Anal. Chem. 1992, 64, 2007-2074
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Emission Characteristics of a Pulsed, Radio-Frequency Glow Discharge Atomic Emission Device Michael R. Winchester+and R. Kenneth Marcus’ Department of Chemistry, Howard L. Hunter Chemical Laboratories, Clemson University, Clemson, South Carolina 29634-1905
The Instrmentatlon, lnltlal observatlorur, and operating characterhtlcr d a pulsed radldroquencyglow discharge atomic emkrlon source are descrlbed. Anomalles In the temporal emkrlon Intendty wave forms for some analyte transttlons are a b reported. These anomalles take the form of a welldeflned Intendty maxlmum located elther near the bqlnnlng of the dkcharge pulse or Just after power termlnatlon, depending on the partlcular transttlon. St111 other analyte tranrltions demonstrate no such temporal Irregularltles. Sehctlve, gated, detectlon of thew emktlon anomdles suggests po#lble analytlcal advantages In terms of Instantan” e ” Intenskbs, which maytrandate IntoImproved analytlcal sensttlvltles.
INTRODUCTION The application of, and interest in, glow discharge (GD) devices as a means of direct solids elemental analysis has increased in recent years. The devices have been applied in various spectroscopic modes including atomic absorption, fluorescence, emission, and mass spectrometries.’ Applications range from analysis of solutions (as residues), bulk metals, layered samples, geological materials, and ceramic materials.2-8 In the vast majority of these applications, the plasma is operated in a steady-state, direct current (dc)mode. As avariation in the operationof GD sources,some researchers have operated the devices in a pulsed (power modulated) mode as a means of improving analytical characteristics and obtainingnovel spectroscopicresponses.+15 Pulsed operation allowsfor the attainment of high instantaneous (and average) ~~~~~~
* Author to whom correspondence should be sent.
+ Present address: Inorganic Analytical Research Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. (1) Harrison, W. W.; Barshick, C. M.; Klingler, J. A.; Ratliff, P. H.; Mei, Y. Anal. Chem. 1990,62,943A-949A. (2) Harrison, W. W.; Prakash, N. J. Anal. Chim. Acta 1970,49, 151159. (3) Chakrabarti,C. L.; Headrick, K. L.; Bertels, P. C.; Back, M. H. J. Anal. At. Spectrom. 1988,3, 713-723. (4) Gough, D. S. Anal. Chem. 1976,423, 1926-1931. (5) Leis, F.; Broekaert, J. A. C.; Steers, E. B. M. Spectrochim. Acta, Part B 1991,46B, 243-251. (6) Jakubowski, N.; Stuewer, D.; Vieth, W. Fresenius’ 2.Anal. Chem. 1988,331,145-149. (7) Bengtaon, A,; Eklund, A.; Lundholm, M.; Saric, A. J. Anal. At. Spectorm. 1990,5,563-567. (8)Brenner, I. B.; Laqua, K.; Dvorachek, M. J.Anal. At. Spectrom. 1987.2.623-627. ----,-. --- - (9) Winchester, M. R.; Hayes, S. M.;Marcus,R. K. Spectrochim. Acta, Part B 1991,46B, 615-627. (10) Glick, M.;Smith, B. W.;Winefordner,J. D. Anal. Chem. 1990,62, 157-161. (11) Wagatauma, K.; Hirokawa,K. Anal. Chem. 1984,56,2732-2735. (12) Djulgerova,R. B. InlmprovedHollow CathodeLanpsfor Atomic Spectroscopy; Caroli, S., Ed.; John Wiley & Sone: New York, 1985. (13) Wagatauma, K.; Hirokawa, K. Spectrochim. Acta, Part B 1988,
Harrison, W. W. J.Am. SOC.Mass
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powers, while avoiding analytically undesirable sample vaporization resultant from excessive cathode heating. (This is due to the fact that the sample is allowed to cool during the discharge-off portion of the pulse cycle.) As a result, sputter atomization rates may be increased relative to those obtained in the nonpulsed mode of operation. In GD atomic absorption applications, in addition to the higher sputter atomization rates, absorbance measurements could be made without background emission during the dischargaoff portion of the cycle.9 These advantages of enhanced sputter rates and background-free analytical measurementshave also been demonstrated in atomicfluorescence applications.10 Atomic emissionapplicationsgenerallyutilize high-power pulses superimposed on top of a continuous, lowpower discharge.11-13 Higher sputter rates and an instantaneously more energetic plasma achieved during the highpower pulse result in emission intensitiesthat are significantly enhanced relative to those obtained with the nonmodulated discharge. Additionally, differences in the response of the emission from analyte and support gas species with respect to the power pulse provide a method for selective detection of analyte emission signals via synchronous detection. Investigations of pulsed GD ion sources for mass spectrometry are most often directed toward the attainment of unique temporal responses for analyte ions.14-16 Specifically, Harrison and co-workers14J5 have found a postpulse signal maximum (“afterpet$”) for ions of the sputtered species immediately after the discharge power is terminated. Conversely, atomic discharge gas ions and polyatomic ions demonstrate no such postpulse signal. Selective detection of ionic analyte species offers simplification of mass spectra. Higher sputter rates are realized in these applications as well. An enhancementin Penning ionizationof the sputtered atomic species was initially suggested as the origin for the afterpeak phenomenon.14 As will be