Anal. Chem. 1997, 69, 4957-4963
Diffusion, Ionization, and Sampling Processes in the Glow Discharge Source for Mass Spectrometry Wei Hang† and W. W. Harrison*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
A microsecond pulsed glow discharge source has been coupled to a quadrupole mass analyzer for plasma diagnostics. Diffusion times of sputtered atoms from the cathode surface to the sampling orifice are measured for different cathodesorifice distances, pressures, and powers. These measured times are found to be close to the values calculated from the theoretical models, indicating the suitability of this approach to describe the diffusion process in the glow discharge. By comparing ion signal profiles of the gaseous species and sputtered particles and the theoretical atom distributions, evidence is shown that most ions measured by the mass spectrometer are ionized in the vicinity of the orifice.
The glow discharge (GD) source was successfully coupled with mass spectrometers in early 1970s,1,2 and the technique of glow discharge mass spectrometry (GDMS) subsequently has become a leading technique for direct solid sample analysis. Direct current glow discharge (dc-GD) and radio frequency glow discharge (rfGD) sources are the two commonly used GD types. Pulsed glow discharges have also been known for many years,3-6 although their possible utility as an analytical GDMS source is of more recent origin.7-9 Certain advantages have been shown for the pulsed GD over dc-GD.3-10 One application of the pulsed technique is to enhance the output intensity of hollow cathode lamps in fluorescence and atomic absorption.3 A 50-100-fold increase in the output intensity of millisecond pulsed discharge lamps vs dc-operated lamps has been reported,4 and a 4 orders of magnitude increase in ion line intensity has been observed for microsecond pulsed hollow cathode lamps in fluorescence spectroscopy.5 The pulsed glow discharge produces a transient burst of sputtered analyte atoms that can be used for atomic absorption, † Current address: Los Alamos National Laboratory, CST 9, Los Alamos, NM 87545. (1) Coburn, J. W.; Kay, E. Appl. Phy. Lett. 1971, 18, 435. (2) Harrison, W. W.; Magee, C. W. Anal. Chem. 1974, 46, 461. (3) Djulgerova, R. B. In Improved Hollow Cathode Lamps for Atomic Spectroscopy; Caroli, S., Ed.; Halsted Press: New York, 1985; Chapter 3. (4) Dawson, J. B.; Ellis, D. J. Spectrochim. Acta 1976, 23A, 565. (5) Huang, B.; Yang, P.; Lin, Y.; Wang, X.; Yuan, D. Chin. Anal. Chem. 1991, 19, 259. (6) Smith, B. W.; Omenetto, N.; Winefordner, J. D. Spectrochim. Acta 1984, 39B, 1389. (7) Klinger, J. A.; Savickas, P. J.; Harrison, W. W. J. Am. Soc. Mass Spectrom. 1990, 1, 138. (8) Hang, W.; Yang, P.; Wang, X.; Yang, C.; Su, Y.; Huang, B. Rapid Commun. Mass Spectrom. 1994, 8, 590. (9) Harrison, W. W.; Hang, W. J. Anal. At. Spectrom. 1996, 11, 835. (10) Walden, W. O.; Hang, W.; Smith, B. W.; Winefordner, J. D.; Harrison, W. W. Fresenius J. Anal. Chem. 1996, 355, 442.
