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Ultrasonic nebulization atmospheric pressure glow discharge — Preliminary study
Spectrochimica Acta Part B 121 (2016) 22–27
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Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Ultrasonic nebulization atmospheric pressure glow discharge — Preliminary study Krzysztof Greda ⁎, Piotr Jamroz, Pawel Pohl Wroclaw University of Technology, Faculty of Chemistry, Division of Analytical Chemistry and Chemical Metallurgy, Wybrzeze Stanislawa Wyspianskiego 27, 50-370 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Received 9 March 2016 Received in revised form 20 April 2016 Accepted 28 April 2016 Available online 07 May 2016 Keywords: Ultrasonic nebulization atmospheric pressure glow discharge Elemental analysis Optical emission spectrometry Solution cathode glow discharge
a b s t r a c t Atmospheric pressure glow microdischarge (μAPGD) generated between a small-sized He nozzle jet anode and a flowing liquid cathode was coupled with ultrasonic nebulization (USN) for analytical optical emission spectrometry (OES). The spatial distributions of the emitted spectra from the novel coupled USN–μAPGD system and the conventional μAPGD system were compared. In the μAPGD, the maxima of the intensity distribution profiles of the atomic emission lines Ca, Cd, In, K, Li, Mg, Mn, Na and Sr were observed in the near cathode region, whereas, in the case of the USN–μAPGD, they were shifted towards the anode. In the novel system, the intensities of the analytical lines of the studied metals were boosted from several to 35 times. As compared to the conventional μAPGD–OES with the introduction of analytes through the sputtering and/or the electrospray-like nebulization of the flowing liquid cathode solution, the proposed method with the USN introduction of analytes in the form of a dry aerosol provides improved detectability of the studied metals. The detection limits of metals achieved with the USN–μAPGD–OES method were in the range from 0.08 μg L−1 for Li to 52 μg L−1 for Mn. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The atmospheric pressure glow discharge (APGD) is a kind of excitation source, which has been extensively explored in the analytical optical emission spectrometry (OES) since 1993/94 [1,2]. It is also known in literature as the electrolyte cathode atmospheric glow discharge (ELCAD) [2,3] and the solution cathode glow discharge (SCGD) [4]. One of the most notable advantages of the μAPGD is that no nebulizer or spray chamber is required for the sample solutions' introduction and the transport of the analytes into this excitation source. Analyzed sample solutions are integral components of the discharge, i.e., a liquid cathode that is bombarded by high energy ions. Subsequently, the droplets of the sample solutions are transferred to the negative glow zone of the discharge, where excitation processes occur [5]. The mechanism of the analytes transport is not entirely clear. Some studies suggest that the electrospray-like nebulization process plays a key role in this process [6], however, the sputtering is also considered [5]. The use of this phenomenon made it possible to manufacture simplified and miniaturized analytical systems that found applications in trace element analysis by analytical OES and atomic absorption spectrometry (AAS) [7]. However, one problem encountered in these systems is that the transport efficiency of the analytes to the phases of the discharge remains still relatively low. Cserfalvi et al. [5] studied the intensities of the emission lines of several metals in an ICP–OES system, in which a conventional pneumatic ⁎ Corresponding author. E-mail address: [email protected] (K. Greda).
http://dx.doi.org/10.1016/j.sab.2016.04.008 0584-8547/© 2016 Elsevier B.V. All rights reserved.
nebulizer, commonly used for the analytes transport to the plasma, was replaced by an APGD close-chamber device that converted the analyzed sample solutions into a respective mist. Unfortunately, in comparison to the IC–OES system with the pneumatic nebulizer, the acquired intensities of the emission lines of the studied metals were lower from 3 to even 30 times. In a similar experiment, Zhu et al. [8] examined the transport efficiency of As, Pb, Se and Sn. Although the liquid cathode solutions introduced to the APGD contained these elements at a level of 10 mg L−1, no measurable signals of As, Pb, Se and Sn were recorded by ICP–OES. In our previous paper [9], it was found that the transport efficiency of As and Sb into the μAPGD operated with a small-sized He jet, assessed by determining the residual concentrations of these elements in the liquid cathode solution treated by the microdischarge, did not exceed 1% (As) and 8% (Sb). Simultaneously, the mass loss of the sample solution introduced as the liquid cathode of the discharge system via the sputtering and evaporation of water was up to 18%. Despite the low transport efficiency of different APGD systems, the detectability of elements competitive with those offered by ICP–OES can still be achieved. The detection limits of alkali metals are usually below 1 μg L−1, and for many other elements, e.g., Ag, Cd, Cu, Hg, In, Mg, Mn, Tl and Zn, they do not exceed 10 μg L−1 [3,10–16]. Bearing all this in mind, it can be expected that the improvement of the transport efficiency of the analytes to the discharge would make the APGD a powerful excitation source for analytical OES. Therefore, in the present paper, an original μAPGD system, operating between a small-sized He nozzle jet as the anode and a flowing liquid cathode solution, was coupled with an ultrasonic nebulizer (USN) to
K. Greda et al. / Spectrochimica Acta Part B 121 (2016) 22–27
enhance the transport efficiency of the analytes and the performance of the μAPGD. The resultant analytes—containing dry aerosol was introduced to the discharge through the He nozzle jet anode. The spatial distributions of the atomic emission lines of the selected metals were studied for the coupled USN–μAPGD system and the μAPGD system with the introduction of analytes from the liquid cathode sample solution. Furthermore, the effect of the He gas flow rate and the addition of the non-ionic surfactant to the liquid cathode solution were investigated. Although the results concerning the USN–μAPGD are of a preliminary nature, they let us prove the validity of such a combination. It should also be mentioned that the concept of the introduction of the dry aerosol into the μAPGD seems to be quite rational. Previously, Manard et al. [17] introduced to the liquid sampling (LS)-APGD the sample particles produced by laser ablation (LA) prior to the determination of Ca and Sr by the analytical OES and mass spectrometry (MS). 2. Experimental 2.1. Instrumentation The μAPGD was operated between a small-sized He jet and a flowing liquid cathode solution in an open-to-air discharge chamber (see Fig. 1). The distance between electrodes (i.e., discharge gap) was 5 mm. Using a peristaltic pump (Masterflex L/S, Cole-Parmer, UK), the liquid cathode solution (acidified with 0.1 mol L−1 HCl) was delivered to the discharge chamber at 3.0 mL min−1 through a quartz tube (ID = 2.0 mm) that was inserted into a graphite tube (ID = 4.0 mm); the edge of the graphite tube was 2 mm above the edge of the quartz tube. As compared to our previous work [18], in the present system, the ID of a stainlesssteel nozzle, used for introducing the He jet-supporting gas, was larger, i.e., 750 μm. For the conventional μAPGD, the He jet-supporting gas flow rate was within the range of 50–300 mL min−1 and was regulated using a flow controller and a digital flow meter. The flow rate of 75 mL min−1 was found to provide the best performance in reference to stabilizing the surface of the liquid cathode solution and the discharge column, and was applied throughout the work, unless otherwise stated. In the case of the coupled USN–μAPGD system, a higher flow rate of the He jet-supporting gas was required (300–1500 mL min−1) because it was also used to carry the resultant dry aerosol from the USN device. For both μAPGD systems, a HV-dc power supply (Dora, Poland) was used and operated in a constant current mode. Stable microdischarges were maintained after supplying a voltage of 1300 V (a voltage drop across the ballast resistor was subtracted) to the stainless-steel nozzle through a 5 kΩ ballast resistor. The liquid cathode solution was grounded
Fig. 1. The experimental set-up of the μAPGD systems operated between a small-sized He jet and a flowing liquid cathode solution: a) — the conventional system, b) — the system combined with the USN device.
