Generation of Volatile Cadmium and Zinc Species Based on Solution

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Generation of Volatile Cadmium and Zinc Species Based on Solution Anode Glow Discharge Induced Plasma Electrochemical Processes Xing Liu, Zhifu Liu, Zhenli Zhu, Dong He, Siqi Yao, Hongtao Zheng, and Shenghong Hu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00126 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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Analytical Chemistry

Generation of Volatile Cadmium and Zinc Species Based

1 2

on Solution Anode Glow Discharge Induced Plasma

3

Electrochemical Processes

4

Xing Liu†, Zhifu Liu†, Zhenli Zhu†*, Dong He†, Siqi Yao†, Hongtao Zheng‡,

5

Shenghong Hu†

6 7

†

8

Sciences, China University of Geosciences, Wuhan, 430074, China

9

‡

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

State Key Laboratory of Biogeology and Environmental Geology, Faculty of

10

Materials Science and Chemistry, China University of Geosciences, Wuhan 430074,

11

China

12

*

13

+86-27-6788-3456. E-mail: [email protected], [email protected]

To whom correspondence should be addressed. Phone: +86-27-6788-3452. Fax:

14 15

ACS Paragon Plus Environment


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ABSTRACT: In this study, a novel high efficiency vapor generation strategy was

2

proposed based on solution anode glow discharge for the determination of Cd and Zn

3

by atomic fluorescence spectrometry. In this approach, a glow discharge microplasma

4

was acted as a gaseous cathode to initiate the plasma electrochemical vapor

5

generation of Cd and Zn. Cadmium/Zinc ions could be converted into molecular

6

species efficiently at the plasma-liquid interface from a supporting electrolyte (HCl,

7

pH = 3.2). It was found that the overall efficiency of the plasma electrochemical vapor

8

generation (PEVG) system was much higher than the conventional electrochemical

9

hydride generation (EcHG) and HCl-KBH4 system. With no requirement for other

10

reducing reagents, this new approach enabled us to detect Cd and Zn with detection

11

limits as low as 0.003 µg L-1 for Cd and 0.3 µg L-1 for Zn. Good repeatability (relative

12

standard deviation (RSD), n = 5) was 2.4% for Cd (0.1 µg L-1) and 1.7% for Zn (10

13

µg L-1) standard. The accuracy of the proposed method was successfully validated

14

through analysis of cadmium in reference material of stream sediment (GBW07311),

15

soil (GBW07401), rice (GBW10045), and zinc in simulated water sample (GSB

16

07-1184-2000). Replacing a metal electrode with a plasma offers the advantage of

17

eliminating potential interactions between the species in liquid and the electrode,

18

which solves the issues associated with electrode encountered in conventional EcHG.

19

The ability to initiate electrochemical vapor generation reactions at the plasma-liquid

20

interface opens a new approach for chemical vapor generation based on interactions

21

between plasma gas-phase electrons and solutions.

22


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Chemical vapor generation (CVG) is a widely adopted sample introduction

2

technique in atomic spectrometry for its enhanced sensitivity and selectivity.1

3

However, conventional CVG methods involve the use of toxic and expensive reagents,

4

such as, tetrahydroborate(III), tin chloride or potassium permanganate. In addition,

5

these reagents are potential source of contamination and their purification for use in

6

ultratrace analysis is time consuming. The replacement of CVG by powerful and

7

greener alternatives has been a hot topic in analytical chemistry.2,3

8

Electrochemical hydride generation (EcHG),4,5 which uses electrons supplied

9

from the cathode as reductant, represents a suitable alternative to conventional CVG.

10

It has been demonstrated that hydrides of several elements, including As, Hg, Bi, Sb,

11

Pb, Ge, Te, Sn, Se, Zn and Cd,6-16 could be achieved by EcHG. The most significant

12

advantage of EcHG is that it obviates the need for chemical reducing reagents.

13

However, the performance of the EcHG process is strongly influenced by the cathode

14

material. Significant interferences from concomitant transition metals ions have been

15

observed due to the reduction and deposition of these ions on cathode surface.17

16

Moreover, adsorption of gaseous reaction products on the electrode surface also

17

reduces vapor generation efficiency.17

18

The interactions between plasmas and liquids have become increasingly more

19

important topics because of its great promise in the fields of analytical chemistry,

20

nanomaterials synthesis, and medical sciences etc.18-20 Plasmas in contact with liquids

21

initiate complex chemistry that leads to the generation of a wide range of reactive

22

species. Especially, in an electrolytic configuration with a cathodic plasma electrode,



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electrons from the plasma are injected into the solution, leading to solvation and

2

ensuing

3

electrochemical cell with an atmospheric-pressure microplasma cathode, and

4

demonstrated the electron-transfer reactions at the plasma-liquid interface.23,24 Atif et

5

al. also investigated the electron transfer events at the solid-gas interface.25 It should

6

be noted that the reactive species may also interact with species in the liquid via

7

chemical pathways in parallel to the electrochemical pathways.26 The plasma

8

chemical processes in dielectric barrier discharge (DBD) has been used for the

9

synthesis of chemicals,27,28 and our group also found that radicals formed in DBD can

