Anal. Chem. 1988, 60, 679-683
avoids the spectral background problems normally encountered in reverse-phase chromatography applications of LC/'H
(9) Buddrus, J.; Herzog, H.; Cooper, J. W. J . Magn. Reson. 1981, 4 2 , 453. (IO) Bayer, E.;Albert, K.; Nieder, M.;Grom, E.; Wolff, G.; Rindlisbacher, M. Anal. Chem. 1982, 5 4 , 1747. (11) Spratt, M. P.; Dorn, H. C. Anal. Chem. 1984, 56, 2038. (12) Spratt, M. P.; Meng. YI; Dorn. H. C. Anal. Chem. 1985, 57. 76. (13) Taft, R. W.; Prosser, F.; Goodman, L.; Davis, G. T. J . Chem. Phys. 1963, 38, 380. (14) Freeman, R.; Frenkiel, T.; Levitt, M. H. J . Magn. Reson. 1982, 5 0 , 345. (15) Shaka, A. J.; Keeler, J.; Freeman, R. J . Magn. Reson. 1983, 53, 313. (16) Fung. B. M. J . Magn. Reson. 1984, 5 9 , 275. (17) Waugh, J. S. J . Magn. Reson. 1982, 50, 30.
NMR. LITERATURE CITED (1) Dorn, H. C. Anal. Chem. 1984, 56. 747A. (2) Laude, D. A.; Wilkins, C. L. Anal. Chem. 1987. 59, 546. (3) Laude, D. A.; Lee, R. W. K.; Wiiklns, C. L. Anal. Chem. 1985, 57, 1464. (4) Laude, D. A.; Wilklns, C. L. Anal. Chem. 1884, 56, 2471. (5) Haw, J. F.; Glass, T. E.; Dorn, H. C. Anal. Chem. 1983, 55, 22. (6) Haw, J. F.; Glass, T. E.;Dorn, H. C. J . Magn. Reson. 1982, 49, 22. (7) Haw, J. F.; Glass, T. E.; Dorn. H. C. Anal. Chem. 1981, 5 3 , 2327. (8) Haw, J. F.; Glass, T. E.; Dorn, H. C. Anal. Chem. 1981, 53, 2332.
679
RECEIVEDfor review August 20, 1987. Accepted December 14, 1987.
Computer-Controlled Source for Atomic Emission Spectrometry Henry Aryamanya-Mugisha, Ronald R. Williams,* and Robert B. Green' Department of Chemistry, Ohio University, Athens, Ohio 45701
The utility of a computer-controlled, high-voltage iow-current (35 mA) microarc as a source for trace metal analysis has been Investigated. I t uses a pair of 0.10 mm diameter tungsten electrodes operating in a helium atmosphere. The stability of the system has been optimized by eliminating oxygen and pressurizing the piasma cell. This was accompilshed by connecting a computer-controlied valve to the exit side of the microarc to control the flow of the support gases. Detection limits of most elements studied hproved by 1 order of magnitude and arc temperature increased from 5200 to 6700 K after this modification. The analytical calibration curves are linear over 3 to 4 orders of magnitude. The wide range of volatilities exhibited by elements in the microarc has been used in the demonstration of a new form of temporal dispersion spectroscopy for multleiement analysis. The temporal emission spectrum of a mixture of barium and calcium is presented.
