In Situ Dielectric Barrier Discharge Trap for Ultrasensitive Arsenic

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An In-situ Dielectric Barrier Discharge Trap for Ultra-sensitive Arsenic Determination by Atomic Fluorescence Spectrometry Yuehan Qi, Xuefei Mao, Jixin Liu, Xing Na, Guoying Chen, Meitong Liu, Chuangmu Zheng, and Yongzhong Qian Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01199 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Analytical Chemistry

An In-situ Dielectric Barrier Discharge Trap for Ultra-sensitive Arsenic Determination by Atomic Fluorescence Spectrometry Yuehan Qi†, Xuefei Mao*†, Jixin Liu*†, Xing Na†,‡, Guoying Chen§, Meitong Liu†,₸, Chuangmu Zheng†, Yongzhong Qian† † Institute of Quality Standard and Testing Technology for Agro-products, Chinese Academy of Agricultural Sciences, and Key Laboratory of Agro-food Safety and Quality, Ministry of Agriculture, Beijing 100081, China ‡ Beijing Ability Technique Company, Limited, Beijing 100081, China §U.S. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, PA 19038, USA ₸ College of Chemistry, Jilin University, Changchun 130012, China Corresponding authors: Tel: +86-10-82106563, Tax: +86-10-82106566. E-mail address: [email protected] & [email protected] (X. F. Mao), [email protected] (J. X. Liu) ABSTRACT: The mechanisms of arsenic gas phase enrichment (GPE) by dielectric barrier discharge (DBD) was investigated via X-ray photoelectron spectroscopy (XPS), in-situ fiber optic spectrometer (FOS), etc. It proved for the first time that the arsenic species during DBD trapping, release, and transportation to the atomic fluorescence spectrometer (AFS) are probably oxides, free atoms and atom clusters, respectively. Accordingly, a novel in-situ DBD trap as a GPE approach was re-designed using three-concentric quartz tube design and a modified gas line system. After trapping by O 2 at 9.2 kV, sweeping for 180 s and releasing by H2 at 9.5 kV, 2.8 pg detection limit (LOD) was achieved without extra pre-concentration (sampling volume = 2 mL), as well as 4-fold enhancement in absolute sensitivity and ~10 s sampling time. The linearity reached R2 > 0.998 in the 0.1 – 8 μg/L range. The mean spiked recoveries for tap, river, lake, and sea water samples were 100% – 106%; and the measurements of the CRMs were in good agreement with the certified values. In-situ DBD trap is also suitable to AAS or OES for fast and on-site determination of multi-elements.

INTRODUCTION To monitor ultratrace arsenic in food and environmental samples, many spectrometric instruments have been employed, such as atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma optical emission spectrometry / mass spectrometry (ICP-OES/MS). Despite high analytical sensitivity and wide dynamic linear range, ICP-MS is not suitable for small laboratories, due to the high cost and extensive training requirements. Hydride generation (HG)-AAS/AFS is an able alternative technique for As analysis, due to high vapor generation efficiency, thorough matrix separation, low cost, and ready miniaturizability. Compared with ICP-MS, the slightly higher detection limit (LOD) posts limitations to its applications in many fields. Hence, it is indispensable to enhance sensitivity of AAS/AFS utilizing simple and rapid pre-concentration approaches. As a typical gas discharge of non-thermal plasma (NTP), dielectric barrier discharge (DBD) has been widely applied to various fields,1 because of ambient-temperature operation, miniaturization, simplicity, and low expense/energy consumption. Due to availability of plasma, free radicals and other activated particles, DBD can also be implemented as ionization source for mass spectrometry,2,3 as atomizer for AAS and AFS,4,5 as chemical vapor generator (CVG),6,7 or as an excitation source for OES.8,9 Zhu et al.10-13 used it as an atomizer of hydride-forming elements or plasma-assisted CVG for AAS/AFS. Hou et

