Anal. Chem. 1996, 68, 2135-2140
Rare Earth Oxide Equilibria in Pulsed Direct Current Glow Discharge Mass Spectrometry Yuan Mei and W. W. Harrison*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
Glow discharge mass spectrometry is used to examine the equilibria existing between La+ and LaO+. A pulsed discharge permitted temporal comparison of spectra taken at varying intervals after discharge initiation. Postdischarge peaks are observed for both atom and oxide ions. By varying the pulse period while sustaining a fixed “on” time, the degree of deposition of gaseous constituents on the cathode surface can be controlled. Injection of normal water and isotopically labeled water for compacted and noncompacted samples allows insight into the source of water signals. As glow discharge techniques continue to show their utility for the elemental analyses of solid samples,1,2 an increased amount of interest and research has been directed at better understanding the glow discharge plasma processes, particularly those involving complex samples such as nonconductive materials.3,4 Currently, the use of radio frequency-powered glow discharges (rf-GD) to analyze nonconductive samples is experiencing fast growth.5,6 While the rf-GD methods are approaching maturity, conventional glow discharge devices powered by direct current (dc) voltages are still recognized as viable tools for handling nonconducting samples such as rare earth oxides (REOs),7-9 given proper sample matrix adjustment. Rare earth oxides represent one of the geological sample groups most difficult to analyze by solution-based spectroscopic methods. Compared with several established methods being used for REO analysis, both solution and solids based, the GD method offers the advantage of much simplified sample preparation procedures and hence significantly shortened sample turnaround times.8 The use of glow discharge mass spectrometry (GDMS) for REO analysis has been demonstrated for several years in this laboratory.7-9 Previously, we have used lanthanum oxide (La2O3) as a representative analyte of the REO group to study its atomization and ionization processes in the glow discharge plasma. Special attention was given to the relationship between the desired atomic ions La+ and the undesired molecular ions LaO+, as measured by the La-LaO redox equilibrium, and how this is (1) Harrison, W. W.; Barshick, C. M.; Klingler, J. A.; Ratliff, P. H.; Mei, Y. Anal. Chem. 1990, 62, 943A. (2) Marcus, R. K., Ed. Glow Discharge Spectroscopies; Plenum Press: New York, 1993. (3) Harrison, W. W. J. Anal. At. Spectrosc. 1992, 7, 75. (4) Marcus, R. K.; Harville, T. R.; Shick, C. R., Jr.; Mei, Y. Anal. Chem. 1994, 18, 902A. (5) Marcus, R. K. J. Anal. At. Spectrosc. 1993, 8, 935-43. (6) Mei, Y.; Marcus, R. K. Trends Anal. Chem. 1993, 12, 86. (7) Mei, Y.; Harrison, W. W. Spectrochim. Acta, Part B 1991, 46, 175. (8) Mei, Y.; Harrison, W. W. Anal. Chem. 1993, 65, 3337. (9) Tong, S. L.; Harrison, W. W. Spectrochim. Acta, Part B 1993, 48, 1237. S0003-2700(96)00019-4 CCC: $12.00
© 1996 American Chemical Society
affected by the chemical environment of the plasma, specifically the redox nature of the plasma constituents. The La-LaO interactions in the glow discharge are generally comprised of two phenomena: (1) the sputter process and (2) the reaction equilibrium. The former is affected by the strong bond between La and O atoms in the La2O3 crystal structure, which results in only partial dissociation of all La-O bonds during sputtering. The outcome of this process is mainly governed by the La-O bond strength, the bombarding energy of Ar+, and the mass ratio of La/O. Under typical discharge conditions, i.e., 1000-1500 V, 1-3 mA, the amounts of sputtered LaO and La neutrals from La2O3 are generally comparable.10 As the sputtered La atoms and LaO molecules are ejected from the cathode surface, they immediately enter the area where the reaction equilibrium is to be established. The many chemical species in the glow discharge plasma (including the discharge supporting gas, sputtered sample species, and gaseous contaminants) act as reactants, and