Instrumentation
R. K. Skogerboe and1 G. N. Coleman Department of Chemistry, Colorado State University, Fort Collins, Colo. 80523
Microwave Plasma Emission Spectrometry F’lasmas produced by the interaction of electromagnetic fields with gases such as argon have been developed rapidly over the past two decades as atomic spectipal excitation media. One that has received extensive attention is the radiofrequency plasma which derives it:; sustaining power by inductive coupling with the magnetic field of the R F source. Fassel and Kniseley extensivel:,, reviewed the capabilities and the instrumentation of the inductively coupled plasmas ( I C P ) in 1971 i f , 2 ) and have since published several paper!; describing spect roanalyt ical applic at ions (3-5 1. F’lasmas produced by t h e interactions of microwave frequency (usually 24,X XfHz) electrica; fields with gases h a t e received less overall attention hut also show considerable potential as spectral excitation sources. T h e 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 applicat ions. and project directions of future deve I opmen t . Plasma Propagation and Coupling
T h e production of a plasma in t h e absence of combustion processes requires provision of a means for ionizing the gaseous medium involved. Rlectricdly produced plasmas depend on 1 he acceleration of electrons t o 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 tems involving: hijrh-frequenchelectric tields: his treatment will not he repeated here. Plasmas may he init iated hy provision o f “seed” electrons wit7 a ‘Tesla discharge and sustained over a wide range of operational con’ tis Lvith a microivav,? frequency m h y appropriate s’?lectionof potential, gas pressure I collision frecluenc,yi. gas flow. anti pcnver coup 1i t-i g co t i d i t i ( I t i s.
Gold Burner Tip (Top View)
-
Plasma P l u m e 4
-Quartz
Containment Tube
I tl
Argon
+
Analyte Figure 1. Schematic of capacitively coupled niicrowave 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 t o form a flamelike plasma a t the tip of the electrode (7-12). This type of capacitively coupled system (CMP) is illustrated schematically in Figure 1. To obtain maxim u m power transfer to the plasma, an impedance matching or tuning stub arrangement must be used. Impedance matching is also essential since magnetrons are easill- damaged by excess reflected power. Electrodeless systems. known as microwave induced plasmas ( M I P ) .comprise the second type. T h e microwave energy is coupled to the gas stream contained in a nonconductive tube (e.g., quartz) with an external cavity
or antenna (13-28). ‘I’he Evenson I/iwave 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, 301 as good choices from those com‘The tapered mercially available (29). cavity configuration (20) illustrated in Figure 3 has also been widely used. Although both the Evensocl and taperNed citvity 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 introducecl a t rates in excess of about 1 mg/s. In all cavity configurations. the plasma itself affects the impedance of the system. T h e introduction of larger amounts of sample materials often causes signifi-
ANALYTICAL CHEMISTRY, VOL. 48. NO. 7, JUNE 1976
611 A
End View
limited data available are summarized in Table 11. These indicate a lack of thermodynamic equilibrium in the plasmas since the spectroscopic temperatures 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) R h e n the argon is saturated with water vapor, both the rotational and line-reversal temperatures undergo proportionate increases. T h e 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 vaporization characteristics of microwave plasmas are comparable to those of combustion flames, whereas their spectroscopic excitation characteristics are generally higher than those of flames. T h e analytical applications discussed below are generally consistent with these observations.
Side View
uning
T y p e N Coaxial Connector
t
Microwave Power
Figure 2. Evenson 'L-wave cavity, transverse configuration
cant changes in the plasma impedance. If the change is too large, the impedance match is disrupted to the extent that the plasma goes out. Consequently, many transverse configurations such as those illustrated in Figures 2 and 3 have relied on operation at reduced pressure and/or the use of microsample introduction systems to alleviate this problem. This has previously been a definite limitation of its analytical utility. T h e development of a coupling system that permitted plasma maintenance a t 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 ,'Y4wave configuration M I P system illustrated 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 receiving the aqueous aerosol produced by a typical flame nebulization device. T h e ability to view the plasma end-on also eliminates problems of changing light transmission characteristic of the transverse configurations (26).
