The conclusions from the electronic spectra are in ac- cord with data from vibrational spectroscopy. Sodium, mercury (I), mercury (II), lead (II), and cadmium [in contrast to aluminum and lathanum (III)] perturb the aromatic system of salicylic acid. The hypsochromic shift of the ~r -~ 7r* bands in the UV spectra occurs in agreement with a decrease in the number, frequency, and intensity of IR and Raman bands characteristic of the aromatic system.

1. B. A. Hess and L. J. Schaad, J. Amer. Chem. Soc. 93, 303 (1971). 2. D. H. Lo and M. A. Whitehead, Can. J. Chem. 46, 2027 (1968). 3. D. H. Lo and M. A. Whitehead, Can. J. Chem. 46, 2041 (1968). 4. M. J. S. Dewar and G. J. Gleicher, J. Amer. Chem. Soc. 87, 685

(1965). 5. R. Zahradnik and J. Michl, Collektion Czechoslov. Chem. Commun.

31, 3442 (1966). 6. G. W. Wheland, Resonance in Organic Chemistry (J. Wiley, New

York, 1955). 7. F. Sondheimer, Pure Appl. Chem. 28, 331 (1971). 8. F. Sondheimer, Proc. Roy. Soc. A297, 173 (1967). 9. D. J. Wilson, V. Boekelheide, and R. W. Griffin, J. Amer. Chem.

Soc. 82, 6302 (1960). 10. H. J. Dauben, J. D. Wilson, and J. L. Laity, J. Amer. Chem. Soc.

91, 1991 (1969). 11. T. Nakajama, S. Kohda, A. Tajiri, and S. Karasawa, Tetrahedron

23, 2189 (1967). 12. J. Kruszewski and T. M. Krygowski, Bull. Acad. Polon. Sci. Ser.

Sci. Chim. 20, 907 (1972). 13. J. Kruszewski and T. M. Krygowski, Tetrahedron Letters 36, 3839

(1972). 14. J. Kruszewski and T. M. Krygowski, Tetrahedron Letters 319 (1970). 15. W. T. Dixon, J. Chem. Soc. (8), 612 (1970). 16. T. M. Krygowski, Przejawy zmian strukturalnych we wtasciwo§-

ciach fisykochemicznych czasteczek zwiazkow v-electronowych. Wydawnictwa Uniwersytetu Warszawskiego, Warszawa (1974).

17. T. M. Krygowski and J. Kruszewski, Ilofieiowe kryteria aromaty- cznofici. Wydawnictwa Politechniki Wro~awskiej, Wroctaw (1978).

18. E. Clar, Polycyclic Hydrocarbons (Academic Press, London, 1964). 19. W. Lewandowski, J. Mol. Struct. 101, 93 (1983). 20. W. Lewandowski, Pol. J. Chem. 58, 1199 (1984). 21. W. Lewandowski and H. Baranska, J. Raman Spectr. 17, 17 (1986). 22. W. Lewandowski, Can. J. Spectr. 37 (1987), in press. 23. S. P. Sinha, J. Inorg. Nucl. Chem. 33, 2205 (1971). 24. M. Lewandowska, A. Janowski, and W. Lewandowski, Can. J. Spectr.

29, 87 (1984). 25. G. Versanyi, Assignments for Vibrational Spectra of 700 Benzene

Derivatives (Akademiai Kiado, Budapest, 1973). 26. K. K. Rohatgi and S. K. Sen Gupta, J. Inorg. Nucl. Chem. 32, 2247

(1970). 27. W. F. Zolin, M. A. Kazanskaya, A. W. Mashinskaya, J. J. Heruze,

and W. I. Capyuk, Opt. Spektrosk. 33, 926 (1972). 28. S. P. Sinha and H. M. N. Irving, Anal. Chim. Acta 52, 193 (1970). 29. W. I. Yermolenko, Zh. Neorg. Khim. 9, 48 (1964). 30. S. A. Durham and F. A. Hart, J. Inorg. Nucl. Chem. 31,145 (1969). 31. A. K. Babko and L. L. Schevchenko, Zh. Neorg. Khim. 9, 42 (1964). 32. Silvia Plostinaru and P. Spacu, Rev. Roum. Chim. 18, 2051 (1973). 33. Silvia Plostinaru and P. Spacu, Rev. Roum. Chim. 19, 567 (1974). 34. M. Sh. Abashamadze, N. I. Pirtshalava, Y. Y. Kharitonow, and R.

