Multielement Determination of Trace Heavy Metals in Water by

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Multielement Determination of Trace Heavy Metals in Water by Microwave-Induced Plasma Atomic Emission Spectrometry after Extraction in Unconventional Single-Salt Aqueous Biphasic System Svetlana Smirnova, Tatiana Samarina, Dmitry Ilin, and Igor V. Pletnev Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01136 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Analytical Chemistry

Multielement Determination of Trace Heavy Metals in Water by Microwave-Induced Plasma Atomic Emission Spectrometry after Extraction in Unconventional Single-Salt Aqueous Biphasic System Svetlana V. Smirnova†, Tatiana O. Samarina‡, Dmitry V. Ilin†, and Igor V. Pletnev†* †Chemistry Department, Lomonosov Moscow State University, Leninskie Gory 1, Moscow 119991, Russia ‡Kajaani University of Applied Sciences, FI-87101, Ketunpolku 5, Kajaani, Finland ABSTRACT: For the first time liquid-liquid extraction was used for the preconcentration of heavy metals prior to their determination in water by microwave-induced plasma atomic emission spectrometry (MP-AES). Extraction of Pb(II), Cd(II), Co(II), Ni(II), Zn(II), and Cu(II) was performed in unconventional aqueous biphasic system (ABS) formed by addition of hydrophobic solid salt, namely, tetrahexylammonium bromide, to aqueous sample, with neither organic solvents nor salting-out agents being used. The metal ions were quantitatively recovered with 4-(2-pyridylazo)-resorcinol (PAR). The extract was diluted with ethanol/HCl and introduced directly into an MP-AES instrument. The factors influencing extraction (pH, reagent concentration, phase contact time, etc.) and MP-AES detection parameters were studied and optimized. For the developed method, limits of detection of 1.3, 4.9, 0.06, 1.2, 4.2, and 3.2 µg L-1 were obtained for cadmium, cobalt, copper, nickel, lead, and zinc, respectively, providing from 60- to 500fold improvement as compared with the analysis without preconcentration. The method was applied for the analysis of two certified reference materials (CRM) of waste water and surface water as well as the samples of well and sea water. Coupling MP-AES with ABS extraction significantly extends the capabilities of the method, especially for the analysis of high salinity waters.

Determination of trace heavy metals in aqueous solutions is a task of permanent interest, particularly in the context of environmental monitoring. Despite the fact that several heavy metals are essential nutrients (cobalt, copper, zinc, etc.), their excess may be dangerous for living organisms. Some other heavy metals (e.g., cadmium and lead) are well-known toxicants, whose emissions have increased dramatically thus leading to the contamination of natural water sources (rivers, lakes, groundwater) or sea water. As heavy metals are not biodegradable, this presents a permanent stress for aquatic (bio)environments.1,2 Monitoring heavy metals in various waters requires simple and affordable yet sensitive and precise analytical approaches. The most popular current methods of environmental analysis are atomic absorption spectroscopy (AAS) in either flame (FAAS) or electrothermal atomization (ETAAS) versions, as well as atomic emission spectrometry or mass spectrometry with inductively coupled plasma (ICP-AES or ICP-MS, respectively). Some recent examples of highly sensitive determination of metal ions using these methods are given in the Supporting Information (Table S1). Nowadays, the most common method of routine analysis is FAAS. Unfortunately, its sensitivity with either conventional line source techniques or high-resolution continuum source3 may be not sufficient for the practical tasks; to attain µg L-1 level, preconcentration is required.4–6 ETAAS is a method of choice for highly sensitive direct determination of trace elements,7–10 but formation of refractory carbides, relatively low reproducibility and performance limit its application for

routine multi-element analysis. AAS methods are typically used for single-element determination and may have some additional drawbacks (e.g., narrow working dynamic range, consumption of flammable gases). ICP-MS and ICP-AES are multielement methods, whose sensitivity is typically enough for the direct water analysis; however, analysis of complex matrices may still require separation or/and preconcentration.11,12 The cost of instrumentation, running costs, and requirements to operator’s skills in case of ICP-MS and ICPAES are significantly higher than in case of AAS methods. Microwave plasma atomic emission spectrometry (MPAES) is a relatively novel analytical technique. A possibility of multielement analysis, versatility, and convenience/safety of nitrogen plasma makes MP-AES a promising alternative to FAAS and flame atomic emission spectroscopy (FAES). As compared to argon plasma ICP-AES, MP-AES is more affordable; however, it is characterized by lower plasma temperature and typically lower sensitivity (~ 1-10 µg L-1 for detecting a number of elements in simple matrices13–15 (Table S2). A particular problem is that the performance of MP-AES may degrade up to the order of magnitude because of the non-spectral interferences of matrix components in the presence of substantial amounts of inorganic salts or organic matter.16–18 Preconcentration performed prior to the analysis in order to increase the concentration of analytes and/or remove the interferents, may be a key step unlocking the potential of MP-AES. Among the preconcentration procedures, liquid-liquid extraction (LLE) is one of the most efficient, simple and affordable methods. LLE preconcentration of heavy metals with com-

