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Nanocomposite-Coated Microfluidic-Based Photocatalyst-Assisted Reduction Device to Couple High-Performance Liquid Chromatography and Inductively Coupled Plasma-Mass Spectrometry for Online Determination of Inorganic Arsenic Species in Natural Water Cheng-Hsing Lin, Yu Chen, Yi-An Su, Yu-Ting Luo, Tsung-Ting Shih, and Yuh-Chang Sun Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 3, 2017
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Analytical Chemistry
Nanocomposite-Coated Microfluidic-Based Photocatalyst-Assisted Reduction Device to Couple High-Performance Liquid Chromatography and Inductively Coupled Plasma-Mass Spectrometry for Online Determination of Inorganic Arsenic Species in Natural Water Cheng-Hsing Lin, Yu Chen, Yi-An Su, Yu-Ting Luo, Tsung-Ting Shih, and Yuh-Chang Sun* Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan ABSTRACT: To selectively and sensitively determine the trace inorganic As species, As(III) and As(V), we developed a nanocomposite-coated microfluidic-based photocatalyst-assisted reduction device (PCARD) as a vapor generation (VG) device to couple high-performance liquid chromatography (HPLC) separation and inductively coupled plasma-mass spectrometry (ICP-MS) detection. Au nanoparticles were deposited on TiO2 nanoparticles to strengthen the conversion efficiency of the nanocomposite photocatalytic reduction. The sensitivity for As was significantly enhanced by employing the nanocomposite photocatalyst and using prereduction and signal-enhancement reagents. Under the optimal operating conditions, the analytical detection limits (based on 3σ) of the proposed online HPLC/nanocomposite-coated microfluidic-based PCARD/ICP-MS system for As(III) and As(V) were 0.23 and 0.34 µg·L–1, respectively. The results were validated using a certified reference material (NIST SRM 1643e) and ground water sample analysis, indicating the good reliability and applicability of our proposed system for the determination of inorganic As species in natural fresh water.
INTRODUCTION th
Arsenic (As) ranks the 20 most abundant element in the earth’s crust, and it is widely distributed in the environment.1 According to Han et al.,2 more than 4.53 million tons of anthropogenic As production was conducted in 2000 and large quantities of As have been released into the environment. Because of the widespread issue of drinking water contaminated by naturally occurring As, in 1980, the International Agency for Research on Cancer provided sufficient evidence to prove that inorganic arsenicals are carcinogenic to humans.3 Accordingly, in 1993, the World Health Organization (WHO) recognized As as a toxic element and specified a guideline for the maximum allowable amount in drinking water (10 ng·mL– 1 , daily intake of 2 L). In addition, as the predominant forms of As in natural waters are trivalent and pentavalent species (i.e., As(III) and As(V)), with As(III) considered more toxic than As(V), considerable attention has been given to the speciation of inorganic As in environmental waters.4,5 In recent years, the most common analytical strategy for As speciation has involved coupling a powerful separation technique and a sensitive and selective detection method. Directly coupled highperformance liquid chromatography (HPLC) and inductively coupled plasma-mass spectrometry (ICP-MS) has been commonly used as an analytical technique for As speciation of a variety of samples.6–9 In addition, a batch-
wise method was also developed to measure inorganic As using solid-phase extraction and ICP-MS determination.10 Although ICP-MS is a very powerful technique for analyzing trace element levels with high sample throughput, the sensitivity for As achieved by conventional ICP-MS is generally poor owing to the high ionization energy of As (9.81 eV). In addition, interference with the molecular ion owing to the presence of Ar and Cl (i.e., 40Ar35Cl+) often disturbs the measurement of the single naturally occurring isotope of As at mass 75. Systematic errors during ICP-MS measurements can also occur because of a shift in the plasma equilibrium caused by introduction of the sample matrix.11 Thus, not all determinations are straightforward; for example, the determination of As in salt-rich samples can be especially complicated by such factors. Hydride generation (HG) techniques coupled with atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and ICP-MS are used frequently for the sensitive determination of various hydride-forming elements.12–14 Several papers have reported that the coupling of HG techniques with ICP-MS overcomes some of difficulties of ICP-MS measurements.14,15 For example, greater sensitivity can be attained because of the improved analyte delivery rate and the absence of sample matrices. The efficiency of forming volatile As hydride species is related to the valence state of the precursor. Presently, sodium tetrahydroborate (NaBH4)-mediated reduction of