I. Mcichkoshvili, Zh. Neorg. Khim. 23, 2650 (1978). 35. M. A. Tinshenko, G. I. Gerasimienko, and N. S. Poluektov, Depow.

Akad. Nauk Ukr. RSR, Ser. B. Geol. Khim. Biol. Nauki 12, 1107 (1976).

36. V. P. Gruzdev and V. L. Ermolaev, Opt. Spektrosk. 42, 586 (1977). 37. K. K. Rohatgi and S. K. Sen Gupta, J. Inorg. Nucl. Chem. 31, 1202

(1969). 38. J. H. S. Green, W. Kynaston, and A. S. Lingsey, Spectrochim. Acta

17, 486 (1961). 39. D. W. Whiffen, J. Chem. Soc. 1350 (1956). 40. J. H. Burns and E. H. Baldwin, Inorg. Chem. 16, 289 (1977). 41. Sflvia Plostinaru and P. Spacu, Rev. Roum. Chim. 18, 2051 (1973). 42. Silvia Plostinaru and P. Spacu, Rev. Roum. Chim. 19, 567 (1974).

Direct Introduction of Aqueous Samples into a Low-Powered Microwave-Induced Plasma for Atomic Emission Spectrometry

G A R Y L. L O N G * and L A R R Y D. P E R K I N S Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

The direct introduction of aqueous samples into a low-power microwave plasma is achieved with the use of a highly efficient TMo~o microwave plasma. A toroidal plasma is sustained in the cavity solely by the Ar gas output of the nebulizer. Samples from a concentric glass nebulizer/ Scott type spray chamber are fed directly into the cavity with no desol- ration apparatus. A toroidal plasma can be sustained from the output of the nebulizer while 1 mL/min water is being aspirated at power levels of 36 W. This plasma is characterized as an atom cell by the study of emission profiles, working curves, and limits of detection. Also, ioniza- tion and vaporization interferences that occur with the use of this plasma are discussed.

Index Headings: Atomic emission spectrometry; Microwave-induced plasma; Sample introduction.

Received 27 October 1986; revision received 8 January 1987. * Author to whom correspondence should be sent.

INTRODUCTION

A significant limitation to the use of the microwave- induced plasma (MIP) for spectrometric determinations is the problem of direct introduction of aqueous samples into the plasma. A microwave-induced plasma system employing a TM010 Beenakker cavity, Ar support gas, and applied power levels of less than 200 W does not possess sufficient plasma energy, density, and kinetic temperature to promote sufficient atomization and ex- citation of the analyte vapor. 1 During the past ten years, various investigators have tried different approaches to the problem of direct aqueous sample introduction.

One such approach focuses on the removal of water vapor from the analyte, so that the limited plasma energy

980 Volume 41, Number 6, 1987 0003-7028/87/4106-098052.00/0 APPLIED SPECTROSCOPY © 1987 Society for Applied Spectroscopy

is used in analyte vaporization and excitation instead of desolvation. This strategy involves the use of nebulizers and desolvat ion systems, 2 e lec t ro thermal atomizers (heated wires and furnaces), :~,4 and micro-arc atomizers3

Another approach to the problem of low energy density and plasma temperature utilizes higher applied power levels in the cavity. In MIP systems where the applied power levels range from 300 to 800 W, sample introduc- tion is achieved without the use of desolvation systems. However, this method also results in severe cavity con- nector heating. Because an elongated plasma is produced with this setup, significant microwave leakage occurs and is attributed to the extended field of the plasma. ~

Using the Beenakker cavity with a lesser applied power of 115 W, Ng and Shen were able to introduce aqueous samples into the plasma using a MAK nebulizer. 7 With this efficient nebulizer and moderate plasma power, no desolvation system was required.

