Flowing Liquid Anode Atmospheric Pressure Glow Discharge as an

Subscriber access provided by Northern Illinois University

Article

Flowing Liquid Anode Atmospheric Pressure Glow Discharge as a Novel Excitation Source for Optical Emission Spectrometry with the Improved Detectability of Ag, Cd, Hg, Pb, Tl and Zn Krzysztof Greda, Krzysztof Swiderski, Piotr Jamroz, and Pawel Pohl Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02250 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

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

Analytical Chemistry

1

Flowing Liquid Anode Atmospheric Pressure Glow Discharge as a Novel Excitation

2

Source for Optical Emission Spectrometry with the Improved Detectability of Ag, Cd,

3

Hg, Pb, Tl and Zn

4 5 6

Krzysztof Greda, Krzysztof Swiderski, Piotr Jamroz, Pawel Pohl*

7

Wroclaw University of Technology, Faculty of Chemistry, Division of Analytical Chemistry

8

and Chemical Metallurgy, Wybrzeze Stanislawa Wyspianskiego 27, 50-370 Wroclaw, Poland

9

*Corresponding author. Tel.: +48-71-320-2494, fax: +48-71-320-2494

10

E-mail address: [email protected] (P. Pohl)

11 12 13

ABSTRACT: A novel atmospheric pressure glow discharge generated in contact with a

14

flowing liquid anode (FLA-APGD) was developed as the efficient excitation source for the

15

optical emission spectrometry (OES) detection. Differences in the appearance and the

16

electrical characteristic of the FLA-APGD and a conventional system operated with a flowing

17

liquid cathode (FLC-APGD) were studied in detail and discussed. Under the optimal

18

operating conditions for the FLA-APGD, the emission from the analytes (Ag, Cd, Hg, Pb, Tl

19

and Zn) was from 20 to 120 times higher as compared to the FLC-APGD. Limits of

20

detections (LODs) established with a novel FLA-APGD system were on average 20 times

21

better than those obtained for the FLC-APGD. A further improvement of the LODs was

22

achieved by reducing the background shift interferences, and as a result, the LODs for Ag,

23

Cd, Hg, Pb, Tl and Zn were 0.004, 0.040, 0.70, 1.7, 0.035 and 0.45 µg L-1, respectively. The

24

precision of the FLA-APGD-OES method was evaluated to be within 2-5% (as the relative

25

standard deviation of the repeated measurements). The method found its application in the

ACS Paragon Plus Environment

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

Page 2 of 38

1

determination of the content of Ag, Cd, Hg, Pb, Tl and Zn in a certified reference material

2

(CRM) of Lobster hepatopancreas (TORT-2), four brasses samples as well as mineral water

3

and tea leaves samples spiked with the analytes. In the case of brass samples, a reference

4

method, i.e., inductively coupled plasma optical emission spectrometry (ICP-OES) was used.

5

A good agreement between the results obtained with FLA-APGD-OES and the certified

6

values for the CRM TORT-2, as well as the reference values obtained with ICP-OES for the

7

brasses samples was revealed, indicating the good accuracy of the proposed method. The

8

recoveries obtained for the spiked samples of mineral water and tea leaves were within the

9

range of 97.5-102%.

10 11 12

INTRODUCTION

13

The direct current atmospheric pressure glow discharge (dc-APGD) is a kind of the

14

excitation source that has been extensively explored in optical emission spectrometry (OES)

15

by Cserfalvi and Mezei since 1993.1,2 As they observed at the very beginning, the emission

16

from the excited Ca, Mg and Na atoms can be observed only when the analyzed solutions

17

acted as a flowing liquid cathode (FLC). In this case, the analyzed solution is bombarded with

18

the positive ions, and in a consequence of the sample sputtering, the dissolved metals can be

19

transferred to the discharge phase where the excitation processes occur. At present, the

20

number of interesting applications of different FLC-APGD systems is large and includes the

21

analysis of various water samples, including tap, mineral, river and waste waters (Ca, K, Mg,

22

Na, Zn),3 river and waste waters (Cr),4 snow, tap and well waters (In, Rh, Te),5 ground waters

23

(Tl),6 as well as brines (Ca, K, Mg, Na),7 human hair and stream sediments (Cd, Cr, Hg,

24

Pb),8,9 honeys (Ca, Cu, Fe, K, Li, Mg, Mn, Na, Rb, Zn),10 titanium dioxide (Ag, Ca, Cu, Fe,


2

Page 3 of 38

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

K, Li, Mg, Na, Pb),11 zirconium-based alloys (Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb),12,13

2

colloidal silica (K, Li, Mg, Na)14.

3

For the APGD operated with a flowing liquid anode (FLA), the solution surface is

4

struck by the electrons, which are not able to sputter the sample. As a result, no emission from

5

the excited atoms of the dissolved metals can be observed.2 For that reason, the FLA-APGD

6

has been almost completely forgotten and the attention of the researchers exclusively focused

7

on the FLC-APGD.15,16 At this point it should be noted that some scientists studied the

8

electrical characteristic and the spectroscopic parameters of the FLA-APGD,17,18 however, for

9

a long time it has not been used as the excitation source for the spectrochemical analysis by

10

OES. It is also worth to mention about the papers showing the analytical application of the

11

alternating current (ac) APGD,19,20 but it generates a small interest as compared with the dc-

12

APGD.

13

In recent years, it has been reported that the redox reactions taking place at the

14

discharge-solution interface of the FLC-APGD system can lead to the production of the

15

volatile species of Hg, I and Os.21-23 It is suspected that this phenomenon may also occur in

16

the case of other elements, e.g., Ag, Pb and Se.24 It can be expected that the inversion of the

17

electrodes polarity would result in intensifying the reduction processes and hence, a greater

18

flux of the volatile species would be generated. Very recently, Liu et al. demonstrated a FLA-

19

APGD system for measuring Cd and Zn.25 As compared to the conventional FLC-APGD

20

systems, the intensities of the atomic emission lines of both metals were significantly

21

enhanced and the limits of detection (LODs) of Cd and Zn were improved by at least one

22

order of magnitude.

23

Since the above-cited study was restricted only to Cd and Zn, we intend to deepen this

24

topic and extend the application of the FLA-APGD for other elements. Therefore, in this

25

paper, the differences in the appearance and the electrical characteristic of the FLA-APGD


3


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 4 of 38

1

and the FLC-APGD were studied in detail. The intensities of the most prominent atomic

2

emission lines of 28 elements, acquired using both APGD systems, were compared and the

3

operating parameters were optimized. Finally, the analytical performance of FLA-APGD-

4

OES was assessed and the optimized and validated, novel FLA-APGD-OES method was

5

applied to determine Ag, Cd, Hg, Pb, Tl and Zn in a certified reference material (CRM) of

6

Lobster hepatopancreas (TORT-2) and four brasses samples. In addition, mineral water and

7

tea leaves samples spiked with Ag, Cd, Hg, Pb, Tl and Zn were analyzed to evaluate the

8

recoveries of the analytes. A reference method, i.e., inductively coupled plasma optical

9

emission spectrometry (ICP-OES) was used to verify the reliability of the described method

10

in the case of analysis of the brasses samples.