the small mean free path in a 1 Torr GD source ensures collisions among the plasma species that lead to both desired and undesired chemical reactions. Desired reactions include those that will eventually result in the generation of atomic analyte ions La+, whereas unwanted reactions will hinder the formation of La+ and promote the formation of LaO+ ions. Of all possible reactions among the analyte atoms and other plasma species, redox reactions controlling the distribution of La and LaO are most important to the elemental analysis capability of GDMS. This concept can be expressed in a simplified fashion by the following expression:
LaO h La + O
The interactions among La, LaO, O, and other species that influence the concentration of oxygen comprise the reaction equilibria that ultimately determine the intensities of the La+ and LaO+ signals as detected by the mass spectrometer. The main objective of this research is to further our understanding of the processes through which the reaction “equilibrium” is established between La and LaO in the glow discharge. The investigations were carred out by using a pulsed dc voltage to power the discharge formed about a sample cathode consisting of La2O3. Klingler et al.11 have shown that pulsed GDMS offers many advantages, including reduced cathode heating, increased discharge voltage, and enhanced sample sputter yield and ion signal intensities. Also reported was the observation of analytically useful anomalies in the pulsed ion signals, as shown in Figure (10) Mei, Y. Ph.D. Dissertation, University of Florida, Gainesville, FL, 1992. (11) Klingler, J. A.; Savickas, P. J.; Harrison, W. W. J. Am. Soc. Mass Spectrom. 1990, 1, 138.
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Figure 2. Illustration of methodology for pulsed glow discharge mass spectrometry. Figure 1. Ion signal profiles in the pulsed discharge: (A) applied voltage, (B) gaseous species, and (C) sputtered species.
1.11 Instead of the anticipated square-wave pattern, as demonstrated by the applied voltage, reproducible peaks were observed at the beginning or end of the pulse, depending on the species being detected. The sudden increase in the sputtered species signal after termination of the pulse, forming an “afterpeak”, was found to be typical for sputtered sample components, metal dimers, and metal argides.11 The analytical and fundamental aspects of these signal anomalies have been further investigated.12,13 EXPERIMENTAL SECTION The basic mass spectrometer system has been described previously.14 The coupling of the pulsing device to the discharge power supply and the corrresponding data acquisition system are shown in Figure 2. A custom-designed pulsed generator and delay circuit were housed in one box (Finnigan MAT, Bremen, Germany). The square-wave function sent out by the pulse generator, with controllable amplitude, frequency, and duty cycle, was used to drive a Kepco OPS3500 operational power supply. The period of the square-wave can be varied between 0.2 ms and 1 s, with the “on” duration controllable between 0.2 and 18 ms. Ion signals from the mass spectrometer can be collected in two modes: (1) as a single ion intensity versus time profile by synchronizing a multichannel analyzer with the pulse trigger signal (shown as the solid line wiring in Figure 2) and (2) as a full mass spectrum of a specified mass range obtained through a data acquisition gate. The acquisition gate can be placed anywhere along the pulse period, with a variable width of 0.2-18 ms. Sample preparation procedures have been detailed in a previous publication.7 Mixtures containing 10% analyte oxide (La2O3, RG, Fisher Scientific Co.) and 90% host matrix powders [Ag (99.999%, Gallard-Schlesinger Chemical Mfg. Corp.), Ta (SG, Spex Industries, Inc.), and Ti (99.9%, Aldrich Chemical Mfg. Corp.)] were pressed into small disks to serve as the glow discharge (12) Klingler, J. A.; Barshick, C. M.; Harrison, W. W. Anal. Chem. 1991, 63, 2571. (13) Pan, C. K.; King, F. L. J. Am. Soc. Mass Spectrom. 1993, 4, 727. (14) King, F. L.; Harrison, W. W. Spectrochim. Acta 1991, 46B, 175.