A third approach to this problem involves the me- chanics of power transfer to the TM01o cavity. Hieftje calculates that, for the Beenakker cavity, only 35 W of applied microwave power is necessary to sustain a plasma with 1 L/min Ar and to support complete analyte de- solvation and vaporization of a 1 mL/min aqueous sam- ple. s This calculation is based on the assumption that the cavity is 100% efficient in power transfer. Unfortu- nately, the Beenakker cavity is much less than 100% efficient. Matus et al. estimate that the inductively cou- pled Beenakker cavity is capable of using 25 W of forward power at 100% efficiency2 Plasma power levels greater than 25 W can be achieved with this cavity, but not at 100% power transfer. Instead, power losses occur and result in substantial cavity and/or tuning stub heating. In an effort to increase the efficiency of the cavity, Matus et al. modified the cavity so it can be more precisely tuned and matched to the generator, thereby increasing the efficiency of power transfer2 This design, termed the highly efficient cavity, features an antenna probe that can be translated along the radial face of the cavity. This translation allows the probe to interact with the electric and magnetic fields to sense a 50-ohm load and thus provide a proper impedance match of the cavity to the generator. In a later study, 1° the cavity was estimated to be 98 % efficient in power transfer of applied power levels of 10 to 100 W. This cavity has been used to introduce Na samples via tungsten filament atomizers.

In this paper, we report our use of this highly efficient TMolo cavity for the direct introduction of aqueous sam- ples from a concentric glass nebulizer with no desolvation system and applied powers of 36 to 70 W. Using a mod- ified tangential flow torch, we produced and sustained a toroidal plasma with the output of the nebulizer alone. For characterization of this low-powered plasma, the lira- its of detection with the use of atomic emission of re- fractory and nonrefractory elements will be presented, along with working curves and plasma profiles of ele- ments of interest. Additionally, studies of ionization and vaporization interferences will be described.

EXPERIMENTAL

Reagents. All chemicals used were analytical reagent grade. Water was deionized. Stock solutions of all metals

CHOPPER

E,v-I

LOCK-IN

F[(;. 1,

tRecorder I The schematic of the MIP-AES system.

were purchased as 1000 ppm (Buck Scientific, Inc.) or prepared according to standard procedures. Volumetric dilution of these solutions was made in order that the desired concentrations could be obtained. The plasma gas used was Airco analytical-grade argon.

Setup. A block diagram of the experimental apparatus is shown in Fig. 1; major components of the experimental setup are listed in Table I. The optimized operational parameters are listed in Table II.

Operating Conditions. For plasma operation, a flow of 1 L/min of argon gas was introduced through the side inlet of the plasma torch with a pressure of 50 psi of argon gas from the nebulizer. To the cavity was applied 70 W of power. A tungsten wire attached to a rubber policeman (for insulation) was inserted into a quartz torch within the TMol0 cavity. The wire was then induc- tively heated by the field, causing a "seed" of the argon gas to ignite the plasma. We tuned the plasma to 0 W reflected power by adjusting a sliding quartz rod and by utilizing the electrical properties of an antenna probe2

It should be noted that after several hours of operation, neither cavity nor connectors were observed to heat up, but remained "cool to the touch." These observations indicated that the cavity was critically coupled to the generator and that no significant power losses were oc- curring in the cavity, cables, or connectors.

The reflected power was monitored by an external me- ter (see Fig. 1). Although the generator provided a built- in means of measuring the incident and reflected power, the instrumental incident power meter revealed the amount of power sent to the generator's magnetron in- stead of the amount of power transferred to the cavity and, thus, the plasma. The resultant reading is not only misleading to the researcher but provides the basis for misleading power transfer data.

To obtain an accurate measurement of the power de- livered to the cavity and reflected from the generator, we employed a "T" circulator, an attenuator, and a ter- mination device (see Table I). In this T configuration, power is sent from the generator to the cavity; then the reflected power travels from the cavity back to the cir- culator, where it is attenuated by a 10-dB attenuator, displayed on the external meter, and terminated by the termination device.

APPLIED SPECTROSCOPY 981

TABLE I. Experimental instrumentation.

Components Model/size Manufacturer

Microwave cavity Highly efficient See Ref. 8 TM(,o

Generator 120 W (70 W actual) Kiva Inst rument Corp., Rockville, MD

Circulator 3-Port T, MA-HC Microwave Assoc. 7238

Attenuator 50 W, 10 dB, Model Englemann Microwave A8510N Co.