11 12 13

EXPERIMENTAL SECTION

14

Instrumentation. The APGD was sustained and stably operated in an open-to-air discharge

15

chamber between a sharpened W rod (OD = 4 mm) and a solution of the flowing liquid

16

electrode that could work as the cathode (FLC) or the anode (FLA) (Fig. 1). In the FLA-

17

APGD system, the W electrode worked as the cathode, hence it was bombarded with the high

18

energy ions, which caused its incandescence and erosion. To extend the life time of the latter

19

electrode, it was inserted into a water cooling block (a tap water flow rate was approx. 300

20

mL min-1). Using a peristaltic pump (Masterflex L/S, Cole-Parmer, UK), the solutions of

21

standards and samples were delivered to the discharge chamber at a flow rate of 3.0 mL min-1

22

through a quartz tube (ID = 2 mm/ OD = 4 mm) that was inserted into a graphite tube (ID = 4

23

mm/ OD = 8 mm). The edge of the graphite tube was 0.5 mm below the edge of the quartz

24

tube. The distance between both electrodes, so called the discharge gap, was 5 mm. The

25

voltage was supplied by a high voltage generator (Dora, Poland), operated in a constant


4

Page 5 of 38

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

current mode and equipped with built-in voltmeter and ammeter. The Pt wires attached to the

2

graphite tube and the W rod provided the electrical contact. To stabilize the APGD, a ballast

3

resistor (2 kΩ) was connected into an electric circuit.

4

Using an achromatic quartz lens (f = 60), the radiation emitted by the APGD was

5

imaged (1:1) on the entrance slit (10 µm) of an imagining spectrograph (Shamrock 500i,

6

Andor, UK), equipped with a holographic grating (1800 lines mm-1) and a UV-Vis CCD

7

camera (Newton DU-920P-OE, 1024 × 255 pixels, Andor, UK). The acquisition time was 10

8

s. The intensities of the band heads of the vibration-rotational spectra of the selected

9

molecules (NO, OH, N2) as well as the atomic emission lines of the studied metals were

10

background-corrected.

11

The concentrations of Ag, Cd, Hg, Pb, Tl and Zn in the APGD-treated solutions,

12

overflowing the graphite tube, and the solutions of the wet digested brass samples were

13

measured using an Agilent bench-top ICP-OES instrument, model 720, with an axial plasma

14

torch. The operating parameters recommended by the instrument producer were applied for

15

the measurements (see Table 1). The concentration ranges for the five working standard

16

solutions used for the external calibration were within 0.01-5.0 mg L-1. The intensity readings

17

were background-corrected using a seven-point fitted background correction (FBC)

18

technique.

19 20

Optical Temperatures and Electron Number Density. The spectroscopic parameters, i.e.,

21

rotational (Trot), vibrational (Tvib) and excitation (Texc) temperatures as well as the electron

22

number density (ne), were determined. The Trot(OH) and the Trot(N2) were assessed by fitting

23

the experimental spectra of the OH (A-X) 0-0 emission band (with the band head at 308.9

24

nm) and the N2 (C-B) 0-2 emission band (with the band head at 380.2 nm) with vibration-

25

rotational spectra of these molecules simulated using Lifbase and Specair computer programs,


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

Page 6 of 38

1

respectively. The Tvib(N2) was calculated with the aid of the Boltzmann plot method using the

2

vibration-rotational bands 0-2, 1-3, 2-4 and 3-5 of the N2 (C-B) system. The ne was calculated

3

from the Stark broadening of the Hβ line at 486.13 nm, while the Texc(H) was determined

4

using two-line radiation ratio method for the Hα and Hβ lines. Others details on the

5

spectroscopic parameters determination are presented in our previous works.26,27

6 7

Reagents and Sample Preparation. De-ionized water was used throughout. An ACS grade

8

concentrated HNO3 solution was obtained from Sigma-Aldrich (Darmstad, Germany) while

9

an ACS grade 28-30% NH3×H2O solution was taken from Avantor Performance Materials

10

(Gliwice, Poland). Sigma-Aldrich single-element stock (1000 mg L-1) standard solutions of

11

Ag, Al, Au, Ba, Bi, Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, Hg, In, K, Li, Mg, Mn, Na, Ni, Pb, Rb,

12

Ru, Sn, Sr, Tl and Zn were used to prepare the working standards solutions. The conductivity

13

and the pH of the solutions were adjusted with HNO3 or NH3×H2O solutions and controlled

14

by using an Elmetron pH-meter and conductivity-meter, model CPC 505 (Zabrze, Poland).

15

The CRM of Lobster hepatopancreas (TORT-2) was obtained from the National

16

Research Council of Canada (NTCC). Its samples (0.1 g) were wet digested in a concentrated

17

HNO3 solution (5 mL) in the open-vessel system using a hot plate at 90 oC. The resulting

18

samples solutions were evaporated nearly to dryness and finally reconstituted with water to 10

19

mL. The concentrations of Hg and Pb were determined in the undiluted samples solutions. For

20

the quantification of Cd and Zn, the samples solutions were diluted 2 and 10 times,

21

respectively. FLA-APGD-OES was calibrated using the simple standard solutions. The

22

samples of brasses were obtained from Cuprum (Wroclaw, Poland). Their shredded samples

23

(0.1 g) were wet digested in a diluted (1:1) HNO3 solution (10 mL) in the open-vessel system

24

at room temperature. The resulting samples solutions were appropriately diluted (1:104) with

25

water prior to the analysis. The method of the two standards additions was used for the


6

Page 7 of 38

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

calibration. The black tea leaves (0.5 g) were wet digested on a hot plate using a concentrated

2

HNO3 solution, subsequently followed by the addition of a 30% H2O2 solution (5 mL). The

3

resulting samples solutions were evaporated to near dryness and finally reconstituted with

4

water to 25 mL. Such undiluted samples solutions were analyzed versus the calibration with

5

the simple standard solutions. Mineral water was analyzed without any prior treatment.

6

Because the analyzed mineral water and black tea leaves did not contain Ag, Cd, Hg, Pb, Tl

7

and Zn, the original samples were spiked with a 1000 µg L-1 multi-element solution so as to

8

increase the concentrations of the analytes in the final samples solutions to 5.0 µg L-1.

9

Prior to the measurements, all the samples solutions were filtered through the 0.45 µm

10

PVDF syringe filters. Their conductivity (4 mS cm-1 or 10 mS cm-1 in the case of Zn) and pH

11

4 were adjusted using HNO3/ NH3×H2O solutions. For each sample matrix, three replicate

12

samples solutions were prepared and measured. The respective reagent blanks were also

13

prepared and considered.

