Enhanced Photochemical Vapor Generation for determination of

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Enhanced Photochemical Vapor Generation for determination of Bismuth by Inductively Coupled Plasma Mass Spectrometry Ying Yu, Yutao Jia, Zeming Shi, Youliang Chen, Shijun Ni, Ruilin Wang, Yurong Tang, and Ying Gao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03681 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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Analytical Chemistry

Enhanced Photochemical Vapor Generation for determination of Bismuth by Inductively Coupled Plasma Mass Spectrometry Ying Yu, †, ‡ Yutao Jia, †, ‡ Zeming Shi, ‡ Youliang Chen, ‡ Shijun Ni, ‡ Ruilin Wang§, Yurong Tang§, and Ying Gao†, ‡, * †

State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University

of Technology, Sichuan 610059, China ‡

College of Earth Sciences, Chengdu University of Technology, Sichuan 610059, China

§ College

of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology,

Sichuan 610059, China

Corresponding author email: [email protected]

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ACS Paragon Plus Environment

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

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ABSTRACT An enhanced photochemical vapor generation (PVG) sample introduction procedure is developed for the determination of trace Bi with inductively coupled plasma mass spectrometry (ICP MS) by the addition of iron. Gas chromatography mass spectrometry (GC-MS) reveals that (CH3)3Bi is the major component of volatile Bi species formed in the presence of 20% (v/v) acetic acid, 5% (v/v) formic acid and 60 μg mL-1 Fe3+ under UV irradiation. The addition of Fe3+ not only largely increases the PVG efficiency of Bi3+ but also accelerates the reaction kinetics of photochemical reduction of Bi3+. The analytical sensitivity was enhanced 30-fold using PVG for sample introduction compared to that for direct solution nebulization detection by ICP MS detection. Furthermore, the proposed method shows much better tolerance of interferes from Cu2+ and Ni2+ than that from conventional hydride generation (HG). Under the optimized conditions, a detection limit of 0.3 ng L-1 was obtained for Bi by ICP MS determination. The relative standard deviations (RSD) was 2.5% for seven replicate measurements of 0.5 ng mL-1 Bi3+ standard solution. The proposed method has been successfully applied for the determination of Bi in environmental samples including water samples and certified reference material of soil (GSS-1) and sediments (GSD-5a and GSD-10) with satisfying results.

Keywords: ferric ion, photochemical vapor generation, Bi, inductively coupled plasma mass spectrometry

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INTRODUCTION High sample introduction efficiency is a perpetual pursuit of analytical chemists of atomic spectrometry.1-4 To realize this old dream, photochemical vapor generation (PVG) was developed by Sturgeon et al. for sample introduction and gained success in the past few years.5-9 For instance, selenium was the first element to be investigated in PVG system for analytical purpose.5 Recently, PVG systems have been established for the determination of transition ions (Fe, Co, Ni and Cu),10-13 hydrideforming elements (As, Te, Sb, Pb, Se, Cd, Hg and Sn),14-21 and non-metal elements (Br, I, Cl and S).22-25 Among them, selenium, mercury and their species were most frequently determined. PVG provides simpler reactions and greener chemistry compared to conventional hydride generation (HG). However, for bismuth (Bi), an important trace element in natural world,26, efficiency is rather low,28,

29

27

the PVG sample introduction

resulting in problems to achieve sensitive detection, especially for the

analysis of natural waters at ng L-1 level.30 The limit of detection of Bi obtained by PVG-ICP MS using PVG reactor was found to be nearly 20 folds higher than As, Sb and Te in a recent publication.29 Interestingly, it was found that transition metal ions can significantly enhance PVG generation efficiency of analytes.19 In the presence of nickel ions, a significant improvement of PVG efficiency was found for Pb with a detection limit of 0.005 ng g-1. This transitional metal ion enhanced PVG has intrigued a recent research focus to improve PVG efficiency for many elements. The signal response has been enhanced from 1.6 to 4130 folds for the determination of Hg, As, Pb, Cl, Se and Te in the presence of transitional metal ions (Table S1).19, 21, 22, 31-33 Metal ion enhanced PVG has been successfully applied for the determination of analytes in environmental samples with satisfying results. However, the field is not sufficiently developed and still at the beginning stage. More researches are required to interpret such observation and study the mechanism. In this work, a largely enhanced PVG by a ferric sensitizer was realized for Bi for the first time. 3