S0003-2700(97)01038-X CCC: $14.00
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fluorescence, and emission measurement.6 Millisecond pulsed glow discharge research in our laboratory also showed an increased sputter yield and ion intensity over the conventional dc source, as measured with a quadrupole mass spectrometer.7 Microsecond pulsed glow discharge (µs-GD) sources have also been coupled to time-of-flight mass spectrometers,8,9 indicating several advantages, including high sample utilization and certain temporal separation advantages. Atomic emission spectroscopy experiments showed that a µs-GD enjoys about 2-4 orders of magnitude higher sensitivity compared to a dc-GD,10 suggesting promise as an emission source for a multichannel optical spectrometer. In a glow discharge ion source, sputtered atoms must diffuse through the adjacent negative glow in order to reach the exit orifice and be sampled. The collision frequency is very high at 1 Torr pressure, such that many of the sputtered atoms are redeposited on the cathode surface11 and those that diffuse away from the cathode are quickly thermalized as they pass through the negative glow. It is thought that a small fraction (0.1-1%) of the sputtered atoms collide with sufficiently energetic electrons or metastable species to effect ionization.11 Duckworth and Marcus proposed that most ions are formed at the transit zone between the cathode fall and negative glow.12 At this point, electrons emitted from the cathode surface will have lost sufficient energy in the cathode fall, thus achieving a higher excitation and ionization cross section more effective for the formation of metastable argon atoms, argon ions, and sample ions. In addition, the highest argon metastable atom density is expected in this region. Sampling from within the cathode fall region is hindered by the fact that the cathode fall potential tends to draw the ions formed near the cathode fall back to the cathode surface due to the existing electric field. Farther into the negative glow, lower ion signals are expected due to lower metastable densities and the decreasing sputtered atomic population. Resonant ionizaion studies using a tunable dye laser indicated that those ions formed in the immediate vicinity of the sampling orifice are more likely to be extracted from the source and transmitted to the mass spectrometer.13,14 By moving the ionizing laser beam away from the sampling orifice along the ion extraction axis, the ion signal obtained in the experiment closely matched its theoretical uncollisional probability:14 e-r/λ, where r is the distance between the laser beam and the orifice and λ is the mean free path. Those ions not experiencing a collision have a higher probability of intact extraction through the sampling orifice, while ions that experience (11) Chapman, B. C. Glow Discharge Processes; Wiley-Interscience: New York, 1980. (12) Duckworth, D. C.; Marcus, R. K. Anal. Chem. 1989, 61, 1879. (13) Harrison, W. W. J. Anal. At. Spectrom. 1988, 3, 867. (14) Hess, K. R.; Harrison, W. W. Anal. Chem. 1986, 58, 1696.
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Figure 1. Instrument layout and the potential at each electrode.
collisions before reaching the orifice are less likely to contribute to the ion signal intensity. However, it has been difficult to determine unambiguously where the sampled ions are actually formed in the glow discharge. In this paper, by using a µs-GD as an ion source for a quadrupole mass spectrometer, we have measured the diffusion time of sputtered atoms from the cathode surface to the sampling orifice, which is of direct interest to studies in our laboratory that combine the pulsed GD with a time-of-flight mass spectrometer. The ion signal profiles of the gaseous species and the sputtered atoms at different cathode-orifice distances and different pressures are presented, providing new information about diffusion and ionization processes in the glow discharge. EXPERIMENTAL SECTION The quadrupole mass spectrometer (Extranuclear Laboratories, Inc., Pittsburth, PA) used in our laboratory has been previously described,15 with various modifications added over the years. Figure 1 shows the current configuration. A stainless steel plate with a 1 mm aperture serves as the ion sampler, followed by a metal grid screen to extract the ion beam from the sampling orifice. The ion beam is focused by an Einzel-type lens into the skimmer with a 1 mm aperture. A Bessel box (Model 616-1, Extranuclear Laboratories, Inc.), which acts as an energy filter, neutral atom and photon stop, plays an important role to lower the noise level and improve the resolution of the instrument. Ions of a selected mass pass through the quadrupole mass analyzer (Model 324-9, Extranuclear Laboratories, Inc.) and are detected by an electron multiplier (Model 4816, Galileo Electro-optics Co., Sturbridge, MA). Signals from the detector are amplified by an analog preamplifier, then monitored, and stored in a fast-speed digital oscilloscope (TDS 620A, Tektronix, Portland, OR). The oscilloscope is triggered by the GD pulse. Signals of a selected mass range are averaged 100 times before being transferred to a PC 386DX computer. The typical potentials applied at each electrode are also shown in Figure 1 and are used to calculate the flight time of the ion packages in the mass spectrometer. Simion (Version 5.0, Idaho National Engineering Laboratory), a software compiled for the electrostatic lens analysis and ion trajectory, was used to simulate the electric field for calculating the flight time of ions. The glow discharge source is constructed from a six-way cross with 2.75 in. flanges. A direct insertion probe is used to introduce (15) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 55, 1523.
