Flow Injection Photochemical Vapor Generation Coupled with

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Flow Injection Photochemical Vapor Generation Coupled with Miniaturized Solution-Cathode Glow Discharge Atomic Emission Spectrometry for Determination and Speciation Analysis of Mercury Jiamei Mo, Qing LI, Xiaohong Guo, Guoxia Zhang, and Zheng Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02214 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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Analytical Chemistry

Flow Injection Photochemical Vapor Generation Coupled with Miniaturized Solution-Cathode Glow Discharge Atomic Emission Spectrometry for Determination and Speciation Analysis of Mercury Jiamei Mo&, Qing Li&, Xiaohong Guo, Guoxia Zhang and Zheng Wang* Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China. ABSTRACT: A novel, compact and green method was developed for the determination and speciation analysis of mercury, based on flow injection photochemical vapor generation (PVG) coupled with miniaturized solution cathode glow discharge atomic emission spectroscopy (SCGD-AES). The SCGD was generated between a miniature hollow titanium tube and a solution emerging from a glass capillary. Cold mercury vapor (Hg(0)) was generated by PVG and subsequently delivered to the SCGD for excitation, and finally the emission signals were recorded by a miniaturized spectrograph. The detection limits (DLs) of Hg(II) and -1 methylmercury (MeHg) were both determined to be 0.2 μg·L . Moreover, mercury speciation analysis could also be performed by using different wavelengths and powers from the UV lamp and irradiation times. Both Hg(II) and MeHg can be converted to Hg(0) for the determination of total mercury (T-Hg) with 8 W/254 nm UV lamp and 60 s irradiation time; while only Hg(II) can be reduced to Hg(0) and determined selectively with 4 W/365 nm UV lamp and 20 s irradiation time. Then, the concentration of MeHg can be calculated by subtracting the Hg(II) from the T-Hg. Due to its similar sensitivity and DL at 8 W/254 nm, the simpler and less toxic Hg(II) was used successfully as a primary standard for the quantification of T-Hg. The novel PVG-SCGD-AES system provides not only a 365-fold improvement in the DL for Hg(II), but also a non-chromatographic method for the speciation analysis of mercury. After validating its accuracy, this method was successfully used for mercury speciation analysis of water and biological samples.

Mercury is one of the most toxic elements, and both its toxicity and transportation are strongly dependent on its chemical forms. Organic mercury, especially methylmercury (MeHg), is considered more toxic than inorganic species (Hg(II)). As a consequence, not only the total mercury (T-Hg) but also MeHg should be determined when evaluating the toxicological and environmental impact. Separation techniques such as gas chromatography (GC) and high-performance liquid chromatography (HPLC), coupled to analytical instrumentations such as atomic fluorescence spectrometry (AFS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and inductively coupled plasma mass spectroscopy (ICP-MS) were the most practical approaches to speciation analysis of mercury, because of their high selectivity 1-4 and sensitivity. However, these techniques are timeconsuming; and they also use large and heavy instruments that require high power, auxiliary gas, and even high vacuum. These drawbacks can be partly resolved by developing miniaturized, portable instruments for routine analysis. Nowadays, great attention has been directed to miniaturized analytical instrumentations for on-site and real-time analysis. The solution cathode glow discharge atomic emission spectrometer (SCGD-AES), which is similar to the atmospheric pressure glow discharge atomic emission spectrometer (APGDAES), is one such promising device due to the small size, low 5,6 power, and freedom from compressed gases. In recent studies, SCGD-AES has been successfully employed to

determine trace metal ions in environmental, biological, and 7-12 other samples. Nevertheless, further improvement of SCGD-AES performance is necessary, because some heavy metals need to be measured at concentrations below the present detection limit (DL). At present, much effort has been devoted to lower the DL for Hg(II). The signal can be increased by adding chemical modifiers into the sample solution, such as lowmolecular weight organic compounds, non-ionic surfactants, 13-17 and ionic surfactants. For example, the intensity of Hg(II) emission lines increased by 10-fold and 4.8-fold with 5% (v/v) HCOOH and 0.15% (m/m) cetyltrimethylammonium chloride (CTAC) added to the sample solution, and the DL values were -1 found to be 2 and 7 μg·L , respectively. It was presumed that the observed amplification effect was due to changes in the surface tension and viscosity of the solution. Another problem with SCGD is that the direct introduction of sample solution into it often results in high matrix interference and low transfer efficiency. Accordingly, separation and pre-concentration techniques based on flow injection (FI) have been applied to resolve matrix interference 18-20 in SCGD-AES. This FI-SCGD-AES technique has significantly improved the sensitivity and selectivity of Hg(II), with the DL as -1 low as 0.75 μg·L . Chemical vapor generation is a widelyadopted sample introduction method for atomic spectrometry, since it could conveniently and efficiently separate the analytes from the condensed phase prior to instrumental measurements, yielding efficient on-line matrix separation,