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through a Pt wire attached to the graphite tube. The resultant discharge current was 40 mA Both microdischarges were ignited by inserting an isolated stainless steel wire into the gap between the electrodes. In the case of the conventional μAPGD system, analytes were introduced to the discharge through the electrospray-like nebulization and/or the sputtering of the flowing liquid cathode sample solutions. For the coupled USN-μAPGD, an additional peristaltic pump was applied to introduce the sample solutions at 3.0 mL min−1 to the ultrasonic nebulizer (U-5000AT +, CETAC Technologies Inc., USA). Nebulization took place in a glass chamber on a transducer unit. Subsequently, the obtained mist was swept out from the chamber by the He jet-supporting gas (at 700 mL min−1, unless otherwise stated), carried to a heated U-tube, vaporized at 140 °C and introduced to a condenser (3 °C). The total length of the glassware module, consisting of the chamber, the U-tube dryer and the condenser, was about 1.5 m. Finally, the resultant dry aerosol was introduced to the μAPGD through a transferring tube attached to the stainless-steel nozzle, acting as the anode. Using a quartz lens (f = 60), the radiation emitted by the μAPGD was imaged (1:1) on the entrance slit (10 μm) of an imagining spectrograph (Shamrock 500i, Andor, UK) equipped with two holographic gratings (1200 and 1800 lines mm−1) and a UV–Vis CCD camera (Newton DU920P-OE, 1024 × 255 pixels, Andor, UK). The acquisition time was 10 s. The intensities of the studied molecular bands as well as the atomic emission lines of the analytes were background-corrected. 2.2. Reagents and sample preparation Compressed He (99.999%) was supplied by Air Products (Poland). Re-distilled water was used throughout. Non-ionic surfactant Triton X-705 and single-element stock (1000 mg L−1) standard solutions of Ca, Cd, In, K, Li, Mg, Mn, Na and Sr were supplied by Sigma-Aldrich (Germany). The analyzed sample solutions containing the studied metals were introduced as the flowing liquid cathode in the case of the μAPGD or through the ultrasonic nebulizer in the case of the coupled UNS–μAPGD system. All these solutions were acidified with ACS grade concentrated HCl (Avantor Performance Materials, Poland) to a final concentration of 0.1 mol L−1. For the USN–μAPGD, the flowing liquid cathode was just a 0.1 mol L−1 HCl solution, used to support the discharge. 3. Results and discussion 3.1. The spatial distribution of the emission The spatial distribution of the intensities of the atomic emission lines of the analytes (Ca, Cd, In, K, Li, Mg, Mn, Na, Sr), within the 5.0 mm discharge gap, was investigated for the μAPGD and the novel coupled USN–μAPGD system. When metals were delivered as the components of the liquid cathode solution, the intensities of their atomic emission lines remained relatively low, likely because the positive metal ions transported from the liquid cathode solution, before recombining to the neutral atoms in the cathode dark space and diffusing into the negative glow region, can be re-attracted to the negatively charged cathode [19]. This process is determined by the pressure in the discharge column, the cathode fall and the average energy of the electrons in the near-cathode region [20]. The distribution profiles of the atomic emission lines of metals reached the maximum in the near cathode region of the discharge, i.e., within the distance of 1–2 mm from the cathode surface (see Fig. 2). These results are consistent with those obtained for other APGD systems operated at higher discharge currents (70– 80 mA) but with narrower discharge gaps (~3 mm) and using metallic rods as anodes instead of the gaseous nozzle jet [7,21,22]. For the USN–μAPGD, where positively charged metal ions were delivered to the discharge in the stream of the He jet-supporting gas as the dry aerosol and through the nozzle anode, the intensities of the atomic emission lines of the studied metals were evidently boosted,
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Fig. 2. The spatial distribution of the intensities of the atomic emission lines of Ca, Cd, In, K, Li, Mg, Mn, Na and Sr acquired for conventional μAPGD–OES and coupled USN–μAPGD–OES. dmax denotes the distance from the liquid cathode solution surface (mm) where the maximum emission occurs.
