Instrumentation Andrew T. Zander Perkin-Elmer Corporation North American Instrument Division 761 Main Ave. Norwalk, Conn. 06859-0905
Atomic Emission Sources for Solution Spectrochemistry Atomic emission spectrometry (AES) is used for elemental determi nations 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 ele ments being sought. As a result, it is somewhat easier to prepare standards, 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 im proved markedly with recent technical advances. "Holographically" produced diffraction gratings and the increased use of high-capability, low-cost micro processors are key examples. The sin gle 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 dis charges that have been optimized principally for solid or powdered sam ples, the inert gas plasma has been de veloped specifically for solution sam ple types. In this regard it is a replace ment for the flame excitation source. Besides being a replacement, the inert gas plasma has performance advan tages so significant that it has rejuve nated the atomic spectrochemical in strumentation market. The market growth for plasma emission instru mentation far exceeds that of any oth er type of atomic analysis equipment 0003-2700/86/A358-1139$01.50/0 © 1986 American Chemical Society
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 preference and associ ated research activity, the principal versions are the inductively coupled plasma (ICP), the direct current plas ma (DCP), and the microwaveinduced plasma (MIP). Other excita tion 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 principal ly associated with analysis of solid samples, such as the Grimm discharge or the hollow-cathode discharge de vices. It is evident that, at least for the short term, no other device will be able to compete successfully for the interest garnered by the more popular and more prevalent plasmas. The following is a comparison, sometimes somewhat subjective, of the ICP, the DCP, and the MIP. How
ever, it is less a comparison of perfor mance figures of merit, which can be found in any number of review arti cles, than an identification of the ways in which the devices are similar and dissimilar. In that regard, this com parison should shed light on the rela tive "usefulness" of the devices, and their consequent commercial accep tance. On the basis of commercial ac ceptance, it might be argued that the MIP does not belong in the compari son. It has held and still does hold, however, a prominent position of in terest within the atomic analysis re search community, certainly far great er than for the DCP, and is included for that reason. Table I presents a list of device characteristics and operational param eters for the DCP, ICP, and MIP. With this information as a basis, we can compare and contrast the analyti cally relevant features of these plas mas. Operating parameters Operating frequency. The DCP operates at dc and the inductive plas mas operate at radio or microwave fre quencies. Direct current power sup plies of comparable output power to ac supplies are smaller, may have few er components, and are generally less expensive (for equivalently stable out put power). The ICP and MIP do not operate at arbitrary frequencies. The Federal Communications Commission has reg ulated the use of the electromagnetic spectrum to well above 30 GHz (a wavelength of about 1000 μπι). Certain narrow bands have been set aside for industrial, scientific, and medical (ISM) uses. The ICP typically oper ates at 27.12 MHz or 40.68 MHz. The
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 · 1139 A
Table 1. Component identification and comparison Direct-current plasma (DCP)
Parameter
Microwave-induced plasma (MIP)
Inductively coupled plasma (ICP)
Operating frequency Discharge structure
dc Multiple electrode set Anodes: Pyrolytic graphite Cathodes: Thoriated tungsten Cooled by carrier gas
27.12 MHz, 40.68 MHz (ISM) RF coil and ceramic torch Coil: Protected copper Cooled by water Torch: Quartz or ceramic, carrier-gas-cooled
915 MHz, 2450 MHz (ISM) Microwave structure and torch Structure: Protected copper Cooled by gas or water Torch: Quartz or alumina, carrier-gas-cooled
Discharge type
Flowing, thermally pinched, transferred dc arc plume Argon, 99.995% or better
Flowing, electrodeless discharge plume Argon, 99.995% or better
Flowing, electrodeless discharge
Carrier gas Gas consumption (total)
About 10 L/min
10 to 20 L/min
Operating pressure
Atmospheric (1.013 kPa)
Atmospheric (1.013 kPa) 220 V, 60 Hz, single phase, 25-30 A 0.5-2.5 kW 3-4 cm 3
Moderate vacuum to atmospheric (0.133-1.013 kPa) 110 V, 60 Hz, single phase, 15 A 0.02-0.5 kW 0.07-0.18 cm 3
0.93-2.5 kW/cm 3
0.13-0.63 kW/cm 3
0.11-2.8 kW/cm 3
About 8 mm 2 4000-6000 Κ stratified 10 1 4 -10 1 5 cm" 3
20-60 mm 2 4000-6000 Κ stratified -ΙΟ^-ΙΟ 15 c m - 3
0.8-7 mm 2 Possibly as high as 3000 Κ 1014cm-3
Power required
110 V, 60 Hz, single phase, 20 A Applied power 0.5-0.75 kW Generated plasma volume 0.2-0.3 cm 3 Power density (power input zone only) Viewing zone size Gas temperature Electron density (Ne)
Helium, 99.9995% or better Argon, 99.995% or better 0.05 to 1.5 L/min
M I P typically operates at 2450 MHz, the frequency of microwave ovens; 915 MHz is not as popularly used. Table II lists the ISM frequencies and their bandwidths. The 461-MHz band 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 the center frequencies. T h e impact of this is that equipment designed for opera tion at these frequencies is complex because of the stringent requirements for radio frequency (rf) shielding of the generators. Shielding require ments are much less severe at micro wave frequencies, where even a few sheets of aluminum foil are generally sufficient to minimize extraneous ra diation surrounding a plasma device. Shielding at lower rf is very difficult to implement properly. Discharge structure. The me-