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hydride-forming elements in acidic media is almost universally employed in HG applications. Unfortunately, As(V) is not reducible to the hydride state when the pH is higher than 3.3.16 Furthermore, the stability of Ar plasma may worsen when NaBH4 is used as the hydride generator because such an online HG system delivers not only hydride vapor but also a large quantity of hydrogen into the ICP.17 Thus, in the past few decades, considerable effort has been invested in addressing the shortcomings of NaBH4-mediated HG techniques by exploring new and alternative vapor generation (VG) techniques. Several reports have described the use of UV-induced photocatalytic reactions to overcome the shortcomings of conventional NaBH4-based techniques for HG. Such reactions not only cause the decomposition of organic materials in aqueous samples,18,19 but also induce photoreduction, e.g., for the volatilization of conventional hydride-forming elements.20–30 To date, various VG techniques have been proposed, with photoinduced VG emerging as one of the most popular techniques. Based on the reaction involved, photoinduced VG techniques can be classified into two types: (i) pure photoinduced VG and (ii) titanium dioxide nanoparticle (nano-TiO2)-enhanced photoinduced VG. In general, pure photoinduced VG can induce photoreduction for the vaporization of hydride-forming elements (As(III), Bi(III), Sb(III), Se(IV), Sn(IV), Pb(IV), and Te(IV)), transition metals (Ni(II), Co(II), Cu(II), Fe(III), Cd(II), and Hg(II)), noble metals (Ag(I), Au(III), Rh(III), Pd(II), and Pt(II)), and nonmetals (I(I) and S(VI)) in the presence of appropriate organic substances, which are present to allow the formation of H⋅ and CO⋅ radicals.18–25 In 2010, Zheng et al.24 reported that the vaporization efficiency of pure photoinduced VG for different species is also strongly dependent on their valence states, with high valence states being more “inert” than low valence states. However, the conversion efficiencies of As(III) and As(V) achieved by pure photoinduced VG were only about 10% and 5% of those achieved by the NaBH4-mediated HG technique, suggesting that the pure photoinduced VG technique is unsuitable for sensitive determination of As species. Recently, nano-TiO2 or TiO2-based nanocomposites enhanced photoinduced VG has been developed as emerging techniques for Se and Hg analysis.26-28 In 2012, Levy et al.29 first used nano-TiO2 as a photocatalyst to convert both As(III) and As(V) into gaseous arsine (AsH3) in the presence of methanol. Another report revealed that the decoration of TiO2 photocatalysts with noble metal nanoparticles (e.g., nano-Ag, nano-Au, and nano-Pt) significantly shifts the Fermi level of the TiO2 photocatalyst, effectively enhancing the photocatalytic reduction efficiency.30 Nonetheless, the low efficiencies of the current photoinduced VG techniques for the conversion of As(V) into volatile products remain an obstacle for the speciation of inorganic As species. Therefore, this study aimed to develop an efficient VG system in conjunction with HPLC and ICP-MS for the direct derivatization of both As(III) and As(V) into volatile species. To simplify the complexity of conventional
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conversion systems during inorganic As speciation, the VG system used in this study exploited the microfluidicbased photocatalyst-assisted reduction device (PCARD) described in our previous work.26 Moreover, to improve the photoreduction efficiency of both As(III) and As(V), a microfluidic-based PCARD coated with nano-Aumodified TiO2 catalyst was employed instead of bare TiO2. After optimizing the operating conditions for the vaporization process, a gas–liquid separator (GLS) was interfaced between the developed nanocomposite-coated microfluidic-based PCARD and the ICP-MS system to establish a simple and sensitive hyphenated system, which facilitated the analytical differentiation of As(III) and As(V) in water samples. EXPERIMENTAL SECTION Chemicals and Materials. Unless otherwise stated, all chemicals were analytical reagent grade and used as received. Deionized water (DI H2O, 18.2 MΩ·cm) was obtained using a Milli-Q apparatus (Millipore, Bedford, MA, USA). Sodium hydroxide (NaOH), nitric acid (HNO3, 69.0–70.0%), methanol (CH3OH, ≥99%), and As(V) stock solution (1000 mg·L−1; As metal in 0.5% HNO3) were obtained from J. T. Baker (Phillipsburg, NJ, USA). Ammonium bicarbonate (NH4HCO3), sodium dithionite (Na2S2O4), sulfuric acid (H2SO4, 95.0–98.0%), sodium citrate tribasic dihydrate (C6H5O7Na3·2H2O, ≥99%), and poly(diallyldimethylammonium chloride) (PDADMAC) (MWav: 