Simultaneous Sensitive Determination of Selenium, Silver, Antimony

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Simultaneous Sensitive Determination of Selenium, Silver, Antimony, Lead and Bismuth in Microsamples Based on Liquid Spray Dielectric Barrier Discharge Plasma Induced Vapor Generation Xing Liu, Zhenli Zhu, Zhengyu Bao, Dong He, Hongtao Zheng, Zhifu Liu, and Shenghong Hu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03966 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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Analytical Chemistry

Simultaneous Sensitive Determination of Selenium, Silver, Antimony, Lead and Bismuth in Microsamples Based on Liquid Spray Dielectric Barrier Discharge Plasma Induced Vapor Generation Xing Liu,1 Zhenli Zhu,*,1 Zhengyu Bao,1 Dong He,1 Hongtao Zheng,2 Zhifu Liu,1 Shenghong Hu1 1

State Key Laboratory of Biogeology and Environmental Geology, School of Earth

Sciences, China University of Geosciences (Wuhan), Wuhan, Hubei 430074, China 2

State Key Laboratory of Biogeology and Environmental Geology, Faculty of

Materials Science and Chemistry, China University of Geosciences (Wuhan), Wuhan, Hubei 430074, China * Phone: +86-27-6788-3452. Fax: +86-27-6788-3456. E-mail: [email protected] or [email protected]

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ABSTRACT: A high-efficient liquid spray dielectric barrier discharge (LSDBD) plasma induced vapor generation technique is developed for the simultaneous determination of selenium, silver, antimony, lead, and bismuth in liquid microsamples (20 μL) by inductively coupled plasma mass spectrometry (ICP-MS). It is demonstrated that the dissolved Se, Ag, Sb, Pb, and Bi ions in solution samples are readily simultaneously converted to volatile species efficiently by LSDBD plasma induced chemical processes under similar conditions. It eliminates the use of unstable and expensive reducing reagents, and only formic acid is required in the proposed LSDBD chemical vapor generation technique. It is also worth to note that this is the first report of using plasma induced chemical processes for the vapor generation of Ag and Bi. The simultaneous sensitive determination of Se, Ag, Sb, Pb, and Bi is realized with sample volume of only 20 μL and the sample throughput could be as high as 180 samples h-1. The limits of detection (LODs) for simultaneous determination of Se, Ag, Sb, Pb, and Bi, are 10 ng L−1 (200 fg), 2 ng L−1 (40 fg), 5 ng L−1 (100 fg), 4 ng L−1 (80 fg), and 3 ng L−1 (60 fg), respectively. The precision of Se, Ag, Sb, Pb, and Bi in the present method are evaluated to be better than 4 %. The utility of the proposed technique is demonstrated by the analysis of ultratrace Se, Ag, Sb, Pb, and Bi in archaea cell and single conodont samples.


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Recently, the analysis of ultratrace elements in microsamples through inductively coupled plasma (ICP) spectrometry has gained great interest.1-6 Increasing the sample volume by dilution can solve this problem to a certain extent, but the analytical signal is directly correlated with the mass of analyte entering the plasma per unit of time, thus when the analyte content is too low the accuracy and precision of the results will be significantly deteriorated.1,2,7 Therefore, it is highly attractive to develop sampling techniques that reduces the consumption of samples while maintaining high sensitivity. Despite solution nebulization being the most popular approach for sample introduction, numerous studies have addressed its limitations, such as low sensitivity, high sample consumption, and serious matrix interferences.8-10 Expectedly, as an excellent sample introduction technique, chemical vapor generation (CVG) techniques provide a potential solution to this conundrum since it significantly alleviates matrix interferences and enhances the sensitivity.11-13 Unfortunately, since its introduction, few studies have been performed to apply CVG to the analysis of microsamples.7 In 1998, Wang et al.14 achieved the hydride generation of As(III), Se(IV) and Sb(III) at microliter sample volume, though the poor sensitivity and reproducibility may account for its limited application. In 2016, Wen et al.15 reported a non-aqueous phase hydride generation technique for the determination of As(III), Hg(II) and Sb(III) in microsamples. In later developments, they used a similar system for trace bismuth determination.16 However, these methods suffer significant memory effect and the reaction vessel should be cleaned after each analysis. In addition, because of the different chemical reaction conditions required for the effective formation of different hydrides, the applicable elements in conventional CVG for simultaneous multi-elemental determination is rather limited.17 For example, in hydride generation (HG), Se(VI), As(V) and Sb(V) need pre-reduction, while Pb(II) often requires pre-oxidation.18,19 Moreover, several additional drawbacks remain in conventional CVG: (a) the use of unstable reducing reagents; (b) interferences from transition and noble metals.20-22 Plasma induced vapor generation (Plasma-CVG),23 which is an emerging green sampling technique with high sensitivity and good selectivity, provides an attractive