chanical configuration t h a t contains or supports the plasmas is quite dif ferent for each of the discharges. T h e D C P (Figure 1) uses a multiple elec trode set: two pyrolytic graphite an odes and one thoriated tungsten cath ode. It should be noted t h a t the threeelectrode D C P has four electrical
poles: two anodes and two cathodes. It uses two power supplies. T h e single cathode connects to the negative pole of both power supplies. Each electrode is efficiently cooled by flowing argon. T h e electrode set tends to lock the generated plasma volume in position, which provides much of the stability
Table II. ISM frequencies and bandwidths Bandwidth limit Frequency ( M H z )
Frequency ( M H z )
%
13.56 27.12 40.68 461.04 915 2450 5800 24125
±0.07 ±0.16 ±0.02 ±0.92 ±13 ±50 ±75 ±125
±0.05 ±0.6 ±0.05 ±0.2 ±1.4 ±2.0 ±1.3 ±0.5
Figure 1 . Schematic diagram of a three-electrode DCP
1140 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Figure 3. Schematic diagram of a 2450-MHz TM 0 io resonant MIP cavity (the Beenakker cavity) A face view is shown at the left, a cross-sectional side view at the right
Figure 2. Schematic diagram of an argon ICP
of the generated signals. The rf ICP uses a radio frequency coil antenna surrounding a ceramic, flow-controlling torch (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 be 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 by plasma AES of any sort that it is rarely considered an analyte. 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.
Figure 4. Schematic diagram of a microwave surface wave launcher (the "surfatron")
The resonant cavity, Figure 3, or electromagnetic field launcher, Figure 4, can be made of anything having a sufficiently shallow skin depth at the operating frequency. Pure copper works well but is hard to machine. Brass and aluminum work best if they are silvercoated. Gold flashing helps significantly. The microwave structure can be gas- or water-cooled, depending on the applied power. Usually water cooling is needed above 200-300 W. The
1142 A · 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. It 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.
Figure 5. Power density in the generated plasma volume of different analytical 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 pressure 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. It 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 110-120-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 pow-
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 7-A arcs at 40-50 V. Consequently, about 0.75 kW (max.) can be applied to the discharge. The ICP is generated from about 2.5 kW or less radiated power at 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 be seen more clearly in Figure 5. It is apparent that the larger the generated plasma volume, the lower will be the power density at a particular power level. The low levels for the ICP are the result of the large size of the fireball. Note that there are no regions of power density overlap among these plasmas. That is, each plasma source has its own characteristic range of useful power. More important, the rate of change of power density with applied power is quite different for each plasma. For the MIP, there is a very steep rate of change of power density with applied power. This can 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-
1144 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
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 between two pinched arcs. The crosssectional area imaged on the spectrometer slit block containing the most useful emission is about 8 mm 2 . In the ICP, analyte emission can be 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 1 mm to 3 or 4 mm in diameter, leading to a very small total viewing zone. Unfortunately, the discharge is often heterogeneously distributed
across the viewed area, making the useful viewed area quite a bit smaller still. Gas temperature. One of the lead ing features of inert gas electrical dis charges for atomic emission is their high thermal temperatures. T h a t is, they are convenient means to obtain much hotter reservoirs of hot gas t h a n combustion flames, furnaces, arcs, and sparks. Comparisons would be much simpler if they were truly thermal sources in equilibrium, b u t they are not. Accurate gas temperatures of the ICP, D C P , and MIP are not available. T h e ICP and D C P have almost equiv alent kinetic temperatures, with the M I P generally accepted to be thermal ly cooler. A temperature comparison a t best can only tell whether progress over earlier excitation sources for A E S has been achieved. This is easily seen if in ert gas electrical discharges alleviate most of the classical chemical interfer ences in emission analyses and provide more intense analyte emission. Both of these features have been document ed extensively, and so it can be as sumed they are " h o t t e r " t h a n previous emission sources. Electron density. Inert gas dis charges in thermodynamic equilibri um will have electron densities on the order of 10 16 c m - 3 or above. T h e elec
tron density of the plasmas and their N e gradients can provide some insight into the excitation mechanisms domi n a n t in them. T h e D C P and I C P only barely a p proach the level of N e associated with equilibrium discharges. In some limit ed instances, L T E (local thermody namic equilibrium) might be invoked; but, in general, the D C P and I C P can not be assumed to be equilibrium dis charges. As a result, clear, uncompli cated models of their fundamental op eration leading to analyte excitation will be developed only slowly, if a t all. T h e M I P exhibits electron density characteristics t h a t definitely place it in the nonequilibrium discharge cate gory. Only for low-pressure M I P s might a complete description of exci tation be possible.
