Anal. Chem. 1994,66, 1249-1253
Frequency, Current, and Amplitude Maps of Oscillating-Plasma Glow Discharge GC Detectors Bryan Cookt and Edward H. Piepmeier' Department of Chemistry, Oregon State University, Gilbert Hall 153, Corvallis, Oregon 9733 1-4003
This study maps the complex relationships between the oscillating frequency (210-800 W z ) , amplitude (to 4 V p-p), and current 0.18-0.7 mA) for an oscillating-plasma glow discharge GC detector as a function of pressure (0.35-0.85 Torr), electrode spacing (0.8-2.8 cm), and cathode shape in order to show the range of useful operating conditions and to gain insight into cell mechanisms. Concave cathodes lead to amplitude roll-offs and appear to cause an increase in local pressure between the electrodes when the gas enters via a hole in the anode. The similarityprinciple explains pressureinduced shifts in frequency gaps and amplitude roll-offs. A simple dispersion equation helps to account for frequency maxima. There appears to be more that one mode of oscillation in regions where the current is relatively constant, indicating that the mechanismsthat control frequency and current have significant differences. Oscillating glow discharges are being used as detectors for gas chromatography.' One design introduces the gas from the chromatograph via a small hole in the anode and directs the flow of gas toward the cathode. The drop in pressure (to a fraction of a Torr) that occurs as the gas enters the detector causes a gas flow similar in shape to a supersonicjet expansion.2 This study maps the complex relationships between the oscillating frequency and amplitude and the cell operating conditions in order to show the range of useful operating conditions and to gain insight into cell mechanisms. Oscillationsof the type studied are the result of local regions of adjacent ion excesses and depletions along the axis of the discharge. These local regions produce local electrical fields that add to or substrate from the main electric field. The local electrical fields cause local ionization rates to increase or decrease from the mean ionization rate in such a way as to sustain and nourish the already existing localized electric fields and ion excesses and depletions and to cause the phase of this waveform to travel toward the cathode. The frequency of these ionization waves is partially determined by a balance between the rate of ionization and the rate of ion 1 0 ~ s . ~
EXPERIMENTAL SECT1ON A schematic of the oscillating-plasma glow discharge cell is shown in Figure 1. The system is the same as described in ref 1 except for the cathodes. The four brass cathodes studied + Present address: Analyte Corp., 910 Chevy Way, Medford, OR 97504. (1) Kuzuya, M.; Piepmeier, E. H. Anal. Chem. 1991,63, 1763-1766. ( 2 ) Campargue, R. J . Phys. Chem. 1984.88, 4466-4474. ( 3 ) PekBrek, L. Ion Waves and Ionization Waves. In lOfhInternntionnlConference on Phenomena in Ionized Gases 1971; Donald Parsons & Co. Ltd.: Oxford, England, 1971; pp 365-403.
0003-2700/94/0366-1249$04.50/0 0 1994 American Chemical Society
5 cm
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Figure 1. Schematic dlagram of the cell showing the cross section of a concave cathode. Crosshatched areas are Delrln.
were 1.5 cm in diameter and the end faces were either flat or concave with radii of curvature of 1.02,1.27, or 1.78 cm. The argon (99.995%) mobile phase from the gas chromatograph enters the cell through a pointed brass nozzle that has a 1.5mm-long, 0.34-mm-diameter orifice in its end. A 0.34-cmi.d. glass tube 17 mm long is slipped over the nozzle. The distance from the tip of the nozzle to the open end of the glass tube is 0.76 cm. Since oscillations are known to occur in the positive column of a glow discharge, the purpose of the glass tube is to initially restrict the eluate to this region to improve the sensitivityof the detector. The electrode spacing is defined as the axial distance between the tip of the anode and the surface of the cathode. The power supply voltage was held constant at 410 V. The gas flow rate and pressure in the cell were adjusted to the desired values, and then the oscillation frequency, amplitude, and cell current were measured at each of a series of equally spaced electrode spacings. The electronics are discussed in detail in ref 1. Basically, a 10-kQ resistor is connected in series with the cell in order to sample the cell current, which has dc and ac components. The voltage across this resistor is sent to two amplifiers. The first amplifier includes a lowpass filter that essentiallyeliminates the oscillating component of the current so that the average dc current can be measured by an A/D converter interfaced to a computer. The second amplifier includes a high-pass filter that eliminates the dc and passes the oscillating component of the signal. The amplitude of the oscillation is observed with an oscilliscope at a point in the circuit before the oscillating signal is converted to a square wave. The amplitude of the ac component at the series resistor is then calculated. After the ac signal is Analytical Chemistry, Vd. 06, No. 8, April 15, 1994
1249
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converted to a square wave, the frequency of the square wave is determined by a digital counter interfaced to a computer.