discussed in detail later, other processes seem to now be underlying such enhancements. In this laboratory, recent efforts have been focused on the development of radio-frequency (rf) powered glow discharge devicesfor mass and opticalspectrometries.1+21 The primary advantage of the rf source is the ability to sputter-atomize nonconductingmaterials directly,without any sample matrix modification. Thus the device is applicable for the analysis of bulk and layered materials ranging from alloys to ceramics. In order to increase the instantaneous (and average)operating powers of the rf plasma emission devices while avoiding a significant increase in sample surface temperature, pulsedmode operation is being investigatedfor this familyof devices. (16) Klingler, J. A.; Harrison,W. W. Anal. Chem. 1991,63,2982-2984. (17) Duckworth, D. C.; Marcus, R. K. Anal. Chem. 1989, 61, 18791886. (18) Duckworth, D. C.; Marcus, R. K. Appl. Spectrosc. 1990,44,649655. (19) Winchester, M. R.; Marcus, R. K. J. Anal. At. Spectrom. 1990, 5. -, 57.5479. - .- - .-. (20) Winchester, M. R.; Lazik, C.; Marcus, R. K. Spectrochim. Acta, Part B 1991,46B, 483-499. (21) Lazik, C.; Marcus, R. K. Spectrochim. Acta, Part B, in press. 0 1992 American Chemical Society
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low power level was set to 0 W,which constitutesan extinguished plasma. Optical monitoring of the rf plasma was performed using a 0.24-m Czemy-Turner monochromator(Digik" 240, CVI Laser, Albuquerque, NM) equipped with a 1200 g/mm grating and an RCA 1P28photomultipliertube. The emissionwave forms were processed in two ways. Firstly, they were fed to a 60-MHz, dual channel, digital storage oscilloscope (Model 3350A, Philips Electronic Instruments, Mahwah, NJ) and output to an XY recorder (Model 200, Houston Instruments, Austin, TX). Secondly, temporally gated detection and processing were accomplished with a boxcar averager (Model 162, EG&G Princeton Applied Research, Princeton, NJ). In this manner, selective monitoringof the aforementionedemission anomalies,as well as steady-state signals, could be accomplished. Since the boxcar averager was equipped with two gated integrators (both EG&G Model 162),differenceand ratio values between the two selected gates could be attained in real time.
RESULTS AND DISCUSSION Similar to the prior research concerning the use of pulsed dc GD sources in atomic emission Spectroscopy, sputter rates and instantaneous emission intensities of analyte species are increased for these pulsed rf discharges relative to continuous discharges of the same average power. Aside from these advantages, the most notable characteristic of this mode of operation includes the existence of anomalies in the temporal emission profiles for various transitions. In particular, some analyte transitions exhibit an emission maximum near the start of the discharge pulse, while others exhibit a maximum just after the discharge power is terminated. Still other analyte transitions, as well as discharge gas transitions, exhibit no such temporal anomalies. Although not well understood, these differences present interesting opportunities for selective detection of analyte transitions. We describe here the instrumentation, initial observations, and operating characteristics of a pulsed, rf glow discharge atomic emission device.
EXPERIMENTAL SECTION Theinstrumentationemployed in the palaed, rfglow discharge atomic emission experiments is depicted in Figure 1. The rf glow dischargeatomicemissionsourceemploysan extemalsample mount geometry and has been described in detail elsewhere.20 Briefly, the source consists of a 7.5-cm X 5-cm X 5-cm hollow, stainlesssteelbody which servesas the sourcechamber. A torque bolt holds the solid sample (either oxygen-free copper or stock brass in these studies) against a Teflon o-ring which forms a vacuum seal. Radio-frequency power is applied to the back of the sample via an insulated feedthrough running the length of the bolt. Radio-frequency power is applied to the feedthrough by means of a coaxial connection to the bolt assembly. Optical sampling of the plasma is accomplishedthrough a quartz window mounted in front of the sample surface. The power to the rf plasma is provided by a radio-frequency power generator and coupled through an automatic matching network (Models RF 5s and AM5, respectively, RF Plasma Products, Marlton, NJ). The generator is capable of delivering up to 500 W of continuous rf power at a 13.56-MHz operating frequency. The generatoris also capableof internally controlled pulsed operation. In this mode, the low and high power levels and pulse frequency are programmed through front panel controls, but the 50% duty cycle can not be changed. Alternatively, pulsed operation can be driven by simple external circuitry consisting of a pair of voltage dividers (0-5 V) which set the low and high power levels and a wave form generator which triggers the pulsed operation (i.e., determines the pulse frequency and duty cycle). The latter approach was chosen for this set of experimentssince it allows for variation of the plasma duty cycle and synchronization of the signal processor to the pulsingof the discharge. A squarewave of preselected frequency and duty cycle is provided by the wave form generator (Model 3017,B&K Precision,Chicago,IL) to trigger both the rf generator and the boxcar averager. In the experimentsdescribedhere, the