14 15 16

RESULTS AND DISCUSSION

17

Appearance and Electrical Characteristic. Comparing the appearance of the APGD

18

operated in contact with the FLC and the FLA, numerous differences can be observed (Fig.

19

2). As can be seen, the FLC-APGD has a conical shape and a bright pink color, whereas a

20

regular, pale violet roller is formed for the FLA-APGD. In the case of the FLA-APGD, the

21

discharge covered a larger area of the W electrode tip, which was warmed to red when the

22

water-cooling was stopped. In both cases, when the solution contained Tl(I) ions (200 µg L-1),

23

the emission of the greenish light from the liquid-gas boundary region was observed for the

24

FLA-APGD. In the case of the FLC-APGD, much lower emission from the Tl atoms was

25

observed, which indicated a lower performance of this excitation source in reference to the


7


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 8 of 38

1

excitation conditions and the efficiency of the transport of the analytes. Based on the CCD

2

images, it was established that the spatial distribution of the emission from the studied metals

3

(Ag, Cd, Hg, Pb, Tl and Zn) was almost identical in both cases. Indeed, using the same

4

discharge system (see Fig. 1) but reversing the polarity of the electrodes, it was found that the

5

position of the maximums in the intensities distributions of the analytical lines of the studied

6

elements was not affected and always appeared in the solution vicinity. Although it is less

7

significant, in comparison with the conventional FLC-APGD, which sounds noisy, the FLA-

8

APGD remains completely silent.

9

Essential differences between the two types of the APGDs were observed in their

10

voltage-current characteristics (Fig. 3). Despite a constant discharge gap, the burning voltage

11

of the discharge with the FLC was significantly larger than this with the FLA. The FLC-

12

APGD remained stable within the range of the relatively low discharge currents, i.e., 20-70

13

mA, and above this value, the arcing with the graphite tube was observed. By contrast, the

14

FLA-APGD could be generated at 30 mA to 140 mA, and surprisingly, it was more stable

15

when the discharge current exceeded 80 mA.

16 17

Spectra Morphology, Analyte Transport and Plasma Parameters. To evaluate the

18

usability of the FLA-APGD for the analytical applications, its emission spectrum within the

19

range of 200-400 nm was recorded and compared with this acquired for the FLC-APGD (see

20

Fig. 4). It was established that there was no qualitative difference between the spectra; the

21

main components were the emission bands of the NO (A-X), OH (A-X) and N2 (C-B)

22

molecules with the main band heads at 237.0, 308.9 and 337.1 nm, respectively. It should be

23

noted, however, that the intensities of the NO and N2 molecular bands were over a dozen

24

times higher when the discharge was generated in contact with the FLA. A likely reason for

25

this could be a limited air access to the discharge zones in the case of the FLC-APGD, which


8

Page 9 of 38

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

is recognized to be sustained and operated in the saturated water vapor atmosphere.28 Indeed,

2

it was found that for the FLC-APGD system, the weight loss of the solution transferred

3

through the discharge chambers and treated by the APGD, primarily related to the water

4

evaporation, was 16.1±0.4% (at the discharge current = 60 mA, the sample flow rate = 3 mL

5

min-1). For comparison, the same factor for the FLA-APGD system was merely 5.2±1.3% (the

6

same operating parameters were used). Hence, it was presumed that in the case of the FLC-

7

APGD, the evaporation of the mass flux of water molecules from the surface of the FLC

8

solution was more intense, forming in the near-cathode zone of the discharge a potential

9

barrier that could hinder the diffusion of the surrounding air into its phases.26,29 Moreover, the

10

Trot(N2) determined in the liquid-gas boundary region, considered as an approximation of the

11

kinetic plasma gas temperature17 in the outer layers of APGD, was about 2200 K for the FLC-

12

APGD and merely 1400 K for the FLA-APGD (see Table 2). Additionally, the temperatures

13

of the solutions treated by both APGD systems were determined. Higher temperatures of the

14

solutions (39-42 oC) were noted for the FLC-APGD system, while for the FLA-APGD system

15

it was only 30-32 oC. This also indicated that the flow heat flux from the discharge to the

16

liquid phase was higher in the case of the FLC system. Bearing all this in mind, it can be

17

assumed that the FLC-APGD produces a relatively large mass flux of the water vapor that

18

effectively limits the diffusion of N2 into the core of the discharge. In the case of the APGD

19

operated with the FLA, where the surface of the flowing solutions is irradiated with electrons,

20

the evaporation of the mass flux of water molecules, as measured in the present work, was

21

over 3-fold lower. Hence, it was presumed that the air surrounding the discharge column

22

easier penetrated its phases. In a consequence, the formation of the excited NO and N2

23

molecules and the resultant intensities of their molecular bands in the FLC-APGD emission

24

spectrum, as compared to the FLA-APGD, were significantly lower.26 It also seems


9


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 10 of 38

1

reasonable that the expanding water vapor mass flux may be responsible for the mentioned

2

noise accompanying the FLC-APGD.

3

Single-element, 1 mg L-1 standard solutions prepared in 0.1 mol L-1 HNO3 were

4

introduced to the FLA-APGD and the FLC-APGD (operated in the same discharge system)

5

and the intensities of the atomic emission lines of 28 different metals within the spectral range

6

of 200-860 nm, including alkali metals (Cs, K, Li, Na, Rb), alkaline earth metals (Ba, Ca, Mg,

7

Sr), d-block metals (Ag, Au, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Ru, Zn) and p-block metals (Al,

8

Bi, Ga, In, Pb, Sn, Tl), were measured. Among the studied metals, only weak emission from

9

some alkali metals (K, Li, Na) and extraordinary emission from Ag, Cd, Hg, Pb, Tl and Zn

10

was observed in the case of the FLA-APGD (see Fig. 5). For the remaining metals, practically

11

no intensities were observed for their most prominent atomic emission lines. To explain this

12

behavior, the percentage of the metals released from the solution to the discharge phase of the

13

FLA-APGD and the FLC-APGD was assessed. For this purpose, the solutions treated by both

14

APGDs and overflowed the graphite tube were collected and analyzed by the ICP-OES

15

method. Considering the concentrations of Ag, Cd, Hg, Pb, Tl and Zn in the initial solutions

16

introduced to the FLA-APGD and the FLC-APGD, and the mass losses of these solutions

17

after the APGD treatment, related to the water evaporation in both discharge systems, the

18

respective transport efficiencies of the analytes were calculated. On the basis of the results

19

achieved, it was found that the emission enhancement for Ag, Cd, Hg, Pb, Tl and Zn in the

20

case of the FLA-APGD corresponded to the increase in the efficiency of their transport. For

21

example, in the case of Cd, only 1.1±0.5% of its initial amount was released from the FLC

22

and as many as 19.0±0.6% from the FLA (the discharge current = 60 mA for both discharge

23

systems). On the other hand, in the case of Hg, the efficiency of its transport was higher for

24

the FLC-APGD (33.1±4.7%) than for the FLA-APGD (12.8±1.2%). Nevertheless, the atomic


10

Page 11 of 38

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

emission for Hg in the case of the FLA-APGD was considerably greater, likely indicating the

2

better excitation conditions in the discharge operated with the FLA.