Termination 10 W, 50 ~, Model Bird Electronics 80M

Discharge tube Tangential flow See Ref. 5 Coaxial cable 50 ~, RG 214 Times Fiber Communi-

cation Monochromator ModelEU-700, 0.35 m Heath I/V Model A1 Thorton EMI PMT Model EU-701-93 Heath Nebulizer Concentric J .C. Meinhard Spray chamber Scott Laboratory built Chopper Model 125A EG & G Lock-in Model 5101 EG & G Lens f/3, Suprasil Oriel Corp.

As a result, the T configuration has proven useful in four ways:

1. No power is reflected back to the generator; thus a longer life time for the generator's magnetron is as- sured.

2. Since the cavity can be accurately tuned to 0 W re- flected power, the T configuration provides a means of monitoring power levels to achieve a "critical cou- pling" between the generator and the resonance cav- ity.

3. The configuration provides a means of calibrating the generator by connecting the attenuator/meter/ter- minator directly to the generator. The actual power leaving the generator is indicated on the external me- ter and can be used to calibrate the instrument's in- cident power meter.

4. The configuration provides an accurate means of re- porting applied and reflected power, thus preventing misleading analytical results. This allows researchers to better compare data.

It should be noted that the plasma can be lit while one is aspirating i mL/min H20 with power levels of 50 W. (On the rare occasions when the plasma went out during these studies, it was easily relit with the accu- mulated water exiting the top of the torch in a spiral "sprinkler" fashion. The presence of air in the spray chamber did not affect the operation of the plasma. Methanol could be directly introduced into the cavity, and propane/Ar mixtures could be introduced into the plasma without causing any problems. The organic gases produced a blue tail similar to that of a Bunsen burner.)

Sample Introduction. Aqueous samples were directly aspirated into the plasma with the use of a Meinhard concentric nebulizer and Scott-type spray chamber. No desolvation system was employed. After ignition of the plasma, the auxiliary argon flow was turned off, so that the plasma was sustained only by the nebulizer gas flow (1.0 L/rain Ar). Sample uptake rate was approximately 0.9 mL/min. The presence of this amount of H20 in the

TABLE II. Operational parameters for MIP-AES.

Forward power 70 W Reflected power 0 W Observation height 1-2 mm above cavity Nebulizer uptake 0.91 mL/min Aux. argon flow 0 mL/min Total argon flow 1 L/min Probe penetration 75%

plasma gas was not observed to affect the plasma with the use of this cavity.

Torch Design. With the use of a modified design of the MIP torch described by Deutsch and Hieftje, a stable tangential plasma was produced in the cavity2 This plas- ma could be sustained alone by the gas output of the concentric nebulizer. The plasma uniformly filled the discharge tube and extended 3 to 4 cm from the top of the resonance cavity.

Limits of Detection. The limits of detection were cal- culated according to the method prescribed by IUPAC guidelines. For each calculation, 20 background readings were taken. The analytical sensitivities were calculated from the working curves of the element, which spanned a range of at least two orders of concentrative magnitude. Although not suggested, a value of 2 has been used for k, so that these values may be compared with those al- ready in the literature for MIP-AES.

Data Collection. The plasma was translated in the X and/or Y direction via translation stages (NRC) to yield the optimum observation zone. The optimum height was found to be 1-2 mm above the cavity. Signals were pro- cessed via a lock-in amplifier and an Apple IIe computer equipped with a 12-bit A/D converter. Data for limits of detection and emission profiles were stored in memory.

Stability. Operation time of the plasma exceeded 8 h with a cavity temperature not exceeding 30°C. To date, the torch life time has exceeded two years, with little or no etching.

Microwave Leakage. Microwave leakage was inspected and was found to be 0.5 mW/cm 2 and well below that of most moderate- and high-power MIPsA 11 We found that this value is less than the allowable leakage for household microwave ovens (1 mW/cm ~ at 5 cm). 12

Excitation Temperature. The excitation temperature of the plasma used in this study was measured by a slope method? 3 This method involves the emission in- tensities of 10 Fe lines, each line representing the relative population of an Fe excited state.