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high transport efficiency, selectivity, and sensitivity. Both 22 23 cold vapor generation (CVG) and hydride generation (HG) have been successfully coupled to APGD-AES for the determination of Hg, As, Se, and Sb. More recently, the sensitivity for Se, Sn, and Ge has been significantly improved 24 Furthermore, HG-SCGD-AES was by HG-SCGD-AES. successfully applied to the valence analysis of selenium 24 without the requirement for chromatographic separation. However, traditional HG systems suffer from some problems, such as relatively expensive and unstable reagents, large amounts of H2 generated, and serious interference from transition elements. Recently, photochemical vapor generation (PVG) was demonstrated to be a powerful alternative to conventional 25-27 In PVG, volatile species are chemical vapor generation. generated from non-volatile precursors by ultraviolet (UV) irradiation in the presence of a low-molecular weight organic acid (LMWOA). PVG not only retains the primary advantages of HG techniques, but is also less toxic, simpler, and subject to little interference from transition metals. Moreover, the instability of plasma arising from the introduction of large amounts of H2 generated during HG is reduced in PVG. Therefore, PVG is an ideal sampling method for improving the performance of microplasma-based AES, since it is easier to miniaturize, safer to operate, and more environmentally friendly. To date, PVG has been successfully coupled to dielectric barrier discharge atomic emission spectrometer 28,29 and point discharge atomic emission (DBD-AES) 30 spectrometer (PD-AES) for the determination of Hg(II), Fe(III), Hg(II), and Ni(II). In addition, PVG can be used as a nonchromatographic technique in the speciation analysis of mercury, due to the different PVG efficiency of mercury 31 species under different conditions. Zheng et al. applied PVG with formic acid to the speciation analysis of mercury in water by AFS. They reported that, when UV light irradiated sample solutions containing formic acid, both Hg(II) and MeHg were reduced to mercury cold vapor (Hg(0)) for the determination of total mercury (T-Hg). Meanwhile, only Hg(II) was reduced to Hg(0) under natural room light (Vis), thus determining Hg(II) selectively. Then, the concentration of MeHg can be calculated by subtracting the Hg(II) concentration from that of T-Hg. Unfortunately, the PVG efficiency under Vis light (with a 32 comparative efficiency of 10%) is much lower than that under UV, and the technique is not sensitive enough for ultra trace speciation analysis of mercury by other analytical instrumentations, especially miniaturized AES. Therefore, in this work we intend to utilize flow injection PVG coupled with SCGD-AES to establish a miniaturized analytical instrumentation for the ultra trace level determination and speciation analysis of mercury. In this installation, the SCGD operates between a miniature hollow titanium tube and a solution emerging from a glass capillary,

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and a miniature spectrometer equipped with charge coupled device (CCD) detector was integrated with the SCGD. An automated stopped-flow operation is employed to increase the UV irradiation time on-line. Volatile mercury vapor generated by PVG is transported to the SCGD for excitation, and finally the emission signals are recorded by the miniaturized spectrograph. The feasibility of the PVG-SCGDAES and the method of transportation were evaluated. The parameters affecting PVG and SCGD excitation efficiency were investigated. Different wavelengths and power of the UV lamps were evaluated, and the PVG-SCGD-AES was tested in the speciation analysis of Hg(II) and MeHg. In addition, the accuracy and applicability of the proposed approach were investigated by using real samples.

EXPERIMENTAL Instrumentation. The full instrumental setup for the PVGSCGD-AES system is shown in Figure 1. It mainly consists of a flow injection PVG, a SCGD excitation source, and a miniaturized CCD spectrograph (Maya 2000 Pro, Ocean Optics Inc., USA). The CCD spectrograph had a spectral detection range of 189–413 nm, a 1200 line/mm grating, a 25 µm entrance slit, and a resolution of 0.35 nm. The flow injection PVG system used in this work consisted of a commercial flow injection (FI) instrumentation that was equipped with a six-port valve (FIA-3110, Beijing Titan Instruments, China), a UV reaction system, and a gas-liquid separator (GLS). The UV reaction system was made from a coiled quartz tube (inner diameter: 2.0 mm, outer diameter: 3.0 mm, internal volume: 3.2 mL) wrapped around a UV lamp of 16 W/254 nm (or 4 W/254 nm, 8 W/254 nm, 9 W/311 nm, 4 W/365 nm, 8 W/365 nm, and 15 W/365 nm, Philip, Holland) and subsequently surrounded with aluminum foil. The aluminum foil was used to protect the human operator from UV irradiation, reflect UV radiation from the lamp, and stabilize the reaction temperature to enhance PVG efficiency. Samples were pumped through the reactor with the aid of the peristaltic pump A. Hg(0) was generated after UV irradiation in the UV reactor. An automated stopped-flow operation was employed to increase the UV irradiation time on-line. One stream of argon was controlled by an XD-600 mass flow meter (Shanghai Cixi Instruments Co., China) through the GLS to maintain the SCGD, and another stream of argon was controlled by peristaltic pump B, which was used to transport the reaction mixture from the UV reactor to the GLS. The Hg(0) isolated from the GLS was then directly transported via a short length of plastic tubing into the SCGD by the argon streams for excitation.