alternatives to conventional CVG for analyzing limited amounts of samples. In 2011, we reported a dielectric barrier discharge (DBD)-plasma induced vaporization technique for the analysis of Hg2+, MeHg+, and EtHg+ with the sample consumption of only 6 μL.24 Afterwards, our group further extended this DBD system for thiomersal determination.25 In 2014, Yang et al. proposed a low-temperature hydrogen plasma assisted vapor generation method for measuring As, Te, Se, and Sb, and the most attractive characteristic is microsampling.26 Soon after, Zheng et al. reported a single drop solution electrode glow discharge (SD-SEGD) induced vapor generation technique for the determination of Cd, Zn,7 and Hg27 in microsamples (5–20 µL). All these reported plasma-CVG indicated that they are suited to be used for the analysis of microsamples, however, these methods retain some shortcomings including limited coverage of elements, low sensitivity of some elements, interferences from coexisting ions, or the need for hydrogen as a reaction gas. Recently, we reported a liquid spray dielectric barrier discharge induced plasma (LSDBD) vapor generation technique, which offers the advantage of high sensitivity and high tolerance to coexisting ions. It has been used for determination of trace Pb and Cd in geological and biological samples.28,29 In principle, it is suitable for simultaneous determination of multiple elements, and may be applied to expand the application of this technique in microsample analysis. In this work, a LSDBD-CVG technique is developed for simultaneous sensitive determination of Se, Ag, Sb, Pb, and Bi in microsamples using only formic acid. To the best of our knowledge, this is the first report on the generation of volatile species of Ag and Bi using plasma-CVG. In addition, the generation efficiencies for Se and Sb were also greatly improved over the reported plasma-CVG. After investigating the influences of solution matrix and operating parameters, the simultaneous sensitive determination of Se, Ag, Sb, Pb, and Bi was realized with sample volume of only 20 μL and the sample throughput could be as high as 180 samples h-1. This green analytical method is applied to the simultaneous determination of ultratrace Se, Ag, Sb, Pb, and Bi in micro amounts of samples, such as single conodont, thousands of archaea cells.