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Figure 3. Signal profiles of La+ and LaO+ in the pulsed glow discharge.
cathodes in different experiments. Pure Cu metal, both in the forms of a bulk pin and a pressed disk from powders, was also used in certain experiments as described in the text. RESULTS AND DISCUSSION In this study, pulsed glow discharges were initially applied to oxide samples to determine whether signals of the elemental ion and the monoxide ion (e.g., La+ and LaO+) might show distinguishably different time profiles. It would be analytically advantageous if one species demonstrated an afterpeak and the other one did not. Under such a circumstance, not only could a discriminative La+/LaO+ ratio be obtained at different regions of the pulse period, but also information about the differential ionization mechanisms of the two species might be gained. Pulsed Signal Profiles. In pulsed discharge operation, the signals of both La+ and LaO+ showed an afterpeak when the pulse was terminated (Figure 3). Afterpeaks are thought to arise from Penning ionization,11-13 since electrons are rapidly removed upon plasma termination, leaving argon metastable atoms as active ionization agents. The presence of afterpeaks for La+ (IE ) 5.28 eV) and LaO+ (IE ) 4.9 eV) suggests that both species were ionized primarily through Penning ionization. There does not seem to be any significant difference in the shape of the two signal profiles in Figure 3. For further comparison, the La+/LaO+ ratio
Figure 4. Signal ratio profiles of La+/LaO+ for the La2O3/Ag and La2O3/Ti samples during pulsed discharge operation.
was calculated from signal intensities obtained through a 1 ms data gate placed sequentially on several positions along the pulse period. The calculated La+/LaO+ ratio is plotted as a function of time in Figure 4, where data obtained from two different sample matrices are presented. The purpose of using two matrices (Ti and Ag) was to create and compare different plasma environments. As a getter reagent, Ti was expected to help create a plasma more reducing in its chemical nature than that of Ag, hence yielding a much larger relative population of La,7 as is demonstrated in Figure 4. The discharges served as two representative plasma environments in which the La-LaO equilibrium was dominated by different species in each discharge. As shown in Figure 4, the magnitude of the La+/LaO+ ratio obtained with the Ti matrix is larger by a factor of 7 than that obtained with the Ag matrix. Both ratios reach a steady-state value after the first couple of milliseconds of the discharge and show little change even into the afterpeak region. The factors that control the distribution of lanthanum between La+ and LaO+ remain consistent for both the silver and titanium matrices. While we cannot be certain without the corresponding data, the assumption follows that the ratio of La and LaO neutral species is generally comparable to that of the ions. This is in general agreement with the observation made with a dc discharge.7 No significant difference in the La+/LaO+ ratio was observed, in either case, between the “on” region and the afterpeak region, which suggests that mechanisms responsible for the ionization of La and LaO were comparable in these regions (later part of the plateau and the afterpeak). This may rule out electron impact as a significant ionization step, as the electrons are removed from the plasma almost instantaneously after discharge termination. During the initial period of the pulse “on” time, species sputtered off the cathode surface tend to include any gaseous contaminants that were deposited during the “off” time of the previous pulse cycle. In the case of the readily oxidizable Ti matrix (Figure 4), this may have caused a temporary lack of gettering action on the sample surface at this point until sputter cleaned, resulting in a slight shift of the La-LaO equilibrium toward the formation of LaO and a lower La+/LaO+ ratio. As the discharge becomes more stable (i.e., beyond 2 ms of the “on” portion), a relatively “clean” surface develops, and a steady-state gettering environment is maintained. As a result, the La+/LaO+ ratio increased and stayed relatively stable in the remaining portion of the pulse “on” time. Since Ag does not demonstrate an obvious gettering ability, such a phenomenon was not observed with the Ag matrix. In fact, the La+/LaO+ ratio was slightly lower
Figure 5. Schematic illustration of discharge processes on the cathode surface at various pulse duty cycles.