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Figure 1. Schematic diagram of the PVG-SCGD-AES system.

The miniature spectrometer equipped with CCD detector was integrated with the SCGD to make the instrument more compact and portable, due to its small volume and absence of moving parts. Parts of the SCGD-AES used here were similar to 24 those used in our former studies, but modified to use miniaturized CCD spectrograph in place of the monochromator equipped with photomultiplier (PMT) detector. The SCGD microplasma was generated and sustained between a 3.5-mm vertical gap between a hollow titanium tube (inner diameter: 1.0 mm, outer diameter: 2.5 mm) and a solution emerging from a glass capillary (inner diameter: 0.38 mm, outer diameter: 1.1 mm). The SCGD operation was sustained by a high-voltage power supply (HSPY-600, Beijing Hanshengpuyuan Instruments Co., China), which was used in the constant voltage mode at ~75 mA with an output voltage of 1060 V. A 1.2 kΩ ballast resistor was placed in series with the Ti anode to limit the discharge current. To reduce fluctuations, an elastic air chamber and knotted tubing acting as pulse damper were included between the pump and the SCGD cell. The SCGD was ignited by squeezing the elastic air chamber, as this reduced the gap between the hollow titanium tube anode and the liquid cathode. Further details on the 10,24 SCGD cell can be found in earlier papers. Optical emission from the SCGD was collected and directed to the CCD spectrograph by a fused silica lens (25 mm diameter, 50 mm focal length) and an optical fiber. The optical fiber probe was mounted on a platform equipped with three independent micrometer screw gauges, so that it could be adjusted precisely in the x, y, and z directions to obtain the maximum signal output. The centers of the lens, the optical fiber probe, and the discharge area were on the same level. The silica lens was located 100 mm from the plasma and 65 mm from the optical fiber probe. Spectra Suite software was used to operate the spectrograph and acquire the signal. Reagents and solution. All reagents were of analytical reagent grade or better. Deionized water (18.25 MΩ·cm resistivity) was ® obtained after passage through a Milli-Q water system (Millipore, Bedford, MA, USA). Stock standard solutions of −1 Hg(II) (1000 mg·L ) were supplied by Sigma-Aldrich (Germany). MeHg (GBW08675) and EtHg (GBW(E)081524) were purchased from National Research Center for Standard

Materials (Beijing, China). The interference study was carried out by dissolving appropriate amounts of the following compounds: Ni(II), Fe(III), Al(III), Zn(II), Cu(II), Co(II), Mg(II), −1 Na(I), and Mn(II) (1000 mg·L , Sigma-Aldrich, Germany). Formic acid (98%) and acetic acid (99%) were sourced from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Two certified reference materials, GBW09101b (human hair) and GBW10029 (fish tissue), were obtained from the Shanghai Institute of Applied Physics, Chinese Academy of Sciences (Shanghai, China) and National Research Center for Standard Materials (Beijing, China), respectively. Sample preparation. A sonication-assisted acid leaching procedure was used to liberate Hg(II) and MeHg from the certified biological reference materials. The sample (0.5 g of fish tissue or 0.3 g of human hair) was accurately weighed into −1 a 50-mL plastic centrifuge tube, and 5 mL of 5.0 mol·L HCl solution was added. The centrifuge tubes were left overnight and then put in a water bath at room temperature and sonicated for 120 min. After centrifugation at 6000 rpm for 15 min, the supernatant was quantitatively transferred to a 50-mL −1 polyethylene terephthalate (PET) bottle. 10 mg·L Ni(II) was added to the samples, which was then diluted to 20 mL by adding 20% (v/v) formic acid. For water samples, Hg(II) and -1 MeHg (10 μg·L of each) were spiked to evaluate the recovery in the proposed method. Meanwhile, a blank sample was processed under the same conditions. Analysis procedure. The PVG procedure was controlled through the pumps, and the operating program is presented in Table 1. In step 1, solutions of samples containing Hg(II), 20% -1 (v/v) formic acid and 10 mg·L Ni(II) were initially pumped into the coiled quartz pipe through the six-port valve, and propelled using a pumping rate of 8.4 mL·min-1 for 15 s. In step 2, both peristaltic pumps A and B were stopped for 60 s to allow the sample to flow into the UV reactor. The aim of this step was to increase the time for which the sample zone was subjected to UV irradiation while it was transported to the GLS. In step 3, the mixture from the UV reactor was flushed into the GLS with Ar, using a pumping rate of 75 mL·min-1 for 30 s. The volatile species were separated in the GLS and further transported into the SCGD microplasma by an argon flow for the AES detection. Step 4 was a 20-s break to wash the system and prepare for the next measurement. Steps 5–8