10

be used to reduce metal ions to their vapors.29,30 However, the doping of hydrogen is

11

necessary for the vapor generation of Cd and Zn in DBD.29,31 We also demonstrated

12

that the vapor generation could be achieved by solution cathode glow discharge

13

induced plasma chemical process. However, it is only limited to Hg, Os and I.32-34 In

14

principle, the plasma electrochemical system, in which plasmas are used as gaseous

15

cathode, have a great potential of integrating both the advantages of plasma

16

processing and those of electrochemistry.

reduction

reactions.21,22

Richmonds

et

al.

reported

an

aqueous

17

Herein we describe a new strategy for the generation of volatile species of Cd

18

and Zn based on solution anode glow discharge induced plasma electrochemical

19

processes. The use of plasma cathode eliminates the potential interactions between the

20

species in liquid and the metal electrode, and thus the problems of conventional EcHG

21

caused by cathode materials are avoided. The dissolved Cd and Zn ions could be

22

readily converted into molecular volatile species efficiently by the plasma


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1

electrochemical vapor generation (PEVG). In addition, it was found that charge

2

transfer depends on the properties of the discharge; for example, the conversion

3

(vapor generation) efficiency of Cd and Zn is found to increase with discharge current,

4

which in turn is related to the flux of plasma electrons to the solution surface.

5

EXPERIMENTAL SECTION

6

Instrumentation. A homemade plasma electrochemistry cell (PEC) (cf. Figure 1)

7

was applied for the vapor generation of Cd and Zn. The H-type divided cell (2 cm

8

internal diameter, about 5 mL reaction volumes) consisted of two compartments, the

9

cathodic and anodic. A Pt foil immersed in an anolyte acted as the anode and a

10

microplasma was used as cathode. The microplasma was initiated by temporarily

11

reducing the distance between the tip of a stainless steel capillary tube (i.d., 1 mm;

12

o.d., 2 mm and 5 cm length) and the solution surface. The gap, or height of the

13

discharge, was set to about 1 mm in the present study. A 5-kΩ resistor inserted

14

between the microplasma cathode and the negative output of the high-voltage power

15

supply stabilizes the discharge current.

16

A syringe pump (Harvard Apparatus PHD 2000, Holliston, MA) and a peristaltic

17

pump (BT 100-1L, Baoding Langer Constant Flow Pump Co., Ltd., China) were

18

employed to drive the sample and waste solutions respectively. The flow velocities of

19

Ar and H2 were monitored by mass flow controller. The microplasma was sustained

20

between the tip of a stainless steel capillary tube and the solution surface by using a

21

HV dc power supply (HB-Z202-200AC, Tianjin Hengbo High Voltage Power Supply

22

Plant, China). The volatile species produced from this cell were swept by an argon



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1

stream and mixed with auxiliary H2 and directed to a quartz furnace atomizer (QTA)

2

and detected by AFS (AFS 9130, Beijing Titan Instruments Co. Ltd., Beijing, China).

3

The Cd/Zn high performance hollow cathode lamp (General Research Institute for

4

Nonferrous Metals, Beijing, China) was used as the radiation source.

5

Analytical

Procedures.

The

analytical

procedures

of

the

plasma

6

electrochemistry vapor generation atomic fluorescence spectrometry (PEVG-AFS) are

7

schematically shown in Figure 1. First, the sample solution was continuously

8

introduced into the cell through a glass capillary (0.38 mm inner diameter) by the

9

syringe pump. Second, the high voltage dc power supply was turned on; a

10

microplasma was then ignited by temporarily reducing the distance between the tip of

11

the stainless steel capillary and liquid surface. During the discharge process, the

12

plasma was spatially stationary and the discharge current was kept at 10 mA

13

(corresponding voltage, ∼550 V), controlled by adjustment of the power supply

14

voltage. Third, the generated volatile species produced from the plasma

15

electrochemical process were swept by an argon stream through a gas-liquid separator

16

(GLS) and then to a T-tube and mixed with auxiliary H2. Last, the gas containing

17

volatile species was directed to the atomizer and detected by AFS. The following

18

operating conditions are employed: photomultiplier voltage, -280/-270V; lamp current,

19

60/60 mA for Cd/Zn, respectively.

20

Reagents and Samples. All chemicals used were at least of analytical grade and

21

all solutions were prepared using high-purity water with a resistivity of 18.2 MΩ cm,

22

obtained from a water purification system (Labconco, Kansas, USA). Stock standard


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1

solutions of 1000 mg L-1 of Cd were supplied by National Analysis Center for Iron

2

and Steel (Beijing, China). Working standards were prepared from stepwise dilution

3

of the stock solution with diluted acid. For the interference study, the following

4

chemicals: NaNO3, KCl, NiCl2·6H2O, CoCl2·6H2O, Pb(NO3)2, Cu(NO3)2·3H2O,

5

SnCl2·2H2O, Zn(NO3)2·6H2O and Cd(NO3)2·4H2O were employed. All the

6

concentrations of the interferences were expressed as metal ions in the present study.