100 000−200 000, 20 wt% in H2O, d = 1.040) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Ammonium hydroxide (NH4OH, 33%), formic acid (HCOOH, 98–100%), and sodium borohydride (NaBH4) were obtained from Riedel-de Haën (Seelze, Germany). Hydrogen tetrachloroaurate (HAuCl4·3H2O, 99.99%; metal basis, Au 49.5% min., crystalline), sodium perchlorate monohydrate (NaClO4·H2O, >97%), and sodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O, 98%) were obtained from Alfa Aesar (Ward Hill, MA, USA). Titanium dioxide nanoparticles (nano-TiO2, Aeroxide® TiO2 P25, average primary particle size: ~21 nm, specific surface area: 50 ± 15 m2·g−1) were purchased from Evonik Industries AG (Essen, Germany). A certified reference material (CRM, NIST SRM 1643e, artificial saline water) was obtained from the National Institute for Standards and Technology (Gaithersburg, MD, USA). The coating reagent was prepared by mixing an appropriate quantity of nano-TiO2 (500 mg·L–1) and colloidal PDADMAC (0.5 wt%) with DI H2O via ultrasonication for 30 min, and then adding the proper amount of nano-Au into the mixture while stirring. (Note: Nano-Au was synthesized using the method developed by Jana et al.,31 and the particle number concentration of nano-Au was calculated via Beer’s Law using the UV-visible absorbance and the particle molar absorptivity reported for nano-Au particles31). Figure S-1 (see Supporting Information) shows a transmission electron microscope (TEM, TECNAI 20, Philips, Netherlands) image of the synthesized nano-Au. The mobile phase for chromatographic separation of in-
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organic As species was prepared by dissolving an appropriate quantity of NH4HCO3 and NaClO4 in DI H2O and then adjusting to the desired pH via adding appropriate amount of HNO3 or NH4OH. Prior to use, the solution was filtered through a poly(tetrafluoroethylene) (PTFE) membrane (Acrodisc®, 0.45 µm, 25 mm O.D., Pall Corp., Port Washington, NY, USA) and degassed via ultrasonication. The prereduction solution was prepared by dissolving an appropriate quantity of Na2S2O4 in DI H2O. The mixture of hole-scavenger solution and signalenhancement reagent was prepared by dissolving concentrated HCOOH and CH3OH in DI H2O and then adjusting to the desired pH. A stock solution of As(III) was prepared by dissolving Na2HAsO4·7H2O in 0.2% H2SO4. All stock solutions were stored at 4°C in high-density polyethylene bottles wrapped with aluminum foil. Working standards for calibration were prepared by serial dilution of the stock solutions with the mobile phase and used directly. All reagent preparations were carried out in a class 100 laminar flow hood. All containers and pipette tips used in this study were cleaned by overnight immersion in concentrated HNO3, followed by rinsing with DI H2O. The tubes used to connect the components of the apparatus were flushed with DI H2O until all contaminants were eliminated. To avoid additional contamination, fully plastic Norm-Ject syringes (Henke Sass Wolf, GmbH, Tuttlingen, Germany) were used for subsampling throughout this study. Fabrication of Nanocomposite-Coated Microfluidic-Based PCARD. The PCARD fabrication procedures were similar to those described in our previous work.26 The network of the microfluidic-based PCARD was designed using geometric modeling software (AutoCAD, Autodesk Inc., Sausalito, CA, USA) and then patterned on poly(methyl methacrylate) (PMMA) substrates (Kun Quan Engineering Plastics Co. Ltd., Hsinchu, Taiwan) using a carbon dioxide laser micromachining system (LES-10, Laser Life Co. Ltd., Taipei, Taiwan). Figure S-2 shows the layout of the developed device (150 mm (L) × 28 mm (W) × 4 mm (H)) with introduction ports for the column effluent and hole-scavenger reagent/signalenhancement reagent, and an outlet for the confluent (E, H/S, and O). The length of the effective reaction channel for the photocatalytic reduction reaction, defined as the distance from the point where the flows of the column effluent and the electron-hole scavenger/signalenhancement reagent converge (C) to the confluent outlet (O), was 154 cm. The channel features were inspected using a high-resolution optical microscope (FS-880ZU, Ching Hsing Computer-Tech Ltd., Taipei, Taiwan). The channel dimensions were 582 μm (width) and 902 μm (depth). (Note: Extreme care was taken in handling the substrates to prevent scratching, as a damaged surface could decrease light transmittance, and hence lower the photocatalytic efficiency.) Modification of the channel in the fabricated microfluidic-based PCARD was carried out via dynamic coating procedures. Briefly, the channel interior was first flushed with saturated NaOH for 12 h, and then flushed with DI