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The presence of formic acid, acetic acid together with Fe3+ was found to be essential for highly sensitive determination of Bi. Furthermore, the addition of Fe3+ efficiently accelerates the reaction kinetics of photochemical reduction for Bi. The possible mechanism of the PVG reaction was discussed. The proposed method shows better performance in terms of anti-interference capability against Cu2+ and Ni2+ compared to that from conventional HG. Certified references materials of soil (GSS-1) and sediments (GSD-5a and GSD-10) were analyzed to validate the accuracy of the proposed method. EXPERIMENTAL SECTION Instrumentation An ELAN DRC™-e ICP-MS (Perkin Elmer, Inc., Shelton, CT, USA) was employed for Bi determination. The commercial sample introduction set-up was replaced by a PVG-system in this work. Sample solutions were introduced into the PVG system by a model IFIS-D injection pump system (Xi’an Remex Analysis Instrument Co. Ltd., Xi’an, China). The PVG photo-reactor was interfaced to the ICP MS as reported before (Figure 1). 31 The PVG reactor, providing 185 nm and 254 nm irradiation, was a 19 W thin film flow-through lamp (Beijing Titan Instruments Co., Beijing, China), which was loosely covered by aluminum foil to protect the operator from UV irradiation. A tandem set of two gasliquid separators for the separation of volatile Bi from sample solution (GLSs, ~2 mL internal volume) were immersed in an ice bath to minimize any transport of liquid droplets derived from condensation of water vapor or co-existing volatile organic compounds to the ICP torch. The generated volatile Bi species were transported from the outlet of the last GLS to ICP-MS through a Teflon lined Tygon tubing by argon carrier gas. The system parameters of ICP-MS were selected as recommended by the manufacture. Typical operating conditions are summarized in Table 1. Optimum conditions for PVG system were investigated independently.

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Figure 1. Schematic diagram of PVG-ICP MS system for the determination of Bi3+. An Agilent 7890A gas chromatograph (GC) equipped with an Agilent 5975C mass selective detector (MS) was used for the identification of the volatile Bi species after separation by a 30 m × 0.25 mm o.d. × 0.25 µm i.d. capillary GC column.34 The GC-MS was operated in the split mode and a gastight Hamiliton syringe (5 mL) was used for collecting volatile productions from headspace of the first GLS. The operation parameters were set as following: injector temperature at 150 oC, oven temperature program, 35 oC, hold for 10 min, heated to 150 oC at 30 oC min-1; transfer line temperature at 150 oC. 34 The carrier gas of He was at 1.2 mL min-1.34 Reagents and solutions Deionized water (DIW) was used throughout. All reagents were of analytical reagent grade or better. ACS grade formic acid (FA) and acetic acid (AA) were obtained from Aladdin Industrial Corporation (Shanghai, China). Stock solution of Bi3+ was purchased from the National Research Center for Standard Materials (NRCSM) of China. A 2 M BrCl solution was prepared in a fume hood by dissolution of 27 g of reagent grade KBr (Aladdin Industrial Corporation, Shanghai, China) in 2.5 L of HCl in a glass container followed by slow addition of 38 g reagent grade KBrO3 (Aladdin Industrial Corporation, Shanghai, China). A rinse solution containing 0.04 M BrCl was prepared by dilution of the 2 M BrCl with DIW. Tap water, river water, and lake water were sampled from Chengdu, and stored in 1% (v/v) acetic acid. 5



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Table 1. Experimental parameters of ICP MS. Parameters