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EXPERIMENTAL SECTION Instrumentation. The schematic diagram of the modified LSDBD-CVG-ICPMS experimental setup is shown in Fig. 1. In this new LSDBD generator, a modified homemade glass device (30 mm o.d.×70 mm length) was designed, which is similar to double-pass spray chamber and can significantly reduce the number of large droplets entering into the ICP. In addition, a concentric micronebulizer (Glass Expansion, USA) is employed here to further reduce sample consumption rate while keeping the high sample introduction efficiency. For the LSDBD, a piece of copper wire surrounding the outside of the nozzle of the micronebulizer serves as one electrode, while the other electrode is inserted into the inner tube of a glass tube. The sample solution was continuously introduced into the micronebulizer through a sample introduction tube (i.d 0.9 mm, length 20cm) by an ICPMS self-contained peristaltic pump at a flow rate of 0.3 mL min−1. Argon is introduced into the micronebulizer at a rate of 0.75 L min−1 for solution nebulization and microplasma generation. The distance between the nozzle and the tip of the glass dielectric barrier was about 3 mm. The DBD plasma was ignited and sustained by a home-made AC power supply. Fig. S1 shows the output voltage waveform of the power supply obtained when it is connected to the DBD device. The peak−peak output voltage of this power supply is about 6800 V with a frequency of ca. 23 kHz, when the input voltage is set at 90 V. Although the new LSDBD reactor can effectively reduce the number of large droplets entering into the ICP, large droplets would also form on the pipeline between the ICP torch and the generator as the operation time increases. Therefore, considering the potential risk, a PC3 cyclone spray chamber (ESI, USA; volumes: ~20 mL) was added between the generator and ICP torch as a gas liquid separator (GLS) to further reduce the large droplets. After that, the volatile species produced from LSDBD generator were swept by an argon stream through a pipeline (i.d 4.0mm, length 15cm) , and directed to ICPMS for detection. A PerkinElmer Elan DRC-e ICPMS (PerkinElmer, USA) was used in our experiments. The detailed operating conditions are listed in Table S1. Reagents and Sample Pretreatment. The details of the reagents and sample pretreatment are shown in the Supporting Information.



RESULTS AND DISCUSSION Generation of Volatile Species of Se, Ag, Sb, Pb, and Bi by Using LSDBD-CVG. The research work of our group28,29 and Yu et al.30,31 demonstrated that improving the interaction between the solution and plasma would greatly enhance the plasma induced vapor generation efficiency and plasma excitation capability. Therefore, a micronebulizer was used in the present study to further improve the nebulization efficiency, and more importantly, to reduce the sample consumption. In our preliminary experiments, 2 μg L−1 Pb standard solutions prepared in 7 % formic acid was introduced into the system to verify the vapor generation efficiency of the modified reactor at a flow rate of 0.3 mL min−1. It was found that at the same sample flow rate, the Pb signal obtained from LSDBD-CVG increased by a factor of 14 compared to conventional pneumatic nebulization (PN) ICPMS (Fig. 2a). Compared with our previous report28, the Pb sensitivity enhancement factor in this modified LSDBD reactor increased by nearly 20 %. This result confirmed that the change of nebulizer and reactor design is effective in improving the sensitivity of Pb and reducing the sample consumption. As an emerging green sampling technology, the capability of DBD-CVG for the sensitive determination of wide range of elements is well appreciated. However, since its introduction, limited elements can be detected with satisfied sensitivity, including Hg,24,25,32 Cd,29,33,34 Zn,35 and Pb28. Although vapor generation of As, Se, Te, and Sb by DBD-CVG has also been reported, the poor sensitivity hinders its application.26,36 Subsequently, in an effort to test if the vapor generation efficiency of As, Se, Te, and Sb can be improved in this LSDBD device, 10 μg L−1 As, Se, Te, and Sb single element solutions were detected by LSDBD−CVG−ICPMS separately. It was found that significant Se and Sb signal enhancement (greater than 16 times, Fig. 2a) were readily obtained in LSDBD−CVG compared with conventional PN-sample introduction. This result implied that the vapor generation of Se and Sb were readily achieved with LSDBD-reactor. However, no significant signal improvement were observed for As and Te, which is likely a results of different redox potentials to initiate the vapor generation. The gaseous product from the LSDBD-CVG was also directed to an atomic fluorescence spectrometer (AFS-9130, Beijing Titan Instruments Co. Ltd., Beijing,