Spectroanalytical Applications T h e development of C M P and M I P systems as spectral excitation sources has essentially followed two paths. Elemental analyses have been the primary focus of the C M P since its origination by Jecht and Kessler ( 7 , 8 )and T r a p p and Van Calker ( 4 5 ) .Subsequent reports have described improvements (9, I O ) , dealt with physical characterization ( 11 , 12, .32-37), assessed analytical capabilities (8, 1 1 , 12, ,113-38, d5), and examined interference effects ( 1 1 , 12, 33, 3 4 ) . Initial reports on the M I P accepted the difficulty in sustaining the plasma and concentrated on applications in which small amounts of sample were delivered in the gas phase. McCor-
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 conclusions: temperature increases with increasing 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 plasmas are similar to the -4800 K value reported for the ICP ( 4 0 ) To obtain a more accurate assessment of the gas temperatures characteristic of such plasmas, measurements based on line-reversal or OH band intensities (rotational temperatures) should also be considered. T h e
Side Section
End View
+
Microwave Power Type N Coaxial ..c- Connector Quartz Containment Tube
+
Plasma Characteristics
Microwave plasmas have physical properties that are quite similar for either general plasma type, and these are nearly the same as those associated with the ICP. Measurements of electron densities for various microwave configurations indicate levels in the 1012-1015/cc range depending on operating pressure, support gas identity, and input power (27,32-38). Spectroscopic temperatures, based on the
U
+
H 2 cm
Argon
+
Analyte
Figure 3. Tapered rectangular cavity, transverse configuration
U
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mack et al. i l : ~ ' ) t'ollowed , closely by Hache and Lisk 11-1-1 7. 20,21 i,used the plasma t o fragment compounds eluted by gas chromatographs and excite atomic or diatomic fragment spectra for quantitaticin by intensity measurements at appropriate wavelengths. For hydrocarhons, detection relied o n observation of spectral features characteristic of carbon or hydrogen ( 1 % 1 8 ) , For compounds containing heteroatoms such as the halogens. N, S . or €'. selective detection \vas hased on spectra characteristic of the heteroatoms. T h e high sensitivity and selectivity obtained with the G G -MIP combination have led to its use for analysis of organometallic compounds of environmental signifiand for metals cancw h y Talmi (.16-191 through the formation ( i f volatile chelates i l P , ,50, .51i. A n abbreviated summary of analytical applications carried out with GC-hlIP systems is given in Table 111. T h e data indicate that the simple and inexpensive escitem offer? a high degree of . coupled with selectivity in many instances that permits the solution of many rigorous analytical problems. 0 1 her developments involving the 1111' have also relied on delivery o f a mi c r ()sam p 1e. H u n n el s and G i l)s(1n i 191 demonstrated that trace elements could he 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 various modifications of this thermal !'aporization approach t o achieve high sensitivity determinations o r to analyze microsamples. Layman and Hieftje ( 3 1 ) have developed a comput e r - con t r 01 1e d . d c mi c r oar c system for sample vaporization and have achieved precise measurements for several elements in the low picogram range. Detection limits for such delivery systems are generally in the 10-10-10-14 g range for a wide variety of elements: this coincides with a concentration range of IO-IW' ng/ml when IO-pl solution samples are used for analysis. Such analytical capabilities clearly indicate the reason f o r c011tinued interest in microsample AIIP systems of this type. T h e use of a loa-power. atmospherure hlIP system for the direct s of solutions was reported i n 197 1 ( 2 8 ) .A n aerosol desolvation unit ( 6 3 )was used t o remove about 97"~) of t h e water vapor while delivering IO20"(, of the analyte. T h e sample stream was diluted with an equal flow of dry argon t o help maintain the plasma. Detection limits of 0.02-0.04 pglml Lvere reported for the five elements studied ( 2 8 ) .Kawaguchi et al. ( - ! - I ) 614A
Side Section
End View
Cooling Gas (Air)
H 2 cm
C-
Type N Coaxial Connector
t
Microwave Power from Double Stub Tuner Figure 4. Evenson 3&wave cavity, axial plasma configuration
Table I. Temperaturesa Reported for 2450-MHz Microwave Plasma Systems Plasma type
M IP
Support gas
Argon
Helium
CMP
Helium and oxygen Neon Oxygen Nitrogen Argon Nitrogen
Operating p r e s , torr
Power level, W
3 12 12 25 630b 760 760 1.07 2 1.07-He 0.4-0, 2 2 2 760 760 760 760
25 25 100 25 100 100 100 ?
