goode and co-workers achieved speciation of chromium and sensitive detection for several elements using an ion exchange membrane combined with LIBS.22,23 They performed the pretreatment not only by passive extracting but also by vacuum assisted filtration with an LOD of 40 ng for Cu, and the pretreatment time was much shorter than that of passive extraction. However, the demand of a vacuum pump for filtration decreased the feasibility of this method in the field application. In addition, it costs too much to use the commercially available ion exchange membrane for routine environmental monitoring. The presented research is aimed to establish a fast, less equipment, low cost, and sensitive method based on membrane separation combined with LIBS for elemental analysis in liquid samples. Instead of employing the membranes with chemical activity, the metal ions reacted with a chelating reagent in the first step and was separated by a filtration membrane subsequently in the presented research. 2,4,6-Trimercapto1,3,5-triazine, also called TMT, was employed as the chelating reagent in this research. Currently TMT in the sodium salt form (TMT) Na3 has been extensively used to remove the univalent and divalent heavy metal ions from wastewater.26,27 Since metal ions have empty electronic orbits to receive the lone pair electrons of the nitrogen atoms and the sulfur atoms in TMT molecules, they have strong interactions and can form stable metal complexes. The metal complexes with TMT groups are usually insoluble and tend to form precipitates in water solution.26 After the complexation procedure, mixed cellulose ester microfiltration membrane was utilized to separate the precipitated metal complexes from aqueous liquid.
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EXPERIMENTAL SECTION Chemicals and Reagents. Stock solutions (100 μg·mL−1 of Cu(NO3)2, AgNO3, Mn(NO3)2, and Cr (NO3)3) were prepared by dissolving analytical reagents in ultrapure water (18.25 MΩ·cm) and stored in the dark. The standard solutions were prepared by diluting the stock solutions. 98% (TMT) Na3 was purchased from Taitan Reagent Company (Shanghai, China). 15% (TMT) Na3/H2O (m/v) solution was prepared just before use and filtered by a 0.4 μm filter membrane to remove insoluble impurities. The pH levels were adjusted by 1 mol·L−1 HNO3 water solution and 1 mol·L−1 NH3 water solution. Membrane Supporter and the Filtration Membrane. A specific membrane supporter with a smaller aperture than the commercially available needle filter was designed and assembled in the presented research. As shown in Figure 1,
Figure 1. Membrane supporter and the filtration membrane. a. the structure of the filter supporter manufactured in the presented research; b. a piece of membrane unloaded from the supporter after the separation procedure; and c. the membrane supporter connected with a plastic syringe.
this supporter is compatible with a standard medical plastic syringe and allows the sample liquid to flow through a 3 mm diameter aperture. The two parts of the supporter hold the filter membrane between them and can be screwed tightly without any water leakage. Mixed cellulose ester microfiltration membranes (0.2 μm) used to separate the precipitated metal ions from aqueous samples were purchased from Jinteng Company (Tianjin China) and cut into 6 mm diameter pieces. Commercial needle filters with a larger aperture (11 mm) for 5578
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Figure 2. SEM photos of filtration membranes. (A and B, the 10000× magnified pictures of membranes used for reagent blank filtration and sample filtration; C and D, the 50000× magnified pictures of membranes used for reagent blank filtration and sample filtration.)
comparison were also purchased from Jinteng Company, and the same filter membranes were loaded. The Separation Procedure. A 1 mL liquid sample was placed in a plastic test tube, and the pH value was controlled near 7. A 200 μL prepared 15% (TMT) Na3/H2O solution was pipetted into the sample and mixed uniformly to form the precipitable metal complexes, and, then, one piece of filtration membrane was loaded on the specific designed filter supporter. The liquid sample treated by TMT was transferred into a medical plastic syringe and pushed through the filter membrane or the needle filter. After the filtration, the membrane was unloaded from the filter supporter, and the extra liquid was absorbed by a clean napkin. At last, the dried membrane was fixed on a metal free plastic sample plate by double sided tape for LIBS analysis. LIBS Apparatus and Analyzing Procedure. Laser pulses were delivered by an Nd:YAG laser (Lotis TII) at 1064 nm with 8−9 ns pulse width and 5 Hz repetition rate. Pulse energy was monitored by Thorlabs laser energy meter PM100D with the sensor ES200C. The laser beams were focused onto a sample surface through a plano-convex quartz lens; the focal length is 50 mm. The diameter of the ablation spot was approximately 0.2 mm. To perform the LIBS analysis, the sample plate was placed on an automatic x-y translation stage to ensure a fresh point on the sample surface for every laser ablation event. Each time of analysis was performed by accumulating the signal from 100 ablation events to enhance the signal-to-noise ratio. The array of ablation craters was 10 × 10 and could cover the whole area of the membrane the sample passed through. The plasma light was collected by a 74UV probe (Ocean optics) and coupled with optical fiber for light
transmission. Eventually the collected emission light was detected by an echelle spectrometer (Ayelle 400 butterfly, LTB Berlin) coupled with an electron-multiplying CCD (EMCCD) detector. The delay time between the laser pulse and the detector exposure was performed relying on a mechanical chopper in front of the spectrometer entrance slit.
