Sensitive Determination of Cd in Small-Volume Samples by

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Sensitive determination of Cd in small-volume samples by miniaturized liquid drop anode atmospheric pressure glow discharge optical emission spectrometry Piotr Jamroz, Krzysztof Greda, Anna Dzimitrowicz, Krzysztof Swiderski, and Pawel Pohl Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 06 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Analytical Chemistry

Sensitive determination of Cd in small-volume samples by miniaturized liquid drop anode atmospheric pressure glow discharge optical emission spectrometry

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

Wroclaw University of Science and Technology, Faculty of Chemistry, Division of Analytical Chemistry and Chemical Metallurgy, Wybrzeze Stanislawa Wyspianskiego 27, 50-370 Wroclaw, Poland

*

Corresponding author. Tel.: +48-71-320-3807, fax: +48-71-320-2494.

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

Abstract A novel liquid drop anode (LDA) direct current atmospheric pressure glow discharge (dcAPGD) system was applied for direct determination of Cd in small-volume samples solutions by optical emission spectrometry (OES). The microdischarge was generated in open-to-air atmosphere between a solid pin type tungsten cathode and liquid drop placed on a graphite disk anode. The arrangement of the graphite disk placed on a PTFE chip platform as well as the solid pin type cathode was simple and robust. The limit of detection (LOD) of Cd for the developed LDA-APGD-OES method was 0.20-0.40 µg L-1 and 7 µg kg-1 for liquid and solid samples, respectively, while precision (as the relative standard deviation for the repeated measurements) was within 2-5%. By using the liquid drop of 50 µL, the linearity range of 11000 µg L-1 was achieved. The effect of addition of the low-molecular weight (LMW) organic compounds, easily ionized elements (EIEs), i.e. Ca, K, Mg, and Na, as well as the foreign ions 1 ACS Paragon Plus Environment


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(Al, Cu, Fe, Mn, Zn) to the solution on the in-situ atomization and excitation processes occurred during operation of the LDA-APGD system and the response of Cd was studied. Validation of the proposed method was demonstrated by analysis of the Lobster hepatopancreas (TORT-2), pig kidney (ERM-BB186) and ground water (ERM-CA615) certified reference materials (CRMs) and recoveries of Cd from water samples spiked with 25 µg L-1 of Cd. Very good agreement between the found and certified values of Cd in the CRMs (the recoveries were within the range of 96.3-99.6%) indicated trueness of the method and its reliability for determination of traces of Cd. In the case of the spiked water samples the recoveries obtained were in the range from 95.2 to 99.5%.

Introduction Determination and monitoring of cadmium (Cd) in environmental, food and clinical samples is very important due to extremely high toxicity of this element and its possibility to accumulate in the environment as well as in human or animal organs (e.g. liver, kidney).1 Apparently from literature, atomic spectrometry techniques, including inductively coupled plasma optical emission/mass spectrometry (ICP-OES/ICP-MS), flame/graphite furnace atomic absorption spectrometry (F-AAS/GF-AAS) or atomic fluorescence spectrometry (AFS), have been widely applied for determination of Cd in various samples and materials. However, these techniques typically require bulky and costly devices that are expensive and energy-intensive in use due to large power and inert or flammable gases consumption. They also require special sample introduction systems, comprising nebulizers along with spray chambers in case of pneumatic nebulization or a graphite furnace along with graphite tubes in case of electrothermal vaporization. Therefore, development of compact, low-cost and lowpower consumption portable instruments for element analysis of liquid samples is of special importance and worth of investigation.2,3

2 ACS Paragon Plus Environment

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Recent work indicates that atmospheric pressure microplasmas and microdischarges can be successfully applied as efficient excitation sources in atomic and molecular optical emission spectrometry.4,5 Among various such sources, the greatest interest has risen in miniaturized atmospheric pressure glow discharge (APGD) generated in contact with a solution of the flowing liquid cathode (FLC).5 In this case, it is possible to carry out element analysis of samples solutions without any special sample introduction system, e.g. a nebulizer with a spray chamber, because the analytes are inherently transported into the discharge phases due to the discharge-sustaining processes. Additionally, this discharge is fully operated in open-to-air atmosphere with no need of additional discharge-supporting gas. The miniaturized APGD systems are inexpensive in manufacture, maintenance and operation (power consumption of ~10-50W), therefore, the cost of element analysis with such systems are also very low.5 Other alternative microplasma sources that can be used for analysis of liquid microsamples in the non-flow through systems are dielectric barrier discharge (DBD)3,6,7, glow discharge (GD)8 and others, e.g. a microplasma device (MPD).9 To the best of our knowledge, only one non-flow through system based on DBD was applied so far for direct determination of Na, K, Zn and Cd in small-volume samples due to transport of the analytes into the discharge phase by the inherent processes related to its operation.3 In other systems cited above, microsamples were transported to the detector with the aim of the additional sample introduction systems.6-9 Accordingly, Li et al.8 applied single drop solution electrode glow discharge assisted chemical vapor generation (SD-SEGD-CVG) that was responsible for transport of Cd and Zn to AFS, ICP-OES and ICP-MS prior to their determination in the liquid microsamples. In another work,6 the small-volume samples and the analytes included in them were electrothermally vaporized (ETV) and introduced to DBD (ETV-DBD-OES) prior to determination of Cd and Zn. Weagant et al.9 also used ETV and coupled it with MPD for element analysis of the small-volume samples. In addition, to