50 ?
50 50 50 200 ?
600 550
Temp, K
Ref
4150 4285 4060 4535 4850 6280C 2300 8550 2350
27 27 41 27 41 28 28 42 35 42
5550 4400 4550 6200 4500 4080 5700
35 35 35 34 9 7 8
4980C
a O n l y temperature measur ments based on multiplet line intensities of the support gas are included herein (39). Local atmospheric pressure for Fort Collins, Colo. CTemPerature of 4 9 8 0 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
1 4-10 630 630 760 760 760 760 760 760
CMP
Power input,
W
Water vapor present?
120 120 100 100 50 100 50 100 200 400
NO No Yes No No Yes Yes Yes Yes
No
Temperature, K Rotationala
Line-reversal
Ref
... ...
2100b 1750b 1700C 2850C
38 38 43 43 44 28 44 28 34 34
2270 3560 1650 1440 2150 2440 4 5 0 0 t 9OOd 4600 i lOOOd
... ... ...
... ... ...
- ??r sysaBased on use o f a portion o f the R z branch o f the (0,O)band in the tem of the neutral H molecule. bReversal o f the 535.0-nm T I line. CReversal of the 589.0-nm Na line. SBased on the Doppler broadening o f C a l l line at 3 9 3 . 4 n m .
ANALYTICAL CHEMISTRY. VOL. 48, NO. 7, JUNE 1976
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Table Ill. Selective Determinations Based on MIP Excitation of Gas Chromatograph Effluents
. I
b
Tern-Pres Reactor Vessels
Element
Compound type
AI As Be Br C CI Cr
Chelate Pesticide Chelate Pesticide Organics Organics Chelates Organics Organics Organics Organics Organics Organics 0 rga nics Pesticide Organics Organics Environmental Environmental
F H D Hg I N 0 P
S Sb Se
Detection limita
Sensitivitya
Ref
... ...
50 47, 48 50 17 52 52, 17, 55 50, 42, 51 13 55 55 49 52, 13 56, 55 55 16 17 53, 54 48 46
100 ng 20 P9 10 Pg
...
...
2 x 10-11b 2 x lo-” 6X
10 ng 50 ng 1 ng
1.5 X
...
3 x lo-” 3 x 10-”b 9 x 10-”b
...
...
...
0.5 Pg 7x 2.9 3x 6X 9x 4x
20 ng 100 ppbb
... ... ...
0.2 ngb 50 Pg 40 P4
10-14 x lom9’ 10-~b 10-1*b lo-”
... ...
aDetection limits are in absolute amount that must be present; sensitivities are defined in terms o f g/s that must be present for detection. bValues obtained with a reduced pressure piasma in helium; a i l others at atmospheric pressure in argon.
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Table IV. Comparison of Detection Limits for Direct Solution Analysis Detection limits, Wglml
AI
As B Cd co Cr cu
Fe Hg
Solubility Determinations Electrical Conductivity Measurements Syntheses at Elevated Temperatures and Pressures Stabilit and Phase Compatibility 0 eyer m inat ions
AASQ
Element
Mg Mn Pb Se Ti V Zn
ICPQ
CMPb
...
