Anal. Chem. 1996, 68, 1148-1152
Microsecond Pulsed Glow Discharge as an Analytical Spectroscopic Source Wei Hang, W. O. Walden, and W. W. Harrison*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
A pulsed glow discharge, operating in the microsecond regime, has been found to be advantageous for the examination of solid samples. We have studied the spectroscopic response by atomic emission, absorption, fluorescence, and mass spectrometries. Results to date show enhanced efficiency for analytical response of the sputtered sample atoms. This type of discharge also permits acquisition of useful diagnostic information concerning glow discharge processes. The rise of glow discharge methods for trace elemental analysis of solids has stimulated considerable interest in various power modes for this plasma source. The traditional approach, reaching back about 100 years, incorporates a direct current (dc) power supply to break down the discharge and supply the ion and electric currents necessary for successful operation. This requires that the analytical sample (normally the cathode) be a conducting material or be capable of incorporation into some conducting matrix. More recently, radio frequency (rf) discharges have attracted interest because of their ability to accept nonconducting samples directly. Both the dc and rf modes yield an atom/ ion production that is essentially a steady-state process.1 An interrupted or pulsed discharge offers certain advantages and disadvantages with respect to constant operation. Turning off a glow discharge quickly terminates the formation of the atomic species of analytical use. Therefore, to the extent that the duty cycle is reduced, so likewise is the time of useful signal. However, if during the on cycle of a pulsed operation it is possible to secure an even larger net signal than is possible at a comparable power level of dc operation, an analytical advantage may be obtained. Further, the more intense conditions possible during pulsed discharge operation may create a different distribution of species or yield some temporal condition that may offer some advantages. The general features of both pulsed dc2-4 and pulsed rf5-7 glow discharges have been known for years. The pulsed glow discharges reported to date have generally featured the millisecond regime, during which time the discharge reaches a form of equilibrium and operates similarly to a dc mode. Depending on the duty cycle, proportionately higher power levels can be employed during the on cycle to produce a more energetic (1) Harrison, W. W.; Barshick, C. M.; Klingler, J. A.; Ratliff, P. H.; Mei, Y. Anal. Chem. 1990, 62, 943A-949A. (2) Smith, B. W.; Omenetto, N.; Winefordner, J. D. Spectrochim. Acta 1984, 39B, 1389-1393. (3) Dhali, S. K. IEEE Trans. Plasma Sci. 1989, 17, 603-611. (4) Klingler, J. A.; Savickas, P. J.; Harrison, W. W. J. Am. Soc. Mass Spectrom. 1990, 1, 138-143. (5) Klingler, J. A.; Harrison, W. W. 17th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, Oct 9, 1990. (6) Pan, C.; King, F. L. Anal. Chem. 1993, 65, 3187-3193. (7) Winchester, M. R.; Marcus, R. K. Anal. Chem. 1992, 64, 2067-2074.
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plasma. Overall, however, the enhancements are often only a factor of 2 or 3 greater than the levels obtained with dc because of power limitations of the discharge, usually evidenced by overheating of the cathode. Microsecond time scale pulse discharges have been studied for their effect on hollow cathode lamp emission intensities. Dawson and Ellis8 studied pulses of 15-40 µs. Piepmeier and De Galan9 used pulses ranging from microseconds to the low milliseconds to study hollow cathode line shapes interferometrically. Araki and co-workers10 found pulsed hollow cathode operation to permit useful background corrections. The use of rf-boosted pulsed hollow cathode discharges was detailed by Farnsworth and Walters,11,12 and Mixon et al.13 showed advantages to the use of fast pulsed discharges for atomic emission from a microcavity hollow cathode emission source. Hang et al.14 reported the use of a high-power, microsecond regime discharge applied to glow discharge mass spectrometry. We have found advantages in applying a similar power mode to other glow discharge applications. By driving the discharge on very rapidly and to high power and current levelssup to 0.5 Asa