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were similar to steps 1–4 except for the UV lamp, sample, and irradiation time. In steps 1–4, the UV lamp was 8 W/254 nm, while in steps 5–8 it was 4 W/365 nm. In step 5, the sample solutions contained Hg(II), MeHg, 20% (v/v) formic acid, and 10 -1 mg•L Ni(II). In step 6, the irradiation time was 20 s. The whole procedure took about 210 s. The spectral peak height at

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253.7 nm was used for quantification. An integration time of 300 ms and an average of five scans were employed. Safety precautions. Hg(0), Hg(II), MeHg, and EtHg are very toxic, and the full range of toxic compounds produced in this study is currently unknown. Essential safety precautions must be taken during all manipulations, and an adequate ventilation/exhaust system should be used.

Table 1. Flow-injection/stopped-ﬂow program.

Step

Pump

1 2 3 4 5 6 7 8

A active A and B stopped B active B active A active A and B stopped B active B active

Delivered medium Samplea --Ar Ar Sampleb --Ar Ar

Pumping rate (mL·min-1) 8.4 --75 75 8.4 --75 75

a

Containing Hg(II), 20% (v/v) formic acid and 10 mg·L-1 Ni(II)

b

Containing Hg(II), MeHg, 20% (v/v) formic acid and 10 mg·L-1 Ni(II)

UV lamp 8 W/254 nm 8 W/254 nm 8 W/254 nm 8 W/254 nm 4 W/365 nm 4 W/365 nm 4 W/365 nm 4 W/365 nm

Time (s) 15 60 30 20 15 20 30 20

Function Transport sample Irradiation Transport volatile Washing system Transport sample Irradiation Transport volatile Washing system

(RSD) remained below 3.0% (n = 11) after background correction. All these results demonstrated that PVG has good compatibility with the miniaturized SCGD-AES.

RESULTS AND DISCUSSION Analytical characteristics. In microplasma AES analysis, the background spectra and the plasma stability generally affect the analysis of target elements. Since this was a novel attempt to determinate Hg(II) by the PVG technique coupled with miniaturized SCGD-AES, the analytical characteristics (including the background spectra, the Hg emission spectra, and the SCGD plasma stability) were investigated to assess its feasibility and practicality. As can be seen from Figure 2, the low spectral background in the wavelength range of 240–260 nm and the sensitive Hg emission spectral line at 253.7 nm can be observed in the presence of 10% HCOOH in the solution.

Transport of reaction mixture to GLS. Effective transport of volatiles from the UV reaction system to GLS is the key in this 29 30 hyphenated technique. In PVG-DBD-AES or PVG-PD-AES systems, the reaction mixture was transported to GLS by a carrier solution (H2O), and further transported into the microplasma by an argon flow. We compared using the carrier solution (H2O) vs. carrier gas (air) to transport the reaction mixture to GLS, and the results showed that the carrier gas was the better choice (see Figure 3). In this work, when using the carrier solution, Hg(0) would accumulate in the GLS and cannot be effectively transported to SCGD for excitation. Thus, the Hg signal intensity was very weak or even absent, and the reproducibility was poor. The separation efficiency, transport efficiency, and the SCGD excitation power were significantly influenced by the type of carrier gas, which served not only as the discharge gas, but also as carrier of the reaction mixture to the GLS and further transport Hg(0) to the SCGD excitation. Three carrier gases were investigated: air, Ar, and He, and the resulting Hg signal intensities are summarized in Figure 3. Since Ar had the maximum responses, it was used as the carrier gas in all subsequent experiments.

Figure 2. Emission spectra of PVG-SCGD-AES recorded (a) without Hg(II) or HCOOH in the solution and (b) with 0.5 mg·L-1 Hg(II) and 10% HCOOH in the solution.

In practice, the stability of the SCGD microplasma was not satisfactory when the two streams of argon meet in the GLS, causing fluctuations in both the background and Hg emission intensity. Thus, a background correction method described in Figure S1 (See section 1 of Supporting Information, SI) was used to improve the stability. The relative standard deviation


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improved minimally. The reason is that the PVG can be a competitive process between the formation of volatile 26 compounds and their decomposition. Therefore, when a longer irradiation time is used, the decomposition of volatiles becomes dominant, leading to reduced signal. In addition, a multitude of bubbles were generated at longer irradiation times, which led to SCGD instability and signal fluctuation. It is worth noting that a measurement error also affects the variability of DL. Thus, the irradiation time was set at 60 s for subsequent experiments and reasonable sample throughput.

Figure 3. Effect of different reaction mixture carrier for the GLS.