in the later stages of the discharge “on” time than during the initial phase of the pulse cycle. This is believed to reflect cathode heating, which affects the plasma by releasing adsorbed water vapor, an important oxidant in the La-LaO equilibria. The La+/LaO+ ratios obtained with two types of discharges showed differences during the early portion of discharge “on” time (Figure 4), a period when the chemical condition of the cathode surface plays an important role in affecting the signals of the sputtered species, prompting the second part of our study of applying pulsed discharges to the oxide sample, i.e., to determine how the La-LaO equilibria are affected by the chemical environment of the cathode surface. Pulsing the discharge provides a means of changing and monitoring the chemical composition on the cathode surface. Redox Equilibria on Cathode Surface. Pulsing Schemes. In a dc glow discharge, the cathode surface develops a steady-state equilibration between sputter ablation and redeposition of the discharge constituents.12 After the initial step of sputter “cleaning” the sample surface, a process that can extend from 5 to 30 min, depending upon the sample, the chemical composition on the cathode surface can be considered to be at equilibrium and remains relatively stable with sputtering time. The extent of gaseous molecules residing on the cathode surface of a dc discharge is also at equilibrium, but their absolute amounts are difficult to obtain. Under such conditions, little information can be obtained on how the chemical constituents affect the La-LaO equilibrium on the cathode surface. Although the absolute amounts of gaseous impurities on the cathode surface are difficult to measure, it is possible to induce systematic changes in the concentration of these adsorbed gases by pulsing the discharge. In this way, a relative indication of their amounts can be obtained. This concept can be illustrated using Figure 5, where four pulsing schemes are shown which have the same “on” time but different “off” times for each pulse cycle. During the pulse “on” time, the sputter action removes various species from the cathode surface, and the discharge current heats the cathode body. During the pulse “off” time, both sputtering and cathode heating are stopped, resulting in a temporary cooling of the cathode upon whose surface water and other gas molecules are deposited. If the pulse “on” time is held constant and the “off” time is increased, as shown in Figure 5, the pulse cycle with longer “off” time will result in more impurities accumulated on the cathode surface between pulses. In this sense, surface effects caused by heating and sputtering are enhanced when employing Analytical Chemistry, Vol. 68, No. 13, July 1, 1996
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Figure 6. H3O+ signal as a function of pulse period for four types of samples: Cu pin, compressed Cu disk, La2O3/Ag disk, and La2O3/ Ta disk. Initial reference point is for a 10 ms period, 50% duty cycle.
a higher duty cycle (more “on” time); effects caused by cooling and surface deposition are promoted when changing to a lower duty cycle (more “off” time). As the “off” time is carefully controlled and varied, a series of cathode surfaces with controlled changes in their chemical compositions can be created for studying the corresponding behavior of the oxides. Water Signal as a Function of Pulse Period. To evaluate the surface cleaning/deposition concept for its effect on glow discharge spectra, experiments were carried out to monitor the water ion signal as a function of the pulse period. The pulse “on” time was held at a constant 5 ms, and the “off” time was varied between 5 and 175 ms. The water signal was collected through a 2 ms data gate placed in the middle of the pulse “on” time. Experiments were performed on four different types of samples: a solid copper pin, a compressed copper powder disk, and compressed disks of La2O3/Ag and La2O3/Ta mixtures, presenting a range of sample matrix situations. The copper pin was the “cleanest” in the sense of little impurity incorporation within the sample. By contrast, a compressed copper powder will always contain entrapped water vapor and air to some extent. The lanthanum oxide samples present the same impurity incorporation problems, allowing a test of the effect of bulk matrix (silver versus tantalum) on the surface problem. The results are shown in Figure 6. The response of the water ion signal to the increase in the pulse period, relative to the water signal observed for a symmetrical 10 ms pulse period (5 ms “on” time), can be classified into two groups among these samples. For cathodes in the first group, consisting of a Cu pin and the La2O3/Ta disk, the water signal increased with the pulse period. That is, for a fixed pulse “on” time, increasing the “off” time resulted in an increased water signal relative to that observed for a symmetrical pulse. This observation was in agreement with the proposition that a prolonged “off” time allows accumulation of water molecules on the cathode surface, causing a “burst” in the water concentration during the next discharge initiation. Contrary to the first group, relative water signals observed in the second group, comprised of the compressed Cu and La2O3/ Ag cathodes, showed a decreasing water signal as the pulse period increased. This response is thought to mean that at shorter “off” times, where discharge duty cycle and the cathode bulk temperature increase, significant quantities of water are released from within the electrode. The dominant form of water release is from the entrapped water within the electrode, not a surface accumula2138 Analytical Chemistry, Vol. 68, No. 13, July 1, 1996
Figure 7. Normalized H318O+ and H316O+ signals (to their respective values at 10 ms period) as a function of pulse period with and without H218O addition; Cu cathode.
