W. W. Harrison K. R. ness R. K. Marcus F. L. King Department of Chemistry University of Virginia Charlottesville,Va. 22903
MASS SPECTROMETRY Glow discharge mass spectrometry (MS) has gained attention recently, but it is hardly new. In fact, it is perhaps the oldest form of MS.Electric discharges were commonly used as “natural” ion sources in the 1920s and 1930s for the early mass spectrographs by Aston, Thomson, Bainhridge, and other pioneers in the field (I). The demise of these sources was hastened b) difficulties in controlling the highvoltage discharge used and by the development of the simple, reliable electron impact ion source, which dominated MS for the next 30 years. Ionization by an electron beam sufficed because MS was overwhelmingly devoted to the analysis of organic materials, such as petroleum products, that exhibited relatively high vapor pressures. Creating a suitable vapor pressure from more intractable materials (e.g., stainless steel) required a much different source and eventually led to revisiting the glow discharge. The elemental analysis of bulk solids by MS-an area that has been loosely termed solids MS by its practitioners-requires a source with sufficient energy to atomize and ionize the sample. Drawing on the extensive use of arcs and sparks for such samples in atomic emission spectroscopy, similar sources were adapted as ionization sources, with the pulsed rf spark finding greatest application (2). This 20-60-kV source demonstrates high sensitivity, broad applicability, and relatively few interferences, hut its ion yield is erratic and has a wide energy spread. In recent years, the glow dis0003-2700/86/03583341AS01.50/0 @ 1986 American Chemical Sociew
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Flgure 1. Schematic representation of a glow discharge and associated components charge has been developed as a more stable, low-energy alternative ion source which, coupled with a quadrupole mass filter, serves as a simple and inexpensive solids mass spectrometer (3).
he glow discharge There are many types of glow discharges ( 4 ) , including the common neon light, In general, the glow discharge is a simple two-electrode device, filled with a rare gas to about 0.1-10.0 torr. A few hundred volts applied across the electrodes causes breakdown of the gas and formation of the ions, electrons, and other species that make the glow discharge useful in analytical chemistry. Figure 1is a sim-
plified sketch that shows the basic components and discharge regions. The sample to he analyzed serves as the cathode; the anode material is not particularly critical. Only two plasma zones need concern us here (5):the cathode dark space and the negative glow. The cathode dark space is a thin layer that shows relatively little light emission from electron atom collisions because of the high energy of the accelerated electrons. By contrast, the negative glow is a region of bright emission arising from excitation of atoms by lower energy electrons that have been slowed by inelastic collisions. The negative-glow region extends diffusely away from the cathode and, in the
ANALYTICAL CHEMISTRY, VOL. 58. NO. 2, FEQRUARY 1986
341 A
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+)
-
- m m
Cathode SamDle
Figure 2. Sputter atomization and ionization processes occurring in a glow dis-
charge M = cathode metal atom;
= potential gradient 01 cathode fall
case of small analytical sources, tends to fill much of the remaining discharge volume. Nearly all of the discharge voltage is dropped across the cathode dark space, leaving the negative glow as an essentially field-free region. Positive gas ions (e.g., AI+) are accelerated across the field established in the cathode dark space and collide with the cathode surface, releasing a variety of secondary particles required to maintain the discharge. Glow discharge sputtering. The sample is atomized into the discharge by a process called sputtering (6). As illustrated in Figure 2, positive ions and fast neutrals strike the cathode sample, penetrating a few atomic distances before losing their momentum through a sort of three-dimensionalbilliards cascade of atomic lattice col-
lisions. Some atoms at the surface receive sufficient energy through these collisions to overcome their binding energy and are normally ejected as neutral atoms, although a small percent may come off as ions. Positive ions are returned to the surface by the cathode dark space field, but the neutral atoms diffuse into the negativeglow region. Glow discharge analytical methods are based on the utilization of this large sputtered neutral population. The glow discharge is a collisionrich environment. At 1torr, atomic mean free paths are in the 0.050.10-mm range, meaning that both incoming sputter agents and outgoing sputtered species are subject to frequent collisions. For example, homharding argon ions originating at the
