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Fundamental studies of electrolyte-as-cathode glow discharge-atomic emission spectrometry for the determination of trace metals in flowing water1
Spectrochimica Acta Part B 53 (1998) 1167–1179
Fundamental studies of electrolyte-as-cathode glow discharge-atomic emission spectrometry for the determination of trace metals in flowing water1 Yang S. Park a, Soo H. Ku a, Sung H. Hong a, Hyo J. Kim a,*, Edward H. Piepmeier b a
Department of Pharmacy, Dongduk Women’sUniversity, 23-1, Sungbuk-ku, 136-714Seoul, Korea b Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA Received 15 January 1998; accepted 27 April 1998
Abstract The fundamental characteristics and analytical performance were investigated of a new glow discharge emission source (GDES) for the determination of trace metals in flowing water. The application of an atmospheric glow discharge in argon gas between an electrolyte solution cathode and a platinum rod anode led to the development of a stable discharge. The intensity of the lines was found to depend strongly on the acidity of the water, the current and the discharge gap. The spectrum emitted from the tap water contained the basic atomic lines of the dissolved metals and OH band peaks, but no emission lines of argon from the discharge gas. The strong emission lines for an element and no emission lines from the discharge gas in the electrolyteas-cathode glow discharge (ELCAD) were different from those in a solid-as-cathode discharge, suggesting a different excitation mechanism. Sub-parts-per-million detection limits of Al, Cd, Cr, Cu, Mn, Pb and Hg were obtained. The continuous-flow sample system demonstrates the possibility to develop a device for the continuous analysis of water and waste water solutions. q 1998 Elsevier Science B.V. All rights reserved Keywords: Electrolyte-as-cathode glow discharge (ELCAD); Atomic emission spectrometry (AES); Water analysis; Metals
1. Introduction Glow discharge electrolysis (GDE) is a type of glow discharge generated between an electrolyte solution cathode and a solid anode at a distance of a few millimeters in the gas phase. Gubkin [1] demonstrated the first possibility of the electrolysis of aqueous solutions of metallic salts using a glow discharge in 1887. Hickling and Ingram [2] extensively reviewed the phenomenon in GDE. They found that one characteristic feature of GDE is that the * Corresponding author. 1 Published in honor of Professor C.L. Chakrabarti
electrolysis yield usually exceeds that expected from Faraday’s law, often about sevenfold. The important factor that led to such high yields was the acceleration energy of the positive ions, which was about 100 eV. The principal experimental parameters that influenced the yields were the gas pressure and the concentration of the reactants. The first paper to mention a relationship between the spectrum emitted by the GDE and the composition of the electrolyte was that of Couch and Brenner [3] in 1959. They found an unusual glow discharge produced between a tungsten electrode and an aqueous solution of cupric and indium salts. The discharge was obtained at 0.05–0.2 A and 500 V with low pressures
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 15 4 -2
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of 5–10 torr. The spectra contained various atomic lines of Cu and In and some molecular bands of InCl and OH. They explained this phenomenon as being due to a volatile copper or indium compound forming in the solution and evaporating into the evacuated space to produce the spectrum. Cserfalvi et al. [4] developed electrolyte-as-cathode glow discharge (ELCAD)-atomic emission spectrometry (AES) and demonstrated a possible analytical application in the continuous analysis of water and waste water solutions. They found that the spectral lines emitted from the discharge are a consequence of the cathodic sputtering of water during the discharge. The intensity of the lines was found to depend strongly on the pH of the water. They also found that the calibration curves of line intensities versus metal concentrations were linear in the 1–50 ppm range and showed a strong dependence on the discharge current and hydrogen ion concentration of the solution. They also showed that pulse-modulated operation with time-resolved signal processing improved the signalto-background ratio. Cserfalvi and Mezei [5] also described the operating mechanism of the electrolyte-as-cathode atmospheric pressure glow discharge and found that the cathode fall is the most important factor in explaining the influence of the pH and discharge current on the emission intensity of an analyte. Cserfalvi and Mezei [6] measured the cathode fall as a function of pH and found that it is almost constant in the pH region 4–8 but decreases significantly as the pH decreases below 4. They also showed that the emission intensity decreases as the cathode fall increases. The observed decrease of the cathode fall with decreasing pH was explained by a model that uses two different electron emission processes near the electrolyte cathode; emission coupled with hydrated electrons is dominant below pH 2.5, while protonindependent emission of poor efficiency operates above pH 3. In our laboratory, analytical methods for the continuous monitoring of heavy metals in drinking and waste water are of interest, and ELCAD may prove to be a useful technique. Various analytical techniques are available for determining trace elements in drinking water, such as GF-AAS, ICP-AES and ICP-MS. However, these techniques are inadequate for the online monitoring of trace elements due to the high cost
of operation, robustness, and ease of operation. Electrochemical methods have been widely used for the on-line monitoring of trace metals in processors and flowing water, and show good detection limits and precision. However, most electrochemical methods (voltammetry, polarography) suffer from matrix effects, and determine only three to four elements with one electrode; therefore three or four instruments are needed for determining all the elements in drinking water that are required by government regulatons to be quantified. ELCAD is a potentially new analytical source for the on-line monitoring of trace elements in flowing solutions. In our laboratory, ELCAD has been further developed and its fundamental characteristics and analytical performance have been studied with a high-resolution monochromator. The effect of the discharge gas on the spectrum has been studied in order to elucidate the excitation mechanism.
2. Experimental 2.1. Glow discharge electrolysis-atomic emission spectrometry system Fig. 1 shows a schematic diagram of the ELCAD cell for the direct analysis of solutions. The liquid sample was introduced using a peristaltic pump (Master Flux, Niles, IL, USA) at a flow rate of 10 ml min −1. The optimum sample introduction rate was found to be 5–10 ml min −1. In the case of a lower sample flow rate, continuous overflowing of the water sample on the cathode was difficult due to surface tension. A higher sample flow rate also helped to reduce the boiling of the water. The gas flow rate was controlled by a mass flow controller (MKS, Andover, MA, USA; 0–500 ml min −1). A highvoltage d.c. power supply (Fug Co. MCN 350, Rosenheim-Langenpfunzen, Germany) capable of supplying up to 2000 V and 250 mA was used. To stabilize the discharge current, a 500 Q ballast resistor was placed in series with the anode discharge. The glow discharge cell was made of glass and was similar to that reported previously [2]. The anode consisted of a 3 mm o.d. platinum rod with a rounded end located just above the cathode. The distance between the anode and the water surface could be adjusted by
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Fig. 1. Schematic diagram of the ELCAD cell. 1, Monochromator; 2, focusing lens; 3, quartz window; 4, sample solution dome; 5, cathode tube (2–10 mm o.d.); 6, sample introduction tube (2.0 mm o.d.); 7, waster water 8, waster valve; 9, solution (from peristaltic pump); 10, plasma; 11, mass flow controller; 12, platinum anode (3 mm o.d.); 13, graphite cathode (5.0 mm o.d.).
rotating a micrometer attached to the anode. The water was connected to the cathode of the power supply through a graphite rod. This electrode was in a separate glass compartment with o.d. 5 cm in order to prevent the reduction of the product at the cathode. A continuous flow of argon served to prevent the accumulation of hydrogen and oxygen in the gas phase, with their possible risk of explosion. The distance between the graphite rod and the cathode spot on the water surface was 5 cm. The liquid sample was introduced through a 2 mm o.d. capillary tube and formed a dome of flowing solution. Various sizes of glass tubes from 2 to 10 mm o.d. could be attached to the top of the capillary tube, resulting in different sizes of the cathode surface. When the cathode tube was less than 5 mm o.d., the plasma was easily spread over the water surface. Therefore, a cathode tube of 10 mm o.d. was used in this study. To prevent overheating of the water sample during the discharge, it was circulated continuously through an overflowing inlet tube and was disposed of through an outlet tube. Various discharge gases such as argon, helium and air were studied, and one of the gases was introduced near the anode to localize the discharge more
effectively between the anode and cathode. A second gas inlet was located near the emission window to prevent the deposition of water droplets in the optical path. Reducing the distance between the anode and cathode to about 1 mm initiated the discharge, and a stable discharge was obtained when operated with a discharge gap of 3–5 mm, at 1500 V and 80 mA. 2.2. Optical spectrometry system The emission spectra of the discharge plasma were collected and foccused one-to-one by a 25 mm diameter symmetric convex spherical silica lens with a focal length of 150 mm onto a 1.0 m Czerny–Turner grating monochromator (Spex model 1000M; Edison, NJ, USA). Radiation was detected with a photomultiplier tube (model R928; Hamamatsu, Japan). The f/8, 1.0 m monochromator employs a 110 mm square plane grating (blazed for 500 nm) with 1200 lines per millimeter, giving it a reciprocal linear dispersion of 0.8 nm mm -1 in the first order. Data acquisition and monochromator control were performed with a DS1000 computer interface board (Spex; Edison, NJ, USA) and homemade software
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written in QuickBASIC language. In the investigation of the effect of the operating parameters on the emission intensity at each wavelength, the monochromator scanned at a rate of 0.005 nm every 0.5 s to acquire the emission spectra. Scan step sizes of 0.01 nm and integration times of 0.1 s were employed to acquire the emission spectra from 200 to 800 nm.