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China) for detection, and it was found that the sensitivity of Se and Sb was improved at least 10 times compared with that of Xing’s report.26 The details on operating conditions of LSDBD-CVG-AFS are described in Supporting Information (Table S2). This result demonstrated that the proposed LSDBD-CVG is advantageous for vapor generation of Se and Sb probably because of the increased interaction of solution and plasma. In recent years, poisoning caused by Ag37 and Bi38 have also been reported, so achieving high-sensitive detection of Ag and Bi is also of great significance. However, to the best of our knowledge, analyses of Ag and Bi with plasma-CVG have not been reported. In follow-up experiments, the vapor generation capability of Ag (10 μg L−1) and Bi (2 μg L−1) from the LSDBD reactor is also evaluated. It was observed that significant signal enhancement of Ag (about 20 times) and Bi (about 25 times) was readily obtained in the presence of the LSDBD plasma (Fig. 2a). In our previous studies, it has been confirmed that plasma does not significantly change the nebulization efficiency of the solution28, so we believe that the vapor generation of Ag and Bi indeed be readily achieved with LSDBD−CVG. It is well known that Bi is classical hydrideforming element and Bi ions could be transformed to BiH3 in acid-K/NaBH4 system.39 In contrast, in HNO3-NaBH4 Ag vapor generation, the volatile species was demonstrated to be Ag nanoparticles.40 As no other volatile species of Ag and Bi have been reported in conventional CVG, and only the presence of ·H or other simple reducing radicals in our plasma chemical process, the volatile species of Ag and Bi in this LSDBD-CVG technique was thus presumed to be Ag nanoparticles and BiH3, respectively. Further investigation of the mechanism is necessary for understanding of the process, but it is beyond the scope of the current study. Compared with the conventional CVG, the proposed method eliminates the use of unstable and expensive KBH4 and the high efficient generation of volatile species of Ag and Bi is achieved only by the use of DBD plasma in the presence of small amount of formic acid. In addition, different from the conventional CVG, the proposed LSDBD−CVG does not produce a large amount of hydrogen, which is also beneficial to maintain the stability of ICP and more compatible with ICPMS. All these results demonstrated that our



proposed LSDBD provides a green and highly efficient vapor generation method for Pb, Se, Sb, Ag and Bi. For volume-limited samples, it is highly desirable to develop sensitive methods which could detect multi-elements simultaneously. However, it is important to recognize that simultaneous detection of different hydride-forming elements is not an easy task. For example, in conventional CVG, As(V), Se(VI) and Sb(V) need prereduction by thiourea, while Pb(II) often requires pre-oxidation by potassium hexacyanoferrate(III). Apart from their complexity, these protocols also present drawbacks such as the increased handling time, the production of toxic waste, and the risk of contamination. Additionally, CVG for Bi was the best at low acidity (1 mol L-1 HCl), whereas the maximum Se response is typically acquired using high HCl concentration (6 mol L-1).41 For the above reasons, conventional CVG can only achieve simultaneous analysis of As + Bi + Sb, and As + Sb + Se,41 or Pb + Cd +Hg,42 etc, but cannot simultaneously analyze Se + Sb + Ag + Bi + Pb. In our preliminary experiments, we have found that Se, Ag, Sb, Pb, and Bi all have good signal intensities under 7 % formic acid, which indicates that this method is suited to the simultaneous analysis of these five elements. To test this, multi-element working standard solution of Se(VI), Ag, Sb(V), Pb, and Bi (10 μg L−1) were prepared in 7 % formic acid and measured by LSDBD-CVG-ICPMS. As we expected, in our LSDBD-CVG technique, these five elements all show good sensitivities when measured simultaneously (Fig. 2b). Moreover, it should be noted that there is no significant change in signal intensity when the five elements are measured simultaneously compared to separate measurements. Furthermore, in an effort to test if the vapor generation efficiency is affected by the valance state of Sb and Se in this LSDBD device, 10 μg L−1 Se(VI), Se(IV), Sb(V), and Sb(III) single element solutions were introduced to the LSDBD−CVG reactor separately. Compared with conventional PN-sample introduction, significant signal enhancement factors (16, 22, 14, and 24) of Se(VI), Se(IV), Sb(V), and Sb(III) were readily obtained in LSDBD−CVG. This result indicated that the valence state of Sb and Se has effect on the vapor generation efficiency. However, it should be noted that the sensitivity of Sb(V) is only about 35 % of Sb(III), and Se(VI) is not reduced at all to