Optimization of PVG Parameters. Several important parameters affecting the PVG efficiency, such as the type and concentration of LMWOA, UV irradiation time, and Ar flow rate, were evaluated by a univariate approach using the 16 W/254 nm UV lamp. The SCGD parameters of the electrolyte, flow rate, discharge voltage, and discharge gap were -1 maintained at pH = 1.0 HNO3, 1.8 mL·min , 1040 V and 3.5 mm, respectively. Type and concentration of LMWOA. The PVG efficiency has been determined to be strongly dependent on the type and concentration of LMWOA used. Mercury vapor had been generated in formic acid, acetic acid, and propionic acid media, but propionic acid has an unpleasant odor and typically causes frothing in the GLS.33 Therefore, 0.5 mg·L-1 Hg(II) solutions containing various concentrations of formic acid or acetic acid were investigated with 30 s irradiation time. The pumping rate of pump B was 85 mL·min-1 and Ar flow rate was 80 mL·min-1. The DL was used as the optimization index for PVG and SCGD parameters, which can show both the PVG efficiency and signal stability. The DL was defined according to the formula DL = 3 SD·k-1, where SD is the standard deviation (n = 11) of the blank samples and k is the slope of the calibration curve. In the results (See section 2 of the SI) the DL values using formic acid were obvious lower than those using acetic acid. The best DL value was best achieved when using 20% (v/v) formic acid. A possible reason is that the kinetics of PVG efficiency of Hg(II) is noticeably different between these two acids: the transformation from Hg(II) to Hg(0) needs a longer irradiation time in the acetic acid media.33 Thus, 20% (v/v) formic acid was selected for all subsequent studies. UV irradiation time. The irradiation time in this study is the duration in which the sample zone is subjected to UV irradiation, reflected by the stopped-flow time of the peristaltic pump and accurately controlled using an FI system. In order to keep the time and sample consumption to a minimum, the pumping rate of pump A was 8.4 mL·min -1 for 15 s. The peristaltic pump in the FI instrumentation was adjusted to produce a residence time of 5–90 s, in order to investigate the effect of irradiation time on the PVG process. As shown in Figure 4, only a few seconds was enough for the generation of mercury vapor, i.e., producing free radicals to reduce the Hg2+ to mercury vapor. The DLs were initially reduced as irradiation time increased from 5 to 60 s; thereafter, the DLs only

Figure 4. Effect of UV irradiation time for 16 W/254 nm UV lamp (LMWOA: 20% (v/v) formic acid, pump B revolution: 85 mL·min-1, Ar flow rate: 80 mL·min-1).

Ar flow rate. The efficiencies of gas-liquid separation and vapor transport were significantly influenced by the flow rate of the carrier gas. In this study, two streams of Ar that were controlled separately by peristaltic pump B and mass flow meter were used for transporting the volatile products and assisting gas-liquid separation. Low flow rates lead to inefficient separation and transport of Hg(0) from the GLS to SCGD, while high flow rates significantly dilute the Hg(0) and make the plasma unstable or even extinguishing the SCGD. The effect of flow rate on the response is shown in Figure 5. The -1 minimum DL values were obtained for 80 and 100 mL·min Ar flow rate. Therefore, these two flow parameters were selected for subsequent experiments.

Figure 5. Effect of Ar flow rate (LMWOA: 20% (v/v) formic acid, irradiation time: 60 s).

Signal response of different species of mercury. One key issue is whether the system can respond to organic mercury compounds by decomposing them. The PVG efficiency for each



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mercury species was evaluated under the optimized PVG and SCGD experimental conditions (See section 3 of the SI), by comparing the net intensity at the equal concentration of 0.1 -1 mg·L . The experiment results showed that there was no significant difference in the signal responses for Hg(II), MeHg, and EtHg solutions when using the 16 W/254 nm UV lamp and 60 s irradiation time (see Figure 6). This suggests that the PVG process of these three species in 20% (v/v) formic acid possibly follows the same mechanism and yields similar production efficiencies for the volatile species, which resembles the 34 results reported by Zhang et al. Hence, it could be deduced that the total mercury (T-Hg) could be detected by the PVG technique after pretreating the sample.

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Potential application to speciation analysis of mercury. For the 4 W/365 nm UV lamp, the effect of radiation time on the response is shown in Figure 8. No signals were detected for either MeHg or EtHg when the radiation time was less than 20 s. This means that the responses from the two mercury species (Hg(II) and O-Hg) were clearly separated when the radiation time was below 20 s. Hence, Hg(II) can be selectively determined by the 4 W/365 nm UV lamp with 20 s radiation time. Considering the sensitivity and the above experimental results, the 8 W/254 nm UV lamp and 60 s radiation time were chosen to determine T-Hg. Finally, the concentration of O-Hg can be calculated by subtracting the Hg(II) concentration from that of T-Hg.