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the sample.16 The disk sample is mounted with a double-sided carbon tape (STR tape, E. F. Fullam Inc., Latham, NY) onto the copper cylindrical-shaped holder. A machinable ceramic (Macor, Coning Works, New York, NY) sleeve is placed around the holder to restrict the discharge only to the sample disk upper surface. The anode-cathode distance can be adjusted by a series of aluminum stops on the direct insertion probe. Ultrapurity-grade argon (99.995%, Liquid Air Corp., San Francisco, CA) was used in the experiment, with an operating argon pressure range from 0.7 to 2.6 Torr. The disk sample was prepared from metal powders, Al (99.8%, -325 mesh, Johnson Matthey Inc. Seabrook, NH), Ti (99.9%, 100 mesh, Aldrich Chemical Co., Wilwaukee, WI), Zn (99.9%, 100 mesh, Johnson Matthey Inc.), and Ag (99.9999%, 100 mesh, Johnson, Matthey Inc.). These four powders of the same weight are mixed evenly in a shaker (Spex, 3110B, Edison, NJ) for 5 min and then compacted in an in-house-built stainless steel die. The final disk has a diameter of 5 mm and thickness of 1 mm. Before the experiment, the compacted disk sample was placed in a hot vacuum oven (Napco, 5831, Tualatin, OR) for 1 h to reduce entrapped water vapor. A high-voltage pulse power supply (Model 350, Velonex, Santa Clara, CA) is used throughout the experiment. It has a maximum output peak power of 26 kW. The pulse has a rise time of 30 ns and fall time of 50 ns for a 200 Ω load. The rise time for the pulsed GD is 100 ns, but the fall time is 20-50 µs due to the capacitive nature of the GD. The fall time has little effect on our diffusion experiment, as shown later. The same source was modified for the cathodic sampling glow discharge. As shown in Figure 2, the hollow cathode (iron) was mounted at the sampling plate with a 1 mm orifice at the bottom as the ion exit. A positive potential is applied at the anode, while the cathode was kept at ground potential. In order to prevent arcing and to keep the glow discharge inside the hollow cathode, a glass tube was used in such a way that it enclosed both the anode and the hollow cathode but left a certain gap to permit argon flow into the discharge chamber. RESULTS AND DISCUSSION Diffusion Model. The glow discharge produces atoms and ions by means of sputtering and collisional processes, as has been described in considerable detail.11,17 When the positive discharge gas ions are accelerated to the cathode, their kinetic energies are (16) Ohorodnik, S. K.; Harrison, W. W. Anal. Chem. 1993, 65, 2542. (17) Harrison, W. W.; Hess, K. R.; Marcus, R. K.; King, F. L. Anal. Chem. 1986, 58, 341A.
Figure 2. Schematic of reversed hollow cathode ion source.
transferred to the lattice atoms and cause an ejection of cathode material, primarily neutral atoms, into the GD. The GD plasma, a weakly ionized gas, may be considered as a mixture of three plasma species (atoms, ions, electrons). When these concentrations are not uniform, the pressure gradient causes more particles to pass in one direction than in the opposite, resulting in diffusion. Although the glow discharge involves sputtering, excitation, and ionization processes, it still can be described reasonably well by the equilibrium gas laws, because the majority of particles are in their neutral ground state.11,18 The transport of the sputtered material is believed to be governed by a diffusion process.12,19 The variation in particle density is given by the equation20,21
∂n/∂t ) D∇2n
(1)
where D is the diffusion coefficient and n is the atom density. For a three-dimensional spherical model, if, at time t ) 0, there are N particles originating from the cathode, the density of such particles at a distance r from the origin at time t is given by the relation20
n(r,t) )
N exp(-r2/4Dt) (4πDt)3/2
(2)
To find a solution for the time of the maximum atom density, the partial derivative with respect to time t is taken and must satisfy the following condition:
∂n(r,t) ) 0|t)tmax ∂t
(3)
tmax ) r2/6D
(4)
Equation 4 is suitable for a three-dimensional spherical model. Equations 5 and 6 are used for a two-dimensional cylindrical model and a one-dimensional infinite plane model, respectively: (18) Gerhard, W.; Oechsner, H. Z. Phys. B 1975, 22, 41. (19) Van Dijk, C.; Smith, B. W.; Winefordner, J. D. Spectrochim. Acta 1982, 37B, 759. (20) Howatson, A. M. An Introduction to Gas Discharge, 2nd ed.; Pergamon Press Ltd.: Oxford, England, 1976. (21) Papoular, R. Electrical Phenomena in Gases; Elsevier Publishing Co. Inc.: New York, 1965.