3. Reagents The pH of the sample was adjusted by adding HNO 3, and was measured with a pH meter (Analab 88 ATC; SunYil Co. Korea). A deionized water sample was obtained from an Elgastat purifier (model UHQOS; UK). Working standards for nine analyte elements were prepared by the dilution of 1000 ppm stock solutions (Showa, Japan).
4. Results and discussion 4.1. Glow discharge with liquid cathode and solid anode A typical glow discharge operates at several hundred volts and several millitorr pressure with a solid anode and cathode. However, a glow discharge with a liquid cathode and solid anode, or a liquid anode and solid cathode, can also be obtained. To initiate the discharge between the liquid cathode and the solid anode at atmospheric pressure, the distance between the anode and cathode can be reduced to less than 1.0 mm or a 20 kV high-voltage spark applied. The ignition voltage increases greatly as the distance between the cathode and anode increases, and a voltage above 1500 V is required to initiate the discharge at a distance of 1.0 mm. After the discharge is established, a stable discharge is sustained at a distance of several millimeters between the solid anode and water as the cathode. When the discharge is operated using tap water spiked with 50 ppm of Cu and with the pH adjusted to 1.0 with HNO 3, a typical glow discharge plasma appears at about a 2 mm gap between the solid anode and the water cathode. The discharge is obtained with a current of 80 mA, a voltage of 1200 V, and argon as the discharge gas. An anode glow can be seen just
below the rounded anode rod located at the top. A bright green color, due to the excitation of Cu, is obvious in the middle of the discharge. However, the typical glow regions that appear in a low-pressure glow discharge, such as the negative glow and the positive column, may not be identified visually in this type of discharge; Cu excitation may occur in part of the area of the negative glow. This glow discharge is obtained when the discharge gap is reduced to about 1 mm under typical operational conditions of several hundred volts and a few milliamps [7]. In this high-pressure glow discharge, the length of the positive column decreases with the distance between anode and cathode. A second bright portion of the plasma occurs on the water surface where cathodic sputtering occurs. This plasma shrinks and shows no green emission when the pH of the water sample is above 3.0 at a constant discharge voltage of 1500 V. The discharge current decreases to 20 mA to maintain the constant voltage at 1500 V. This indicates that the pH of the solution is an important factor for sustaining the discharge. Increasing the conductivity of the solution also helps to maintain the plasma. Fig. 2 shows characteristic current–voltage curves for various discharge gaps. The discharge was operated at atmospheric pressure with argon gas between the platinum anode and the cathode was deionized water adjusted to a pH of 1.0 with HNO 3. The deionized water sample was continuously circulated at 10 ml min −1 through an overflowing inlet tube, and was disposed of through an outlet tube using a
Fig. 2. Current–voltage characteristic curves for various discharge gaps. (A) 1.0 mm; ( × ) 2.0 mm; ( + ) 3.0 mm; (W) 4.0 mm. (Discharge gas, Ar; sample, deionized water at pH 1.0 adjusted with HNO 3.)
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peristaltic pump. The discharge ignition was obtained by decreasing the distance between the anode and cathode to less than 1.0 mm by moving the anode towards the water surface while applying a constant voltage of 1500 V. A stable discharge was sustained with several hundred volts and several tens of milliamps. When the discharge was on, a rounded glow spot sat on the surface of the electrolyte solution cathode. The size of this glow spot increased as the discharge current increased. The current density at this spot was 400–600 mA cm −2, which agrees with the value of 350–600 mA cm −2 with air as the discharge gas. During operation, the distance between the cathode and anode varied from 1 to 4 mm. When the discharge gap was increased further than 5 mm, the discharge was unstable. The discharge voltage increased as the discharge current increased when the discharge gap was between 1 and 4 mm. The atmospheric ELCAD using air as the discharge gas is known to be an abnormal glow region because the discharge voltage increases as the discharge current increases [4]. With argon as discharge gas, the voltage–current relationship was similar to that with air. Not many differences in the voltage–current relationships were found among discharge gases such as helium, argon and air. As the discharge gap increased, the discharge voltage increased at constant current. 4.2. Background spectra Most of the background spectral features in ICPs and DCPs arise from trace components in the plasma gas or water vapor. The background spectrum in a glow discharge contains emission lines from both the plasma gas and the metal cathode due to sputtering of the cathode. Fig. 3 shows a typical spectrum of deionized water obtained with a discharge current of 50 mA, a voltage of 1200 V and a gap of 1.5 mm at atmospheric pressure. A 100 ml min −1 flow of argon was used for the discharge gas. The water sample was acidified to pH 1.0 with HNO 3 and introduced into the cathode at 10 ml min −1. These spectra were collected using scan step sizes of 0.01 nm and integration times of 0.1 s for the spectral regions 200–300 nm (Fig. 3a), 300–400 nm (Fig. 3b) and 400–500 nm (Fig. 3c). In the 200–300 nm region, the spectrum is dominated by emission bands of OH at 283 nm (Fig. 3a).
Fig. 3. Spectrum of deionized water obtained with a discharge current of 50 mA, a gas flow rate of 100 ml min −1 and a discharge gap of 1.5 mm in argon at atmospheric pressure. (a) 200–300 nm; (b) 300–400 nm; (c) 400–500 nm.
There are no other prominent lines in this region. In the 300–400 nm region (Fig. 3b), the spectrum is also dominated by emission bands of OH at 306.4 nm. It was reported that the OH bands seen in this spectrum are attributed to a recombination process of the precursor species of OH + ions [8]. These are reported to be a major product of the electron impact ionization of the H 2O molecules in water vapor [9]. Various species formed upon the dissociation of the water molecules
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(i.e. H+3, O +, OH + etc.) can also be detected using lowpressure glow discharge-mass spectrometry. In the 400-500 nm region (Fig. 3c), the H b 486.1 nm spectral line is apparent. Cserfalvi et al. [4] monitored the emission intensity of the H b line as a function of the solution pH, it becoming more intense with decreasing pH. An increase in the H + ion concentration in the solution may result in an increasing amount of H in the plasma and an increasing H line intensity. The spectrum shows many lines due to oxygen ions in the 400–500 nm region when using argon as the discharge gas. Mezei et al. [8] also reported the same oxygen ion lines in a nitrogen and air atmosphere. Since the excitation energies of oxygen ions are very high (around 25–28 eV) and the only possible source of the oxygen is water, the oxygen ions are produced from water vapor by electron impact [8,9]. One of the interesting results found from the spectrum of the blank was the absence of emission lines from argon gas atoms and ions. Since the glow discharge plasma is sustained because of excitation and ionization between the discharge gas and electrons, there are many emission lines from discharge gases in low-pressure GDs. Therefore, the excitation conditions are different in this device. The spectrum appears relatively simple compared with emission spectra from other plasma sources such as the GD, ICP and MIP, due to the absence of emission lines from the discharge gas. 4.3. Spectrum of tap water Fig. 4 shows a spectrum of tap water collected from the laboratory, spiked with 10 ppm of Cu and Pb, obtained with a discharge current of 50 mA, a voltage of 1200 V, a gas flow rate of 100 ml min −1 and a discharge gap of 1.5 mm at atmospheric pressure. A discharge voltage of 1200 V was required to keep a 50 mA discharge current for the tap water, and 1500 V was required for the deionized water. The reason for the lower discharge voltage in the case of tap water might be due to its higher conductivity. The tap water sample was also acidified to pH 1.0 with HNO 3. These spectra were collected using scan step sizes of 0.01 nm and integration times of 0.1 s for the spectral regions 200–400 nm (Fig. 4a), 400–600 nm (Fig. 4b), and 600–800 nm (Fig. 4c). The most
Fig. 4. Spectrum of tap water spiked with 10 ppm of Cu and Pb obtained with a discharge current of 50 mA, a gas flow rate of 100 ml min −1 and a discharge gap of 1.5 mm in argon at atmospheric pressure. (a) 200–400 nm; (b) 400–600 nm; (c) 600– 800 nm.