7

Argon of 99.999% purity (Oxygen Co. of Wuhan Iron & Steel Group, Wuhan, China)

8

was used as the carrier gas. Standard reference materials for stream sediment

9

(GBW07311, Cd), soil (GBW07401, Cd), rice (GBW10045, Cd) obtained from

10

National Research Center for Standard Materials (Beijing, China), and simulated

11

water sample (GSB 07-1184-2000, Zn) developed by the Institute for Environmental

12

Reference Materials Ministry of Environmental Protection (Beijing, China) were

13

applied to evaluate the accuracy of the present method.

14

Sample Preparation. The simulated natural water sample GSB 07-1184-2000

15

was simply diluted and was adjusted to the pH of 3.2 before analysis. The three solid

16

samples were firstly dried at 105 °C for more than 24 h to remove its moisture content.

17

The procedures used for sample decomposition were as follows: Sample powder (0.05

18

g for GBW07311 and GBW07401, 0.25 g for GBW10045) was weighed and placed

19

in a 10mL home-made PTFE-lined stainless steel bomb. After wetting with a few

20

drops of ultra-pure water, 1 mL of HNO3 and 1 mL of HF were added and the bomb

21

was sealed and placed in an electric oven and heated to 190 °C for 48 h. After cooling,

22

the bomb was opened and placed on a hotplate at 115 °C and evaporated to incipient



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1

dryness (but not baked). This was followed by adding 1 mL HNO3 and evaporating to

2

the second round of incipient dryness. Then, the resultant salt was re-dissolved by

3

adding 3 mL of 30% HNO3 and resealed and heated in the bomb at 190 °C for 12 h.

4

After cooling, the vessels were taken to an electrical hot plate at 115 °C to heat to

5

almost dryness; then, 5 mL of diluted HCl (pH = 3.2) was added to redissolve all the

6

analyte. The final solution was transferred to a polyethylene bottle and diluted to an

7

appropriate volume with HCl (pH = 3.2). A reagent blank solution was prepared in

8

the same way.

9


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1

RESULTS AND DISCUSSION

2

Generation of Volatile Species of Cd and Zn by Using Plasma Cathode. In

3

conventional EcHG, both the cathode and anode are connected to the electrolyte, and

4

metal

5

electrode/electrolyte interfaces. In our PEVG, a microplasma were used as cathode to

6

initiate the plasma electrochemical vapor generation of Cd and Zn, where only the

7

anode (Pt foil) is immersed in the electrolyte solution. To test whether Cd and Zn ions

8

in the solution could be reduced to the volatile species by using the microplasma

9

cathode, the gases in the cathodic cell was introduced to an atomic fluorescence

10

spectrometer. It was found that significant fluorescence signal of Cd and Zn could be

11

obtained even with concentration as low as 0.2 µg L-1 and 5 µg L-1, respectively,

12

when the quartz tube atomizer is heated by hydrogen-argon entrained flame (Figure 2).

13

In an effort to ensure that all signals detected during plasma exposure indeed

14

originated from the generation of volatile metal species, rather than as an aerosol

15

produced by plasma-liquid interaction, the volatile products were also directed to an

16

ICP-OES. Significant Cd/Zn signal were also obtained, but no measurable signal of

17

spiked Na/Mg was observed even at a solution of 100 mg L-1. These results

18

demonstrated that the vapor generation of Cd and Zn indeed be readily achieved with

19

PEVG.

volatile

species

are

generated

by

charge

transfer

at

the

metal

20

In subsequent experiments, vapor generation of Cd and Zn was also performed

21

by using microplasma as anode (e.g. solution cathode glow discharge, SCGD) with

22

the same discharge cell and experimental conditions, simply by inversing the polarity



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1

of the electrode. It was found that no measurable signal of Zn was observed even at a

2

concentration of 1 mg L-1. Although the Cd signal was observed at the concentration

3

of 100 µg L-1 by SCGD, its efficiency was at least 1200 times lower than that of

4

PEVG (Figure S1). This result indicated that the charge-transfer process using

5

microplasma cathode is necessary/preferable for the vapor generation of Cd and Zn.

6

Meanwhile, it is interesting to note that the change of polarity of electrodes can

7

significantly enhance the emission sensitivity of some elements(Cd, Zn and Ag

8

etc).35,36 All these results demonstrates that the use of plasma cathode presents a

9

different behavior compared with solution cathode and it is preferable for the vapor

10

generation of Cd and Zn.

11

Furthermore, the vapor generation efficiency of the PEVG system was also

12

compared with traditional EcHG using Cd (Figure S2, supporting information). It

13

should be noted that the EcHG was performed with the same electrolyte condition and

14

cathode material as reported in the reference.10 With the discharge current of 10 mA,

15

significant Cd atomic fluorescence signal was obtained from PEVG even with 0.5 µg

16

L-1, whereas no measurable Cd signal was observed from EcHG even with cadmium

17

concentration as high as 20 µg L-1. It was found that the Cd signal obtained from

18

traditional EcHG operated with 150 mA was still decreased by a factor of 35

19

compared to PEVG with an operation current of 10 mA. These results suggested that

20

electron utilization efficiency and vapor generation efficiency of PEVG were much

21

higher than EcHG, which also implied that the plasma chemical process may also be

22

involved in PEVG in addition to electron-transfer electrochemical process. Replacing


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1

a metal cathode with a plasma in PEVG has the unique advantage of eliminating any

2

potential metal deposition and contamination of the metal electrode, which is common

3

in traditional EcHG.