al.14 attempted to utilize a tungsten coil electrothermal vaporizer (ETV) coupled to a DBD atomizer for Cd and Zn detection. Hence, it has been proved that DBD is an available tool of elemental atomization. However, Kratzer et al.15 reported that analytical stability suffered from severe elemental residue in DBD atomizer. If elemental residue can be accurately and precisely regulated, DBD would be converted into an efficient gas phase enrichment (GPE) approach for ultratrace elemental analysis. So, Mao et al.16 first utilized a novel tubular DBD reactor (DBDR) as an arsenic trap that was coupled to AFS, leading to stable, precise, and accurate trap/release via a simple switching of O2 or H2 into argon carrier gas. Such an HG-DBD-AFS system was applied to measure ultratrace arsenic in surface water, with a 1 ng/L LOD via 8-fold enrichment in 20 mL sampling volume. Soon after, Kratzer et al.17 used a planar DBD atomizer coupled to HG-AAS to enrich arsenic based on similar principle, and confirmed that arsenic analyte can be retained in the DBD atomizer by the Ar-O2 discharge and subsequently released by the Ar-H2 discharge. They also realized a pre-concentration efficiency of ~100%, reaching 10 ng/L LOD via 300 s pre-concentration. Later, Kratzer et al. proved HG-DBD-AAS is suitable for Sb trapping/releasing.18 Compared to those thermal GPE methods using the graphite furnace,19,20 quartz tube,21-23 or metal materials,24-27 the DBD trap consumes less energy. When heating a quartz tube trap/atomizer, the temperature gradient on the

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tube interior would reduce the efficiency and stability of releasing analyte, leaving some elemental residue in dead areas of the tube.28 In contrast, NTP renders DBD trap no temperature gradient.9,10 Furthermore, repeated heating and cooling that degrades quartz crystal structure and its trapping and releasing capacity,23 was completely avoided by the ambient temperature DBD trap. Similar to sputtering procedure, quick DBD release sharpen the signal peak and increase absolute sensitivity. However, the releasing speed of arsenic was not significantly accelerated by the DBD trap devices 16, 17 mentioned above. So, the previous DBD-AFS/AAS can not satisfy the demand of ultratrace elemental analysis for micro samples, such as biological samples. It resulted from large volume and long time of sample introduction. On the other hand, the H2 generation from the HG system broadens the released arsenic peak in Kratzer’s work 17 vs. the O2/H2 switching model from Mao et al.16 Sensitivity suffered from slow H2 generation as well as long distance transportation between the DBDR and the detector.17 Due to the difficulty to alternate a commercial AAS/AFS’s detector, the most viable way is to optimize the DBD structure and position. Jiang et al.29 employed a novel compact tandem atomizer comprising an ArH2 flame atomizer and an ETV device without a transportation tube. In-situ ETV-AFS technique improved sensitivity, precision and LOD significantly due to the lack of analyte loss and atom congregation, and highly enhanced atomization efficiency. Hence, if the in-situ observation of atomic spectrometric signals is achieved for DBD-AFS, no loss or dilution of released arsenic during the transportation from the DBD trap to the detector can lead to the best signal intensity acquisition. Such a design of the in-situ DBD trap is reported here for the first time. Meanwhile, the trapping and releasing mechanisms of DBD have not yet clarified fully, which are essential in DBD trap design and applications to enrichment of arsenic species. In this work, these mechanisms were investigated by X-ray photoelectron spectroscopy (XPS), in-situ fiber optic spectrometry (FOS), etc. Consequently, the DBD system was re-designed and modified to fit the commercial AFS to attain optimal trapping and releasing efficiency. Under optimal DBD conditions, we achieved 1.4 ng/L LOD without an extra pre-concentration (2 mL sampling volume within 10 s), which was equivalent to that of the original manner with 200 s trapping even close to ICPMS’s level. The in-situ DBD trap is applicable to both AAS and OES, as well as a wide array of atomic spectrometers capable of on-site and fast analysis. No doubt it is a considerable commercial potential to atomic spectrometer producers. EXPERIMENTAL SECTION DBDR. The DBD reactor is used for the study of arsenic species and distribution during the trap/release procedures. The DBDR contained two concentric quartz tubes; the bigger tube was wrapped with a copper mesh as the ground electrode; the smaller tube was inserted with a copper bar as the high voltage electrode. The DBDR was coupled to a high frequency and high voltage pulse power supplier with frequency of 3×104 Hz (Mev40kV, Xi’an MEV, Xi’an, China). Here, the voltage is changed to control discharging. All gas lines were precisely controlled by separate gas mass flow controllers (GMFC) (Beijing Seven Star Electronics, Beijing, China), of which Ar carrier gas could be switched to the upstream or downstream of the gasliquid separator (GLS) via a three-way valve controlled by computer with details described previously.16