prominent features of the emission spectrum in tap water are OH bands at 306.4 nm and (second order) 612.8 nm. In the 200–400 nm region (Fig. 4a), the spectra are dominated by emission bands of OH at 283 and 306.4 nm as well as two Cu I resonance lines at 324.7 and 327.4 nm, and an Mg I line at 285.3 nm. The Pb I 217.0 nm line is barely seen in
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the spectrum. However, there are no peaks from Cu ion lines, such as Cu II at 224.7 nm, which are apparent in low-pressure glow discharges having solid electrodes. In the 400–600 nm region (Fig. 4b), the spectra are dominated by Na I lines at 589.0 nm and Mg I (second order; 2 × 285.3 nm) as well as the Ca I 422.7 nm line. Pb I 405.8 nm, H b 486.1 nm and Cu I 510.5 nm lines are also seen in the spectrum. Again, there are no emission lines from argon gas in the spectral ranges from 350 to 500 nm. In the 600– 800 nm region (Fig. 4c), a typical OH band peak is apparent at 612.8 nm and at the second order of 306.4 nm. Cu I 324.7 and 327.4 nm lines (both second order) also occur. Since the grating system is blazed for 500 nm, the second order OH band (2 × 306.4 nm) is much stronger than the first order OH band at 306.4 nm. K I 766.5 and 769.9 nm lines are also observed. The spectral lines arise mainly from the elements dissolved originally (Ca, Na, and K) and spiked (Cu, Pb) in tap water and can be attributed to the effect of cathode sputtering during the discharge. The mechanism of emission has been extensively investigated [4–6,8]. When the discharge is operating, the positive metal ions are released from the electrolyte cathode surface due to cathode sputtering. These positive metal ions cannot pass through the negative space charge near the cathode dark space but escape from the cathode surface and diffuse into the negative glow only if they are recombining (becoming neutral) in the cathode dark space. The recombination of the positive metal ions takes place via a three-body collision involving one positive metal ion and two electrons. The neutral metal atoms produced in this way diffuse into the negative glow, where they are excited by electron collisions. 4.4. Effect of discharge gas on the emission intensity In low-pressure GDs with solid electrodes, the discharge gas plays an important role in both the sputtering process and the gas phase electron characteristics. Wagatsuma and Hirokawa [11] found that the emission intensities of Cu atoms and ions are strongly dependent on the type of foreign gas rather than other discharge conditions such as gas pressure and discharge voltage. Also, the spectrum contains many atomic and ionic gas emission lines which result
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from the excitation and ionization of foreign gases. Cserfalvi et al. [4] investigated a liquid cathode and solid anode glow discharge in air at atmospheric pressure; the spectrum showed typical OH bands at 306 and 612 nm as well as nitrogen band peaks at 358 nm, probably due to the excitation of the air. To investigate the effect of different discharge gases on the ELCAD, argon, air and helium were each introduced into the discharge chamber. Fig. 5 shows the effect of these discharge gases (air, Fig. 5a; argon, Fig. 5b; helium, Fig. 5c) on a spectrum over the range from 200 to 500 nm for a deionized water sample containing 10 ppm of Cu, Pb, Zn, and Cd. Each spectrum was obtained with an 80 mA discharge current, a 1.5 mm discharge gap and a 100 ml min −1 gas flow. Again, one of the interesting results is that the spectra show no significant changes in the emission lines, but the relative emission intensities of the lines varied with the discharge gas. The spectra of three different discharge gases show typical OH bands at 281 and 306 nm. The spectral lines of the elements dissolved in the sample are also obvious for Cu I at 324.7 nm, Cu I at 327.4 nm, Pb I at 368.4 nm and Pb I at 405.8 nm. The emission lines of Cd I at 228.8 nm and Zn I at 213.7 nm are also visible. When air is employed as the discharge gas, molecular band spectra are emitted which result from transitions among various vibrational states in different electronic states of N 2. No emission lines from argon and helium discharge gases occur. In the case of a solid cathode and an anode glow discharge with argon as the discharge gas, many argon atomic and ionic lines appear in the wavelength range 400– 500 nm, resulting from transitions from 3p 54s (11.55–11.83 eV) to 3p 55p (14.46–14.74 eV). Emission lines from singly ionized species (Ar II) also appear in the wavelength region 294.29– 496.51 nm [11]. There are no emission peaks from argon atoms or ions in the wavelength range from 400 to 500 nm in the spectrum of ELCAD. This suggests that the excitation mechanism of metals in solution is different from that of the normal low-pressure solid cathode glow discharge, where elements are sputtered due to the bombardment of ionized gas and further excited by high-energy electrons. In GDE, Hickling and Ingram [2] mentioned that the discharge current is carried to the solution by positive gaseous ions
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Fig. 5. Effect of the discharge gas on the spectrum obtained with a 80 mA discharge current, a 1.5 mm discharge gap and a 100 ml min −1 gas flow rate. Sample, deionized water containing 10 ppm of Cu and Cd; spectral range, 200–800 nm. (a) Air; (b) argon; (c) helium.
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Fig. 5. Continued.
In the case of nitrogen molecules, many energy levels exist continuously between 0 and 8 eV due to the many vibrational and rotational states belonging to each electronic state. The characteristics of the emission from H and OH radicals and Cu I and Cd I were measured. Table 1 shows an example of the effect of different discharge gases on the emission intensities of H (656.3 nm), OH radicals (306.4 nm), Cu I (324.7 nm) and Cd I (second order of 228.8 nm). The ratio of the intensities of Cu I and Cd I to that of the H radical was calculated for three different discharge gases because the ratio is dependent on the atmosphere. In a metal electrode glow discharge, the emission lines and the relative intensities of the metals are strongly dependent on the kind of foreign gas rather than on the other
which are driven into the liquid from the gas phase and subsequently discharged. The nature of the discharge gas above the solution does not affect the discharge significantly since the local temperature under the glow spot must be relatively high and the discharge must occur through the solvent vapor. With aqueous solutions, the plasma region above the cathode surface is saturated with water vapor, since an electrolyte is the cathode. Therefore, the main positive ions present are likely to be H 2O +. Dolan investigated the collision of H 2O molecules with electrons as a function of electron energy [9] and found electron impact ionization producing H 2O molecular ions to be the most probable process: H2 O + e → H2 O + + 2e
(1)
Table 1 Relative intensities of the emission from OH (306.4 nm), Cu (324.7 nm), Cd (second order 228.8 nm), H (686.3 m) and their ratios a Atmosphere
OH
Cu
Cd
H
OH/H
Cu/H
Cd/H
Ar Air He
128 76 12
120 170 50
50 70 20
25 37 11
5.1 2.1 1.1
4.8 4.6 4.5
2.5 2.3 1.8
a
Conditions: current, 80 mA; gas flow rate, 100 ml min −1; discharge gap, 1.5 mm.
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discharge conditions such as the gas pressure and the discharge voltage, suggesting that the foreign gas energy levels play significant roles in determining the ionization and excitation of the analyte atoms [11]. The ratios of the Cu I 324.7 nm and Cd I 2 × 228.8 nm lines to that of the H 656.3 radical in the ELCAD are 4.8–4.5 and 2.5–1.8, respectively, and are independent of the atmosphere. This result may support the idea that the intensity of the atomic metal lines results from a three-body recombination process: M+ + e + X → M + X
(2)
where M + denotes a positive metal ion, e an electron and X a third body. The intensity of the atomic lines emitted by the negative glow is determined primarily by recombination in the cathode dark space and electron impact excitation of neutral atoms in the negative glow. Therefore, the type of discharge gas may not play an important role in the excitation of metal species in ELCAD. The ratio of the intensity of the 306.4 nm OH radical to that of the 656.3 nm H radical was also calculated because the ratio would also be independent of the atmosphere. However, the ratio was in the order argon . air . helium. These results were unexpected, for the mechanism of ionization and excitation of the OH radicals should be similar to that of metals. In ELCAD, electron impact ionization of the H 2O molecules was found to be the most probable process producing H 2O + molecular ions since the plasma region above the cathode surface is saturated by water vapor. Some possible reactions in the electron impact ionization of water vapor have been proposed by Melton [12]. The total ionization crosssection consists of contributions from the reactions shown as Eq. (3) (producing H 2O +), Eq. (4) (OH +), Eq. (5) (O +) and Eq. (6) (H +). H2 O + e → H2 O + + 2e
(3)
H2 O + e → OH + + H + 2e
(4)
H2 O + e → O + + 2H + 2e
(5)
H2 O + e → H + + OH + 2e
(6)
The occurrence of these reactions is supported by our experimental results that OH bands, and H atom and O + ion lines occur in the spectrum. For the OH radical,
Fig. 6. Effect of the discharge current on the emission intensity from a sample containing 5 ppm of Pb, Cd, Cu, and Cr. Discharge conditions; discharge gas, 100 ml min −1 argon; gap, 2.0 mm; pH of the solution, 1.2. Emission lines: Pb, 405.8 nm; Cd, 228.8 nm; Cu 324.7 nm; Cr, 357.8 nm.
dissociative recombination of these ions might be the dominant process: H2 O + + e → H + OH
(7)
OH + + e → OH
(8)
The cross-sectional data for dissociative recombination of the H 2O + and OH + ions presented by Mul et al. [13] were j7 = 1:7 × 10 − 18 (0:11=W )0:92 m2 W , 0:11 eV j7 = 1:7 × 10 − 18 (0:11=W )1:86 m2 W . 0:11 eV j8 = 2:0 × 10 − 20 =W m2 where W is in electron volts. The rate coefficients for the reactions shown as Eq. (7) and Eq. (8) are 10 −13 and 10 −14, respectively if the electron energy is 1 eV, but 10 −16 and 10 −15, respectively, if the electron energy is 40 eV. Therefore, the amount of OH radicals should increase when the electron energy decreases and the concentration of OH + and H 2O + ions increases. Kanzaki et al. [10] measured the emission intensity of H and OH as a function of discharge gas (Ar, N 2, He and H 2) at a current density of 75 mA cm −2. The ratio of OH to H was in the order Ar . N 2 . He . H 2. They also measured the ionic concentrations in glow discharge electrolysis by the double-probe method in various atmospheres, and found that the ionic concentrations were in the order Ar . N 2 . He . H 2. However, the electron energies were in the order Ar . N 2 . He . H 2 at a low
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Fig. 7. Effect of pH on the emission intensity from Cd, Cu, and Zn. Discharge conditions: discharge current, 80 mA; gas, argon; gap, 2.0 mm. (A) Cd (228.8 nm); (×) Cu (324.7 nm); (+) Zn (213.8 nm).