4

Several experiments were undertaken to determine the identity of volatile species

5

generated in PEVG. Firstly, we found that there was no measurable atomic

6

fluorescence signal from Cd and Zn with the atomizer at room temperature even at 10

7

mg L-1. These results suggested that the generated volatile Cd and Zn species were

8

both in molecular form since free Cd/Zn atom vapor can be detected at room

9

temperature.29 Then, the generated of Cd/Zn vapors was investigated by a Shimadzu

10

AA6300C electrothermal atomic absorption spectrometry. As shown in Figure 3, no

11

measurable signal was observed below the temperature of 300 °C. The cadmium

12

atomic absorption signal increased rapidly with further elevation of the atomizer

13

temperature and plateaued at 500°C. The volatile species of Zn also presented a

14

similar trend. Because there is only one steep slope, perhaps only one molecular

15

species (presumably CdH2/ZnH2) was generated by using PEVG. Although the

16

collected volatile species of Cd and Zn could be preserved for at least 7 hours at room

17

temperature (Figure S3), we failed to characterize them with GC-MS.

18

In our proposed PEVG system, Cd2+ and Zn2+ could be readily converted into

19

volatile molecular species with a microplasma as a gaseous, metal-free cathode. The

20

metallic capillary is not in contact with electrolyte and serves only as an electrical

21

conductor, and thus the properties of this metal capillary do not change significantly

22

during electrolysis and the metallic capillary need not be cleaned or replaced, which is



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1

in contrast to traditional EcHG. In addition, the generated volatile species can be

2

released rapidly from the solution because the reaction occurs at the gas-liquid

3

interface. Furthermore, the proposed PEVG eliminates the use of unstable reducing

4

reagents (i.e., NaBH4) or hydrogen gas (DBD plasma-CVG29,31,37); only a small

5

amount of hydrochloric acid is needed. Therefore, it provides a green technique for

6

vapor generation, which reduces the generation of hazardous waste and also avoids

7

possible contamination from reagents.

8

Optimization of PEVG-AFS.

9

To achieve the maximum signal, the effects of experimental parameters such as

10

material of metal capillary, electrolyte pH, discharge current, carrier gas flow rate,

11

hydrogen flow rate and sample flow rate were evaluated. Unless otherwise specified,

12

three replicates were performed at each set of the studied parameters.

13

Material of capillary. In conventional electrochemistry, electrons in the metal

14

cathode are bound by the work function of the material, however, with a plasma

15

cathode system, the reducing electrons are not bound to a metal electrode.36 Therefore,

16

in principle, the properties of the metal capillary in PEVG should not affect the vapor

17

generation of Cd and Zn. To investigate this, three materials (steel, copper, graphite)

18

were used as the working electrode in the cathodic cell. It was found that comparable

19

fluorescence signal of Cd was obtained for all these three materials (Figure 4a). This

20

result reinforces our belief that the proposed PEVG are not dependent on the

21

characteristics of the working electrode materials but reproducible over various

22

materials. Replacing the metal cathode with a plasma offers a unique advantage of


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1

eliminating potential interactions between the species in liquid and the metal electrode.

2

Based on this result, steel capillary was used in the subsequent studies.

3

pH of Electrolyte. The influence of pH on the vapor generation of Cd and Zn

4

were studied with HCl and HNO3. As shown in Figure 4b, no detectable signal was

5

observed in the pH range from 1 to 2 for both Cd and Zn. A possible reason is that the

6

decrease of pH is beneficial to hydrogen evolution reaction and play down the

7

reduction efficiency of zinc and cadmium. The AFS signal increased with pH varying

8

from 2.0 to 3.2, where the maximum signals were both achieved for Cd and Zn. The

9

signal response then decreased sharply with further increase in pH. In addition, the

10

plasma stability worsened at high pH, probably caused by the low conductivity of the

11

electrolyte under low acid concentrations. Similar phenomenon was observed in both

12

HCl and HNO3 media, which ensured that the effect was caused by pH. On basis of

13

these results, pH 3.2 was selected in the following experiments. Therefore, pH of 3.2

14

of HCl was employed for further experiments.

15

Discharge Current. The dependence of the vapor generation processes on

16

discharge current was also investigated in the range of 6-14 mA (Figure 4c), and the

17

intensity was observed to increase almost linearly with the current up to 14 mA. With

18

raising discharge current, a higher flux of electrons reaches the solution surface, and

19

the rate of electrochemical reduction of Cd/Zn increases. However, when the current

20

was set higher than 15 mA, overheating of the cathode occurred due to the cathode

21

sputtering and the discharge also became unstable. As a compromise, an electrolytic

22

current of 10 mA was used throughout this work.