3

Considering the FLC-APGD, it is believed that the metals are transferred to the

4

discharge phase owing to the bombardments of the solution surface with the positive ions and

5

the sputtering of the solution components.1,30 In the case of the FLA-APGD, the solution

6

surface is irradiated by the electrons and hence the sputtering efficiency is negligible. For this

7

reason, for non-volatile species forming metals like alkali (Cs, K, Li, Na, Rb) and alkaline

8

earth (Ba, Ca, Mg, Sr) metals, the highest intensities of their analytical lines were observed

9

for the FLC-APGD system. For other metals, i.e., Ag, Cd, Hg, Pb, Tl and Zn, the highest

10

intensities of their analytical lines were observed for the FLA-APGD system. This may

11

indicate that the latter group of metals could be transported to the APGD by another mean.

12

Because these metals are known to form different volatile species, e.g., in the reduction

13

reaction with NaBH4, it reasonably seems that some of their volatile species could be

14

produced in the liquid phase and/or the liquid-discharge interfacial zone by reacting with the

15

hydrated electrons and/or the H radicals, which could be readily formed in these conditions.31

16

Liu et al.25 tried to identify the volatile Cd and Zn compounds that are formed in the APGD

17

operated with the FLA containing the ions of these metals. The ultimate volatile species of Cd

18

and Zn were swept by an Ar stream and introduced into a quartz furnace atomizer coupled

19

with an atomic fluorescence spectrometer. Because signals were measured with the quartz

20

furnace heated by a H2-Ar flame but not when it was at room temperature, the authors implied

21

that the produced volatile species of Cd and Zn were molecular in nature. Afterwards, they

22

tried to characterize these species by gas chromatography mass spectrometry (GC-MS),

23

however, no related compounds were detected. In our opinion, the experiments were vitiated

24

because the volatile species produced in the liquid-gas boundary region were likely

25

immediately atomized in the APGD. Hence, it seems to be impossible to determine the


11


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 12 of 38

1

primary volatile compounds of Cd and Zn formed in the FLA-APGD system. It cannot be

2

either excluded that in the outer and cooler regions of the APGD, the obtained atomic vapors

3

of Cd and Zn may participate in the secondary reactions, producing some compounds,

4

however, they may differ from the species created in the liquid-gas boundary region.

5

Nevertheless, the production of the volatile species of Ag, Cd, Hg, Pb, Tl and Zn in the

6

electron-transfer reactions as well as with the H radicals at the interface of the discharge and

7

the liquid phases is highly probable, leading to the improvement of the efficiency of the

8

transport of the analytes into the discharge zone.

9

The optical temperatures as well as the ne were also measured for the near-liquid

10

electrode zone (see Table 2). These spectroscopic parameters were determined in order to

11

elucidate the excitation conditions in the FLA-APGD and the FLC-APGD. The values of the

12

Tvib(N2) as well as the Texc(H) were comparable for both discharge systems, taking into

13

account the precision of the temperatures determination. A higher value of the Trot(N2) was

14

noted for the FLC-APGD (2200 K) as compared to this achieved for the FLA-APGD (1400

15

K). The values of the Trot(OH) were comparable (~3500 K) for both systems. These optical

16

temperatures may lead to the conclusion that the FLA-APGD system is in a higher non-

17

equilibrium thermodynamic state and that the excitation mechanisms of the N2 and OH

18

molecules in both discharges are different. Considering a probably lower water vapor flux in

19

the FLA-APGD, it was suspected that the ne would be higher. Surprisingly, the value of the ne

20

measured for the FLA-APGD was about 2 times smaller than this one evaluated for the FLC-

21

APGD system, but still, it was high, i.e., 2.4×1014 cm-3. However, it should be noted that the

22

electrons accelerated by the electric filed in the near-liquid zone of the FLA could reach

23

higher energies, and such electrons could be responsible for the efficient excitation of the

24

transported metals in the core of the discharge.


12

Page 13 of 38

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

Although the mechanism by which the metals are transferred to the FLA-APGD still

2

remains unclear, we suppose that this excitation source can be successfully used for the

3

spectrochemical analysis, particularly for the sensitive measurements of Ag, Cd, Hg, Pb, Tl

4

and Zn, for which the transport and the respective signals were incredibly high. To verify this

5

statement, the operating parameters of the FLA-APGD system were optimized in the next

6

step, and its analytical performance with the OES detection was evaluated.

7 8

Optimization of Operating Parameters of the FLA-APGD System. The effect of the

9

solution pH and conductivity of the solution, as well as the discharge current on the intensities

10

of the atomic emission lines of Ag (338.3nm), Cd (228.8 nm), Hg (253.7 nm), Pb (368.3 nm),

11

Tl (377.6 nm) and Zn (213.9 nm) was investigated (Fig. 6). Conductivity and pH of the mixed

12

working standard solutions of Ag, Cd, Hg, Pb, Tl and Zn were adjusted with diluted HNO3

13

and NH3×H2O solutions. Except where otherwise stated, the pH of the working standard

14

solution, its conductivity and the discharge current were around 5-6, 5 mS cm-1 and 120 mA,

15

respectively.

16

For most of the examined metals, the highest intensities of their atomic emission lines

17

was observed above the pH 4. It can be assumed that at the lower pH, the oxidation potential

18

of HNO3 prevents the reduction of the metals ions. This could result in the low emission from

19

the analytes atoms. For comparison, it is well known that for the FLC-APGD, the analyzed

20

solution needs to be strongly acidified. Cserfalvi et al.1,30 proved that a high concentration of

21

the H3O+ ions leads to the reduction of the cathode voltage drop, which facilitates the

22

recombination of the sputtered metals ions with the electrons32,33. The observed differences in

23

the optimal pH values indicate that different mechanisms are responsible for the metals

24

transport in the FLA-APGD and the FLC-APGD.


13


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 14 of 38

1

In the next turn, it was established that under the low conductivity conditions of the

2

working standard solution (< 1 mS cm-1), the FLA-APGD was very unstable and only weak

3

emission from Ag, Cd, Hg, Pb, Tl and Zn could be observed. For most of the studied metals,

4

the highest intensities of the analytical lines were achieved when the conductivity of the

5

working standard solution was around 4-5 mS cm-1. This is likely because the conductivity of

6

the solution can have the effect on the I-V (current versus voltage) characteristic of the

7

system, particularly the midpoint voltage related to its potential drop and the appreciable

8

conduction through the gas phase of the discharge. As shown by Jin et al.,34 when the

9

conductivity of the solution increases for the contact glow discharge electrolysis (CGDE), the

10

midpoint voltage decreases and the migration of the ions is enhanced in these conditions.

