Andrew T. Zander Perkin-Elmer Corporation North American Instrument Division 761 Main Ave. Norwalk, Conn. 06859-0905
~tortlicr L
Atomic emission spectrometry (AES) is used for elemental determinations in a variety of sample types. Solution samples, or solid samples that have been prepared for analysis as solutions, comprise a significant portion of the analytical demand met by AES. A solution sample presents a more homogeneous matrix for the elements being sought. As a result, it is somewhat easier to prepare standan and fewer different standards are re quired. Consequently, a substantial amount of effort has been expended on the development of spectrometric instrumentation that preferentially uses solution sample input. There are many components and subsystems in a modern atomic spectrochemical analysis system. All of their performances have been improved markedly with recent technical advances. “Holographically” produced diffraction gratings and the increased use of high-capability, low-cost microprocessors are key examples. The single most important development in atomic spectrochemical equipment in the past 10 to 15 years, however, has been the development of the inert gas, electrical discharge plasma excitation source. Unlike other electrical discharges that have been optimized principally for solid or powdered samples, the inert gas plasma has been developed specifically for solution sample types. In this regard it is a replacement for the flame excitation source. Besides being a replacement, the inert gas plasma has performance advantages so significant that it has rejuvenated the atomic spectrochemical instrumentation market. The market growth for plasma emission instrumentation far exceeds that of any other type of atomic analysis equipment 0003-2700/86/A358-1139$01.50/0 @ 1986 American Chemical Society
a
a
-_rSources€or
L i
i
and is expected to continue to do so for a number of years. The inert gas electrical discharge plasma, commonly using argon, comes in a few different configurations. In order of market Dreference and associated research activity, the principal versions are the inductively coupled plasma (ICP), the direct current plasma (DCP), and the microwaveinduced plasma (MIP). Other excitation sources are popular for AES, of course. None, however, have received the market acceptance of the ICP and DCP or the level of research activity of the MIP. Or they.~ may be principally associated with analysis of solid^ samples, such as the Grimm disrharge or the hollow-cathode discharge devices. It is evident that. at least for the short term, no other devire will be able M compete surcessfully for the interest garnered by the more popular and more prevalent plasmas. The following is a comparison, sometimes somewhat suhjective, of the ICP. the DCP, and the MIP. How.
-er, it is less a comparison of perforance figures of merit, which can he und in any number of review articles, than an identification of the ways in which the devices are similar and dissimilar. In that regard, this comparison should shed light on the relative “usefulness” of the devices, and their consequent commercial acceptance. On the basis of commercial acceptance, it might be argued that the MIP does not helona in the comparison. It has held andstill does hold, however, a prominent position of interest within the atomic analysis research community, certainly far greater than for the DCP, and is included for that reason. Table I presents a list of device characteristics and operational parameters for the DCP, ICP, and MIP. With this information as a basis, we can compare and contrast the analytically relevant features of these plasmas Operathg parameters Operating frequency. The DCP operates at dc and the inductive plasmas operate at radio or microwave frequencies. Direct current power supplies of comparable output power to ac supplies are smaller, may have fewer components, and are generally less expensive (for equivalently stable output power!. The ICP and MIP do not operate at arbitrary frequencies. The Federal Communications Commission ha+regulated the use of the electromagnetic spectrum to well above 30 GHz !a wavelength of ahout 1000 urn). Certain narrow hands have been set aside for industrial, srientific, and mediral (ISM) uses. l’he ICP typically operates at 27.12 MH7 or -10.68MHz. The
ANALYTICAL CHEMISTRV. VOL. 58. NO. 11. SEPTEMBER 1986 * 1139A
Operating frequency Discharge structure
Discharge type
dc Multiple electrode sei Anodes: Fyrolytlc graphite Caihodes: Thofiated tungsten Cooled by Carrier gas Flowing, mermaily pinched. transferred dc arc plume Argon, 99.995% or better
Carrier gas Gas consumption (total)
Operating pressure
About 10 Llmin Atmospheric (1.013kPa)
Power required
110 V, 60 Hz, single phase,
Aooiied .. Dower Generated plasma volume Power denslty (power input zone only) Viewing zone slze Gas temperature Electron density (Ne) 1