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1

H2 Flow Rate. As reported in previous EcHG work,25 only a small amount of

2

hydrogen was produced in the electrolysis process, which is insufficient to maintain

3

an Ar/H2 flame. Therefore, auxiliary H2 is necessary in our PEVG-AFS system for the

4

atomization of volatile Cd and Zn species. The dependence of the Cd and Zn

5

fluorescence intensity on H2 flow rate was tested in the range of 0-90 mL min-1. The

6

result was shown in Figure 5a. It was observed that fluorescence signal could not be

7

obtained when the flow rate of auxiliary hydrogen was set below 20 mL min-1. Both

8

the fluorescence signal of Cd and Zn first increased remarkably along with the H2

9

flow rate from 20 to 40 mL min-1, reached a maximum at 50 mL min-1, then decayed

10

afterwards. In this work, 50 mL min-1 H2 flow rate was chosen.

11

Ar Flow Rate and Sample Flow rate. The influence of Ar flow rate on the

12

fluorescence signal of Cd/Zn was studied in the range of 100-500 mL min-1. Figure.

13

5b shows that, the fluorescence signal reached its maximum value at 300 mL min-1

14

and then declined. The result suggests that the use of high carrier gas flow rates can

15

enhance the transport efficiency, but can also decreases the signal because of

16

volumetric dilution of Cd and Zn vapor. As shown in Figure. 5c, the maximum signal

17

of Cd was observed at a sample flow rate of 2.5 mL min−1, whereas the optimal

18

sample flow rate for Zn was found to be at 2.1 mL min−1. However, the fluorescence

19

signal response decreased with further increase of the flow rate beyond 3.5 mL min-1,

20

as the result of a decrease in the residence time of the analytes inside the cell. Based

21

on these results, Ar flow rate of 300 mL min-1 and sample flow rate of 2.5 mL min−1

22

were selected in the subsequent studies.


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1

Interference Studies. Transition metal (Cu, Co, and Ni) ions are usually

2

considered as serious interferent for EcHG because these ions are easily reduced to

3

their metallic states during electrolysis, subsequently deposited on the cathode surface

4

and then dispersed over it. As a result, the cathode hydrogen overvoltage is altered

5

and affects the efficiency for generation of the volatile analyte species.38 In addition,

6

the vapor generation of Cd/Zn are also highly susceptible to the interference from

7

other hydride forming elements.5 To evaluate the effect of the interference from

8

coexisting ions, the effects of eight ions, including those from transition metal and

9

hydride-forming elements on the determination of 0.5 µg L-1 Cd2+ and 10 µg L-1 Zn2+

10

were studied by PEVG. The interferences from these ions are summarized in Table 1.

11

The values are given as the recovery in the presence of interfering ions relative to the

12

interference-free response.

13

It can be seen that there is no significant interference from these studied ions at

14

concentration of 1 mg L-1. However, severe depression effects were observed when

15

these ions were present at concentrations of 10 mg L−1 especially for Zn. Na+, K+, Ni2+

16

and Co2+ caused the most severe interference. The interference mechanism of Na+ and

17

K+ is unclear at present. Similar observation was also reported in our previously DBD

18

plasma-CVG studies29. One possibility is that high concentrations of ions alter the

19

conductivity of the samples, which in turn affect the electrical characteristics of the

20

plasma. In the case of Sn2+, the decrease in the signal of Zn2+ and Cd2+ may be

21

explained by the formation of tin hydride, which act as competitive reactions to the

22

formation of cadmium/zinc hydride. It should be noted there is no significant effect of



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

1

Ni2+, Co2+ and Cu2+ at concentration up to 1 mg L-1 in our proposed PEVG. However,

2

in traditional EcHG, 33%, 44%, 14% signal enhancement on Cd signal were reported

3

in the presence of 1 mg L-1 Ni2+, Co2+ and Cu2+, respectively.5 The results

4

demonstrated that PEVG has higher tolerable levels for coexisting Ni2+, Co2+ and

5

Cu2+ in the sample compared to EcHG.

6

Analytical Characteristics. Under the optimized conditions (electrolyte, HCl

7

(pH = 3.2), discharge current of 10 mA, Ar gas flow rate of 300 mL min-1, and sample

8

flow rate of 2.5 mL min-1), the analytical characteristics of PEVG-AFS were

9

evaluated. The calibration curves of fluorescence signals versus ion concentration

10

were linear in the range of 0.05-0.5 µg L-1 for Cd (R2 = 0.9910) and 2-20 µg L-1 for

11

Zn (R2 = 0.9980). The repeatability value, expressed as the relative standard deviation

12

(RSD, n = 5) of the signal, was 2.4% for Cd (0.1 µg L-1) and 1.7% for Zn (10 µg L-1)

13

standard. The limits of detection (LODs), were calculated to be 0.003 µg L-1 for Cd

14

and 0.3 µg L-1 for Zn. This suggests that the proposed PEVG is one of the highest

15

sensitive vapor generation techniques and can be used for the determination of

16

ultratrace cadmium and zinc (Table 2).

17

The overall efficiency of the PEVG system was also evaluated by a comparison

18

with conventional CVG system using Cd. For the conventional CVG system, we

19

followed the method reported by Li et al.39 with 3.5% (m/v) KBH4 as reducing

20

solution, 1.5% (m/v) thiourea, and 0.7 mg L-1 cobalt prepared in 0.2% (m/v) KOH,

21

2.5% (v/v) HCl as carrier solution. Identical sample introduction and gas flow rates

22

were used for the conventional HCl-KBH4 CVG system and continuous-flow PEVG.