Layman and Hieftje (1) introduced the microarc as an atomizer for microliter aqueous samples in 1975. It uses a pair of electrodes to support a helium plasma sustained by a high-voltage pulsed dc power supply. Unlike other atomic emission excitation sources, two modes of sample introduction are possible with the microarc. The sample can be placed directly on the cathode prior to the initiation of the microarc (1) or it can be introduced via the support gas stream (2). Aside from its simple design, operation, and low cost, the microarc offers several unusual advantages over common atomic emission sources. (1) Very small, discrete sample volumes may be analyzed. (2) Since aerosol introduction is not used for discrete samples, losses due to inefficient droplet formation do not occur. (3) Intimate mixing of analyte with the plasma occurs because the plasma forms as a glow discharge around the sample on the cathode. This is in contrast to other sources where cold aerosol is injected into a hot plasma. (4) The process of solvent evaporation is separated from sample atomization and excitation, so plasma energy is concentrated on these latter processes. (5) Due to the ac 'Permanent address: Code 3851, Naval Weapons Center, China
Lake, CA 99355.
nature of the current flow, analyte emission is inherently modulated. While this does not remove source flicker noises, it does remove l / f noises from the detection electronics. (6) Lastly, a computer-controlled microarc consumes only a fraction of the normal electrical current and support gas used by other common plasma sources. In this study investigations of a computer-controlled microarc as an alternative source for atomic emission spectrometry are discussed. Two modes of sample introduction were studied, Le., discrete and continuous aerosol sample introduction. Temporal profiles of the elements studied and the analytical characteristics of the instrument are presented. In addition, results on matrix effects of easily ionized elements (EIE) on the emission of analytes will be discussed. Finally, results obtained by temporal resolution of a mixture of elements will be presented.
EXPERIMENTAL SECTION Microarc and Electrode Design. A computer-controlled microarc, based on the design of Hieftje et al. (I, 3 , 4 ) ,was used in this work. A block diagram of the experimental setup is presented in Figure 1 and the equipment used is listed in Table I. A microarc plasms cell 4.5 cm in length, 2.5 cm in width, and 4.5 cm in height was constructed from aluminum. The plasma was supported by helium in a 1.0-cm hole within the cell. As shown schematically in Figure 1, the plasma cell provided an entrance and exit for support gases, a photodiode, as well as a pair of tungsten electrodes and a quartz observation window. The design of the electrodes depended upon the mode of sample introduction. In the first part of this study, microliter samples were placed directly on the cathode and the electrodes were made from a 0.10 mm diameter tungsten wire (John Mattley, Inc., 99.98% pure) instead of the 0.25-mm thoriated tungsten wire used in the original microarc ( I ) . The cathode was formed into a small elliptical loop (ca. 1 mm i.d.). Each electrode was insulated and supported by a threaded Teflon plug that was tightly inserted in the microarc plasma cell. A spacing of approximately 1 mm between the electrodes was found to be optimum. The sample introduction port was located directly over the loop in the cathode allowing easy with a microliter syringe. This port was then sealed with a hollow Teflon plug containing a glass rod serving as a light-pipe to a photodiode which was used to detect total emission (helium + analyte) from the microarc. This signal was used as a reference for demodulating the analyte signal recorded by the photomultiplier tube (PMT).
0003-2700/S8/0360-0679$01.50/00 1988 American Chemical Society
680
ANALYTICAL CHEMISTRY, VOL. 60. NO. 7, APRIL 1. 1988
Table I. Experimental Apparatus support gases
1. oil-free helium (Aga Gas, Inc., Cleveland, OH)
2. oil-free hydrogen (Aga Gas, Inc., Cleveland, OH) microarc plasma cell 1. plasma cell: a hollow 4.5 cm x 2.5 cm X 4.5 cm aluminum block with a 2.5 cm long and 1.0 cm 0.d. hole 2. electrodes: 0.10 mm diameter tungsten wire (John Mattley, Inc., 99.98% pure) 3. interelectrode spacing: -1 mm 4. quartz window: cemented on the exit of the cell with silicone cement (General Electric) 5. photodiode: 2N 5780 for sampling reference signal monochromator 0.5-m Jarrell-Ash Model 82-000 with 1180 lines/mm grating and a reciprocal linear dispersion of 0.002 nm/mm slits 100-rm entrance slit 150 rm exit slit photomultiplier tube RCA 1P28 support gas valves Mekeever (Columhus, OH) electronic relays Electro-Triol, Inc. computers 1. 68HCll (Motorola)with 8K bytes of ROM, 512 bytes of EEPROM, 256 bytes of RAM, eight channel, eight-bit analog-to-digitalconveter 2. IBM-PC
FU Dlgilal 110
Motorola 68HCll
ADC
from ,,om phofod,ode PMT
Figure 1. Schematic diagram of the microarc apparatus showing electrode camgurat!ans: V I , V2, electmnic valves: P, phdodiie; R1. R2. R3, ekctronlc relays.