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the hydride by NaBH4 in conventional HG.43 In analytical testing, strong oxidizing acids such as nitric acid and perchloric acid are often used for the digestion of geological and environmental samples. Considering that nitric acid/perchloric acid can easily oxidize Se(IV) and Sb(III) to Se(VI) and Sb(V) during digestion, we can consider that both Se and Sb in the sample solution after digestion are in its high valence state. Although the vapor generation efficiency of Se(VI) and Sb(V) is lower than that of Se(IV) and Sb(III), it still have much higher efficiency (> 14 times) than conventional nebulization sampling. In addition, the LSDBD-CVG method can also efficiently convert Pb (II) without pre-oxidization step. These results imply that our proposed method can be used for highly sensitive simultaneous detection of Se, Ag, Sb, Pb, and Bi in geological and environmental samples without the pre-reduction of Se and Sb and pre-oxidation of Pb. As the sample does not need to be pre-reduction or pre-oxidation, this method can reduce the reagent consumption and save the test time. Therefore, the proposed LSDBD technique provides a promising method for the low-cost and highsensitive simultaneous determination of Se, Ag, Sb, Pb, and Bi, which is impossible with other CVG methods. Based on the above discussion, and considering that both Se and Sb in the standard solution are in high valence state and the Pb is in the form of Pb(II), the solutions containing these five elements used in subsequent experiments all prepared directly from their standard stock solution for simplification. Optimization of Experimental Conditions. In order to find the optimal conditions for simultaneous determination of Se, Sb, Ag, Bi and Pb by the proposed LSDBD-CVG-ICPMS technique, several parameters such as formic acid concentration, input voltage, sample flow rate and argon flow rate were evaluated by a univariate approach. All the parameters were optimized with 2 μg L−1 mixed standard solutions, and a peristaltic pump was employed for continuous sample introduction. Various enhancement reagents have been employed to improve the performance of plasma-CVG, such as Triton X-114 for Hg and Cd,34 formic acid for Hg44 and Pb28, and methanol for Cd29. After preliminary study, we found that the presence of formic acid can significantly improve the sensitivity of these metal elements. However, only slight enhancement was observed with the use of acetic acid, methanol and ethanol.



And therefore, formic acid was employed and the effect of formic acid concentration on the signal of Se, Ag, Sb, Pb and Bi was investigated firstly. In this case, the mixed standard solutions of these five elements were prepared with different volume percentages (0 %, 1.0 %, 3.0 %, 5.0 %, 7.0 %, 9.0 %) of formic acid. As shown in Fig. 3, significant enhancement of Se, Ag, Sb, Pb and Bi signal intensity was readily achieved with the addition of a small amount of formic acid. This enhancement is probably the consequence of the increasing formation of reducing radicals (e.g., H radicals) in the presence of formic acid in the LSDBD-plasma, which is highly effective in promoting the vapor generation of Se, Ag, Sb, Pb, and Bi. In addition, the formic acid might alter the surface tension of the sample solution and consequently favors the release of the generated volatile species, thereby contributing to the intensity enhancement. However, it was found that the trend and optimum formic acid concentration are different for these five elements. For example, a formic acid concentration of 3-9 % yielded a response plateau for Bi; and the observed Ag, Pb and Sb signal intensities achieved maximum value at a formic acid concentration of 7 %; the optimum signal value of selenium was obtained when the concentration of formic acid was 5 %. And finally, in order to make sure that each element has good signal strength, 7 % formic acid is selected for further investigations. Other operational parameters of LSDBD-CVG-ICPMS including input voltage, sample flow rate and Ar flow rate were also investigated, and the results are summarized in Fig. 4. The effects of these parameters are discussed in the Supporting Information. Interference from Coexisting Ions. Transition metals and mutual vapor-forming elements interference are well known phenomena in conventional HG.45-47 For example, in the presence of 5 mg L-1 Cu (only 50 times that of Ag), the signal of Ag would decrease to 63 %.48 Wen et al. found that the vapor generation of Bi is significantly inhibited by Hg and Ag (also vapor-forming elements), and it could only tolerate 5 and 50 times higher concentrations than Bi, respectively.16 Despite our previous study demonstrated that the proposed LSDBD-CVG technique offered the distinguishing feature of being highly tolerant to coexisting ions in the case of Pb28 and Cd29, we still