tmax ) r2/4D
(5)
tmax ) r2/2D
(6)
When the diffusing gas is bounded by a container, the solution of eq 1 becomes a boundary value problem, and the spatial density distribution is characterized by the geometry of the container. More detailed solutions for different geometry chambers are given by Hasted.22 Some studies of GD diffusion processes have been reported. Ferreira and Human proposed a two-dimensional diffusion model for a Grimm-type glow discharge.23 Van Straaten and Gijbels simulated the one- and two-dimensional diffusion model for the VG 9000 GDMS source.24,25 Their work mainly focused on the study of spatial density distribution in the source chamber for a dc glow discharge. The chambers were relatively small, wherein the boundary had to be taken into consideration. The maximum density in their simulation and measurement was found to be ∼1 mm in front of the cathode, which differs from the expectations derived from eq 2, in which the maximum density should be at r ) 0. The explanation Van Straaten et al. suggested is that the cathode also acts as a sink in the diffusion process after the sputtered atoms are thermalized. Other authors found that the sputtered atom density decreases continuously from the cathode surface, which correlates with the density distribution from eq 2.15 It is difficult to achieve a definitive study of the diffusion of sputtered atoms in a continuous glow discharge source. Since the GD source chamber in our experiment is rather large, the boundary condition plays only a small role in the diffusion process and is neglected in our work. In addition, the glow discharge used here is in a microsecond pulsed mode, so that the glow discharge terminates long before the sputtered atoms collide with the chamber wall, which further minimizes the boundary condition. Different models describe different diffusion processes in different source geometries. Choice of a model for a specific source condition depends on the way that sampling orifice “looks at” the disk sample. If the sample and orifice are very close, the one-dimensional plane model is most suitable, since little lateral diffusion happens before sputtered atoms and ions reach the orifice. On the other hand, if the distance between sample and orifice is very large, a three-dimensional sphere mode is a better fit, since sputtered particles experience the three-dimensional diffusion before they reach the orifice. At intermediate distances, a two-dimensional model is suitable. In our case, some of the experiments, such as the variation of cathode-orifice distance, required different models to describe different conditions. Flight Time. The pulsed discharge permits a decoupling of the two basic categories of discharge species, based on their origin. When the pulse initiates, gaseous discharge components (e.g., argon and traces of water vapor, oxygen, and nitrogen) are subjected to immediate ionization throughout the discharge chamber and a representative sampling of these ions will be registered at the detector after a fixed transit time through the MS optics. However, ions of the sputtered cathode material appear only after some significant delay, comprised of sputter (22) (23) (24) (25)
Hasted, J. B. Physics of Atomic Collision; Butterworths, London, 1964. Ferraira, N. P.; Human, H. G. C. Spectrochim. Acta 1981, 36B, 215. Van Straaten, M.; Vertes, A.; Gijbels, R. Spectrochim. Acta 1991, 46B, 281. Van Straaten, Gijbels, R. Anal. Chem. 1992, 64, 1855.