Emission Wave Forms for Various Optical Transitions. As noted in previous studies of pulsed source GDMS, one possible advantage of the pulsed mode of operation, as compared to continuous mode operation, is the ability to generate different temporal signal responses for analyte and background species. Harrison and co-workers observed that analyte (sputtered) ion signals increased to a maximum approximately 0.5 1118 after the discharge power was terminated, while signals of support gas and polyatomic ions exhibited no such enhan~ement.1~This behavior was attributed to an increase in Penning ionization of analyte atoms due to an increase in argon metastable densities just after power cessation. The authors suggested that the increase in metastable densities could be due to enhanced argon ionelectron recombination,producinghighly excitedargon atoms, many of which then radiatively relax into the metastable levels. Subsequent atomic absorption studies by those researchers,l5 however, indicated no such increase in the metastable population. Conversely,similarabsorption studies with a pulsed Grimm-type discharge by Strauss and coworkers22indicated a slight ( N 10% ) increase in metastable atom densities after pulse cessation, peaking -2200 pa after termination. In the mass spectrometric (MS) studies, the researchers noted that signals of support gas and polyatomicions exhibited a maximum near the beginning of the discharge pulse, while analyte ions exhibited no such maximum.14J5 The authors demonstrated the selective detection of analyte ions by use of a judiciously chosen data gate, effectively eliminating previously troublesome isobaric interferences. The possibility that analytically advantageous anomalies similar to those demonstrated in this previous work might be exhibited in pulsed rf source atomic emission spectroscopy is a major part of the impetus for this work. As noted earlier, some researchers have reported such effects in pulsed dc source atomic emission experiments.12J3 The temporal emission wave forms of a number of Cu(1) and Cu(I1) transitions taken while sputtering a stock copper sample at 10 Hz with a 50% duty cycle are shown in Figure 2. Also shown is a corresponding applied power wave form. The instantaneous dischargepower (i.e., during the dischargeon portion of the cycle) was 40 W, and a source pressure of 6 Torr of argon was utilized. Significant differences are seen in the wave forms for the various transitions. Of note first is the presence of a large peak at the beginning of the pulse for the resonant, Cu(1) 324.7-nm transition. The width of this peak to the point where it reaches the steady-state, or "plateau", region is approximately 2 ma with the intensity at (22) Straw, J. A.; Ferreia, N. P.; Human, H.G.C. Spectrochim. Acta, Part B 1982,37B,947-954.
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Cu II 368.8 nm
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Flgure 2. Temporal emission waveforms for selected Cu species (target = OFHC (Cu), average power = 20 W, pressure = 8 Torr (Ar), pulse frequency = 10 Hr, duty cycle = 50%).
the maximumbeing roughly 3 times that of the plateau region. A similar but far less dramatic response is also seen for the Cu(1) 510.5-nm line. While not resonant, this transition originates from the same energy level (4p 2P3/2’, 3.82 eV) as the 324.7-nm line. Very different from the transitions originating from this relatively low-lying level, the Cu(1) 521.8nm transition shows no “prepeak”but rather a peak just after discharge termination, similar to the postpulse ion signal observed in pulsed source GDMS experiments. This atomic transition originates from a level (4d ‘D5/2) 6.19 eV above the ground state. Finally, it is seen that the Cu(I1) 368.7-nm transition demonstrates the same sort of “postpeak” as the Cu(1) 521.8-nm line. It should be noted that the intensity wave forms in these figures are not on the same intensity scale. Furthermore, it should be noted that the term *prepeak” does not imply that the peak is generated prior to the application of rf power. The presence of the prepeaks exhibited by the Cu(1)324.7nm and Cu(1) 510.5-nm transitions is intriguing. The explanation for their existence cannot be deduced with the present data, since the plasma conditions near the beginning of the discharge pulse are not precisely known. However,let us briefly consider one possible part of the explanation. It is known that during steady-state glow discharge operation, a large majority (estimated to be upwards of 9 5 % 9 of the sputtered material is redeposited onto the sample surface, only to be sputtered again at a later time. The remainder of the sputtered material is either deposited onto the discharge chamber walls or lost into the vacuum system. Sputtering of the sample, apart from that caused by incidental collisions of gas-phase species with the sample, should cease almost immediately upon power disruption since the electric fields present in the discharge should be rapidly extinguished. (23) Bruhn, C.; Harrison, W. W. Anal. Chem. 1978, 50, 16-21.