solution temperature (58C) and Ar = N2 . He = H2 at a high solution temperature (138C). These results suggest that the ion concentration in the gas phase and the electron energy difference among the different discharge gases may explain the increase in the emission intensity of the OH radical in the order Ar . N 2 . He. 4.5. Effect of discharge parameters on emission intensity In ELCAD, the intensities of the spectral lines
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depend on operating parameters such as the discharge voltage and current, the pH of the solution, the pressure, and the discharge gas. The intensity of the spectral lines has been investigated as a function of the pH, discharge current [5] and pressure [8], with air as the discharge gas. In this study, the effect of experimental parameters such as the discharge current and the solution pH on the emission intensity were studied with argon as the discharge gas. Fig. 6 shows the effect of the discharge current on the emission intensity of deionized water samples containing 5 ppm of Cd, Cu, Pb, Cr, and Zn. The emission lines for measuring these metals are 228.8 nm, 324.7 nm, 405.8 nm, 357.8 nm and 213.8 nm, respectively. The emission intensity generally increases as the discharge current increases from 40 mA and then levels off after 80 mA for Cu, Cd, Cr and Zn, and 60 mA for Pb. The intensities reduce to zero at about 20 mA with argon as the discharge gas, and this threshold seems to be independent of the discharge gas. Cserfalvi and Mezei [5] observed the same threshold of about 20–30 mA in air at different pressures. The effect of the discharge current on the emission intensity is significant both because of the number of ions bombarding the surface of the water and the increase in the average accelerating potential for the ions caused by a concomitant increase in the discharge voltage.
Fig. 8. Temporal stability of the emission intensities for deionized water containing 10 ppm of Cu and Pb. Discharge conditions: current, 80 mA; gap, 1.5 mm; gas, argon. Emission lines: Cu, 324.7 nm; Pb, 405.8 nm; H, 486.1 nm.
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Table 2 Analytical performance for heavy metals determined by the ELCAD Element
Wavelength (nm)
Line equation
R2
Blank noise
Detection limit (ppm)
Camax (ppm)
Al Cd Cr Cu Mn Pb Hg
396.1 228.8 357.8 324.7 403.1 405.8 365.0
0.5 + 2.8 × 0.15 + 0.11 × 0.1 + 1.4 × 5.4 + 14 × 1.3 + 9.2 × 0.25 + 2.1 × 0.15 + 0.25 ×
0.992 0.981 0.991 0.996 0.997 0.989 0.980
0.44 0.4 0.6 2.3 0.45 0.37 0.25
0.3 0.7 0.9 0.3 0.1 0.3 1
0.2 0.01 0.05 1 0.3 0.1 0.001
a
C max, maximum concentration of heavy metal in drinking water allowed by the Korean Environmental Protection Agency.
One of the interesting observations about ELCAD is that the discharge strongly depends on the pH of the solution. The discharge is significantly reduced in intensity as the pH increases, and no visual emission is observed when the pH of the solution is above 3.0. Fig. 7 shows the effect of pH on the emission intensity of deionized water samples containing 10 ppm of Cd, Cu and Zn. The emission intensity decreases sharply as the pH increases from 1.0 to 3.0, and no emission is observed when the pH of the solution is above 3.0. The reason for the latter may be explained by the relationship between pH and cathode fall. Cserfalvi and Mezei [5] measured the cathode fall as a function of pH and found that it is almost constant in the pH region 4–8, but significantly decreases as the pH falls below 4. The emission intensity decreases as the cathode fall decreases.
4.6. Analytical performance Fig. 8 shows the temporal stability of the glow discharge electrolyte-atomic emission system for a deionized water sample containing 10 ppm of Cu and Pb. The emission line of H b at 486.1 nm was also measured. The discharge was run at 80 mA, 1300 V and with a 1.5 mm discharge gap. Intensities for these species become detectable immediately after the discharge is initiated. The emission intensity reaches a steady state after 1 min and shows RSDs of below 2.5% and below 2.4% for Cu and Pb, respectively, for 30 min. The RSD for H b is below 4%, twofold higher than for Cu and Pb, possibly because hydrogen has higher mobility than the other elements
and is therefore easily influenced by changes in experimental parameters such as the gas flow rate and the solution temperature. The fluctuation of the emission signal is mainly due to small changes in the water level, which causes a variation in the optical focusing and voltage–current relationship in the discharge. To ascertain the analytical performance of ELCAD, the detection limits for some elements were investigated. Each sample was prepared with deionized water of pH 1.0 adjusted with HNO 3. The limits of detection (LODs) were calculated using the equation LOD = (ksbk )=m where k = 3, s bk is the standard deviation of the signal from the blank, and m is the initial slope of the calibration line. A k value of 3 is used to give a greater than 99% confidence that the signal is due to the analyte and not random error. The discharge was operated at a 80 mA discharge current, a 10 ml min −1 sample flow rate and a 1.5 mm discharge gap. Table 2 summarizes the LOD values of Al, Cd, Cr, Cu, Mn, Pb and Hg using ELCAD. The values are in the range from 0.1 to 1 ppm and, for most elements studied here, are not good enough for the measurement of heavy metals in drinking water at the concentrations suggested by the Korean Environmental Protection Agency. However, this technique shows the potential for on-line monitoring of heavy metals in waste water, since the detection limit requirement for most heavy metals in water is higher than that in drinking water.
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5. Conclusion A stable glow discharge can be produced by applying 1–2 kV in atmospheric argon gas between an electrolyte solution cathode and a platinum rod anode. The spectrum of the discharge contains basic atomic lines of the dissolved metals in water and OH band peaks, but no emission lines of argon from the discharge gas. This situation is different from that pertaining when using a solid cathode, suggesting a different excitation mechanism. The intensities of the spectral lines depend on operating parameters such as the pH of the solution, the discharge current, the pressure and the discharge gas. Detection limits for Al, Cd, Cr, Cu, Mn, Pb and Hg are sub-parts per million. The ELCAD system demonstrates the possibility for the development of a device for the continuous metals analysis of water and waste water solutions. Acknowledgements The authors are indebted to T. Cserfalvi and P. Mezei for helpful criticisms and suggestions. References [1] J. Gubkin,, Ann. Phys. 32 (III) (1887) 114. [2] A. Hickling, D. Ingram, Contact glow discharge electrolysis, Trans. Faraday Soc. 60 (1964) 783.
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[3] D.E. Couch, A. Brenner, Glow discharge spectra of copper and indium above aqueous solutions, J. Electrochem. 106 (1959) 628. [4] T. Cserfalvi, P. Mezei, P. Apai, Emission studies on a glow discharge in atmospheric pressure air using water as a cathode, J. Phys. D 26 (1993) 2184. [5] T. Cserfalvi, P. Mezei, Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry, J. Anal. At. Spectrom. 9 (1994) 345. [6] T. Cserfalvi, P. Mezei, Operating mechanism of the electrolyte cathode atmospheric glow discharge, Fresenius’ Z. Anal. Chem. 355 (1996) 813. [7] H.J. Kim and J.S. Kang, Development of oscillating-plasma glow discharge as detector for GC, Abstr. 127, 24th CSI Conf. Liepzig, Germany, August 1995. [8] P. Mezei, T. Cserfalvi, M. Janossy, Pressure dependence of the atmospheric electrolyte cathode glow discharge spectrum, J. Anal. At. Spectrom. 12 (1997) 1203. [9] T.J. Dolan, Electron and ion collisions with water vapour, J. Phys. D 26 (1993) 4. [10] Y. Kanzaki, N. Nishimura, O. Matsumoto, On the yields of glow discharge electrolysis in various atmosphere, J. Electroanal. Chem. 167 (1984) 297. [11] K. Wagatsuma, K. Hirokawa, Characterization of atomic emission lines from argon, neon, and nitrogen glow discharge plasma, Anal. Chem. 57 (1985) 2901. [12] C.E. Melton, Radiolysis of water vapor in a wide range radiolysis source of a mass spectrometer. I. Individual and total cross sections for the production of positive ions, negative ions and free radicals by electrons, J. Phys. Chem. 74 (1970) 582. [13] P.M. Mul, J.W. McGowan, P. Defrance, J.B.A. Mitchell, Merged electron-in beam experiments. V. Dissociative recombination of hydroxyl, water, hydronium and hydronium-d 3 monocations, J. Phys. B 16 (1983) 3099.