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decrease the DBD overload with the sample components, a pneumatic micronebuliser was applied to introduce the small amounts of the samples in their direct analysis on the concentration of Cd.7 In our recent paper10, a novel system of APGD generated in contact with the flowing liquid anode (FLA) was developed and successfully applied for determination of Ag, Cd, Pb, Tl and Zn in various environmentally relevant samples with extremely high sensitivity and detectability. In the present study, a non-flow through, low-power, low-cost and very straightforward portable system based on the liquid drop anode (LDA) placed on a graphite disk was developed for the first time for reliable determination of traces of Cd in the smallvolume samples. The operating parameters were initially optimized in order to stably sustain the LDA-APGD system. The effect of addition of the low-molecular weight (LMW) organic compounds as well as the easily ionized elements (EIE’s) was studied in reference to analytical performance of the examined system with LDA-APGD.

Experimental Plasma Experimental Setup and Instrumentation The experimental set-up is given in Fig. 1. A high voltage (HV), positively charged direct current (dc) power supply (Dora Electronic Laboratory, Poland), working in a constant current mode (within 0-150 mA), was used to initiate and sustain the discharge. APGD was generated in contact with the LDA in fully opento-air atmosphere. No additional discharge-supporting gas was required. A sharpened solid pin W electrode (external diameter 4 mm, length 10 mm) was used as a grounded cathode without a cooling block. The LDA construction was as follows: a 50-µL aliquot of a sample/standard solution was pipetted onto a graphite disk electrode (external diameter 5 mm, length 3 mm), placed inside a PTFE (polytetrafluoroethylene) chip platform (external diameter 25 mm, thickness 15 mm) (see insert A in Fig. 1 for details). The graphite disk was placed about 1.5


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mm below the edge of the PTFE chip platform, forming a shallow for a small-volume sample, and connected by means of a Pt wire with the positive output of the dc-HV power supply. Additionally, a 50 W 10 kΩ ballast resistor (Tyco, USA) was used in the anode circuit to stabilize the discharge current. The voltage supplied to the LDA was changed within 9001000 V. The Pt wire connection with the dc-HV generator were isolated to avoid any electric shock. The LDA-APGD system combined the in-situ evaporation with maximum efficiency of analyte transport directly into the discharge and excellent excitation capability of the Cd atoms in the discharge zone. The detailed experimental set-up (instrumentation, emission spectra acquisition), as well as reagents, samples and their preparation are given in the Supporting Information.

Results and discussion Optimization of experimental parameters. Size of the drop in the Teflon-graphite platform was found to be critical for stable operation of LDA-APGD. It appeared that the optimal solution volume was 50 µL because the liquid drop was formed inside the shallow of the PTFE chip platform and completely wetted the graphite disk (see Fig. 1). No memory effects were observed when such 50-µL liquid drop was evaporated. Reproducibility of the background-corrected intensity signal for Cd, present in a working standard solution at a concentration of 25 µg L-1, repeatedly recorded during operation of the LDA-APGD system for 50-µL liquid drops of this solution was better than 5% (n=5). Application of the lower volumes of the solutions, i.e. 10 and 20 µL, was useless; the discharge was less stable while evaporation of the drop was relatively short. In case of higher volumes of the solution, i.e. >100 µL, the drop was partially spilled on the Teflon platform and stability of the discharge was either lower. As a result, the Cd response was not enhanced while its reproducibility was deteriorated.