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RESULTS AND DISCUSSION The Membrane Separation Mechanism. Scanning electron microscope (SEM) observation of the membrane surface can provide the most direct evidence to prove that the metal precipitates were separated by the membrane. The 10000× magnified pictures of membranes filtered reagent blank and the TMT treated standard solution are shown in Figure 2A and 2B, respectively. The images clearly show that the fibers of the membrane in Figure 2B became thicker and the gaps between the fibers were almost fulfilled. On the contrary, the fibers and the gaps in Figure 2A had no significant change. This phenomenon can be interpreted as that the membrane can separate insoluble substances from the sample solution, and considering the atom emission spectral lines of target elements can be observed by LIBS analysis for the membrane surface, a conclusion can be drawn that the metal precipitates are or at least are contained in the separated substances. In further observations, it is notable that the morphology of the separated substances is in the form of small irregular particles (shown in Figure 2D, a 50000× magnified picture of membrane for the TMT treated sample), and because the size is much smaller than the pore size of the membrane, the particles cannot be blocked by the lattice structure. Comparing Figure 2C and 2D, it can be found that the small insoluble 5579
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Analytical Chemistry particles were accumulated on the surface of the membrane fibers. According to this finding, it could be speculated that the separation of the metal precipitates is mainly based on the interaction between insoluble particles and the membrane fibers, rather than a regular size-dependent obstruction. Influence of the Diameter of the Filter Membrane on the Intensity of Emission Spectra. In LIBS analysis, stronger emission spectral intensity can be obtained if more analytes were ablated and excited by each laser shot. In this research, because of the metal precipitates the amount was very small, thus the precipitate layer accumulated on the membrane surface can be very thin; each laser shot can ablate throughout the precipitate layer. Hence to increase the diameter of the laser ablation spots or the metal precipitate layer thickness can obtain a larger ablation amount by each laser pulse and consequently stronger emission spectral intensity. In a regular LIBS system with a nanosecond pulse laser source, the pulse energy is usually under 0.2 J; the diameter of the ablation spot on the sample surface is commonly a few hundreds of micrometers or even smaller to provide a sufficient irradiance to generate laser-induced plasma. Higher energy output allows larger ablation spots, but the cost of equipment increases dramatically. Because the metal precipitate layer thickness, namely the precipitates amount on the unit area of the filter membrane, was directly related to the diameter of the filter supporter’s aperture when the sample volume was fixed, therefore, to decrease the aperture is a preferred method with much less cost than to employ a larger laser source. Thus, in the presented research, the smallest commercially available needle filter with 11 mm diameter was used for filtration in the first place. Besides, in order to further decrease the aperture of water flow during the filtration procedure, a specific membrane supporter with a 3 mm aperture was designed and manufactured as described in the Membrane Supporter and the Filtration Membrane section. The supporter was compatible with the standard medical plastic syringes, and the cut filtration membranes can be loaded and unloaded easily. With this supporter, signal enhancement was expected for the emission lines of the target elements. To compare the spectral intensities between the cases applying filters in different scales, a 1 mL standard solution with 50 ng·mL−1 target elements was treated and analyzed according to the procedure mentioned in the Experimental Section. As shown in Figure 3, when the total number of laser shots is fixed, the signal intensities obtained by the smaller membranes were approximately 5× higher than that of the larger ones, which proved that the smaller filtration membrane diameter can increase the ablation amount by each laser pulse and subsequently improve the spectral signal intensities and the method sensitivity. On the other hand, more laser shots are necessary to cover the surface of the 11 mm membrane to obtain the equivalent signal intensities; however, negative effects such as longer analytical time and higher background signals will appear consequently. Optimization of the Analytical Conditions. As a LIBS method coupled with sample pretreatment, both the precipitation conditions and the LIBS system parameters were optimized in this research to obtain satisfying analytical performance. The pH value of a liquid sample and the chelating reagent amount were optimized as the critical parameters in the precipitation procedure. The influences of laser pulse energy and the delay time between the laser pulse
Figure 3. Comparison of spectral intensities for target elements between the cases applying 3 mm filters (green lines) and 11 mm filters (red lines).
and the detector on the signal to noise ratio (S/N) were also investigated in the LIBS analysis procedure. The optimization was performed by analyzing a standard solution at different experimental conditions according to the procedures described in the Experimental Section. The concentrations of target elements were all 50 ng·mL−1, and the spectral lines at 324.764 nm, 328.068 nm, 403.088 nm, and 425.441 nm were selected for the quantitative analysis of Cu, Ag, Mn, and Cr, respectively. In SNR calculation, the noise is defined as the standard deviation of the background value in the range of 2 nm adjacent to the emission line peak, and the signal is the peak height. The statistical results of the experimental data for optimization are shown in Figure 4. Although a lower pH value of the sample solution can provide a better analytical performance for Ag and Cu, extra pH adjustment was required, thus the pretreatment procedure would be more complicated. Under the premise of method feasibility and analytical performance, the precipitation was carried out with 1.0 mL pH neutral samples and 200 μL 15% (TMT) Na3/H2O (m/v) solution. The LIBS parameters were selected for the purpose of acquiring higher SNR; consequently, the optimal delay time and pulse energy for quantitative analysis was 3.5 μs and 70 mJ, respectively. Calibration Curves and Limits of Detection. Calibration curves were built from 10 to 120.0 ng·mL−1 to perform quantitative analysis of Cu2+, Ag+, Mn2+, and Cr3+ by plotting the signal intensities versus the concentrations of metal ions in liquid samples. Six parallel samples at different concentrations were measured; the average of the signal intensities corresponded to each point in the calibration curves, and the error bar at each point indicated the standard deviation of six parallel measurements. The calibration curves presented linear behavior in the concentration range from 10 to 120 ng mL−1, for Cu, Ag, Mn, and Cr; the curves can be described as y = −27.43 + 25.33x, r2 = 0.994; y = −161.58 + 72.49x, r2 = 0.990; y = 993.14 + 114.10x, r2 = 0.996; and y = 1917.44 + 123.59x, r2 = 0.997, respectively, where y is the emission spectral intensity, and x is the elemental concentration in the water sample. 5580
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Figure 4. Optimization of the analytical conditions. A. The effect of the pH value of sample solutions on the spectral intensities of target elements; B. The effect of the 15% TMT/water solution amount on the spectral intensities; C. The influence of laser energy on the SNR of LIBS signals; D. The influence of time resolution, namely the delay time between the exciting laser and the detector of the spectrometer, on the SNR of LIBS signals.