Values

RF power /W

1175W

Nebulizer (Carrier) gas flow

0.95 L min-1

Cool gas flow

15 L min-1

Auxiliary gas flow

1.2 L min-1

Resolution

0.7 amu

Dead time

50 ns

Dwell time

30 ms

Scanning mode

Peak hopping

Sampling pump rate

3.8 mL min-1

Isotope monitored

209Bi

Sweeps per reading

1

Readings per replicate

1

Number of replicates

1000

Analysis procedure The sample digestion of certified references materials of sediments was according to previous reports. 35 The digests were subsequently evaporated to near dryness at 200 ℃ to eliminate excess acids. Then the digests were leached back and filled up to 10.0 mL with DIW. After dilution, AA, FA as well as Fe3+ were added into digest solutions prior to analysis. Three sample blanks were processed along with samples. For the determination of Bi, the procedure was carried out as followed. Briefly, the samples were introduced to the PVG reactor with a sample flow of 3.8 mL min-1 for 20 s. Then the irradiated sample was delivered to the GLSs for analyzing by ICP MS at the same sample flow rate

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without stopping pump. Since the generated volatile Bi species may be absorbed on the inner wall of the Teflon tubing connector between the PVG reactor and the first GLS due to its hydrophobic nature, a 0.04 M BrCl solution for 10 s followed by 5% (m/v) oxalic acid (70 s) and DIW (20s) was used to clean the PVG system for next analysis. For samples analysis, the peak area of the

209Bi

isotope was used to

construct a standard additions calibration curve. Safety Considerations. Some of the generated volatile species are toxic.

Proper ventilation and

personal protective equipment should be employed for all manipulations. RESULTS AND DISCUSSION Ferric Ions-Assisted PVG Recently, metal ions assisted PVG was reported for the determination of Se, Pb, As, Cl and Te (Table 1).22, 31-33 The addition of ferric ions provides approximately 10-fold enhancement of generation efficiency for As.33 Since Bi and As belong to the same group of the Periodic Table of Elements, ferric ions-assisted PVG may occur for Bi. Therefore, Fe3+ was added to PVG solution for preliminary experiments. PVG efficiencies of elements are highly depended on the type and concentration of low molecular organic compounds used. AA was reported to be the most efficient medium for Bi in PVG system.29,

36

Therefore, the effect of concentration of AA from 1% (v/v) to 30% (v/v) on the

photochemical reduction of 2 ng mL-1 Bi3+ was investigated in the presence of 60 μg mL-1 Fe3+. Interestingly, the signal response from AA in the investigated range was almost ignored when the sample solution was continuously introduced to PVG reactor without stopping (corresponding to 13 s irradiation time) at a flow rate of 3.8 mL min-1. The influence from FA at the same concentration range on the detection of 2 ng mL-1 Bi was also evaluated with 13 s irradiation time in this work. As shown in Figure 2a, the maximum signal response was obtained at 5% (v/v) FA. Nerveless, the signal intensity at

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the optimized FA concentration was rather low. It was reported that the use of mixed low monocular weight organic acids (LMWOAs) can efficiently enhance the PVG efficiencies of As and Te. The effect of the concentration of AA and FA on the Bi measurement was studied. By fixing the concentration of FA at 5% (v/v), the signal response of Bi sharply enhanced when AA concentration increased from 0 to 20% (v/v), and declined slightly at 30% (v/v) as shown in Figure 2b. Therefore, 5% (v/v) FA together with 20% (v/v) AA was required for efficient photochemical reduction of Bi. In order to investigate the roles of mixed acids played in the PVG of Bi, more experiments were carried out. When prolonging the UV irradiation time from 13 s to 120 s in AA medium without FA and Fe3+, obvious signal response was observed (Figure 3). The maximum signal was obtained at 20% (v/v) AA, although the signal response was only 2.1% of that from mixed acids media in the presence of Fe3+ and FA. In subsequent, FA with varied concentration was added to PVG solution of Bi in the presence of 20% (v/v) AA and 60 μg mL-1 Fe3+. The highest signal response was also recorded at 5% (v/v) FA, confirming that the improvement of PVG efficiency of Bi3+ was due to the use of mixed acids medium in the presence of Fe3+. According to the first law of photochemistry, photons must be absorbed by compounds to arise of a photochemical reaction.7 The UV absorption spectra of the diluted mixed solution of FA and AA towards irradiation below 250 nm was obviously higher than those when only FA or AA was used. A new absorption peak at 290 nm was also observed when Fe3+ was added (Figure S1). The increased UV absorption may facilitate the photochemical reduction of Bi and partly account for the enhanced PVG yield of Bi. More details of the possible mechanism would be discussed in the later part. Therefore, 20% (v/v) AA and 5% (v/v) FA was selected for following experiments.