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However, redeposition will continue as long as there is sputtered material in the gas phase. Therefore,it seemslikely that there may be a larger redeposited layer on the sample surface after disruption of power than when the discharge is operatingin a steady-state mode. How much larger this postpulse redeposited layer is may depend upon the discharge conditions. Upon redeposition,particlesmay chemicallybond with surface material provided they collide with the surface with sufficient energy. Redeposited material which is not significantly bonded with the surface will require relatively little energy in order to be sputtered again. Therefore, this material should be relatively easily sputtered at the onset of the next discharge pulse. If a substantially larger post-pulse redeposited layer does exist, and if a substantial portion of this layer is not significantly chemically bonded, a prepeak could result due to a temporarily large gas phase number density of sputtered material immediatelyafter the beginning of the discharge pulse. In reality, the actual explanation for the existence of the prepeak is probably not this simple since considerations of gas phase excitation processes early in the discharge pulse must be included. Clearly, there is some temporal dependence of the electron energy distribution function in the evolving plasma. This is supported to some extent by the observation that the intensity of the prepeak relative to the plateau region intensity is dependent on the discharge conditions employed. In fact, conditions which exhibit no prepeak may be found. Since only the two aforementionedtransitions which demonstrate prepeaks were extensivelyinvestigated,no conclusion concerningthe species dependency of this parametric response can be drawn at this time. The postpeak exhibited by the Cu(I1)368.7-nm transition is most likely to be the result of either enhanced ionization of neutral copper atoms and/or enhanced excitation of copper ions immediately after power cessation. The upper level of this transition (4s21G.d lies 11.85 eV above the ionic ground state, almost resonant with the argon metastable levels of 11.55 and 11.72 eV. The very small energy defect can be easily provided by kinetic energy; therefore, the population of this ionic level would be enhanced by an increase in argon metastable density, which would result in enhanced emission. As noted earlier,pulsed source GDAASIMSexperimentshave indicated no increase in the density of argon metastables immediately after power disruption. Therefore, neither enhanced Penning excitation of copper ions nor high-energy electron impact ionization/excitationis likely. Alternatively, as electrons are depleted from the collapsingplasma, the ionelectron recombination pathway of a copper ion is removed, possibly making photorelaxation of that state a favorable mechanism which would account for the enhanced ion emission. An absence of a recombination pathway seems to be the ultimate conclusionfor the postpeak in Harrison’sMS work.I5 There is, though, no direct experimental evidence of this mechanism, or lack thereof, for this system. The postpeak seen in the temporal profile for the Cu(1) 521.8-nm transition would seem to be somewhat more easily explained. This transition originates from a high-lying state (4d ‘D5/2,6.19 eV) and feeds into the upper level of the Cu(1) 324.7-nm and Cu(1) 510.5-nm transitions (4p2P3/2O). It would appear that the population of this highly excited state may be due to a copper ion-electron recombination rate enhancement after power disruption, owing to the rapid collisional deceleration of the plasma electrons. However, due to the short lifetimes of these excited states, on the order of nanoseconds, relative to the time scales of the emission wave forms, it would seem that any enhancement in the 521.8-nm emission should also result in detectable enhancements of the 324.7- and 510.5-nm emission intensities. This would be especially true for the 324.7-nm transition, which has the highest transition probability of all copper atomic transitions.
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Flguro 3. Temporal emission wave forms for selected Ar species (average power = 20 W, pressure = 6 Torr, pulse frequency = 10 Hz, duty cycle = 50%).
However, no such enhancement is observed for the 324.7- or 510.5-nm transitions. Perhaps there is an alternativepathway for depopulation of the 4p 'P3/zo level, which would account for the observed data. Further work is needed to assess adequately these results. Wave forms for Ar(1) and Ar(I1) emission transitions are illustrated in Figure 3. These wave forms were collectedwhile sputtering the same sample, under identical discharge conditions, that was used in the acquisition of the wave forms in Figure 2. Presented first is the profile for the Ar(1) 420.1nm transition, which originates from a 14.50 eV (5p [5/21 (J = 3) in Racah term notation) atomic energy level. The second profile is for the Ar(1) 696.5-nm transition, which originates from a 13.33 eV (4p' [1/21 (J= 1))energy level. Both of these transitions feed into a metastable level at 11.55eV (4s [3/21° (J = 2); 4s 3Pz0 in L-S coupling term notation). If overpopulation of the metastable level occurs as a result of recombination processes, a postpeak should be observed for both of these atomictransitions as highly excited argon atoms radiatively relax. Neither transition exhibitsa postpeak. This observation would tend to reinforce the findings of Harrison and co-workers,l6but contradicts the findings of Strauss et al." in terms of the 420.1-nm transition. Studies by that group with a Grimm-type lamp classified this as a "recombination transition", indicative of recombination in the collapsing plasma. They observed a nearly 2X increase in intensity for such lines, maximizing -150 MS after power cessation. At this point, no reason other than dischargedesign and powering schemescan be given for the differencesbetween the Grimm lamp and those studies presented here and in the MS work.15 The trace of the Ar(I1)434.8-nm transition shown in Figure 3c also exhibits no postpeak. This transition originates from the 4p 4D7/20level,which lies 19.49eV above the ionicgroundstate level and 35.25 eV above the atomic ground-state level. Since the argon metastables lie at energies of only 11.55 and 11.72eV, Penning processescannot accountfor any significant population of this level. Therefore, this level can only be effectively populated through electron-impact processes. Owing to the fact that the plasma electrons are expected to collisionally decelerate immediatelyafter pdwer termination, meaning that electron-impactexcitation and ionizationrates