3 4 2 ~ ANALYTICAL CHEMISTRY, VOL. 58, NO. 2. FEBRUARY 1986
cathode dark space-negative glow interface (Figure 2) may be subject to one or more collisions before striking the cathode. Thus for a 500-V discharge, relatively few 500-eV ions reach the cathode; the average energy might be closer to 100-200 eV (7),but this is sufficient to sputter-remove copious quantities of sample. Similar collisional effects are experienced by sputtered particles that are deflected or even knocked back onto the sample surface for subsequent resputtering. Sputter yield depends on the mass and energy of the sputter ion (one or two ejected atoms per incoming particle are not unusual), but with glow discharges one must consider the effective sputter yield, wherein the net weight loss per unit of current incoroorates redepusition effects. Glow discharge ionization. The glow discharge not only atomizes the solid sample but also provides the means by which these atoms are ionized. Sputtered atoms diffuse into the negative glow, a rich reaction zone that contains energetic electrons, ions, and metastable atoms. Our interest here is directed toward the ionization of the sputtered atoms rather than to the bulk ionization processes by which the discharge sustains itself. The two major ionization steps for the analytical sample atoms appear to he the electron impact and Penning (metastable impact) modes ( 4 ) : M M
-
+ e-
+ Ar*
M+
M'
+ 2e-
+ Ar + e-
(1) (2)
where Ar' renresents a metastable argon atom. Electron densities of l O I 4 ~ 3 1 are 1 ~ ~ reported in glow discharges, with electron energies ranging from fast to 'thermal. Metastable rare-gas atoms, because of their long lifetimes, may play a disproportionately large role in the ionization of sputtered neutrals. Argon, a common glow discharge gas, has metastable states at 11.55 and 11.72 eV, values exceeding nearly all of the elemental first-ionization potentials. Although metastable populations are generally several orders of magnitude lower than electron populations, Penning ionization has been identified as the dominant ionization process in some glow discharge configurations. The net result of these ionization steps is shown in Figure 3, a typical low-gain mass spectrum of a steel cathode in an argon glow discharge. One sees primarily peaks arising from isotopes of argon, argon hydride, and the major elemental constituents of steel. At higher sensitivities, shown in the insets, minor and trace constituents appear. Background contrihutions are normally also present from
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Figure 3. Typical glow discharge mass spectrum Shown we low-galn scan and expanded sensitivity regions of NBS 418A steel sampk. Mo lo present at 2000 ppm, Sn at 110 ppm. Fuloed argon discharge. 0.7 Iwr. 3 mA. 800 V
in
- J+++
residual nitrogen, oxygen, and water vapor. Glow discharge mass spectra are composed primarily of singly charged atomic ions, although certain molecular ions (MAr+, Mz+, and MO+) do appear at higher detection sensitivities. For example, Fez+ in Figure 3 is present a t about the equivalent of 5 ppm in the discharge; such dimers can be an order of magnitude higher in other matrices (see Figure 6). The ratio of peak heights among the sample elements corresponds to their bulk concentration. Glow discharge sputter yields and ionization efficiencies are rather uniform, producing generally similar relative sensitivity factors (within a factor of 3X) used in quantitative analysis. For example, in Figure 3 the ion intensity of W n + at 15 ppm is roughly an order of magnitude smaller than that of IIOMo+ a t 190 ppm. A comparison of the40Arf and "Fe+ peak heights in Figure 3 does not necessarily give a true picture of the respective atom and ion populations in the discharge. The sputtered elements are enhanced by the previously described Penning ionization process. Also, the ion energy window of the mass spectrometer can be set to favor ions of the sputtered elements, which are formed a t a slightly different energy than the discharge gas ions.
-l*
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+-
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Anaiytical sample Flgure 4. Simpllfled representation of flve common source conflguratlonsfor glow dlscharge MS (see text for detalls) 844A
* ANALYTICAL
-~=me-spac(rometry In this review, it is assumed that the reader is familiar with the general principles of MS (8)by which ions are extracted from an ion source for subsequent separation according to their mass-to-charge ratio, mle, allowing qualitative and quantitative analysis. It is the unique properties of the glow discharge as an ionization source for solid samples that will be emphasized here.