In the second part of the study, solution samples were introduced into the helium stream in aerml form by using a cros~Aow pneumatic nebulizer described by Kniseley et al. (5). The tungsten cathode was replaced with a 16-gauge hypodermic needle which had an 80-mesh stainless steel screen tightly wrapped over its end and the helium/aerml mixture was introduced through the needle (2). The plasma was supported on the screen and the aerosol was forced to flow through it. Microarc Power Supply. A microarc power supply similar to that reported by Layman and Hieftje (I) was contructed. The only modification to the power supply was the use of a 900-V power transformer (T-101 Espey 18-109, Saroga IN). Since &e transformer was scavenged from old equipment, the total cost of the power supply was less than $30. A solid-state relay was used to allow computer control of the initiation and termination of the microarc current. Optical System and Data Acquisition. The analyte emission signal was isolated by a Jarrell-Ash monochromator and detected by an RCA 1P28 photomultiplier tube operated by a Jarrell-Ash power supply (Model 82 3750,Jarrell-Ash Co., Waltham MA) at -600 V. The current produced by the emission of analyte was converted to voltage by an operational amplifier (P233LM 741CN) and then sampled by the analog-to-digitalconverter (ADC). The reference photodiode signal was sampled on a separate ADC channel. After each determination, data were sent to an IBM Personal Computer.
Computer Control. A Motorola M68HCll (Motorola, Inc., Microprocessor Products Group, Austin, TX) single-chip microcomputer contained in the M68HCll evaluation hoard (EVB) was used for experimental control and data acquisition. Memory systems on the microcomputer include an 8K read only memory (ROM) containing a Forth interpreter, 256 bytes of static random access memory (RAM), and 512 bytes of electrically erasable programmable ROM (EEPROM). In addition, an eight-channel eight-bit ADC with a 16-psconversion time, serial communications interface, and serial peripheral subsystems are provided on the microcomputer (6). Other main features include five eight-hit I/O DOI~S. An additional 8K of static RAM were nrovided bv the EVB as well as level conversions for RS232 communications at a total cost of ahout $175. Electronic relays for controUi the microarc power supply and gas valves were programmed by the EVB. Data from each analysis were collected by the microcomputer and stored in EVB RAM before transmission to an IBM Personal Computer for storage and processing. Software. M68HCll software for experimental control, data acquisition, cross-correlation, and digital low-pass filtering was written in Max Forth (New Micros, Inc., Dallas, TX). All other data manipulation wa8 performed on the IBM Personal Computer using programs written in Turbo Pascal (Borland, Inc., Scotts Valley, CA). Procedures. A. Discrete Sample Introduction. Prior to a determination, the microcomputer turned on the helium support gas and the power supply and allowed the microarc to warm-up for 20 min. The electrodes were automatically cleaned by bleeding small amounts of hydrogen into the helium stream for 5 s. After the system had stabilized (ea. 30 s), the microarc was extinguished and the w r was signaled to inject the sample. Microliter samples were placed on the cathode with a 10-SL Hamilton syringe and allowed to evaporate from the hot cathode into the flowing helium for ahout 30 s. After desolvation, the exit valve was closed and the microarc was initiated Two sets of data were obtained during each determination: one from the analyte (PMT) and one from the total microarc emission (photodiode). Each datum in these sets consisted of the sum of four consecutive 16-rs conversions to improve the dynamic range of the data. The analyte signal was recorded as 500 successive samples of the PMT output. The total emission (helium + analyte) was acquired simultaneously as 500 successive samples of the reference photodiode. These data were cross-correlated and digitally low-pass filtered to produce the demodulated signal. At the end of the determination, the microarc power supply and helium flow rate were then automatically turned off and the data were sent to the IBM Personal Computer for storage, processing, and display. In some studies the exit valve was not closed during data acquisition. Usually, the first 300 data points contained the emission of the analyte. For quantitative analysis, raw data were averaged in blocks of 4, 9, or 14 depending upon the nature of emission. E. Continuous Aerosol Sample Introduction. For these studies electrical power and helium were supplied to the microarc continuously and the exit valve remained open. The instrument wa8