Figure 6. Signal response of different mercury species by PVG-SCGD-AES. Figure 8. Effect of UV irradiation time when using the 4 W/365 nm UV lamp.

Influence of the wavelength and power of the UV lamp. The wavelength and power of the UV lamp affect the energy and intensity of the photons. Different wavelengths and powers were evaluated under the optimized conditions, and the results are shown in Figure 7. The responses to inorganic mercury (Hg(II)) and organic mercury (O-Hg, namely MeHg and EtHg) were different at different wavelengths and powers of the UV lamp. For both Hg(II) and O-Hg, the response is in the order of 365 nm < 311 nm < 254 nm. At 254 nm, there was no significant difference between Hg(II) and O-Hg at different powers, although the maximum net intensity can be obtained at 8 W. Thus, the 8 W/254 nm UV lamp and 60 s radiation time can be used to detect T-Hg. However, Hg(II) and O-Hg showed different responses under the 365 nm UV lamp at different powers, and the difference was most obvious at 4 W. Therefore, the 4 W/365 nm UV lamp was chosen to investigate the conditions for the speciation analysis of mercury.

Figure 7. Signal response of different wavelengths and powers of the UV lamp.

Influence of Ni(II) on PVG efficiency. A nano-TiO2 suspension and immobilized nano-TiO2 or ZrO2 were demonstrated to be highly effective catalysts for improving the PVG efficiency.35-37 However, the operation is complicated and time-consuming. Additionally, transition metal ions such as Fe(III), Co(II), and Ni(II) can be used as enhancers to improve the PVG efficiency for As(III)38 and Pb(II).39 In this study, the effects of three transition metal ions (Fe(III), Co(II), and Ni(II)) on the response were investigated using 0.1 mg·L-1 Hg(II) or MeHg. It was found that both Co(II) and Ni(II) can improve the PVG efficiency, with Ni(II) being better than Co(II). However, Fe(III) has an inhibitory effect on the PVG efficiency. The effect of Ni(II) concentration in the medium is shown in section 4 of the SI . Upon increasing the Ni(II) concentration from 0 to 10 mg·L-1, the PVG efficiency increased. However, an Ni(II) concentration above 10 mg·L-1 reduced the PVG efficiency, because a high concentration of Ni(II) consumes parts of the UV light and free radicals. The experimental results demonstrated that adding 10 mg·L-1 Ni(II) could increase the signal intensity by almost a factor of 1.5–1.6 compared to that without nickel ion. Further investigation of the mechanism may improve our understanding of the process, however it is beyond the scope of the current study. Therefore, a 10 mg·L-1 Ni(II) solution was chosen as the optimal enhancement reagent in this work, in order to yield maximum response at a minimum concentration. The influence of other ions can be seen in section 5 of the SI. Possible mechanisms of enhancement and speciation analysis. Many reports have been devoted to the possible mechanisms of PVG to reduce Hg(II). The mechanisms may be different with and without TiO2 catalyst,27,40 by going through the non-radical


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and radical pathways, respectively. Our proposed method here uses formic acid without TiO2 catalyst. Therefore, the photolysis of formic acid produces CO and H2 via the free radical pathway: HCOOH + hν → ·H + ·COOH → H2 + CO2 and/or HCOOH + hν → HCO· + ·OH → CO + H2O The produced hydrogen and/or CO· radical can reduce Hg(II) to Hg(0). Hence, the volatile Hg(0) is generated from nonvolatile Hg(II) precursors by PVG. The efficient on-line matrix separation and high transport efficiency contribute to the enhanced SCGD-AES performance. Although both inorganic and organomercury species can be efficiently converted to Hg(0) in the presence of formic acid under UV irradiation, the PVG efficiency of mercury species are different under different conditions. The light emitted from -1 the 254 nm UV lamp (corresponding to 4.9 eV or 471 kJ·mol ) has sufficient energy to cleave the C–Hg bond (since the bond energy of ClHg–CH3 is 280.0 ± 12.6 kJ·mol-1 and that of ClHg– -1 37 C2H5 is 264.8 ± 12.6 kJ·mol ). This cleavage results in the formation of Hg(II), which is subsequently reduced to Hg(0). Theoretically speaking, the 365 nm UV lamp (corresponding to -1 3.4 eV or 327 kJ·mol ) is also powerful enough for cleaving the C–Hg bond. However, the transmission of the UV light through the quartz is not 100%, and the light also decays in the air. Therefore, the light emitted from the 4 W/365 nm UV lamp could not significantly cleave the C–Hg bond when the radiation time is less than 20 s, meaning that it cannot convert organomercury to Hg(II). This is the basic principle for the speciation analysis of mercury. However, this method cannot distinguish MeHg from Et-Hg, because the distinction of bond energy is not obvious. Meanwhile, the detailed reaction sequences and mechanisms are still unclear at this point.