equipment were required. Besides, the shorter pretreatment time and smaller sample volume were also the advantages of the presented method. Method Validation. In this section, the accuracy and the repeatability were evaluated by analyzing spiked environmental water samples and comparing with the results obtained by ICPOES (PerkinElmer Optima 8000). The recoveries were calculated by using ICP-OES results as the reference values. The raw samples were spiked to the concentration of 65 ng· mL−1 analytes, and five parallel samples of the spiked samples were analyzed under the optimal experimental conditions. As listed in Table 2, the mean recoveries of the spiked samples were in the range of 94.8%−105.1%, and the relative standard deviations in the groups of parallel spiked samples were all lower than 11.8%, which means that the accuracy and the repeatability were satisfying for LIBS analysis. The real environmental samples always contain various metal elements, and the chelating reagent TMT used in the present study has very good affinity for many other divalent metal ions besides the targets elements. For this reason, potential interference of some elements was investigated, and the possibility of quantitative analysis of these elements was also discussed. Different concentration levels (50 ng·mL−1, 100 ng· mL−1, 200 ng·mL−1, 500 ng·mL−1, 1000 ng·mL−1) of Ni, Ba, Cd, and Pb were added as the interferences into samples with spiked 50 ng·mL−1 targets elements, which means that the concentrations of interferences are up to 20-fold of the targets. All the samples with interferences were analyzed under the optimized conditions, and the results were summarized in Figure 5. According to Figure 5A, the presence of interferences in this concentration range has no significant effect on the analytical results of the targeted elements. As can be seen in Figure 5B, the intensities of spectral emission lines for Ni and Ba also showed linearity in the concentration range tested. Besides, the emission lines of Cd and Pb were also observed in high concentration samples. Consequently, it can be predicted that these elements can also be sensitively analyzed with certain condition optimization.
The LODs were estimated according to the IUPAC definition: LOD = 3σB/s. σB is the standard deviation (SD) of 11 replicated measurements of reagent blank samples, and s is the slope of the calibration curve. The slopes, standard deviation values, and LODs calculated for the four elements are listed in Table 1. As discussed in the Introduction section, only a few LIBS methods reported in the literature for liquid analysis have the comparative LODs with the presented method.15,21,22,24 Research published by Chen and co-workers28 reached LODs as low as 0.083−5.62 ng·mL−1 for Cr, Mn, Cu, Zn, Cd, and Pb by using the electrical-deposition technique. Although some lower LODs were achieved in their work, extra experimental apparatus such as electrical power supply and magnetic stirrer
CONCLUSIONS In the presented research, we have developed a fast, low cost, and sensitive sample pretreatment method particularly designed for LIBS based on metal precipitation and membrane separation for elemental analysis in liquid samples. Different from other membrane based separation strategies reported for LIBS analysis, instead of using the membranes with chemical activities, in the presented research, the metal ions reacted with a chelating reagent in the first step and then were separated by a filtration membrane subsequently. A low cost and widely used chelating reagent, (TMT) Na3, was selected, and its 15% water solution was used in the sample pretreatment. After the filtration by the mixed cellulose ester microfiltration membrane,
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Table 1. Standard Deviations of Blank Samples (n = 11), the Slopes of the Calibration Curves, and the Detection Limits Calculated According to LOD = 3σB/s for Cu2+ (324.764 nm), Ag+ (328.068 nm), Mn2+ (403.076 nm), and Cr3+ (425.435 nm) Lines element
SDs of blank samples (counts)
slopes of the calibration curve (counts·mL·ng−1)
LODs (ng·mL−1)
Cu Ag Mn Cr
21.86 23.13 36.45 53.08
25.330 72.490 114.10 123.59
2.59 0.957 0.958 1.29
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Analytical Chemistry Table 2. Concentrations and Recoveries of Cu, Ag, Mn, and Cr in 1.0 mL Spiked Water Samples (n = 5) elements
spiked (ng·mL−1)
LIBS mean found (ng·mL−1)
LIBS found RSDs (%)
ICP-OES mean found (ng·mL−1)
mean recoveries (%)
Cu Ag Mn Cr
65 65 65 65
73.3 66.6 72.7 72.4
10.3 11.8 3.9 3.4
74.8 70.2 69.9 68.9
97.9 94.8 104.0 105.1
additional elements should be considered in practical sample analysis. With the rapid development of portable LIBS instruments, the presented method shows great potential in field water quality monitoring.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86-28-85418180. Fax: 86-28-85418180. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for financial support from the National Major Scientific Instruments and Equipment Development Special Funds (No. 2011YQ030113), the National Recruitment Program of Global Experts (NRPGE), the Hundred Talents Program of Sichuan Province (HTPSP), and the Startup Funding of Sichuan University for setting up the Research Center of Analytical Instrumentation.