should rapidly decrease, this level is not expected to be significantly populated after power disruption. As a result, the postpulse behavior for this transition illustrated in Figure 3c is not surprising. Finally, as is seen in the wave forms in Figure 3, none of the argon transitions demonstrate a prepeak. Since all of these transitions originate from high-lyingenergy levels, this implies that rates of excitation into high-lying levels are low near the start of the pulse. Since excitation into these levels is expected to be accomplished principally through electronimpact excitation, this is not surprising due to the fact that the plasma electrons must be accelerated from thermal energies once the discharge is initiated. These results are in agreement with the pulsed source GDMS results reported by Harrison et al.14 in which argon ion signals demonstrate no enhancements near the beginning of the discharge pulse, although electron-impact ionization of residual gases upon plasma initiation is suggested by the same group.15 Certainly more work is needed to assess adequately these results. Effect of Pulse Length. The presence of the prepeak and postpeak phenomena in Figure 2 presents interesting opportunities for the analyst. As can be seen in the traces, the instantaneous intensities of these emissionanomalies can be substantially higher than the steady-state emission intensities (Le., where the profiles are flat), thereby possibly providing improved analytical sensitivity if gated detection is employed. Additionally, the gated detection of either the prepeak or postpeak may be advantageous in certain cases in terms of reducing spectral interferences since transitions which do not exhibit such anomalies would be discriminated against. Furthermore, another interesting possibility exists. As is seen in Figure 2a,b, beyond the first approximately 2 me of the pulsed discharge cycle, the emission profiles which exhibit the prepeak phenomenon become relatively flat. Therefore, if an analysis were based on the gated detection of the prepeak, pulse lengths longer than this would be unnecessary. Decreasing the pulse length would allow for the use of higher instantaneous discharge powers with lower duty cycles at the same pulse frequency. The higher instantaneousdischargepowersshould result in higher sputter atomization rates. In addition, the lower duty cycles would help to maintain a reasonably low sample temperature, thereby avoidingthe analyticallydeleteriouseffects of sample vaporization due to excessive heating. Additionally, the higher instantaneous discharge powers should also produce higher plasma excitation rates. This effect coupled with the higher atomization rates should produce higher instantaneous emissionintensities. Since the pulse frequency would remain unchanged, any concomitant increase in analysis time might be avoided. Alternatively,dependingon the sample's thermal properties, higher pulse frequencies with no change in duty cycle could be employed, thereby resulting in shorter pulse lengths. Before studies of the analytical advantages of the shorter pulse length, lower duty cycle, and higher instantaneous discharge power plasmas are assessed, it would be informative to evaluate the effects of pulse length on the shapes of the wave forms. The effect of pulse length on the emission wave forms was studied by collecting wave forms for the various opticaltransitionsat a range of pulse frequencieswhile holding the duty cycle constant (50%). Under these conditions, an increase in the pulse frequency is equivalent to a decrease in the pulse length. The duty cycle was kept constant so that both the average and instantaneous powers could be kept constant. This was thought to be important for two reasons. First, a change in average power would result in a change in sample temperature which might distort the emission wave forms. Second,a change in instantaneous power would result in a change in instantaneous atomizationand excitation rates,
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
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Figure 4. Effect of pulse frequency (length) on the temporal emlsslon wave forms for (a)Cu(1) 324.7 nm, (b) Cu(1) 521.8 nm, and (c)Ar(1) 696.5 nm (average,power = 40 W, pressure = 6 Torr, duty cycle = 50%).
which would make comparisons of emission intensities between the various wave forms difficult. Figure 4 is a compilation of the emission wave forms for the Cu(1) 324.7-nm, Cu(1) 521.8-nm and Ar(1) 696.5-nm transitions for pulse frequencies from 10 to 2000 Hz. The discharge was maintained with an average power of 40 W and a source pressure of 6 Torr of argon. Except for the 2000-Hz Cu plots, the scales of the time (abscissa) axes have been adjusted such that the discharge-on time fills approximately the same print space. Therefore, in order to know the time scale for any particular plot, it must be calculated from the given pulse frequency. In the 2000-Hz Cu plots, approximately four full cycles were employed in the print space to illustrate the acceptable reproducibilitiesof the prepeaksand postpeaks. In each group of plots, the emission intensity (ordinate) axes are on the same scale. As can be seen in the wave forms, the existence and overall shapes of the prepeak and postpeak are independent of pulse length. For the Cu(1) 324.7-nm transition, 2000-Hz pulsing produces a wave form that is almost exclusivelyprepeak,while for the Cu(1) 521.8-nm transition, this frequency produces a wave form that is almost exclusivelypostpeak. The intensity of the prepeak is relatively independent of the pulse length. In contrast, the postpeak intensity is reduced at pulse frequencies higher than 10 Hz (pulse lengths shorter than 50 ma), principally due to the fact that the 521.8-nm emission intensity at the instant the power is terminated is reduced at these frequencies. This intensity reduction appears to be
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Figure 5. Effect of pulsed discharge power (plottedas average power) on the emission Intensity of Cu(1) 324.7 nm In the (a)prepeak (gate A) and (b) plateau (gate 6 )regions of the pulse wave form and the (c) ratbs of A/B for a range of source pressures (pulse frequency = 10 Hz, duty cycle = 50%).