H2O and evacuated with air for at least 1 h in each step. The coating reagent, containing nano-Au, nano-TiO2, and PDADMAC, was immediately delivered into the reaction channel and incubated for 5 h, after which the channel was flushed with DI H2O for at least 1 h to eliminate the non-stick nanocomposite and dried under a gentle stream of air. The morphology of the modified channel and the composition of the coating material were inspected using a field-emission gun scanning electron microscope (FEGSEM, JSM-6330F, JOEL Ltd., Tokyo, Japan) and an energy dispersive spectrometer (EDS), respectively. However, as the thin film of Au coated on the samples for FEG-SEM analysis disturbed the Au composition of the nanocomposite coated on the channel interior, a coupled laser ablation (LA, UP-213, New Wave Research, Inc., Fremont, CA, USA) and ICP-MS (Agilent 7500a, Agilent Technologies, Inc., Tokyo, Japan) system was utilized to evaluate the distribution of nano-Au in the nanocomposite. The samples for FEG-SEM mapping were cross-sections of the channel obtained by cutting a fabricated nanocompositecoated microfluidic-based PCARD, and the samples for LA-ICP-MS measurements were PMMA substrates prepared via the same coating process as described above for the nanocomposite-modified channel. Apparatus and Instrumentation. Experiments were performed using an HPLC/nanocomposite-coated microfluidic-based PCARD/ICP-MS hyphenated system, as depicted in Figure 1. An HPLC pump (Model 426, Alltech Associates Inc., Deerfield, IL, USA) equipped with a metal-free six-port injector (Rheodyne®, Model 9010, IDEX Corp., Cotati, CA, USA) and a 100 µL poly(etheretherketone) (PEEK) sample loop was employed. Sample filtrates were separated using an analytical column packed with a polymer-based anion-exchange resin (PRP-X100, 10 µm, 250 × 4.1 mm i.d., Hamilton Company, Reno, NV, USA). After the analytes were separated by HPLC, the As species in the effluent were mixed with a stream of Na2S2O4 solution (0.1%) in a PTFE heating coil (Alltech Associates Inc., Deerfield, IL, USA) in a boiling water bath (100°C) for analyte prereduction. Then, the resulting mixture was delivered into the nanocomposite-coated microfluidicbased PCARD and reduced in the presence of a mixture of HCOOH (400 mM) and CH3OH (150 mM) under UV irradiation (UV-A lamp, 40 W, maximum emission at 365 nm, Great Lighting Corp., Taipei, Taiwan). (Caution! Throughout the experiment, the photocatalytic reaction was carried out in an opaque box to block stray UV radiation. An appropriate exhaust system is recommended because of the production of ozone during UV irradiation.) Na2S2O4 or the mixture of HCOOH and CH3OH were added to the reaction path using a peristaltic pump (Minipuls 3, Gilson Inc., Middleton, WI, USA). After vaporization, the gaseous As products were carried into the ICP-MS system via the GLS by a stream of Ar. PEEK tubes (Upchurch Scientific, Oak Harbor, WA, USA) were used to connect all the components of the system. Adjustment of the sampling position and ion lenses to optimize the signal for As at m/z 75 was performed
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using a commercial As standard solution. Detailed descriptions of the instrumental system and conditions are
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provided in Table 1.
Figure 1. Schematic of the HPLC/nanocomposite-coated microfluidic-based PCARD/ICP-MS hyphenated system.
Table 1. Optimized operating conditions for the HPLC/nanocomposite-coated microfluidic-based PCARD/ICP-MS system HPLC Separation Hamilton PRP X-100 anion-exchange 20 mM NH4HCO3 + 1 mM NaClO4, pH -1 8, 100 μL·min sample volume 100 μL Nanocomposite-Coated Microfluidic-Based PCARD modify method dynamic coating for 5 hours -1 coating reagent 500 mg·L Degussa P 25 TiO2 + 0.114 nM nano-Au + 0.5%(w/v) PDADMAC, pH 6 Prereduction and Photocatalytic Reduction -1 prereduction 0.1% Na2S2O4, 100 μL·min , online prereduction in boiling water for 3 min hole-scavenger + signal400 mM formic acid + 150 mM -1 enhancement methanol, 800 μL·min reagent pH of the mixture pH 4 illumination time 15 s illunmination -1 UV365, 11 mW·cm density analytical column mobile phase
plasma forward power coolant gas flow auxiliary gas flow carrier gas flow sample cone skimmer cone
The presence of titanium in the material of the continuous bed, as evaluated by EDS, indicated that nano-TiO2 was indeed embedded on the channel interior wall (Figure 2B). Furthermore, to avoid interference from the Au film coated on the samples for FEG-SEM mapping, the surface of a PMMA substrate prepared using the same coating procedure (Figure 2C) was evaluated using LAICP-MS. This analysis indicated that both nano-TiO2 and nano-Au are present in the coated photocatalyst (Figure 2D). These results also show that the nanocomposite adheres well to the channel interior wall using our developed dynamic coating method.