RESULTS AND DISCUS!BION Figure 2 shows the oscillation frequency maps for the four cathodes (across) at three different gas flow rates (down). Each curve represents a different cell pressure. Gaps in the curves are shown when the oscillation either quits or is no longer a repetitive simple waveform, but rather a complex, noisy signal. Frequencies range fromjust above 200-800 kHz. A general observation for all of the cathode shapes is that higher gas flow rates allow oscillations to occur at higher pressures. Most curves show a decrease in frequency as the electrode spacing increases. Curves that show an increase appear in the graphs for the flat cathode (Figure 2 a-c) or for the lowest flow rate (Figure 2 c, f, i, and 1). When maxima are observed, the maxima occur on either side of an electrode spacing of 1.5 cm, and mimima are observed near 1.5 cm. The change from an increase to a decrease in frequency with electrode spacing may be explained if it is assumed that there is an integral number of effective wavelengths between the electrodes. ("Effective" is used because the discharge is not homogeneous and the wavelengths in the plasma may change from anode to cathode.) An integral number of wavelengths is expected because the arrival of an exceas of ions at the cathode pulls extra electrons from the cathode (to neutralize the ions), which in turn pulls extra electrons from the anode via the power supply circuit, which in turn causes a corresponding ex-8 of ions at the anode? In this way the cathode and anode are tightly coupled to each other so that
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the peaks and valleys of the oscillations occur simultaneously at both electrodes-forcing an integral number of effective wavelengths between the electrodes. Experimental evidence for an integral number of wavelengths exists in the literature,s where the frequency of oscillation was found to change in a discontinuous manner as the length of a constant-current discharge is changed by a length equal to an oscillation wavelength. In another study,6 a plot of frequency versus discharge length in argon is roughly sawtooth in form, as would be expected if the wavelength changed at first in a linear manner and then discontinuously by one cycle to always keep the total circuit feedback phase shift equal to an integral multiple of 2a. In a similar manner, periodic changes in frequency have been observed' as plasma current is changed in a fixed-length plasma. In this case, to maintain the oscillation by positive feedback, the oscillating frequency varies to keep the wavelength constant until the number of waves in the plasma column can change discontinuously by one. When there is an integral number of wavelengths between the electrodes, the frequency behavior can be explained with the help of the dispersion relation obtained from eq 3.5 in PeMrek's theory of ionization waved w
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(4) Zh? Z.; Piepmeicr, E. H.,in preparation. (5) hw,A. A. Bulk MWCOWState UI&.1-3 93 55; 1% 1 4 4 1 . (6) Gtrtzsnstcin, M.E.;potcmhn ' ,V. V. Zh. Ekap. T w . Fig. 1953, 24, 6. ( 7 ) Pupp, W.Z . Tech. Phys. USSR 1934, 7,257.
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where h is the wavelength), a is the inverse of the electron temperature relaxation length, and A is a coefficient related to plasma conditions. This equation shows that the frequency increases with wavelength (electrode spacing) when a < k and decreases when a > k. When a2 = k2the frequency does not change with wavelength. This may explain why there is a point where the frequency does not change when the electrode spacing (proportional to wavelength) is changed. It may also explain why the frequency sometimes increases with electrode spacing and sometimes decreases. The presence of two maxima may be caused by changes in A that occur when the current changes with the electrode spacing. The breaks in the curves, where the signal becomes noisy, may be caused when the number of wavelengths between the electrodes is changing and both oscillations are competing for the plasma resources. Figure 3 shows the average cell currents corresponding to the frequencies in Figure 2. Currents above 0.7 mA saturated the A/D converter and therefore appear as horizontal lines. The gaps in the lines are shown to indicate that the simple oscillation waveform has stopped or become noisy, not that the cell current has stopped. These gaps were included in the current graphs because the currents generally increase with cell pressure, causing less overlap of these lines than in the frequency and amplitude graphs, thereby making the gaps in the lines easier to identify. Although currents were present beyond the largest electrode spacing where oscillations occurred, they were not measured because the primary interest in this study is the behavior of the frequency. There are several distinct regions in these curves. At the shortest electrode spacings the current at a given pressure generally increases with electrode spacing whether the frequency is increasing or decreasing. This indicates that there is a difference in the mechanisms that control current and those that control the oscillations.
This region is followed by a sigmoidal increase in the current which may or may not be accompanied by a gap that indicates a noisy oscillation signal. The gaps usually occur at higher cell pressures, and the width of the gap tends to increase with cell pressure. The current curves also show a trend in the locationsof the gaps toward shorter electrode spacings at higher cell pressures. This trend might be expected from the similarity principle: which predicts that at higher gas densities (pressures at constant temperature) plasmas with similar characteristics require higher current densities and smaller cells. Near an electrode spacing of 1.5 cm, a second sigmoidal increase in current occurs that usually has no gap in the simple-waveform oscillation signal, although at higher pressures the oscillations may stop before this second sigmoidal region is reached. After each sigmoidal increase the current tends to level off or go through a shallow maximum into a gradual decline. At the same time the frequency continues to decrease in this region. The sigmoidal increases in current correspond to similar, but often not so obvious distortions in the frequency curves. Figure 4 shows the peak-to-peak amplitudes of the oscillationsignals, which provide additional information about the gaps in the simple-waveform oscillation signal. Usually the amplitudes increase with electrode spacing and are relatively independent of pressure below 0.60 Torr. Since current changes with pressure, the amplitude is relatively independent of current also below 0.60 Torr. This dependence of amplitude on only electrode spacing (for a given cathode shape) could be explained as follows if there is an integral number of wavelengths between the electrodes. Amplitude reflects the magnitude of the ion excess and depletion in adjacent half cycles of the oscillation. An increase in (8) Franklin, R.N.Plamra PheMmeMinGasDischarges;ClaredonPreas:Oxford, England, 1976. vonEnge1,A.IonizedGases;ClarcndonPress,Oxford,England, 1955.