plasma is created that exhibits unusual and attractive analytical features, including ion currents and photon emission much higher than those possible with dc operation at the same power. The spectral characteristics are still those of a true glow discharge. We also find that the rapid production of ionssessentially a pulsed injectionspermits useful studies of diffusion processes in the glow discharge source. Reported here are some preliminary results for atomic emission and mass spectrometries, along with certain observations for atomic fluorescence and atomic absorption, using the fast pulse source. EXPERIMENTAL SECTION A schematic diagram for the microsecond pulsed glow discharge (GD) in a configuration for optical measurements is shown in Figure 1. The pin sample is mounted onto the tip of a direct insertion probe, which is inserted into the top port of a 6-way cross that serves as the GD chamber. At 90° to the probe is the window port leading to the entrance slit of the monochromator. Opposite the monochromator, also 90° to the sample, is a hollow cathode lamp used for atomic absorption. For atomic fluorescence studies, a xenon flash lamp (Model 457A, Xenon Corp., Woburn, MA) was (8) Dawson J. B.; Ellis D. J. Spectrochim. Acta 1967, 23A, 565-569. (9) Piepmeier, E. H.; De Ganlan, L. Spectrochim. Acta 1975, 30B, 263-279. (10) Araki, T.; Uchida, T.; Minimi, S. Appl. Spectrosc. 1977, 31, 150-155. (11) Farnsworth, P. B.; Walters, J. P. Anal. Chem. 1982, 54, 885-890. (12) Farnsworth, P. B.; Walters, J. P. Spectrochim. Acta 1982, 37B, 773-788. (13) Mixon, P. D.; Griffin, S. T.; William, J. C., Jr.; Cai, X. J.; Williams J. C. J. Anal. At. Spectrom. 1994, 9, 697-700. (14) Hang, W.; Yang, P.; Wang, X.; Yang, C.; Su, Y.; Huang, B. Rapid Commun. Mass Spectrom. 1994, 8, 590-594. 0003-2700/96/0368-1148$12.00/0
© 1996 American Chemical Society
spectra were also taken using a detection system that centered around a wave form generator (Hewlett Packard, Model HP3325A) that produced a trigger to synchronize the applied voltage to the data collection time.
Figure 1. Representation of the glow discharge system used for atomic emission, absorption, and atomic fluorescence measurements.
placed at 90° to the monochromator and hollow cathode lamp. In each case, the analytical region was imaged onto the entrance slit of the monochromator. A scanning monochromator (GCA/McPherson, Acton MA, EU700) having a grating with a blaze wavelength of 250 nm is employed, in conjunction with a photomultiplier tube (Model R955, Hamamatsu, Bridgewater, NJ), to produce an output current that is then converted to a voltage by a small transimpedance amplifier (Model A-1, Thom EMI, Rockaway, NJ). The signal is collected by a gated boxcar (Model 250, Stanford Research Systems, Sunnyvale, CA). All spectra are digitized and sent to a computer via an analog-to-digital converter (Model 245, Stanford Research Systems). The glow discharge was operated in both the continuous and pulsed modes. A high-voltage dc power supply (Kepco, Flushing NY; Model OPS-3500) powered the discharge in the continuous mode. In the pulsed mode, a high-power pulse generator was employed (Model 350, Velonex, Santa Clara, CA). The pulse width and frequency of the pulse generator were varied (1-300 ps and 10-5000 Hz). Voltages up to 3000 V could be applied to the cathode. The GD pressure was optimized for each mode of power delivery. Comparisons between dc and pulsed spectra were done at average power levels of 2.5 W. A machinable ceramic sleeve shielded the sample holder on the probe tip, allowing only 5 mm of the exposed 2 mm diameter cathode pin sample to be sputtered. The reference samples used in this work were NIST 661 steel and 1109 brass (National Institute of Standards and Technology, Gaithersburg, MD) and Cu (Johnson Matthey Chemicals; 99.999% pure). Ultrahigh-purity argon (Alfagaz, Walnut Creek, CA; 99.995% pure) was used as the sputter gas. Quartz windows and lenses were used. The pulsed GD source was also used in conjunction with a quadrupole mass spectrometer that featured a six-way cross discharge chamber.15 Time-resolved studies of selected ions were performed by monitoring a particular m/z and profiling its evolution with respect to the initiation of the electric field. Gated (15) Bruhn, C, G.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1978, 50, 373375.