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plexing reagents is a well-developed field. However, to the best of our knowledge, there are no examples of using LLE preconcentration prior to the MP-AES analysis. LLE is often considered as not a “green” procedure due to the use of volatile organic solvents, which typically have a number of drawbacks such as the ability to pollute environment, flammability, toxicity, etc. However, the current interest to “greener” analytical chemistry19 already resulted in the development of the novel “greener” LLE techniques. Microextraction, 20,21 which is available in several versions (nowadays, the most actively studied one is a dispersive liquid-liquid microextraction or DLLME),22,23 radically reduces the extract volume and solvents consumption. Cloud-point extraction, CPE, fully avoids the use of LLE organic solvents by using the surfactants instead (primarily, nonionic and zwitterionic ones). Upon heating above the cloud-point temperature, aqueous surfactant solution separates into two immiscible layers, namely, small-volume surfactant-enriched phase and bulky aqueous phase; the first one may serve as a media for the extraction of hydrophobic metal chelates, organic compounds, and proteins.24,25 However, application of this promising technique may be limited by thermal stability of analytes and by the need in time-consuming heating and cooling steps; in some cases, the addition of high concentrations of salts is required. Ionic liquids (IL), which are low-melting organic salts, present another approach26–29 providing water-immiscible media composed of ions and allowing one to avoid the drawbacks of the conventional extraction solvents. However, ILs are still not as easily available as the common solvents; they are typically not cheap and may exhibit low biocompatibility/biodegradability. Aqueous biphasic system (ABS) is another LLE platform of current interest. ABS are liquid-liquid heterogeneous systems typically formed by adding inorganic salt (salting-out agent in high concentration) to the aqueous solutions of some polymers like polyethyleneglycole30,31 or ILs32–34 or zwitterionic compounds.35 The most significant drawback of ABS is a need to introduce high concentration of the salting-out agent, which may be unacceptable in analytical applications, especially in trace analysis. In addition, the volume of non-aqueous phases and extraction properties of ABS can vary significantly depending on the salt content in the sample (sea water, etc.). Moreover, high salt content is especially disadvantageous for the methods of atomic emission spectroscopy, which are rather sensitive to the presence of salts in the sample.12,36 Surprisingly and not widely known, there are some organic salts, which produce water-rich liquid-liquid biphasic systems being mixed with water in the absence of any salting-out agents; the example is tetrahexylammonium bromide, THABr.37 Phase separation occurs spontaneously at ambient temperature upon adding the solid salt to water (phase diagrams of this and related systems were reported in physicochemical study by Nakayama37 in 1981). Notably, even the “less aqueous” of two phases consists mainly of water. It is noteworthy to mention also that neither THABr is a roomtemperature IL (as its melting temperature is 81-83°C), nor the formed biphasic system is a conventional ABS (as no saltingout agents were added). THABr-water is an unconventional single-salt ABS which combines the advantages of IL and ABS. Ionic nature of components implies non-volatility. Single-salt system is easy in preparation; as it avoids the use of salting-out agents, the associated drawbacks are eliminated.

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Since the volume of the formed THABr-enriched phase is much smaller than the original sample volume, this ABS is perfectly suitable for microextraction with potentially high enrichment factor. Herein, we report on ABS microextraction of heavy metals (Cd, Co, Cu, Ni, Pb, and Zn) followed by their simultaneous determination using MP-AES. The extraction is performed in ABS formed by adding THABr to the aqueous sample, with neither organic solvents nor salting-out agents being used. Metal ions are extracted as hydrophobic chelates with a wellknown reagent, 4-(2-pyridylazo)resorcinol, PAR.38 To the best of our knowledge, this is the first report on the successful use of liquid-liquid extraction for enhancing the performance of MP-AES analysis, as well as on the use of unconventional single-salt ABS in extraction. EXPERIMENTAL Materials and instrumentation. Tetrahexylammonium broTHABr, (99 %, Sigma, USA), 4-(2mide, pyridylazo)resorcinol, PAR, (97 %, Sigma, USA), ethanol (95%, Ekros, Russia) were used as supplied. The stock standard solutions (1 g L−1) of Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Fe(III), Mn(II), Cr(III) and Zn(II) were from Inorganic Ventures (USA). The standard solutions were obtained by diluting the stock standard solutions with 0.2 mol L-1 hydrochloric acid. The solutions of Ca, Na, Mg (0.1 - 500 mg L-1) were prepared by dissolving proper amount of respective analytical grade chlorides in water; they were used for the preparation of artificial sea water (the preparation procedure is described in Supporting Information). The stock solution of PAR (25 mmol L-1) was prepared by dissolving its proper amount in high purity water. All working solutions were prepared daily by diluting the stock solution. Hydrochloric, boric, acetic, orthophosphoric acids, and sodium hydroxide (98%, Panreac, Germany) were used for preparing Britton–Robinson buffer solutions or pH adjustments. A buffer solution was prepared from 0.04 mol L-1 solutions of boric, orthophosphoric, acetic acids and 0.2 mol L-1 solution of sodium hydroxide. All used chemicals were of analytical reagent grade. High purity water (18.2 MΩ×cm, Milli-Q Academic system, Millipore) was used throughout the experiments. The certified reference materials of waste water SPS-WW1 and surface water SPS-SW2 were purchased from LGC Standards (UK). Baltic sea water sample was taken from Gulf of Bothnia. Well water was collected outside Moscow. All the samples were filtered, acidified, and kept in plastic containers at 4 °C prior to the analysis. An Agilent ICP-OES 5100 SVDV inductively coupled plasma optical emission spectrometer (Agilent Technologies, Australia) was used for the detection of metal content in water phase after the extraction. An Agilent MP-AES 4200 microwave plasma atomic emission spectrometer (Agilent Technologies) was used for metal ions determination in organic phase after the extraction; wavelengths (nm): Cd 228.802, Cu 324.754, Pb 283.305, Zn 213.857, Co 345.351, Ni 305.082. For all used wavelengths, nebulizer gas flow was 0.4 L/min and air flow was minimal. The complete list of optimum operating parameters for ICP-AES and MP-AES are given in Table S3. A U-2900 UV-vis spectrophotometer (Hitachi) was used for studying Reichardt’s dye solvatochromism. A Flash EA 1112 CHNS Analyzer (Thermo Electron Corp.) was used for elemental analysis. The measurement of water content in water-saturated tetrahexylammonium bromide was made using a