tion between pulses. Given a fixed period of time, more water was released into the discharge as cathode heating became more dominant due to a longer “on” time portion rather than from the surface accumulation during the “off” time. Therefore, as the cathode was allowed to cool less (shorter “off” time), a larger amount of water was released into the discharge. Overall, the water signal behavior as a function of pulse period seemed to be determined by the origin of the water: in the discharge gas phase or inside the cathode body. An exception was in the case of a compressed Ta disk, which deserves further discussion. Although the cathode was compressformed, the changing trend of its water signal followed that of a pin cathode which did not have water trapped within. Here, increased heating did not seem to enhance the amount of water released from the cathode. The difference is related to the high chemical activity of the cathode matrix,15 tantalum, which is evidently able to bind reactively with the internally generated water, particularly at the higher temperatures of the electrode associated with the cathode heating. Use of Isotopically Labeled Water. Seeking additional evidence of water’s dual origin in the glow discharge, a further experiment was carried out in which both modes of water origin were created in the same discharge and differentially identified by the mass spectrometer. A Cu powder compressed disk was used as the cathode, which served as the source for internally generated water. Copper was used because of its relatively low reactivity, avoiding any gettering effect inside the cathode. Isotopically labeled water (H218O, 97 atom %, Sigma Chemical Co.) was bled into the Ar gas at 5%, serving as the “gas phase” water. The H316O+ signal was monitored (see Figure 7) as a function of the pulse period before adding H218O into the discharge. With longer “off” time (increasing pulse period), the H316O+ signal of the “internal” water decreased due to less cathode heating, in keeping with the results of Figure 6. Also shown in Figure 7 is the H318O+ signal as a function of the pulse period. The H318O+ signal showed the characteristic cathode deposition response that increases with pulse period, indicating cathode surface deposition from the gas phase water. The distinct difference between the changing trends of the H316O+ and H318O+ signals reinforces the observation that water originating from the two different sources does respond differently to the changing pulse period. La+/LaO+ as a Function of Pulse Period. The purpose of studying the La+/LaO+ ratio as a function of the pulse period was (15) Giorgi, T. A. Jpn. Phys. Suppl. 1974, 2, 53.
Figure 8. H3O+ signal and La+/LaO+ ratio as a function of pulse period for a La2O3/Ag sample. H3O+ signal is normalized to the value at 10 ms period.
to relate the La-LaO redox equilibrium to chemical changes induced on the cathode surface. Two samples were used to represent the two “water origin” situations: La2O3/Ag and La2O3/ Ta, due to the different water signal behavior previously noted with these samples. Again, the pulse “on” time was held constant as the pulse period was varied. The La2O3/Ag sample was used mainly to demonstrate the effect of the “internal” water on the La+/LaO+ ratio. The water signal and the signal ratio of La+/LaO+ are plotted as a function of the pulse period in Figure 8. The decreasing trend of the water signal with the increasing pulse period (increased “off” time) has been described in the previous section as resulting from reduced cathode heating and consequent lower water evolution out of the sample bulk. For the pulse periods from 10 to 35 ms, the water signal showed the sharpest decrease, corresponding to an increase in the La+/LaO+ ratio. The opposing trends of the water signal and the La+/LaO+ ratio are not unexpected, since decreased water vapor causes a shift in the gas phase equilibrium toward enhanced La+. For the pulse periods beyond 35 ms, the La+/LaO+ ratio trailed off to a somewhat lower value, possibly because the surface water deposition became the more significant source of water in the plasma at the longer “off” times, as previously reflected in Figure 6. With the surface water concentration increasing at longer periods, the La+/LaO+ ratio became smaller. As observed earlier, in the discharge using a compressed Ta disk, water ion signal was not affected greatly by the sample heating, due to the gettering ability of the Ta. Hence, when using a La2O3/Ta sample, monitoring the La+/LaO+ ratio as a