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In-situ DBD system. An in-situ DBD system was re-designed as a GPE trap, whose schematic diagram is presented in Fig. 1. The DBD trap was installed replacing the AFS atomizer, consisting of three concentric quartz tubes. A shield tube was added as the outermost layer of the three-concentric quartz tubes to supply shield gas to isolate the outside air for the AFS signal. Carrier gas lines, power supplier, and the analytical procedures of the in-situ DBD system were similar to the DBDR. After a 10~20 s (sampling time can be altered according to preconcentration requirement) HG introduction by peristaltic pump, 9.2 kV discharge under O2-Ar for trapping, 180 s sweeping by Ar and 9.5 kV discharge under H2-Ar for release were performed orderly. Here, 200 mL/min H2 was mixed with 600 mL/min of Ar shield gas to support a diffusion flame for arsenic atomization after ignition by an electronic igniter. Hydride Generator. HG system was on the upstream of the DBD as a sample introduction approach. A peristaltic pump (PP) connecting to a GLS provided a continuous arsine flow. The reductant concentration optimized in our previous work 16 at 0.5% KBH4 in 0.15% KOH, provided not so many H radicals to interfer arsenic trapping under O2 atmosphere. The blank was 5% HCl (v/v); the Ar carrier gas flow rate was 500 mL/min.

Figure 1 The schematic diagram of the in-situ DBD trap. PMT, photomultiplier; As HCL, arsenic hollow cathode lamp; G, ground electrode; HV, high voltage electrode. Table 1 Optimal instrumental parameters of HG-AFS. Parameters

Values

HCl (v/v) KBH4/KOH (g/L) HCL current (total/main) (mA) PMT voltage (V) PP speed (mL/min) Argon carrier gas flow rate (mL/min)

5% 5/1.5 60/30 -260 12 500

Shield gas flow rate (mL/min)a 800 The shield gas in DBDR is Ar; the shield gas in in-situ DBD system is Ar/H2 (v:v=3:1). AFS. Atomic fluorescence signals were measured by an AFS (AFS-8220, Beijing Titan Instrumental, Beijing, China) equipped with a high-intensity As hollow cathode lamp (HCL, a


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Analytical Chemistry Beijing Research Institute of Nonferrous Metals, Beijing, China). The detailed parameters are listed in Table 1. XPS. An axis ultra X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, ThemoFisher, England, USA) was employed to measure the arsenic species trapped on inner surface of the DBD quartz tube. Al K (2 keV) X-ray radiation was used as the excitation source. In-situ FOS. A miniaturized fiber optic spectrometer (FOS) (USB2000+, Ocean Optics, USA) was installed to monitor the atomic emission spectrum of released arsenic. The FOS probe was positioned coaxially near the outlet of the DBD tube, and the signal was acquired from the discharge area of the DBD tube via a lens. Gas lines and power supplier of the DBD-FOS system were similar to the DBDR, and adequate ventilation protected personnel from toxic arsenic gas. The DBD-FOS system is described in Fig. 2. The USB2000+ covered 200 to 1100 nm with a 200 μm slit.