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870 KF Titrino plus titrator (Metrohm, Switzerland). A pH410 pH-meter (Akvilon, Russia) supplied with an ELSK-13.7 combined glass microelectrode was used for the pH measurements. Phase separation after the extraction procedure was facilitated using a Hettich EBA 20 centrifuge (Tuttlingen, Germany). The mixing in biphasic systems was carried out with an ELMI S – 3 orbital shaker (ELMI, Latvia). Extraction in aqueous biphasic system THABr –H2O. A 50 µL aliquot containing the required amount of the analytes (Cd, Co, Ni, Pb, Zn, and Cu) was placed in 10-mL polypropylene centrifuge tube, and 160 µL of 25 mmol L−1 PAR and 3.8 mL of Britton–Robinson buffer solution (pH 5.5) were added successively; the mixture was stirred and maintained for 1 min. Further, 0.110±0.001 g of THABr was added to the prepared solution, and cloudy solution was obtained. Then the tube was placed on a shaker for 5 minutes to equilibrate the mixture completely. Separation of the phases was facilitated by the centrifugation for 5 min at 6000 rpm. After the centrifugation, the aqueous phase was easily poured out, acidified to pH 2.0 and collected for the ICP-AES measurements. 100±5 µL of the organic phase was diluted by 300 µL of the mixture containing 77 % ethanol (v/v) and 0.04 M HCl. Blank solutions used for the baseline measurement by ICP-AES and MPAES were prepared in a similar way to avoid the discrepancies in the matrix composition. Analysis of real samples. A 4.0 mL aliquot of the real sample or certified reference water materials was placed in 10-mL polypropylene centrifuge tube, and 320 µL of 25 mmol L−1 PAR and 3.8 mL of Britton-Robinson buffer solution (pH 5.5) were added in the given order; the mixture was shaken and maintained for 1 min. 0.220±0.001 g of THABr was added to the prepared solution, which resulted in obtaining a cloudy solution. The tube was placed on a shaker for 5 minutes and then centrifugated for 5 minutes at 6000 rpm. After centrifugation, the aqueous phase was poured out and the remaining extract was easily collected from the bottom of the tube. The volume of separated organic phase was 200±20 µL. 100±5 µL of the organic phase with preconcentrated analytes was transferred into 2-mL plastic test-tubes and diluted by 300 µL of the mixture containing 77 % ethanol (v/v) and 0.04 M HCl. The metal contents were determined directly in the final solution by MP-AES. To obtain calibration curves, instead of an aliquot of real sample, 4 mL of solution containing proper concentration of metals were used. Blank solution used for the baseline measurement by MP-AES was prepared in with the same way as a 4 mL aliquot of high purity water. Calculation of performance figures. The extraction efficiency (E, %) of the metal ions was calculated using the following Eq.:

where c0w and cw are initial and final (equilibrium) concentrations of the metal ion in the aqueous phase, respectively (µg L-1); V0w and Vw are initial and final volumes of the aqueous phase, respectively (mL). All experiments were performed three times, and the standard deviation of the result did not exceed 5% (unless otherwise indicated by error bars). Enhancement factors were calculated as a ratio of the calibration curves slopes obtained with and without the use of the proposed extraction procedure. RESULTS AND DISCUSSION

Characterization of single-salt aqueous biphasic system THABr–H2O. At ambient temperature tetrahexylammonium bromide (THABr) is a solid salt, which is poorly soluble in water and produces two-phase liquid-liquid system being mixed with it. Phase diagrams of such and related systems were reported by Nakayama.37 Both liquid layers are composed of THABr and water; the lower liquid layer is THABrdepleted, while the upper one being much smaller in volume is rich in THABr. Notably, the separation into two phases occurs in the absence of the additional salting-out agents, which are typically required for ABS. We characterized THABr-H2O ABS in the conditions appropriate for the possible microextraction applications. It was found that upon adding the increased amounts of THABr in 30 to 220 mg range to 4.00 ml of water the volume of the forming upper liquid layer vupper increases from 30 to 200 µL, while the volume of the lower phase vlower remains nearly equal to the initial water aliquot volume. Obviously, high vlow:vupper ratios are beneficial for the potential use of ABS in analytical preconcentration (supposing that the upper layer would be a concentrate). According to elemental analysis of the upper phase, the contents of C, H, N, and Br were 59.84, 12.39, 2.92, and 16.19 wt %, respectively, while the C/N, C/Br, Br/N, and C/H ratios were 23.94, 24.59, 0.97, 0.41, respectively. The data of elemental analysis agree well with the calculated contents (wt %) of C (58.04), H (12.12), N (2.82), and Br (16.09) in the upper phase and obtained C/N (24.00), C/Br (24.00), Br/N (1.00), and C/H (0.40) ratios. This confirms that newly formed phase is composed of tetrahexylammonium bromide. The measured density of the upper and lower phases are 0.993 and 1.02 g L-1 at 20°C, respectively. Solid THABr has low solubility in water, which is 0.8 wt % at 25 °C.37 It is worth mentioning that even being less aqueous from two phases, the upper THABr-rich layer still consists mainly of water. The measured water content is 12.5% wt., that is, χ = 78% on a molar basis. This is significantly higher than the values, which are characteristic for even very “wet” molecular solvents (χ = 42%, tri-n-butyl phosphate; χ = 52%, nbutanol39). One may expect that such a hydrophilic medium should be a comfortable environment for solubilization and extraction of relatively hydrophilic compounds. Polarity of THABr-rich phase was characterized by solvatochromic response of the well-known probe, Reichardt’s betaine dye. The Dimroth–Reichardt polarity parameter ET(30)40 is equal to 47.0 kcal mol-1. This value corresponds to the medium, which is slightly more polar than acetonitrile (45.4) but less polar than ethanol (52.4). This allows one to expect good solubilizing/extraction ability of THABr-rich liquid layer with respect to polar analytes including metal ion complexes. Extraction of heavy metal ions in THABr–H2O ABS. Extraction in the aqueous biphasic system THABr – H2O was studied and optimized for preconcentration of metal ions. The effect of pH, ionic strength, complexing reagent concentration, and time of phase contact on the simultaneous preconcentration of metal ions was investigated. Extraction of metals was carried out from a mixture (Cd, Ni, Co, Cu, Pb and Zn) containing 200 µg L−1 of each metal. Extraction efficiency was calculated by determining a residual content of metals in the aqueous phase by ICP-AES.