RESULTS AND DISCUSSION Pulsed glow discharge studies, which have been of interest in this laboratory for over 20 years,16 were stimulated by the prospect of larger ion currents for the then infant glow discharge mass spectrometry. While increased analytical signals were, indeed, obtained in the pulse mode, unanticipated phenomena were observed during the discharge termination phase that led us to have greater interest in these studies.17 The relatively long (millisecond) pulses employed yielded different spectral responses for several discharge species, primarily due to distinct phenomena occurring during discharge ignition, steady-state equilibrium, and termination. Of particular interest was the enhanced ion signal from sputtered species during the collapse of the discharge, attributed primarily to increased metastable argon formation.18 In the experimental configuration available at that time, ∼1-2 ms was required for discharge “equilibration” after ignition. To obtain optimum performance from pulsed discharge operation, two aspects of instrumentation become important. First, a gated detection system is required to permit temporal selection of the analytical signal, the accumulation and averaging of which creates a high signal-to-noise ratio for analytical measurements. Second, an adequate programmable power supply is necessary to ignite and sustain the discharge for the desired period and of sufficient speed to facilitate a range of pulse rates. To take maximum advantage of the pulsed method, one would wish a discharge to operate at a high frequency of short, intense pulses. This calls for a fast-response (low-impedance), high-current, highvoltage supply. The acquisition of such a unit has permitted the extension of our earlier pulsed discharge studies into a shorter time regime (microseconds). Our interest in fast pulse discharges extends to both their potential analytical application and their fundamental study by several complementary atomic spectroscopic methods used in our laboratory, including emission,19 absorption,20 fluorescence,21 and mass spectrometries.22 We report here some preliminary results that illustrate the interesting phenomena and potential analytical advantages that may be gained. Atomic Emission. The pulse power supply used in these studies can drive the glow discharge as high as 0.5 A operating current within 200 ns. Figure 2a shows an oscilloscopic trace of the voltage profile for an 8 µs pulse. The voltage pulse shows a higher leading edge that subsequently drops toward an equilibrium level. Note that the impedance of the discharge network extends the voltage decay for up to 25 µs after pulse termination. The discharge exhibits several time-dependent phenomena. Emission from the argon fill gas is observed almost instanta(16) Harrison, W. W.; Mattson, W. A. Proceedings of the 23rd ASMS Conference on Mass Spectrometry and Allied Topics, Houston, TX, May 28, 1975; N11. (17) Klingler, J. A.; Barshick, C. M.; Harrison, W. W. Anal. Chem. 1991, 63, 2571-2576. (18) Strauss, J. A.; Ferreira, N. P.; Human, H. G. C. Spectrochim. Acta 1982, 37B, 947-954. (19) Marcus, R. K.; Harrison, W. W. Anal. Chem. 1986, 58, 797-802. (20) Hoppstock, K.; Harrison, W. W. Anal. Chem. 1995, 67, 3167-3171. (21) Walden W. O.; Harrison, W. W.; Smith, B. W.; Winefordner, J. D. J. Anal. At. Spectrom. 1994, 9, 1039-1043. (22) De Gendt, S.; Van Grieken, R. E.; Ohorodnik, S. K.; Harrison, W. W. Anal. Chem. 1995, 67, 1026-1033.
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Figure 2. Comparison of (a) the discharge voltage for the microsecond pulse glow discharge and (b) the atomic emission signal for copper.