Figure 5. Interference experimental results: A. the emission intensities of target elements with different concentrations of interferences; B. the emission intensities of interference elements Ni, Ba at different concentrations.
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REFERENCES
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sensitive detection of Cu, Ag, Mn, and Cr in water samples was achieved by LIBS analysis. In order to increase the method sensitivity, a specific membrane supporter with a smaller aperture than the commercial available needle filter was designed, and the expected signal enhancement was achieved. When the membranes after the separation procedure were observed by the SEM, it can be found that the separation is mainly based on the interaction between insoluble particles and the membrane fibers, rather than a regular size-dependent obstruction. The detection limits for Cu, Ag, Mn, and Cr obtained in this research were 2.59 ng·mL−1, 0.957 ng·mL−1, 0.958 ng·mL−1, and 1.29 ng·mL−1 respectively, which were greatly improved from direct liquid analysis by LIBS. The analytical performance of the proposed method is at an advanced level or at least comparable to the reported research, and, moreover, it also has the superiorities in low cost, better feasibility, less pretreatment time, and small sample volume. Besides, the Ayelle 400 butterfly spectrometer used in this research has a very small entrance slit aiming at obtaining high spectral resolution instead of great sensitivity, thus it is not perfect for ultrasensitive quantitative analysis. Because an excellent analytical performance especially on sensitivity was obtained by employing Ayelle 400 butterfly as the spectrometer, it is predictable that the presented pretreatment method has great applicability for different LIBS systems, and even lower LODs could be obtained if a detector with higher sensitivity was engaged. According to the interference testing results, although the experimental conditions of the pretreatments might need some slight modifications to include other additional elements, compromise between method sensitivities and the ability of simultaneous determination of multiple 5582
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Simple, Fast Matrix Conversion and Membrane Separation Method for Ultrasensitive Metal Detection in Aqueous Samples by LaserInduced Breakdown Spectroscopy Xu Wang,†,‡ Yin Wei,†,‡ Qingyu Lin,‡ Ji Zhang,‡,§ and Yixiang Duan*,‡ †
Analytical & Testing Center, ‡Research Center of Analytical Instrumentation, Key Laboratory of Bio-resource and Eco-environment, Ministry of Education, College of Life Sciences, and §College of Chemistry, Sichuan University, Chengdu, Sichuan 610065, P.R. China ABSTRACT: A fast, low cost, and sensitive sample pretreatment method was specially developed for laser-induced breakdown spectroscopy (LIBS) based on metal precipitation and membrane separation for simultaneous elemental analysis in liquid samples. The metal elements were reacted with the chelating reagent 2,4,6-trimercapto-1,3,5-triazine (TMT) in the first step and separated by mixed cellulose ester microfiltration membrane subsequently. A specific membrane supporter with smaller aperture than the commercially available needle filter was designed and assembled in the presented research to increase the sensitivity. As a result, the detection limits of Cu, Ag, Mn, and Cr obtained in this research were 2.59 ng·mL−1, 0.957 ng·mL−1, 0.958 ng· mL−1, and 1.29 ng·mL−1 respectively, which were greatly improved from direct liquid analysis by LIBS. Satisfactory linearity, reproducibility, and accuracy were also obtained in the concentration range of 10−120 ng·mL−1 for all four elements tested at the optimized experimental conditions. The analytical figures of merits of the proposed method are at an advanced level and are comparable to those reported among other non-LIBS research. In addition, the separation mechanism in this research was initially explored and further varied to be based on metal precipitates adsorption by membrane fibers, rather than a regular sizedependent obstruction.
L
research concerning matrix conversion based on different mechanisms has been reported.11−14 Evaporation of the sample solvent can accomplish matrix conversion and analyte preconcentration simultaneously. Filter paper,12 amorphous graphite,13 and CaO11 were also used as the solid substrate for LIBS analysis. Pasquini and co-workers employed the ring oven technique to preconcentrate Na, Fe, and Cu from a fuel ethanol sample into a very small area on filter paper; the limits of detection (LODs) for Na, Fe, and Cu obtained from a 0.6 mL ethanol sample were 0.7 μg·mL−1, 0.4 μg·mL−1, and 0.3 μg· mL−1 respectively.14 As described above, the sample volume should be small, and the heating apparatus was required to ensure the method practicability. If the volatility of the liquid sample, water for instance, is weak, the evaporation time could be even longer. To overcome the problems mentioned above, some pretreatment methods developed for other instrumental analysis techniques were adapted for LIBS, and good results have been obtained. De Jesus applied dispersive liquid−liquid microextraction (DLLME) with LIBS to avoid the sample solvent evaporation procedure, and 5 ng·mL−1 of V was
aser-induced breakdown spectroscopy (LIBS) technique was initially introduced in the 1960s,1,2 and, then, the hardware system became more and more powerful and portable as a result of fast development of laser source and electronic technology. With the available portable3 or even handheld4 apparatus, the LIBS technique shows great potential for field deployable and in situ analysis. Although LIBS achieved successful qualitative5 and quantitative analysis, or even trace level elemental detection6 for samples in solid phase, it failed to provide a satisfying performance in direct liquid sample analysis. In bulk liquid analysis, emission lights generated from LIBS plasma are usually quenched so that the spectral intensity also decreased7 significantly. If the laser breakdown occurred near the liquid surface, splashed water droplets might even contaminate the optical receiver of the LIBS instrument.8 Changing the sample configuration into liquid jet9 or microdroplets10 can reduce the negative impacts mentioned above to some extent; however, since solvent vaporization consumes part of the laser pulse energy, the energy used for elements excitation was considerably reduced and so was the detection capability. Besides changing the sample configuration, because solid samples are more suitable for LIBS analysis, transferring the analytes from the liquid phase to solid phase is the most popular strategy for liquid sample pretreatment,11 and much © 2015 American Chemical Society
Received: January 19, 2015 Accepted: May 12, 2015 Published: May 12, 2015 5577
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This filtration membrane has the advantages such as low cost (the purchase price in China is only 0.5% of the 3 M Empore Extraction Disks used by Goode), soft texture that it can be cut or manipulated easily unlike the Al2O3 membrane Shi et al. used, and no metal contamination for a clean spectral background. Several common water pollutant elements were selected as examples to be tested by the proposed method. According to the experimental results, besides the univalent and divalent metal ions, the trivalent ion Cr3+ can also be detected by the presented method. The experimental conditions were optimized, and the mechanism related to the membrane separation procedure was investigated. According to the final experimental results, excellent linearity in a certain linear range and very low detection limits were obtained in short analytical time.