Figure 2 The schematic diagram of the HG-DBDR-insitu FOS system. S, sample in 5% (v/v) HCl; R, 5g/L KBH4 solution; PP1 or PP2, peristaltic pumps; M, a four-way mixer; C, a reaction coil; GLS, a gas-liquid separator; W, waste; V1, V2 or V3, three-way switch valves, which are in charge of switching in/off argon, oxygen or hydrogen, respectively. DBD, dielectric barrier discharge trap/release set, which contains two concentric quartz tubes; on the outer surface of the bigger tube, a copper mesh is wrapped as the ground electrode; in the middle of the smaller tube, a copper bar is inserted as the high voltage electrode. FOS, a fiber optic spectrometer; P, a fiber probe; PC, a personal computer; PS, high voltage power supply. Chemicals and standards. All chemicals were of reagent grade and purchased from Sinopharm Chemical Reagent (Beijing, China) unless otherwise stated. Mg(NO3)2 was used as inner surface coating of the DBDR to retain arsenic. HCL (5%, v/v) was used to clean the DBDR. Deionized water (18 MΩ) was prepared using a Milli-Q integral purification system (Millipore, Billerica, MA, USA). HgBr2 test paper and filter paper were cut into tiny pieces (3 mm × 3 mm), then placed downstream to the DBDR. Standard stock solutions (1000 mg/L) of sodium arsenite were purchased from the National Research Center for Certified Reference Materials (NRCCRM) (Beijing, China), and diluted as required using 5% HCl. Simulated natural water samples (CRMs) containing arsenic included GBW08605 (matrix is water solution with 0.2% (v/v) H2SO4, 2.2 mg/L K+, 23 mg/L Na+, 37 mg/L Cl-), and GBW(E)080390, GBW(E)080391, and GSB07-3171-2014 (matrix is 1% HNO3). Tap water, river water and lake water samples were collected from Beijing, and sea water was collected from Tianjin, and

preserved using 0.1% HNO4 (v/v). The river, lake and sea water samples were cleaned using filter paper prior to usage, respectively. RESULTS AND DISCUSSION Arsenic species by DBD trapping. In our previous work,16 the optimal arsine trapping would be in an oxygen atmosphere without H2 by DBD. O2 can promote trapping arsenic. So, the arsenic species and the role of oxygen atmosphere on trapping arsenic were investigated here. However, due to the fluorescence depression by O2, the trapping efficiency was evaluated based on the releasing arsenic signals rather than arsenic breakthrough. To directly validate the trapping efficiency, the FOS system (Fig. 2) was utilized; the emission spectrum in the discharge area of the DBDR tube was monitored using a 5 μg/mL arsenic solution. Fig. 3 reveals complete trapping of arsenic species in the discharge area under 40 mL/min O2 atmosphere.

Figure 3 Comparison of the emission spectra for trapping, release, and second release. The FOS monitored the emission spectrum in the discharge area of the DBDR tube using a 5 μg/mL arsenic solution. Table 2 Bonding energy and concentrations of As in DBDR surface after trapping. Sputtering time (s)

Sputtering number

As concentration (%)

Bonding energy (eV)

0

0

0.71

45.48

150.45

1

0

301.09

2

0

To investigate the arsenic species by trapping further, the trapped arsenic on the quartz surface of the DBD tube was tested by XPS with sputtering. The bonding energy and sputtering can be employed to prove what species and how deep on the quartz surface when trapping arsenic, respectively. The results are shown in Table 2. When sputtering by Ar for 150 s, no arsenic was found on the quartz surface, while 0.71% surface concentration of arsenic before sputtering. It demonstrated that As was trapped on the immediate surface of the quartz tube. So the trapped As was easily ablated by Ar sputtering. In addition, the bonding energy 30 of As correct by C1s at about 45.48 eV implied that the trapped analyte might be arsenic oxide. To verify