Analytical Performance and Merit. Table 2 summarizes the analytical performance of the PVG-SCGD-AES using the calibration curves of various standard solutions under optimal operating conditions. All linear correlation coefficients are better than 0.99. The RSD for five replicate measurements of -1 50 μg·L of Hg(II) and MeHg was better than 3.0%. In the presence of 8 W/254 nm UV and 60 s irradiation, the sensitivity for Hg(II) is the same as that for MeHg, and the DL -1 values are both 0.2 μg·L . It means that the sensitivity and DL are not dependent on the mercury species, and thus a simpler, cheaper and less toxic Hg(II) standard series can be used for the determination of both Hg(II) and T-Hg, similar to that

34

reported by Zhang et al. Therefore, MeHg can be decomposed and T-Hg can be determined. However, only Hg(II) can be selectively determined using 4 W/365 nm UV and -1 20 s irradiation, with the DL of 1.6 μg·L . Then, the concentration of MeHg can be obtained by subtracting Hg(II) from T-Hg. The whole procedure took about 210 s. The proposed method not only determines the Hg(II) or T-Hg contents with high sensitivity, but also provides a nonchromatographic technique method for the speciation analysis of mercury. Table 2. Analytical Performance of the PVG-SCGD-AES system.

Analy te Hg(II) Hg(II) MeHg

Conditions 4W/365 nm, 20 s 8W/254 nm, 60 s 8W/254 nm, 60 s

Line equation

R2

RSD (%)

DL (μg·L-1)

74.28C+869.36

0.9983

2.9

1.6

340.46C+1167.76

0.9992

2.3

0.2

358.77C+1702.38

0.9983

3.0

0.2

The DL values of PVG-SCGD-AES for the determination of Hg(II) were compared to recent literature values in Table 3. Compared to single SCGD-AES, the sensitivity for Hg(II) was significantly enhanced (365-fold) by the PVG-SCGD-AES technique, and the calculated DL for Hg(II) is 0.2 µg·L-1. The proposed method is much better than the chemically enhanced SCGD-AES, because the former might be able to resolve matrix interference and enhance the transport efficiency by PVG. Meanwhile, its DL value for Hg(II) is superior to that of FI-SCGD-AES and comparable to those of CVG-APGDAES and PVG-ICP-AES. Furthermore, the proposed method has -1 a low DL (0.2 µg·L ) similar to those obtained using other microplasma sources such as DBD or PD. To date, the PVG28 30 and PVG-PD-AES were developed for the DBD-AES determination of Hg, with the respective DL of 0.2 and 0.1 -1 μg·L . In addition, the proposed method provides a novel nonchromatographic technique method for the speciation analysis of Hg(II) and MeHg. It is worth noting that the DLs obtained here for selective Hg(II) and T-Hg analysis are much worse than those reported from PVG-AFS. Nevertheless, our method here has a flower power consumption, higher throughput, and better potential for instrumental miniaturization than the commercial AFS.



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Table 3. Comparison of the DLs of the PVG-SCGD-AES system with those reported in the literature. -1

Analytical techniques PVG-SCGD-AES SCGD-AES

DL (μg·L ) MeHg 0.2f

Hg(II) 1.6e, 0.2f

a

a

Reference

T-Hg 0.2f

This work

73

---

---

This work

SCGD-AESa, c

2

---

---

14

b, d

7

---

---

17

b

22

---

---

7

SCGD-AES

SCGD-AES

FI-SCGD-AES

b

0.8

---

---

18

CVG-APGD-AESb

0.1

---

---

22

PVG-DBD-AESb

0.2

---

---

28

0.1

---

---

30

PVG-ICP-AES

0.1

---

---

41

PVG-AFS

0.2g

---

0.003h

31

PVG-PD-AES

a

a

Miniature spectrometer with CCD detector

b

Monochromator with PMT detector

c

Chemically enhanced by adding HCOOH to solutions

d

Chemically enhanced by adding CTAC to solutions

e

4 W/365 nm UV lamp and 20 s irradiation time

f

8 W/254 nm UV lamp and 60 s irradiation time

g

Vis and 20 s irradiation time

h

125 W UV lamp and 20 s irradiation time

Table 4. Analytical results of certified reference material, water and biological samples by PVG-SCGD-AES.

Founda (μg·L-1)

Added (μg·L-1)

Certified (μg·g-1)

Recov ery (%)

Sample Hg(II)

MeHg

Hg(II)b

MeHgc e

GBW09101b

_

_

0.26 ± 0.05

Tap water

10

0

9.64 ± 0.15

nd

0

10

nd

10

10

9.63 ± 0.15

GBW10029

_

_

nd

0.72 ± 0.08

Mean ± SD, n = 5

b

Obtained by 4 W/365 nm UV lamp and 20 s irradiation time

c

Obtained by subtracting b from d

d

Obtained by 8 W/254 nm UV lamp and 60 s irradiation time

e

Concentration, μg·g-1

f

Obtained by Gao et al.42

e

Hg(II)

0.98 ± 0.13

e

MeHg f

T-Hg f

0.24 ± 0.04

0.72 ± 0.08

1.06 ± 0.28

_

9.70 ± 0.25

_

_

_

96.4

9.87 ± 0.33

9.87± 0.33

_

_

_

98.7

9.89 ± 0.08

19.52 ± 0.23

_

_

_

97.6

e

e

_

_

_

_

0.82 ± 0.09

a

T-Hgd

0.82 ± 0.09

nd: Not detectable.

