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W. W. Harrison, Chenglong Yang, Eric Oxley University of Florida
he glow discharge (GD) is a simple and efficient way to generate atoms. Long known for its ability to convert solid samples into gas-phase atoms, GD techniques provide ground-state atoms for atomic absorption or atomic fluorescence, excited-state atoms for atomic emission, and ionized atoms for MS (1, 2). Commercial instrumentation has been developed for all these methods, except atomic fluorescence. In fact, GD spectroscopies are mature techniques that have been extensively discussed and are well recognized within the analytical community for elemental analysis (3, 4). 480 A
A N A LY T I C A L C H E M I S T R Y / S E P T E M B E R 1 , 2 0 0 1
S E P T E M B E R 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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(a) Photons O –V
Ions
dc
Sputtered atoms
Negative glow
(b) Photons O
Ions
–V Pulse FIGURE 1. General scheme of a glow discharge source for optical emission or MS. Atom density decreases as the black fades to orange. (a) dc mode. The device consists of a negative cathode and a grounded anode immersed in a low-pressure rare gas, usually argon. A potential (V) breaks down the discharge fill gas, yielding Ar+ that is attracted to the cathode—which, in this case, is the sample. The ions collisionally sputter neutral atoms from the surface into the adjacent GD plasma. As these sputtered atoms diffuse through the GD plasma, collisions with electrons, ions, and metastable atoms excite and ionize the sample atoms. (b) Pulsed mode operates on the same principles described for dc mode, but in short, repetitive dc pulses.
Most GD devices are operated in the continuous mode, as illustrated in Figure 1a, producing a steady-state population of sputtered atoms that reaches a maximum concentration close to the cathode and steadily decreases with distance. However, it is also possible to run a GD in a pulse-mode operation (Figure 1b), generating with each pulse a packet of sample atoms that expands as it diffuses across the cell. High transient atom densities are thereby obtained. Both continuous (or dc) and pulsed-mode operation provide effective analyte atom populations for analysis, but the latter method opens up creative new approaches that take advantage of temporal effects (5). Why pulse a GD source? It provides short-term, high-power operation, which is why other devices such as magnets are pulsed. A pulsed GD device offers three basic advantages over dc operation: high peak voltages and currents to enhance sample sputtering, excitation, and ionization; gated detection options for temporal discrimination of analytical signals; and fewer problems with thermal effects because of the lower average power. The millisecond or microsecond pulse discharges generate high instantaneous voltages
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A N A LY T I C A L C H E M I S T R Y / S E P T E M B E R 1 , 2 0 0 1
and currents, but these results are acceptable because the average power remains at a safe level due to the short duty cycles employed. Moreover, the short duty cycles allow the application of higher currents and voltages than continuous operation. Gated detection methods measure signals only during the short pulse period (or portions of that pulse). These pulses can be averaged, yielding better analytical results. Continuousmode GD devices operate at a power of a few watts; pulsed GD systems can be run at kilowatt peak power. Thus, the GD can be transformed into a powerful source, even if just for microseconds (6, 7 ). In this feature, we discuss the advantages of pulsed GD, provide some example applications, and list areas for future research.
Pulsed GD properties The average power of a pulsed GD is no greater, or even less, than the continuous dc counterpart. So, how can there be any net advantage analytically? After all, the number of sputter events is determined by the net discharge current, and, if the average powers are equivalent over a given time period for two techniques, samples under pulsed or continuous discharges are hit with the same number of ions. However, because the voltage is greater for pulse than continuous mode, higher-energy ions strike the sample, which, in turn, increases the sputter yield. A more energetic plasma is created in a pulsed experiment by restricting the on-time of the discharge, and that increases the numbers of excited and ionized sputtered atoms. Indeed, the emission spectra of a pulsed GD show different and unusual lines as a result of populating higher-order atom and ion states. So, not only are more sample atoms formed, but a higher percentage of them yield photons and ions for analytical measurements. Observations have shown 10- to 100-fold signal increases.