Extraction in the absence of additional complexing reagents. It is known that some metal ions forming stable anionic complexes with bromide 41 (the stability constants, lg β4, of CdBr42- and PbBr42- are equal to 3.70 and 1.1, respectively) can be extracted from aqueous solution to water-immiscible solvent with tetraalkylammonium bromides, in the form of ionic associates; extraction of such ionic associates makes it possible to separate transition metals which form bromide complexes from the ones that do not (e.g., lead and cadmium from cobalt, copper, iron and zinc).42 In view of this, we studied extraction of cadmium, copper, nickel, zinc, cobalt, and lead in a two-phase system THABr-water without any additional reagents. Extraction of metal ions was studied as a function of pH. Cadmium and lead are quantitatively extracted into the upper phase in the whole studied pH range 2.1–11.8. Extraction efficiency (%) of Cu, Ni, Co, and Zn increases with increasing pH, but remains lower than quantitative one: 37, 40, 41, 55 (at pH 3.08) and 68, 65, 58, 65 (at pH 6.72). It is worthy of note that the decrease of residual metal concentration in the aqueous phase at high pH could result from the formation of poorly soluble metal hydroxides and their precipitation on the phase interface. Furthermore, extraction of metals from relatively acidic solutions is required in practice more often. To maximize the recovery of all the metal ions at neutral and lower pH, we performed an extraction in the presence of additional complexing reagent, PAR. Extraction in the presence of PAR Extraction of metal ions with PAR and effect of pH on the extraction. Complexation of metal ions with pyridylazo reagents has been successfully applied in extraction separation43,44 due to sensitivity and low price of those reagents, broad pH range suitable for complexation, and fast kinetics of complex formation. 4-(2-pyridylazo)-resorcinol (PAR) is a nonselective reagent forming water-soluble stable complexes with zinc, cadmium, cobalt, nickel, lead, cooper, and other metals.38,45 Usually, PAR acts as a tridentate ligand (reactive groups are pyridine nitrogen, azo group, and o-hydroxyl group) and forms chelates with metal to ligand stoichiometry of 1:2. It is well known that PAR complexes can be extracted into the conventional solvents as ion associates with longchain quaternary ammonium salts.46,47 The alkylammonium component of THABr, THA+, is an already present counterion facilitating effective extraction of anionic complexes of metal ions with PAR. Formation of metal-PAR complexes is strongly dependent on pH of the aqueous phase. The effect of pH on the extraction of metal-PAR complexes in two-phase binary THABr – H2O system was studied in the range of 2.2–11.6. In order to maintain a constant working pH during the complexation and extraction process, Britton–Robinson buffer solutions were selected. Figure 1 presents an increase of the extraction efficiency for copper, nickel, zinc, and cobalt from pH 2.2 to 4.5. At low pH the protonated form of PAR (H3L+) dominates and its deprotonated form, which is capable of complexation, is present in insufficient quantities. For cadmium and lead extraction efficiency does not depend on pH and ionic state of PAR due to the formation and extraction of chlorometallates from acidic aqueous solutions. All the studied metals are extracted quantitatively into THABr-enriched upper phase in the presence of PAR (1·10-3 mol L-1) at pH 5.4–11.6. Therefore, pH 5.5 was chosen for further experiments.

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Effect of PAR concentration on the extraction. Quantitative extraction of metal ions in aqueous two-phase system based on

Figure 1. Effect of pH on the extraction of 4 ml solution containing 200 µg L−1 of each metal ion, 1·10-3 mol L-1 PAR, 110 mg THABr.

THABr depends on the formation of metal–PAR complexes, therefore, the effect of PAR concentration was studied in the range of 1·10-5–5·10-3 mol L-1. The recovery for Cd, Pb, Co, and Ni increased up to PAR concentration of 2.5·10-5 mol L−1, for Cu and Zn that went up to 1·10-3 mol L−1(Figure S1). Above that concentration the recovery remained constant. A concentration of 1·10-3 mol L−1 was selected as an optimum one to guarantee maximum recovery of the analytes (R.S.D. values at this concentration ranged from 1.0% to 3.0%). It was noted that extraction of this complex in two-phase system based on THABr does not require any specially introduced counter-ion or salting-out agent. Effect of the phase contact time. We studied the effect of extraction time, which is defined here as time elapsed from the moment of adding THABr to the aqueous samples containing metal ions and PAR to the beginning of the centrifugation stage. The results showed that quantitative extraction of Cd, Co, Cu, Ni, and Pb was achieved in 1 min (Figure S2). Extraction efficiency for Zn increased with increasing extraction time up to 5 min and remained constant with further increase of extraction time up to 60 min. So, in order to keep analysis time as short as possible, the centrifugation was performed after 5 min of phase contact. The extraction is practically instantaneous, which confirms the efficiency of the proposed two-phase system based on THABr with in situ formation of extraction solvent. The results showed that extraction in this system is a very fast process; the addition of solid THABr to aqueous solution results in the release of a new upper phase enriched with THABr and simultaneous extraction of Me-PAR complexes; one may conclude that the surface area of the interface between the THABr-riched droplets and the aqueous phase is actually so large that the diffusion of extracted species into the formed concentrated phase is almost immediate. Effect of ionic strength on the extraction (effect of NaCl concentration). The effect of ionic strength might negatively influence the formation and extraction of Me-PAR complexes 48 or even change the volume of formed ABS-rich phase because of the salting-out effect, which could cause additional