11

There were two relevant exceptions to this; the Zn 213.9 nm atomic emission line had the

12

greatest intensity when the conductivity of the solution exceeded 8 mS cm-1, while the

13

intensity of the Ag 338.3 nm atomic emission line was the strongest at the conductivity within

14

a quite narrow range of 1-2 mS cm-1. Although the intensity of the Ag atomic emission line

15

was considerably boosted at the low conductivity, the precision of the measurements was

16

rather poor and the discharge had a tendency to distinguish.

17

Finally, upon the increase of the discharge current, a remarkable enhancement in the

18

intensities of all atomic emission lines was observed. This effect can partly be explained by an

19

improved efficiency of the metals transport, as determined by analyzing the APGD-treated

20

solutions on the remaining content of the metals. For example, at the discharge current equal

21

to 60 mA, about 13% of the initial amount of Hg, 20% of Cd and 30% of Tl was released

22

from the working standard solution and introduced to the discharge phase. By doubling the

23

discharge current, the transport efficiency was established to increase to 42%, 52% and 65%,

24

respectively for Hg, Cd and Tl. Furthermore, the increase in the discharge current was found

25

to cause the greater evaporation of water. As a result of a higher mass flux of the water vapor


14

Page 15 of 38

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

that entered the discharge phase, a potential barrier was formed in the discharge and likely

2

prevented its phases from the penetration with the surrounding air. In these conditions, the

3

formation of the NO and N2 excited atoms was hindered and therefore the emission from

4

these molecules was notably suppressed. The only drawback of the FLA-APGD generated at

5

the high discharge currents (> 120 mA) was its less stability.

6

Taking into account all these outcomes, the following settings of the operating

7

parameters of the FLA-APGD were recognized as optimal and used in the further part of the

8

work: the pH of the standards and samples solutions = 4-7, the conductivity of the solutions =

9

4 mS cm-1 (in the case of Ag, Cd, Hg, Pb, Tl) or 10 mS cm-1 (in the case of Zn) and the

10

discharge current = 120 mA. In the case of the conductivity of the solutions, the first value

11

was a compromise for the simultaneous measurements of Ag and Cd, Hg, Pb and Tl. The

12

need for adjusting the conductivity of the solutions to another value in the case of the

13

measurements of Zn could be considered as a kind of inconvenience in the sample preparation

14

prior to the element analysis by FLA-APGD-OES.

15 16 17

Analytical Performance. Under the optimal operating parameters of the FLC-APGD system

18

and the FLA-APGD system, the sensitivities of the analytical lines of Ag, Cd, Hg, Pb, Tl and

19

Zn and the LODs of the metals were determined (as three times the standard deviation of ten

20

consecutive measurements of a respective blank solution dived by the sensitivity of the

21

calibration curve for a given analytical line) and compared (Table 3). Additionally, for the

22

FLA-APGD, the upper linearity range (ULR) and the repeatability, expressed as the relative

23

standard deviation (RSD) of the replicated (n=10) measurements of the response for Ag, Cd,

24

Hg, Pb, Tl and Zn, were evaluated.


15


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 16 of 38

1

As can be seen, in comparison to the FLC-APGD, the FLA-APGD provides higher

2

sensitivity of the studied metals, i.e., from over 20-times in the case of Pb and Zn to almost

3

120-times in the case of Tl. Unfortunately, due to relatively more intensive vibration-

4

rotational bands of the NO and N2 molecules, the background shift interferences in the case of

5

the FLA-APGD were greater. As a result, the background level and its fluctuation in the

6

vicinity of the analytical lines of Ag, Cd, Hg, Pb, Tl and Zn were higher and hence, the

7

detectability of these metals was not enhanced as much as expected, i.e., the LODs were

8

improved only 3 to 47 times. None of the selected analytical lines of Ag, Cd, Hg, Pb, Tl and

9

Zn was overlapped with the lines of the molecular bands of the NO and N2 molecules. To

10

achieve a further enhancement, an additional horizontal diaphragm, enabling to exclude the

11

light from the near-cathode region that comprises the strongest emission from the NO and N2

12

molecules, was used to decrease the mentioned background shift interferences. By making

13

this, the LODs were much improved and covered the range of 0.004 to 1.7 µg L-1. In this way,

14

they were among the lowest LODs reported for this type of the excitation sources (Table 4). It

15

is worth noting that in the case of Ag, Cd and Tl, their LODs achieved with the FLA-APGD-

16

OES method were competitive with (Hg, Pb, Zn) or much better (Ag, Cd, Tl) than those

17

offered by the ICP-OES instrument (Agilent 720) used in the present work as the reference

18

method.

19

The calibration plots for the analytical lines of Cd and Zn were estimated to be linear

20

(R2>0.995) up to 200 µg L-1, which gives only 2-3 orders of magnitude. The linearity ranges

21

for the analytical lines of other metals varied from 4 to at least 6 orders of magnitude in the

22

case of Tl. The obtained values were similar to those reported in the literature for the APGDs

23

generated in contact with the FLC.37-41 The repeatability of the FLA-APGD measurements

24

was within the range of 2-5% and it was slightly worse than this for the FLC-APGD, which

25

usually was better than 2%.


16

Page 17 of 38

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

The Effect of Foreign Ions. Brass is a metal alloy mainly consisting of Cu and Zn, but it may

3

also contain small proportions of a range of other metals, e.g., Ag, Al, Fe, Mn, Ni and Pb. To

4

evaluate the susceptibility of FLA-APGD-OES to the matrix-induced interferences in the

5

analysis of brass and other metallurgical samples, the effect of the presence of Al, Cu, Fe, Mn

6

and Ni on the measurements of Ag, Cd, Hg, Pb, Tl and Zn was studied (Fig. 7). In addition,

7

the effect of the presence of Ag, Pb and Zn on the measurements of the remaining analytes

8

was also examined. The potential interfering ions were added separately (at a concentration of

9

10 mg L-1) to the single-element working standard solutions (at a concentration of 200 µg L-1)

10

of Ag, Cd, Hg, Pb, Tl and Zn.

11

It was found that in the case of the measurements of Hg and Tl no interferences from

12

any foreign ions, i.e., Ag, Al, Cu, Fe, Mn, Ni, Pb and Zn, were observed; the recoveries

13

assessed for both metals were ~95-105%. The metals being the most susceptible to the matrix

14

effects were Pb and Zn. Simultaneously, when the latter metals used as the interferents, they

15

did not affect the recoveries of other metals. Because of a high content of Cu in the brass

16

samples, it was recognized as potentially the most disruptive interferent (especially in the

17

quantifications of Ag or Pb). Unfortunately, it is difficult to judge the cause of this type of

18

interferences and it rather lies beyond the scope of this work. Certainly, this is related to a

19

complex and unexplained hitherto mechanism of the release of the analyes from the surface of

20

the FLA solution, likely including the charge-transfer reactions leading to the formation of

21

their volatile species and the further atomization processes and/or the spattering of the metals

22

ions and their neutralization by the impact with electrons in the gas phase of the discharge.