CHEMISTRY, VOL. 58, NO. 2. FEBRUARY 1988
Glow discharge sampling. As noted previously, the glow discharge is adaptable to many discharge types and ion source configurations. Sometimes the sample type itself dictates a certain cathode geometry; other source models arise from more fundamental considerations, Most of the glow discharge sources to date can be assigned to one of the five simplified models shown in Figure 4. In each case, ions are extracted from the negative glow of the plasma, which contains ions representative of the sample composition. A sampling orifice is located near the negative glow or may take the form of a sampling cone, which can advantageously probe specific regions of the discharge. The most versatile of the source configurations is prohahly the pin source, which uses a spring clip or pinvise-type mount adaptable to samples ranging from thin wires to rods. Placed near the ion exit orifice, a small sample cathode (e.g., 2-mm X 10-mm pin) a t a low discharge current (1-5 mA) produces sufficient ion signal for sensitive elemental analysis. For sample materials more easily cut into discs, the disc ion source may be preferable. Compared with the pin source, a somewhat different shape of negative glow is observed, but at equivalent current densities comparahle mass spectra are obtained from the two sources. The hollow-cathode source is another form of glow discharge (9).It can be thought of as two plane cathodes brought close enough together so that their negative glows overlap and coalesce into a single common negative glow. This phenomenon enhances cathodic sputtering and plasma ionization. Hollow-cathode ion sources yield excellent sensitivity, but compared with pin or disc preparation, the ma-
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chining of a hollow cathode from a bulk metal is sometimes more cumhersome. This configuration is particularly useful, however, for samples in the form of a metal foil or fragments that can be inserted into the cathode cavity for glow discharge MS analysis. The bollow-cathode plume (HCP) (10) makes much different use of the hollow-cathode effect. The hollowcathode base is formed by a sample disc with a 1-2-mm orifice. Selection of suitable gas pressures and discharge currents causes a highly energetic plume to be ejected out of the orifice. A constricted hollow-cathode discharge, localized in the narrow orifice, creates intense sputter action on the surface of the disc and direct transport of the atoms into the plume for excitation and ionization. The Grimm glow discharge (11) is known as an obstructed discharge. By bringing the tubular anode within the cathode dark space, the discharge is obstructed or prevented from striking between the adjacent electrodes, restricting the discharge. This source can analyze large samples without cutting and shaping of the material because the anode defines the sample area to be sputtered. Glow discharge ion sources have been operated in the de, pulsed-dc, and rf modes. The simplest arrangement uses a low-cost dc supply; discharge powers are often as low as 5-10 W. Plasma species are formed with a steady-state stability of approximately 1-2%. Advantages may be gained by pulsing the discharge, albeit a t certain additional expense. Some commercial dc power supplies are essentially power operational amplifiers that can be triggered by an external
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
signal source. A pulse generator permits the power supply to he driven a t selectsble repetition rates and duty cycles. For a given average current, pulsed operation uses a higher voltage, bigber current discharge during its ON cycle than is the case in dc. More energetic argon ions are produced, causing enhanced sputter yield and a higher sample atom density in the plasma. Pulsed discharges also offer the possibility of time-resolved mass spectra by gating the detector a t the desired interval along the pulse envelope. For example, more favorable ion signals from the sputtered species are obtained in the immediate postpulse interval. Glow discharges are also readily powered by rf generators. These power supplies are more expensive than de units and provide more complex coupling considerations, but they also offer the ability to run nonconducting samples and to operate at lower discharge pressures because of the greater electron collisional action in the rf field. In plasma-etching applications (4).rf glow discharges are routinely used. Instrumental considerations. The glow discharge source presents a flowing gas load to the mass spectrometer, requiring differential pumping to interface the 1-torr source with the 10-6-10-'-torr analyzer chamber. Fortunately, gas flow rates can be quite low, typically a few cubic centimeters per minute. A cryopumping stage may also be included to help eliminate residual water vapor, an impurity which, upon dissociation in the discharge, can affect sputter yield by formation of surface oxides. Figure 5 is a general representation