function of the pulse period should reveal information about how the LaLaO equilibrium on the sample surface was affected by the deposition of water molecules originating in the gas phase. The water signal and the La+/LaO+ ratio obtained in these experiments are plotted as a function of the pulse period in Figure 9. Increasing the pulse period (“off” time) allowed water molecules to build up on the cathode surface, resulting in a higher H3O+ signal during the next discharge “on” time. Water accumulating on the cathode surface acted as an “oxidation zone” for the cathode species. Part of the oxidation may occur through chemical bonding directly on the cathode surface. Another oxidation process may be considered to arise in the gas phase as oxygen (from the water) attachment to the sputtered species, especially those metals that form stable monoxides (e.g., La), as they leave the cathode surface. As a result, the La-LaO equilibrium was shifted toward
Figure 9. H3O+ signal and La+/LaO+ ratio as a function of pulse period for a La2O3/Ta sample. H3O+ signal is normalized to the value at 10 ms period.
Figure 10. H318O+ signal and La+/La18O+ ratio as a function of pulse period for a La2O3/Ta sample with addition of H218O. H318O+ signal is normalized to the value at 10 ms period.
the formation of LaO, showing a decrease in the La+/LaO+ ratio (Figure 9). To confirm the rationale developed with the compressed Ta sample, 18O-labeled water was again injected into the discharge to induce the formation of La18O, which did not exist in the bulk cathode disk. The process of forming La18O should happen only on the uppermost surface of the cathode or in the gas phase. Pulse studies allowed probing the gas phase mechanism8,10 at the cathode surface. It may be noted that the residual amount of H218O mixed in the discharge gas responded in a similar fashion to the pulse period change as did H216O (Figure 10). Consequently, the La+/La18O+ ratio also showed the same changing trend as did the La+/La16O+ ratio in responding to the pulse period change. Because the origin of H218O was only in the discharge gas phase, but not from within the cathode, the strong similarities between Figure 9 and Figure 10 suggest that water molecules of the two forms were functioning in a similar mechanism, that is, affecting the chemical composition on the cathode surface. CONCLUSIONS The fundamental processes occurring on the surface of a glow discharge cathode have been studied by introducing transient plasma perturbations. This is accomplished by powering the plasma in a controlled time-resolved fashion. Pulsing the discharges creates a systematic change in the chemical compositions and chemical reactions on the cathode surface, which results in the redox equilibration between La and LaO on the sample Analytical Chemistry, Vol. 68, No. 13, July 1, 1996
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surface. These studies have also shown that, when using compressed disks as glow discharge cathodes, water contamination arises primarily from within the disk body. It has been known that this contamination can be reduced by using getter reagents as sample matrices where sputtered getter atoms will react with and hence remove water molecules in the plasma gas phase. The study described here offers an indication that the gettering action may commence in the solid state, even before the getter atoms enter the gas phase. The self-heating of the cathode during the operation of the glow discharge may induce binding reactions between the getter reagent and the water contaminants enclosed within the disk cathode. On the other hand, vaporized water molecules in the plasma gas phase can be redeposited back onto the cathode surface once the cathode cools. These deposited water molecules tend to oxidize the sputtered atoms as they leave
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the cathode surface, causing the La-LaO equilibrium to shift toward forming the LaO molecules. A clean cathode surface created by getter matrices is favorable for preventing La atom oxidation. While this study does not include specific analytical applications, there are implications here for the practical world. The findings demonstrate the advisability of using either physical (e.g., cryogenic cooling) or chemical (e.g., getter) means to ensure low water contamination in a glow discharge source to achieve best performance. Received for review January 11, 1996. Accepted April 12, 1996.X AC960019D X
Abstract published in Advance ACS Abstracts, May 15, 1996.