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need to verify the anti-interference capability of our modified LSDBD-CVG for these four newly reported elements. To evaluate the susceptibility of the proposed LSDBDCVG to matrix-induced errors, the effect of 8 coexisting ions on the recoveries of Se, Ag, Sb, and Bi signals was studied (Fig. 5). The values were given as a percentage of relative response obtained from a 2 μg L−1 mixed standard solutions (Se, Ag, Sb, and Bi) in the presence of interfering elements to the absence of these coexisting elements. The results showed that 10 mg L−1 of coexisting ions did not interfere with the simultaneous determination of Se, Ag, Sb, and Bi under the optimized chemical conditions within the error range of ± 10 %. Mutual vapor-forming element interference is a well-known phenomenon in vapor generation coupled techniques and also being a great challenge for multi-elemental determination in CVG. In this work, the mutual interferences in the determination of Se, Ag, Sb, and Bi were also investigated at an interference ion concentration of 1 mg L-1 (analyte concentration, 2 μg L−1). No significant effect was observed. All these results demonstrate that the proposed LSDBD-CVG technique offers excellent anti-interference capability for coexisting ions in the sample, and it indicates that the proposed method can be used for the analysis of Se, Ag, Sb, and Bi in complex samples. Analysis of microsamples. Considering the high sensitivity and fast chemical reaction kinetics of our method, it is attractive to apply the developed LSDBD-CVG to microsamples by transient analysis in order to reduce sample consumption and boost sample throughput. In an effort to achieve the detection of microsamples, we first transferred fixed volume (20 μL) of samples containing 10 μg L-1 of Sb, Pb, and Bi to a clean glass slide using a pipette. Subsequently, the sample pipe was switched manually between the sample droplets and the blank solution. This not only avoids the use of expensive flow syringe systems, but also makes it easier to select different sampling volume as needed without being limited by the size of the injection loop. Because air introduced during switching has a negative impact on the plasma chemical reaction in the experiment, a quick manual operation is required to switch the sample pipe between the sample and the blank solution. Although the discharge fluctuated slightly during switching, it appeared to be stabilized before the solution reached it



about 0.5 s later. As can be seen in Fig. 6, the resulting transients (10 μg L-1 Sb, Pb, and Bi) are reproducible, the RSDs of 20 μL Sb, Pb, and Bi ranged from 2.4~4.8 % by peak height (n=5). In addition, it is worth noting that the peak height intensities were all about 120 % of the values obtained with continuous sample introduction. In other words, the signals obtained by this injection mode are higher than continuous sampling mode. The possible reason is that the sampling pipe inevitably enters a small amount of air when the pipe switched from the blank to the sample solution, and the instantaneous nebulization efficiency of the sample will be increased since there is no solution in the front of the sample. This speculation is supported by the observing similar signal enhancement with the introduction of Li, Cs, and In. Application of the lower volumes of the solutions, i.e., 5 and 10 μL, was not available under such manual sampling conditions. The main reason is that the lower sample volume will increase the relative influence of air on the signal and reduce the stability of the signal. It can be expected that the sample consumption of the LSDBD−CVG could be further reduced by using the micro flow injection sampling device. As shown in Fig. 6, one sample could be analyzed within 20 s in the proposed manual injection mode, which equals to a 0.05 Hz sampling rate, or 180 samples h-1. It should be noted that a peak tailing could be clearly observed from Fig. 6, which should resulted from the dead volume of the GLS. It can be expected that the analysis time could be improved further by reducing the dead volume of the total LSDBD-CVG device. This indicates that the proposed LSDBDCVG technique can easily be used for high throughput determination of trace elements in small amount of samples. Analytical Characteristics. Under the optimal experimental conditions, the analytical response curves were generated using peak height as the analytical parameter. Solutions concentration ranging from 0.1 to 20.0 μg L-1 and the blank solutions were used to determine the LODs and relative standard deviations (Fig. S3). It was found that the calibration curves of Se, Ag, Sb, Pb and Bi were all linear over the studied range of 0.1–20 μg L-1 with a correlation coefficient better than 0.9988. The relative standard deviations (RSDs, n=5) of 2 μg L-1 Se, Ag, Sb, Pb, and Bi in this method, are 3.7 %, 2.9 %, 1.9 %, 3.5 %, and 3.4 %, respectively (Table S3). The limits of detection