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Figure 2. a: Effect of the concentration of FA and AA on Bi response: 13 s irradiation time, 60 μg mL-1 Fe3+, and 2 ng mL-1 Bi3+; b: effect of the concentration of AA on Bi response: 5% (v/v) FA, 13 s irradiation time, 60 μg mL-1 Fe3+ and 2 ng mL-1 Bi3+.

Figure 3. Comparison of signal response of Bi in different photochemical reduction media with different irradiation time: 5% (v/v) FA, 20% (v/v) AA, 60 μg mL-1 Fe3+ and 2 ng mL-1 Bi3+. The effects of ferric ions as enhancement reagents for PVG of Bi were studied in detail. As shown in Figure 4a, the PVG efficiency of Bi3+ was largely enhanced when the concentration of Fe2+ and Fe3+ increased from 0 to 40.0 μg mL-1, and then almost kept constant above 60.0 μg mL-1. The similar enhancement effect of Fe3+ and Fe2+ may due to the co-reduction of Fe and Bi since both Fe3+ and Fe2+ could be transformed into volatile species under UV irradiation. Interestingly, the signal response of 9



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Bi3+ in the absence of ferric ions was near blank which may due to the very short UV irradiation time used (Figure 3). The irradiation time plays an important role for the photochemical reduction of analytes. Therefore, the influences of UV irradiation time from 13 s to 180 s were investigated with or without 60 μg mL-1 Fe3+, respectively. As shown in Figure 4b, the response kept almost constant in the range of 13 s (the shortest time required by introducing PVG solution through PVG reactor at the sample flow rate of 3.8 mL min-1) to 140 s, there after slight decrease was found probably due to the decomposition of the produced volatile species. However, the PVG efficiency of Bi3+ in the absence of Fe3+ is highly depended on the irradiation time. The signal response of Bi enhanced as the increasing irradiation time, and the maximum signal response was obtained above 120 s irradiation time. But the signal intensity was only about 8 % of that from the system with 60 μg mL-1 Fe3+. Furthermore, the analytical precision of replicate determination of Bi for four times was up to 14% in the absence of Fe3+. Considering the analytical sensitivity, sample throughout and the stability of used reagent, 13 s irradiation time in the presence of 60 μg mL-1 Fe3+ was selected for subsequent experiments. The carrier argon flow influences the ICP sampling depth and the GLS efficiency of volatile Bi as well. The effect of carried gas from 0.80 to 1.10 L-1 min-1 was investigated in mixed acid medium using 2 ng mL-1 Bi3+. The maximum signal response of Bi was obtained at 0.95 L min-1. Higher gas flow leads to the volatile species dilution, while lower gas flow results in poor transfer efficiency of volatile Bi species. Thus, 0.95 mL min-1 of argon gas flow was selected for subsequent studies.

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Figure 4. a: Effect of the concentration of ferric ions on Bi response: 5% (v/v) FA, 20% (v/v) AA, 13 s irradiation time; b: Effect of irradiation time on Bi response with or without the presence of 60 μg mL-1 Fe3+, 5% (v/v) FA and 20% (v/v) AA. The mechanism of the ferric ion-assisted PVG for Bi