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The effect of solution conductivity and pH on the Cd response at 228.8 nm was also investigated and established to be important in reference to evaporation of the drop, atomization efficiency and other plasma-chemical processes. The maximal Cd response was established at solution conductivity being within 4-6 mS cm-1 and pH of 6-7. Nevertheless, in the pH ranges changed from 4 to 5 and from 8 to 10, the Cd response was also high; the differences in these responses as compared to the maximal signals received at pH 6-7 were lower than 6%. At low solution pH, i.e. 60 mA, the discharge was established to be unstable, while the solid W pin electrode began to immediately overheat. Bearing in mind that the design and operation of the developed LDA-APGD system should be simple, no cooling block of the cathode was used as it was applied in the previously reported FLA-APGD system.10 Therefore, the discharge current within 50-55 mA was considered as optimal in reference to stability of the discharge, efficient atomization and excitation of the Cd atoms and the highest analyte response in the system. With respect to all of this, the discharge


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current of 55 mA, solution conductivity of 5 mS cm-1 and pH of 6 were applied in further experiments.

Effect of addition of low molecular compounds In case of APGDs generated in contact with the FLC, it was established that the presence of the LMW organic compounds5,11 or surfactants5,12 in the solutions introduced to these systems is responsible for significant enhancement of the response of different metals, among others Cd.5,11,12 The LMW organic acids were also desired in the photo chemical vapor generation (photo-CVG) processes of Cd.11 Generally, addition of the LMW organic compounds was recognized to likely promote formation of the H radicals in the liquid phase and/or in the liquid-discharge interfacial zone and change the physical properties of the FLC solutions.5,11,12 Here, for the first time, the effect of addition of the LMW organic compounds, i.e. HCHO, HCOOH and CH3OH, each at a concentration of 2% (v/v), to the analyzed solutions was examined in case of the LDAAPGD system. Admixture of the non-ionic surfactant (Triton X-405 at 0.5% m/v) to the solution was also tested, however, the discharge was very unstable in this case, probably due to a sudden change in surface tension and viscosity of the drop. Thus this aiding substance was neglected in further experiments. It was established that addition of CH3OH to the solution caused an increase in the intensity of the Cd I emission line by about 20% as compared to this acquired for the solution without any LMW organic compound. Contrary, addition of HCHO and HCOOH to the solution caused a fall of the intensity of the Cd I emission line by about 3% and 30%, respectively. The background level in vicinity of the Cd I emission line was found in these conditions to be slightly lower for CH3OH (by ~10%) and HCHO (by ~15%) or higher in case of HCOOH (~20%) as compared to this with no LMW organic compound. Quite similar results in reference to the differences in action of CH3OH and HCOOH on the Cd response were observed by Cai and co-workers,7 who used DBD



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generated on the nozzle of a pneumatic micronebulizer prior to in-situ atomization/excitation and direct analysis of the nebulized solutions. The transient Cd signals, i.e. the intensity of its analytical line, were acquired within 35 s during LDA-APGD operation and drop evaporation along with analyte transport and insitu atomization/excitation in the discharge phase (Fig. 2). As can be seen, there are significant differences in the course of these processes depending of the added LMW compound. The intensity of the Cd I emission line was observed to suddenly grow up and reach the maximal value at about 0.5 s (no LMW compound or HCHO present) or 5.5 s (CH3OH present). Then, the Cd response started to lower but the speed of these changes was different. In case of HCOOH, the Cd response was found to gradually increase with time, reached the highest value at 30 s and next rapidly lowered. It should be noted that in all cases, the mean time required for evaporation of the whole drop and transport of the analyte along with in-situ atomization/excitation in the discharge phase was 32.0±0.5 s (at the discharge current of 50 mA). When the discharge current was lower, i.e. 30 mA, this mean time was higher but less reproducible, i.e. 55±2 s.

Possible in-situ atomization and excitation mechanism In our lately published work it was established that the surface of the FLA solution was irradiated by the high-energy electrons (egas) originating from the discharge phase and accelerated under the influence of the applied potential between the electrodes.10 Similarly in case of DLA-APGD, the surface of the liquid drop, being the LDA of the discharge system, was irradiated by the egas. In these conditions, the hydrated electrons (eaq) and the H radicals could readily be formed.10,13 In consequence, the electron-transfer reactions, e.g. Cd2++2eaq=Cd0, and the direct reduction processes of the Cd(II) ions by the H radicals, e.g. Cd2++2H=Cd0+2H+, in the liquid phase and/or in the interfacial liquid-discharge zone could be responsible for formation of Cd cold vapor and/or