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uncertainty and limitations for further analytical applications. The salinity effect in ABS based on THABr was investigated using NaCl in the concentration range of 0.1–10% (w/v). The electrolyte was added to the solutions containing 200 µg L-1 of Cd, Ni, Co, Cu, Pb, and Zn. As shown in Figure S3, the addition of up to 10% of NaCl has no effect on the extraction efficiency of metal ions. Accordingly, extraction into THABr– H2O system can be an excellent solution of sample pretreatment prior to determining metals by methods having difficulties in working with high salinity solutions (highly saline brines, marine, salty lake or produced waters). Determination of heavy metals using microwave-induced plasma atomic emission spectroscopy after LLE preconcentration Optimization of MP-AES analysis conditions. In earlier studies, MP-AES technique was successfully applied for the determination of the elements in various objects with complex high-salinity matrices such as artificial sweat extraction solution16 or Mehlich’s soil extraction solution.49 In those cases, observed limits of detection and limits of quantitation were significantly higher than the expected instrumental detection limits obtained by the measurements of fully microwavedigested samples or pure blank solutions. However, even in the acidic aqueous solutions the detection limits (LOD) of the studied elements varied significantly depending on the matrix composition and applied calculation method (Table S2). To evaluate sensitivity of the direct MP-AES determination of heavy metals without preconcentration in highly saline matrix, LODs were obtained using artificial sea water which imitates a complex matrix (the preparation procedure is described in Supporting Information). For estimates, the most intense lines of elements, which are not influenced by the presence of foreign ions, were used wherever possible. Standard calculation procedure was applied (3s0/slope, where s0 is a standard deviation of the blank prepared with artificial sea water, n =20). Comparison of the obtained results with the declared characteristic values for deionized water50 and 1% HNO351 shows at least 2–10-fold increase of LOD for Cd, Ni, Cu, and Pb, and 60-fold increase for Zn (Table S4). Thus, a considerable amount of salts inevitably leads to increasing background signal due to the presence of non-spectral interference, and, as a result, LODs are noticeably worse than expected. High level of the total dissolved salt (TDS) concentration not only affects sensitivity significantly, but might become a problem for the methods involving atomization, since the nebulizer and inner torch injector may be blocked by the accumulated salts. Using extraction techniques would improve sensitivity of MP-AES by eliminating concomitant element influence as well as by increasing the emission signals intensities due to preconcentration. The replacement of solvent also does not guarantee better sensitivity. Several studies have used MP-AES for the direct determination of metal ions in organic gasoline and ethanol fuel,17 petroleum crude oil,18 diesel and biodiesel.52 As well as in ICP-AES, replacing aqueous phase for organic one reduces sensitivity of the elements determination by an order of magnitude. This is certainly related to the changes of the physical properties of the sample (such as viscosity, density, surface tension) affecting the efficiency of its supply to plasma, nebulization, and evaporation as compared with aqueous samples. However, the introduction of carbon into plasma has much greater effect on the change of the emission lines intensity.53

Moreover, high carbon content might even be a reason of the torch damage due to devitrification or overheating of the torch. Taking into account that power of microwave device is fixed (1 kW) for the commercial MP-AES, plasma conditions could only be changed by varying operating parameters such as nebulizer gas pressure, plasma viewing position, feed pump rate, and air supply. Furthermore, the vaporization of organic diluents in MP-AES plasma together with the nebulization process may be more efficient as compared with aqueous solutions, but it is strongly affected by volatility and viscosity of the input liquid. In our case, the liquid concentrate, THABr– rich phase, has relatively high viscosity, which hindered its introduction into plasma. To reduce viscosity, the concentrate was diluted with a low-viscosity solvent. In the systematic study devoted to coupling DLLME to ICP-AES, Martinez et al. 54 have explored how the nature and dilution ratio might have influence on analytical figure of merits. Although injection of extractant/alcohol mixtures (up to 1:3) did not affect plasma temperature, the use of more volatile diluents resulted in higher emission signals. In this study, octanol, nisopropanol, and ethanol were examined as diluents, and ethanol was found to be the best one (Figure 2A). It should be noted that 77% ethanol was a minimum alcohol content, which allows dissolving the extract and gives completely homogeneous solution. Ethanol was acidified with HCl (0.04 mol L-1 final concentration) to prevent the loss of ions due to precipitation and hydration. That was the minimal acid concentration, which did not reduce analytical characteristics of merits. On the average, the emission signal intensities for ethanol/HCl were 15-20% higher in comparison with other diluents. For further study, the concentrate phase of ABS obtained after extraction was diluted with ethanol/HCl mixture and introduced directly into plasma. Sample supplying line of MP-AES spectrometer consisted of the concentric nebulizer One Neb made of chemically inert polymer and double-pass cyclonic spray chamber ensuring the formation of a fine aerosol with a narrow droplet size distribution and enhancing nebulizer efficiency while minimizing transport losses. By the use of the flow blurring technology, this nebulizer design55 not only reduces the clogging, which allows one to use the solvents with salinity of up to 10% TDS, but also has high spray efficiency even at low sample flow rates (5 rpm) required for supplying organic solution.56 Reducing the sample flow by decreasing peristaltic pump feed rate also declines carbon emission and soot deposition in the atomizer. Additionally, to eliminate carbon emission and soot accumulated on the torch, an external gas control module (EGCM) was used for continuing air supply into plasma. It was found that the minimal possible air flow is sufficient for preventing the deposition of combustion/decomposition products of the organics at the torch and obtaining reliable and stable emission signal (Figure 2B). Nebulizer gas pressure and plasma viewing position play a key role in obtaining sensitive emission signals. It is possible to optimize these parameters by using a software option comparing the recorded signals with background signals under different conditions (signal to root background ratio, SRBR). It was found that for all analytes the nebulizer gas flow of 0.4 L min-1 is optimal (Figure S4). Although an MP-AES instrument has the axial configuration, plasma viewing position might be changed. For most of the used emission lines the maximum of emission intensity was achieved directly in the



center of plasma view. However, for Pb (283.305 nm) and Ni (305.082 nm) the maxima of the desired signals were reached