23

Considering the first work on the FLA-APGD by Liu et al.,25 and the results of these authors

24

obtained with GFAFS and GC-MS, we can only hypothesize that the source of the

25

interferences could be the secondary reactions and/or processes occurring in the gas phase of


17


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 18 of 38

1

the discharge and resulting in the production of some compounds that cannot be further

2

excited. Nevertheless, the observations made here about the interferences provided the useful

3

information on how to calibrate the element analysis of the brass samples.

4 5

The Samples Analysis. The results of the analysis of the CRM TORT-2 revealed a good

6

accuracy of FLA-AGD-OES. Accordingly, the determined concentrations were 26.6±0.4,

7

0.25±0.02, 0.37±0.02 and 181±3 µg g-1, respectively for Cd, Hg, Pb and Zn, and well

8

corresponded to the certified values for these metals that were 26.7±0.6, 0.27±0.06, 0.35±0.13

9

and 180±6 µg g-1, correspondingly.

10

In the case of the brasses, the preliminary analysis indicated that the content of Hg (in

11

all brasses samples) and Tl (in the brasses samples no. 1, 3 and 4) were below the respective

12

LODs obtained with FLA-APGD-OES. Hence, these samples solutions were spiked with

13

known amounts of Hg and Tl. The prepared samples solutions were analyzed by FLA-APGD-

14

OES and ICP-OES as the dependable reference method. To determine the concentrations of

15

Ag, Cd, Hg, Pb and Tl, the standard addition method was used at two stages, considering the

16

outcomes of the interferences study. For the quantification of Zn, a greater dilution of the

17

samples solutions was needed (1:2×106). Therefore, the matrix effects could be neglected and

18

the external standard calibration curve method was used for determining the Zn content. As

19

can be seen from Table 5, a good agreement between the concentrations of the metals

20

determined with FLA-APGD-OES and the reference values was revealed. Moreover, the

21

recoveries of the metals added to the samples solutions were within the ranges of 100-106%

22

and 95.7-109%, respectively for Hg and Tl. In the case of this analysis, the possible error

23

sources of the combined uncertainty were identified applying the BIMP GUM “bottom-up”

24

approach and included the measurement repeatability, the calibration, the sample preparation

25

repeatability, and the recovery rate. All the relative uncertainties assessed were calculated for


18

Page 19 of 38

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

each analyte and the expanded uncertainties (U) were retrieved by multiplying the combined

2

uncertainty by the factor k=2 (p=95%). The U values for the studied metals were commonly

3

in the range of 15.8-22.3%, except for few cases, in which it was from 33.6 to 45.2%, i.e., for

4

Pb in the brasses samples 1, 2 and 3 or Ag in the brasses samples 3 and 4. A reasonable

5

explanation for such relatively high U values could be the sample preparation step and the

6

necessity of a three-step dilution (20×20×25) of the original solutions of the digested brasses

7

samples.

8

Finally, the recoveries assessed for Ag, Cd, Hg, Pb, Tl and Zn, added to mineral water

9

were also quantitative, i.e., 97.5±1.2% (Ag), 101±1% (Cd), 97.5±2.0% (Hg), 100±2% (Pb),

10

99.1±1.8% (Tl) and 98.9±1.4% (Zn). In the case of tea leaves, the following recoveries were

11

evaluated: 102±1% (Ag), 99.5±0.9% (Cd), 98.9±2.4% (Hg), 101±3% (Pb), 99.3±1.5% (Tl)

12

and 101±2% (Zn). In addition, the concentration of Zn determined by FLA-APGD-OES in

13

teal leaves, i.e., 25.2±0.3 µg g-1, well corresponded to the result established by using ICP-

14

OES, i.e., 24.9±0.2 µg g-1.

15 16 17

CONCLUSIONS

18

It has been shown that, as compared to the FLC-APGD, the FLA-APGD offers greater

19

sensitivity and significantly improves the detectability of Ag, Cd, Hg, Pb, Tl and Zn. The

20

reason for the observed enhancement in the emission from the atoms of these metals could be

21

a higher percentage of the analyte released from the solution to the discharge (the transport

22

efficiency), and the favorable excitation conditions resulting from a smaller concentration of

23

the water vapor in the discharge phase. The developed FLA-APGD system was applied as an

24

independent and self-sufficient excitation source for the analytical OES and showed the very

25

good sensitivity, precision and accuracy in the element analysis of the samples having


19


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 20 of 38

1

different matrices. Certainly, the need for adjusting the pH of the sample solutions and their

2

conductivity could be considered as a kind of inconvenience in the sample preparation prior to

3

the element analysis carried out by this method. In a further work, the coupling of the FLA-APGD with the other plasmas, e.g., ICP or

4 5

microwave induced plasma (MIP), seems to be worthy of consideration.

6 7 8

ACKNOWLEDGEMENTS

9

We gratefully acknowledge the Polish National Science Centre (NCN) for the financial

10

support (UMO-2014/13/B/ST4/05013). This work was also co-financed by a statutory activity

11

subsidy from the Polish Ministry of Science and Higher Education for the Faculty of

12

Chemistry, Wroclaw University of Technology (Poland).

13 14 15

REFERENCES

16

(1) Cserfalvi, T.; Mezei, P. J. Anal. At. Spectrom. 1994, 9, 345-349.

17

(2) Cserfalvi, T.; Mezei, P.; Apai, P. J. Phys. D: Appl. Phys. 1993, 26, 2184-2188.

18

(3) Yu, J.; Yang, S.; Sun, D.; Lu, Q.; Zheng, J.; Zhang, X.; Wang, X. Microchem. J. 2016,

19

128, 325-330.

20

(4) Ma, J.; Wang, Z.; Li, Q.; Gai, R.; Li, X. J. Anal. At. Spectrom. 2014, 29, 2315-2322.

21

(5) Bencs, L.; Laczai, N.; Mezei, P.; Cserfalvi, T. Spectrochim. Acta Part B 2015, 107, 139-

22

145.

23

(6) Shekhar, R.; Madhavi, K.; Meeravali, N. N.; Kumar, S. J. Anal. Methods 2014, 6, 732-

24

740.


20

Page 21 of 38

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

(7) Yang, C.; Wang, L.; Zhu, Z.; Jin, L.; Zheng, H.; Belshaw, N. S.; Hu, S. Talanta 2016, 155,

2

314-320.

3

(8) Li, Q.; Zhang, Z.; Wang, Z. Anal. Chim. Acta 2014, 845, 7-14.

4

(9) Zhang, Z.; Wang, Z.; Li, Q.; Zou, H.; Shi, Y. Talanta 2014, 119, 613-619.

5

(10) Greda, K.; Jamroz, P.; Dzimitrowicz, A.; Pohl, P. J. Anal. At. Spectrom. 2015, 30, 154-

6

161.

7

(11) Wang, Z.; Gai, R.; Zhou, L.; Zhang, Z. J. Anal. At. Spectrom. 2014, 29, 2042-2049.