detected;15 but after the extraction, the extract solvent with analytes still cannot be analyzed directly by LIBS, and another 5 min was necessary to evaporate the solvent. Solid phase extraction or solid phase microextraction (SPE/ SPME) was also employed as a sample pretreatment for LIBS as an alternative method.16,17 In SPE/SPME, solid phase substrates with or without chemical activity were usually merged in the liquid samples, and the analytes can be adsorbed by the substrates. Owing to that the adsorbents of SPE/SPME can be directly analyzed by LIBS without the elution procedure, SPE/SPME were widely used in LIBS related research. Various materials such as wood,18,19 bamboo charcoal,20 nanographite,21 ion exchange membranes,22,23 modified plasticized PVC membrane,24 and nanochannel porous membrane25 were used as the adsorbents in SPE/SPME-LIBS elemental analysis for liquid samples. Among the research reported in the literature, membrane separation really overshadowed other branches of SPE methods in consideration of not only the sensitivity but also the method feasibility. Recently, plasticized PVC membrane containing a complex reagent was synthesized and employed as the SPE adsorbent in a selective SPE-LIBS analysis for Cu2+ in water. An LOD of 15 μg·L−1 was obtained by simply immersing the membrane in the liquid samples, but the extraction time was as long as 50 min, meaning the problem of time-consuming.24 Shi et al. developed a pretreatment method based on 3D nanochannel porous Al2O3 membranes. Sensitive detection for Cu, Ag, Pb, and Cr was accomplished by simply transferring a 100 μL sample onto the membrane surface and evaporating the sample solvent; the LOD obtained for Cr was as low as 81 ng·mL−1.25 However, both research failed to utilize the greatest advantage of the membrane separation technique, which is the full interaction between the membrane and large volume of liquid sample in a short time by passing the liquid through the membrane. Goode and co-workers achieved speciation of chromium and sensitive detection for several elements using an ion exchange membrane combined with LIBS.22,23 They performed the pretreatment not only by passive extracting but also by vacuum assisted filtration with an LOD of 40 ng for Cu, and the pretreatment time was much shorter than that of passive extraction. However, the demand of a vacuum pump for filtration decreased the feasibility of this method in the field application. In addition, it costs too much to use the commercially available ion exchange membrane for routine environmental monitoring. The presented research is aimed to establish a fast, less equipment, low cost, and sensitive method based on membrane separation combined with LIBS for elemental analysis in liquid samples. Instead of employing the membranes with chemical activity, the metal ions reacted with a chelating reagent in the first step and was separated by a filtration membrane subsequently in the presented research. 2,4,6-Trimercapto1,3,5-triazine, also called TMT, was employed as the chelating reagent in this research. Currently TMT in the sodium salt form (TMT) Na3 has been extensively used to remove the univalent and divalent heavy metal ions from wastewater.26,27 Since metal ions have empty electronic orbits to receive the lone pair electrons of the nitrogen atoms and the sulfur atoms in TMT molecules, they have strong interactions and can form stable metal complexes. The metal complexes with TMT groups are usually insoluble and tend to form precipitates in water solution.26 After the complexation procedure, mixed cellulose ester microfiltration membrane was utilized to separate the precipitated metal complexes from aqueous liquid.