20 A 0.5-0.75kW
cm3 0.93-2.5kW/cm3
0.2-0.3
About 8 mm2 4000-6000 K stratified 10"-10'5
cm-3
MIP typically operates at 2450, MHz, the frequency of microwave ovens; 915 MHz is not as popularly used. Table I1 lists the ISM frequencies and their bandwidths. The 461-MHz hand is not consistently listed on all charts as an ISM frequency. What should be noted is the very narrow width of the allowed band across t h e center frequencies. The impact of this is that equipment designed for operation at these freauencies is comnlex because of the stringent requirements for radio frequency (rf) shielding of the generators. Shielding requirements are much less severe at microwave frequencies. where even a few sheets of aluminum foil are generally sufficient tu minimize extraneou radiation surrounding a plasma device. Shielding at lower rf is very difficult to implement properly. Discharge structure.'I'he me-
27.12 MHz, 40.68 MHz (ISM) RF coil and ceramic torch Coll: protected copper Cooled by water Tach: Quartz or ceramic,
carrier-gas-cooled Flowing, electrodeless discharge plume Argon, 99.995% or bener
0.5-2.5 kW 3-4 cm9 0.13-0.63 kWIcm9
Helium. 99.9995% or bener Argon, 99.995% or better 0.05io 1.5LImin Moderate vacuum io atmospheric (0.133-1.013kPa) 110 V, 60 Hz. single phase. 15 A 0.02-0.5 kW 0.07-0.18 cm3 0.1 1-2.8 kWIcm9
20-60 mm2
0.8-7 mm2
4000-6000 K stratified
Possibly as high as 3000 K io" cm-3
10 io 20 Llmin
Atmospheric (1.013kPa) 220 V, 60 Hz. single phase, 25-30 A
iOl'-iOls
0111-3
chanical configuration that contains or supports the plasmas is quite different for each of the discharges. The DCP (Figure 1)uses a multiple electrode set: two pyrolytic graphite anodes and one thoriated tungsten cathode. It should be noted that the threeelectrode DCP has four electrical
Table II. ISM trequencies and bandwidths
13.56 27.12 40.68 461.04 915 2450 5800 24125 1140A
f0.07 *0.16
f0.05 fO.6
*0.02
*0.05
f0.92 *13 f50 *75 *I25
f0.2 *1.4 f2.0 *1.3
f0.5
915 MHz. 2450 MHz (ISM) Mlcrowave structure and twch Structure: protected copper Cooled by gas or water Torch Quartz or alumina, carrier-gaecooled Flowing. electrodeless dlscharga
Figure 1. Schematic diagram
ANALYTICAL CHEMISTRY, VOL. 58. NO. 11. SEPTEMBER 1986
poles: two anodes and two cathodes. It uses two power supplies. The single cathode connects to the negative pole of both power supplies. Each electrode is efficiently cooled by flowing argon. The electrode set tends to lock the generated plasma volume in position, which provides much of the stability
Figure 3. Schematic diagram of a 2450-MHz TMo~o resonant MiP cavity (the Beenakker cavity) A hoe view is ahown at
rlgure z.
me ibn. a ~~css-sectimisue view at me dght
scnemauc aiagram 01an
argon ICP of the generated signals. The rf ICP uses a radio frequency coil antenna surrounding a ceramic, flow-controllingtorch (Figure 2). Usually, the coil is made of copper, is protected from oxidation and corrosion in some manner, and is water-cooled. The torch, generally a concentric arrangement of quartz tubes, is efficiently cooled by flowing argon. The pneumatic arrangement tends to hold the generated plasma volume in position, but not nearly as well as a set of electrodes does. The trade-off, of course, is that there is no electrode material present in the ICP discharge. This cannot he considered very much of an advantage, though, since the electrode material present in the DCP is carbon. The amount of carbon that can be found in a DCP discharge is so small that it has virtually no influence on the background structure. Carbon is quantitated so poorly hy plasma AES of any sort that it is rarely considered an anal@. The MIP uses some form of short circuit or open circuit for transmission of microwave radiation, or it uses a device to confine the microwave field. 1142A
Figure 4. Schematic diagram of a microwave surface wave launcher
(the “surfatmn”) The resonant cavity, Figure 3, or electromagnetic field launcher, Figure 4, can be made of anything having a sufficiently shallow skin depth a t the operating frequency. Purecopper works well hut is hard to machine. Brass and aluminum work best if they are silvercoated. Gold flashing helps significantly. The microwave structure can he gas- or water-cooled, depending on the applied power. Usually water cooling is needed above 2M300 W. The
ANALYTICAL CHEMISTRY. VOL. 58, NO. 11, SEPTEMBER 1986
MIP is contained in a quartz or ceramic tube, which is cooled by the flowing carrier gas. Concentric tube torches and devices to swirl the carrier gas are used quite successfully. Obviously, the hardware requirements for each of the plasmas are markedly different. I t is not possible to switch between plasma types in the same way that it is possible to switch flame types on the same burner system.