Fundamental studies of electrolyte-as-cathode glow discharge-atomic emission spectrometry for the determination of trace metals in flowing water1 Yang S. Park a, Soo H. Ku a, Sung H. Hong a, Hyo J. Kim a,*, Edward H. Piepmeier b a
Department of Pharmacy, Dongduk Women’sUniversity, 23-1, Sungbuk-ku, 136-714Seoul, Korea b Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA Received 15 January 1998; accepted 27 April 1998
Abstract The fundamental characteristics and analytical performance were investigated of a new glow discharge emission source (GDES) for the determination of trace metals in flowing water. The application of an atmospheric glow discharge in argon gas between an electrolyte solution cathode and a platinum rod anode led to the development of a stable discharge. The intensity of the lines was found to depend strongly on the acidity of the water, the current and the discharge gap. The spectrum emitted from the tap water contained the basic atomic lines of the dissolved metals and OH band peaks, but no emission lines of argon from the discharge gas. The strong emission lines for an element and no emission lines from the discharge gas in the electrolyteas-cathode glow discharge (ELCAD) were different from those in a solid-as-cathode discharge, suggesting a different excitation mechanism. Sub-parts-per-million detection limits of Al, Cd, Cr, Cu, Mn, Pb and Hg were obtained. The continuous-flow sample system demonstrates the possibility to develop a device for the continuous analysis of water and waste water solutions. q 1998 Elsevier Science B.V. All rights reserved Keywords: Electrolyte-as-cathode glow discharge (ELCAD); Atomic emission spectrometry (AES); Water analysis; Metals
1. Introduction Glow discharge electrolysis (GDE) is a type of glow discharge generated between an electrolyte solution cathode and a solid anode at a distance of a few millimeters in the gas phase. Gubkin [1] demonstrated the first possibility of the electrolysis of aqueous solutions of metallic salts using a glow discharge in 1887. Hickling and Ingram [2] extensively reviewed the phenomenon in GDE. They found that one characteristic feature of GDE is that the * Corresponding author. 1 Published in honor of Professor C.L. Chakrabarti
electrolysis yield usually exceeds that expected from Faraday’s law, often about sevenfold. The important factor that led to such high yields was the acceleration energy of the positive ions, which was about 100 eV. The principal experimental parameters that influenced the yields were the gas pressure and the concentration of the reactants. The first paper to mention a relationship between the spectrum emitted by the GDE and the composition of the electrolyte was that of Couch and Brenner [3] in 1959. They found an unusual glow discharge produced between a tungsten electrode and an aqueous solution of cupric and indium salts. The discharge was obtained at 0.05–0.2 A and 500 V with low pressures
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 15 4 -2
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of 5–10 torr. The spectra contained various atomic lines of Cu and In and some molecular bands of InCl and OH. They explained this phenomenon as being due to a volatile copper or indium compound forming in the solution and evaporating into the evacuated space to produce the spectrum. Cserfalvi et al. [4] developed electrolyte-as-cathode glow discharge (ELCAD)-atomic emission spectrometry (AES) and demonstrated a possible analytical application in the continuous analysis of water and waste water solutions. They found that the spectral lines emitted from the discharge are a consequence of the cathodic sputtering of water during the discharge. The intensity of the lines was found to depend strongly on the pH of the water. They also found that the calibration curves of line intensities versus metal concentrations were linear in the 1–50 ppm range and showed a strong dependence on the discharge current and hydrogen ion concentration of the solution. They also showed that pulse-modulated operation with time-resolved signal processing improved the signalto-background ratio. Cserfalvi and Mezei [5] also described the operating mechanism of the electrolyte-as-cathode atmospheric pressure glow discharge and found that the cathode fall is the most important factor in explaining the influence of the pH and discharge current on the emission intensity of an analyte. Cserfalvi and Mezei [6] measured the cathode fall as a function of pH and found that it is almost constant in the pH region 4–8 but decreases significantly as the pH decreases below 4. They also showed that the emission intensity decreases as the cathode fall increases. The observed decrease of the cathode fall with decreasing pH was explained by a model that uses two different electron emission processes near the electrolyte cathode; emission coupled with hydrated electrons is dominant below pH 2.5, while protonindependent emission of poor efficiency operates above pH 3. In our laboratory, analytical methods for the continuous monitoring of heavy metals in drinking and waste water are of interest, and ELCAD may prove to be a useful technique. Various analytical techniques are available for determining trace elements in drinking water, such as GF-AAS, ICP-AES and ICP-MS. However, these techniques are inadequate for the online monitoring of trace elements due to the high cost
of operation, robustness, and ease of operation. Electrochemical methods have been widely used for the on-line monitoring of trace metals in processors and flowing water, and show good detection limits and precision. However, most electrochemical methods (voltammetry, polarography) suffer from matrix effects, and determine only three to four elements with one electrode; therefore three or four instruments are needed for determining all the elements in drinking water that are required by government regulatons to be quantified. ELCAD is a potentially new analytical source for the on-line monitoring of trace elements in flowing solutions. In our laboratory, ELCAD has been further developed and its fundamental characteristics and analytical performance have been studied with a high-resolution monochromator. The effect of the discharge gas on the spectrum has been studied in order to elucidate the excitation mechanism.
2. Experimental 2.1. Glow discharge electrolysis-atomic emission spectrometry system Fig. 1 shows a schematic diagram of the ELCAD cell for the direct analysis of solutions. The liquid sample was introduced using a peristaltic pump (Master Flux, Niles, IL, USA) at a flow rate of 10 ml min −1. The optimum sample introduction rate was found to be 5–10 ml min −1. In the case of a lower sample flow rate, continuous overflowing of the water sample on the cathode was difficult due to surface tension. A higher sample flow rate also helped to reduce the boiling of the water. The gas flow rate was controlled by a mass flow controller (MKS, Andover, MA, USA; 0–500 ml min −1). A highvoltage d.c. power supply (Fug Co. MCN 350, Rosenheim-Langenpfunzen, Germany) capable of supplying up to 2000 V and 250 mA was used. To stabilize the discharge current, a 500 Q ballast resistor was placed in series with the anode discharge. The glow discharge cell was made of glass and was similar to that reported previously [2]. The anode consisted of a 3 mm o.d. platinum rod with a rounded end located just above the cathode. The distance between the anode and the water surface could be adjusted by
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Fig. 1. Schematic diagram of the ELCAD cell. 1, Monochromator; 2, focusing lens; 3, quartz window; 4, sample solution dome; 5, cathode tube (2–10 mm o.d.); 6, sample introduction tube (2.0 mm o.d.); 7, waster water 8, waster valve; 9, solution (from peristaltic pump); 10, plasma; 11, mass flow controller; 12, platinum anode (3 mm o.d.); 13, graphite cathode (5.0 mm o.d.).
rotating a micrometer attached to the anode. The water was connected to the cathode of the power supply through a graphite rod. This electrode was in a separate glass compartment with o.d. 5 cm in order to prevent the reduction of the product at the cathode. A continuous flow of argon served to prevent the accumulation of hydrogen and oxygen in the gas phase, with their possible risk of explosion. The distance between the graphite rod and the cathode spot on the water surface was 5 cm. The liquid sample was introduced through a 2 mm o.d. capillary tube and formed a dome of flowing solution. Various sizes of glass tubes from 2 to 10 mm o.d. could be attached to the top of the capillary tube, resulting in different sizes of the cathode surface. When the cathode tube was less than 5 mm o.d., the plasma was easily spread over the water surface. Therefore, a cathode tube of 10 mm o.d. was used in this study. To prevent overheating of the water sample during the discharge, it was circulated continuously through an overflowing inlet tube and was disposed of through an outlet tube. Various discharge gases such as argon, helium and air were studied, and one of the gases was introduced near the anode to localize the discharge more
effectively between the anode and cathode. A second gas inlet was located near the emission window to prevent the deposition of water droplets in the optical path. Reducing the distance between the anode and cathode to about 1 mm initiated the discharge, and a stable discharge was obtained when operated with a discharge gap of 3–5 mm, at 1500 V and 80 mA. 2.2. Optical spectrometry system The emission spectra of the discharge plasma were collected and foccused one-to-one by a 25 mm diameter symmetric convex spherical silica lens with a focal length of 150 mm onto a 1.0 m Czerny–Turner grating monochromator (Spex model 1000M; Edison, NJ, USA). Radiation was detected with a photomultiplier tube (model R928; Hamamatsu, Japan). The f/8, 1.0 m monochromator employs a 110 mm square plane grating (blazed for 500 nm) with 1200 lines per millimeter, giving it a reciprocal linear dispersion of 0.8 nm mm -1 in the first order. Data acquisition and monochromator control were performed with a DS1000 computer interface board (Spex; Edison, NJ, USA) and homemade software
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written in QuickBASIC language. In the investigation of the effect of the operating parameters on the emission intensity at each wavelength, the monochromator scanned at a rate of 0.005 nm every 0.5 s to acquire the emission spectra. Scan step sizes of 0.01 nm and integration times of 0.1 s were employed to acquire the emission spectra from 200 to 800 nm.