the result of a failure to attain steady-state conditions with the shorter pulse lengths. This conclusion is supportedfurther by the observation that no flat plateau region is present in the Cu(1) 521.8-nm wave forms at the higher frequencies. Interestingly, the Ar(1) 696.5-nm wave forms exhibit a reasonably flat plateau region even at frequenties as high as 600 Hz (pulse lengths as short as 830 ma). This may indicate that the levels from which these two transitions originate are populated through somewhat different mechanisms. This would be expected since the upper level of the copper transition lies only 6.19 eV above the ground state, while the upper level of the argon transition lies 13.33 eV above the ground state. The Cu(1) 324.7-nm wave forms demonstrate a reasonably flat plateau region at only the two lowest pulse frequencies (the two longest pulse lengths). Comparisons of the Cu(1) 324.7-nm wave forms and the Cu(1) 521.8-nm wave forma in terms of the attainment of steady-state conditions are difficult to make, since the prepeak and postpeak are obviously produced through different mechanisms. Although either the prepeak or postpeak characteristics can be exploited analytically, the prepeak may be more analytically desirable since its intensity is conserved at the higher pulse frequencies (shorter pulse lengths). As a result,
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Flgure 6. Effect of duty cycle (pulse length) on the emission intensHy of Cu(1) 324.7 nm In the plateau region of the pulse wave form (average power = 20 W, pressure = 6 Torr, pulse frequency = 10 Hz).
the investigations presented in the remainder of this paper will not consider the postpeak, but will emphasize the prepeak and ita behavior relative to the steady-state plasma. Future investigations, including studies of peak intensity reproducibility and the like, are needed to evaluate further the analyticalutility of the prepeak and postpeak phenomena. ParametricDependencies of the Prepeak andPlateau Regions. Another point of interest in the evaluation of the analyticalcharacteristicsof the prepeak relativeto the plateau (i.e., steady-state) region is their respective parametric dependencies. Therefore, studies have been performed to quantitate the parametric responses of these two regions. The capability of the boxcar averager to set two data gates was employed. Both the prepeak and plateau gates (gates A and B, respectively) were set to widths of 30 MS, with gate A positioned such that it straddled the most intense portion of the prepeak and gate B opened 5 ms from the leading edge of the waveform. Shown in Figure 5 are the responses of Cu(1) 324.7-nm emission intensities to increases in pulsed discharge power (plotted as average power) for a range of source pressures. The source was operated with a 10-Hz repetition rate and a 50% duty cycle. The emission intensities for the prepeak and plateau regions (parts a and b of Figure 5, respectively) are seen to increase continually with applied power. Analogous plots for the same transition obtained for the continuously (steadystate) operated rf sourceshowedsevere rollover above approximately 15-W discharge power at similar source pressures, presumably due to analyte self-absorption.202l Selfabsorptionhas been noted as well in dc poweredglow discharge emission sowces (24). The lack of any indication of selfabsorption here seems to indicate a slow build up of gasphase sputtered material within the source volume, such that a “cool” absorbing region has not had time to be established (24)West, C.D.; Human,H. G. C . Spectrochim. Acta, Part I3 1976, 31B,81-92. (25)Striganov, A. R.;Sventitakii, N. S. Tables of Spectral Lines of Neutral and IonizedAtom; IFI/Plenum: New York-Washington, 1968. (26)Meggers,W.F.;Corliee, C. H.;Scribner, B. F. Tables of SpectralLine Intensities (Part I . Arranged by Elements); U.S. Government Printing Office: Washington, DC, 1975. (27)Moore, C. E. Atomic Energy Leoels, Vol. 11; U.S. Government Printing Office: Washington, DC, 1971. (28)Wiese, W. L.; Martin, G. A. Wauelengths and Transition Probabilities for A t o m and Atomic Ions (Part II. Transition Probabilities); U.S.Government Printing Office: Washington, DC, 1980. (29)Malakhov, Yu.I. Opt. Spectroc. (USSR)1978,44(2), 125. (30)Kerkhoff,H.; Schmidt,M.;Zimmermann,P. Z.Phy.9. A: At.Nucl. 1980,298,249. (31)Osherovich,A.L.; Anisimova,G. P.; Burshtein, M. L.; Verolainen, Ya. F.; Szigeti,J.; Ledovskaya,E. A. Opt. Spectrosc. (USSR)1971,30( S ) , 429.