EALABASSMANTWTH 7LdYEARS OF EXPERIENCE.
C
hemBaseT The personal database manager for chemists. Created by a team of chemists who, collectively, represent over 728 years of lab experience. Chemists who realize that a chemist’s place is in the lab. Not hunting through files. That’s why ChemBase works the way you think.Graphically. With structures. Let’s say you need to find a speafc group of related compounds
Normally that might take hours searchingthrough files and notebooks.But ChemBase can store all that data on any of the IBM family of personal computers. And it understands chemistry. Bonds. All 103elements. Isotopes. Valence. Even reaction transformations. In seconds, it rams through your data.Recognizes what vou want. And dimlavs the results. in customized &s &d tables. And ChemBase is just one of ourfamilyof~emistrysoftware
tools. Used by thousands of chemists worldwide. Making us the leadmg supplier of chemistry software. For details about ChemBase, write or call Molecular Design Limited, 2132 Farallon Drive, San Leandro, CA 94577, (415)895-1313. Because every chemist needs experienced help around the lab.
.;.;.;. h# I
.
Molecular @;!;$ . . Design Ltd
IBM k a registeredtrademark of International BuinessMachinesCartim CIRCLE 138 ON READER SERVWX CMlD
Figure 5. Power density in the generated plasm volume of differentanalytical plasmas
Carrier gas requirements. Each device is continually regenerated because of its flowing character. Consequently, memory effects from prior samples are negligible. The argon carrier gas for the DCP and ICP should be at least 99.995%pure. When funding allows, liquid argon is recommended as the reservoir. The tank boil-off is used, resulting in effectively purer argon. The MIP typically uses argon, but newer cavities and launchers allow atmospheric preasure operation with helium. Quite high-purity He is required due to the high excitation capabilities of the He MIP, which makes impurity spectra quite intense. The DCP uses nearly 10 L/min of argon, with the bulk of that going to the single type of nebulizer developed for it. Most ICPs use between 10-20 L/min of argon, with the bulk of that going to the gas that cools the quartz torch. Newer low-flow versions use as little as 5 L/min. The same gas also supplies the fireball, but it is directed first to provide primary cooling. The MIP uses very little carrier gas, 50 mL/min to a few liters per minute. I t goes predominantly to the discharge. The more popular versions of MIP use more expensive helium, which greatly offsets its reduced gas consumption. Power requirements. External service to the plasma generators is most similar for the DCP and MIP. Standard llO-l2O-V, 20-A power is sufficient. Quite frequently, the stability of this supply is inadequate. It is always wise to use a power regulator between the plasma line and the power source. The ICP requires more pow1144A
-
er capacity since it generally uses more power: 220-V,20-30-A, singlephase service is typical. The generation of a DCP within a 3-electrode set requires two I-A arcs a t 40-50 V. Consequently, about 0.75 kW (max.) can he applied to the discharge. The ICP is generated from about 2.5 kW or less radiated power a t a coil antenna. Rf current and voltage are rarely monitored by the operator. The MIP is generated from a few watts to a few hundred watts of radiated power within a confinement structure. Microwave current and voltage also are not monitored. Power density. The power density in the volume of power input (the generated plasma volume) is quite different for each discharge. This can he seen more clearly in Figure 5. It is apparent that the larger the generated plasma volume, the lower will be the power density a t a particular power level. The low levels for the ICP are the result of the large size of the fiieball. Note that there are no regions of power density overlap among these plasmas. That is, each plasma source has its own characteristic ranee of usefulpower. More imoortant. the rate of chanee of power density k t h applied pow& is quite different for each plasma. For the MIP, there is a very steep rata of change of power density with applied power. This ran partially account for both the poorer signal precision obtained from the MIP and the large fluctuation of background signal upon sample introduction. Sample injection into the MIP, at all but the very low-
ANALYTICAL CHEMISTRY. VOL. 58, NO. 11. SEPTEMBER 1986
I
est injection rates, causes an alteration of the power transfer efficiency to the plasma and consequent power density fluctuations. As a result, sample introduction into an MIP with retention of power stability, and thereby, signal stability, is quite difficult. Even with MIPS operated at relatively high power, the small size of the plasma and the viewing of the plasma itself make sample introduction a complicated task. The DCP has a larger generated plasma volume, the confines of which are mostly controlled by laminar-flowing hot argon. The device, though, is current controlled, not power controlled. Consequently, the power density can fluctuate if the resistance of the material in the electrical gap changes measurably. High concentrations of easily ionized matrix elements, for example, can alter the ohmic heating zones of the discharge. The resultant swelling of the generated plasma volume results in reduced power density; the consequent temperature drop of the discharge under