3. Reagents The pH of the sample was adjusted by adding HNO 3, and was measured with a pH meter (Analab 88 ATC; SunYil Co. Korea). A deionized water sample was obtained from an Elgastat purifier (model UHQOS; UK). Working standards for nine analyte elements were prepared by the dilution of 1000 ppm stock solutions (Showa, Japan).
4. Results and discussion 4.1. Glow discharge with liquid cathode and solid anode A typical glow discharge operates at several hundred volts and several millitorr pressure with a solid anode and cathode. However, a glow discharge with a liquid cathode and solid anode, or a liquid anode and solid cathode, can also be obtained. To initiate the discharge between the liquid cathode and the solid anode at atmospheric pressure, the distance between the anode and cathode can be reduced to less than 1.0 mm or a 20 kV high-voltage spark applied. The ignition voltage increases greatly as the distance between the cathode and anode increases, and a voltage above 1500 V is required to initiate the discharge at a distance of 1.0 mm. After the discharge is established, a stable discharge is sustained at a distance of several millimeters between the solid anode and water as the cathode. When the discharge is operated using tap water spiked with 50 ppm of Cu and with the pH adjusted to 1.0 with HNO 3, a typical glow discharge plasma appears at about a 2 mm gap between the solid anode and the water cathode. The discharge is obtained with a current of 80 mA, a voltage of 1200 V, and argon as the discharge gas. An anode glow can be seen just
below the rounded anode rod located at the top. A bright green color, due to the excitation of Cu, is obvious in the middle of the discharge. However, the typical glow regions that appear in a low-pressure glow discharge, such as the negative glow and the positive column, may not be identified visually in this type of discharge; Cu excitation may occur in part of the area of the negative glow. This glow discharge is obtained when the discharge gap is reduced to about 1 mm under typical operational conditions of several hundred volts and a few milliamps [7]. In this high-pressure glow discharge, the length of the positive column decreases with the distance between anode and cathode. A second bright portion of the plasma occurs on the water surface where cathodic sputtering occurs. This plasma shrinks and shows no green emission when the pH of the water sample is above 3.0 at a constant discharge voltage of 1500 V. The discharge current decreases to 20 mA to maintain the constant voltage at 1500 V. This indicates that the pH of the solution is an important factor for sustaining the discharge. Increasing the conductivity of the solution also helps to maintain the plasma. Fig. 2 shows characteristic current–voltage curves for various discharge gaps. The discharge was operated at atmospheric pressure with argon gas between the platinum anode and the cathode was deionized water adjusted to a pH of 1.0 with HNO 3. The deionized water sample was continuously circulated at 10 ml min −1 through an overflowing inlet tube, and was disposed of through an outlet tube using a
Fig. 2. Current–voltage characteristic curves for various discharge gaps. (A) 1.0 mm; ( × ) 2.0 mm; ( + ) 3.0 mm; (W) 4.0 mm. (Discharge gas, Ar; sample, deionized water at pH 1.0 adjusted with HNO 3.)
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peristaltic pump. The discharge ignition was obtained by decreasing the distance between the anode and cathode to less than 1.0 mm by moving the anode towards the water surface while applying a constant voltage of 1500 V. A stable discharge was sustained with several hundred volts and several tens of milliamps. When the discharge was on, a rounded glow spot sat on the surface of the electrolyte solution cathode. The size of this glow spot increased as the discharge current increased. The current density at this spot was 400–600 mA cm −2, which agrees with the value of 350–600 mA cm −2 with air as the discharge gas. During operation, the distance between the cathode and anode varied from 1 to 4 mm. When the discharge gap was increased further than 5 mm, the discharge was unstable. The discharge voltage increased as the discharge current increased when the discharge gap was between 1 and 4 mm. The atmospheric ELCAD using air as the discharge gas is known to be an abnormal glow region because the discharge voltage increases as the discharge current increases [4]. With argon as discharge gas, the voltage–current relationship was similar to that with air. Not many differences in the voltage–current relationships were found among discharge gases such as helium, argon and air. As the discharge gap increased, the discharge voltage increased at constant current. 4.2. Background spectra Most of the background spectral features in ICPs and DCPs arise from trace components in the plasma gas or water vapor. The background spectrum in a glow discharge contains emission lines from both the plasma gas and the metal cathode due to sputtering of the cathode. Fig. 3 shows a typical spectrum of deionized water obtained with a discharge current of 50 mA, a voltage of 1200 V and a gap of 1.5 mm at atmospheric pressure. A 100 ml min −1 flow of argon was used for the discharge gas. The water sample was acidified to pH 1.0 with HNO 3 and introduced into the cathode at 10 ml min −1. These spectra were collected using scan step sizes of 0.01 nm and integration times of 0.1 s for the spectral regions 200–300 nm (Fig. 3a), 300–400 nm (Fig. 3b) and 400–500 nm (Fig. 3c). In the 200–300 nm region, the spectrum is dominated by emission bands of OH at 283 nm (Fig. 3a).
Fig. 3. Spectrum of deionized water obtained with a discharge current of 50 mA, a gas flow rate of 100 ml min −1 and a discharge gap of 1.5 mm in argon at atmospheric pressure. (a) 200–300 nm; (b) 300–400 nm; (c) 400–500 nm.
There are no other prominent lines in this region. In the 300–400 nm region (Fig. 3b), the spectrum is also dominated by emission bands of OH at 306.4 nm. It was reported that the OH bands seen in this spectrum are attributed to a recombination process of the precursor species of OH + ions [8]. These are reported to be a major product of the electron impact ionization of the H 2O molecules in water vapor [9]. Various species formed upon the dissociation of the water molecules
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(i.e. H+3, O +, OH + etc.) can also be detected using lowpressure glow discharge-mass spectrometry. In the 400-500 nm region (Fig. 3c), the H b 486.1 nm spectral line is apparent. Cserfalvi et al. [4] monitored the emission intensity of the H b line as a function of the solution pH, it becoming more intense with decreasing pH. An increase in the H + ion concentration in the solution may result in an increasing amount of H in the plasma and an increasing H line intensity. The spectrum shows many lines due to oxygen ions in the 400–500 nm region when using argon as the discharge gas. Mezei et al. [8] also reported the same oxygen ion lines in a nitrogen and air atmosphere. Since the excitation energies of oxygen ions are very high (around 25–28 eV) and the only possible source of the oxygen is water, the oxygen ions are produced from water vapor by electron impact [8,9]. One of the interesting results found from the spectrum of the blank was the absence of emission lines from argon gas atoms and ions. Since the glow discharge plasma is sustained because of excitation and ionization between the discharge gas and electrons, there are many emission lines from discharge gases in low-pressure GDs. Therefore, the excitation conditions are different in this device. The spectrum appears relatively simple compared with emission spectra from other plasma sources such as the GD, ICP and MIP, due to the absence of emission lines from the discharge gas. 4.3. Spectrum of tap water Fig. 4 shows a spectrum of tap water collected from the laboratory, spiked with 10 ppm of Cu and Pb, obtained with a discharge current of 50 mA, a voltage of 1200 V, a gas flow rate of 100 ml min −1 and a discharge gap of 1.5 mm at atmospheric pressure. A discharge voltage of 1200 V was required to keep a 50 mA discharge current for the tap water, and 1500 V was required for the deionized water. The reason for the lower discharge voltage in the case of tap water might be due to its higher conductivity. The tap water sample was also acidified to pH 1.0 with HNO 3. These spectra were collected using scan step sizes of 0.01 nm and integration times of 0.1 s for the spectral regions 200–400 nm (Fig. 4a), 400–600 nm (Fig. 4b), and 600–800 nm (Fig. 4c). The most
Fig. 4. Spectrum of tap water spiked with 10 ppm of Cu and Pb obtained with a discharge current of 50 mA, a gas flow rate of 100 ml min −1 and a discharge gap of 1.5 mm in argon at atmospheric pressure. (a) 200–400 nm; (b) 400–600 nm; (c) 600– 800 nm.