within the pulse time frame of these experiments. Atomic absorption measurementa are required to study the residence times of sputtered atoms within the source volume. The intensity responses for changes in source pressure for a set power level are quite similar for the pulsed and nonpulsed systems. Conditions of high pressure and high discharge power are seen to yield optimum instantaneous signal intensitiesfor both the prepeak and plateauregions. However, the dischargebecomes somewhat unstable at average powers above approximately 50 W; thus, average powers less than this value should probably be used for analytical purposes. In order to quantitate roughly the analytical advantage (in terms of instantaneous emission intensities) of selectively monitoring the prepeak rather than the plateau region, the ability of the boxcar signal processor to perform simple mathematical processing of the two gate inputs was utilized. Shown in Figure 5c is the parametric response of the ratio of the prepeak intensity to the plateau region intensity (Le., gate A/gate B). The ratio is seen to be inversely related to both discharge power and source pressure. Therefore, even though higher instantaneous emission intensities are found at conditionsof high power and pressure,the prepeak provides the greatest analyticaladvantagerelativeto the plateau region at conditions of low power and pressure. As noted in a previous section, the origin of the prepeak is unknown at present. Therefore, it is impossible to assess adequately the parametric dependencies of the prepeak in terms of the actual processes which dominate at the onset of the discharge pulse. If the rapid sputtering of a previously redeposited layer were the dominant factor, the opposite parametric dependencies would be expected. Redeposition is expected to be more efficient at higher pressures. In addition, sputter removal should increase with discharge power owing to the more rapid steady-state sputtering at higher powers. Obviously, more work is needed to deduce the mechanisms responsible for the existence of the prepeak. Any adequate attempt to explain the phenomenon should alsoinclude considerationof the gas-phaseprocessesoccurring in the early plasma, including the monitoring of electron population characteristics (density, distribution, etc.). Effect of Instantaneous Discharge Powers and Duty Cycles on InstantaneousEmission Intensities. As mentioned previously, the use of gated detection of the prepeak may allow for the stable utilization of very high power, short plasma pulses providing analytically advantageous increases in instantaneous emission intensities. Therefore, it is informative to investigate the extent to which instantaneous emission intensities may be improved in this way. Furthermore,the comparisonof the instantaneousemissionintensities of the pulsed dischargesto those of a nonpulsed discharge is of particular interest since most glow discharge sources are operated in a nonpulsed mode. Owing to the fact that nonpulsed discharges exhibit no temporal anomalies and that different transitions showdifferent behavior,this comparison can only be made by employing a sampling gate positioned over the plateau region rather than over the prepeak. Figure 6 illustrates such data for the Cu(1) 324.7-nm transition. A constant average power of 20 W and pulse frequency of 10 Hz were maintained while varying the duty cycle. The pulse length is varied as a result. Considering the short integration time (10 ma), these data roughly represent the instantaneous Cu(1) 324.7-nm emission intensities provided by the various discharges. The instantaneous intensity is shown to decreasemonotonicallywith instantaneous power from 100 (20% duty cycle) to 20 W (100% duty cycle). If continuous signal integration were employed, the factor of 8 increasein emission intensity provided by the 20% duty cycle plasma relative to the nonpulsed plasma would more than offsetthe factor of 5 decreasein plasma duty cycle. Therefore, the pulsed rf glow dischargemay provide analyticaladvantages
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992 Cu I
a
2079
324.7 nm
channel A
b
cu I 521.8 n m Cu I
channel B
324.7 n m
Cu I
327.4 n m
2n Zn I
C u I 510.5 n m 481.0 nm \
472.2 nm
Cu
I
578.2 n m
d C
A - B
Cu I
324.7 nm
/I
cu
'
327'4 n m
Zn
I
472.2 nm
Zn I I
481.0 nm
I
Cu I
578.2 n m
4
Zn I
213.8 nm Cu I
510.5 nm
Cu I
521.8 nm
~lgur. 7. optical emisslon spectra of a stock brass target derived from the (a) prepeak (gate A) and (b) plateau (gate B) regbns Of the Pulse wave form and (c) the difference spectra (gate A gate 6) (average power = 40 W, pressure = 12 Torr, frequency = 10 Hz, d w cycle = 50%).
-
even if gated detection of the prepeak is not employed. A complete evaluation must include determination of signalto-noise ratios, etc., for the various plasmas utilizing suitable detection schemes. Characteristics of Spectra Acquired with Gated Detection. It has been shown here that the use of pulsed rf glow discharge devices in conjunction with gated detection may provide analytical advantagesin terms of instantaneous emission intensities, particularly for transitions which demonstrate the prepeak phenomenon. It follows that spectra acquired while selectively monitoring the prepeak should emphasis the emission from transitions which exhibit the prepeak. Given the narrow line widths, spectral interferences are relatively rare in GD atomicemission spectroscopy except in cases of self-absorption. Spectra acquired in the pulsed
mode, however, could be advantageousin those certain cases where overlap or self-absorption does occur. Shown in parta a-c of Figure 7 are the emission spectra of a stock brass sample sputtered at an average power of 40 W, a source pressure of 12 Torr, a pulse frequency of 10 Hz, and a duty cycle of 50 % Although higher average powers may provide higher emission intensities asdiscwed in a preceding section, lower average powers provide a somewhat more stable discharge;therefore,the lower power was chosen. The boxcar averager was used to set two data gates, each 30 ps wide. Gate A was positioned such that it straddled the most intense portion of the prepeak for the Cu(1) 324.7-nm transition, and gate B was opened approximately6 ma from the start of the discharge pulse (Le., over the plateau region). Parts a and b of Figure 7 show the spectra taken while monitoring the
.
2074
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
Table I. Analyte Spectral Lines Observed in Figure 7c spectral line
transitiona
energy of upper level0
resonance line?"
