R. K. Skogerboe and G. N. Coleman Department of Chemistry, Colorado State University, Fort Collins, Colo. 80523
Instrumentation
Microwave Plasma Emission Spectrometry Plasmas produced by the interaction of electromagnetic fields with gases such as argon have been developed rapidly over the past two decades as atomic spectral excitation media. One that has received extensive attention is the radiofrequency plasma which derives its sustaining power by inductive coupling with the magnetic field of the RF source. Fassel and Kniseley extensively reviewed the capabilities and the instrumentation of the inductively coupled plasmas (1CP) in 1974 (1, 2) and have since published several papers describing spectroanalytical applications (.3-5). Plasmas produced by the interactions of microwave frequency (usually 2450 MHz) electrical! fields with gases have received less overall attention but also show considerable potential as spectral excitation sources. The purpose of the present report is to discuss means that have been used to produce microwave coupled plasmas, examine the operational characteristics of various plasma configurations, summarize their present status in terms of spectroanalytical applications, and project directions of future development. Plasma Propagation and Coupling
The production of a plasma in the absence of combustion processes requires provision of a means for ionizing the gaseous medium involved. Electrically produced plasmas depend on the acceleration of electrons to sufficient velocities so that their collision with gas atoms or molecules results in ionization. Brown (6') has discussed in detail the electron acceleration and collision energy exchange mechanisms for systems involving high-frequency electric fields; his treatment will not be repeated here. Plasmas may be initiated by provision of "seed" electrons with a Tesla discharge and sustained over a wide range of operational conditions with a microwave frequency sysl em by appropriate selection of field potential, gas pressure (collision frequency), gas flow, and power coupling conditions.
Gold Burner Tip (Top View) Plasma
Adjustable Tuning Stub
/
Quartz Containment Tube
Figure 1. Schematic of capacitively coupled microwave plasma system
Two general types of microwave frequency plasma systems have received the most attention as spectrochemical excitation sources. In one, the microwave generated from a magnetron and conducted through a coaxial waveguide to the tip of a coaxial (conductive) electrode is used to form a flamelike plasma at the tip of the electrode (7-12). This type of capacitively coupled system (CMP) is illustrated schematically in Figure 1. To obtain maximum power transfer to the plasma, an impedance matching or tuning stub arrangement must be used. Impedance matching is also essential since magnetrons are easily damaged by excess reflected power. Electrodeless systems, known as microwave induced plasmas (MIP), comprise the second type. The microwave energy is coupled to the gas stream contained in a noneonduetive tube (e.g., quartz) with an external cavity
or antenna (13-28). The Evenson lUwave cavity (29) system illustrated in Figure 2 utilizes what will be referred to herein as the transverse configuration. Although other cavity arrangements of this general type have been used, a comparison of the coupling efficiencies of several configurations indicates this and the Broida type (22, 30) as good choices from those commercially available (2.9). The tapered cavity configuration (20) illustrated in Figure 3 has also been widely used. Although both the Evenson and tapered cavity systems include provisions for impedance matching, several investigators have found that it is difficult to sustain a plasma with them when a foreign gas or material is introduced at rates in excess of about I mg/s. In all cavity configurations, the plasma itself affects the impedance of the system. The introduction of larger amounts of sample materials often causes signifi-
ANALYTICAL CHEMISTRY, VOL. 48. NO. 7, JUNE 1976 · 611 A
End View
Side View Fine Tuning Probe
Cooling Air
Quartz Containment Tube Coarse Tuning Probe
.Type Ν Coaxial Connector Microwave Power
Figure 2. Evenson 1/4-wave cavity, transverse configuration
cant changes in the plasma imped ance. If the change is too large, the im pedance match is disrupted to the ex tent that the plasma goes out. Conse quently, many transverse configura tions such as those illustrated in Fig ures 2 and 3 have relied on operation at reduced pressure and/or the use of microsample introduction systems to alleviate this problem. This has pre viously been a definite limitation of its analytical utility. The development of a coupling sys tem that permitted plasma mainte nance at atmospheric pressure in the presence of relatively large amounts of sample, including aqueous aerosols, was reported by Lichte and Skogerboe (26). This has evolved to the axial '·%wave configuration MIP system illus trated in Figure 4 which is similar to the Broida cavity used by Layman and Hieftje (31). It has been demonstrated that this system can be used to sustain atmospheric pressure plasmas in argon, helium, or nitrogen while re ceiving the aqueous aerosol produced by a typical flame nebulization device. The ability to view the plasma end-on also eliminates problems of changing light transmission characteristic of the transverse configurations (26). Plasma Characteristics Microwave plasmas have physical properties that are quite similar for ei ther general plasma type, and these are nearly the same as those associ ated with the ICP. Measurements of electron densities for various micro wave configurations indicate levels in the 10 12 -10 I5 /cc range depending on operating pressure, support gas identi ty, and input power (27, 32-38). Spec troscopic temperatures, based on the
measurement of the intensities of multiplet wavelengths of the support gases (39), are summarized for various operational conditions in Table I. These indicate three general conclu sions: temperature increases with in creasing ionization potential of the support gas, reduction in operating pressure causes a small decrease in temperature, and the spectroscopic temperatures of the two types of plas mas are similar to the ~4800 Κ value reported for the ICP (40). To obtain a more accurate assess ment of the gas temperatures charac teristic of such plasmas, measure ments based on line-reversal or OH band intensities (rotational tempera tures) should also be considered. The