prominent features of the emission spectrum in tap water are OH bands at 306.4 nm and (second order) 612.8 nm. In the 200–400 nm region (Fig. 4a), the spectra are dominated by emission bands of OH at 283 and 306.4 nm as well as two Cu I resonance lines at 324.7 and 327.4 nm, and an Mg I line at 285.3 nm. The Pb I 217.0 nm line is barely seen in
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the spectrum. However, there are no peaks from Cu ion lines, such as Cu II at 224.7 nm, which are apparent in low-pressure glow discharges having solid electrodes. In the 400–600 nm region (Fig. 4b), the spectra are dominated by Na I lines at 589.0 nm and Mg I (second order; 2 × 285.3 nm) as well as the Ca I 422.7 nm line. Pb I 405.8 nm, H b 486.1 nm and Cu I 510.5 nm lines are also seen in the spectrum. Again, there are no emission lines from argon gas in the spectral ranges from 350 to 500 nm. In the 600– 800 nm region (Fig. 4c), a typical OH band peak is apparent at 612.8 nm and at the second order of 306.4 nm. Cu I 324.7 and 327.4 nm lines (both second order) also occur. Since the grating system is blazed for 500 nm, the second order OH band (2 × 306.4 nm) is much stronger than the first order OH band at 306.4 nm. K I 766.5 and 769.9 nm lines are also observed. The spectral lines arise mainly from the elements dissolved originally (Ca, Na, and K) and spiked (Cu, Pb) in tap water and can be attributed to the effect of cathode sputtering during the discharge. The mechanism of emission has been extensively investigated [4–6,8]. When the discharge is operating, the positive metal ions are released from the electrolyte cathode surface due to cathode sputtering. These positive metal ions cannot pass through the negative space charge near the cathode dark space but escape from the cathode surface and diffuse into the negative glow only if they are recombining (becoming neutral) in the cathode dark space. The recombination of the positive metal ions takes place via a three-body collision involving one positive metal ion and two electrons. The neutral metal atoms produced in this way diffuse into the negative glow, where they are excited by electron collisions. 4.4. Effect of discharge gas on the emission intensity In low-pressure GDs with solid electrodes, the discharge gas plays an important role in both the sputtering process and the gas phase electron characteristics. Wagatsuma and Hirokawa [11] found that the emission intensities of Cu atoms and ions are strongly dependent on the type of foreign gas rather than other discharge conditions such as gas pressure and discharge voltage. Also, the spectrum contains many atomic and ionic gas emission lines which result
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from the excitation and ionization of foreign gases. Cserfalvi et al. [4] investigated a liquid cathode and solid anode glow discharge in air at atmospheric pressure; the spectrum showed typical OH bands at 306 and 612 nm as well as nitrogen band peaks at 358 nm, probably due to the excitation of the air. To investigate the effect of different discharge gases on the ELCAD, argon, air and helium were each introduced into the discharge chamber. Fig. 5 shows the effect of these discharge gases (air, Fig. 5a; argon, Fig. 5b; helium, Fig. 5c) on a spectrum over the range from 200 to 500 nm for a deionized water sample containing 10 ppm of Cu, Pb, Zn, and Cd. Each spectrum was obtained with an 80 mA discharge current, a 1.5 mm discharge gap and a 100 ml min −1 gas flow. Again, one of the interesting results is that the spectra show no significant changes in the emission lines, but the relative emission intensities of the lines varied with the discharge gas. The spectra of three different discharge gases show typical OH bands at 281 and 306 nm. The spectral lines of the elements dissolved in the sample are also obvious for Cu I at 324.7 nm, Cu I at 327.4 nm, Pb I at 368.4 nm and Pb I at 405.8 nm. The emission lines of Cd I at 228.8 nm and Zn I at 213.7 nm are also visible. When air is employed as the discharge gas, molecular band spectra are emitted which result from transitions among various vibrational states in different electronic states of N 2. No emission lines from argon and helium discharge gases occur. In the case of a solid cathode and an anode glow discharge with argon as the discharge gas, many argon atomic and ionic lines appear in the wavelength range 400– 500 nm, resulting from transitions from 3p 54s (11.55–11.83 eV) to 3p 55p (14.46–14.74 eV). Emission lines from singly ionized species (Ar II) also appear in the wavelength region 294.29– 496.51 nm [11]. There are no emission peaks from argon atoms or ions in the wavelength range from 400 to 500 nm in the spectrum of ELCAD. This suggests that the excitation mechanism of metals in solution is different from that of the normal low-pressure solid cathode glow discharge, where elements are sputtered due to the bombardment of ionized gas and further excited by high-energy electrons. In GDE, Hickling and Ingram [2] mentioned that the discharge current is carried to the solution by positive gaseous ions
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Fig. 5. Effect of the discharge gas on the spectrum obtained with a 80 mA discharge current, a 1.5 mm discharge gap and a 100 ml min −1 gas flow rate. Sample, deionized water containing 10 ppm of Cu and Cd; spectral range, 200–800 nm. (a) Air; (b) argon; (c) helium.
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Fig. 5. Continued.
In the case of nitrogen molecules, many energy levels exist continuously between 0 and 8 eV due to the many vibrational and rotational states belonging to each electronic state. The characteristics of the emission from H and OH radicals and Cu I and Cd I were measured. Table 1 shows an example of the effect of different discharge gases on the emission intensities of H (656.3 nm), OH radicals (306.4 nm), Cu I (324.7 nm) and Cd I (second order of 228.8 nm). The ratio of the intensities of Cu I and Cd I to that of the H radical was calculated for three different discharge gases because the ratio is dependent on the atmosphere. In a metal electrode glow discharge, the emission lines and the relative intensities of the metals are strongly dependent on the kind of foreign gas rather than on the other
which are driven into the liquid from the gas phase and subsequently discharged. The nature of the discharge gas above the solution does not affect the discharge significantly since the local temperature under the glow spot must be relatively high and the discharge must occur through the solvent vapor. With aqueous solutions, the plasma region above the cathode surface is saturated with water vapor, since an electrolyte is the cathode. Therefore, the main positive ions present are likely to be H 2O +. Dolan investigated the collision of H 2O molecules with electrons as a function of electron energy [9] and found electron impact ionization producing H 2O molecular ions to be the most probable process: H2 O + e → H2 O + + 2e
(1)
Table 1 Relative intensities of the emission from OH (306.4 nm), Cu (324.7 nm), Cd (second order 228.8 nm), H (686.3 m) and their ratios a Atmosphere
OH
Cu
Cd
H
OH/H
Cu/H
Cd/H
Ar Air He
128 76 12
120 170 50
50 70 20
25 37 11
5.1 2.1 1.1
4.8 4.6 4.5
2.5 2.3 1.8
a
Conditions: current, 80 mA; gas flow rate, 100 ml min −1; discharge gap, 1.5 mm.
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discharge conditions such as the gas pressure and the discharge voltage, suggesting that the foreign gas energy levels play significant roles in determining the ionization and excitation of the analyte atoms [11]. The ratios of the Cu I 324.7 nm and Cd I 2 × 228.8 nm lines to that of the H 656.3 radical in the ELCAD are 4.8–4.5 and 2.5–1.8, respectively, and are independent of the atmosphere. This result may support the idea that the intensity of the atomic metal lines results from a three-body recombination process: M+ + e + X → M + X
(2)
where M + denotes a positive metal ion, e an electron and X a third body. The intensity of the atomic lines emitted by the negative glow is determined primarily by recombination in the cathode dark space and electron impact excitation of neutral atoms in the negative glow. Therefore, the type of discharge gas may not play an important role in the excitation of metal species in ELCAD. The ratio of the intensity of the 306.4 nm OH radical to that of the 656.3 nm H radical was also calculated because the ratio would also be independent of the atmosphere. However, the ratio was in the order argon . air . helium. These results were unexpected, for the mechanism of ionization and excitation of the OH radicals should be similar to that of metals. In ELCAD, electron impact ionization of the H 2O molecules was found to be the most probable process producing H 2O + molecular ions since the plasma region above the cathode surface is saturated by water vapor. Some possible reactions in the electron impact ionization of water vapor have been proposed by Melton [12]. The total ionization crosssection consists of contributions from the reactions shown as Eq. (3) (producing H 2O +), Eq. (4) (OH +), Eq. (5) (O +) and Eq. (6) (H +). H2 O + e → H2 O + + 2e
(3)
H2 O + e → OH + + H + 2e
(4)
H2 O + e → O + + 2H + 2e
(5)
H2 O + e → H + + OH + 2e
(6)
The occurrence of these reactions is supported by our experimental results that OH bands, and H atom and O + ion lines occur in the spectrum. For the OH radical,
Fig. 6. Effect of the discharge current on the emission intensity from a sample containing 5 ppm of Pb, Cd, Cu, and Cr. Discharge conditions; discharge gas, 100 ml min −1 argon; gap, 2.0 mm; pH of the solution, 1.2. Emission lines: Pb, 405.8 nm; Cd, 228.8 nm; Cu 324.7 nm; Cr, 357.8 nm.
dissociative recombination of these ions might be the dominant process: H2 O + + e → H + OH
(7)
OH + + e → OH
(8)
The cross-sectional data for dissociative recombination of the H 2O + and OH + ions presented by Mul et al. [13] were j7 = 1:7 × 10 − 18 (0:11=W )0:92 m2 W , 0:11 eV j7 = 1:7 × 10 − 18 (0:11=W )1:86 m2 W . 0:11 eV j8 = 2:0 × 10 − 20 =W m2 where W is in electron volts. The rate coefficients for the reactions shown as Eq. (7) and Eq. (8) are 10 −13 and 10 −14, respectively if the electron energy is 1 eV, but 10 −16 and 10 −15, respectively, if the electron energy is 40 eV. Therefore, the amount of OH radicals should increase when the electron energy decreases and the concentration of OH + and H 2O + ions increases. Kanzaki et al. [10] measured the emission intensity of H and OH as a function of discharge gas (Ar, N 2, He and H 2) at a current density of 75 mA cm −2. The ratio of OH to H was in the order Ar . N 2 . He . H 2. They also measured the ionic concentrations in glow discharge electrolysis by the double-probe method in various atmospheres, and found that the ionic concentrations were in the order Ar . N 2 . He . H 2. However, the electron energies were in the order Ar . N 2 . He . H 2 at a low
Y.S. Park et al./Spectrochimica Acta Part B 53 (1998) 1167–1179
Fig. 7. Effect of pH on the emission intensity from Cd, Cu, and Zn. Discharge conditions: discharge current, 80 mA; gas, argon; gap, 2.0 mm. (A) Cd (228.8 nm); (×) Cu (324.7 nm); (+) Zn (213.8 nm).
