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Spectrochemical Analysis by Using Discharge Devices with Solution Electrodes Michael R. Webb and Gary M. Hieftje Indiana University Solution electrode discharge instruments offer a low-cost, portable, small platform for fast spectrochemical analysis. (To listen to a podcast about this feature, go to the Analytical Chemistry website at pubs.acs.org/journal/ancham.) Solution electrode discharge systems are emerging tools in atomic spectrometry because they offer potential advantages over commercially and analytically successful plasma source techniques. They provide an alternative to nebulizer-based sample introduction, which is the Achilles’ heel of plasma source spectrometry.1-3 Conventional nebulizers generate a range of droplet sizes, which are introduced into a discharge in an uncontrolled manner. As a result, they can be a source of imprecision, memory effects, and matrix interference. In contrast, solution electrode discharge devices tend to be relatively small, operate at modest power levels, and require little or no added gases. These attributes make them less costly, portable, and fast. We aim here to provide an overview of the field, highlighting similarities and differences in design, comparing strengths and weaknesses, and showing the evolution of this class of discharge systems. Quantitative comparisons among the sources should be treated with some caution because they are not all optimized for the same purpose. Electrical discharge devices having solution electrodes are not limited in use to analytical spectrometry. For example, such discharge devices have been used for water purification (through degradation of organic compounds and destruction of microbes and viruses), synthesis, and surface modification.4-8 Except for a few cases offered to provide historical perspective, we will concentrate on discharge systems used for elemental analysis. BEGINNINGS OF GDE The first glow discharge apparatus to use a solution as one of the electrodes was described by Gubkin in 1887.9 The electrochemical technique based on this phenomenon became known as glow discharge electrolysis (GDE), which received significant attention in the middle of the 20th century.10 In a simple GDE apparatus, one electrode placed in the solution holds the solution at a low 862
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potential (Figure 1a). A second electrode is separated from the solution by a gap filled with air or other gas at either reduced or atmospheric pressure. When a high potential is applied to the latter electrode, a glow discharge is formed in the gap. Also in the middle of the last century, contact GDE (CGDE) was introduced.11 In CGDE, a thin wire serving as the anode is inserted into a solution. When a high voltage is applied, the solution near the anode is heated to above its boiling point. A discharge is then maintained in the vapor surrounding the electrode. Couch and Brenner provided the first hint that these techniques, or similar ones, might be useful for atomic spectrometry.12 In that work, atomic emission lines characteristic of copper and indium (which were present in the original solution) were observed from the plasma in a GDE-type setup. In 1964, a similar observation was made by Hickling and Ingram with CGDE;11 a 10.1021/ac801561t CCC: $40.75 2009 American Chemical Society Published on Web 01/12/2009
Figure 1. Simplified diagrams for (a) GDE and (b) ELCAD systems. Dashed line, solution level; 1, metal anode; 2, solid counter electrode; 3, high-voltage power supply; 4, current-limiting resistor; 5, plasma; 6, diaphragm (glass frit or cotton fiber), not always present.
yellow emission, ascribed to sodium, was observed from a sodium phosphate solution. ELECTROLYTE-CATHODE DISCHARGE Despite the observation of atomic emission from GDE and CGDE in the 1950s and 1960s, not until 1993 was a GDE-like system specifically designed for elemental analysis.13 This glow discharge between a metal electrode and a flowing solution became known as an electrolyte-cathode discharge (ELCAD).14 Research on discharge apparatus designs with an emphasis on fundamental processes and characteristics has recently been reviewed.15 Figure 1b portrays a simple ELCAD apparatus that differs somewhat from the original. The sample solution is delivered through a tube that opens directly below the metallic anode. The cathodic electric connection is most often made to a reservoir of waste solution. Two strategies are used to connect this waste reservoir to the solution emerging from the tube. One is to fabricate a channel in the tube wall and to fill that channel with an electrically conductive material, such as a wet plug of cotton fiber. Because of the higher pressure in the tube, solution from the waste reservoir supposedly does not enter the tube.16 In the second strategy, the portion of the solution not consumed by the discharge overflows into the reservoir, creating a conductive path. Even when a conductive channel is used, an overflow is usually maintained so the solution consumption does not have to be exactly balanced with the solution supply. In addition, for discharge devices with diameters greater than the i.d. of the solution-transport tube, completely covering the tube with solution results in greater discharge stability and correspondingly lower detection limits.16 Solution electrode discharge devices, including the ELCAD, generally produce simple spectra with a low continuum background but strong emission from the hydroxyl radical (Figure 2). Detection limits for alkali metals are particularly good, because the ionization temperature of the ELCAD apparatus is low and because background emission in the visible region, where neutral atoms of the alkalis emit, is very low.
The earliest ELCAD experiments showed that the concentrations and identities of electrolytes in the sample solution have a strong effect on the emission intensity of analytes. Also, analyte emission increases with rising electrolyte concentration at first but declines at very high concentrations. The concentration at which this reversal occurs is electrolyte-sensitive and possibly apparatus-sensitive but has not been observed at acid concentrations 400 nm have been studied. Precision of 5% RSD was reported for 100 ppm indium and 1 ppm sodium. A portion of the noise was correlated between elements, however, so using indium as an internal standard reduced fluctuations in the sodium results to 2%. Calibration plots were nonlinear but smooth, with log-log slopes of 0.8. ELECTROLYTE JET CATHODE GLOW DISCHARGE Yagov et al. have also explored other discharge systems in which both of the electrodes are solutions. In the electrolyte jet cathode
Figure 4. Cross-jet design of the electrolyte jet cathode glow discharge apparatus. A high potential applied between two solution streams creates a discharge in the gap between them. (Adapted with permission from Ref. 35.)