Analytical validation and applications. To demonstrate the accuracy of the developed PVG-SCGD-AES system, the analysis was applied to the certified reference material of GBW09101b (human hair) by standard curve method. The

results are summarized in Table 4. The results show that Hg(II) can be selectively determined by the 4 W/365 nm UV lamp and 20 s radiation time, and T-Hg can be determined with the 8 W/254 nm UV lamp and 60 s radiation time.


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Therefore, the concentration of MeHg can be calculated by the difference between these two. There was no significant difference between the found and the certified values. Meanwhile, the contents of Hg(II) and MeHg are in agreement with the values obtained by Gao et al.42 The proposed method was also applied to fish tissue and tap water samples. It was necessary to spike the tap water -1 sample with 10 μg·L of both Hg(II) and MeHg, because their endogenous concentrations were below the DLs of the proposed method. As shown in Table 4, Spike recoveries in the range of 96–98% were obtained using this approach for the tap water sample, and the found value of MeHg is in agreement with the certified values

CONCLUSIONS A flow injection photochemical vapor generation (PVG) method was used as the sampling technique for the first time to couple with miniaturized solution cathode glow discharge atomic emission spectroscopy (SCGD-AES) for the determination and speciation analysis of mercury. Nickel ion was used as an enhancement reagent for the PVG efficiency of Hg(II) and MeHg, resulting in 1.5–1.6 times the signal intensity compared to that without nickel ion. The proposed method provided a DL value of 0.2 µg·L-1 for Hg(II), a 365-fold enhancement over SCGD-AES. This novel PVG-SCGD-AES system not only significantly improves the sensitivity for the determination of Hg(II), but also provides a compact method for the speciation analysis of Hg(II) and MeHg. The mechanisms of both the enhancement and speciation analysis are not fully understood. However, PVG is also amenable to other hydride-forming elements (As, Se, Cd, etc.) as well as some transition metals (Fe, Ni, etc.). Thus, the PVG-SCGD-AES technique could be potentially extended to other elements. Furthermore, PVG can decompose organic compounds, thus GC or HPLC could be combined to PVG-SCGD-AES for speciation analysis. In a future study, it would be worthwhile to consider the mechanism and potential applications of this system.

ASSOCIATED CONTENT Supporting Information The Supporting Information (SI) includes the background correction method, optimization of SCGD parameters, and interference study.

AUTHOR INFORMATION Corresponding Author *Tel.: (+86) 021 52413503. Fax: (+86) 021 52413016. E-mail: [email protected] Notes The authors declare no competing financial interest. & These authors contributed equally to this work and should be considered co-first authors.

ACKNOWLEDGMENTS This work was supported by the Instrument Developing Project of Chinese Academy of Sciences (No. YZ201539), the National Natural Science Foundation of China (No. 21175145), and Shanghai technical platform for testing and characterization of inorganic materials (14DZ2292900).

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For TOC only


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Figure 1. Schematic diagram of the PVG-SCGD-AES system. 404x161mm (96 x 96 DPI)



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Figure 2. Emission spectra of PVG-SCGD-AES recorded (a) without Hg(II) or HCOOH in the solution and (b) with 0.5 mg•L-1 Hg(II) and 10% HCOOH in the solution. 287x201mm (300 x 300 DPI)


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Figure 3. Effect of different reaction mixture carrier for the GLS. 287x201mm (300 x 300 DPI)



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Figure 4. Effect of UV irradiation time for 16 W/254 nm UV lamp (LMWOA: 20% (v/v) formic acid, pump B revolution: 120 r•min-1, Ar flow rate: 80 mL•min-1). 355x249mm (300 x 300 DPI)


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Figure 5. Effect of Ar flow rate (LMWOA: 20% (v/v) formic acid, irradiation time: 60 s). 287x201mm (300 x 300 DPI)



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Figure 6. Signal response of different mercury species by PVG-SCGD-AES. 287x201mm (300 x 300 DPI)


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Figure 7. Signal response of different wavelengths and powers of the UV lamp. 287x202mm (300 x 300 DPI)



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Figure 8. Effect of UV irradiation time when using the 4 W/365 nm UV lamp. 355x249mm (300 x 300 DPI)


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Flow Injection Photochemical Vapor Generation Coupled with

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