this speculation, MgO was employed to react with arsenic oxides to form stable magnesium arsenate.31 A saturated solution of Mg(NO3)2 was coated on the inner surface of the outer quartz tube at the DBDR discharge zone. After 8 h drying at 200 °C, Mg(NO3)2 turned into MgO forming a thin white layer. Then the coated DBDR was employed to trap/release arsenic by introducing 20 ng arsine with 1 mL of 20 μg/L arsenic standard solution. The As signals with MgO coating was cut to approximately one fourth because the trapped arsenic oxide reacted with MgO to form magnesium arsenate. Hence, the evidences mentioned above have proved that arsenic oxides are trapped on the quartz surface after oxidation by O2 with arsine. Arsenic species by DBD releasing. HgBr2 test paper and the DBD-FOS were used to verify the arsenic species during DBD releasing. The HgBr2 trail was based on the reaction of HgBr2 with AsH3.32 For HG-AFS, the HgBr2 test paper and the common filter paper were cut into 3 mm × 3 mm pieces, and placed downstream to GLS. The AFS intensity using HgBr2 was obviously lower than that using only filter paper (Fig. 4), demonstrating immobilization of AsH3 by HgBr2. For HG-DBDRAFS, these two kinds of papers were placed at the downstream DBDR for measurement separately. Similar AFS intensities between two treatments (Fig. 4) proved AsH3 was not the species released from the DBD tube. To identify the arsenic species released from DBD, the insitu FOS was employed to monitor the emission from the discharge area during releasing on two successive releases (Fig. 3). The characteristic As lines at 228.812 nm and 234.984 nm were observed on the first release, proving presence of free atoms. These atomic lines disappeared in the second release, implying the first release was complete. Although NO peaks from air appeared due to the DBDR-FOS opening to the ambient atmosphere, they had little influence on As releasing signals. However, we had no idea about releasing arsenic in the non-discharge area via this FOS manner. So, the study of As spatial distribution was performed next. The discharging area of the DBD was kept a distance from the acquisition position of fluorescence signal to avoid the emission interference by DBD plasma. However, there was no As AF signal when no H2 flame. In the transport channel, no O2 or other interferents existed to react with arsenic compounds. So, free arsenic atoms may change into certain stable state during

Figure 4 Effect of the HgBr2 test paper on arsenic by HGAFS and HG-DBDR-AFS. The previous optimized HGDBDR-AFS conditions are adopted, and the HG-AFS is set as the same parameters as HG-DBDR-AFS. The AF intensity of arsenic detected by HG-AFS with the filter paper is set as 100; other results are normalized to this value. transportation. On the basis of the theory of atom cluster reported previously,33 the transported arsenic state is probably atom cluster. The free As atoms likely collided with each other and aggregated into clusters under ambient temperature, which after transported to the AFS emitted no AFS signal without H 2 flame. When the H2 flame ignited, the arsenic atom clusters changed into free atoms under high temperature, and AF signals was acquired again. Finally, to obtain the optimal AF intensity, arsenic species were atomized and excited by an H2 flame supported by 200 mL/min H2 in the Ar stream. Spatial distribution of As in the DBD tube after trapping/releasing. The DBD quartz tube can be divided into the discharging and non-discharging zones.16 To investigate the spatial distribution of arsenic in the DBD tube, two sets of DBDR comprising the outer quartz tube (i.d. 4 mm) and the inner tube (i.d. 1 mm) was employed to trap/release arsenic after 8 h soaking in 20% nitric acid. Arsenic standard solution, 100 mL of 20 μg/L, was introduced into the HG-DBDR-AFS for

Figure 5 Spatial distribution of arsenic after trapping/releasing in the DBD tube. Panel A shows the spatial distribution of arsenic after trapping in the DBD tube; while, Panel B shows that after releasing. Here, the AFS parameters are shown in Table 1 using 5% HCl and 5 g/L KBH4.