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other molecular Cd species, e.g. CdH2. Transport of such volatile Cd species into the discharge phase and their in-situ atomization/excitation likely contributed to the response of the LDA-APGD system. The maximum response of Cd was observed in the middle part of the discharge, near the solid cathode zone, probably due to the favorable excitation conditions in this part of the discharge. It seems reasonable that addition of CH3OH to the solution could increase its volatility, promote production of the H radicals13 and quenche the highly oxidative OH radicals.14

Analytical figures of merit and application Analytical figures of merit were determined for Cd under the optimized conditions (i.e. solution conductivity of 5 mS cm-1, solution pH of 6, the discharge current of 55 mA, the liquid drop volume of 50 µL). The limit of detection (LOD) was calculated using a 3σ of background criterion, while repeatability of the Cd response was expressed as the relative standard deviation (RSD) of 10 replicate measurements of the integrated intensity of the Cd I emission line. As can been seen in Fig. S2 (Supporting Information), good linear correlation (R2 = 0.9991) was found for the integrated intensity (I) of the Cd I emission line and the concentration (C) of this element in the standards solutions (10 points) in the range of 1-1000 µg L-1. The LOD and measurement precision are given in Table 1 for the conditions when different LMW organic compounds were added to the solutions or no such compounds were added. In the latter case, the LOD and measurement precision were established to be 0.28 µg L-1 and within 2-5%. Slightly better LOD (0.20 µg L1

) and precision (1-4%) were established when 2% CH3OH admixture was used in the

solutions. In general, the LOD of Cd assessed for the developed LDA-APGD-OES method was comparable or better than those reported for other miniaturized discharge systems generated for small-volume samples or commercially available instruments for GF-AAS or ICP-OES (see Table S1 for comparison in Supporting Information). It should also be noted



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that the LOD of Cd determined here for the LDA-APGD-OES system was almost 140-190 times better than this reported for the LF-DBD-OES system generated in contact with liquid film.3 The effect of the EIEs (Ca, K, Mg, Na ions) as well as the foreign ions (Al, Cu, Fe, Mn, Zn), separately added into the solutions, on the Cd response is given in Fig. S3 and Fig. S4, respectively (in Supporting Information). The studied EIEs and foreign ions are the main components of many environmental, food and biological samples. Additionally, the presence of the EIEs in the drop could change the course of the processes taking place during LDAAPGD operation and hence, influence plasma excitation-ionization equilibrium and the in-situ atomization/excitation processes.12 The Cd concentrations in the solutions containing the EIEs at different concentrations and the foreign ions were measured against the simple standards solutions and expressed as recoveries of Cd. It was found that no interferences from the EIEs were observed even if their concentrations were up to 200 mg L-1. The respective recoveries of Cd assessed in these conditions were within 95-104 %. Unfortunately, the recoveries obtained at the highest studied concentration of the EIEs, i.e. 500 mg L-1, were 55%, 41%, 81%, 92%, respectively for Ca, Mg, Na, K, and pointed out necessity to use the standards additions method for calibration. The foreign ions studied here had rather a little influence on the Cd recoveries. The recoveries of Cd (50 µg L-1) from the solutions containing 200-fold higher concentrations of Al, Cu, Fe, Mn and Zn were (the mean value for n=3): 91%, 97%, 92%, 101%, 90%, respectively. Trueness and suitability for trace analysis of the proposed LDA-APGD-OES method was assessed by analysis of the CRMs (TORT-2 and ERM-BB186) and the ground CRM (ERM-CA615), mineral, tap, river and artesian well waters, all spiked with Cd. For simplicity of the sample preparation procedure, no LMW organic compound was added to the samples and standards solutions. The concentrations of Cd, obtained by the two standards additions


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method, in the analyzed CRMs were consistent with the certified values (see Table S2 in Supporting Information). The t-test for comparison with the reference value showed that the obtained results of the Cd concentrations in the TORT-2 and ERM-BB186 CRMs well agreed with the certified values at the 95% confidence level. The Cd concentrations in the ground water (ERM-CA615) CRM as well as tap, mineral, river and artesian well water samples were found to be below the respective LOD, i.e. 0.28 µg L-1. Therefore, these samples were spiked with 25 µg L-1 of the Cd(II) ions and the recoveries of Cd were determined. The Cd concentrations in all spiked waters samples solutions were measured by LDA-APGD-OES against the simple standard solutions. The recoveries assessed for Cd were 99.5±2.8 % (mineral water), 98.1±5.4% (tap water) and 95.2±4.3% (ERM-CA615 CRM) proving very good trueness of the developed LDA-APGD-OES method (see Table S2 and S3 in Supporting Information). However, in the environmental water samples, i.e. river and artesian well waters, the Cd recoveries was worse (respectively 56.3±5.9 % and 79.5±4.8 %), probably due to existence of the complexes of Cd with humic acids. In this case, the two standards additions method was applied for calibration and the recoveries of 97.1±5.1% and 98.3±4.5% for river and artesian well water, respectively, were obtained.