glow discharge apparatus, a conductive sample solution is sprayed from a nozzle and serves as the cathode of a glow discharge.35 Some experiments were performed with a metallic anode, but a cross-jet design, in which the anode is also a solution spray, was more extensively evaluated (Figure 4). Detection limits were 0.2-2 ppm, not as low as those for the DSD device. The system was also evaluated as a detector for flow injection analysis. Detection limits were not obtained, but a precision of 8% RSD was found for repeated injections of 12 ppm (4 µg) potassium. CHANNEL DISCHARGES Zuev et al. named a third approach “discharge on boiling in a channel”.36,37 As with the DSD and the cross-jet glow discharge systems, both electrodes of the discharge are solutions, and like in the DSD system, this discharge is pulsed. The channel discharge device is similar to CGDE in that both operate in an environment produced by a boiled solution. In the channel discharge apparatus, the sample solution vaporizes because of Joule heating in a 0.1-1 mm channel; a discharge then forms across the channel. Because the discharge is somewhat isolated from the surrounding air, nitrogen bands are absent, and background in the 350-450 nm region is lower than with the DSD device. Detection limits ranged from 20 ppb to 70 ppm for a variety of metals at the comparatively low solution flow rate of 0.85 mL/ min. The shape and composition of the channel were found to be important. Quartz was used because of its durability. Less-robust materials eroded over time, which caused drift due to the changing channel size and elevated blanks from the eroded elements. Analytical Chemistry, Vol. 81, No. 3, February 1, 2009
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Wu et al. have studied a similar system.38 In their liquid electrode discharge apparatus, two beakers containing the sample solution are connected by a 0.8-mm-i.d. quartz capillary. A high potential is applied between the beakers, and Joule heating causes a bubble to form within the capillary. No pumping is involved; instead, a tilt causes the bubble to migrate toward one of the beakers. When it reaches the end of the capillary, the bubble breaks apart, which narrows the gap and begins an electrical breakdown that lasts 1-2 s before the cycle begins again. The researchers found the effect of the solution pH on emission intensity to be similar to that in the ELCAD apparatus, so they evaluated the system using pH 1 HNO3. Detection limits were not calculated but were estimated to be ∼1 ppm for several metals. LIQUID ELECTRODE SPECTRAL EMISSION CHIP Another solution electrode discharge system on a chip is the liquid electrode spectral emission chip (LEd-SpEC).39 Unlike the designs discussed so far, the LEd-SpEC does not use a flowing cathode, although one might be added in the future. The present staticsolution configuration precludes the possibility of on-line separation or preconcentration; however, removing the pumping apparatus reduces the size of the overall package, potentially making it more portable. The gap between the electrodes is comparable to that in other systems (2.5 mm), but the instrument (excluding power supply and spectrometer) is smaller than most (10 mm × 20 mm). The LEd-SpEC therefore holds promise as a field instrument. Evaluation has so far been at modest concentrations (>1 ppm), and no detection limits have been reported. Integrated optics (consisting of an aperture and a grating) have been included on some LEd-SpEC designs. Chromium (425 nm) could be viewed in the presence of N2 (358 nm) but with significant spectral overlap. Such interferences will arise in samples of only moderate complexity, and the broad background features add noise to the signal, which is likely a major factor in the apparent low sensitivity of the device. Poor lightcollection efficiency also plays a role. No detection limits were given, but concentrations of 1000-10,000 ppm chromium or sodium were used for characterization. Additional optics would probably improve the performance of the device by raising both light collection and resolution, but the extra parts would also increase size and complexity. An interesting application outside elemental analysis has been demonstrated in which the LEd-SpEC discharge is used as a “tunable” light source for fluorescence excitation.40 The excitation wavelength is adjusted by varying the composition of the cathode solution and by using an appropriate bandpass filter. Elemental emission from the LEd-SpEC has a very narrow bandwidth, which might be useful to avoid excitation of fluorescence interferences. It also allows access to the deep UV. For example, the 280 nm lead line was used to excite tryptophan. The same group has reported a microglow discharge device, which is a nonlithographic device very similar to the LEd-SpEC.41 In this design, a 2.4-mm-i.d. capillary is bonded to a steel shaft to create a reservoir with a steel electrode. A pointed graphite anode is held 0.8 mm above the reservoir, and a voltage is applied between it and the steel electrode. As the discharge burns at 2.5 mA, it consumes the sample solution. After ∼15 s, the solution level drops below the top of the steel electrode, and the discharge 866
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self-terminates. The process consumes ∼0.3 mL of sample solution. The microglow discharge device is extremely simple because (like the LEd-SpEC) it requires no flowing solution. Spectra from 20 elements at unknown concentrations were shown. A detection limit of 170 ppb (lowered to 80 ppb by using an internal standard) was reported for sodium, but the noise used in the calculation was the standard deviation during consumption of a single sample rather than the standard deviation from sampleto-sample variability. Because the discharge gap changes as the sample is consumed and because the analyte emission intensity and location probably vary with this gap, the within-sample fluctuations are likely to be greater than the sample-to-sample fluctuations.24 Hence, the true detection limit is likely to be somewhat better. Similarly, the 12-42% RSD quoted for individual sodium samples is likely to be a pessimistic estimation of the sample-to-sample reproducibility of the microglow discharge. Additionally, the optical arrangement appears to have collected light mainly from the positive column of the discharge; in similar discharges, background emission tends to be higher and analyte emission tends to be lower in this region than in the negative glow.24,42 FUTURE PERSPECTIVE All of these discharge devices are smaller than a few millimeters. Indeed, the LS-APGD and SCGD devices can be operated as microplasmas (plasmas with at least one dimension