i.e., higher from several times in the case of In, K, Mg and Na, to even 18, 23 and 35-times for Li, Sr and Ca, respectively (Table 1). The reason for that could be the higher concentrations of the analytes atoms in the near-anode region. It was probably the consequence of the analytes enrichment in the dry aerosol produced by the USN device and a partial removal of the water vapor. Although this explanation seems to be reasonable, because of differences in the operating conditions, i.e., the He jet-supporting gas flow rate in the USN–μAPGD was almost 10-times higher than in the case of the conventional μAPGD, it cannot be explicitly stated. Contrary to the μAPGD, in the USN–μAPGD system, the maxima of the intensities distribution profiles for the atomic emission lines of the studied metals were shifted towards the anode, i.e., they occurred at the distance of 2–4 mm from the liquid cathode solution surface. It can be noticed that in the case of the analytical lines of metals that
intensities were only modestly enhanced, i.e., In, K, Mg and Na, their distribution profiles across the distance between the electrodes had two partially resolved maxima. The first one in the near-cathode region (the negative glow) was lower and the second one, of a greater intensity, was in the anode vicinity (the positive column). Also for other metals, relatively high emission in the near-cathode region can be observed, probably as a result of an increasing flux of the analytes atoms into this discharge zone. 3.2. The effect of the jet-supporting gas flow rate The effect of the jet-supporting gas flow rate on the distribution of the Na I 589.0 nm emission line in the conventional μAPGD was investigated for the range of 50–300 mL min−1. It was found that by increasing
Table 1 Comparison of the sensitivities of the atomic emission lines, the LODs of metals (Ca, Cd, In, K, Li, Mg, Mn, Na, Sr) and the relative standard deviations (RSD) of the net intensities of the analytical lines evaluated for conventional μAPGD–OES and coupled USN–μAPGD–OES methods. Metal
Ca Cd In K Li Mg Mn Na Sr
Sensitivity (a. u. per μg−1 L)
LOD (μg L−1)
RSD (%)
μAPGD
USN–μAPGD
USN–μAPGD/μAPGD ratio
μAPGD
USN–μAPGD
USN–μAPGD/μAPGD ratio
μAPGD
USN–μAPGD
0.12 0.40 2.0 66 69 3.2 0.12 1.5 × 102 0.066
4.0 4.7 2.5 2.3 × 102 1.3 × 103 14 1.6 3.9 × 102 1.5
34 12 1.3 3.4 18 4.3 13 2.6 23
2.5 × 102 54 11 0.85 0.55 15 1.9 × 102 0.23 3.0 × 102
22 13 16 0.78 0.08 5.0 52 0.13 13
12 4.2 0.66 1.1 6.7 2.8 3.7 1.7 24
1.05 2.69 1.43 0.66 1.54 0.56 0.83 0.36 0.63
5.32 6.10 4.42 3.45 8.76 10.8 10.4 1.70 3.59
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the He flow rate, the measured distribution profile of the intensity of the Na 589.0 nm atomic emission line was not changed but its intensity was significantly decreased likely due to a hindered transport of the metal ions in the conditions of the higher He atoms flux that possibly moved across the potential barrier of the water vapor flux as reported in refs. [18]. The USN–μAPGD system behaved in a different manner. At the lowest studied He jet-supporting gas flow rate, i.e., 300 mL min−1, two resolved low-intensity maxima were observed (Fig. 3). The position of the first maximum was around 1 mm from the liquid cathode solution surface and did not change with the alternation of the gas flow rate. The second maximum appeared at the distance of 3 mm from the liquid cathode solution surface and its position moved towards the cathode when the He flow rate was increased, possibly as the effect of the higher flux of the He atoms and the lower pressure of the water vapor in the near-cathode region [18]. The highest intensity of the studied Na I 589.0 nm emission line was observed at the He jet-supporting gas flow rate within the range of 600–1200 mL min−1. In a similar way, the spatial distributions of the intensities of the molecular band heads of the NO, OH and N2 species at 237.0, 309.4 and 337.1 nm, respectively, were examined versus the He jet-supporting gas flow rate for the USN–μAPGD system. It was established that regardless of the gas flow rate used, the emission profile for the OH molecules looked like a single broad peak with the intensity maximum either in the middle of the discharge gap or in the near-anode region. Nevertheless, quite strong emissions from the OH molecules could also be observed in the vicinity of the cathode, especially at the highest He flow rates studied here, i.e., 1200 and 1500 mL min−1. As opposed to the OH radicals, the emissions from the N2 and NO molecules in the near-cathode region were hardly observed. It is understandable when one considers that the main component of the discharge gas in the near-cathode region of the μAPGD is the expanding water vapor, which limits the diffusion of the molecules coming from the ambient air, e.g., nitrogen and its compounds [18,20,23]. At the high flow rates of the He jet-supporting gas, the outer layers of the discharge containing the N2 and NO species could be sucked into the core of the μAPGD, where the high-speed electrons, capable of the excitation processes, exist. Because of the observed enhancement of the emissions from the molecular components, too high He flow rates were undesirable. In the further studies, the He flow rate was set at 700 mL min−1.
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3.3. The surfactant addition study It has already been stated that in the case of the APGD operated with metallic rod-anode being in contact with the liquid cathode, the addition of the surfactant to the liquid cathode solution typically results in the improvement of the intensities of the atomic emission lines of analytes and a substantial decrease of the emissions from the molecular components [11,12,24,25]. Therefore, in next turn, the effect of the nonionic Triton X-705 addition (at a final concentration of 1%, m/v) to the liquid cathode solution on the emission spectrum of the μAPGD operated with the small-sized He jet (with and without the coupled USN device) was studied. What must be emphasized, in the case of the USN–μAPGD system, the surfactant was added to the liquid cathode solution, i.e., a 0.1 mol L−1 HCl solution. For the μAPGD operated with the small-sized He jet, the presence of Triton X-705 in the liquid cathode solution was found to cause a noticeable decrease in the intensities of the vibrational–rotational bands of the NO, OH, N2 molecules and a 2–5 fold increase in the intensities of the atomic emission lines of the studied metals. The greatest enhancements were observed for In, Mg and Mn, whereas the lowest for Ca, Li and Sr. These results were consistent with what was obtained for the APGD operated with a solid-rod anode with no gaseous jet [24,25]. Against this background, it was surprising that in the case of the USN–μAPGD system, the addition of Triton X-705 to the liquid cathode solution did not affect the intensities of the molecular bands at all and even caused a reduction of the intensities of the atomic emission lines of the studied metals by 10–70%. The sputtered fragments of the surfactant molecules were previously supposed to affect the excitation conditions of the μAPGD as indicated by changes in the optical temperatures and the electron number density [24]. Now, it can be concluded that in the present μAPGD arrangement, with the introduction of the analytes through the anodic compartment in the stream of the He jet-supporting gas, the presence of the non-ionic surfactant results in deterioration of the excitation conditions. It seems that the benefits associated with the addition of the non-ionic surfactant to the liquid cathode solution in the conventional μAPGD are primarily related to the improvement of the metals ions transport from the surface of the liquid cathode to the discharge zones. In the case of the USN–μAPGD, the use of Triton X-705 in the solution of the liquid cathode was not reasonable. The surfactant was not added to the solution containing the metal ions (ultrasonically