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
allowed to warm up for approximately 20 min after which a sample solution was aspirated into the helium stream. A helium flow rate of about 900 mL/min was required for transporting about 2.5 mL/min of solution into the plasma/screen. The analyte emission and the total microarc emission were simultaneously sampled by the ADC as described above, crm-correlated, low-pass filtered, and stored in computer memory. The signal was sampled for approximately 1 s at 1.6 ms per point. The microarc power supply and support gas were then turned off. Data were sent to an IBM Personal Computer for storage and display. Reagents. All chemicals were ACS reagent grade and used without further purification.
Table 11. Absolute Detection Limits
wavelength, nm
this work
Pb Mn
405.8 403.2 589.0 280.3 324.8 422.7 213.8 451.1 328.1
1.80 0.10 0.50 0.10
Mg
cu Ca Zn
A. Discrete Sample Introduction. Optimization. In-
In Ag 6400 5600
%
detection limits, ng previous
element
Na
RESULTS AND DISCUSSION vestigations into the emission characteristics of the microarc with flowing helium were carried out initially. Analyte emission occurred in the first 10-15 oscillations of the applied voltage waveform except a t low microarc currents (about 25 mA) where delays of about 320 ms in analyte emission were observed. This may be due to slower heating of the cathode which would indicate that vaporization is more important than sputtering in the microarc. A decrease in analyte intensity and an increase in microarc instability and memory effects were also observed under these conditions. This is consistent with a reduced rate of sample vaporization caused by low temperatures a t these low currents. Additionally, the decreased stability of the plasma probably leads to intermittent excitation of sample and a concomitant decrease in analyte emission and increased memory effects. At high currents (greater than 40 mA), decreased stability and memory effects were also observed which again probably resulted from inefficient sample vaporization. Consequently, all the sample deposited on the cathode would not be vaporized and low analyte emission intensities and increased memory effects would be observed. Plots of He emission intensity versus microarc current when 0.10 and 0.25 mm diameter tungsten electrodes were used at a constant optimum helium flow rate (171 mL/min) showed that 35 mA resulted in maximum microarc stability, minimum memory effects, and most intense analyte emission signals. The smaller electrodes produced analyte intensities approximately 50% larger than that from the 0.25 mm diameter electrodes. This may be due either to higher current densities that would be associated with smaller tungsten electrodes or to an increase in analyte concentration in the plasma caused by a reduction in plasma volume or both. An optimum current of 35 mA and 0.10 mm diameter tungsten wire electrodes were used in all discrete sample analyses. Excitation Temperature. The emission intensities of 667.8, 587.6, 501.6, 492.2, 471.3, 447.2, and 388.9 nm He lines were used to compute plasma excitation temperatures of the microarc using standard methods (7-9). When helium was allowed to flow during data acquisition, the microarc temperature was calculated to be about 5200 K. With the flow of helium stopped during signal acquisition, the plasma temperature could be increased substantially. However, a quartz viewport and computer-controlled valve on the exit side of the gas flow were necessary to exclude oxygen from the microarc. This also pressurized the microarc about 4 psi above ambient pressure. After these modifications temperatures of 6780 K were obtained which are similar to those reported by other workers for similar plasmas (3, 8-10). Plasma Stability. After these modifications the stability of the microarc was checked by measuring the emission of He I line (587.6 nm) under optimum conditions. This emission was reproducible over a 4-h working period after an initial warm-up of 20 min. The percent relative standard deviation (%RSD) of the peak He line emission improved from 12 to 4 after optimization.