Time-resolved spectroscopy If pulsed operation of the GD were only to achieve enhanced photon and ion yield, it would offer real significance. However, there’s another—and in some ways more significant—feature of pulsed discharge spectra. The pulsed GD consists of a series of repetitive events, where time-dependent phenomena are observed. These temporal effects allow time-resolved spectroscopy. When a cold, quiescent GD cell is pulsed by a high-voltage, high-current probe, a cascade of physical and chemical interactions is triggered. For example, as the pulse is initiated, the discharge gas becomes partially ionized and begins supplying sputtering ions to dislodge analyte atoms from the sample’s surface. An induction period of ~0.5–1 µs precedes the spectral measurement of the first sputtered atoms. Interestingly, how each element responds in this induction period varies because of different sputter rates and excitation and ionization pathways. The result is a degree of temporal elemental differentiation due to these time-dependent phenomena. In the case of atomic emission, these differences are small but potentially useful. For MS, the advantage is much greater. Atomic emission. Figure 2 shows an example of temporal differentiation among typical plasma species in a pulsed GD. As
In Figure 3, a selected time delay of 150 µs produces a mass spectrum comprised almost exclusively of analyte species. A comparison of Figures 3a and 3b demonstrates improvements obtained in the spectrum. The pulsed GD device combined with a TOF mass spectrometer provides a complete mass spectrum for each pulse. By averaging many such pulses, S/B is greatly enhanced. At a nominal 500-Hz pulse rate, significant accumulations are made within seconds. Even without TOF access, temporal resolution can be used to remove mass spectral interferences. For example, King et al. have determined 40Ca+ in the presence of 40Ar+ with a quadrupole system by means of time-gated detection (12). In the afterpeak region of a pulsed millisecond discharge, little or no 40Ar+ was present to interfere with the desired Ca+ signal, and a linear working curve can be obtained for Ca+ under such conditions. Thin-film analysis. GD methods are intrinsically surface analysis methods. That is, the sputter release of atoms by ion bombardment is a surface phenomenon. The GD device acts as an ion milling machine that methodically erodes the sample surface. For bulk analysis of metals and alloys, presuming a homogeneous sample, the ablated atoms in each pulse represent the bulk sample composition, and a pulsed discharge will continue to deliver reproducible, consistent ion packets for analysis. For a thin-film sample, the basic GD process is the same; however, the temporal results are quite different because sequential sample layers are exposed by the discharge (13). Figure 4 shows a typical response from a GD analysis of a thin layer on a host matrix. Instead of recording analyte emission spectra or ion
Pulse on Cu Fe Ar D A T A
Intensity
expected, the discharge gas, argon, appears first, because it requires only the breakdown of the fill gas for photon release. The emission from the sample sputtered atoms, most of which appear at quite similar time scales (represented by copper and iron in this figure), comes later. The emissions from metal ions often are delayed in time, as shown in Figure 2 for copper ion. Multielement samples present additional separation opportunities. A narrow data gate window allows the selection of an optimal measurement point for a selected spectral line and the possibility for discrimination against some other line. Wider data gates permit inclusion of all the emission signals in cases where differentiation is not desired or advantageous. Bengtson showed that signal-to-background (S/B) for selected elements in steel samples improved with timeresolved measurements (8). Nitrogen analyses in metals often suffer from interference due to background nitrogen in the discharge gas, but time-resolved procedures in pulsed GD allow the separation of the sputtered nitrogen from the background. Atomic fluorescence/absorption. Within a GD pulse— typically, 10 µs—most sputtered atoms do not diffuse very far from the cathode. After the pulse, these atoms maintain a relatively long lifetime, measured in milliseconds, as they diffuse across the discharge cell, slowed by collisions with the argon fill gas. However, all the electrons and most ions have been removed by then, and a greatly reduced background is available for spectroscopic measurements in the post-pulse period. For example, the S/B for atomic fluorescence measurements of sputtered atoms can improve during the “dark” period when the pulsed plasma is off (9). Atomic absorption techniques can benefit similarly from measurements in the dark period. MS. Pulsed discharges offer arguably the greatest reward for MS experiments, particularly with time-of-flight (TOF) instruments. Because MS inherently requires the physical transport of ions from the source, and because TOF requires a pulsed introduction of ions, time-dependent events with pulsed GD offer special opportunities (10). Figure 3 shows time-resolved TOF spectra as a function of increasing delay time after the pulse discharge period. Initially (~ first 100 µs), the ions extracted from the GD source are those originating from the discharge argon gas and other contaminant gases such as air and water vapor. No measurable analyte species have yet reached the ion exit orifice because of the time required for sputter release and diffusion across the GD cell. As the delay time is increased beyond 100 µs, analyte ions begin to appear and the discharge gas ions disappear. Thus, by acquiring TOF mass spectra at the optimum delay time for the sample ions, the desired analyte species can be discriminated from potentially interfering discharge gas species. This is particularly important for low-mass elements such as magnesium, aluminum, and silicon, which are prone to interferences from GD gas-phase contaminants (11).