ICP-MS 1500 W -1
15 L·min -1 1.0 L·min -1 1.14 L·min nickel, 1 mm orifice nickel, 0.4 mm orifice
RESULTS AND DISCUSSION Characterization of the Nanocomposite-Coated Microfluidic-Based PCARD. To determine the success of the coating procedure and the morphology of the obtained nanocomposite, the homemade nanocompositecoated microfluidic-based PCARD was characterized using FEG-SEM, EDS, and LA-ICP-MS after optimization of the coating reagent. Figure 2A shows FEG-SEM micrographs of the channel cross-section of the nanocomposite-coated device: the main image shows the channel wall and the inset shows a cross-section of the whole channel. The observed morphology is similar to that obtained in our previous work,26 with the channel interior containing a continuous bed with a fairly rough surface.
Figure 2. Characterization of the fabricated nanocomposite-coated microfluidic-based PCARD. (A) SEM micrograph of a cross-section of the nanocomposite-coated microfluidic-based PCARD. EDS elemental analysis results (B) for the region denoted by a red square in (A) for native and nanocomposite-coated microfluidic chips. (C) Photograph of the nanocomposite-coated PMMA sample chip for LA-ICP-MS analysis. (D) Ti and Au elemental signal intensities from LA-ICP-MS analysis. The data in
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(D) were normalized to the maximal average values for each element. Influence of Nano-Au on Conversion Efficiency. In this study, to determine trace inorganic As species using a simple, robust and environment-friendly method, we extended our previous work26 and improved the method for coating a TiO2 film on the channel interior of microfluidic-based PCARD devices by adding appropriate amount of nano-Au in the coating reagent. Nano-Au/nano-TiO2 composite can provide elevation of the Fermi level for reduction, and might provide more active cites for photoredox reaction in the region of Au/TiO2 interfaces. The advantages of using nano-Au/nano-TiO2 composite enable the analytical capability required for the specific needs of As speciation. As mentioned above, photoinduced VG techniques face difficulties in the determination of inorganic As species, especially As(V). Figure 3A shows the effect of pH on As conversion efficiency under photocatalytic reduction with bare nano-TiO2, using the design of our previous work.26 In this preliminary experiment, we found not only poor conversion efficiencies for both inorganic As species, but also different optimal pH values for the conversion of As(III) and As(V). Speculatively, the poor As conversion efficiencies are due to the insufficient reducing power of the free electrons which were excited to the conduction band during the photocatalytic reaction (eCB− at approximately –0.3 V vs. SHE)32,33 for the conversion from elemental As(0) to volatile AsH3 (As(0)/AsH3 = –0.533 V vs. SHE at pH 5,34 calculated by the Nernst equation from the reported E0 value). As depicted in Figure 3A, the difference in the pH values for obtaining optimal As(III) and As(V) signals might result from the difference in the arsenical adsorption affinities for the TiO2 surface and the competition between As(III), As(V), and formate ions for the positively charged sites on the TiO2 surface (point of zero charge at pH 6.935). These results indicated that the pure TiO2-coated microfluidic-based PCARD was not a feasible device for the determination of inorganic As species. To enhance the vaporization efficiency of inorganic As species, we utilized a nano-Au/nano-TiO2 composite as a binary photocatalyst coated on the channel interior of the
designed microfluidic device. According to Jacob et al.,36 Au-TiO2 nanocomposite can improve charge separation by elevating the Fermi level, thus enhancing the photocatalytic reduction efficiency. Furthermore, Subramanian et al.37 reported that the shift of the Fermi level is dependent on the size of nano-Au. For nano-Au ranging from 3 to 8 nm, smaller sizes of nano-Au in the nanocomposite result in greater shifts of the Fermi level, resulting in improved photocatalytic reduction efficiencies and higher photocurrents. Therefore, 4 nm nano-Au was used to prepare the nanocomposite in this study. As shown in Figure 4, a significant enhancement in the As(III) and As(V) signals was obtained when a suitable amount of nano-Au was present in the coating reagent. When less than 0.114 nM nano-Au (calculated particle number concentration) was present in the coating reagent, the intensities of the As(III) and As(V) signal increased as the nano-Au concentration increased. According to Jacob et al.,36 when the amount of nano-Au in the TiO2 film coated on the interior of microfluidic channel increases, the average energy level of the free electrons is also increased after UV irradiation, and consequently, the conversion efficiency increases. However, when more than 0.114 nM nano-Au was present in the coating reagent, the obtained signals decreased as the concentration of nano-Au increased. We supposed it might cause by (1) too much nano-Au averaging the quantity of photocatalytic excited free electrons and reduced the elevated Fermi level of the composite, and/or (2) the reduced active sites for arsenical or formate ion when too much nano-Au landed on TiO2. However, t0 the best of our knowledge, the origin of this phenomenon is still unclear. Thus, to achieve optimal conversion efficiencies for the analyte As species, we used a nano-TiO2 suspension solution containing 0.114 nM nano-Au as the coating reagent in all subsequent experiments. Subsequently, the manufacturing reproducibility of our homemade devices was examined. The variation in the As(III) signal intensities obtained using five PCARD devices fabricated according to the same procedure was 12.6% (relative standard deviation, RSD), indicating the high comparability of the performance of each device.