neously, because the initial electron current that precipitates discharge breakdown also stimulates photon emission. A delay is seen, however, for emission from the elements in the cathodic sample. The discharge process must first clean the surface by stripping away absorbed layers of impurities, such as water vapor. Then, a sufficient quantity of sample atoms must be sputtered, followed by collision with electrons and metastable argon atoms to promote the emission seen in Figure 2b for copper at 327.4 nm. At the scale sensitivity shown, the first sputtered copper atom emission is detected at ∼1.5 µs, building then rapidly toward a maximum, which is interrupted by discharge termination at 8 µs. The loss of sputtering at that point causes an emission decay that is generally symmetrical with the growth in emission signal, indicating the dependence of both steps on atomic diffusion processes in the argon gaseous medium. No postpulse peak is apparent for the sputtered atom, in contrast to that observed with the millisecond pulse. This may be related to reduced metastable argon formation in that region, which will have to be experimentally confirmed. The fast pulse discharge offers many opportunities to study plasma interactions, mechanisms, and relationships. The plasma chemistry that creates photons and ions exhibits a temporal pattern that may be observed by setting increasing boxcar delays for spectral registration. Figure 3 shows a segment of an emission spectrum taken from a copper cathode, indicating the changing pattern of spectral lines as the discharge forms, stabilizes, and then decays. The delay times shown are measured from the initiation of the discharge. Copper ion emission, for example, forms slightly later in the period and sustains its intensity after copper neutral atom emission has dropped significantly. The analytical potential of the fast pulse discharge is reflected in the high emission intensity observed during the on cycle. Comparing the microsecond pulse discharge with a dc discharge at a common average power level, signals from the former are up to 2 orders of magnitude greater than the dc emission. This is evidently due not only due to enhanced sputtering during the high current pulse but also to a more efficient utilization of the atoms produced. The high currents and high voltages evidently yield enhanced efficiency of atom excitation. The emission advantage of the pulsed discharge is shown in Figure 4, a comparison of results from an NIST 1109 brass sample. Note the difference in scales between the two spectra, as well as the different ratios of 1150 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996
Figure 3. Copper atomic emission spectra in a microsecond pulse glow discharge as a function of time after discharge initiation.
Figure 4. Comparison of copper atomic emission from (a) a microsecond pulse glow discharge and (b) a dc glow discharge. An average power of 4 W was used for both discharge configurations.
lines. The major copper peaks at 324.7 and 327.4 nm are saturated in the pulsed discharge spectrum. Extended studies are underway to explore the relationships between dc and pulsed operation. Mass Spectrometry. The major interests in our laboratory have involved mass spectrometric analysis. The well-known advantages of this analytical approach have led to the development of several commercial GDMS instruments. As in the case of atomic emission spectrometry, a gated detection system is required to take full advantage of a pulsed glow discharge ion source. Complicating the acquisition of temporal mass spectra, however, is the relatively slow transit time of ions compared to photons. This is particularly true in a 1 Torr source, such as the glow discharge, where atom/ion movement is diffusion limited. The analytical sample is normally positioned 3-10 mm from the ion exit orifice. This requires that the analyte atoms be sputtered,
Figure 5. Effect of the diffusion time in a glow discharge cell as a function of cathode to ion-exit distance: (a) total time for argon ions and (b) total time for copper ions.
drift across the source chamber to the orifice, and there be ionized and extracted with their charge intact. Ions formed at positions significantly removed from the exit orifice have only correspondingly smaller chance to be extracted from the source before collisional neutralization. The effect of this is seen in Figure 5, which shows the difference between the ion time profiles of argon gas versus the corresponding responses of the copper sample at several cathode to ion-exit orifice distances. Time of ion transport within the source and the quadrupole analyzer is measured from the initiation of the pulse discharge. Reflected in the overlapping symmetrical
ion profiles in Figure 5a, the changing distance makes little or no difference for argon, as this does not affect significantly the population of argon atoms near the orifice upon discharge initiation. The time required for argon to produce a signal response is determined by the ion transit time from the ion orifice through the quadrupole to the detector. These ions are first recorded about 80 µs after discharge initiation. Over the next several hundred microseconds, the ion signal reaches a maximum and then exhibits a long tail before once again attaining background levels. For copper (Figure 5b), the sputtered atoms must diffuse across the discharge cell and be ionized before entering the quadrupole mass spectrometer. At very close cathode-orifice distances, copper ions are first detected at only slightly later times than those for argon, but the effects of diffusion are clearly seen in the ion signal maxima. Increasing the cathode-orifice distance causes a shift in the total time required to reach the detector, based on the extra diffusion time in the gaseous medium. What becomes obvious is the extended ion signal produced from the initial 8 µs pulse. This ability to inject ions into the system by means of a short, energetic pulse permits the study of diffusion phenomena in glow discharge mass spectrometry. We are currently involved in a study of differential diffusion processes as a function of atom mass for a series of analyte materials. The microsecond pulse discharge produces a large ion yield, the effect of which is tempered by its signal spread from the diffusion processes. Instead of a repetitive 8 µs burst of ions reaching the detector, the ion packet can extend to nearly 1 ms. At a pulse frequency of 100 Hz, the 1 ms ion bands create an “effective” duty cycle of 10%, albeit at the expense of short term ion intensity. A measure of the elemental distribution over the ion band can be obtained by using a gated delay to sample each packet at a given interval after discharge initiation. Figure 6 shows nine time slices through the band, yielding mass spectra that reveal time-dependent plasma ionization processes using a NIST 661 steel sample. Initially, the spectrum is dominated by background impurity gases in the chamber. The 12C+ increases rapidly during the discharge cycle (150-250 us) but then begins to decay as, in turn, the sputtered iron atoms are ionized and become the largest ion species. By time-resolved methods, GDMS spectra
Figure 6. Mass spectra from an iron cathode in a pulsed glow discharge as a function of time after discharge initiation.