Cu+
G A T E
5
10 Time (µs)
15
20
FIGURE 2. Optical emission of discharge gas and sputtered species from a microsecond-pulsed GD device. S E P T E M B E R 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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(b) Signal
OH
H O+ + 2
H 3O +
Ar+
ing that broadens the layer transition zone. Mg+
Al+
Other analytical considerations
Signal
(a)
Del
ay t ime (µs )
Signal
No commercially available systems are yet N2H+ O2+ ArH+ Si+ Ar+ ArH+ available for pulsed GD techniques. HowCu+ ever, commercial pulsed power supplies and 15 15 25 36 45 25 36 45 m/z m/z gated detection equipment can be interfaced with existing equipment for pulsed GD (c) measurements. If GD techniques already work well in 250 their present configuration, why convert to pulsed operation? The need to shift will not be compelling for those satisfied with con150 tinuous mode. However, in cases where additional sensitivity, reduced spectral interferences, and enhanced sputter control are valuable, the pulsed GD source is attractive. 50 Moreover, in addition to the mainstream applications of time variation just described, 15 30 45 60 75 other valuable uses for pulsed GD have m/z been reported. Pulsed GD with pulsed laser ablation. FIGURE 3. Time-resolved mass spectra from a microsecond-pulsed GD device as a funcUsing lasers as ablation sources extends GD tion of delay time after pulse termination. spectroscopy to nonconductors such as glasses, soils, and ceramics, which otherwise (a) A low mass region at a short delay time, with interferences from the discharge gas and other backrequire sample pretreatment to convert them ground gases, versus (b) the same region taken at a longer delay time, allowing temporal resolution of ions from sputtered analytes. (c) Full mass spectrum showing the relationship between signal ininto conducting forms for conventional GD tensity and delay time. analysis. Lasers, having no such conductive requirement, easily ablate and atomize a broad gamut of sample types. By combining two temporal sources, the analyst can experspectra, the signal of interest in this case is the time-dependent iment with optimal overlap of the two pulses—in both space intensity of the pure (or major) element in the layer. As the GD and time—to achieve the desired results. These configurations ablation nears removal of the first layer (copper in Figure 4), and timing sequences may differ depending on whether photons the signal drops rapidly toward zero, while the signal compo- or ions are being measured. nent of the substrate component (iron in this case) simultaneFocusing a laser pulse on a sample surface produces extenously rises. At the layer interface, signals arise from both sive atomization of the material in a powerful “fireball”, generelements due to atomic mixing and GD nonuniating a broad background emission. This interfering broadformity on the sample surface. By converting band emission is removed or reduced by waiting a selected sputter time to a corresponding erosion period after terminating the laser pulse to take a measdepth, the crossover point in Figure 4 can urement. This approach provides a rich concentration be used to calculate the thickness of the of sample atoms that can then be probed in a spectralinitial layer (14). ly “dark” region with a high-voltage, high-current GD In principle, either continuous or pulse. pulsed GD will work for this application, Ingeneri explored several fundamental parameters but in the case of very thin layers (