Figure 3. Effect of pH on the conversion efficiency of inorganic As species in a nano-TiO2-coated microfluidic-based PCARD (A) and nanocomposite-coated microfluidic-based PCARD (B). The data were normalized to the maximal average
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value in each figure. The operating conditions for (A) and (B) are as shown in Table 1, except that no prereduction and signal-enhancement reagents were added for (A), and no signal-enhancement reagent was added for (B).
Figure 4. Variation of As(III) and As(V) signals with the amount of nano-Au added to the coating reagent. The data were normalized to the maximal average value. This series experiment was carried out under the conditions shown in Table 1, except that 200 mM HCOOH was used and no signal-enhancement reagent was added. Optimization of Operating Conditions. For the determination of trace inorganic As species using the developed nanocomposite-coated microfluidic-based PCARD, the composition of the solution flowing through the reaction device has a significant effect on the vaporization efficiency of the analytical system. The solution was a mixture of the column effluent and prereduction (Na2S2O4), hole-scavenger (HCOOH), and signalenhancement (MeOH) reagents. For anion-exchange column separation of As species, as reviewed by Chen and Belzile,38 phosphate buffer is a common mobile phase. However, phosphate ions can compete with As ions for the active sites on the TiO2 surface,39 which could dramatically inhibit As adsorption. In fact, we observed that 0.3 mM phosphate ion in solution decreased the As(III) conversion efficiency to 30% of the maximum conversion efficiency (Figure S-3). In contrast, carbonate buffer is reported to act as a good eluent in anion-exchange column separation with limited interference for photocatalytic VG.26 Thus, to achieve rapid inorganic As separation, a mobile phase containing 20 mM NH4HCO3 and 1 mM NaClO4 at pH 8.0 was utilized as the chromatographic eluent. Furthermore, to optimize the analytical performance of the proposed system, the influence of the prereduction, hole-scavenger, and signal-enhancement reagents on the analytical signals was investigated, as elucidated below.
porate an efficient prereduction process prior to the photocatalytic reduction to enhance the analytical sensitivity of As(V). Na2S2O4 solution, which is as an effective reagent for online prereduction that can virtually instantaneously convert As(V) to As(III),6,40 was selected as the prereductant for the proposed online system. During the offline investigation of the prereduction operating conditions, we found that heating the mixture of sample and prereductant in boiling water significantly improved the signal of As(V) compared with that obtained at room temperature. Moreover, as no significant pH effect (pH 3.5, 8, and 10) was observed during the prereduction process, the pH of the Na2S2O4 solution was not adjusted in this study. Accordingly, for the online investigation, the optimal Na2S2O4 concentration was evaluated at 3 min online heating in boiling water (Figure S-4). The experimental results showed that the As(III) vaporization efficiency was not obviously affected by Na2S2O4, as reported by Sun et al.40 In contrast to As(III), the vaporization efficiency of As(V) could be increased by increasing the Na2S2O4 concentration, reaching a plateau at 0.03% (w/v) Na2S2O4. Notably, the signal intensities of both As(III) and As(V) decreased with 0.2% (w/v) Na2S2O4. Likely, this suppression is due to excess sulfur-containing compounds when Na2S2O4 is too concentrated,41,42 which might affect the redox reaction of arsenicals on the surface of the nanocomposite. Based on these observations, a 0.1% (w/v) Na2S2O4 solution was selected for the prereduction process in all subsequent experiments. Influence of the Hole-Scavenger on Conversion Efficiency. In photocatalytic reduction processes, formate and methanol are the most frequently used holescavengers for improving the reduction efficiency. The conversion efficiency of inorganic As species by 200 mM HCOOH was nearly 100 times better than that by 200 mM methanol under the operating conditions shown in Table 1. Hence, formate was selected as the hole-scavenger, and the optimal HCOOH concentration was determined. Figure 5 depicts the variation of the As relative intensities as a function of HCOOH concentration. As shown, the conversion efficiencies of both As(III) and As(V) increase with increasing HCOOH concentration, reaching a plateau at 400 mM HCOOH. Thus, to attain the most sensitive analytical performance, a HCOOH concentration of 400 mM was adopted in all subsequent experiments.