RESULTS AND DISCUSSION

Power Levels. In order to be able to accurately report the power levels at which this low-powered MIP was operated, we calibrated the forward power meter of the microwave generator by directly feeding the output into the attenuator/meter/terminator setup (as described in the experimental section). The generator is capable of producing forward power levels of up to 72 W. With the meters calibrated, it was noted that the plasma could easily be sustained by the Ar flow of the nebulizer alone, thus allowing for the introduction of 0.9 mL/min of sam- ple using only 50-70 W of forward power. Neither the

982 Volume 41, Number 6, 1987

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1 2 .

11,

10,

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8 -

7

6

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2 0 3 0 4 0 5 0

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The emission profile of a 10-ppm Na solution with the use of direct nebulization. Zero millimeter represents flush with the top of the microwave cavity.

introduction of the sample nor the reduction in the ap- plied power levels resulted in reflected power (0 W); therefore no adjustments in the cavity tuning were nec- essary. Power levels be low 50 W did necessitate tuning adjustments; however, with careful tuning we were able to produce a toroidal plasma using only 36 W of forward power (0 W reflected power) while aspirating the sample. This confirms the earlier calculation s of the minimum energy required to operate an MIP with direct nebuli- zation and the efficiency of the highly efficient cavity.

Exc i ta t ion Temperature. The temperature of the plas- ma at 70 W forward power with the use of 1.0 L/min Ar and the aspiration of 0.9 mL/min aqueous solutions was measured by the Fe slope method? 3 At an observation height of 2 mm above the cavity, the excitation temper- ature was measured to be 4500 + 100 K.

Prof i les . In order to evaluate the cavity as an atom and excitation cell, we constructed plots of analyte emis- sion vs. observation heights for a variety of elements. In Fig. 2, a plasma emission profile of Na(I) is shown that derived from a 70-W plasma and direct sample intro- duction. In this profile, the relative intensity is plotted against the viewing height above the radial face of the cavity with the torch flush against the face. The optimum response for emission was observed at a height of 2 ram, while the response was greatly diminished at higher view- ing regions of 10 to 20 mm. This optimum region at 2 mm was noted for the other elements studied and was chosen as the viewing height for measurement of working curves and calculation of detection limits.

W o r k i n g Curves . With this method of direct sample introduction, there existed some concern regarding the effect of high salt samples on the plasma's ability to vaporize and excite the analyte. In Fig. 3, a working curve for Na is shown, for which the operating conditions out- lined in Table II were used. This log-log plot, which extends from 0.01 ppm to 1000 ppm, is quite linear over five orders of concentrative magnitude and possesses a slope of 0.9998. At large Na concentrations, no bending

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I I I / I. - 2 - 1 0 1 2 3

Log C o n c e n t r a t i o n ( u g / m L )

FE(~. 3. Na emission calibration curve (slope = 0.9998).

in the working curve is noted, indicating that the plasma is able to vaporize and excite these high analyte concen- trations and is free from self reversal.

Limi t s of Detection. An important parameter in gaug- ing the analytical utility of this direct sample introduc- tion MIP system is the limit of detection for an element. In Table III, a listing of detection limits for this system (column 1) and for other MIP systems is presented. The values were calculated in accordance with IUPAC guide- lines, as described in the experimental section. The data in column 2 were obtained by Beenakker et al., ~4 with the use of a 150-W plasma with a crossflow nebulizer and heated spray chamber. These values, however, were cal- culated with only 10 background readings and an as- sumed RSD of 0.004 to 0.006. r'~ The data in column 3 were obtained by Haas and Caruso, with the use of a higher-power MIP (300-500 W) cavity with direct sam- ple introduction# The data in the fourth column were

TABLE III. MIP-AES limits of detection (~g/mL)."