limited data available are summarized in Table II. These indicate a lack of thermodynamic equilibrium in the plasmas since the spectroscopic tem peratures are much higher. In dry argon, the line-reversal temperatures are essentially constant over a wide pressure range and are affected very little by power input (35, 37, 38). When the argon is saturated with water vapor, both the rotational and line-reversal temperatures undergo proportionate increases. The presence of water vapor consequently appears to "thermalize" the plasma, probably via a rotational coupling mechanism. These two sets of temperature data generally indicate that the sample va porization characteristics of micro wave plasmas are comparable to those of combustion flames, whereas their spectroscopic excitation characteris tics are generally higher than those of flames. The analytical applications discussed below are generally consis tent with these observations. Spectroanalytical Applications The development of CMP and MIP systems as spectral excitation sources has essentially followed two paths. El emental analyses have been the pri mary focus of the CMP since its origi nation by Jecht and Kessler (7, 8) and Trapp and Van Calker (45). Subse quent reports have described improve ments (9, 10), dealt with physical characterization (11, 12, 32-37), as sessed analytical capabilities (8, 11, 12, 33-38, 45), and examined interfer ence effects ( 11, 12,33, 34). Initial reports on the MIP accepted the difficulty in sustaining the plasma and concentrated on applications in which small amounts of sample were delivered in the gas phase. McCor-
End View
Side Section Microwave Power Type Ν Coaxial — Connector Quartz Containment ι Tube
2 cm
Figure 3. Tapered rectangular cavity, transverse configuration
612 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
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mack et al. (VA), followed closely by Hache and Lisk (14-17, 20, 21 ),'used the plasma to fragment compounds eluted by gas chromatographs and ex cite atomic or diatomic fragment spec tra for quantitation by intensity mea surements at appropriate wave lengths. For hydrocarbons, detection relied on observation of spectral fea tures characteristic of carbon or hy drogen (73-/8). For compounds con taining heteroatoms such as the halo gens. N, S, or P, selective detection was based on spectra characteristic of the heteroatoms. The high sensitivity and selectivity obtained with the GO MIP combination have led to its use for analysis of organometallic compounds of environmental signifi cance by Talmi (-16- 49) and l'or metals through the formation of volatile che lates (42, 50, 51). An abbreviated sum mary of analytical applications carried out with GC-MIP systems is given in Table HI. The data indicate that the relatively simple and inexpensive exci tation system offers a high degree of sensitivity coupled with selectivity in many instances that permits the solu tion of many rigorous analytical prob lems. Other developments involving the Mil 1 have also relied on delivery of a mierosample. Runnels and Gibson (19) demonstrated that trace elements could be determined by deposition of small volumes of solution on a metal filament, evaporation of the solvent, and vaporization of the residue into the plasma for excitation of the atomic spectra. Others (57-62) have used var ious modifications of this thermal va porization approach to achieve high sensitivity determinations or to ana lyze microsamples. Layman and Hieftje (31) have developed a com puter-controlled, dc microarc system for sample vaporization and have achieved precise measurements for several elements in the low picogram range. Detection limits for such deliv ery systems are generally in the 1 0 - l ( , - 1 0 - 1 '' g range for a wide variety of elements; this coincides with a con centration range of 10-10~ 3 ng/ml when 10-μ1 solution samples are used for analysis. Such analytical capabili ties clearly indicate the reason for con tinued interest in mierosample MIP systems of this type. The use of a low-power, atmospher ic pressure MIP system for the direct analysis of solutions was reported in 1971 (28). An aerosol desolvation unit (63) was used to remove about 97% of the water vapor while delivering 10 20% of the analyte. The sample stream was diluted with an equal flow of dry argon to help maintain the plasma. Detection limits of 0.02-0.04 Mg/ml were reported for the five elements studied (28). Kawaguchi et al. (41)
Side Section
End View
Cooling Gas (Air)
Quartz Containment Tube
Type Ν Coaxial Connector
Microwave Power from Double Stub Tuner
Figure 4. Evenson 3/4-wave cavity, axial plasma configuration
Table I. Temperatures" Reported for 2450-MHz Microwave Plasma Systems Plasma type
MIP
Support gas
Argon
Helium
CMP
Helium and oxygen Neon Oxygen Nitrogen Argon Nitrogen
Operating press, torr
Power level, W
Temp, Κ
Ref
3 12 12 25 630* 760 760 1.07 2
25 25 100 25 100 100 100 ? 50
4150 4285 4060 4535 4850 4980c 6280e 2300 8550
27 27 41 27 41 28 28 42 35
1.07-He 0.4—02 2 2 2 760 760
?
2350
42
50 50 50 200 ?
5550 4400 4550 6200 4500
35 35 35 34 9
600 550
4080 5700
7 8
760 760
" O n l y temperature measurements based on multiplet line intensities of the support gas are included herein (39). " L o c a l atmospheric pressure for Fort Collins, Colo. c Temperature of 4980 in dry argon and 6280 in argon saturated with water vapor.
Table II. Gas Temperatures Characteristic of Microwave Plasmas in Argon Plasma type
Operating press, torr
MIP
Power input, W
Water vapor present?
Temperature, Κ Rotational"
Line-reversal
Ref
1 120 No ... 2100* 38 4-10 120 No ... 1750» 38 630 100 No 2270 1700c 43 630 100 Yes 3560 2850e 43 760 50 No 1650 ... 44 760 100 No 1440 ... 28 760 50 Yes 2150 ... 44 760 100 Yes 2440 ... 28 CMP 760 200 Yes 4500 ± 9 0 0 d ... 34 760 400 Yes 4600 ± 1000d ... 34 "Based on use of a portion of the R2 branch of the (0,0) band in the 2 Σ — 2π sys tem of the neutral OH molecule. » Reversal of the 535.0-nm Tl line. c Reversal of the 589.0-nm Na line.