Commercial availability of systems A m u n d a n e b u t essential concern about these plasmas is their availabil ity. Availability means much more t h a n the presence in the marketplace of the various major components of the plasma devices. It means t h e pres ence of complete, t u r n k e y systems, or as close to t h a t as possible. In effect, such commercial availability is an in dicator of the level of development of the device. It is certainly an indicator of its acceptance.
T h e ICP is available worldwide as a complete atomic emission analysis system from a dozen or more compa nies. Another dozen companies sell just the plasma generator and torch. Buying an I C P system is much like buying a Chevrolet: Lots of people have them; parts are available when needed; and experienced service per sonnel can be found relatively easily. T h e D C P is available worldwide as a complete AES system from just one company. A number of electronic houses sell an acceptable power sup ply for the D C P ; b u t there is no sec ond source for the plasma device it self. This does not mean the D C P sys tem is undeveloped or poorly accepted. T h e D C P has been commer cially successful for more t h a n a dozen years. Well over 1000 units have been sold. At least t o mid-1983, the D C P systems commanded 20-30% of t h e plasma emission marketplace. T h e ab sence of development of t h e D C P by instrument companies other t h a n SpectraMetrics (now Beckman) is probably explainable; suffice it to say, it did not occur. T o continue the auto mobile analogy, buying a D C P might be like buying a L a n d Rover: T h e y can be found all over the world; a lot of t h e m are in distinctly inhospitable lo cations; and in ownership one is con soled by their remarkable durability.
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T h e M I P is available as an emission system only in t h e form of a multi channel chromatographic detector. There are no AES systems built around the M I P or its variants. A number of companies sell acceptable microwave generators, transmission line components, resonant cavities, or launchers. All these pieces can be con figured on a spectrometer system de signed to use ICPs or DCPs, b u t no in strument company provides a system. T h u s , having an M I P is much like owning an Indianapolis race car: It can perform in some really amazing ways; a lot of its parts look like other race car parts; b u t the device is only what the operator decides t o make of it.
Detection limits DCPs, ICPs, and M I P s do n o t per form in exactly the same manner, as can be seen from just one of the more common performance figures of merit. Table III lists detection limits ob tained from each plasma type for 10 frequently determined analytes. T h e plasmas were operated on spectrome ter systems or instrument systems of reasonably equal quality. T h a t is, t h e data are not biased for one plasma or the other due to the use of superior signal retrieval and conditioning equipment. Data for all three plasmas on the same instrumental system were
Table III. Detection limits (M9/L) Element
DCP
ICP
Fe
5
3
1
0.04
Cu
1
2
Pb
13
12
MIP
1
Ca
0.5
0.05
0.2
Zn
4
0.5
Na
2
9
Mg
0.1
0.08
0.04 0.001 0.05
Ni
1
7
80
Cr
1
4
1
13
30
Κ
—
available only for Ni. T h e M I P has better detection limits for 7 of the 10 elements. In addition, the M I P has t h e capability of exciting the halogens well enough t h a t they can be quantitated a t the μg/mL level at their UV a n d visible lines, not their vacuum UV (VUV) lines. T h e D C P and I C P excite t h e halogen VUV lines only with great difficulty. Of course, one figure of merit can not be paramount. Too often, though, it is only the detection limits t h a t are discussed in t h e context of acceptable performance. Researchers must realize t h a t the M I P routinely requires more extensive sample pretreatment, or a
complicated sample introduction tech nique, or a nonstandard one-of-a-kind approach to sample injection for these kinds of detection limits to be seen. Were we t o normalize t h e sample in troduction systems to t h a t used for the conventional ICP, t h e M I P would be, in t h e words of one prominent de veloper, " a mediocre performer." T h e principal point is t h a t the perfor mances of each of these plasmas must be considered on a sample-by-sample, even analyte-by-analyte basis.