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Figure 8. Atomic absorption response of copper atoms in an pulsed glow discharge as a function of time.
Figure 7. Atomic emission response from a copper cathode in an argon pulsed glow discharge comparing argon and copper response times.
can be obtained that discriminate against background gases and promote the relative sensitivity of analyte ions. As illustrated in Figure 6, a spectrum optimized for the analyte would have the gated detector set to accept ions around a time band of 450-550 µs. There is little differentiation among the sample atoms, so that relative sensitivity factors determined by fast pulsed methods are rather similar to those determined by dc discharges. Diagnostic Studies. In addition to the prospective analytical utility of the fast pulse discharge, there exists the opportunity to obtain interesting and valuable diagnostic information concerning glow discharges processes. Unlike dc, rf, or even the slower pulsed methods, the microsecond operating regime effects rapid ignition in a time scale that permits resolution of prime discharge events. As one example, Figure 7 illustrates the temporal development of four important species in an argon glow discharge with a copper cathode. Not surprisingly, argon emission is observed before that of copper, which requires sputter release and subsequent excitation. Less clear is why a maximum for argon ion emission at 294.2 nm appears before that of argon neutral atom emission at 419.8 nm, but both commence photon release within the first microsecond of the discharge pulse. Copper emission at 249.3 nm does not appear until 2 µs into the pulse, after sputter release and collisional excitation. By contrast, copper ion emission at 224.7 nm is not apparent until beyond 3 µs. We plan a more detailed comparison of different spectral lines and elements to gain a better understanding of the discharge events. The advantage of atomic emission measurements compared to mass spectrometry is the immediacy of observation, with no lengthy mass transport of ions during which time resolution is lost. However, atomic diffusion processes may also be monitored by absorption methods to gain some measure of sputtered atom
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densities as a function of time. Figure 8, which has a 10 µs pulse superimposed on a millisecond absorption time scale, shows that the response of ground state atoms is quite different compared to that of their excited analogs. Even though copper 324.7 nm emission maximizes at the termination of the 8 µs pulse (Figure 2), the ground-state copper population appears to increase for up to ∼1 ms before falling over the next 10-12 ms to background levels. This reflects the diffusion-controlled process of the sputtered atoms that are ejected from the cathode and drift into the negative glow, which constitutes the absorption volume sampled in Figure 8. We have confirmed this observation by atomic fluorescence measurements (see Figure 1 methodology), which show the same time scale for maximization of the sputtered atoms. Clearly, pulse repetition rates greater than 100 Hz would find residual sputtered atoms carried over from pulse to pulse. CONCLUSIONS The use of a high-power, short pulse to drive a glow discharge has several interesting features that merit further exploration. Enhanced atomic excitation could make glow discharge atomic emission more competitive with glow discharge mass spectrometry for elemental sensitivity. The short-term nature of the discharge may make it particularly valuable for coupling with a time-of-flight mass spectrometer. Beyond potential analytical advantages, a fast pulse discharge permits diagnostic evaluations of plasma processes that may aid our understanding of the glow discharge processes. ACKNOWLEDGMENT This work was supported by a research grant from the Department of Energy, Basic Energy Sciences, for which we are most grateful. Received for review October 30, 1995. Accepted January 10, 1996.X AC9510797 X
Abstract published in Advance ACS Abstracts, February 15, 1996.