Influence of the Prereduction Reagent on Conversion Efficiency. Our preliminary experimental results showed that the signals of As(V) were apparently lower than those of As(III). In other words, it was vital to incor-
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Analytical Chemistry sites may change. Thus, the pH conditions would directly cause a variation in the vaporization efficiencies of the desired As species. The influence of solution pH on As vaporization efficiency was explored, and the effect of pH on As conversion efficiency in the proposed microfluidicbased PCARD is displayed in Figure 3B. According to these experimental results, we supposed that the interaction between formate ion (pKa = 3.74) and the charged surface of the nanocomposite has a considerable effect on the conversion efficiency in the photocatalytic reduction process, i.e., more formate ions on the surface of the nanocomposite increases the reducing power. As shown in Figure 3B, the conversion efficiency of both As(III) and As(V) increased with the increasing dissociation rate of HCOOH (from pH 2.0 to pH 4.0), whereas the conversion efficiency decreased with the decreasing surface charge density of the nanocomposite (from pH 4.0 to pH 6.0). Hence, for optimal conversion of As(III) and As(V) to their gaseous products, a solution at pH 4.0, which provided the highest signal intensities for both As(III) and As(V), was selected for all subsequent experiments.
Figure 5. Variation of As(III) and As(V) signal intensities with the concentration of the hole-scavenger, HCOOH. The data were normalized to the maximal average value. The operating conditions are shown in Table 1, except that signal-enhancement reagent was added.
Matrix Interference. To further demonstrate the applicability of our proposed method for natural fresh water analysis, we estimated its tolerance to interference from several major elements and three transition metals ions. Transition metals are very common contaminants in natural fresh water system, we chose three typical redoxsensitive and photoreduction competing transition ions (Fe, Cu, and Ni) as examples to estimate the tolerance of our proposed system on the simulated contaminated water samples. The interference of each metal was evaluated by determining the As species concentrations in spiked synthetic samples containing 5 µg·L–1 of the As species of interest and major elements at their mean concentrations in river water or transition metals (Fe, Cu or Ni ions) at concentrations several to hundreds of times higher than their typical concentrations in river water.44 All the synthetic samples were measured under the operating conditions shown in Table 1, and details of the synthetic samples are provided in Table 2. As shown, the recoveries of As(III) and As(V) in all the synthetic samples are between 80% and 105%. From the viewpoint of the applicability of our developed method to real sample analysis, these matrix elements do not severely interfere with inorganic As determination.
Influence of the Signal-Enhancement Reagent on Signal Intensity. Interestingly, when 400 mM HCOOH was used as the hole-scavenger, the As signal intensities were further enhanced by the presence of methanol in solution. According to Hu et al.,43 a suitable amount of methanol in the plasma can enhance the As signal intensities owing to C+-species–analyte atom charge transfer reactions. Hence, as a signal-enhancement reagent, the effect of the concentration of methanol on the signal intensity was investigated to attain better analytical performance. Figure S-5 displays the variation of As signal intensities with different concentration of methanol added to the hole-scavenger solution. The signal intensities of both As(III) and As(V) increased with the increasing concentration of methanol, reaching a plateau at 50–150 mM methanol. Thus, to achieve the most sensitive analytical performance, 150 mM methanol was added to the holescavenger solution in all subsequent experiments. Influence of pH on Conversion Efficiency. As the solution pH may affect the dissociation of the analyte species and the surface charge density of the nano-Au/nanoTiO2 composite, the adsorption affinities of inorganic As ions and formate ions for the surface of the nanocompo-
Table 2. Interference effects of coexisting ions on As conversion efficiency interference ions major elements
transition metal ions a
As(III) (%)
-1
compositions of coexisting ions (mg·L ) HCO3 58.4
-
SO4
2-
11.2
-
Cl
NO3
7.8
1.0
-
2+
Mg
Na
15.0
4.1
6.3
Ca
2+
+
a
As(V) (%)
+
K
92.4
91.9
Fe (1.0)
104.5
80.3
Cu (5.0)
93.8
102.5
Ni (5.0)
104.5
105.0
2.3
-1
Relative signal intensities of As in each samples comparing with the simple As standard solution, 5 µg·L As.
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Analytical Performance. To characterize the analytical performance of the proposed online system, we evaluated the long-term analytical stability, the analytical features of merit, standard reference material (NIST SRM 1643e) analysis, and real sample analysis. To establish the signal intensity drift during analysis over several hours, we performed a long-term stability test for the proposed system. During 20 hours of continuous analysis, the RSDs (%) of the As(III) and As(V) signal intensities in a sample containing 5 µg·L–1 of As(III) and As(V) were 6.0% and 7.3%, respectively, which suggests that this homemade nanocomposite-coated microfluidic-based PCARD is robust, only slightly susceptible to interference, and compatible with ICP-MS. Subsequently, we evaluated the analytical characteristics of the entire online system, and the resultant analytical features of merit are shown in Table 3. The arsenical calibration curves has satisfactory linearities with correlation coefficients higher than 0.998. Moreover, based on the 3σ criterion, the limits of detection for the determination of As(III) and As(V) were 0.23 and 0.34 µg·L–1, respectively, as estimated using the standard deviation for seven repeated analysis with the proposed system. The precision of the system for analyzing both inorganic As species was better than 4% based on the RSD of the signal intensities for three replicate measurements of 5 µg·L–1 As(III) and As(V). These results show that our proposed system has sufficient sensitivity and precision for analytical purposes.