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a
b
Figure 3. Signal profile of (a) O and (b) Al. The pulses applied have a frequency of 100 Hz, a width of 7 µs, and a magnitude of 2.4 kV. The argon pressure is 1 Torr.
release from the cathode surface and diffusion across the cathode-sampler distance for subsequent extraction and MS registration. By increasing the cathode-sampler distance, measurements should show a longer diffusion time for ions of sputtered atoms, while the times for background gases should be influenced much less by this change in configuration. Therefore, any diffusion theory would need to satisfy the actual experimental measurements of ion signals as a function of origin. Shown in Figure 3 are ion signal profiles of representative sputtered atoms and gas species. The argon pressure in the source chamber is 1 Torr, and the mass analyzer is fixed at each selected mass during the diffusion measurement. The pulse frequency is 100 Hz, with a magnitude of 2.4 kV and a width of 7 µs. Also indicated in the figures are the cathode-sampler distance when the signal profiles were recorded. These figures thus provide information about the flight time of ions in the mass spectrometer, the diffusion process, and density distribution of the sputtered atoms in the source chamber. To represent the intrinsic gaseous species in the discharge, data were taken for O+, Ar2+, and Ar+. Figure 3a shows for O+ that, following initiation of the 7 µs pulsed discharge, the ion signal shows a fairly sharp initiation after 100 µs, peaks at ∼200 µs, and then exhibits a long tail out to 1000 µs. The time between the initial appearance and the maximum signal remains almost the same at different distances, limited to differences in plasma shape and length. The slight extension of the ion signal with cathode4960
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Figure 4. Calculated and measured flight time of different elements in the quadrupole mass spectrometer: b, calculated time; 9, measured time.
sampler distance arises from ions that are formed nearer the cathode and still make their way to the orifice for extraction. Since there exists an energy distribution and spatial distribution for both the gas species and sputtered atoms and ions, all the signals recorded have a time spread of several hundred microseconds. Figure 3b shows the quite different effect presented for ions of the sputtered species. Data were taken for Al+, Ti+, Zn+, and Ag+, all of which followed the basic patterns shown in Figure 3b for Al+. Here the initiation times are similar to the gaseous species, indicating that some atoms are sputtered and ionized and traverse the discharge plasma without significant collisional resistance. A small number of the ions are formed in the sputtering process with a high kinetic energy and may experience no collisions on their way to the orifice. According to their thermal energy (5-10 eV), only several microseconds are required to reach the orifice, such that the initiation time (60-90 µs) indicates primarily the ion flight times in the mass spectrometer. The maximum ion signals are related to cathode-sampler distance, since most atoms/ions would reach the sampler at longer diffusion times with increasing distance. Also, at the increased cathode distance, the ion signals are reduced in amplitude and spread over a larger time scale due to the additional diffusional distance. Flight times of selected gas species ion and sample ions were measured and compared to calculated values for each as obtained from Simion calculations based on the various lens potentials. Figure 4 shows the results of this comparison, indicating general agreement, centering around a straight line according to the expression
t ) 11.7xM + 0.2
(7)
where t is in µs and M is in amu. Diffusion Calculations. In evaluating and comparing the three diffusion models previously described, it was necessary to calculate the mean free paths and diffusion coefficients of various species under the conditions used in the pulsed glow discharge. Listed in Table 1 are some relevant characteristics of selected sputtered atoms and gas species atoms. The mean free path is calculated from (20)
Table 1. Ionization Potential, Atom Diameter, Mean Free Path, and Diffusion Coefficient of Sputtered Atoms and Some Gas Species Atomsa element
ioniz potentl (eV)
atom diam (Å)
mean free path (mm)
O Ar2+ Ar Al Ti Zn Ag
13.6 27.6 15.76 5.98 6.82 9.39 7.57
0.66 0.9b 0.9b 1.43 1.46 1.33 1.44
1.24 0.77 0.64 0.43 0.45 0.34
diff coeff (cm2/s)
345 324 307 257
a One Torr argon, 399 K. b Derived from argon covalent radius van der Waals radius.