Element CL ~' CL ~ e~ d c,#

Ag 0.12 0.006 A1 1.4 0.4 0.65 Ba 0.18 Ca(I) 0.040 Ca(II) 0.27 Co 1.8 0,15 Cr 8.0 0.15 0.004 Fe 0.65 0.008 K 0.024 Li 0.043 0,001 Mg O.63 0,005 Mn 6.9 0,05 0.0039 Na 0.002 Sr 0.025 0,01 Zn 0.42 0.0023

0.062

0.018

0.013

. Note: Two significant figures are used for comparative purposes only. b In this work, k = 2 with the use of 20 background observations. k = 2 and RSD of 0.004-0.006. See Ref. 13.

"k = 2. See Ref. 6. °k = 3. See Ref. 7.

A P P L I E D S P E C T R O S C O P Y 9 8 3

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FIG. 4. The effeet of Na on the emission signal of a 10-ppm calcium solution.

obtained by Ng and Shen, with the use of a Beenakker cavity and moderate power (115 W), and a MAK neb- ulizer for direct sample introduction. 7 The limits of de- tection are described here as 3~.

As noted by a comparison of the data, the data from the system described in this paper (column 1) are gen- erally an order of magnitude inferior to the other data. This lack of sensitivity may result from the fact that (1) the generator used in this study had a limitation of 72 W forward power and/or (2) a 0.35-m monochromator with a resolution of 2 /~ was employed. In the other studies, a monochromator of 0.5 m or more was used. Applied powers of 100 to 150 W produce a greater energy density in the plasma and lead to enhanced analyte sig- nals. A monochromator with finer resolution decreases the stray light registered in the detection system, thus enhancing the sensitivity.

With respect to the data in the fourth column, Ng and Shen used, in addition to the 0.5-m instrument, a MAK nebulizer for sample introduction into the l15-W plas- ma. 7 In terms of droplet size distributions, the MAK produces a much smaller sized droplet than does the Meinhard used in this work.15 Because droplet size affects the desolvation and vaporization times of the analyte as well as the solvent loading, smaller droplet sizes should result in decreased detection limits. In particular, the detection limits are 2 orders of magnitude superior for Cr and Mg with the use of the MAK nebulizer. However, the detection limit for Sr is not statistically different. Since these metals have similar dissociation energies for their oxides, the decrease in sensitivity for Cr and Mg in this work may be the result of spectral noise rather than insufficient plasma power, as compared with the results of Ng and Shen's work.

Interferences. The utility of this direct sample intro-

R 1 4 - e 1 a t i v 10 e

8" I n t 6" e n .s 1 4" t Y 2-

I I I I 0 1 10 100 1000

04 P / Ca M o l a r R a t i o

FIG. 5. The effect of P04 :~ on Ca 2~ a tomic emission signal (Ca ~ con- cent ra t ion = 10 ppm).

duction MIP as an atom and excitation cell for atomic spectrometry was estimated by examination of several interferences that occur in flames and plasma. For ion- ization interference, the effect of Na on Ca signals was studied; for vaporization interference, the effect of P043 on Ca signals was studied.

Ionization. The effect of an easily ionizable element, Na, on Ca(I) emission signals is shown in Fig. 4. With the use of the operating parameters outlined in Table II and a 10-ug/mL Ca solution, the effect of Na concentra- tions from 0.01 #g/mL to 1000/~g/mL was observed. At low Na levels, 0.01-1.0 ~g/mL, no discernible effect of the easily ionizable element (EIE) on Ca(I) is seen. At levels of 10 to 100/~g/mL Na, a maximum 10 % increase in Ca(I) was recorded with the presence of the EIE. How- ever, at 1000/~g/mL Na, the Ca(I) increased by 300% with this level of EIE present in the plasma. This large increase indicates that there is significant Ca ionization occurring in the plasma; at these low plasma powers, matrix matching of standards and samples must be con- sidered before this plasma is used as an analytical tool.

Vaporization Interferences. To evaluate the ability of this plasma to vaporize refractory compounds, we con- ducted the classic Ca2+/P043 vaporization experiment using the operating conditions outlined in Table II and a 10-/~g/mL Ca solution. The results of this experiment are plotted in Fig. 5. As seen in this plot, the Ca(I) signal falls by 30% with the addition of P04 ~-, achieving a PO43-/Ca 2÷ ratio of 1:1. This level of depression remains essentially unchanged through the PO43-/Ca 2÷ ratio of 100:1. At a level of 1000:1, however, the Ca(I) signal drops by 8O%.