Figure 5. Mass spectra of volatile Fe species and volatile Bi species generated by photochemical reduction of 5 μg mL-1 Bi3+ in the presence of 20% AA, 5% FA and 60.0 μg mL-1 Fe3+. Recently, metal ions-assisted PVG has been successfully applied for the determination of As, Te, Se, Pb and Cl. 22, 31-33, 36 However, the field is not sufficiently developed and the mechanism is needed to be further studied to interpret such results.7 When Bi3+ was added to the PVG system of Fe, the depression of Fe signal response was observed as in the cases of Te and As.10 Nevertheless, the presence

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of ferric species in PVG systems of As and Te can greatly improve the generation efficiencies of As and Te31,33 In this work, the addition of Fe3+ into the PVG solution caused an obvious increase of UV absorption towards radiation below 250 nm,31, 33 which may partly cause the increase in PVG yield of Bi (Figure S1). The photochemical reduction of Fe was also observed in the PVG system. The major fragments recorded by GC-MS at m/z 84, 112, 140, 168 and 196 correspond to FeCO+, Fe(CO)2+, Fe(CO)3+, Fe(CO)4+ and Fe(CO)5+, implying formation of iron pentacarbonyl Fe(CO)5 in this PVG system37. The PVG efficiency of Fe3+ was found to be 4% under the selected conditions. The GC-MS results suggest that the major products formed from irradiation of Bi3+ in the presence of 20% (v/v) AA, 5% FA(v/v) and 60 mg L-1 Fe3+ is (CH3)3Bi (bp 109 oC) , which was similar with that for the PVG of As in mixed formic and acetic acid. The structure of (CH3)3Bi and Fe(CO)5 was confirmed by the major mass fragments as shown in Figure 5.38, 39 Furthermore, the investigation of the pH on photochemical reduction of Bi3+ showed that the change of pH value of the PVG solution lead to the decrease of signal response for Bi (Figure S2). The lowest signal response of Bi was observed in the pH range of 2.0 to 3.5, which was the optimal condition for the PVG of Fe. It is speculated that the presence of iron or the generated intermediate products of photochemical reduction of iron other than produced Fe(CO)5 promote the ligand to metal charge transfer reactions (LMCT) of Bi causing the enhanced PVG for Bi.7 As reported before, hydrides or carbonyls generate in FA medium in PVG, whereas methylated species yield in AA solution.7, 40 Based on the GC-MS measurement results, two possible scenarios may account for the PVG of Bi3+ as in the case for PVG of Se(IV): first reduction of Bi3+ to form intermediate Bi0 species by hydrated electrons/reactive radicals from photodecomposition of carboxylic acids and subsequent nucleophilic attacks by •R and rearrangements to form volatile compounds.7, 40 Ferric ions may serve as sensitizers for accelerated photo-oxidation of FA since Fe3+ can enhance the rate of oxidation of FA 100 folds more than it does that of AA.7, 41 As a consequence, the presence of Fe3+ can

12


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greatly accelerate the reduction reactor of Bi3+ to Bi0. The PVG efficiency of Bi was near blank value in 5% (v/v) FA without Fe3+ under the irradiation time ranging from 13 to 120 s (Figure S1). The similar phenomenon was observed for Bi in 20% AA (v/v) under 13 s irradiation times in the absence of Fe3+ (Figure 3). Obvious signal of Bi could be obtained when prolonging the irradiation time to 120 s in AA medium. When Fe3+ was added, weak signal of Bi could be obtained with 13 s irradiation in FA solution. Whereas, 120 s irradiation times was also required for photochemical reduction of Bi in AA solution in the presence of Fe3+, demonstrating the enhanced oxidation rate of FA by adding Fe3+. In this work, the photodecomposition of FA and the following reduction of Bi3+ to form Bi0 by generated hydrated electrons/ reactive radicals supposed to be the rate-determined step for efficiently PVG of Bi. Subsequently, Bi0 was attacked by CH3• generated from the photodecomposition of AA, leading to the form of (CH3)3Bi. Therefore, the presence of FA, AA together with Fe3+ contributes the enhanced PVG of Bi. The optimum irradiation times of mixed FA and AA with or without Fe3+ further confirmed the proposed speculation. Interference Study The potential interferences from 1.0 μg mL-1 of K+, Ca2+, Na+, Mg2+, Mn2+ and Zn2+ were studied using 0.5 ng mL-1 Bi3+ in this work. As shown in Table 2, no obvious interferences were found. In conventional HG, serious interferences take place for the determination of Bi in the liquid phase from some transition metal species as a result of formation of finely dispersed metal or metal boride colloids absorbing and decomposing the generated hydride.3 Especially for Cu2+, one fold excess of Cu2+ can cause obvious depression of signal for Bi detection.3 In this work, the interfering from Cu2+, Ni2+ and Co2+ for the measurement of Bi was evaluated. As evident from Table 2, the proposed method showed much better performance for the tolerance interferences from Cu2+ and Ni2+ compared to that of HG. Up to 2000 folds of Cu2+ and Ni2+ did not cause appreciable effect for 0.5 ng mL-1 Bi3+ detection. The major