Fig. 3. The spatial distribution of the intensities of the atomic Na emission line and the band heads of the OH, NO, N2 molecules acquired for coupled USN–μAPGD–OES versus the He jetsupporting gas flow rate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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nebulized into the μAPGD) in order to avoid its condensation in the glass module and the transferring tube of the USN device. 3.4. Detectability improvement As mentioned above, the combination of the USN device with the μAPGD resulted in the enhanced signals of the studied metals as compared to those obtained with the conventional μAPGD system. On the other hand, their maxima were usually shifted towards the nearanode region, where relatively strong emissions from the N2 and NO molecules also occurred. This can be clearly seen in Fig. 4 for the atomic In 451.2 nm emission line (He flow rate = 700 mL min−1). Accordingly, although the use of the USN device caused an increase in the intensity of this line by 30%, the maximum of the emission from this line, observed in the near-anode region, was accompanied by the vicinity of the intensive N2 molecular bands. As a result, the limit of detection (LOD) of In obtained with the USN–μAPGD was reduced by about 30% as compared to this evaluated with the μAPGD. A quite different situation was observed for the atomic Sr 460.7 nm emission line. Using the coupled USN–μAPGD system, the intensity of this line was improved over 20-times. Because the maximum of the intensity of this line observed in the near-anode region did not interfere with the emissions from the N2 molecular bands, a proportional improvement of the LOD of Sr was noted for the USN–μAPGD. To evaluate the LODs of the studied metals, the 0.2 mm parts (4–5 lines of the CCD) of the imaged emission spectra of the μAPGD and the USN-μAPGD), corresponding to the near-cathode and near-anode regions of both microdischarges and providing the highest emissions from the analytes atoms, were acquired and taken into account in the calculations. The LODs were calculated as the concentrations giving the signals equal to 3 times of the standard deviations (SDs) of the repeated measurements (n = 10) of the respective blanks. The comparison of the LODs evaluated with the OES for all the studied metals using the conventional μAPGD and the coupled USN–μAPGD is given in Table 1. It can be seen that although the use of the USN device results in a distinct detectability improvement, against the literature background, most of the obtained LODs are not satisfactory. A reasonably explanation for the observed discrepancies in the LOD values is the use of different modes for the emission spectra recording. In present paper
each of the 255 lines of the CCD was read separately. This approach enabled to obtain the information about the spatial distribution of the intensities of the emission lines and the molecular band heads, which was useful for the diagnostic purpose, but simultaneously the generated readout noise was summed and increased. In order to increase the signal-to-noise ratio, it would be desired to read all 255 lines of the CCD in one measurement cycle. This readout mode would result in obtaining the LODs better on average 4 times than those presented in Table 1. Bearing that in mind, it can be concluded that by combining USN with the μAPGD the LODs of Ca, Li and Sr can be significantly improved. It is all the more valuable because neither the addition of the surfactant [24,25] nor the low-molecular-weight organic compounds [14,15], nor other known modification of the APGD system, cause so high enhancement of the LODs of these three elements. In addition, the reproducibility of measurements for both systems was evaluated (as the relative standard deviation, RSD) and compared (see Table 1). It was found that the average RSDs for μAPGD–OES and USN–μAPGD–OES were 1.1% and 6.1%, respectively.
4. Conclusions The use of USN allows us to improve the sensitivity of μAPGD-OES about one order of magnitude. To fully exploit the possibilities offered by the novel system, the reduction of the spectral interferences originated from the N2 and NO molecular bands is necessary. To achieve this, a semi-closed discharge chamber can be used, cutting off the access of the ambient air. Based on the results of this paper, we also suppose that a decrease of the discharge gap would also help in protecting the discharge zones from the ambient air penetration. In a further study, it would be worth to consider the use of a low dead-volume or low-consumption pneumatic nebulizer mounted into a heated spray chamber to decrease the amount of the water vapor introduced into the microdischarge. It may seem that it is a step backwards in relation to the presented USN–μAPGD system. However, the distance from the aerosol chamber to the μAPGD would be shortened, eliminating analyte losses related to the possible deposition of the dry aerosol in the transferring tube.
Fig. 4. The spectrum image of the discharge gap acquired for the μAPGD coupled with and without the USN device. The concentrations of In and Sr in standard solutions used were the same.
K. Greda et al. / Spectrochimica Acta Part B 121 (2016) 22–27
Acknowledgments This work was funded by the Polish National Science Center (NCN) based on decision no. UMO-2014/13/B/ST4/05013. It was also cofinanced by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology. References [1] T. Cserfalvi, P. Mezei, P. Apai, Emission studies on a glow discharge in atmospheric pressure air using water as a cathode, J. Phys. D. Appl. Phys. 26 (1993) 2184–2188. [2] T. Cserfalvi, P. Mezei, Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry, J. Anal. At. Spectrom. 9 (1994) 345–349. [3] L. Bencs, N. Laczai, P. Mezei, T. Cserfalvi, Detection of some industrially relevant elements in water by electrolyte cathode atmospheric glow discharge optical emission spectrometry, Spectrochim. Acta B 107 (2015) 139–145. [4] C.G. Decker, M.R. Webb, Measurement of sample and plasma properties in solutioncathode glow discharge and effects of organic additives on these properties, J. Anal. At. Spectrom. 31 (2016) 311–318. [5] T. Cserfalvi, P. Mezei, Investigations on the element dependency of sputtering process in the electrolyte cathode atmospheric discharge, J. Anal. At. Spectrom. 20 (2005) 939–944. [6] A.J. Schwartz, S.J. Ray, E. Elish, A.P. Storey, A.A. Rubinshtein, G.C.Y. Chan, K.P. Pfeuffer, G.M. Hieftje, Visual observations of an atmospheric-pressure solution-cathode glow discharge, Talanta 102 (2012) 26–33. [7] K. Gyorgy, L. Bencs, P. Mezei, T. Cserfalvi, Novel application of the electrolyte cathode atmospheric glow discharge: atomic absorption spectrometry studies, Spectrochim. Acta B 77 (2012) 52–57. [8] Z. Zhu, G.C.-Y. Chan, S.J. Ray, X. Zhang, G.M. Hieftje, Use of a solution cathode glow discharge for cold vapor generation of mercury with determination by ICP-atomic emission spectrometry, Anal. Chem. 80 (2008) 7043–7050. [9] K. Greda, P. Jamroz, D. Jedryczko, P. Pohl, On the coupling of hydride generation with atmospheric pressure glow discharge in contact with the flowing liquid cathode for the determination of arsenic, antimony and selenium with optical emission spectrometry, Talanta 137 (2015) 11–17. [10] A.J. Schwartz, S.J. Ray, G.M. Hieftje, Automatable on-line generation of calibration curves and standard additions in solution-cathode glow discharge optical emission spectrometry, Spectrochim. Acta B 105 (2015) 77–83. [11] R. Shekhar, K. Madhavi, N.N. Meeravali, S.J. Kumar, Determination of thallium at trace levels by electrolyte cathode discharge atomic emission spectrometry with improved sensitivity, Anal. Methods 6 (2014) 732–740. [12] Z. Zhang, Z. Wang, Q. Li, H. Zou, Y. Shi, Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge-atomic
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Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Ultrasonic nebulization atmospheric pressure glow discharge — Preliminary study Krzysztof Greda ⁎, Piotr Jamroz, Pawel Pohl Wroclaw University of Technology, Faculty of Chemistry, Division of Analytical Chemistry and Chemical Metallurgy, Wybrzeze Stanislawa Wyspianskiego 27, 50-370 Wroclaw, Poland
a r t i c l e