A subjective measure of relative performance A broader list of performances has been created for t h e comparison of these plasmas. Using current research literature a n d reviews describing t h e use of DCPs, ICPs, a n d M I P s , a list of operational parameters most fre quently commented upon has been compiled (Table IV). With t h e diversi ty of systems involved, a n d t h e diver sity of experience supporting the liter ature, a nonquantitative b u t nonethe less informative means of comparing these parameters was used. Each plas ma is given a score of from 1 to 10 for each of the points of comparison. This is done as much to keep track of the device with the better performance in the description of each parameter, as it is to actually rate t h e devices
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Table IV. Subjective performance comparisons (scale: 1-10, 10 = best) Parameter
DCP
1CP
MP
Sample form Solids
4
3
1
Liquids Aqueous
10
10
7
Organic
10
9-10
2
Saline
10
9
5
Slurry
10
4
1
Gases
10
10
10
Linearity
10
10
6
Chemical
9
10
4
Physical
10
9
5
6
9
3
8-9
10
5-6
Operational costs
8
7
9
Instrumental complexity
9
7
7
10
5
9
2
10
1
10
2-3
1-2
Tolerance to matrix effects
Interelement Precision
Cost of plasma Commercial availability Data base size
1-2
Level of development Plasma Theoretical
1
8
Practical
3
8
1-2
8
4-5
Sample introduction
4-5
Potential for development Plasma Theoretical Practical Sample introduction
against each other. No one has yet provided a particularly reliable measure with which to do that; this is not an attempt to do so. Some of the comparisons will be explained in detail. Sample form. The ability of a plasma to provide acceptable results when the sample is in the listed state is compared with this parameter. All of the plasmas do poorly with solid sample introduction. They are not as robust as dc arcs and graphite atomizers. (Laser vaporization of solids is not considered here.) The DCP and ICP were specifically developed for solution analyses. The MIP does not perform as well for solution analyses. This is often the result of the sample introduction being improperly matched to the material input rate limitations of the MIP. Organic liquids are handled nearly equally well by DCPs and ICPs. The ICP requires slightly more fine adjustment than the DCP, although the newer free-running ICPs are more durable. Organics rapidly extinguish the MIP at all but the very lowest of input rates or at the highest of applied powers. Saline matrices have a tendency to clog ICP sample input tubes faster than DCP components. MIPs can only
10
8
9
8-9
2-3
4-5
7-8
2-3
8-9
just manage a saline sample. The simplified sample introduction device and the geometric rigidity of the DCP allow slurry samples to be run reasonably well. Except for use with some versions of the Babington nebulizer, conventional ICPs have difficulty with very high total-dissolved-solids samples. MIPs cannot handle slurries. All of the plasmas perform well when the sample is gaseous or completely vaporized prior to entering the excitation volume. Separation of sample introduction, vaporization, and excitation steps is clearly an advantageous route. But it has been offset traditionally by the greater instrument system complexity in this approach. Linearity. Linearity refers to the range of concentration over which signal response is linearly predictable to within experimental error. The wide range and linearity of ICP standard curves have been commented upon repeatedly. The DCP provides equivalent linearity, but few publications have described this, and, more importantly, in the first versions of D C P AES systems, instrument electronics did not permit unhindered use of the inherent linearity, so that it was commonly assumed the DCP source did
1148 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
not possess this feature. The latest version of the DCP-AES system does allow use of the total DCP linear range. The MIP shows shorter linear dynamic ranges. In many cases, the material rate limit of the MIP is reached before the standard curve becomes nonlinear. The small size of the MIP is the cause of this limitation. Precision. Routinely obtained short-term and long-term precision is better with an ICP than with a DCP, according to most of the published reports. The DCP can be shown to be as precise as the ICP; there is no fundamental reason why it should be less so. However, the unsophisticated sample introduction system may be the cause. Further development would be helpful. The MIP suffers poor precision because the injected sample causes a disruption of the generation of the plasma itself. Instrumental complexity. The number of adjustable variables required for normal operation is greatest for the ICP. The MIP, depending on the control of the device desired, will have quite a few adjustments. Unfortunately, they are usually interdependent to a large extent. A large number of papers have been published describing the numerous parameter trade-offs involved in obtaining various levels of optimized operation for ICPs and MIPs. For the DCP, only infrequently will more than the viewing height have to be adjusted. The operator does not have access to the power supply beyond the on/off switch. Cost of the plasma device. DCP sources could retail for approximately $7500, but it is not certain that they are available outside of total instrument systems. An ICP source costs two or three times more than a DCP. The MIP probably can be configured for as low as about $2000, but more likely would be