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at the viewing position set equal to 60 (arbitrary units, as defined by manufacturer) in both cases. We assume

Figure 2 (A) Effect of the diluent on the intensity of emission line (Cu 324.754 nm, 50 µg L–1 Cu, diluent : extract ratio = 3:1); (B) The influence of oxygen supply on the intensity of emission line (Cu 324.754 nm, 200 µg L–1 Cu).

that at these wavelengths the emission of molecular ions of the solvent (background signal value) is maximum in the central viewing position, but its intensity decreases with increasing the distance from the center of the torch because of the lower plasma temperature in the end viewing positions. At the same time, the emission signals of the analytes at this viewing position are still significant. Thus, changing the position of viewing the torch, one can choose the optimal signal-to-noise ratio and increase sensitivity of determining some elements. The temperature of MP-AES plasma is at least twice lower than that of conventional ICP-AES plasma. On one hand, that could increase the influence of non-spectral matrix effects, but on the other hand, it could result in simpler atomic emission spectra. To avoid spectral overlapping of emission lines of the analytes and possible interferents, the following strategy of choosing analytical lines was employed: 1) maximizing the distance from the emission line of possible interfering components (such as iron or manganese) and 2) maximizing the intensity of the selected line (to maximize SRBR, i.e. to improve detection limits). It is worth mentioning that emission lines often used for ICP-AES determination of target analytes are not presented in MP-AES spectrometer software. Moreover, their number is limited and a user can not insert them manually. For example, only five lines of cadmium and lead are available in the MPAES instrument used. Furthermore, the analytical lines, which provided the best detection sensitivity in aqueous solutions, did not give a positive result for analyzing concentrates. For instance, Pb (405.787 nm) was used for analysis of lead in high-salinity waters, with LOD being 11 µg L-1 (Table S4), but the intensity of that line in organic phase was extremely low and Fe+3 interference was significant. The same is true for the determination of cobalt and nickel. We screened the possible analytical lines and selected the optimal ones (see Table S3) along with other instrumental parameters. Other possible major factors such as the magnetic field strength or plasma, supporting and cooling gas flows are made invariant by the manufacturer. Thus, the effect of these factors on the emission signal intensity has not been studied. Some other instrumental parameters (feed pump rate, background

pixelization, and plasma stabilization time) have also been investigated, and were found to be of low importance for optimization of the emission signals. Effect of coexisting ions. The effects of possible interferents on the recovery of heavy metals was examined, see Table 1. The tolerated amounts of each ion were tested concentration values that caused less than 5 % alteration in the analytical signal. As can be seen from Table 1, the ions commonly present in natural water samples at correspondent levels did not interfere under the experimental conditions (recoveries 95 – 105 %). It should be noted that iron could interfere with lead and the recovery value did not exceed 86 % at the metal-tointerferent ratio (w/w) of 1:10 (0.2 mg L-1) (for Fe(III) and Pb(II)). This is primarily due to the spectral overlapping of Pb (283.305 nm) and Fe (283.244 nm) emission lines. The presence of iron in the concentrate is caused by co-extraction of its PAR complex to the THABr-phase. In addition, Na+, Cl-, SO42, and CO32- ions were also studied, and no influence on the analytical results was observed even at the metal-tointerferent ratio (w/w) of 1:1000. Those results also indicate that the proposed preconcentration method for the studied trace elements could be applied to various samples. Table 1. Effect of coexisting ions on the recovery of target analytesa Ions

Fe(III) Mn(II) Ca(II) Mg(II) SO42CO32-

Interference/metal ratio

Cd

Co

100 100 10000 10000 1000 1000

103 105 101 100 98 102

103 105 97 99 101 102

a

Recoveryb (%) Cu Ni 90 102 99 100 102 100

102 105 100 100 102 100

Pb

Zn

86b 98 101 100 103 100

98 105 100 99 105 97

50 µg L−1 each metal ion recovery experiments were performed in triplicates, the standard deviation of the results does not exceed 5%; c Interference/metal ratio 10, 0.2 mg L-1 Fe(III)

b


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Analytical performance. Analytical performance (detection limits, ranges of linearity, enhancement factors) of the proposed method was evaluated under the optimum conditions (Table 2). The volumes of the initial aqueous phase and organic phase formed after the extraction procedure were 8 mL and 0.2 mL, respectively, and the total volume of organic phase after dilution was 0.4 mL. The calibration curves were obtained after the standard series were subjected to the extraction in THABr – H2O system followed by multielement determination by microwave induced plasma atomic emission spectrometry. In order to determine the analytical characteristics of the proposed method, the pH of the blank solutions was adjusted to pH 5.5 using buffer solution, required amount of 1 mmol L-1 PAR and 200 mg THABr were added, and then the proposed method was applied. LODs and LOQs were calculated as the ratio of three and ten times standard deviation for twenty blank readings, respectively, to the slope of the calibration curves. Average blank signals for proposed method and their standard deviations are given in Table S5. The results presented show that the preconcentration stage makes it possible to significantly reduce LODs of metals for MP-AES determination (Table 2), which probably occurs due to the elimination of the significant influence of salt background in sea water. The enhancement factor calculated as a ratio of the slopes of the calibration curves obtained with and without proposed extraction procedure is in the range from 60 to 503 depending on the metal ion (Table 2). Thus, the combination of MP-AES method with extraction in

THABr-H2O system allows one to apply this method for determining low concentrations of metals in saline solutions. A linear relationship between the analytical signal and the concentration of the studied metals was obtained over the range of 4-100 µg L–1 (Table 2). For the metal ions studied regression coefficients (R2) of the calibration curves were between 0.9998 and 0.9986. The intra-day and inter-day repeatability RSDs obtained by performing three replicates at 50 µg L–1 of the studied metals on the same day and on three consecutive days did not exceed 2% and 5%, respectively (for 3–4 parallel runs). The suggested method provides LODs, which are close to or better than those of the existing methods for the determination of the studied metals by ICP-AES and FAAS which use modern “green” solvents (Table S1). Application of the proposed method. The accuracy of the developed procedure was verified by the analysis of the certified reference materials (CRM) of surface water SPS-SW2 and waste water SPS-WW1. The observed values given in Table 3 are in good agreement with the certified values of the CRMs. The recovery values calculated are always higher than 90% except for lead recovery result in SPS-WW1. In the studied sample the iron content was high and exceeded the tolerance limit, which could lead to the interference with lead determination on the chosen emission line.