8

(12) Manjusha, R.; Reddy, M. A.; Shekhar, R.; Kumar, S. J. Anal. Methods 2014, 6, 9850-

9

9856.

10

(13) Manjusha, R.; Reddy, M. A.; Shekhar, R.; Kumar, S. J. J. Anal. At. Spectrom. 2013, 28,

11

1932-1939.

12

(14) Wang, Z.; Schwartz, A. J.; Ray, S. J.; Hieftje, R. G. J. Anal. At. Spectrom. 2013, 28, 234-

13

240.

14

(15) Jamroz, P.; Greda, K.; Pohl, P. Trend. Anal. Chem. 2012, 41, 105-121.

15

(16) Broekaert, J. A. C.; Reinsberg, K.-G. Spectrochim. Acta Part B 2015, 106, 1-7.

16

(17) Bruggeman, P.; Liu, J.; Degroote, J.; Kong, M. G.; Vierendeels, J.; Leys, Ch. J. Phys. D:

17

Appl. Phys. 2008, 41, 215201 (11pp).

18

(18) Xiong, Q.; Yang, Z.; Bruggeman, P. J. J. Phys. D: Appl. Phys. 2015, 48, 424008 (12pp).

19

(19) Huang, R.; Zhu, Z.; Zheng, H.; Liu, Z.; Zhang, S.; Hu, S. J. Anal. At. Spectrom. 2011, 26,

20

1178-1182.

21

(20) Xiao, Q.; Zhu, Z.; Zheng, H.; He, H.; Huang, C.; Hu, S. Talanta 2013, 106, 144-149.

22

(21) Zhu, Z.; Chan, G. C.-Y.; Ray, S. J.; Zhang, X.; Hieftje, G. M.; Anal. Chem. 2008, 80,

23

7043-7050.

24

(22) Zhu, Z.; He, Q.; Shuai, Q.; Zheng, H.; Hu, S. J. Anal. At. Spectrom. 2010, 25, 1390-1394.


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

Page 22 of 38

1

(23) Zhu, Z.; Huang, C.; He, Q.; Xiao, Q.; Liu, Z.; Zhang, S.; Hu, S. Talanta 2013, 106, 133-

2

136.

3

(24) Doroski, T. A.; Webb, M. R. Spectrochim. Acta Part B 2013, 88, 40-45.

4

(25) Liu, X.; Zhu, Z.; He, D.; Zheng, H.; Gan, Y.; Belshaw, N. S.; Hu, S.; Wang, Y. J. Anal.

5

At. Spectrom. 2016, 31, 1089-1096.

6

(26) Greda, K.; Jamroz, P.; Pohl, P., J. Anal. At. Spectrom. 2013, 28, 1233-1241.

7

(27) Jamroz, P., Zyrnicki, W., Pohl, P., Spectrochim. Acta B. 2012, 73, 26-34.

8

(28) Mezei, P.; Cserfalvi, T. Appl. Spectrosc. Rev. 2007, 42, 573-604.

9

(29) Jamroz, P.; Greda, K.; Pohl, P.; Zyrnicki, W., Plasma Chem. Plasma Process. 2014, 34,

10

25-37.

11

(30) Cserfalvi, T.; Mezei, P. Fresenius J. Anal. Chem. 1996, 355, 813-819.

12

(31) Tochikubo, F., Shimokawa, Y., Shirai N., Uchida,S., Jpn. J. Appl. Phys.2014, 53,

13

126201.

14

(32) Mezei, P.; Cserfalvi, T.; Janossy, M. J. Anal. At. Spectrom. 1997, 12, 1203-1208.

15

(33) Mezei, P.; Cserfalvi, T.; Kim, H. J.; Mottaleb, M. A. Analyst 2001, 126, 712-714.

16

(34) Jin, X.-L.; Wang, X.-Y.; Zhang, H.-M.; Xia, Q.; Wie, D.-B.; Yue, J.-J., Plasma Chem.

17

Plasma Process. 2010, 30, 429-436.

18

(35) Greda, K.; Kurcbach, K.; Ochromowicz, K.; Lesniewicz, T.; Jamroz, P.; Pohl, P. J. Anal.

19

At. Spectrom. 2015, 30, 1743-1751.

20

(36) Shekhar, R.; Madhavi, K.; Meeravali, N. N.; Kumar, S. J. Anal. Methods 2014, 6, 732-

21

740.

22

(37) Webb, M. R.; Andrade, F. J.; Hieftje, G. M. J. Anal. At. Spectrom. 2007, 22, 766-774.

23

(38) Webb, M. R.; Andrade, F. J.; Hieftje, G. M. Anal. Chem. 2007, 79, 7807-7812.

24

(39) Shekhar, R.; Karunasagar, D.; Ranjit, M.; Arunachalam, J. Anal. Chem. 2009, 81, 8157-

25

8166.


22

Page 23 of 38

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

(40) Shekhar, R.; Karunasagar, D.; Dash, K.; Ranjit, M. J. Anal. At. Spectrom. 2010, 25, 875-

2

879.

3

(41) Jamroz, P.; Pohl, P.; Zyrnicki, W. J. Anal. At. Spectrom. 2012, 27, 1032-1037.

4


23


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 24 of 38

1

Table 1. Operating parameters for the ICP OES measurements using an Agilent 720

2

spectrometer with the axially viewed plasma Parameter

Setting

Plasma torch

Horizontally oriented, quartz, one-piece with 2.4 mm injector

Sample introduction system

Glass, one-pass cyclonic chamber with OneNeb nebulizer

Optical/detection system

High resolution Echelle-type, 400 mm focal length thermostated polychromator with cooled cone interface (CCI), and custom-designed, image mapping (IMAP) and adaptive integration (AIT) technologies featuring CCD detector

Supplied RF power

1.2 kW

Plasma Ar flow rate

15.00 L min-1

Auxiliary Ar flow rate

1.50 L min-1

Nebulizing Ar flow rate

0.75 L min-1

Sample uptake rate

0.75 mL min-1

Stabilization delay

15 s

Solution uptake delay

30 s

Replicate read time

1s

Replicates

3

Rinse time

10 s

Wavelengths

Ag 328.1 nm, Cd 228.8 nm Hg 253.7 nm, Pb 405.8 nm, Tl 377.6 nm and Zn 213.8 nm

3


24

Page 25 of 38

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

Table 2. The plasma parameters determined for the FLC-APGD and FLA-APGD systems

2

(the discharge current 60 mA and the sample flow rate 3 mL min-1) Trot(N2), K

Trot(OH), K

Tvib(N2), K

Texc(H), K

ne , cm-3

FLC-APGD

2200 ± 100

3500 ± 100

5400 ± 400

5300 ± 500

(5.3±0.8)x1014

FLA-APGD

1400 ± 100

3500 ± 100

5700 ± 300

4900 ± 500

(2.4±0.2)x1014

3


25


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 26 of 38

1

Table 3. The analytical performance of the atmospheric pressure glow discharge generated in

2

contact with the flowing liquid cathode (FLC-APGD) and the flowing liquid anode (FLA-

3

APGD). Element λ (nm)

FLC-APGD

FLA-APGD

Slope

LOD

Slope (a. u.) LODa

(a. u.)