λ1 )
πnrd21r(1
1 + m1/mr)1/2
(8)
where nr is the density of argon, d1r ) 1/2(d1 + dr), d1 is the diameter of a sputtered atom, dr is the diameter of an argon atom, and mr is the mass of argon. The diffusion coefficient is calculated by26
D)
3(2πk3T3/µ)1/2 16Pπσ2
Figure 5. Calculated and measured maximum signal of Al. Same condition as Figure 3, in which line 1 is the one-dimensional infinite plane model, line 2 is the two-dimensional cylindrical model, and line 3 is the three-dimensional sphere model.
(9)
where k is Boltzmann constant, T is the thermal temperature, µ is the reduced mass of the collision partners, P is the pressure, and σ is the collision cross section. The bulk thermal temperature in the pulsed glow discharge is not easily measured during the very short on-time. One may assume, however, that the temperature is only a slightly above the room temperature (∼295 K), due to the fact that the duty cycle of the pulsed GD is 0.07% and the average GD power in this experiment is only 0.05 W. The temperature of a 4 W dc-GD was found to be only 350 K in the negative glow region,27 so we have used here an estimation of T ) 300 K in eq 9. Diffusion, Ionization, and Sampling in GD Source. Presented in Figure 5 are the measured ion signal maximum times at four different cathode-sampler distances for Al+. Calculations derived from the three diffusion models are plotted on the same figure for comparison. We have made similar measurements for three other metal ions. At very close distances, the infinite plane model might be expected to apply, and the numbers show a reasonable approximation to that model at 3 and 5 mm distances. At greater cathode-sampler distances, the diffusion processes might be expected to more closely resemble a cylindrical and then a spherical model. As shown in Figure 6, and also observed for the other metal ions, the measured times tend to shift away from the plane model and approach the cylindrical model. When the cathode is far from the orifice, it may be thought of as a point source relative to the sampler orifice, which would lead to a spherical model being best suited. All we can really say within the distance constraints of the experiment is that the values are shifting toward other models at longer distances. Comparison of Ion and Atom Profiles. The ion profile plots shown in Figure 3 indicate a well-defined and discrete package (26) Maitland, G. C.; Rigby, M.; Smith, E. B.; Wakeman, W. A. Intermolecular Force; Clarendon Press: Oxford, U.K., 1981; Chapter 5. (27) Ohorodnik, S. K.; Harrison, W. W. J. Anal. At. Spectrom. 1994, 9, 991.
Figure 6. One-dimensional mode of atom density vs distance, D ) 300 cm2/s (see Table 1), and T ) 300 K. N is the atoms sputtered out per pulse, ∼1 × 1013.
of ions being produced at the sampler. This is somewhat misleading with respect to atom distribution as indicated previously by atomic absorption measurements10 and now by diffusion calculations. Even though the GD is pulsed, during which time the cathode generates sample atoms for a fixed time interval, those atoms are not transported as distinctive atom packets. Instead, they distribute throughout the source chamber according to diffusion processes. If we plot the calculated atom density vs distance following GD termination, as shown in Figure 6, even after 1 ms the most concentrated atom region is found close to the cathode. A 10 µs, 100 Hz pulsed discharge, is fired at 10 ms intervals. Atoms released in the first 10 µs of that 10 ms period will have dissipated to virtually zero before the next pulse occurs. At faster repetition rates, some residual atoms from the previous pulse will still be present in the argon when the next pulse fires, which would affect the temporal resolution of sputtered atoms and discharge gas species. Calculated atom densities at several cathode-sampler distances as a function of time are shown in Figure 7. At 5 mm, our standard distance, the plot shows that the atom density rises to a maximum at the sampler after 300-400 µs, in keeping with our ion profiles shown in Figure 3. However, the atom density is Analytical Chemistry, Vol. 69, No. 24, December 15, 1997
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Figure 7. One-dimensional mode of atom density vs time. Same condition as Figure 6.