Table 2. Analytical characteristics of the proposed method for the determination of metal ions by MP-AES Parameter

Cd 228.802

Co 345.351

Cu 324.754

Ni 305.082

Pb 283.305

Zn 213.857

Linearity range (с, µg L )

4 – 400

15 – 200

0.5 – 200

4 – 100

15 – 100

40 – 100

Slope ± SD (a, µg L–1) Intercept (b) Correlation coefficient (r) LOD (с, µg L–1)* LOQ (с, µg L–1)** Enhancement factor Intraday precisionb Interday precisionb

500±11 2754 0.9984 1.3 4.4 394 0.5 0.9

281±6 282 0.9991 4.9 16.0 93 0.7 1.1

2306±43 3316 0.9984 0.06 0.2 100 0.4 0.7

190±2 59 0.9996 1.2 4.0 165 0.5 0.7

56±3 78 0.9958 4.2 14.0 58 1.1 4.0

412±3 17 0.9998 3.2 9.9 503 1.0 2.9

–1 a

a

For the equation I = ac + b (n=6), where I is an intensity (a.u.), and c is a concentration of metal ion, µg L–1. For 50 µg L–1 of each metal ion. These statistical parameters are obtained by using artificial sea water as analytical samples (%RSD, n = 3). * Limit of detection calculated as 3s0/slop, where s0 is a standard deviation of the blank (n = 20). ** Limit of quantitation calculated as 10s0/slop, where s0 is a standard deviation of the blank (n = 20).

b

Table 3. Determination of metals in the certified samples SPS-SW2 and SPS-WW1 Sample

Cd

Co

Cu

Ni

Pb

Zn

20.0±0.1

60.0±0.3

400±2

1000±5

100.0±0.5

600±6

Found (µg·L–1)

21±2

56 ±14

391±10

960±47

75±3

592±10

Recovery (%)

104

94

98

96

75

99

2.50±0.02

10.00±0.05

100±1

50.0±0.3

25.0±0.1

100±2

Found (µg·L )

2.6±0.1

11±2

97±4

50±2

23±2

99±5

Recovery (%)

104

110

97

100

95

99

–1

Certified value (µg·L ) SPS-WW1a (waste water)

Certified value (µg·L–1) SPS-SW2b (surface water)

–1



Page 8 of 25

a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SPS-WW1 composition: 2 mg L–1 Al, 1 mg L–1 Fe, 0.4 mg L–1 Mn; concentrations of other foreign ions were less when 0.4 mg L–1; SPS-SW2 composition: 10 mg L–1 Na, 10 mg L–1 Ca, 2 mg L–1 Mg, 1 mg L–1 K; concentrations of other foreign ions were less when 0.4 mg L–1.

b

Table 4. Determination of studied metals in model and natural water samples Sample Well water Model sea water Natural sea water a b

Added (µg·L–1)

Cd

Co

a

a

Cu a

Ni

Pb a

Zn a

Found (µg L–1)

R (%)

Found (µg L–1)

R (%)

Found (µg L–1)

R (%)

Found (µg L–1)

R (%)

Found (µg L–1)

R (%)

Founda (µg L–1)

R (%)

25

24.9±0.2

100

23.7±1.3

95

24.1±1.1

96

24.9±0.3

100

24.0±1.4

96

26.4±0.7

105

50

49.3±0.8

99

49.0±1.1

98

49±2

98

49.6±0.4

99

48.3±1.8

97

49.9±1.5

100

20

20.3±0.5

101

20.1±0.3

100

20.4±0.6

102

19.5±0.8

98

22±2

110

20.4±1.0

102

50 100

50.6±0.8 100.3±0.7

101 51±2 100 101.2±1.4

102 101

51±4 101.9±1.9

102 102

50±2 101.1±1.2

100 101

53±4 98±7

107 98

50±2 97±3

100 97

0 25 50

n.d.b 24.9±0.2 49.5±0.6

100 99

101 103

n.d.b 24.5±0.5 49.0±1.1

98 98

n.d.b 23.8±1.2 48.9±1.4

95 98

64±2 90.5±1.2 113±2

105 98

299±12 321±13 346±13

88 94

15.3±1.1 40.6±1.2 66.7±1.5

Mean and standard deviation (n=3) n.d. – Not detected

Based on the satisfactory validation data, the developed procedure was applied for the determination of analytes in the real samples with different salinity of matrix such as well, model and natural sea water samples. The results are summarized in Table 4. Concentrations of the elements in the model sea and well water samples were found to be lower than their corresponding detection limit values, while concentration of Co(II), Pb(II), and Zn(II) in sea water was 15.3 µg L-1, 64.3 µg L-1 and 299.1 µg L-1, respectively. In order to verify accuracy of the proposed procedure for the samples with different matrix composition the recovery study was also conducted. For that reason, the known amounts of the analytes were added to water samples and then the proposed method was applied. The results are shown in Table 4. On average, the percent recoveries obtained for spiked water samples were between 95-110%. This certifies that studied elements present in natural waters or highly saline solutions can be accurately determined within the limits of the method developed.