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(LOD), using the definition 3 s m-1 (s is the standard deviation corresponding to 11 blank measurements and m is the slope of the calibration graph), were calculated to be 10 ng L−1 (200 fg), 2 ng L−1 (40 fg), 5 ng L−1 (100 fg), 4 ng L−1 (80 fg), and 3 ng L−1 (60 fg) for Se, Ag, Sb, Pb, and Bi, respectively. These are all comparable to or as much as 1 order of magnitude better than that of conventional HG-ICPMS: 10 ng L−1 for Se, 12 ng L−1 for Ag,49 10 ng L−1 for Sb50, 8 ng L−1 for Pb46 and 15 ng L−1 for Bi50. It is also worth to note that the detection limits in absolute (mass) terms are extremely low, ranging from 40 fg (Ag) to 200 fg (Se), which is beneficial for the analysis of ultratrace elements in volume-limited samples. These results demonstrate that our proposed method provides a promising highly sensitive, high throughput approach for the simultaneous determination of Se, Ag, Sb, Pb and Bi in volume-limited samples. The overall vapor generation efficiency of these 5 elements was estimated by determining the amount of analyte retained in the waste effluent after the generation process using LSDBD−CVG. For example, in the case of Ag, 10 μg L-1 feed solution of Ag+ was used and the waste solution was collected into a precleaned container. And then the vapor generation of Ag was estimated from the feeding amount of Ag and the amount of Ag in the waste solution obtained with PN-ICP-MS. It was found that the total sample introduction efficiency was calculated to be 41±5 %, 67±6 %, 46±4 %, 47±5 %, and 87±4 % for Se(VI), Ag, Sb(V), Pb, and Bi, respectively. These results further demonstrated that our proposed LSDBD-CVG provides a highly efficient vapor generation method for Se, Ag, Sb, Pb and Bi. In addition, it is worth pointing out that the vapor generation efficiency for Ag is even higher than that of Se, Sb and Pb. In contrast, the conversion efficiency of Ag is usually much lower than those hydrideforming elements (such as As, Sb, and Se) in conventional HG (around 10 to 20 %). These results indicated that the vapor generation mechanism of the proposed LSDBD-CVG is different from that of conventional HG. In addition, the plasma chemical process between the fine droplets with plasma in LSDBD also favors the release of the generated volatiles species, which might also contributes the high vapor generation efficiency especially the case of Ag. All these results further demonstrated