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Article history: Received 9 March 2016 Received in revised form 20 April 2016 Accepted 28 April 2016 Available online 07 May 2016 Keywords: Ultrasonic nebulization atmospheric pressure glow discharge Elemental analysis Optical emission spectrometry Solution cathode glow discharge
a b s t r a c t Atmospheric pressure glow microdischarge (μAPGD) generated between a small-sized He nozzle jet anode and a flowing liquid cathode was coupled with ultrasonic nebulization (USN) for analytical optical emission spectrometry (OES). The spatial distributions of the emitted spectra from the novel coupled USN–μAPGD system and the conventional μAPGD system were compared. In the μAPGD, the maxima of the intensity distribution profiles of the atomic emission lines Ca, Cd, In, K, Li, Mg, Mn, Na and Sr were observed in the near cathode region, whereas, in the case of the USN–μAPGD, they were shifted towards the anode. In the novel system, the intensities of the analytical lines of the studied metals were boosted from several to 35 times. As compared to the conventional μAPGD–OES with the introduction of analytes through the sputtering and/or the electrospray-like nebulization of the flowing liquid cathode solution, the proposed method with the USN introduction of analytes in the form of a dry aerosol provides improved detectability of the studied metals. The detection limits of metals achieved with the USN–μAPGD–OES method were in the range from 0.08 μg L−1 for Li to 52 μg L−1 for Mn. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The atmospheric pressure glow discharge (APGD) is a kind of excitation source, which has been extensively explored in the analytical optical emission spectrometry (OES) since 1993/94 [1,2]. It is also known in literature as the electrolyte cathode atmospheric glow discharge (ELCAD) [2,3] and the solution cathode glow discharge (SCGD) [4]. One of the most notable advantages of the μAPGD is that no nebulizer or spray chamber is required for the sample solutions' introduction and the transport of the analytes into this excitation source. Analyzed sample solutions are integral components of the discharge, i.e., a liquid cathode that is bombarded by high energy ions. Subsequently, the droplets of the sample solutions are transferred to the negative glow zone of the discharge, where excitation processes occur [5]. The mechanism of the analytes transport is not entirely clear. Some studies suggest that the electrospray-like nebulization process plays a key role in this process [6], however, the sputtering is also considered [5]. The use of this phenomenon made it possible to manufacture simplified and miniaturized analytical systems that found applications in trace element analysis by analytical OES and atomic absorption spectrometry (AAS) [7]. However, one problem encountered in these systems is that the transport efficiency of the analytes to the phases of the discharge remains still relatively low. Cserfalvi et al. [5] studied the intensities of the emission lines of several metals in an ICP–OES system, in which a conventional pneumatic ⁎ Corresponding author. E-mail address: [email protected] (K. Greda).
http://dx.doi.org/10.1016/j.sab.2016.04.008 0584-8547/© 2016 Elsevier B.V. All rights reserved.
nebulizer, commonly used for the analytes transport to the plasma, was replaced by an APGD close-chamber device that converted the analyzed sample solutions into a respective mist. Unfortunately, in comparison to the IC–OES system with the pneumatic nebulizer, the acquired intensities of the emission lines of the studied metals were lower from 3 to even 30 times. In a similar experiment, Zhu et al. [8] examined the transport efficiency of As, Pb, Se and Sn. Although the liquid cathode solutions introduced to the APGD contained these elements at a level of 10 mg L−1, no measurable signals of As, Pb, Se and Sn were recorded by ICP–OES. In our previous paper [9], it was found that the transport efficiency of As and Sb into the μAPGD operated with a small-sized He jet, assessed by determining the residual concentrations of these elements in the liquid cathode solution treated by the microdischarge, did not exceed 1% (As) and 8% (Sb). Simultaneously, the mass loss of the sample solution introduced as the liquid cathode of the discharge system via the sputtering and evaporation of water was up to 18%. Despite the low transport efficiency of different APGD systems, the detectability of elements competitive with those offered by ICP–OES can still be achieved. The detection limits of alkali metals are usually below 1 μg L−1, and for many other elements, e.g., Ag, Cd, Cu, Hg, In, Mg, Mn, Tl and Zn, they do not exceed 10 μg L−1 [3,10–16]. Bearing all this in mind, it can be expected that the improvement of the transport efficiency of the analytes to the discharge would make the APGD a powerful excitation source for analytical OES. Therefore, in the present paper, an original μAPGD system, operating between a small-sized He nozzle jet as the anode and a flowing liquid cathode solution, was coupled with an ultrasonic nebulizer (USN) to
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enhance the transport efficiency of the analytes and the performance of the μAPGD. The resultant analytes—containing dry aerosol was introduced to the discharge through the He nozzle jet anode. The spatial distributions of the atomic emission lines of the selected metals were studied for the coupled USN–μAPGD system and the μAPGD system with the introduction of analytes from the liquid cathode sample solution. Furthermore, the effect of the He gas flow rate and the addition of the non-ionic surfactant to the liquid cathode solution were investigated. Although the results concerning the USN–μAPGD are of a preliminary nature, they let us prove the validity of such a combination. It should also be mentioned that the concept of the introduction of the dry aerosol into the μAPGD seems to be quite rational. Previously, Manard et al. [17] introduced to the liquid sampling (LS)-APGD the sample particles produced by laser ablation (LA) prior to the determination of Ca and Sr by the analytical OES and mass spectrometry (MS). 2. Experimental 2.1. Instrumentation The μAPGD was operated between a small-sized He jet and a flowing liquid cathode solution in an open-to-air discharge chamber (see Fig. 1). The distance between electrodes (i.e., discharge gap) was 5 mm. Using a peristaltic pump (Masterflex L/S, Cole-Parmer, UK), the liquid cathode solution (acidified with 0.1 mol L−1 HCl) was delivered to the discharge chamber at 3.0 mL min−1 through a quartz tube (ID = 2.0 mm) that was inserted into a graphite tube (ID = 4.0 mm); the edge of the graphite tube was 2 mm above the edge of the quartz tube. As compared to our previous work [18], in the present system, the ID of a stainlesssteel nozzle, used for introducing the He jet-supporting gas, was larger, i.e., 750 μm. For the conventional μAPGD, the He jet-supporting gas flow rate was within the range of 50–300 mL min−1 and was regulated using a flow controller and a digital flow meter. The flow rate of 75 mL min−1 was found to provide the best performance in reference to stabilizing the surface of the liquid cathode solution and the discharge column, and was applied throughout the work, unless otherwise stated. In the case of the coupled USN–μAPGD system, a higher flow rate of the He jet-supporting gas was required (300–1500 mL min−1) because it was also used to carry the resultant dry aerosol from the USN device. For both μAPGD systems, a HV-dc power supply (Dora, Poland) was used and operated in a constant current mode. Stable microdischarges were maintained after supplying a voltage of 1300 V (a voltage drop across the ballast resistor was subtracted) to the stainless-steel nozzle through a 5 kΩ ballast resistor. The liquid cathode solution was grounded
Fig. 1. The experimental set-up of the μAPGD systems operated between a small-sized He jet and a flowing liquid cathode solution: a) — the conventional system, b) — the system combined with the USN device.