681
0.60
precision of
work concn [ref]
emission intensity
f1.85 k2.37 f1.20 f0.55 k9.70 *9.93 f3.54 k2.70 f1.30
0.10 [12]
0.9 [ll]
5.00 0.10
0.05 [12] 4.5 [6] 0.075 [12]
0.16
0.24
1.00
0.50
1
.-
:MOO j
.+J
i'
'C I
.E 5200 C
.-
2400
Ifi 1600 800
0
0.00
0.08
0.52
0.40
nrne (sec.) Figure 2. Effect of sodium on the emission signal of iron at 374.8 nm: (A) emission signal for 250 ng of iron; (B) emission signal for 250 ng of iron in 50 ng of sodium; (C)emission signal for 250 ng of iron in 1000 ng of sodium.
Analytical Curves. Analytical characteristics for representative elements are presented in Table 11. Most elements exhibit 3 to 4 orders-of-magnitude of linearity which is the limit available with four samples of an eight-bit ADC. Table I1 also shows a comparison of the detection limits obtained in this study with those of other authors (11,12) using similar plasmas. Detection limits obtained with the computer-controlled microarc are better or in good agreement with those previously reported. The detection limits from this study were calculated by using 50-ms integration times instead of the more usual 1-s integration times. Precision. The reproducibility of analyte intensities varies between f0.5% and &lo%, depending upon the analyte being studied (see Table 11). Imprecision of emission intensities is probably a reflection of the imprecision of measuring microliter samples with a syringe. This might be reduced to less than f l %by using an automatic sampling system. Effects of Easily Ionized Substances on Analyte Emission. When analyses were done in the presence of EIE's such as sodium, complex enhancements in signal intensities were observed. Figure 2 shows emission versus time plots of iron (374.8 nm) in the presence of increasing amounts of sodium. Memory effects also decreased as the concentration of sodium increased. When large amounts of sodium (1000 ng) were added, the increase in iron intensity and decrease in memory effects were accompanied by a change in temporal pattern of iron emission spectrum as shown in curve C of Figure 2. One possible explaination is that an increase in current density in the plasma is caused by the EIE. This increases the excitation and atomization efficiency of the microarc and so increase in both intensity and duration of iron emission. Similar effects have been reported by other workers using
682
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
1
200
Table 111. Emission Enhancements Observed due to Presence of an Easily Ionized Concomitant ?& enhancement due to 500
species (100 ng)
wavelength, nm
NaNO,
KNO,
RbCl
Mg 1 Mg I1 Zn I Zn I1 Ca I Ca I1
285.2 279.6 213.9 206.2 422.7 396.9
22.9 20.0 22.5 137.5 21.1 38.9
12.5 85.4 4.9 18.9
25.0 106.2 16.0 25.4
ng of
high-powered microwave discharges (13, 14) and dc plasmas (15, 16). In order to elucidate possible mechanisms for the above observations, further investigations into the effects of EIE's on the emission of analyte atom and ion lines were done. A summary of results obtained for several analytes in various EIE's is given in Table 111. Enhancements in intensities of all atom and ion lines and a decrease in microarc memory effects were observed for all elements studied in the presence of EIE's. Sodium gave the highest signal enhancement and the lowest memory effects followed by rubidium and potassium. These observations are in agreement with the results reported by Kawaguchi and co-workers (17) using a low-power microwave discharge in the study of calcium emission in the presence of potassium and sodium concomitants. Since both ion and atom emission lines are enhanced by the EIE, the mechanism of signal enhancement is probably due to an increase in excitation rather than atomization or sample vaporization. Studies on the effects of sodium chloride and sodium nitrate on emission intensities of various analytes and ionization temperature of the plasma were performed in order to understand the effects of EIE volatility on