Page 16 of 30

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1

It was observed that the fluorescence signal of cadmium obtained from PEVG was

2

improved by a factor of 2.3 compared to the conventional HCl-KBH4 CVG system,

3

which indicates that the PEVG system is a high-efficiency vapor generation technique.

4

Additionally, the vapor efficiency was also estimated from a comparison of the

5

relative concentrations of cadmium and zinc in the feed and waste solutions after the

6

sample was subjected to PEVG. The vapor generation efficiency was calculated to be

7

73 ± 5% for Cd and 30 ± 2% for Zn, which are much higher than those obtained with

8

EcHG.21,40 These results further demonstrated that our proposed PEVG system

9

provided a highly efficiency vapor generation method for Cd and Zn.

10

To evaluate the accuracy of the present method, certified reference materials

11

including stream sediment (GBW07311, Cd), soil (GBW07401, Cd), rice

12

(GBW10045, Cd) and simulated water sample (GSB 07-1184-2000, Zn) were

13

analyzed. In the initial experiment, the external standard curve method was used to

14

analyze all the four certified reference materials. However, the obtained concentration

15

for the stream sediment sample was only about 63% of the certified value. Therefore,

16

stream sediment was analyzed by the standard addition method to overcome the

17

matrix suppression. The analytical results obtained are given in Table 3. The values of

18

the certified reference materials determined by the developed method agreed well

19

with the reference values. All these results suggested that the proposed method can be

20

used for the determination of Cd/Zn in environmental and geological samples.

21

CONCLUSIONS

22

In summary, a novel vapor generation strategy for Cd and Zn was developed



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Page 18 of 30

1

based on solution anode glow discharge induced plasma-liquid electrochemical

2

reaction. It was found that the generated volatile species of Cd and Zn were both

3

molecular species in nature and the reaction efficiency depended on discharge

4

parameters (e.g., discharge current and solution pH). This PEVG method achieves

5

high vapor generation efficiency at a low current, and the absence of a solid electrode

6

also successfully solves the issues associated with electrode encountered in EcHG.

7

However, the applicability of PEVG should be improved further, although we also

8

found similar effects in the case of Hg, Pb, Ag and Tl; in addition, the underlying

9

mechanism of this method should be studied in the future (e.g., overpotential of

10

hydrogen evolution on the plasma-liquid interface, detailed reaction sequence).

11

Acknowledgement

12

We acknowledge the financial support from the National Nature Science

13

Foundation of China (No. 41673014, 21375120, and 41521001), Program for New

14

Century Excellent Talents in University from the Ministry of Education (No.

15

NCET-13-1015), Nature Science Foundation of Hubei Province (2016CFA038) and

16

International

17

(2014DFA20720).