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through a Pt wire attached to the graphite tube. The resultant discharge current was 40 mA Both microdischarges were ignited by inserting an isolated stainless steel wire into the gap between the electrodes. In the case of the conventional μAPGD system, analytes were introduced to the discharge through the electrospray-like nebulization and/or the sputtering of the flowing liquid cathode sample solutions. For the coupled USN-μAPGD, an additional peristaltic pump was applied to introduce the sample solutions at 3.0 mL min−1 to the ultrasonic nebulizer (U-5000AT +, CETAC Technologies Inc., USA). Nebulization took place in a glass chamber on a transducer unit. Subsequently, the obtained mist was swept out from the chamber by the He jet-supporting gas (at 700 mL min−1, unless otherwise stated), carried to a heated U-tube, vaporized at 140 °C and introduced to a condenser (3 °C). The total length of the glassware module, consisting of the chamber, the U-tube dryer and the condenser, was about 1.5 m. Finally, the resultant dry aerosol was introduced to the μAPGD through a transferring tube attached to the stainless-steel nozzle, acting as the anode. Using a quartz lens (f = 60), the radiation emitted by the μAPGD was imaged (1:1) on the entrance slit (10 μm) of an imagining spectrograph (Shamrock 500i, Andor, UK) equipped with two holographic gratings (1200 and 1800 lines mm−1) and a UV–Vis CCD camera (Newton DU920P-OE, 1024 × 255 pixels, Andor, UK). The acquisition time was 10 s. The intensities of the studied molecular bands as well as the atomic emission lines of the analytes were background-corrected. 2.2. Reagents and sample preparation Compressed He (99.999%) was supplied by Air Products (Poland). Re-distilled water was used throughout. Non-ionic surfactant Triton X-705 and single-element stock (1000 mg L−1) standard solutions of Ca, Cd, In, K, Li, Mg, Mn, Na and Sr were supplied by Sigma-Aldrich (Germany). The analyzed sample solutions containing the studied metals were introduced as the flowing liquid cathode in the case of the μAPGD or through the ultrasonic nebulizer in the case of the coupled UNS–μAPGD system. All these solutions were acidified with ACS grade concentrated HCl (Avantor Performance Materials, Poland) to a final concentration of 0.1 mol L−1. For the USN–μAPGD, the flowing liquid cathode was just a 0.1 mol L−1 HCl solution, used to support the discharge. 3. Results and discussion 3.1. The spatial distribution of the emission The spatial distribution of the intensities of the atomic emission lines of the analytes (Ca, Cd, In, K, Li, Mg, Mn, Na, Sr), within the 5.0 mm discharge gap, was investigated for the μAPGD and the novel coupled USN–μAPGD system. When metals were delivered as the components of the liquid cathode solution, the intensities of their atomic emission lines remained relatively low, likely because the positive metal ions transported from the liquid cathode solution, before recombining to the neutral atoms in the cathode dark space and diffusing into the negative glow region, can be re-attracted to the negatively charged cathode [19]. This process is determined by the pressure in the discharge column, the cathode fall and the average energy of the electrons in the near-cathode region [20]. The distribution profiles of the atomic emission lines of metals reached the maximum in the near cathode region of the discharge, i.e., within the distance of 1–2 mm from the cathode surface (see Fig. 2). These results are consistent with those obtained for other APGD systems operated at higher discharge currents (70– 80 mA) but with narrower discharge gaps (~3 mm) and using metallic rods as anodes instead of the gaseous nozzle jet [7,21,22]. For the USN–μAPGD, where positively charged metal ions were delivered to the discharge in the stream of the He jet-supporting gas as the dry aerosol and through the nozzle anode, the intensities of the atomic emission lines of the studied metals were evidently boosted,
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Fig. 2. The spatial distribution of the intensities of the atomic emission lines of Ca, Cd, In, K, Li, Mg, Mn, Na and Sr acquired for conventional μAPGD–OES and coupled USN–μAPGD–OES. dmax denotes the distance from the liquid cathode solution surface (mm) where the maximum emission occurs.