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Figure 6 The effects of different air or H2 flow rates on trapping or release of arsenic. The previous DBDR discharge potentials and 500 mL/min carrier gas of Ar are adopted. For Panel A, the released arsenic is measured. 9.2 kV discharge potential is used for trapping; 9.5 kV discharge potential and 200 mL/min H 2 are used for releasing. The AF intensity of arsenic at 50 mL/min air is set as 100; other results are normalized. For Panel B, the released arsenic is also measured. 9.2 kV discharge potential and 50 mL/min air are used on trapping; 9.5 kV discharge potential is used on release. The AF intensity of arsenic at 50 mL/min H2 is set as 100; other results are normalized. trapping without or with releasing. Next, the outer and inner tubes were cut into three parts designated as 1, 2 and 3 from upstream to downstream. Among them, Section 2 was the discharging area. During the trapping process, no arsenic breakthrough was observed revealing complete trapping by DBDR. To measure trapped or residual arsenic on different tube segments, the detached tubes were extracted for 30 min in 5% hydrochloric acid; the leachates were measured for arsenic presence by HG-AFS (Fig. 5). For the spatial distribution of As in the DBD tube after trapping in Fig. 5A, Section 2 had the highest AF signal accounting for 92% of the total arsenic, in sharp contrast to Sections 1 and 3. The overwhelming majority of arsenic was trapped on the discharged area, revealing uneven DBDR trapping over quartz tube surface, likely corresponding to plasma intensity distribution. In Fig. 5B, only Section 3 had As signal, implying incomplete release of arsenic by the original DBDR. Partial releasing or retrapping might result in arsenic residue in non-discharge area downstream. A long rear DBDR tube is thus adverse to complete release of arsenic and DBD-AFS signal. Accordingly, the original DBDR had to be re-designed. Optimization of the in-situ DBD trap. The DBD tube rear was changed into discharge area, and the gas line from the DBD tube to AFS was removed. Meanwhile, the in-situ DBD trap was not utilized for atomization; instead, a mixture of H2-Ar was used to provide an H2 flame for arsenic atomization. All the changes made to the DBD system (Fig. 1) were aimed to avoid analyte loss and to promote the instrumental sensitivity. Operational conditions of the in-situ DBD trap, the discharge voltage, working gas, carrier gas flow rate, and time of sweeping water vapor, should all be reconsidered. After the verification, the discharge voltages of 9.2 kV on trapping and 9.5 kV on releasing for the previous DBDR16 were still appropriate. To simplify the gas supply system, air from a miniaturized air pump was utilized replacing O2-Ar mixture. Flow rate of air was

optimized as shown in Fig.6A. As signals rose with the increase of the air flow rate before 50 mL/min, then reached a plateau. Hence, 50 mL/min of air mixed with 500 mL/min of Ar was set as the optimal with a favorable relative standard deviation (RSD) of 3.3%. After trapping, the air flow was shut off, and Ar carrier gas was kept for 180 s to sweep interfering water vapor.16 For arsenic release, H2 was introduced with Ar carrier gas under 9.5 kV discharge potential. In Fig. 6B, the flow rates of H2 were optimized, and 50 mL/min was chosen as optimal. Unlike previous study,16 more than 75% of arsenic could be released from the in-situ DBD tube at 0 mL/min H2, due to possible H2 intro-

Figure 7 Spectrograms of arsenic by HG-AFS, HGDBDR-AFS and HG-in-situ DBD-AFS. The parameters of AFS for the three systems are listed in Table 1; Using 5% HCl and 15 g/L KBH4 for HG-AFS, and 5% HCl and 5 g/L KBH4 for HG-DBDR-AFS and HG-in-situ DBD-AFS.



duction from the mixed H2-Ar shield gas. During arsenic releasing, H2 might play an essential role in supplying H radicals for As atomization or ionization.34 Comparison of the half peak width among different designs. The absolute analytical sensitivity of the previous DBDAFS/AAS was compromised by unsatisfactory half peak width and the residual analyte in rear non-discharge area of the DBD set.16, 34 After optimization of the in-situ DBD trap, the half peak width of the in-situ DBD trap was compared with that of the original DBD in Fig. 7 and Table 3. The in-situ DBD trap presented a sharper peak of 0.3 s from 5 ng arsenic vs. those of HGAFS (1.3 s) and HG-DBDR-AFS (1.2 s) systems from 20 ng arsenic with similar peak heights. The in-situ DBD trap enhanced absolute analytical sensitivity by more than 4 times based on the pre-concentration factor explanation by Camara.35 Such enhancement was achieved without increasing the sampling time or volume, which accordingly brought us a high analytical efficiency. Table 3 Comparison of half peak widths in different methods. Spectral instrument HG-AFS HG-DBD-AFS HG-in-situ DBD-AFS

Half peak width (s) 1.3 1.2 0.3

Arsenic (ng) 20 20 5

Height 2823 2873 2794

Table 4 Spiked recoveries and measured arsenic presence in real water samples. Samples