This behavior is fairly consistent with other data ob- served for the MIP. At a PO43-/Ca ~+ ratio of less than 1:1, the condensed phase compound is formed. The ad- dition of more PO43- does not affect the Ca(I) signal, except at extremely high P04 s concentrations. It should be noted that the P043 depression caused only a 30 % reduction in Ca(I) signal as compared with a 70% depres- sion which occurred with an earlier MIP system. A1-

984 Volume 41, Number 6, 1987

though the Ca~÷/P04 ~ depression is very sensitive to drople t size format ion and distr ibut ion, TM the alleviation or reduct ion of the depression is indicative of the power of the p l a sma to p romote analyte vaporizat ion and dis- sociation. This system, using direct sample in t roduct ion into the MIP, is still prone to vaporizat ion interferences, bu t less so than are other MIPs; hence it suppor ts the claim of Burns and Boss t ha t the highly efficient TMolo M I P is very efficient in power transfer . 1°

C O N C L U S I O N S

With the use of the highly efficient TMolo cavity, the direct in t roduct ion of aqueous samples into a low-power M I P with no desolvat ion sys tem has been achieved. T h e combina t ion of nebul izer / torch and cavity permi t s a to- roidal p la sma susta ined by the Ar ou tpu t of the nebulizer alone. The plasma, with direct aqueous sample intro- duction, could be sustained on as little as 36 W appl ied power. Although the limits of detect ion with the use of this sys tem in the emission mode are worse, by a factor of 10, than those of previous M I P - A E S studies, the fact t h a t ex t remely low forward powers and a medium-res - olution monoch roma to r were employed to obtain these values m a y explain these inferior values. In t e rms of ionization and vaporizat ion interferences, the results of this sys tem compare favorably with those of other MIP- AES systems.

Fu tu re work using the highly efficient TMolo cavity with direct sample in t roduct ion will focus on greater ap- plied forward powers and the use of the cavity for a tomic

f luorescent work. The fact t ha t the M I P is inherent ly less energetic than the ICP may prove to be an advantage for the M I P ' s funct ion as an a tom reservoir for a tomic fluorescence spec t romet ry .

ACKNOWLEDGMENTS

A VPI & SU small projects grant for partial support of this work is acknowledged. The authors also wish to thank Andrew Mollick for construction of the microwave torches.

1. A. T. Zander and G. M. Hieftje, Appl. Spectrosc. 35, 357 (1981). 2. R. K. Skogerboe and G. N. Coleman, Anal. Chem. 48, 611A (1976). 3. H. Kawaguchi, I. Atsuya, and B. L. Vallee, Anal. Chem. 49, 266

(1977). 4. O. Rose, D. N. Muncey, A. M. Yacynych, W. R. Heineman, and J.

A. Caruso, Analyst 101,753 (1976). 5. L. R. Layman and G. M. Hieftje, Anal. Chem. 47, 194 (1975). 6. D. L. Haas and J. A. Caruso, Anal. Chem. 55, 2014 (1984). 7. K. C. Ng and W. L. Shen, Anal. Chem. 58, 2084 (1986). 8. G. M. Hieftje, Department of Chemistry, Indiana University, per-

sonal communication. 9. L. G. Matus, C. B. Boss, and A. N. Riddle, Rev. Sci. Instrum. 54,

1667 (1983). 10. B. Burns and C. B. Boss, North Carolina State University, unpub-

lished work. 11. D. L. Haas, J. W. Carnahan, and J. A. Caruso, Appl. Spectrosc. 37,

82 (1983). 12. Tom Smithwick, Department of Safety, Virginia Polytechnic In-

stitute and State University, personal communication. 13. D. J. Kalnicky, V. A. Fassel, and R. N. Knisely, Appl. Spectrosc.

31, 137 (1977). 14. C. I. M. Beenakker, B. Bosman, and F. W. J. M. Bowmans, Spec-

trochim. Acta 33B, 373 (1978). 15. R. F. Browner, Department of Chemistry, Georgia Tech, personal

communication. 16. G. L. Long and C. B. Boss, Anal. Chem. 54, 624 (1982).

APPLIED SPECTROSCOPY 985