i.e., higher from several times in the case of In, K, Mg and Na, to even 18, 23 and 35-times for Li, Sr and Ca, respectively (Table 1). The reason for that could be the higher concentrations of the analytes atoms in the near-anode region. It was probably the consequence of the analytes enrichment in the dry aerosol produced by the USN device and a partial removal of the water vapor. Although this explanation seems to be reasonable, because of differences in the operating conditions, i.e., the He jet-supporting gas flow rate in the USN–μAPGD was almost 10-times higher than in the case of the conventional μAPGD, it cannot be explicitly stated. Contrary to the μAPGD, in the USN–μAPGD system, the maxima of the intensities distribution profiles for the atomic emission lines of the studied metals were shifted towards the anode, i.e., they occurred at the distance of 2–4 mm from the liquid cathode solution surface. It can be noticed that in the case of the analytical lines of metals that
intensities were only modestly enhanced, i.e., In, K, Mg and Na, their distribution profiles across the distance between the electrodes had two partially resolved maxima. The first one in the near-cathode region (the negative glow) was lower and the second one, of a greater intensity, was in the anode vicinity (the positive column). Also for other metals, relatively high emission in the near-cathode region can be observed, probably as a result of an increasing flux of the analytes atoms into this discharge zone. 3.2. The effect of the jet-supporting gas flow rate The effect of the jet-supporting gas flow rate on the distribution of the Na I 589.0 nm emission line in the conventional μAPGD was investigated for the range of 50–300 mL min−1. It was found that by increasing
Table 1 Comparison of the sensitivities of the atomic emission lines, the LODs of metals (Ca, Cd, In, K, Li, Mg, Mn, Na, Sr) and the relative standard deviations (RSD) of the net intensities of the analytical lines evaluated for conventional μAPGD–OES and coupled USN–μAPGD–OES methods. Metal
Ca Cd In K Li Mg Mn Na Sr
Sensitivity (a. u. per μg−1 L)
LOD (μg L−1)
RSD (%)
μAPGD
USN–μAPGD
USN–μAPGD/μAPGD ratio
μAPGD
USN–μAPGD
USN–μAPGD/μAPGD ratio
μAPGD
USN–μAPGD
0.12 0.40 2.0 66 69 3.2 0.12 1.5 × 102 0.066
4.0 4.7 2.5 2.3 × 102 1.3 × 103 14 1.6 3.9 × 102 1.5
34 12 1.3 3.4 18 4.3 13 2.6 23
2.5 × 102 54 11 0.85 0.55 15 1.9 × 102 0.23 3.0 × 102
22 13 16 0.78 0.08 5.0 52 0.13 13
12 4.2 0.66 1.1 6.7 2.8 3.7 1.7 24
1.05 2.69 1.43 0.66 1.54 0.56 0.83 0.36 0.63
5.32 6.10 4.42 3.45 8.76 10.8 10.4 1.70 3.59
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the He flow rate, the measured distribution profile of the intensity of the Na 589.0 nm atomic emission line was not changed but its intensity was significantly decreased likely due to a hindered transport of the metal ions in the conditions of the higher He atoms flux that possibly moved across the potential barrier of the water vapor flux as reported in refs. [18]. The USN–μAPGD system behaved in a different manner. At the lowest studied He jet-supporting gas flow rate, i.e., 300 mL min−1, two resolved low-intensity maxima were observed (Fig. 3). The position of the first maximum was around 1 mm from the liquid cathode solution surface and did not change with the alternation of the gas flow rate. The second maximum appeared at the distance of 3 mm from the liquid cathode solution surface and its position moved towards the cathode when the He flow rate was increased, possibly as the effect of the higher flux of the He atoms and the lower pressure of the water vapor in the near-cathode region [18]. The highest intensity of the studied Na I 589.0 nm emission line was observed at the He jet-supporting gas flow rate within the range of 600–1200 mL min−1. In a similar way, the spatial distributions of the intensities of the molecular band heads of the NO, OH and N2 species at 237.0, 309.4 and 337.1 nm, respectively, were examined versus the He jet-supporting gas flow rate for the USN–μAPGD system. It was established that regardless of the gas flow rate used, the emission profile for the OH molecules looked like a single broad peak with the intensity maximum either in the middle of the discharge gap or in the near-anode region. Nevertheless, quite strong emissions from the OH molecules could also be observed in the vicinity of the cathode, especially at the highest He flow rates studied here, i.e., 1200 and 1500 mL min−1. As opposed to the OH radicals, the emissions from the N2 and NO molecules in the near-cathode region were hardly observed. It is understandable when one considers that the main component of the discharge gas in the near-cathode region of the μAPGD is the expanding water vapor, which limits the diffusion of the molecules coming from the ambient air, e.g., nitrogen and its compounds [18,20,23]. At the high flow rates of the He jet-supporting gas, the outer layers of the discharge containing the N2 and NO species could be sucked into the core of the μAPGD, where the high-speed electrons, capable of the excitation processes, exist. Because of the observed enhancement of the emissions from the molecular components, too high He flow rates were undesirable. In the further studies, the He flow rate was set at 700 mL min−1.
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3.3. The surfactant addition study It has already been stated that in the case of the APGD operated with metallic rod-anode being in contact with the liquid cathode, the addition of the surfactant to the liquid cathode solution typically results in the improvement of the intensities of the atomic emission lines of analytes and a substantial decrease of the emissions from the molecular components [11,12,24,25]. Therefore, in next turn, the effect of the nonionic Triton X-705 addition (at a final concentration of 1%, m/v) to the liquid cathode solution on the emission spectrum of the μAPGD operated with the small-sized He jet (with and without the coupled USN device) was studied. What must be emphasized, in the case of the USN–μAPGD system, the surfactant was added to the liquid cathode solution, i.e., a 0.1 mol L−1 HCl solution. For the μAPGD operated with the small-sized He jet, the presence of Triton X-705 in the liquid cathode solution was found to cause a noticeable decrease in the intensities of the vibrational–rotational bands of the NO, OH, N2 molecules and a 2–5 fold increase in the intensities of the atomic emission lines of the studied metals. The greatest enhancements were observed for In, Mg and Mn, whereas the lowest for Ca, Li and Sr. These results were consistent with what was obtained for the APGD operated with a solid-rod anode with no gaseous jet [24,25]. Against this background, it was surprising that in the case of the USN–μAPGD system, the addition of Triton X-705 to the liquid cathode solution did not affect the intensities of the molecular bands at all and even caused a reduction of the intensities of the atomic emission lines of the studied metals by 10–70%. The sputtered fragments of the surfactant molecules were previously supposed to affect the excitation conditions of the μAPGD as indicated by changes in the optical temperatures and the electron number density [24]. Now, it can be concluded that in the present μAPGD arrangement, with the introduction of the analytes through the anodic compartment in the stream of the He jet-supporting gas, the presence of the non-ionic surfactant results in deterioration of the excitation conditions. It seems that the benefits associated with the addition of the non-ionic surfactant to the liquid cathode solution in the conventional μAPGD are primarily related to the improvement of the metals ions transport from the surface of the liquid cathode to the discharge zones. In the case of the USN–μAPGD, the use of Triton X-705 in the solution of the liquid cathode was not reasonable. The surfactant was not added to the solution containing the metal ions (ultrasonically