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component of volatile Bi species was (CH3)3Bi instead of BiH3 which may lead to the improved tolerance ability of Bi3+ towards Ni2+ and Cu2+. Whereas, more than 20 folds of Co2+ would bring positive effect for Bi3+ detection, which was 5 times lower than that in HG system.3 The interferences from hydride forming elements were investigated for the determination of Bi3+. Te, Se, As, Pb, Sn and Sb at the concentration of 0.1 μg mL-1 had no significant effect for the determination of Bi. Additionally, 50.0 μg mL-1 NO3-, Cl-, PO43- and SO42- did not cause interferences on the determination of 0.5 ng mL-1 Bi3+. Table 2. Effect of coexisting substances for the detection of 0.5 ng mL-1 Bi3+.

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Interfering ions

Concentration/μg mL-1

[Interferent]/[Bi]

Recovery/%, n=3

Na K

1.0 1.0

2000 2000

98±2 100±3

Ca

1.0

2000

97±1

Mg

1.0

2000

103±2

Zn

1.0

2000

91±2

Ni

2.0

4000

101±3

Mn

1.0

2000

104±2

Cu

1.0

2000

92±1

Co

0.01

20

110±2

Sb

0.1

200

98±1

Te

0.1

200

103±3

Se

0.1

200

94±2

Sn

0.1

200

96±1

As

0.1

200

88±1

Pb

0.1

200

97±3

Cl-

10.0

20000

99±3

NO3-

10.0

20000

100±3

SO42-

10.0

20000

100 ±2


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Analytical Figures of Merit and Sample Analysis Under optimum experimental conditions, the analytical figures of merit were evaluated. Thirty folds of enhancement in sensitivity are realized using PVG for sample introduction compared to that for direct solution nebulization. By comparison of the signal intensities of Bi3+ solutions from ICP MS detection using solution nebulization sample introduction before and after UV irradiation, the vapor generation efficiency was estimated to be 55±2%, which was about 12-fold enhancement than that in the absence of Fe under the optimized condition (Figure 3). Besides, only 13 s irradiation time was needed in the proposed system. The precision (relative standard deviations, RSD) obtained from 7 replicate measurements of 0.5 ng mL-1 Bi standard solution was found to be 2.5%. By measuring a series of standard solutions of Bi3+, the calibration curve was established using peak height and area respectively (Figure S3). The linear regression coefficients (R2) were 0.999. The formation of dispersed black precipitate of Bi0 at high concentration of Bi3+ in HG, which can cause rollover of calibration curves and limit the linear range to less than 1.0 μg mL-1 by AAS detection,42 was not observed in the proposed system even with the concentration of standard solution up to 5 μg mL-1. But the PVG efficiency of Bi3+ at higher concentration greatly relies on the irradiation time. The PVG efficiency of Bi3+ at 5 μg mL-1 was estimated to be 15% using 13 s UV irradiation by inductively coupled plasma optical emission spectrometry (ICP OES) detection using solution nebulization sample introduction. With 120 s irradiation time, the PVG efficiency of Bi3+ increased to 48±1% as in the case for PVG of Se(IV).5 A method limit of detection (LOD) was found to be 0.3 ng L-1 for Bi by ICP MS detection, which is significantly improved by comparing with previous reports of Bi determination and about 10-fold lower than that obtained from solution nebulization sample introduction. As shown in Table 3, the developed method is among the most sensitive Bi measurement techniques, due to the powerful elemental