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EXPERIMENTAL SECTION Chemicals and Reagents. Stock solutions (100 μg·mL−1 of Cu(NO3)2, AgNO3, Mn(NO3)2, and Cr (NO3)3) were prepared by dissolving analytical reagents in ultrapure water (18.25 MΩ·cm) and stored in the dark. The standard solutions were prepared by diluting the stock solutions. 98% (TMT) Na3 was purchased from Taitan Reagent Company (Shanghai, China). 15% (TMT) Na3/H2O (m/v) solution was prepared just before use and filtered by a 0.4 μm filter membrane to remove insoluble impurities. The pH levels were adjusted by 1 mol·L−1 HNO3 water solution and 1 mol·L−1 NH3 water solution. Membrane Supporter and the Filtration Membrane. A specific membrane supporter with a smaller aperture than the commercially available needle filter was designed and assembled in the presented research. As shown in Figure 1,
Figure 1. Membrane supporter and the filtration membrane. a. the structure of the filter supporter manufactured in the presented research; b. a piece of membrane unloaded from the supporter after the separation procedure; and c. the membrane supporter connected with a plastic syringe.
this supporter is compatible with a standard medical plastic syringe and allows the sample liquid to flow through a 3 mm diameter aperture. The two parts of the supporter hold the filter membrane between them and can be screwed tightly without any water leakage. Mixed cellulose ester microfiltration membranes (0.2 μm) used to separate the precipitated metal ions from aqueous samples were purchased from Jinteng Company (Tianjin China) and cut into 6 mm diameter pieces. Commercial needle filters with a larger aperture (11 mm) for 5578
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Figure 2. SEM photos of filtration membranes. (A and B, the 10000× magnified pictures of membranes used for reagent blank filtration and sample filtration; C and D, the 50000× magnified pictures of membranes used for reagent blank filtration and sample filtration.)
comparison were also purchased from Jinteng Company, and the same filter membranes were loaded. The Separation Procedure. A 1 mL liquid sample was placed in a plastic test tube, and the pH value was controlled near 7. A 200 μL prepared 15% (TMT) Na3/H2O solution was pipetted into the sample and mixed uniformly to form the precipitable metal complexes, and, then, one piece of filtration membrane was loaded on the specific designed filter supporter. The liquid sample treated by TMT was transferred into a medical plastic syringe and pushed through the filter membrane or the needle filter. After the filtration, the membrane was unloaded from the filter supporter, and the extra liquid was absorbed by a clean napkin. At last, the dried membrane was fixed on a metal free plastic sample plate by double sided tape for LIBS analysis. LIBS Apparatus and Analyzing Procedure. Laser pulses were delivered by an Nd:YAG laser (Lotis TII) at 1064 nm with 8−9 ns pulse width and 5 Hz repetition rate. Pulse energy was monitored by Thorlabs laser energy meter PM100D with the sensor ES200C. The laser beams were focused onto a sample surface through a plano-convex quartz lens; the focal length is 50 mm. The diameter of the ablation spot was approximately 0.2 mm. To perform the LIBS analysis, the sample plate was placed on an automatic x-y translation stage to ensure a fresh point on the sample surface for every laser ablation event. Each time of analysis was performed by accumulating the signal from 100 ablation events to enhance the signal-to-noise ratio. The array of ablation craters was 10 × 10 and could cover the whole area of the membrane the sample passed through. The plasma light was collected by a 74UV probe (Ocean optics) and coupled with optical fiber for light
transmission. Eventually the collected emission light was detected by an echelle spectrometer (Ayelle 400 butterfly, LTB Berlin) coupled with an electron-multiplying CCD (EMCCD) detector. The delay time between the laser pulse and the detector exposure was performed relying on a mechanical chopper in front of the spectrometer entrance slit.
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RESULTS AND DISCUSSION The Membrane Separation Mechanism. Scanning electron microscope (SEM) observation of the membrane surface can provide the most direct evidence to prove that the metal precipitates were separated by the membrane. The 10000× magnified pictures of membranes filtered reagent blank and the TMT treated standard solution are shown in Figure 2A and 2B, respectively. The images clearly show that the fibers of the membrane in Figure 2B became thicker and the gaps between the fibers were almost fulfilled. On the contrary, the fibers and the gaps in Figure 2A had no significant change. This phenomenon can be interpreted as that the membrane can separate insoluble substances from the sample solution, and considering the atom emission spectral lines of target elements can be observed by LIBS analysis for the membrane surface, a conclusion can be drawn that the metal precipitates are or at least are contained in the separated substances. In further observations, it is notable that the morphology of the separated substances is in the form of small irregular particles (shown in Figure 2D, a 50000× magnified picture of membrane for the TMT treated sample), and because the size is much smaller than the pore size of the membrane, the particles cannot be blocked by the lattice structure. Comparing Figure 2C and 2D, it can be found that the small insoluble 5579
DOI: 10.1021/acs.analchem.5b00253 Anal. Chem. 2015, 87, 5577−5583
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Analytical Chemistry particles were accumulated on the surface of the membrane fibers. According to this finding, it could be speculated that the separation of the metal precipitates is mainly based on the interaction between insoluble particles and the membrane fibers, rather than a regular size-dependent obstruction. Influence of the Diameter of the Filter Membrane on the Intensity of Emission Spectra. In LIBS analysis, stronger emission spectral intensity can be obtained if more analytes were ablated and excited by each laser shot. In this research, because of the metal precipitates the amount was very small, thus the precipitate layer accumulated on the membrane surface can be very thin; each laser shot can ablate throughout the precipitate layer. Hence to increase the diameter of the laser ablation spots or the metal precipitate layer thickness can obtain a larger ablation amount by each laser pulse and consequently stronger emission spectral intensity. In a regular LIBS system with a nanosecond pulse laser source, the pulse energy is usually under 0.2 J; the diameter of the ablation spot on the sample surface is commonly a few hundreds of micrometers or even smaller to provide a sufficient irradiance to generate laser-induced plasma. Higher energy output allows larger ablation spots, but the cost of equipment increases dramatically. Because the metal precipitate layer thickness, namely the precipitates amount on the unit area of the filter membrane, was directly related to the diameter of the filter supporter’s