around $5000 or more. Its effective cost would be even more, since what is purchased is not turnkey and the purchaser is required to supply further components. The total cost of an atomic emission system, of course, is significantly greater than just that of the plasma. By the time an appropriate spectrometer, detector, electronics subsystem, control computer, and adequate software are incorporated into the instrument array, the fact that an ICP might cost two to three times more than a DCP is not quite as significant. Other considerations, such as the volume of the DCP power supply being less than 10% of that of an ICP power supply, begin to come into play. Data base size. The most quantitative parameter listed in Table IV is data base size. It is the number of research publications produced over the previous four years, as documented in
the ANALYTICAL C H E M I S T R Y bian-
nual reviews. Taking the ICP citations as the base, there are only 20-30% that many MIP papers, and only 10-20% that many DCP papers. The number of DCP papers is as high as it is as a result of the recently established biennial Winter Plasma Conference. The size of the data base is an indication that more people know more about ICPs than MIPs and DCPs combined. Level of development. The DCP has been commercially available for a longer period of time than the ICP. A dc plasma jet was available in the early 1950s. Even the most recent version of DCP has been around longer than most of the ICPs. As characterized by the data base material, however, the ICP has been described more extensively at a fundamental level. There are numerous papers describing microwave devices, but very few report on any single, specific configuration. Consequently, the theoretical development of the MIP is still fragmented. The theoretical treatment of the DCP is only now starting to appear. Some recent DCP papers have examined the DCP in the sort of detail with which the ICP has always been reported. From a practical point of view, characterized by the type of tinkering that results in physical alterations to the source or its power supply, not much has been done with the DCP. This is probably because there is only one version available. Without a second or third variation on the theme, very little curiosity has been shown about how the device might be rearranged. Further, the DCP provides little to be adjusted in normal operation. So the device does not lend itself to inventive tweaking that might indicate the diversity of modifications from which it could benefit. The ICP has evolved through many stages of development; it continues to do so. Some of the latest practical developments involve lower power, lower argon usage, and different support gases. The MIP probably offers the greatest opportunity for configuration development. Except for the TMoio cavity, no single variation of MIP has become widely accepted. With regard to sample introduction, there are more versions of sample input device for the ICP than for DCPs and MIPs. The MIP is interfaced best to a capillary column gas chromatograph; it requires quite a bit of development for direct solution input. The sample introduction device used on the DCP is reasonably effective considering it wasn't developed for that purpose originally. The DCP sample introduction scheme could use much more development. The MIP has yet to find a consistent need in plasma AES. It is an an-
swer looking for a problem. Even for the halogen detection with which it does so well, adequate demand does not exist to warrant commercial development of an MIP system at this time. The DCP provides a less expensive, more operationally forgiving source for plasma AES. It is easy to run and maintain. And it is an exceptionally clever optical match to the échelle spectrometer to which it has always been mated. It suffers from lack of a wider interest base among the publishing research community and its unwarranted connection to dc arc technology. The ICP is the de facto standard source of plasma AES. It may not be the best. But there are more of them around. And all the latest, hottest work is being done with them. Suggested reading Inductively coupled plasmas Fassel, V. A. Science 1978,202(13), 183. Barnes, R. M. CRC Crit. Rev. Anal. Chem. 1978, 7. Thompson, M.; Walsh, J. N. A Handbook of Inductively Coupled Plasma Spectrometry; Chapman and Hall: New York, 1984. Direct current plasmas Miller, M. H.; Eastwood, D.; Schulz-Hendrick, M. Spectrochim. Acta 1984,39B, 13. Zander, A. T.; Miller, M. H. Spectrochim. Acta 1985,40B, 1023. Miller, M. H.; Zander, A. T. Spectrochim. Acta 1986,41B, 453. Microwave-induced plasmas Zander, A. T.; Hieftje, G. M. Appl. Spectrosc. 1981,35(4), 357. Matousek, J. P.; Orr, B. J.; Selby, M. Prog. Analyt. Atom. Spectrosc. 1984, 7, 275.
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800-621-0851 Ext. 204 Andrew T. Zander is a senior staff engineer at Perkin-Elmer, working in analytical instrumentation development. He holds a B.S. degree in chemistry from the University of Illinois and received a Ph.D. in analytical chemistry in 1976 from the University of Maryland. He held a postdoctoral appointment at Indiana University, spent two years on the faculty at Cleveland State University, and was an application engineer with Spectrometries, Inc., prior to his present position. His research interests center on radiation sources for atomic spectrometry, diffraction devices, and signal detection and processing schemes.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 · 1149 A