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Table 3. Analytical characteristics of the proposed HPLC/nanocomposite-coated microfluidic-based PCARD/ICP-MS system As(III) As(V) Analytical Features of Merit linear correlation coefficients, R 0.998 0.999 -1 0.23 0.34 limits of detection, µg·L a RSDs , % 1.4 3.6 NIST SRM 1643e (Artificial Acidity Water) -1 60.45 ± 0.72 certified value, µg·L c b -1 ND 55.83 ± 0.36 measured values , µg·L d d spike recoveries, % 103 87 Industrial Ground Water c b -1 ND 737.2 ± 51.5 real sample , µg·L e e spike recoveries, % 93 81 a -1 RSDs of the signal intensities of 5 µg·L As(III) and As(V), n b c d -1 = 3. n = 3. ND: Not detected. 2 µg·L As(III) or As(V) were e -1 spiked in SRM samples. 1 µg·L As(III) or As(V) were spiked in industrial ground water samples.
Furthermore, we determined inorganic As species in the certified reference material (SRM) NIST SRM 1643e to validate the analytical accuracy of the system, as shown in Table 3. Owing to the lack of information about the concentrations of the individual As species in this artificial saline solution, the analytical reliability was validated by comparing the sum of the concentrations measured for the two arsenicals of interest with the total As concentration in the SRM. The SRM determination showed that the measured total As concentration was 92% of the certified value. In addition, by spiking the SRM aqueous sample with 2 µg·L–1 As(III) and As(V), recoveries of 103% and 87% were achieved for As(III) and As(V), respectively. These results indicated that our proposed system provided remarkable accuracy for inorganic As species analysis in fresh water samples. Finally, we also analyzed a ground water sample collected from a suburban area in southern Taiwan to determine the applicability of this system for natural water measurements. As shown in Table 3, although only As(V) was detected in the real sample, spiking of this sample showed that both inorganic As species could be detected in ground water using our proposed system. In addition, acceptable spike recoveries of 93% and 81% were obtained for As(III) and As(V), indicating that our proposed system accurately determines the concentrations of As(III) and As(V) with satisfactory robustness for natural fresh water measurements.
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Analytical Chemistry CONCLUSIONS
REFERENCES
We have developed an HPLC/nanocomposite-coated microfluidic-based PCARD/ICP-MS hyphenated system for online VG and determination of two trace inorganic As species, As(III) and As(V), in natural fresh water. The channel interior of a high-optical grade PMMA microfluidic-based PCARD was modified with a photocatalyst using a dynamic coating technique. For efficient conversion of inorganic As ions into volatile As species, 4 nm Au nanoparticles were embed on the surface of nano-TiO2 to obtain a nanocomposite photocatalyst with enhanced reducing power for photocatalytic reduction. Furthermore, to improve sensitivity, sodium dithionite prereduction and methanol signal-enhancement were also incorporated into the hyphenated online system to enhance the arsenical VG efficiency and ICP ionization efficiency. The developed system, which coupled HPLC separation and ICP-MS detection with the nanocomposite-coated microfluidic-based PCARD, was suitable for the determination of trace inorganic As species in natural fresh water samples. The perplexing conversion of As(III) and As(V) simultaneously to volatile species via photocatalyst assisted vaporization system was achieved. The results obtained in this study showed that nano-Au/nano-TiO2 composite can directly reduce and vaporize As(V), we suppose that it has potential to break the deadlock on As(V) vaporization by photocatalytic reduction with a suitable modification of nano-TiO2. Based on the analytical results obtained, the proposed hyphenated system has excellent reliability and applicability for the determination of As(III) and As(V) in natural fresh water samples. Furthermore, because As(III) and As(V) in the column effluent can be directly converted into volatile species with low toxicity reagents and without consumption of the photocatalyst, our proposed system can be considered not only an accurate, sensitive, and selective analytical system, but also a cheap, simple, and environmentfriendly method for the determination of inorganic As species in natural fresh water samples.
ASSOCIATED CONTENT Supporting Information Figure S-1−S-5. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author *Tel: +886-3-5715131 ext 35596; Fax: +886-3-5723883; E-mail:
[email protected]
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT The authors would like to convey their gratitude to Prof. MoHsiung Yang for his advice. We are also grateful for the financial support provided by the Ministry of Science and Technology of the Republic of China (Taiwan).
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