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,fa glow discharge mass spectrometer Jased on a quadrupole analyzer. The ?xpanding gas jet from the source, :ontaining ions and neutral atoms, is .educed in pressure several orders of nagnitude by the pumping stack beow the intermediate chamber. Ion transfer optics (e.g., an einzel lens) may he used to enhance ion throughput. A portion of the source beam, still :ontaining significant numbers of neutrals, passes through an orifice into the lower pressure analyzer chamber. where an energy pass filter selects an energy window for transmission to the quadrupole for isotopic resolution. A central stop in the energy filter prevents most of the neutral atoms and glow discharge photons from reaching the quadrupole and detector, thus reducing background noise. As a further aid to noise reduction, the detector is mounted off axis. Most of the reported work in glow discharge MS has been carried out using quadrupoles, although the only commercial instrument (VG Isotopes) on the market is a magnetic-sector instrument. There are well-known advantages and limitations to both approaches. Experimental systems have often arisen less from optimized design than from judicious application of already available apparatus. Quadrupoles make it possible to design simple, inexpensive instruments with low sampling voltages and high ion transmission, particularly a t lower mass ranges. The weakness of the quadrupoles is their low resolution; ions such as 1602+ and 32S+ cannot be disthguished from each other. Double-focusing magnetic instruments can provide such resolution a t considerable additional expense. For complex interference-prone matrices, high resolution may be quite valuable. For most samples, however, quadrupole users are able to take advantage of alternative isotones to eet around isobaric interferences. Other asDects of nlow discharge MS can also influence &strument design. Because the technique deals with elemental analysis, only a limited mass range, normally up to 250 amu, is necessary. The analyst may he interested in both qualitative and quantitative analysis. This means that instruments should be capable of fast, qualitative mass scans to determine sample makeup as well as peak-selective integration for more precise quantitative comparisons to analytical standards. Repetitive, accumulated scanning is also valuable for smoothing out source instabilities and improving elemental detection limits.
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349 A
w e 6. Glow discb
nass spectrum of an NBS C1102 brass sample
Expanded OBnSltivities show seven eiemmts at W i r r e w c l i v e concentrations(ppm): Mn (45). Fe (110). Ni (50), A8 (40), Cd (45). Sn (BO),and Sb (50). ArgOn discharge. 0.8 twr. 4 mA, 1100 V
able, a newly emerging technique must address some clearly perceived need to he taken seriously. In the case of glow discharge MS, it is the elemental analysis of solids (12). Studies to date with the technique show the following useful features: direct analysis of metals and alloys * analysis of nonconductors by compacting with a graphite or metal matrix responsiveness to both metallic and nonmetallic elements minimal matrix effects * parts-per-billion detection limits attainable sensitivities generally uniform for most elements * isotopic information is obtained spectra are much simpler than optical emission spectra * stable, low-power discharge inert discharge environment economical operation rapid qualitative scan of periodic tahln 1 . 1
stable ion beam for quantitative analysis Limitations of the method appear to he:
gas load requires differential pumping system some spectral interferences from discharge gas and gashatrix molecules applicability is primarily to solids It follows that glow discharge MS should find a wide range of analytical applications, including hulk metals, compacted materials, and, to a lesser extent, thin films and solution residues. Selective, illustrative examples 350A
will be given for each of these general sample types. Bulk metals. The glow discharge is a surface-active technique: it is the surface atoms (see Figure 2) that are ejected into the plasma for subsequent ionization and analysis. However, relatively high sputter rates (0.1-10 fig permit the discharge to tunnel its way through a significant sample volume, averaging out localized sample inhomogeneities and producing data representative of hulk concentrations. By averaging a series of repetitive mass scans during sputter ablation, sample inhomogeneities are further smoothed. Examination of the individual scans can give an indication of elements that may vary in concentration over the sputtered sample volume. The discharge can also serve as an effective self-cleaning device for the cathode, sputtering away surface contamination before actual analytical data are taken from the underlying true hulk composition. The analysis of metals and alloys has been the most frequent use to date of glow discharge MS. More than 10 years ago, Cohurn (3)and co-workers at IBM showed that glow discharge ion signals are representative of the sample composition. Both de and rf discharges were investigated. Considerable previous work had been done by others in sampling glow discharges mass spectrometrically, but little attention had been paid to the sputtered cathode species. Cohurn’s research interests, which were mainly in plasma diagnostics, did not lead him to pursue the potential for trace element
ANALYTICAL CHEMISTRY, VOL. 58. NO. 2. FEBRUARY 1986