sustained with little decrease for a much longer period, which does not fit the ion profile. The net ion population is a result of both the atom density and the number of ionizing agents present. Electrons diffuse away in tens of nanoseconds after the plasma is extinguished.28 Metastable argon atoms have a long lifetime of up to milliseconds and play an important role for the ionization of sputtered atoms in the pulse-off region.14,20,29,30 So at hundreds of microseconds after discharge termination, metastable argon atoms are the principal anticipated ionizing species. The decrease in sputtered atom ion signal beyond 400 µs shown in Figure 3 likely results from a drop in the metastable atom population as their lifetimes become exceeded. One obvious feature of Figure 3b is that the position of the maximum ion signal of sputtered atoms changes with cathodesampler distance, while gas species signals exhibit little change in their maximum signal locations. Atoms on the cathode surface are first sputtered by the fast argon ions and atoms and spend several hundred microseconds in the diffusion process before their densities reach a maximum in the vicinity of the orifice. Those sputtered atoms, ionized by Penning collisions, will be successfully sampled into the mass spectrometer. Thus, the shift of the maximum signal relates to the sputtered atom diffusion and redistribution at different cathode-orifice distances. Gas species atoms are ionized during the pulse-on region. They have little chance to be ionized after discharge termination because of the depletion of electrons and the high ionization potential of these gas species (Table 1). Ions formed far away from the orifice routinely collide with other particles at 1 Torr pressure due to their small mean free path. They usually experience more than 10 collisions before they reach the orifice, so the signal intensities of gas species represent their local ion densities at the different times and the different cathode-orifice distances. Also observed is a large decrease in sputtered ion signal intensities with the increase in distance, while gas species signal intensities show little change. The signal intensities of gas species are almost the same throughout our measured distance range, because gas species ions have a small gradient distribution in the negative glow at low pressure.31 The intensities of the sputtered (28) Johnson, E. O.; Malter, L. Phys. Rev. 1950, 80, 58. (29) Hess, R. K.; Harrison, W. W. Anal. Chem. 1988, 60, 691. (30) Klingler, J. A.; Barshick, C. M.; Harrison, W. W. Anal. Chem. 1991, 63, 2571.
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samples decrease with the increase of the distance mainly because the sputtered atom number density decreases with increase of the distance from the cathode, which is illustrated by eq 2. The Al ion signal as a function of cathode-sampler distance was chosen to compare with three different diffusion models. Figures 3 and 7 considered together show that the relative signal intensity of Al ions at different distances follows generally the theoretical atom distribution, most closely matching the threedimensional model. We obtained similar intensity plots for ions of Ti, Zn, and Ag that showed configurations closely resembling that of Figures 3 and 7, although these data are not included here for article length considerations. Since the relative ion signal closely approximates the relative sputtered atoms density, this suggests that most of the ions sampled by the mass spectrometer are those ionized in the vicinity of the orifice. For the ions formed 1 mm before the orifice, only 14% can reach the orifice without collision at 1 Torr (according to e-r/λ); only 2% for the ions formed 2 mm before the orifice, and so forth. The chance of a sputtered atom to be ionized is proportional to the metastable argon density. Due to the collapsing electrical field, there is a rise of metastable argon population in the afterglow,32 providing more opportunity for the sputtered atoms to be ionized. But after a few hundred microseconds, the metastable argon population begins to decrease, and by ∼1 ms, most metastable argon atoms will have decayed to their ground state. Let us consider an alternative assumption that ions sampled by the mass spectrometer are not formed primarily in the vicinity of the orifice, but are formed in the transition zone between the cathode fall and negative glow (∼1 mm in front of the cathode surface at 1 Torr). If we assume the following two conditions are true (1) both the gas species and sputtered atoms are ionized in this transition zone and (2) they have sufficiently small cross sections to make their lifetime long enough to reach the orifice through diffusion, it would then be reasonable to obtain the signal profiles like Figure 3b for sputtered atoms when the distance varies. It should also be the same for gas species signals if gas species ions are formed at the same location (transition zone) as sputtered particle ions. This assumption is not true, as indicated by the clear differences shown in Figures 3 and 7 between sputtered and gaseous species. Effect of Pressure. Diffusion processes in a glow discharge would be affected by the operating pressure. While an optimum pressure is selected for analytical conditions, we can observe the effect here of varying the discharge pressure, which would be expected to be more significant for the sputtered species than for the discharge gas itself. The results are shown in Figure 8. At the four selected pressures, the ion profiles in Figure 8a for Ar+ reveal changes in intensity but no variation in ion signal arrival time at the detector. However, for ions derived from sputtered atoms, using Al+ once again as shown in Figure 8b, the higher the pressure, the longer the net time required for the ions to make their way through the ion source. Another factor appears to enter in, however, as the pressure increases. Comparing ion intensity to the diffusion models, we find increasing deviation from the model at higher discharge pressures, indicating perhaps that the gas flow contributes to atom transport, decreasing the measured (31) Bogearts, A.; Gijbels, R. Phys. Rev. A 1995, 5, 3743. (32) Strauss, J. A.; Ferreira, N. P.; Human, H. G. C. Spectrochim. Acta 1982, 37B, 947.