Measureda (µg/L)

Added (µg/L)

Founda (µg/L)

Tap water

0.12 ±0.02

10.0

River water

0.68 ±0.04

10.0

Lake water

4.1 ±0.2

10.0

Sea water

8.7 ±0.2

10.0

GBW(E)08039 1b GBW08605c

3999 ±28

—

10.1 0.5 10.8 0.8 14.7 0.14 18.8 0.3

499 ±20

—

Recovery (%)

± 100 ± 102 ± 106 ± 102

GBW(E)08039 509 ±11 — 0d GSB07-317136.0 ±0.7 — 2014(200445)e GSB07-317126.0 ±0.8 — 2014(200446)f a Mean value and standard deviation (n = 3). b Certified value: 4000 ±80 µg/L. c Certified value: 0.500 ±0.020 µg/g. d Certified value: 0.50 ±0.075 mg/L. e Certified value: 34.8 µg/L. f Certified value: 26.0 µg/L. Analytical performance, interference and real samples analysis. Under optimized conditions, the analytical figures of merit were evaluated. A series of standard solutions 0.1 - 8 μg/L were measured, and the liner regression coefficient (R2) was > 0.998. The LOD was 1.4 ng/L or 2.8 pg (sampling volume = 2 mL), calculated by formula 3 σ/m with 11 measurements in

blanks (σ is the standard deviation and m is the slope of the calibration curve). This LOD was one order of magnitude lowered than conventional AFS without increasing sample volume, and the absolute analytical sensitivity was increased by 4 times than the original HG-DBDR-AFS. The sampling time was reduced to ~10 s sharply lower than the original DBDR manner (~100 s) at the same LOD, equivalent to common flow injection/PP HGAFS. Excellent precision was demonstrated by < 1.2% RSD of repeated As measurements of 0.5 µg/L arsenic standard solutions (n = 7). HG introduction eliminates interferences from non-HG-forming elements, so potential interferents from certain HG-forming elements were investigated, such as Se, Sb, Bi, Pb, and Hg. Interference was found insignificant below 100 µg/L. Certified reference materials (CRMs) GBW08605, GBW(E)080390, and GBW(E)080391 were measured for method validation. The detailed results (Table 4) demonstrated good agreement with the certified values. Real samples including tap water, river water, lake water, and seawater samples and spiked samples were measured with 100% to 106% spiked recoveries. Mechanism of the GPE by DBD. During mechanism studies, it was found that As trapping is spatially uneven. Arsine was introduced into the DBD tube and oxidized under O2-Ar atmosphere, then the arsenic oxides were trapped mainly on the discharge area. During the releasing cycle, H2 produced a large number of H radicals and other reactive particles, 34 thereby arsenic was atomized to remove from the quartz surface. Next, free As atoms collided forming clusters,33 which were transported to the H2 flame to be atomized and excited. Transition from excited state to ground state produced AFS signal. This is the first report on the overall mechanism involved in trapping, release, and transportation of the GPE by DBD-AFS, based on which the DBD was re-designed and applied to real-world analysis. CONCLUSIONS An in-situ DBD trap was developed as a GPE approach based on a three-concentric-tube design. Mechanisms in trapping, release, transportation, and atomization steps were also deduced for the first time. Without extra pre-concentration, it achieved exceptionally low 2.8 pg LOD and a 4-fold sensitivity enhancement over the original design. This new DBD trap gained high efficiency, cost, energy consumption, ambient and sensitivity advantages, as well as applicability to AAS and OES, owing a great potential of the miniaturization for the fast and on-site determination of multi-elements.

AUTHOR INFORMATION Corresponding Author *X. F. Mao. Phone: +86 10 82106563. Fax: +86 10 82106566. E-mail: [email protected] & [email protected]. *J. X. Liu. Phone & Fax: +86 10 82106563. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by the National Key Research and Development Program of China (No. 2017YFD0801200), the National Natural Science Fund of China (No. 31571924) and Central Public-interest Scientific Institution Basal Research Fund (No. 1610072017009).


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