solution temperature (58C) and Ar = N2 . He = H2 at a high solution temperature (138C). These results suggest that the ion concentration in the gas phase and the electron energy difference among the different discharge gases may explain the increase in the emission intensity of the OH radical in the order Ar . N 2 . He. 4.5. Effect of discharge parameters on emission intensity In ELCAD, the intensities of the spectral lines
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depend on operating parameters such as the discharge voltage and current, the pH of the solution, the pressure, and the discharge gas. The intensity of the spectral lines has been investigated as a function of the pH, discharge current [5] and pressure [8], with air as the discharge gas. In this study, the effect of experimental parameters such as the discharge current and the solution pH on the emission intensity were studied with argon as the discharge gas. Fig. 6 shows the effect of the discharge current on the emission intensity of deionized water samples containing 5 ppm of Cd, Cu, Pb, Cr, and Zn. The emission lines for measuring these metals are 228.8 nm, 324.7 nm, 405.8 nm, 357.8 nm and 213.8 nm, respectively. The emission intensity generally increases as the discharge current increases from 40 mA and then levels off after 80 mA for Cu, Cd, Cr and Zn, and 60 mA for Pb. The intensities reduce to zero at about 20 mA with argon as the discharge gas, and this threshold seems to be independent of the discharge gas. Cserfalvi and Mezei [5] observed the same threshold of about 20–30 mA in air at different pressures. The effect of the discharge current on the emission intensity is significant both because of the number of ions bombarding the surface of the water and the increase in the average accelerating potential for the ions caused by a concomitant increase in the discharge voltage.
Fig. 8. Temporal stability of the emission intensities for deionized water containing 10 ppm of Cu and Pb. Discharge conditions: current, 80 mA; gap, 1.5 mm; gas, argon. Emission lines: Cu, 324.7 nm; Pb, 405.8 nm; H, 486.1 nm.
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Table 2 Analytical performance for heavy metals determined by the ELCAD Element
Wavelength (nm)
Line equation
R2
Blank noise
Detection limit (ppm)
Camax (ppm)
Al Cd Cr Cu Mn Pb Hg
396.1 228.8 357.8 324.7 403.1 405.8 365.0
0.5 + 2.8 × 0.15 + 0.11 × 0.1 + 1.4 × 5.4 + 14 × 1.3 + 9.2 × 0.25 + 2.1 × 0.15 + 0.25 ×
0.992 0.981 0.991 0.996 0.997 0.989 0.980
0.44 0.4 0.6 2.3 0.45 0.37 0.25
0.3 0.7 0.9 0.3 0.1 0.3 1
0.2 0.01 0.05 1 0.3 0.1 0.001
a
C max, maximum concentration of heavy metal in drinking water allowed by the Korean Environmental Protection Agency.
One of the interesting observations about ELCAD is that the discharge strongly depends on the pH of the solution. The discharge is significantly reduced in intensity as the pH increases, and no visual emission is observed when the pH of the solution is above 3.0. Fig. 7 shows the effect of pH on the emission intensity of deionized water samples containing 10 ppm of Cd, Cu and Zn. The emission intensity decreases sharply as the pH increases from 1.0 to 3.0, and no emission is observed when the pH of the solution is above 3.0. The reason for the latter may be explained by the relationship between pH and cathode fall. Cserfalvi and Mezei [5] measured the cathode fall as a function of pH and found that it is almost constant in the pH region 4–8, but significantly decreases as the pH falls below 4. The emission intensity decreases as the cathode fall decreases.
4.6. Analytical performance Fig. 8 shows the temporal stability of the glow discharge electrolyte-atomic emission system for a deionized water sample containing 10 ppm of Cu and Pb. The emission line of H b at 486.1 nm was also measured. The discharge was run at 80 mA, 1300 V and with a 1.5 mm discharge gap. Intensities for these species become detectable immediately after the discharge is initiated. The emission intensity reaches a steady state after 1 min and shows RSDs of below 2.5% and below 2.4% for Cu and Pb, respectively, for 30 min. The RSD for H b is below 4%, twofold higher than for Cu and Pb, possibly because hydrogen has higher mobility than the other elements
and is therefore easily influenced by changes in experimental parameters such as the gas flow rate and the solution temperature. The fluctuation of the emission signal is mainly due to small changes in the water level, which causes a variation in the optical focusing and voltage–current relationship in the discharge. To ascertain the analytical performance of ELCAD, the detection limits for some elements were investigated. Each sample was prepared with deionized water of pH 1.0 adjusted with HNO 3. The limits of detection (LODs) were calculated using the equation LOD = (ksbk )=m where k = 3, s bk is the standard deviation of the signal from the blank, and m is the initial slope of the calibration line. A k value of 3 is used to give a greater than 99% confidence that the signal is due to the analyte and not random error. The discharge was operated at a 80 mA discharge current, a 10 ml min −1 sample flow rate and a 1.5 mm discharge gap. Table 2 summarizes the LOD values of Al, Cd, Cr, Cu, Mn, Pb and Hg using ELCAD. The values are in the range from 0.1 to 1 ppm and, for most elements studied here, are not good enough for the measurement of heavy metals in drinking water at the concentrations suggested by the Korean Environmental Protection Agency. However, this technique shows the potential for on-line monitoring of heavy metals in waste water, since the detection limit requirement for most heavy metals in water is higher than that in drinking water.
Y.S. Park et al./Spectrochimica Acta Part B 53 (1998) 1167–1179
5. Conclusion A stable glow discharge can be produced by applying 1–2 kV in atmospheric argon gas between an electrolyte solution cathode and a platinum rod anode. The spectrum of the discharge contains basic atomic lines of the dissolved metals in water and OH band peaks, but no emission lines of argon from the discharge gas. This situation is different from that pertaining when using a solid cathode, suggesting a different excitation mechanism. The intensities of the spectral lines depend on operating parameters such as the pH of the solution, the discharge current, the pressure and the discharge gas. Detection limits for Al, Cd, Cr, Cu, Mn, Pb and Hg are sub-parts per million. The ELCAD system demonstrates the possibility for the development of a device for the continuous metals analysis of water and waste water solutions. Acknowledgements The authors are indebted to T. Cserfalvi and P. Mezei for helpful criticisms and suggestions. References [1] J. Gubkin,, Ann. Phys. 32 (III) (1887) 114. [2] A. Hickling, D. Ingram, Contact glow discharge electrolysis, Trans. Faraday Soc. 60 (1964) 783.
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[3] D.E. Couch, A. Brenner, Glow discharge spectra of copper and indium above aqueous solutions, J. Electrochem. 106 (1959) 628. [4] T. Cserfalvi, P. Mezei, P. Apai, Emission studies on a glow discharge in atmospheric pressure air using water as a cathode, J. Phys. D 26 (1993) 2184. [5] T. Cserfalvi, P. Mezei, Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry, J. Anal. At. Spectrom. 9 (1994) 345. [6] T. Cserfalvi, P. Mezei, Operating mechanism of the electrolyte cathode atmospheric glow discharge, Fresenius’ Z. Anal. Chem. 355 (1996) 813. [7] H.J. Kim and J.S. Kang, Development of oscillating-plasma glow discharge as detector for GC, Abstr. 127, 24th CSI Conf. Liepzig, Germany, August 1995. [8] P. Mezei, T. Cserfalvi, M. Janossy, Pressure dependence of the atmospheric electrolyte cathode glow discharge spectrum, J. Anal. At. Spectrom. 12 (1997) 1203. [9] T.J. Dolan, Electron and ion collisions with water vapour, J. Phys. D 26 (1993) 4. [10] Y. Kanzaki, N. Nishimura, O. Matsumoto, On the yields of glow discharge electrolysis in various atmosphere, J. Electroanal. Chem. 167 (1984) 297. [11] K. Wagatsuma, K. Hirokawa, Characterization of atomic emission lines from argon, neon, and nitrogen glow discharge plasma, Anal. Chem. 57 (1985) 2901. [12] C.E. Melton, Radiolysis of water vapor in a wide range radiolysis source of a mass spectrometer. I. Individual and total cross sections for the production of positive ions, negative ions and free radicals by electrons, J. Phys. Chem. 74 (1970) 582. [13] P.M. Mul, J.W. McGowan, P. Defrance, J.B.A. Mitchell, Merged electron-in beam experiments. V. Dissociative recombination of hydroxyl, water, hydronium and hydronium-d 3 monocations, J. Phys. B 16 (1983) 3099.