that the proposed LSDBD-CVG provides a highly efficient vapor generation method for the simultaneous determination of Se, Ag, Sb, Pb and Bi. Method validation and its application to real samples. To validate the accuracy of the present method, the proposed technique was applied to the determination of Sb in simulate water certified reference material (GSB 07-1376-2001), which was diluted 30-fold before analysis. The standard curve method was employed and the measured value of 30.8 ± 1.9 μg L−1 was in good agreement with the certified value of 29.8 ± 1.5 μg L−1. Due to the lack of multi-elemental standard reference materials containing elements of Se, Ag, Sb, Pb and Bi, we added a mixed standard solution to a tap water sample and verified the accuracy of the method by testing its spike recovery. It can be seen that Se, Ag, Sb, Pb, and Bi were not detected in tap water, and the obtained recoveries were in the range of 92.5 - 109.5 % at spiking level of 2 µg L-1 (Table S4). These results validated the accuracy of the proposed method. To further demonstrate the usability of the proposed method, it was also successfully applied to the analysis of microfossil (conodont) and biological samples (archaea cell), with analytical results summarized in supporting information. CONCLUSIONS In this study, a novel LSDBD plasma induced chemical vapor generation technique is developed for simultaneous sensitive determination of Se, Ag, Sb, Pb, and Bi in microsamples. It is demonstrated that Se, Ag, Sb, Pb, and Bi can be readily simultaneous converted to volatile species in LSDBD only using formic acid. Compared to conventional pneumatic nebulization ICP-MS, a minimum of 14-fold sensitivity enhancement is readily achieved with the LSDBD-CVG. To the best of our knowledge, this is the first report of vapor generation of Ag and Bi by plasma chemical process. In addition, the proposed LSDBD-CVG demonstrates its usability without the pre-reduction step (for Sb and Se) or pre-oxidation step (for Pb), which favors simultaneous multi-elemental determination and reduces the analysis time. Moreover, only 20 μL sampling volume is required in the developed approach, and one analysis could be completed within 20 s (180 samples h-1). Furthermore, the proposed LSDBD−CVG also offers excellent anti-interference capability to coexisting ions,


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enabling direct analysis of environmental samples by using external calibration. In summary, the proposed modified LSDBD technique is shown to be particularly well suited to simultaneous multi-elemenal analysis in microsamples by virtue of its high sensitivity, rapid reaction kinetics, similar generation condition, low sample consumption and high throughput.

ACKNOWLEDGEMENT We acknowledge the financial support from the National Nature Science Foundation of China (No. 41673014, and 41521001), National key research and development program (2017YFD0801202) and Nature Science Foundation of Hubei Province (2016CFA038).

ASSOCIATED CONTENT Supporting Information. Reagents and sample pretreatment, optimization of experimental conditions, AFS operating parameters, typical waveforms of the discharge output voltage, calibration curve and temporal profiles of Se, Ag, Sb, Pb and Bi in LSDBD-CVG-ICPMS, analytical characteristics of elements determination with LSDBD-CVG-ICPMS and HG-ICPMS, and analysis of tap water samples, microfossil and biological samples.



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Figure 1. The schematic diagram of LSDBD-CVG-ICPMS for the simultaneous multielemental analysis in microsamples.


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Figure 2. (a) Comparison of the Se, Ag, Sb, Pb, and Bi sensitivity for three sample introduction modes. (b) Typical temporal profile of signal intensity of 10 μg L−1 Se(VI), Ag, Sb(V), Pb, and Bi by continuous-flow LSDBD−CVG−ICPMS.



Figure 3. Dependences of the signal intensities of 2 μg L−1 Se, Ag, Sb, Pb, and Bi on formic acid concentration.


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Figure 4. Effect of input voltage (a), sample flow rate (b), and Ar flow rate (c) on the signal intensities of 2 μg L-1 Se, Ag, Sb, Pb, and Bi.



Figure 5. Influence of the foreign ions (K, Ni, Mg, Co, Zn, Cu, Ca, and Mn all at a concentration of 10 mg L−1; Se, Ag, Sb and Bi all at a concentration of 1 mg L−1) on the recovery of Se, Ag, Sb and Bi (2 μg L-1).


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Figure 6. Temporal profiles of 10 μg L-1 Sb, Pb and Bi solution. Sample flow rate, 0.3 mL min-1; sample volume, 20 μL.



For TOC Only


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Simultaneous Sensitive Determination of Selenium, Silver, Antimony

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