the microarc. Figure 3 shows the emission-time curves for 25 ng of magnesium (285.2 nm) in the absence and presence of 50 ng of sodium chloride and 50 ng of sodium nitrate. A summary of the results obtained in this study is given in Table IV. These results show that htghest emission intensities and minimum microarc memory effects were obtained when the more volatile concomitant, sodium nitrate, was used. In the presence of an EIE, the current density in the plasma is probably increased through rapid volatilization of the concomitant which results in a steeper increase in analyte emission during the first oscillation of the microarc as shown in Figure 3. The data in Table IV show that ion line intensities were enhanced significantly more by the EIE than atom lines. Similar observations have been reported in the literature (17) for dc plasmas. This is especially perplexing since the excitation temperature decreased in the presence of the EIE as
0 :
720
640
.z
,
I
I
I
I
I
I
I
I
1
--
560
A
c
480
.-2 400 --
.-c 320 -.-
240
W
160
E
?------
--
-80 0
I
I
I
I
'
I
a
I
1
Table IV. Effects of Sodium Chloride and Sodium Nitrate on the Emission of Magnesium, Calcium, and Zinc, and Excitation Temperature of the Plasma concomitant Dresent NaCl (100 ng)
none species (50 ng)
wavelength, nm
Mg I' Mg 11'
EI"
T2mb
EI"
285.2 279.6
130.0 101.3
6583
165.0 129.4
Zn I Zn I1
213.9 206.2
194.3 122.3
7377
203.6 135.7
Ca I Ca I1
422.7 396.9
257.1 165.3
6446
280.0 230.0
T~LR*
NaNO, (100 ng)
EIa
T~LR*
6309
180.0 150.0
4593
6304
217.9 147.1
6113
3763
328.6 248.6
4375
nEI, emission intensity of species, in arbitrary units. T ~ L R ,excitation temperature (in kelvin) by using two-line ratio method (18); emission data employed for evaluation of TZLR ref via the two-line ratio method was obtained in ref 7 and 24. CAbsoluteconcentrations of magnesium and concomitant were 25 ng and 50 ng, respectivelv.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
Detection Limits. The limits of detection obtained in this work are presented in Table V. A comparison of the above detection limits with those of other authors using a low-power microwave dicharge (17), an inductively coupled plasma (21, 22), and a nitrogen plasma (23),is also given in Table V.
1
x
c
0 c C
.;.
800
C
ACKNOWLEDGMENT
400
v)
The authors thank Bob Shellman and Dewey King for their technical assistance in the construction and maintenance of the experimental apparatus.
'E w
0 0.00
0.26
0.78
0.52
1.04
LITERATURE CITED
l i m e (sec.)
Flgure 5. Emission-time spectrum obtained from an aerosol sample of 5 pg/mL of sliver (328.1 nm)
Table V. Detection Limits of Analytes with Aerosol Sample Introduction Method
element
Na Mg
Ai2
683
wavelength, nm
this work
previous work concn [ref]
589.0 285.2 328.1
5.0 1.0 5.0
1500 [21] 2 [22], 300 [21], 0.005 [23] 0.005 1171
emission preceds that of calcium because of the higher volatility of barium oxide (bp 3000 "C) than calcium oxide (bp 3500 O C ) (20) which would form from the nitrates of these salts. This also demonstrates that thermal vaporization is the dominant mechanism by which analyte is transported into the plasma in this version of the microarc. In practice this type of analysis will find only limited utility since the temporal resolution is highly matrix dependent. B. Aerosol Sample Introduction. The emission spectrum obtained when 5 pg/mL of Ag (328.1 nm) was aspirated into the microarc is shown in Figure 5. The stability of the emission signals was good for all analytes studied with a RSD of ca. 8.5%. Calibration Curves. The calibration curves obtained were linear over 2 to 3 orders-of-magnitude instead of 3 to 4 orders-of-magnitude observed for discrete analysis. This was caused by the bending of the curves toward the abscissa for analyte concentrations greater than 1000 pg/mL. This is probably due to self-absorption which indicates that the microarc plasma is not as spatially homogeneous for the screen electrode as for the loop cathode.