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sensitivity of ICPMS and the high efficiency sample introduction method ferric ions assisted PVG system. Compared to reported PVG-ICP MS system, the LOD has been improved 43 folds.29 The accurate determination of the background concentration range of Bi in freshwater is important for the relative research in the field of environment science.30,

43

To validate the accuracy of the

proposed method, three environmental water samples were analyzed. Standard addition calibration method was applied to avoid the possible interferences from sample matrix for the accurate detection of Bi and the result was shown in Table 4. Furthermore, a spike recovery test was carried out and the recoveries were from 95% to 100%, demonstrating the capability of the proposed system for the accurate determination of Bi at ng L-1 level. To further evaluate the accuracy of the proposed method, Soil and sediments certified reference materials were analyzed. The analytical results were in good agreement with the certified value as listed in Table 5. Table 3. Comparison of analytical figures of merit.

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Method

Preconcentration

Samples

LOD

Ref.

HG-AFS

-

Urine

20.0 ng L-1

44

EAAS

Solid phase extraction

Urine

13.0 ng L-1

45

HG-miniaturized OES

-

Soil

1000 ng L-1

46

HG-ICP-MS

Solid phase extraction

Aquatic Samples

2 ng L-1

47

PVG-AFS

-

Natural waters

100 ng L-1

28

PVG-ICPMS

-

Water and fish muscle

13 ng L-1

29

Ferric ions enhanced PVG-ICP-MS

-

Water and sediment

0.3 ng L-1

This work


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Table 4. Determination of Bi in the water samples. Sample

Determined/ng mL-1

Added/ng mL-1

Found/ng mL-1 Recovery/%

Tap water

0.0020±0.0001

0.002

0.0040±0.0001

100

Lake water

0.0061±0.0002

0.012

0.0175±0.002

95

Dongfeng River

0.0019±0.0002

0.002

0.0038±0.0001

97

Table 5. Analytical results of Bi in certified reference materials Sample

Certified value (μg g-1)

Measured (μg g-1, n=3)

GSS-1

1.2±0.1

1.1±0.2

GSD-5a

3.0±0.2

2.8±0.1

GSD-10

0.38±0.04

0.38±0.03

Conclusion A highly sensitive method is proposed for the accurate determination of Bi in environmental samples by ICP MS after ferric ion-assisted PVG. Thirty folds of enhancement in sensitivity were obtained using PVG compared to that for direct solution nebulization. The addition of Fe3+ not only largely increases the PVG efficiency of Bi but also accelerates the reaction kinetics of photochemical reduction of Bi. Additionally, the proposed method shows better performance in terms of anti-interfere ability towards Cu2+ and Ni2+ than that of HG. The rollover of calibration curves in HG caused by the formation of Bi0 at high concentration was not observed in the developed method. Nevertheless, the mechanism of PVG of Bi is required further investigation to improve our understanding of metal ionsassisted PVG. The proposed method has great potential for analysis of ultra-trace Bi in a variety of materials.

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ACKNOWLEDGMENT National Natural Science Foundation of China (No. 21205007), Sichuan Youth Science and Technology Foundation (No.2017JQ0043), China Postdoctoral Science Foundation (No. 2016M590870 and No. 2018T110952), State Key Laboratory of Geohazard Prevention and Geoenviroment Protection Independent Research Project (SKLGP2016Z006), and the Education Department of Sichuan Province (Grant No. 17ZA0040) are acknowledged for their financial support. Supporting Information Additional information as noted in text, including the UV absorbance of the mixture of formic acid and acetic acid, effect of pH, calibration curves established by the proposed method and a table listed the enhanced PVG by metal ions in recent publications.

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For TOC only

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Enhanced Photochemical Vapor Generation for determination of

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