Fig. 3. The spatial distribution of the intensities of the atomic Na emission line and the band heads of the OH, NO, N2 molecules acquired for coupled USN–μAPGD–OES versus the He jetsupporting gas flow rate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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nebulized into the μAPGD) in order to avoid its condensation in the glass module and the transferring tube of the USN device. 3.4. Detectability improvement As mentioned above, the combination of the USN device with the μAPGD resulted in the enhanced signals of the studied metals as compared to those obtained with the conventional μAPGD system. On the other hand, their maxima were usually shifted towards the nearanode region, where relatively strong emissions from the N2 and NO molecules also occurred. This can be clearly seen in Fig. 4 for the atomic In 451.2 nm emission line (He flow rate = 700 mL min−1). Accordingly, although the use of the USN device caused an increase in the intensity of this line by 30%, the maximum of the emission from this line, observed in the near-anode region, was accompanied by the vicinity of the intensive N2 molecular bands. As a result, the limit of detection (LOD) of In obtained with the USN–μAPGD was reduced by about 30% as compared to this evaluated with the μAPGD. A quite different situation was observed for the atomic Sr 460.7 nm emission line. Using the coupled USN–μAPGD system, the intensity of this line was improved over 20-times. Because the maximum of the intensity of this line observed in the near-anode region did not interfere with the emissions from the N2 molecular bands, a proportional improvement of the LOD of Sr was noted for the USN–μAPGD. To evaluate the LODs of the studied metals, the 0.2 mm parts (4–5 lines of the CCD) of the imaged emission spectra of the μAPGD and the USN-μAPGD), corresponding to the near-cathode and near-anode regions of both microdischarges and providing the highest emissions from the analytes atoms, were acquired and taken into account in the calculations. The LODs were calculated as the concentrations giving the signals equal to 3 times of the standard deviations (SDs) of the repeated measurements (n = 10) of the respective blanks. The comparison of the LODs evaluated with the OES for all the studied metals using the conventional μAPGD and the coupled USN–μAPGD is given in Table 1. It can be seen that although the use of the USN device results in a distinct detectability improvement, against the literature background, most of the obtained LODs are not satisfactory. A reasonably explanation for the observed discrepancies in the LOD values is the use of different modes for the emission spectra recording. In present paper
each of the 255 lines of the CCD was read separately. This approach enabled to obtain the information about the spatial distribution of the intensities of the emission lines and the molecular band heads, which was useful for the diagnostic purpose, but simultaneously the generated readout noise was summed and increased. In order to increase the signal-to-noise ratio, it would be desired to read all 255 lines of the CCD in one measurement cycle. This readout mode would result in obtaining the LODs better on average 4 times than those presented in Table 1. Bearing that in mind, it can be concluded that by combining USN with the μAPGD the LODs of Ca, Li and Sr can be significantly improved. It is all the more valuable because neither the addition of the surfactant [24,25] nor the low-molecular-weight organic compounds [14,15], nor other known modification of the APGD system, cause so high enhancement of the LODs of these three elements. In addition, the reproducibility of measurements for both systems was evaluated (as the relative standard deviation, RSD) and compared (see Table 1). It was found that the average RSDs for μAPGD–OES and USN–μAPGD–OES were 1.1% and 6.1%, respectively.
4. Conclusions The use of USN allows us to improve the sensitivity of μAPGD-OES about one order of magnitude. To fully exploit the possibilities offered by the novel system, the reduction of the spectral interferences originated from the N2 and NO molecular bands is necessary. To achieve this, a semi-closed discharge chamber can be used, cutting off the access of the ambient air. Based on the results of this paper, we also suppose that a decrease of the discharge gap would also help in protecting the discharge zones from the ambient air penetration. In a further study, it would be worth to consider the use of a low dead-volume or low-consumption pneumatic nebulizer mounted into a heated spray chamber to decrease the amount of the water vapor introduced into the microdischarge. It may seem that it is a step backwards in relation to the presented USN–μAPGD system. However, the distance from the aerosol chamber to the μAPGD would be shortened, eliminating analyte losses related to the possible deposition of the dry aerosol in the transferring tube.
Fig. 4. The spectrum image of the discharge gap acquired for the μAPGD coupled with and without the USN device. The concentrations of In and Sr in standard solutions used were the same.
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Acknowledgments This work was funded by the Polish National Science Center (NCN) based on decision no. UMO-2014/13/B/ST4/05013. It was also cofinanced by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology. References [1] T. Cserfalvi, P. Mezei, P. Apai, Emission studies on a glow discharge in atmospheric pressure air using water as a cathode, J. Phys. D. Appl. Phys. 26 (1993) 2184–2188. [2] T. Cserfalvi, P. Mezei, Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry, J. Anal. At. Spectrom. 9 (1994) 345–349. [3] L. Bencs, N. Laczai, P. Mezei, T. Cserfalvi, Detection of some industrially relevant elements in water by electrolyte cathode atmospheric glow discharge optical emission spectrometry, Spectrochim. Acta B 107 (2015) 139–145. [4] C.G. Decker, M.R. Webb, Measurement of sample and plasma properties in solutioncathode glow discharge and effects of organic additives on these properties, J. Anal. At. Spectrom. 31 (2016) 311–318. [5] T. Cserfalvi, P. Mezei, Investigations on the element dependency of sputtering process in the electrolyte cathode atmospheric discharge, J. Anal. At. Spectrom. 20 (2005) 939–944. [6] A.J. Schwartz, S.J. Ray, E. Elish, A.P. Storey, A.A. Rubinshtein, G.C.Y. Chan, K.P. Pfeuffer, G.M. Hieftje, Visual observations of an atmospheric-pressure solution-cathode glow discharge, Talanta 102 (2012) 26–33. [7] K. Gyorgy, L. Bencs, P. Mezei, T. Cserfalvi, Novel application of the electrolyte cathode atmospheric glow discharge: atomic absorption spectrometry studies, Spectrochim. Acta B 77 (2012) 52–57. [8] Z. Zhu, G.C.-Y. Chan, S.J. Ray, X. Zhang, G.M. Hieftje, Use of a solution cathode glow discharge for cold vapor generation of mercury with determination by ICP-atomic emission spectrometry, Anal. Chem. 80 (2008) 7043–7050. [9] K. Greda, P. Jamroz, D. Jedryczko, P. Pohl, On the coupling of hydride generation with atmospheric pressure glow discharge in contact with the flowing liquid cathode for the determination of arsenic, antimony and selenium with optical emission spectrometry, Talanta 137 (2015) 11–17. [10] A.J. Schwartz, S.J. Ray, G.M. Hieftje, Automatable on-line generation of calibration curves and standard additions in solution-cathode glow discharge optical emission spectrometry, Spectrochim. Acta B 105 (2015) 77–83. [11] R. Shekhar, K. Madhavi, N.N. Meeravali, S.J. Kumar, Determination of thallium at trace levels by electrolyte cathode discharge atomic emission spectrometry with improved sensitivity, Anal. Methods 6 (2014) 732–740. [12] Z. Zhang, Z. Wang, Q. Li, H. Zou, Y. Shi, Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge-atomic
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