Science

&

Technology

Cooperation

18


Program

of

China

Page 19 of 30

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REFERENCES: (1) Sturgeon, R.; Guo, X.; Mester, Z. Anal. Bioanal. Chem. 2005, 382, 881-883. (2) Wu, P.; He, L.; Zheng, C.; Hou, X.; Sturgeon, R. E. J. Anal. At. Spectrom. 2010, 25, 1217-1246. (3) He, Q.; Zhu, Z.; Hu, S. Rev. Anal. Chem 2014, 33, 111-121. (4) Arbab-Zavar, M. H.; CHamsaz, M.; Youssefi, A.; Aliakbari, M. Anal. Chim. Acta 2006, 576, 215-220. (5) Arbab-Zavar, M. H.; Chamsaz, M.; Youssefi, A.; Aliakbari, M. Anal. Chim. Acta 2005, 546, 126-132. (6) Sengupta, M. K.; Sawalha, M. F.; Ohira, S.-I.; Idowu, A. D.; Dasgupta, P. K. Anal. Chem. 2010, 82, 3467-3473. (7) Bolea, E.; Laborda, F.; Castillo, J. R.; Sturgeon, R. E. Spectrochim. Acta, Part B 2004, 59, 505-513. (8) Zhang, W.; Su, Z.; Chu, X.; Yang, X. Talanta 2010, 80, 2106-2112. (9) Sáenz, M.; Fernández, L.; Domínguez, J.; Alvarado, J. Spectrochim. Acta, Part B 2012, 71, 107-111. (10) Zhang, W. B.; Yang, X. A.; Xue, J. J.; Wang, S. B. J. Anal. At. Spectrom. 2012, 27, 928-936. (11) Arbab-Zavar, M. H.; Chamsaz, M.; Youssefi, A.; Aliakbari, M. Anal. Sci. 2012, 28, 717-722. (12) Ding, W.-W.; Sturgeon, R. Spectrochim. Acta, Part B 1996, 51, 1325-1334. (13) Ding, W.-W.; Sturgeon, R. J. Anal. At. Spectrom. 1996, 11, 225-230. (14) Jiang, X.; Gan, W.; Han, S.; He, Y. Spectrochim. Acta, Part B 2008, 63, 710-713. (15) Zhang, W.; Yang, X.; Chu, X. Microchem. J. 2009, 93, 180-187. (16) Jiang, X.; Gan, W.; Wan, L.; Deng, Y.; Yang, Q.; He, Y. J Hazard Mater 2010, 184, 331-336. (17) Denkhaus, E.; Golloch, A.; Guo, X. M.; Huang, B. J. Anal. At. Spectrom. 2001, 16, 870-878. (18) Fridman, G.; Friedman, G.; Gutsol, A.; Shekhter, A. B.; Vasilets, V. N.; Fridman, A. Plasma Process. Polym. 2008, 5, 503-533. (19) Staack, D.; Fridman, A.; Gutsol, A.; Gogotsi, Y.; Friedman, G. Angew. Chem. Int. Ed. 2008, 47, 8020-8024. (20) Yan, T.; Zhong, X.; Rider, A. E.; Lu, Y.; Furman, S. A.; Ostrikov, K. Chem. Commun. 2014, 50, 3144-3147. (21) Nováková, E.; Rychlovský, P.; Resslerová, T.; Hraníček, J.; Červený, V. Spectrochim. Acta, Part B 2016, 117, 42-48. (22) Leng, A.; Lin, Y.; Tian, Y.; Wu, L.; Jiang, X.; Hou, X.; Zheng, C. Anal. Chem. 2017, 89, 703-710. (23) Richmonds, C.; Witzke, M.; Bartling, B.; Lee, S. W.; Wainright, J.; Liu, C.-C.; Sankaran, R. M. J. Am. Chem. Soc. 2011, 133, 17582-17585. (24) Witzke, M.; Rumbach, P.; Go, D. B.; Sankaran, R. M. J. Phys. D: Appl. Phys. 2012, 45, 442001-442005. (25) Zhang, W.; Yang, X. Anal. Chim. Acta 2008, 611, 127-133. (26) Rumbach, P.; Witzke, M.; Sankaran, R. M.; Go, D. B. J. Am. Chem. Soc. 2013, 135, 16264-16267. (27) Yi, Y.; Zhou, J.; Guo, H.; Zhao, J.; Su, J.; Wang, L.; Wang, X.; Gong, W. Angew. Chem. Int. Ed. 2013, 52, 8446-8449. (28) Wang, A.; Qin, M.; Guan, J.; Wang, L.; Guo, H.; Li, X.; Wang, Y.; Prins, R.; Hu, Y. Angew. Chem. Int. Ed. 2008, 47, 6052-6054. (29) Zhu, Z.; Wu, Q.; Liu, Z.; Liu, L.; Zheng, H.; Hu, S. Anal. Chem. 2013, 85, 4150-4156. (30) Liu, Z.; Zhu, Z.; Wu, Q.; Hu, S.; Zheng, H. Analyst 2011, 136, 4539-4544.



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(31) Zhu, Z.; Liu, L.; Li, Y.; Peng, H.; Liu, Z.; Guo, W.; Hu, S. Anal. Bioanal. Chem. 2014, 406, 7523-7531. (32) Zhu, Z.; Chan, G. C.-Y.; Ray, S. J.; Zhang, X.; Hieftje, G. M. Anal. Chem. 2008, 80, 7043-7050. (33) Zhu, Z.; He, Q.; Shuai, Q.; Zheng, H.; Hu, S. J. Anal. At. Spectrom. 2010, 25, 1390-1394. (34) Zhu, Z.; Huang, C.; He, Q.; Xiao, Q.; Liu, Z.; Zhang, S.; Hu, S. Talanta 2013, 106, 133-136. (35) Liu, X.; Zhu, Z.; He, D.; Zheng, H.; Gan, Y.; Belshaw, N. S.; Hu, S.; Wang, Y. J. Anal. At. Spectrom. 2016, 31, 1089-1096. (36) Greda, K.; Swiderski, K.; Jamroz, P.; Pohl, P. Anal. Chem. 2016, 88, 8812-8820. (37) Yang, M.; Xue, J.; Li, M.; Han, G.; Xing, Z.; Zhang, S.; Zhang, X. Talanta 2014, 126, 1-7. (38) Ribeiro, A. S.; Vieira, M. A.; Willie, S.; Sturgeon, R. E. Anal. Bioanal. Chem. 2007, 388, 849-857. (39) Li, R.; Yan, H.; Yang, X.; Li, Z.; Guo, Y. J. Anal. At. Spectrom. 2011, 26, 1488-1493. (40) Greda, K.; Jamroz, P.; Pohl, P. J. Anal. At. Spectrom. 2012, 28, 134-141. (41) Guo, X.; Guo, X. Chinese. J. Anal. Chem 1998, 26, 674-678. (42) Vargas-Razo, C.; Tyson, J. F. Fresenius J. Anal. Chem. 2000, 366, 182-190.

17 18


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1

Figure legends and table captions:

2

Figure 1. Schematics diagram of the instrumental setup of the continuous-flow

3

PEVG-AFS.

4

Figure 2. Typical temporal profile of fluorescence signal of 0.2 µg L-1 Cd2+ (a) and 5

5

µg L-1 Zn2+ (b) by continuous-flow PEVG-AFS. (electrolyte, HCl (pH=3.2); discharge

6

current, 10 mA; Ar gas flow rate, 300 mL min-1; H2 gas flow rate, 50 mL min-1;

7

sample flow rate, 2.5 mL min-1)

8

Figure 3. Effect of the atomizer temperature on the absorption signal of 20 µg L-1

9

Cd2+ and 100 µg L-1 Zn2+ obtained by PEVG-AAS.