these conditions has been verified. This sensitivity of the power density does not directly lead to reduced signal stability, though, since the viewing zone of the DCP is the plume exhaust of the thermally pinched arcs, not the arcs themselves. But some disturbance to the signal can occur, since the temperature and electron density gradients defining the plume are altered. The ICP has a relatively constant power density over its typical operating range. This is very much the result of the large size of the ICP, in which power or volume fluctuations that would be significant in the smaller MIP or DCP cause very little alteration of the ICP power density. That is, the useful power in the ICP is very nearly always the same. The exhaust plume used for observation will be even less affected. This can partially account for the exceptional short-, medium-, and long-term precision seen with ICPs. Viewing zone size. A result of the size of the generated plasma volume is the size of the viewing zone for spectroanalytical observation. The DCP viewing zone is a volume confined hetween two pinched arcs. The crosssectional area imaged on the spectrometer slit block containing the most useful emission is about 8 mm2. In the ICP, analyte emission can he viewed in a tall, narrow region above the load coil. The total viewing zone is tens of square millimeters. The MIP discharge itself is used for analyte emission viewing. The containment tubes are 1mm to 3 or 4 mm in diameter, leading to a very small total viewing zone. Unfortunately, the discharge is often heterogeneously distributed
a c r w the viewed area, making the useful viewed area quite a bit smaller still. Gp8 temperature. One of the leading features of inert gas electrical discharges for atomic emission is their high thermal temperatures. That is, they are convenient means to obtain much hotter reservoirs of hot gas than combustion flames, furnaces, arcs, and sparks. Comparisons would be much simpler if they were truly thermal sources in equilibrium, but they are not. Accurate gas temperatures of the ICP, DCP, and MIP are not available. The ICP and DCP have almost equivalent kinetic temperatures, with the MIP generally accepted to he thermally cooler. A temperature comparison a t best can only tell whether progress over earlier excitation sources for AES has been achieved. This is easily Been if inert gas electrical discharges alleviate most of the classical chemical interferences in emission analyses and provide more intense analyte emission. Both of these features have been documented extensively, and so it can be assumed they are “hotter” than previous emission sources. Electron density. Inert gas discharges in thermodynamic equilibrium will have electron densities on the order of 10’8 cma or above. The elec-
i-..; ........ ~
....
I
i
,i I
.., ...
.* ........
tron density of the plasmas and their Ne gradients can provide some insight into the excitation mechanisms dominant in them. The DCP and ICP only barely approach the level of Ne associated with equilibrium dis,charges.In some limited instances, LTE (local thermodynamic equilibrium) might be invoked; but, in general, the DCP and ICP cannot be assumed to be equilibrium discharges. As a result, clear, uncomplicated models of their fundamental operation leading to analyte excitation will be developed only slowly, if a t all. The MIP exhibits electron density characteristics that definitely place it in the nonequilihrium discharge category. Only for low-pressure MIPS might a complete description of excitation be possible.
canerclalavallabMy ol systems A mundane but essential concern about these plasmas is their availability. Availability meana much more than the presence in the marketplace of the various major components of the plasma devices. It means the presence of complete, turnkey systems, or as close to that as pessible. In effect, such commercial availability is an indicator of the level of development of the device. It is certainly an indicator of its acceptance.
’
?he new defirdition 04 high perfornjance. ! It sh ‘ws mfi.mul . ~ win~pur n ~4o2l~Awcontroller.! ~ w ~ tm~A~aa* t ~ ~ n
9
tackle very c$ompl+x grab GI x e L . . . w . . .
The ICP is available worldwide as a complete atomic emission analysis system from a dozen or more companies. Another dozen companies sell just the plasma generator and torch. Buying an ICP system is much like buying a Chevrolet: Lots of people have them; parts are available when needed; and experienced service personnel can be found relatively easily. The DCP is available worldwide as a complete AES system from just one company. A number of electronic houses sell an acceptable power supply for the DCP; but there is no second source for the plasma device itself. This does not mean the DCP system is undeveloped or poorly accepted. The DCP has been commercially successful for more than a dozen years. Well over loo0 units have been sold. At least to mid-1983, the DCP systems commanded 203096 of the plasma emission marketplace. The absence of development of the DCP by instrument companies other than SpectraMetrics (now Beckman) is probably explainable; suffice it to say, it did not occur. To continue the automobile analogy, buying a DCP might be like buying a Land Rover: They can be found all over the world; a lot of them are in distinctly inhospitable locations; and in ownership one is consoled by their remarkable durability.