a
b
Figure 8. Signal profiles of (a) Ar and (b) Al at different pressures. Same condition as Figure 3; cathode-orifice distance 5 mm.
diffusion times. At a practical operating pressure in the GD source, typically at 1 Torr, the flow rate does not interfere significantly with diffusion.15 Cathodic Extraction. The hollow cathode GD ion source has drawn much less attention than other types of GD source, because it requires machining the sample into a cylindrical, hollow geometry. This source does, however, exhibit very interesting plasma reactions and processes that have not been sufficiently explored. One report recently focuses on the suppression of cluster ion interferences in GDMS by sampling high-energy ions from this source using a double-focusing mass spectrometer.33 We have elected to use the hollow cathode GD in this study in a (33) Deng, R.; Williams, P. Anal. Chem. 1994, 66, 1890. (34) Marcus, R. K.; Harrison, W. W. Spectrochim. Acta 1985, 40B, 933. (35) Hang, W.; Baker, C.; Smith, B. W.; Winefordner, J. D.; Harrison, W. W. J. Anal. At. Spectrosc. 1977, 12, 143.
reversed geometry mode. That is, instead of the normally anode sampling configuration, a cathodic extraction system was prepared, as shown in Figure 2. In this way, ions of sputtered materials are prepared in large quantities at or near the sampler, which in this case is the orifice in the bottom of the cathode. Our purpose is to provide more evidence of the diffusion process in a GD. After the GD operated for several hours, we examined the hollow cathode and found that little sputtering was observed in the main body of the cathode. The most intense sputter removal occurred at the edge of the orifice facing the source. It is not similar to a hollow cathode plume, where most erosion is at the exit side of the orifice,34 indicating that little or no external plume was generated, which if present could possibly complicate ionization locations and processes. Since the cathode also serves as the ion sampler through its exit orifice, a change in anode-cathode distance does not affect ion signal timing profiles, though the ion intensities will be modified as the plasma volume changes slightly with location of the anode. The responses for gaseous ions and sputtered species ions are quite similar. Also, in a study of pressure effects, we found virtually no shift for sputtered atom ions, although argon ions showed ion signals extending to longer diffusion times as the pressure was lowered, reflecting the expanded GD plasma. CONCLUSION These experiments offer some insight into the diffusion, ionization, and sampling process in a pulsed GD source, exploring the different behavior of intrinsic gas species and sputtered particles. The data show significant differences between these species based on their origin and subsequent ionization mechanisms. This information has been already been shown to be of particular value in time-of-flight mass studies in our laboratory.35 The comparison of experimental data with several diffusion models indicates that multiple effects must be considered, in that the atomic populations do not reflect the measured ion signals. Metastable argon populations must also play a significant role in the net ion response. We are currently studying the temporal separations and ion formation processes by time-of-flight methods as a valuable and complementary set of experiments to these quadrupole-based measurements. ACKNOWLEDGMENT This work has been supported by grants from the U.S. Department of Energy, Division of Chemical Sciences, for which we are most grateful. Received for review September 18, 1997. October 28, 1997.X
Accepted
AC9710386 X
Abstract published in Advance ACS Abstracts, December 1, 1997.
Analytical Chemistry, Vol. 69, No. 24, December 15, 1997
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