10

Figure 4. Effect of different electrode materials on the fluorescence signal intensities

11

of Cd (a)；influence of pH (b), and the effect of discharge current (c) on fluorescence

12

signal of 0.2 µg L-1 Cd2+ and 5 µg L-1 Zn2+.

13

Figure 5. Effect of H2 flow rate (a), Ar flow rate (b), and sample flow rate (c) on the

14

fluorescence signal of 0.2 µg L-1 Cd2+ and 5 µg L-1 Zn2+.

15 16

Table 1. Influence of concomitant ions on recovery of response from 0.5 µg L-1

17

cadmium and 10 µg L-1 zinc

18

Table 2. Analytical characteristics of element determination with PEVG and other

19

vapor generation techniques

20

Table 3. Analytical results for Cd/Zn determination in certified reference materials

21



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

1 2

Figure 1. Schematics diagram of the instrumental setup of the continuous-flow

3

PEVG-AFS.

4 5 6 7


Page 22 of 30

Page 23 of 30

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1 2

Figure 2. Typical temporal profile of fluorescence signal of 0.2 µg L-1 Cd2+ (a) and 5

3

µg L-1 Zn2+ (b) by continuous-flow PEVG-AFS. (electrolyte, HCl (pH=3.2); discharge

4

current, 10 mA; Ar gas flow rate, 300 mL min-1; H2 gas flow rate, 50 mL min-1;

5

sample flow rate, 2.5 mL min-1)



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

1 2

Figure 3. Effect of the atomizer temperature on the absorption signal of 20 µg L-1

3

Cd2+ and 100 µg L-1 Zn2+ obtained by PEVG-AAS.


Page 24 of 30

Page 25 of 30

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1 2

Figure 4. Effect of different electrode materials on the fluorescence signal intensities

3

of Cd(a)；influence of pH (b), and the effect of discharge current (c) on fluorescence

4

signal of 0.2 µg L-1 Cd2+ and 5 µg L-1 Zn2+.

5



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

1 2

Figure 5. Effect of H2 flow rate (a), Ar flow rate(b), and sample flow rate(c) on the

3

fluorescence signal of 0.2 µg L-1 Cd2+ and 5 µg L-1 Zn2+.


Page 26 of 30

Page 27 of 30

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1

Table 1. Influence of concomitant ions on recovery of response from 0.5 µg L-1

2

cadmium and 10 µg L-1 zinc Interferences

Concentration -1

Na+ +

K

Cu2+ 2+

Co

Ni2+ Sn2+ 2+

Pb

2+

Zn

Cd2+

Recovery(%)

(mg L )

Cd

Zn

1

97.9 ± 1.5

87.7±2.4

10

17.5 ± 0.3

32.0±1.5

1

99.6 ± 2.5

83.6±4.0

10

21.0 ± 1.7

7.1±0.4

1

98.5 ± 2.1

80.0±2.6

10

70.7 ± 1.5

30.9±1.7

1

106.1 ± 2.5

78.7±3.6

10

41.4 ± 1.0

25.7±4.7

1

100.5 ± 2.9

105.1±4.9

10

18.6 ± 0.7

51.7±4.2

1

98.7 ± 1.7

92.9±4.6

10

79.8 ± 2.0

40.4±3.7

1

102.9 ± 1.9

109.1±4.5

10

96.0 ± 3.4

53.2±1.8

1

103 ± 2.4

—

10

76 ± 2.5

—

1

—

89.0±3.2

10

—

60.2±4.3

3 4



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

Page 28 of 30

1

Table 2. Analytical characteristics of element determination with PEVG and other

2 3

vapor generation techniques VG technique

LOD -Zn

Detector

References

-1

Detector

References

AAS

42

-1

(µg L ) NaBH4 + acid

LOD-Cd (µg L )

2.0

AFS

41

0.016

EcHG

11

AAS

11

0.15

AFS

10

DBD plasma-CVG

0.2

AFS

31

0.03

AFS

29

This work(PEVG)

0.3

AFS

0.003

AFS

4


Page 29 of 30

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1

Table 3. Analytical results for Cd/Zn determination in certified reference materials Sample GBW07311 (Sediment) GBW07401(Soil)

Element Cd Cd

Certified value -1

Found value -1

2.3 ± 0.2 µg g

2.2 ± 0.1 µg g

-1

Standard addition

-1

4.3 ± 0.4 µg g

4.1 ± 0.2 µg g -1

Method

External calibration -1

GBW10045 (Rice)

Cd

0.19 ± 0.02 µg g

0.20 ± 0.01 µg g

External calibration

GSB 07-1184-2000 (water)

Zn

0.507±0.021 mg L-1

0.493±0.032 mg L-1

External calibration

2 3



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1

For TOC only

2

3 4 5

Generation of volatile cadmium and zinc species based on plasma electrochemical processes.


Page 30 of 30

Generation of Volatile Cadmium and Zinc Species Based on Solution

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