0.03 0.1
0.002 0.04
4
0.03
6 0.001
0.005 0.002
0.03 0.5
0.01 0.0004
0.005
0.003
0.003 0.002 0.005 0.5 0.0001 0.002 0.01 0.1 0.09 0.02 0.002
0.001 0.001 0.005 0.2 0.0007 0.0007
0.008 0.03 0.003 0.006
0.002
0.02
... ...
0.02 0.05 0.04 0.01 0.1 0.2
...
0.2 0.05 0.1
MlPd
MlPC
0.06
...
0.01 0.0006
...
0.06
... ...
0.01 0.001 0.01 0.001 0.001 0.001 0.001
0.05 0.003
...
0.005 0.005 0.04 0.1
...
0.1
...
0.08
0.0006
0.00 1
QData taken from ref. 1. b D a t a taken from refs. I 2 and 3 4 . CSingl element determinations, 0.5-m monochromator; data taken from refs. 26 and 64. Simuitaneous multielement determinations, 1.5-m direct reader; data taken from refs. 6 4 and 65.
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used an ultrasonic. nebulizer kvith a similar desolvation system and determined t h a t several elements could be detected at comparable concentration levels. Lichte and Skogerhoe ( 2 6 )subsequently used a cavity similar to that of Figure 1 and an aerosol desolvation system ( 6 3 )for solution analysis without dilution of the argon. Sinple-element detection limit.< ohtained with a half-meter monochromator are summarized in Table IY. These values and those reported for the CMP (Table Ii’) led t o the evaluation of the MIP as a multielement source for direct read-
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
ing spectrometry ( 6 4 , 65) T h e results (also given in Table IV), when compared i+ith detection limits cited for the ICP or flame atomic absorption, demonstrate that sensitive multielement analysis of solution samples can be accomplished a t levels competitive with those of the other t w o techniques. T h e ultimate utility of any analytical system must be determined on the basis of interference effects. hlurayama ( 3 4 )reported rather prominent effects for the CMP due to sodium. T h e presence of K a caused a small re-
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duction in the excitation temperature and a concomitant decrease in the electron concentration of the plasma, but increased both ion and neutral atom excitation. Such reductions are inconsistent with the enhancements observed. In a similar investigation, Boumans et al. ( 1 2 ) reported both enhancement and suppression effects due to the presence of easily ionized elements such as cesium and calcium. In these instances ( 1 2 , 3 4 ) ,the effects are due to a combination of phenomena. Both C M P systems have design deficiencies in that the sample aerosol is injected into the plasma along its peripheral boundaries (see Figure I), while the light emitted in the central portion of the plasma is selected for measurement. The problem is consequently that of entry of sample into the central plasma which is overcome with the ICP by use of the toroidal configuration ( I , 2 ) . Moreover, the degree of lateral diffusion of aerosol into the central region must change with solution composition as shown by West et al. (66,67).T h e importance of this is particularly apparent for the C M P used by Murayama ( 3 4 ) .Boumans et al. ( 1 2 ) have also noted that as the amount of Cs present in solution increases, the geometry and zonal structure of the plasma undergo rather striking changes. As a result, the region of maximum excitation shifts accordingly. T o overcome or alleviate these problems, it would seem advisable to arrange for sample injection through the central core.of the plasma. Enhancement effects on calcium and barium due to the presence of easily ionized elements were also noted for a M I P by Kawaguchi and associates ( 4 4 ) .This appears to have been due to typical ionization repression effects because the only