Analytical Chemistty, Vol. 66,No. 8, April 15, 1994
1251
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wavelength provides more distance along the axis of the discharge for the ion density to increase without requiring the local ion density gradients to change. Therefore, when an increase in electrode spacing increases the wavelength, a larger ion excess, and therefore amplitude, could easily occur at the peak of an oscillation cycle without changing anything else in the plasma, such as the gradients in electron density and electron energy responsible for ionization or ion density gradients responsible for local electric fields that help determine electron energies. Changing the wavelength without changing the amplitude would require the density gradients to change. Therefore an increase in amplitude with electrode spacing would seem to be a likelyway for the plasma to respond since other plasma conditions can remain relatively constant. An increase in curvature of the cathode is accompanied by a decrease in amplitude for most of the curves. For the concave cathodes at pressures above 0.60 Torr, the amplitude tends to roll o€€(decrease)rather quickly above an electrode spacing near 1.5 cm. Apparently under these conditions the currentdensity increases that accompany higher pressures are too high to support excess concentrations of ions because of self repulsion, and the amplitude (proportional to excess ion concentration) decreases accordingly. Higher pressures cause the roll-off to begin at shorter electrode spacings, as might be expected from the similarity principle. The roll-off occurs for lower pressures as the radius of curvature of the concave cathodes increases. It might be that the gas that flows from the anode nozzle to the cathode causes an increase in the local pressure between the electrodes that is higher than the measured cell pressure. A cathode with greater curvature would confine the gas more, producing a higher pressure in the plasma region, as is indicated by these results. The transition between the presence and absence of rolloff appears to be a sensitive function of pressure and flow rate. For example, in Figure 4d the two lines for 0.61 Torr 1252 Analytical Chemistty, Vol. 66, No. 8, April 15, 1994
had flow rates of 5.99 and 6.02 mL/min. The curve for the 0.5% higher flow rate shows the roll-off while the other shows only a dip. In Figure 4k a 5% increase in pressure from 0.52 to 0.55 makes the difference between roll-off and a continuing increase in amplitude with electrode spacing. At short electrode spacings, low pressures (0.35-0.40Torr) and the lowest flow rate (3 mL/min; Figure 4c, f, i, and l), the amplitude starts out unusually high and decreases with an increase in electrode spacing until the gap is reached. After the gap, the amplitude drops abruptly (by up to a factor of 8) to join the main group of curves that show increasing amplitudes with electrode spacing. These abnormally high amplitudes correspond to abnormally low frequencies (Figure 2c, f, i, and 1) that increase abruptly (by up to a factor of 2.5) after the gap. The magnitudes of these amplitude drops and frequency gains increase with cathode curvature. These discontinuities in amplitude and frequency occur while the current shows relatively little change, indicating relatively little change in the average plasma conditions. Therefore this discontinuous behavior in the oscillations is most likelycaused by a change in oscillating modes from one side of the gap to the other. The mode change could be caused by a change in the number of effective wavelengths between the electrodes, as mentioned earlier, or by a change in the relative contributions that different ionization mechanisms have in supporting the oscillations.
CONCLUSIONS These systematic studies have revealed trends in plasma responses over the useful range of experimental variables for these plasma cells and have identified gaps in frequencies, sigmoidal increasesin current, amplitude roll-offs, and maxima and minima in the responses. Up until now these features have been little more than hindrances to obtaining reliable oscillating plasmas. Now these features have provided
additional insight into plasma mechanisms in these flowing gas cells. Concave cathodes lead to amplitude roll-offs and appear to cause an increase in local pressure between the electrodes with this particular gas inlet system. The similarity principle explains the pressure-induced shifts in frequency gaps and amplitude roll-offs. A simple dispersion equation helps to account for frequency maxima. There appears to be more that one mode of oscillation in regions where the current undergoes relatively little change. These modes are being investigated in another study. Although there may be similarities in the mechanisms, the mechanisms that control frequency and current contain significant differences. These
differences have led to the simultaneous use of current and frequency signals to help identify analytes (impurities) in the plasma gas.9
(9) Smith, D. L.; Piepmcicr, E. H. Anal. Chem., companion paper in this issue.
Abstract publbhcd in Advance ACS Abstracts, March 1, 1994.
ACKNOWLEDGMENT The financial support of the National Science Foundation (Grant CHE-9013929) is gratefully acknowledged. Received for review September 2, 1993. Accepted January 20, 1994.'
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