gA valueb
Cu(1) 324.7 nm Cu(1) 327.4 nm Cu(1) 510.5 nm Cu(1) 515.3 nm Cu(1) 521.8 nm Cu(1) 578.2 nm Zn(1) 213.8 nm Zn(1) 472.2 nm Zn(1) 481.0 nm
4s 2s1/2 4p 2P3/20 4s 2s1,2 4p 2P1/20 4s' 'D6/2 4p 2P3/20 4p 'Pipo 4d 2D3p 4p 2P3p0 4d 2Dsp 482 'D3p 4p 2P1/20 482 'So 4p 1PlO 4p 3Plo 5s 3% 4p 3P20 5s 3S1
3.82 3.79 3.82 6.19 6.19 3.79 5.80 6.65 6.65
yes yes no no
5.56' 2.74' 0.08od 2.4d 4.5d 2.8 f 0.4e 21.3c 0.43' 2.5 0.28
--
+
+
+
+
+ +
no no Yes no no
*
*
Copper data from ref 25. Zinc data from refs 26 and 27. From ref 28, unless otherwise indicated. Estimated uncertainty stated in reference aa 10%. Estimated uncertainty stated in reference aa 25 %. e Calculated from experimentally determined radiative lifetime from ref 29. Stated uncertainty is not detailed in reference. f From ref 30. No uncertainty given in reference. 8 Calculated from experimentally determined radiative lifetime from ref 31. Stated aa 95% confidence interval.
emission with gates A and B, respectively. These spectra are not on the same intensity scale. However, as shown in Figure 3, the argon transitions appear to attain steady-state intensities relatively early in the plasma lifetime such that their absolute intensities are quite similar in the two spectra. Therefore, these transitions in the 400-500-nm region may be used as a benchmark in assessing the relative intensities of analyte transitions in the two spectra. Since the plateau emission appears to behave similarly to that obtained with the nonpulsed plasma, except in terms of absolute intensity, the spectrum taken with gate B should adequately approximate spectra taken with a nonpulsed discharge in terms of composition. The capabilityof the boxcar averagerto perform simple mathematical processing of the two gate inputs was utilized to obtain the spectrum in Figure 7c, which is the difference spectrum of the two gates (i.e., gate A - gate B). The three spectra were taken for consecutive scans of the same plasma over a 10-min time frame. Comparing parta a and b of Figure 7 indicates that the selective detection of the prepeak does in fact emphasize emission from those transitions which demonstrate the prepeak phenomenon. In particular, the intensity of the Cu(1) 324.7-nm transition relative to the intensities of the argon transitions is substantially enhanced in the "prepeak" spectrum as compared to in the "plateau" spectrum. Other analyte transitions, such as the Cu(1) 521.8-nm line, possess weaker relative intensities in the "prepeak* spectrum than they do in the "plateau" spectrum. This relationship is supported by the observation that the plateau region of the emission wave form for the Cu(1) 521.8-nm line shown in Figure 2c is not reached until approximately 10 ms after dischargeinitiation. In contrast, the plateau region is attained for all of the argon transitions shown in Figure 3 in less than 4 ms. The remaining analyte transitions, such as the Cu(1) 578.2-nm line, appear to behave similarly to the argon transitions in the early plasma as evidenced by the similarity in their relative intensities in the two spectra. These sorta of observationsagain point to a temporally evolvingelectron energy distribution and the possibility for various excitation mechanisms. The difference spectrum depicted in Figure 7c provides a quick method for surveying the early plasma behavior of transitions within a wide spectral range, Transitions demonstrating the prepeak phenomenon are characterized by positive difference intensities, while transitions requiring a relatively long time to reach steady state are characterized by negative difference intensities. Those transitions exhibiting similar intensities in the prepeak and plateau regions are characterized by difference intensities near zero. A comparison of these data with the characteristics of the observed transitions may be helpful in identifying transition characteristics which might be associated with a particular type of emissionbehavior. Pertinent transition characteristics for severalof the prominent analyte transitions are tabulated
in Table I. A comparison of the data in the table with Figure 7c illustrates the fact that none of these transition characteristics appears to be associated with a particular emission behavior. These results amplify the need for further investigations in order to gain an understanding of the dynamics of the early plasma which lead to the phenomenon of the prepeak. Future studiesof the pulsed, radio-frequencyglow discharge atomic emission device should include determinations of emission intensities, analyticalprecisions,and signal-to-noise ratios for a variety of discharge conditions and for a variety of analytical transitions. The mechanism responsible for the generation of the prepeak remains unknown at the present time. It seems reasonable that a relatively large, but not significantly chemically bonded, redeposited layer may be a part of the explanation. However, any serious attempt to explain the phenomenon must include consideration of the gas-phase excitation conditions near the beginning of the discharge pulse.
CONCLUSIONS The data presented here indicate that pulsed rf glow discharges may provide analytical advantages over conventional nonpulsed systems. In particular, the enhanced emissionintensities of the prepeak and postpeak may provide improved sensitivity if selective detection of either of these anomalies is employed. Additionally, the acquisition of spectra in this way may be advantageous in certain cases in terms of reducing spectral interferences, since transitions which do not exhibit anomalous behavior are discriminated against. The prepeak may be more analytically advantageous than the postpeak, since its intensity is relatively unaffected by changes in pulse length or frequency. Therefore, gated detection of the prepeak would allow the use of short pulse lengths, low duty cycles, and high instantaneous discharge powers. In this way, relatively high sputter atomization and excitation rates can be attained, while still maintaining a reasonably low sample temperature and short analysis time.
ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant number CHE-8901788. Financial support from Jobin-Yvon,Division of Instruments SA, is also gratefully appreciated. Furthermore, we wish to thank Dr. Gregory C. Turk of the Inorganic Analytical Research Division of the National Institute of Standards and Technology for the loan of the boxcar averager.
RECEIVED for review February 18, 1992. Accepted May 26, 1992.