analysis, hut he did demonstrate the analytical possibilities, which others have since developed. Over the past decade, the glow discharge ion source in many different configurations has been investigated in our laboratories a t the University of Virginia (12).After initial work with hollow-cathode sources, we turned to a coaxial electrode system using pin or disc cathodes formed directly from the sample material. This source is highly applicable to elemental analysis of metals and alloys with minimum sample preparation. The material is cut or machined to size (not critical), loaded in the source, and sputter-cleaned. Mass spectra are then obtained. Qualitative analysis requires only a few minutes. Figure 6 shows a portion of a glow discharge mass spectrum taken of an NBS C1102 brass sample. Prominent are the major copper and zinc peaks plus others arising from 4oAr+2and ‘OArlHt. The ratio of “AI+ to “ArlHt is greatly influenced hy the amount of residual water vapor in the source. Expanded sensitivity insets show ion signals for manganese, iron, nickel, arsenic, cadmium, tin, antimony, and a group of matrix dimers. These spectra were taken as broad qualitative scans, so that sensitivity was not maximized, but for reference it can be noted that 12*Sn+is present at about 2.8 ppm. Other studies using NBS 662 steel samples have found sensitivities for 18 elements averaging about 0.5 ppm; reproducibilities showed an average standard deviation of 3-4%. Glow discharge mass spectra are simple and
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Figure 7. Glow discharge mass spectrum showing trace elements in high purity nines”) indium Pb Is estimated to be present a1 1.5 ppm. Bi at 70 ppb, TI at 2.5 ppm
easily interpreted. Comparing the brass spectrum in Figure 6, or more particularly the iron spectrum in Figure 3, to equivalent optical spectra (13) shows the sharp difference in spectral complexity. Cantle and associates a t VG Isotopes, Ltd., using the commercial instrument they developed, have reported parts-per-hillion sensitivities in a series of reports (14).Typical of the demands on solids elemental analysis is the determination of trace impurities in high-purity indium as used in the semiconductor industry. Carrier mobility of indium compounds prepared from the pure indium may be
seriously affected by trace impurities. Figure 7 shows a spectral segment of a mass spectrum taken of “six nines” (nominal 99.9999%) indium. The lead concentration is estimated a t about 1.5 ppm and the bismuth a t 70 ppb. Cantle et al. have also reported thorium and uranium analyses in aluminum matrices to 2-ppb sensitivities. This indicates that with state-of-theart instrumentation, glow discharge MS can match the sensitivity of the more complex spark source MS and that it is a very competitive analytical technique for direct metals analysis. Compacted samples. Although bulk metals and alloys are easily
Figure 8. Analysis of thin metal films on a copper substrate Shown is me la, signal with time lor each of lour elemem as !he glow discharge spunera throwh Uw sample. Film thicknesses: Co = 5500 A Au = 10,200 A Ag = 19,700 A Ta = 5750 A. Adapted from Reference 15
352A
ANALYTICAL CHEMISTRY, VOL. 58. NO. 2, FEBRUARY 1986
formed into an appropriate electrode shape, many other important materials are in forms that require special treatment before analysis. Metal powders or turnings can be compressed in a suitable die to form a disc or pin that is conducting and stable in the glow discharge. Nonconducting materials, such as soils, glasses, and ores, require the addition of a conducting matrix before the electrode is pressed. Sample-to-matrix ratios can be as high as 91, depending on the sample material. Graphite is probably the best matrix material because of its high purity, low background contribution, and its availability in very finely divided form. Metal powders (Ag and Au) are also quite effective matrices, usually giving even greater sensitivity because of their high sputter rate. But their large particle size, compared with graphite, may cause electrode inhomogeneity problems. Also, metal powders tend to produce greater spectral hackground (M+, MO+, MAr+, M2+, etc.) than does graphite. At this point little has been reported concerning analysis of compacted samples by glow discharge MS. Work in our laboratory, however, indicates that such applications are quite feasible and could extend the scope of this technique to many other materials hesides metals. Thin-film analysis. The analysis of thin metal films attracts wide commercial interest, and many surface analysis techniques have been applied to such problems. As yet another surface method, glow discharge MS brings certain advantages and limitations to thin-film analysis. Ideally, one wishes for a technique that erodes evenly through the layers with no method-induced atomic mixing to blur film interfaces. In this regard, the previously described redeposition that occurs in glow discharge sputtering can be a problem. Glow discharges also sputter at rather rapid rates with their mA-range currents, compared with controlled pA-regime ion beam sputtering. Nevertheless, glow discharge MS does have certain attractive features for thin-film analysis (15),including minimal matrix effects, uniform sensitivity, ability to vary etch rate hy changing discharge gas, and little surface damage when using low-energy sputter ions. By operating at low pressures (