aperture when the sample volume was fixed, therefore, to decrease the aperture is a preferred method with much less cost than to employ a larger laser source. Thus, in the presented research, the smallest commercially available needle filter with 11 mm diameter was used for filtration in the first place. Besides, in order to further decrease the aperture of water flow during the filtration procedure, a specific membrane supporter with a 3 mm aperture was designed and manufactured as described in the Membrane Supporter and the Filtration Membrane section. The supporter was compatible with the standard medical plastic syringes, and the cut filtration membranes can be loaded and unloaded easily. With this supporter, signal enhancement was expected for the emission lines of the target elements. To compare the spectral intensities between the cases applying filters in different scales, a 1 mL standard solution with 50 ng·mL−1 target elements was treated and analyzed according to the procedure mentioned in the Experimental Section. As shown in Figure 3, when the total number of laser shots is fixed, the signal intensities obtained by the smaller membranes were approximately 5× higher than that of the larger ones, which proved that the smaller filtration membrane diameter can increase the ablation amount by each laser pulse and subsequently improve the spectral signal intensities and the method sensitivity. On the other hand, more laser shots are necessary to cover the surface of the 11 mm membrane to obtain the equivalent signal intensities; however, negative effects such as longer analytical time and higher background signals will appear consequently. Optimization of the Analytical Conditions. As a LIBS method coupled with sample pretreatment, both the precipitation conditions and the LIBS system parameters were optimized in this research to obtain satisfying analytical performance. The pH value of a liquid sample and the chelating reagent amount were optimized as the critical parameters in the precipitation procedure. The influences of laser pulse energy and the delay time between the laser pulse
Figure 3. Comparison of spectral intensities for target elements between the cases applying 3 mm filters (green lines) and 11 mm filters (red lines).
and the detector on the signal to noise ratio (S/N) were also investigated in the LIBS analysis procedure. The optimization was performed by analyzing a standard solution at different experimental conditions according to the procedures described in the Experimental Section. The concentrations of target elements were all 50 ng·mL−1, and the spectral lines at 324.764 nm, 328.068 nm, 403.088 nm, and 425.441 nm were selected for the quantitative analysis of Cu, Ag, Mn, and Cr, respectively. In SNR calculation, the noise is defined as the standard deviation of the background value in the range of 2 nm adjacent to the emission line peak, and the signal is the peak height. The statistical results of the experimental data for optimization are shown in Figure 4. Although a lower pH value of the sample solution can provide a better analytical performance for Ag and Cu, extra pH adjustment was required, thus the pretreatment procedure would be more complicated. Under the premise of method feasibility and analytical performance, the precipitation was carried out with 1.0 mL pH neutral samples and 200 μL 15% (TMT) Na3/H2O (m/v) solution. The LIBS parameters were selected for the purpose of acquiring higher SNR; consequently, the optimal delay time and pulse energy for quantitative analysis was 3.5 μs and 70 mJ, respectively. Calibration Curves and Limits of Detection. Calibration curves were built from 10 to 120.0 ng·mL−1 to perform quantitative analysis of Cu2+, Ag+, Mn2+, and Cr3+ by plotting the signal intensities versus the concentrations of metal ions in liquid samples. Six parallel samples at different concentrations were measured; the average of the signal intensities corresponded to each point in the calibration curves, and the error bar at each point indicated the standard deviation of six parallel measurements. The calibration curves presented linear behavior in the concentration range from 10 to 120 ng mL−1, for Cu, Ag, Mn, and Cr; the curves can be described as y = −27.43 + 25.33x, r2 = 0.994; y = −161.58 + 72.49x, r2 = 0.990; y = 993.14 + 114.10x, r2 = 0.996; and y = 1917.44 + 123.59x, r2 = 0.997, respectively, where y is the emission spectral intensity, and x is the elemental concentration in the water sample. 5580
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Figure 4. Optimization of the analytical conditions. A. The effect of the pH value of sample solutions on the spectral intensities of target elements; B. The effect of the 15% TMT/water solution amount on the spectral intensities; C. The influence of laser energy on the SNR of LIBS signals; D. The influence of time resolution, namely the delay time between the exciting laser and the detector of the spectrometer, on the SNR of LIBS signals.
equipment were required. Besides, the shorter pretreatment time and smaller sample volume were also the advantages of the presented method. Method Validation. In this section, the accuracy and the repeatability were evaluated by analyzing spiked environmental water samples and comparing with the results obtained by ICPOES (PerkinElmer Optima 8000). The recoveries were calculated by using ICP-OES results as the reference values. The raw samples were spiked to the concentration of 65 ng· mL−1 analytes, and five parallel samples of the spiked samples were analyzed under the optimal experimental conditions. As listed in Table 2, the mean recoveries of the spiked samples were in the range of 94.8%−105.1%, and the relative standard deviations in the groups of parallel spiked samples were all lower than 11.8%, which means that the accuracy and the repeatability were satisfying for LIBS analysis. The real environmental samples always contain various metal elements, and the chelating reagent TMT used in the present study has very good affinity for many other divalent metal ions besides the targets elements. For this reason, potential interference of some elements was investigated, and the possibility of quantitative analysis of these elements was also discussed. Different concentration levels (50 ng·mL−1, 100 ng· mL−1, 200 ng·mL−1, 500 ng·mL−1, 1000 ng·mL−1) of Ni, Ba, Cd, and Pb were added as the interferences into samples with spiked 50 ng·mL−1 targets elements, which means that the concentrations of interferences are up to 20-fold of the targets. All the samples with interferences were analyzed under the optimized conditions, and the results were summarized in Figure 5. According to Figure 5A, the presence of interferences in this concentration range has no significant effect on the analytical results of the targeted elements. As can be seen in Figure 5B, the intensities of spectral emission lines for Ni and Ba also showed linearity in the concentration range tested. Besides, the emission lines of Cd and Pb were also observed in high concentration samples. Consequently, it can be predicted that these elements can also be sensitively analyzed with certain condition optimization.