Conclusions A simple and compact microplasma, LDA-APGD-based analytical system, combining the insitu evaporation of the liquid samples with their excellent atomization and excitation capability, was successfully applied for highly sensitive determination of Cd in small-volume samples by OES. The newly developed LDA-APGD-OES method enabled to analyze the liquid microsamples with good detectability, which is comparable to this obtained with ICPOES and GF-AAS. The studied system was found to be resistant to the presence of the relatively high concentrations of Ca, K, Mg and Na (up to 200 mg L-1) as well as such foreign



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ions as Al, Cu, Fe, Mn and Zn in the solutions. Surprisingly, addition of the LMW organic compounds, (i.e. HCHO or HCOOH) to the solutions had no significant effect on the Cd response and analytical performance of the LDA-APGD-OES system. Only in case of CH3OH, the LOD of Cd and the RSD were slightly better than those assessed for the system without any LMW organic compound added to the solutions. Both, precision and trueness of the results of analysis of the CRMs proved reliability of this novel method.

Acknowledgments Research project funded by the National Science Center (NCN), Poland based on decision No. 2014/13/B/ST4/05013. The work was also financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Science and Technology (Poland).

References (1) Sigel, A.; Sigel, H.; Sigel, K.O. Cadmium: From toxicity to essentiality, Springer; Dordrecht, 2013. (2) Yuan, X.; Zhan, X.; Li, X.; Zhao, Z.; Duan, Y. Sci. Rep. 2016, 6, 19417. (3) He, Q.; Zhu, Z.; Hu, S.; Zheng, H.; Jin, L. Anal. Chem. 2012, 84, 4179-4184. (4) Luo, D.; Duan Y. TrAC Trends Anal. Chem. 2012, 39, 254-266. (5) Jamroz, P.; Greda K.; Pohl, P. TrAC Trends Anal. Chem. 2012, 41, 105-121. (6) Jiang, X.; Chen, Y.; Zheng, C.; Hou, X. Anal. Chem. 2014, 86, 5220-5224. (7) Cai, Y.; Zhang, Y.; Wu, D.; Yu, Y.; Wang, J. Anal. Chem. 2016, 88, 4192-4195. (8) Li, Z.; Tan, Q.; Hou X.; Xu, K.; Zheng, C. Anal. Chem. 2014, 86, 12093-12099. (9) Weagant, S.; Chen, V.; Karanassios, V. Anal. Bioanal. Chem. 2011, 401, 2865-2880. (10) Greda, K.; Swiderski, K; Jamroz, P.; Pohl, P. Anal. Chem. 2016, 88, 8812-8820.


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(11) Decker, C.G.; Webb, M.R. J. Anal. At. Spectrom. 2016, 31, 311-318. (12) Greda, K.; Jamroz, P.; Dzimitrowicz, A.; Pohl, P. J. Anal. At. Spectrom. 2015, 30, 154161. (13) Shirai, N.; Uchida, S.; Tochikubo, F. Jap. J. Appl. Phys. 2014, 53, 046202. (14) Jamroz, P.; Greda, K.; Pohl, P.; Zyrnicki, W. Plasma Chem. Plasma Process. 2014, 34, 25-37.



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Fig. 1. The experimental set-up of the LDA-APGD-OES system (not to scale).


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1000 without 800

Intensity (a.u.)

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HCHO HCOOH

600

CH3OH

400

200

0 -5

0

5

10

15

20

25

30

35

40

45

Time (s)

Fig. 2. The effect of addition of low-molecular weight organic compounds on the in-situ atomization and vaporization processes of Cd in the LDA-APGD.



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Table 1. The limit of detection (LOD) of Cd and precision (as RSD in %) in the measurements by LDA-APGD-OES with and without LMW organic compounds added to the solutions. LOD (µ µg L-1)

LOD (pg)

RSD (%)

Aqueous solution

0.28a (7b)

14

2-5

5% (v/v) CH3OH

0.20a

10

1-4

5% (v/v) HCHO

0.26a

13

2-6

5% (v/v) HCOOH

0.40a

20

3-8

Matrix

a

Evaluated for water samples.

b

In µg kg-1. Evaluated for solid samples (0.4 g).


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For Table of Contents Only


Sensitive Determination of Cd in Small-Volume Samples by

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