observed enhancements involved elements for which ion line intensities were measured. A subsequent investigation ( 2 6 ) examined the effect of sodium on emission and reported no effect a t sodium levels 1000-fold above the concentration of the analyte. Later studies have demonstrated that avoidance of such interference effects depends on the choice of the nebulizer and aerosol transport facilities ( 4 3 ) . T h e primary requirement is the use of a nebulizer which produces droplets in the 1-2 fi mean size range. Solute vaporization effects such as the phosphate repression of calcium emission have been reported for C M P ( 4 4 ) and M I P ( 2 6 , 4 4 )systems. Based on this overview, plasmas produced with low-power microwave supplies show considerable promise as spectral excitation sources. T h e relatively simple spectra emitted are comprised primarily of lines originating
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from ground state transitions. T h e facilities required to product. such plasmas are quite inexpensive; C h l P systems are available and MIlP units can be assembled from commercial components. As indicated above, design changes in the sample introduction systems appear appropriate for the alleviation of some interference problems associated with the CMP. T h e demonstrated sensitivity, selectivity, and universality of the M I P as a GC detector make i t an obvious choice over other types of selectice detectors with less universality. T h e sensitive microsample analysis capabilities of the M I P should also encourage its adoption. Finally, the recent results on the use of the M I P as a multielement excitation source offer considerable promise that will surely encourage its further development
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References V.A. Fassel and R. N.Kniseley, Anal. ('hem., 46, 111OA (1974). i 2 1 V. A. Fassel and R. N.Kniselev. ibid., i 11
p 1155.4. ( 3 ) G . F. Larson, V.A. Fassel, R. H. Scott, and R. N.Kniseley. ibid.. 47, 238 (1975). ( 4 ) C. C. Butler. R.N.Kniseley. and V, A , Fassel, ibid.. p ,325. tt5) V.A. Fassel, C, A. Peterson. F. N. Abercrombie. and R. N.Kniseley. ihid., 48, 516 (1976).
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1959.
~
7i 7 ) U.Jecht and W.Kessler. Z Anal.
('hem.. 198. '27 (1963). 18) L.Jecht and IT Kessler. Z. Phxs . , 178. 133 (1964).
Ratio TLC
1
(9)H. Goto. K. Hirokawa. and M. Suzuki. Frpspnius Z . Anal. ('hpm., :!25. 130 (1967) (10) S. llurayama. J A p p l t'h?, , 39,54i8 (19681. ( 1 1 ) RI. Sermin. .-lnaiusis, 2, 186 11973). ( 1 2 ) P.\V,J.hl. Boumans. F. ,J, DeBoer. F. .J. Dahmen, H. Hoelzel, and A. Meier. Spectrochim. A c t a , 30B, 44.9 (1975). ( 1 3 ) .A. J . McCormack. 5 . C. 'Tong, and LT, I). Cooke, Ana/. Chern., 37, 1470 11965). (1.1) C. A. Bache and I). .J. 1,isk. ihid , p 1 'l77
'e:
A. Bache and I). -J. Lisk. ibid , 38, 783 (1966). (16) C. A. Rache and 1). .J. Lkk. ibid.. D (1,;
_ _
-
( 1 7 ) C. A . Bache and I). J . Li!;k, i b i d . , 39,
11
Quantitation of TLC is much more accurate with an SD-3000
786 (1967). (18) H. A. Moye. ibid., p 144:,:, i 19) .I. H. Runnels and .J. H. 1 Iibscln. ibid p 1:198. ( 2 0 ) C. A . Bache and I). .J. 1,kk. ibid , 40, 2224 (1968). (21 C. A. Bache and D. .J. Lkk, J . G a s ('hr(JmUtiJgr.,6 , :101 (1968). ( 2 2 ) H. McCarrol. R e v Sei. Instrum., 41, 279 (1970I. ( 2 3 ) C. A. Bache and I). .J. I,i:;k, .Anal ( ' h e m . , 33, 950 (1971) . (2.1) F. E. Lichte and R. K.Sktrgerhoe, i h t d . , 3?, 1321 I 19721.
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i2,5) F. E. Lichte ihid., p 1380. i 2 6 ) F. E. Lichte
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