The LODs were estimated according to the IUPAC definition: LOD = 3σB/s. σB is the standard deviation (SD) of 11 replicated measurements of reagent blank samples, and s is the slope of the calibration curve. The slopes, standard deviation values, and LODs calculated for the four elements are listed in Table 1. As discussed in the Introduction section, only a few LIBS methods reported in the literature for liquid analysis have the comparative LODs with the presented method.15,21,22,24 Research published by Chen and co-workers28 reached LODs as low as 0.083−5.62 ng·mL−1 for Cr, Mn, Cu, Zn, Cd, and Pb by using the electrical-deposition technique. Although some lower LODs were achieved in their work, extra experimental apparatus such as electrical power supply and magnetic stirrer
CONCLUSIONS In the presented research, we have developed a fast, low cost, and sensitive sample pretreatment method particularly designed for LIBS based on metal precipitation and membrane separation for elemental analysis in liquid samples. Different from other membrane based separation strategies reported for LIBS analysis, instead of using the membranes with chemical activities, in the presented research, the metal ions reacted with a chelating reagent in the first step and then were separated by a filtration membrane subsequently. A low cost and widely used chelating reagent, (TMT) Na3, was selected, and its 15% water solution was used in the sample pretreatment. After the filtration by the mixed cellulose ester microfiltration membrane,
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Table 1. Standard Deviations of Blank Samples (n = 11), the Slopes of the Calibration Curves, and the Detection Limits Calculated According to LOD = 3σB/s for Cu2+ (324.764 nm), Ag+ (328.068 nm), Mn2+ (403.076 nm), and Cr3+ (425.435 nm) Lines element
SDs of blank samples (counts)
slopes of the calibration curve (counts·mL·ng−1)
LODs (ng·mL−1)
Cu Ag Mn Cr
21.86 23.13 36.45 53.08
25.330 72.490 114.10 123.59
2.59 0.957 0.958 1.29
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Analytical Chemistry Table 2. Concentrations and Recoveries of Cu, Ag, Mn, and Cr in 1.0 mL Spiked Water Samples (n = 5) elements
spiked (ng·mL−1)
LIBS mean found (ng·mL−1)
LIBS found RSDs (%)
ICP-OES mean found (ng·mL−1)
mean recoveries (%)
Cu Ag Mn Cr
65 65 65 65
73.3 66.6 72.7 72.4
10.3 11.8 3.9 3.4
74.8 70.2 69.9 68.9
97.9 94.8 104.0 105.1
additional elements should be considered in practical sample analysis. With the rapid development of portable LIBS instruments, the presented method shows great potential in field water quality monitoring.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86-28-85418180. Fax: 86-28-85418180. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for financial support from the National Major Scientific Instruments and Equipment Development Special Funds (No. 2011YQ030113), the National Recruitment Program of Global Experts (NRPGE), the Hundred Talents Program of Sichuan Province (HTPSP), and the Startup Funding of Sichuan University for setting up the Research Center of Analytical Instrumentation.
Figure 5. Interference experimental results: A. the emission intensities of target elements with different concentrations of interferences; B. the emission intensities of interference elements Ni, Ba at different concentrations.
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REFERENCES
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sensitive detection of Cu, Ag, Mn, and Cr in water samples was achieved by LIBS analysis. In order to increase the method sensitivity, a specific membrane supporter with a smaller aperture than the commercial available needle filter was designed, and the expected signal enhancement was achieved. When the membranes after the separation procedure were observed by the SEM, it can be found that the separation is mainly based on the interaction between insoluble particles and the membrane fibers, rather than a regular size-dependent obstruction. The detection limits for Cu, Ag, Mn, and Cr obtained in this research were 2.59 ng·mL−1, 0.957 ng·mL−1, 0.958 ng·mL−1, and 1.29 ng·mL−1 respectively, which were greatly improved from direct liquid analysis by LIBS. The analytical performance of the proposed method is at an advanced level or at least comparable to the reported research, and, moreover, it also has the superiorities in low cost, better feasibility, less pretreatment time, and small sample volume. Besides, the Ayelle 400 butterfly spectrometer used in this research has a very small entrance slit aiming at obtaining high spectral resolution instead of great sensitivity, thus it is not perfect for ultrasensitive quantitative analysis. Because an excellent analytical performance especially on sensitivity was obtained by employing Ayelle 400 butterfly as the spectrometer, it is predictable that the presented pretreatment method has great applicability for different LIBS systems, and even lower LODs could be obtained if a detector with higher sensitivity was engaged. According to the interference testing results, although the experimental conditions of the pretreatments might need some slight modifications to include other additional elements, compromise between method sensitivities and the ability of simultaneous determination of multiple 5582
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