(µg L-1)

ULR

RSDa

(µg L-1)

(µg L-1)

(%)

Ag

338.3

181

1.1

10.7×103

0.055 (0.004)b

500

2.04

Cd

228.8

65.4

2.3

6.49×103

0.049 (0.040)b

200

4.96

Hg

253.7

3.31

86

275

2.4 (0.70)b

5×103

2.26

Pb

368.3

4.18

87

91.4

8.0 (1.7)b

50×103c

4.78

Tl

377.6

50.9

5.8

6.05×103

0.42 (0.035)b

50×103c

2.10

Zn

213.9

26.0

4.9

650

1.9 (0.45)b

200

2.38

4

LOD Limit of detection (3σ criterion, n=10). URL Upper linearity range. RSD Repeatability

5

expressed as the relative standard deviation (n=10).

6

100×LOD.

7

The highest studied concentration.

b

a

For the analytle concentration being

LODs obtained for a system with the reduced background shift interferences.

c

8


26

Page 27 of 38

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

Table 4. The comparison of the limits of detection (LODs) of Ag, Cd, Hg, Pb, Tl and Zn

2

obtained for APGD-OES reported in different papers. LOD (µg L-1)

Element This paper

Reference ICP-OESa

Best reported

3

Ag

0.004

0.1

[24]

0.41

Cd

0.040

0.05

[25]

0.056

Hg

0.70

0.066

[35]

0.85

Pb

1.7

1

[24]

0.92

Tl

0.035

1

[36]

1.6

Zn

0.45

0.14

[25]

0.36

a

Assessed for an Agilent 720 spectrometer.

4


27


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

Page 28 of 38

Table 5. The comparison of the Ag, Cd, Hg, Pb, Tl and Zn content in different brasses samples determined using FLA-APGD-OES and ICPOES. Metal

Ag

Cd

Hg

Pb

Tl

Zn

% (m/m)

Method

1

2

3

4

FLA-APGD-OES

0.0183±0.0004

0.0197±0.0040

0.0430±0.0031

0.0940±0.0590

ICP-OES

0.0195±0.0040

0.0203±0.0034

0.0346±0.0006

0.0871±0.0010

FLA-APGD-OES

0.0175±0.0021

0.0082±0.0001

0.0147±0.0002

0.0204±0.0005

ICP-OES

0.0163±0.0007

0.0086±0.0003

0.0142±0.0003

0.0195±0.0005

FLA-APGD-OES

< 0.007

< 0.007

< 0.007

< 0.007

ICP-OES

< 0.008

< 0.008

< 0.008

< 0.008

FLA-APG`D-OES

5.30±0.49

2.65±0.37

2.37±0.16

6.78±0.07

ICP-OES

5.42±0.05

2.54±0.08

2.42±0.08

6.80±0.10

FLA-APGD-OES

< 0.0004

0.0398±0.0013

< 0.0004

< 0.0004

ICP-OES

< 0.016

0.0372±0.0045

< 0.016

< 0.016

36.7±1.4

37.9±1.2

39.7±0.9

36.6±0.8

36.8±1.1

38.5±1.0

39.3±0.6

36.3±0.8

FLA-APGD-OES a

Reference value

Average values (n=3) ± standard deviation. a Reference value given by Cuprum (Wroclaw, Poland).

28 ACS Paragon Plus Environment

Page 29 of 38

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


Captions for figures

Fig. 1. A schematic diagram of the FLA-APGD system.

Fig. 2. The images of the APGD generated in contact with the flowing liquid cathode (FLC, left) and the flowing liquid anode (FLA, right).

Fig. 3. The current-voltage characteristics of the APGD with the flowing liquid cathode (FLC) (pH=1, flow rate of 3 mL min-1) or the flowing liquid anode (FLA) (pH=1, conductivity 5 mS cm-1, flow rate of 3 mL min-1). A voltage drop across the ballast resistor was subtracted.

Fig. 4. The background spectra of the APGD generated in contact with the flowing liquid cathode (FLC, blue) and the flowing liquid anode (FLA, red) in the spectral range of 200-400 nm.

Fig. 5. The atomic emission lines of Ag, Cd, Hg, Pb, Tl and Zn recorded for the FLA-APGD system.

Fig. 6. The effect of the solution pH (measured at 120 mA and 3 mS cm-1), its conductivity (measured at pH=6 and 120 mA) and the discharge current (measured at pH=6 and 4 mS cm1

) on the intensities of the atomic emission lines of Ag, Cd, Hg, Pb, Tl and Zn acquired for the

FLA-APGD.


29


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 30 of 38

Fig. 7. The effect of the foreign ions (Ag, Al, Cu, Fe, Mn, Ni, Pb or Zn, all at a concentration of 10 mg L-1) on the intensities of the atomic emission lines of Ag, Cd, Hg, Pb, Tl and Zn (expressed as the % recovery) acquired for the FLA-APGD.


30

Page 31 of 38

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


Fig. 1. A schematic diagram of the FLA-APGD system.


31


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 32 of 38

Fig. 2. The images of the APGD generated in contact with the flowing liquid cathode (FLC, left) and the flowing liquid anode (FLA, right).


32

Page 33 of 38

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


Fig. 3. The current-voltage characteristics of the APGD with the flowing liquid cathode (FLC) (pH=1, flow rate of 3 mL min-1) or the flowing liquid anode (FLA) (pH=6, conductivity 5 mS cm-1, flow rate of 3 mL min-1). A voltage drop across the ballast resistor was subtracted.


33


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 34 of 38

Fig. 4. The background spectra of the APGD generated in contact with the flowing liquid cathode (FLC, blue) and the flowing liquid anode (FLA, red) in the spectral range of 200-400 nm.


34

Page 35 of 38

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


Fig. 5. The atomic emission lines of Ag, Cd, Hg, Pb, Tl and Zn recorded for the FLA-APGD system.


35


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 36 of 38

Fig. 6. The effect of the solution pH (measured at 120 mA and 3 mS cm-1), its conductivity (measured at pH=6 and 120 mA) and the discharge current (measured at pH=6 and 4 mS cm1

) on the intensities of the atomic emission lines of Ag, Cd, Hg, Pb, Tl and Zn acquired for the

FLA-APGD.


36

Page 37 of 38

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


Fig 7. The effect of the foreign ions (Ag, Al, Cu, Fe, Mn, Ni, Pb or Zn, all at a concentration of 10 mg L-1) on the intensities of the atomic emission lines of Ag, Cd, Hg, Pb, Tl and Zn (expressed as the % recovery) acquired for the FLA-APGD.


37


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 38 of 38

For TOC only


38

Flowing Liquid Anode Atmospheric Pressure Glow Discharge as an

Recommend Documents