(FAAS/FAES) providing full versatility as a state-of-the-art multielement spectroscopic method that allows one to expand the range of analyzed objects and units (major and minor elements, non-metals). The additional merits of MP–AES mode are selectivity, sensitivity and safety of analysis as compared with flame methods, which makes it a promising instrument for routine analysis. However, in order to obtain highly accurate, reliable, and sensitive results, development of separation and preconcentration procedures for the trace elements determination is required.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

CONCLUSIONS The first application of the binary tetrahexylammonium bromide–water system in liquid-liquid extraction for the heavy metal ions preconcentration is presented. The proposed extraction procedure implies adding solid salt (THABr) to the solution containing the target metal ions and PAR as a chelating agent. Direct determination (introduction into microwave plasma of atomic emission spectrometer without backextraction) of cadmium, copper, nickel, cobalt, zinc, and lead by MP-AES was demonstrated in the presence of organic matrix. In addition, preconcentration stage makes it possible to significantly reduce detection limits of metals for MP-AES by eliminating significant influence of the salt background in sea water. Accuracy of the proposed procedure and its applicability to water samples with satisfactory results was shown. Proposed preconcentration procedure is versatile and can be combined with other suitable spectroscopy techniques such as ICPAES or ETAAS. MP-AES could become an acceptable alternative to flame atomic-absorption and atomic-emission spectroscopy

* E-mail: [email protected] , Tel. +7-495-939-54-64

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Authors thank Agilent Technologies Russia for providing MPAES 4200 microwave plasma atomic emission spectrometer. We are grateful to Prof. M.A. Proskurnin and Dr. D.S. Volkov for their instructive help and technical assistance concerning AES measurements.

Supporting Information File "SupportingInformation(SI)forPublication-revised-ac-201801136w.docx". Comparison of extraction-atomic-spectrometric methods for the determination of studied metals using nonconventional solvents; analytical performance data on heavy met-


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al analysis by MP-AES without preconcentration. Details of extraction experiments and MP-AES operation.

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For Table of Contents only



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"

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10

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For Table of Contents 90x50mm (96 x 96 DPI)



Figure 1. Effect of pH on the extraction of 4 ml solution containing 200 µg L−1 of each metal ion, 1·10-3 mol L-1 PAR, 110 mg THABr. 51x44mm (600 x 600 DPI)


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Figure 2 (A) Effect of the diluent on the intensity of emission line (Cu 324.754 nm, 50 µg L–1 Cu, diluent : extract ratio = 3:1); (B) The influ-ence of oxygen supply on the intensity of emission line (Cu 324.754 nm, 200 µg L–1 Cu). 45x35mm (600 x 600 DPI)



Figure 2 (A) Effect of the diluent on the intensity of emission line (Cu 324.754 nm, 50 µg L–1 Cu, diluent : extract ratio = 3:1); (B) The influ-ence of oxygen supply on the intensity of emission line (Cu 324.754 nm, 200 µg L–1 Cu). 45x35mm (600 x 600 DPI)


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Figure S-1. Effect of PAR concentration on the extraction of 4.0 ml of solution containing of 200 µg L−1 each metal ion, 110 mg THABr, pH 5.5. 45x35mm (600 x 600 DPI)



Figure S-2. The effect of phase contact time on the extraction efficiency of metals with PAR (concentration of each metal 200 µg L−1, 1·10-3 mol L-1 PAR, 110 mg THABr, aqueous phase volume 4.0 ml, pH 5.5). 45x35mm (600 x 600 DPI)


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Figure S-3. Effect of ionic strength on the extraction efficiency of metals with PAR (concentration of each metal 200 µg L−1, 1·10-3 mol L-1 PAR, 110 mg THABr, aqueous phase volume 4.0 ml, pH 5.5). 45x35mm (600 x 600 DPI)



Figure S-4. The influence of nebulizer gas flow on the intensity of emission line (Cd 228.802 nm). 45x35mm (600 x 600 DPI)


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Figure 1. Effect of pH on the extraction of 4 ml solution containing 200 µg L−1 of each metal ion, 1·10-3 mol L-1 PAR, 110 mg THABr. 206x177mm (300 x 300 DPI)



Caption : Figure 2 (A) Effect of the diluent on the intensity of emission line (Cu 324.754 nm, 50 µg L–1 Cu, diluent : extract ratio = 3:1); (B) The influ-ence of oxygen supply on the intensity of emission line (Cu 324.754 nm, 200 µg L–1 Cu). 183x140mm (300 x 300 DPI)


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Caption : Figure 2 (A) Effect of the diluent on the intensity of emission line (Cu 324.754 nm, 50 µg L–1 Cu, diluent : extract ratio = 3:1); (B) The influ-ence of oxygen supply on the intensity of emission line (Cu 324.754 nm, 200 µg L–1 Cu). 183x140mm (300 x 300 DPI)



Figure S-1. Effect of PAR concentration on the extraction of 4.0 ml of solution containing of 200 µg L−1 each metal ion, 110 mg THABr, pH 5.5. 183x140mm (300 x 300 DPI)


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Figure S-2. The effect of phase contact time on the extraction efficiency of metals with PAR (concentration of each metal 200 µg L−1, 1·10-3 mol L-1 PAR, 110 mg THABr, aqueous phase volume 4.0 ml, pH 5.5). 183x140mm (300 x 300 DPI)



Figure S-3. Effect of ionic strength on the extraction efficiency of metals with PAR (concentration of each metal 200 µg L−1, 1·10-3 mol L-1 PAR, 110 mg THABr, aqueous phase volume 4.0 ml, pH 5.5). 183x140mm (300 x 300 DPI)


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Table S-4. Comparison of LODs for determination of metal by MP-AES in pure deionized water, 1%HNO3 and in artificial sea water without preconcentration. 183x140mm (300 x 300 DPI)


Multielement Determination of Trace Heavy Metals in Water by

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