(1) (2) (3) (4)
Layman, A. T.; Hieftje, G. M. Anal. Chem. 1975,4 7 , 194-202. Williams, R. R.; Green, R. B. Anal. Chim. Acta 1986, 187, 301-305. Zander, A. T.; Hieftje, Gary, M. Anal. Chem. 1878,5 0 , 1257-1260. Bystroff, R. I.; Layman, L. R.; Hieftje, G. M. Appl. Spectrosc. 1979, 3 3 , 230-240. ( 5 ) Kniseiey, Richard N.; Amenson, Harry; Butler, C. C.; Fassel, V. A. Appl. Spectrosc. 1974,28, 285-286. (6) Williams, R. R. 68Micro J . 1986,8 , 45-47. (7) Aider, J. F.; Bombeika, R. M.; Kirkbright, G. F. Spectrochim. Acta, Part 8 1080,3 5 8 , 163-175. (8) Rippetoe, W. E.; Johnson, E. R.; Vlckers, T. J. Anal. Chem. 1975,47, 436-440. (9) Tanabe, T.; Haraguchi, H.; Fuwa, K. Spectrochim. Acta, Part 8 1983, 3 8 8 , 49. (10) Workman, J. M.; Brown, P. G.; Caruso, J. A. Appl. Spectrosc. 1988, 4 0 , 857. (1 1) Brackett, J. M.; Vickers, T. J. Spectrochim, Acta, Part 6 1982,3 7 8 , 84 1-847. (12) Pfluger, C. E.: Nessel, T. Ana/yst (London) 1984, 109, 593-596. (13) Murayama, S. Spectrochlm. Acta, Part6 1970,2 5 8 , 191-200. (14) Murayama, S.; Matsuno, H.; Yamamoto, M. Spectrochim. Acta, Part B 1968,2 3 8 , 513-520. (15) Nygaard, Danton D.; Gilbert, Thomas R. Appl. Spectrosc. 1981,3 5 , 52-56. (16) Nygaard, Danton D. Anal. Chem. 1979,5 1 , 881. (17) Kawaguchi, Hiroshi; Hasegawa, Masayasu; Mizuike, A. Spectrochim. Acta, Part 8 1972,2 7 8 , 205-210. (18)Eastwood, DeLyte; Hendrick, Martha S.; Sogiiero, G. Spectrochim. Acta, Part 8 1980,3 5 8 , 421-430. (19) Boumans, P. W. J. M. Theory of Spectrochemical Excitation; Plenum: New York, 1966. (20) Lange's Handbook of Chemistry, 12th ed.; Dean, John, Ed.; McGrawHill: New York, 1979. (21) Rezaaiyaan, R.; Hieftje, Gary, M. Anal. Chim. Acta Ig85, 173, 63-75. (22) Wendt, R. H.; Fassel, V. A. Anal. Chem. 1065,3 7 , 920-922. (23) West, C. D.; Hume, D. N. Anal. Chem. 1964,3 6 , 412. (24) Wiese, W. L.; Martin, G. A. Part JI. Wavelengths and Transition Probablllties for Atoms and Atomic Ions, NSRDS-NBS; National Bureau of Standards: Washington, DC, 1980.
RECEIVED for review June 19, 1987. Accepted December 12, 1987. This research was sponsored by the Ohio University Research Challenge Grant RC86-15. This work was presented in part at the 1987 Pittsburgh Conference & Exposition in Atlantic City, NJ.
