Figure 3 shows the mechanism of Kanamaru and Nagakura for the phototautomerization of anthrone ( 4 ) . Step 1, is the abstraction of a hydrogen atom from ether by the anthrone triplet; this was substantiated by the detection of the anthrone ketyl radical as the reaction intermediate. Steps 2 and 4 account for the formation of anthrapinacol (AP) and 9-anthrol photodimer. Step 3 produces the tautomer, 9-anthrol, which was shown by product studies not to be formed via disproportionation of the anthrone ketyl radical. Our results for anthrone in ethanol, except for the formation of 362 intermediate, are satisfied by the mechanism of Kanamaru and Nagakura. The most likely precursor to the 362 intermediate is anthrone because formation of the 362 intermediate was more than 10 times faster with irradiation at 300 nm (where anthrone absorbs strongly) t h a n a t 350 nm (where 9-anthrol absorbs strongly). Since the anthrone triplet should form the anthrone ketyl radical in ethanol, a dimerization pathway of this radical might account for 1. The fact that the 362 intermediate is formed upon flashing (Le., it is a primary photoproduct) is compatible with this. It should also be noted that the quantum yield for formation of the 362 intermediate would have to be high (>0.02) to compete with the
other photoreactions of the anthrone ketyl radical. Some direct formation of the 362 intermediate may come from photolysis of anthrapinacol; but it is more probable that AP photolysis forms anthrone, which in turn photolyzes in low yield to the 362 intermediate. Figure 4 summarizes the major pathways for the photolysis of anthrone in ethanol and the production of the blue fluorescing species observed in the photoinduced luminescence. Anthrone is initially produced by photolysis of oxanthrone, the tautomeric form of 9,lO-dihydroxyanthracene (3). Rapid equilibrium is established between anthrone and 9-hydroxyanthracene. 9-Hydroxyanthracene photolyzes primarily to the dimer. 9-Anthrone undergoes photolysis to the ketyl radical which dimerizes to form the anthrapinacol or the 362 intermediate. Anthrapinacol under photolysis forms 9-hydroxyanthracene as one of its major products. There exists a possibility that the 362 intermediate is formed from the photolysis of anthrapinacol although this probably occurs in very low yield. Received for review September 2, 1973. Accepted November 28, 1973. This work was supported in part through funds provided by the US.Army Research Office-Durham.
Easily Operated Direct Current Argon Plasma Arc for Atomic Spectrometric Analysis D. A. Murdick, Jr.,l and E. H. Piepmeier Department of Chemistry, Oregon State University, Corvallis, Ore. 9733 7
An easily operated and stable dc argon plasma arc has been designed and studied. The detection limit for hafnium is 1.5 ppm, for copper 0.06 ppm, and for calcium 0.01 ppm. Phosphate does not interfere with calcium except in the cooler outer regions of the arc. Cost of operation is low compared to flames and the flow rate of the sample stream can be reduced to nearly zero.
An easily operated and stable d c plasma arc has been designed and studied. The arc discharge is similar to the one of Marinkovic and Vickers ( I ) , b u t appears to be much easier to ignite. The detection limit for hafnium, a n element known to readily form oxides in flames ( 2 ) , is 12 times better than emission and comparable to atomic a b sorption detection limits for chemical flames. The arc is sufficiently energetic t h a t phosphate interferes with calcium only in the cooler regions of the arc. The cost of operation of the arc is low, about 756/hr, compared to flames. Because flow rates of the sample stream can be reduced to nearly zero, this arc is a good candidate for use with a n ultrasonic nebulizer.
EXPERIMENTAL The Arc Chamber. A cut-away view of the argon plasma arc is shown in Figure 1. The arc consists of two sections-a moveable Present address, Dow Chemical Company, Midland, Mich. 48640. ( 1 ) M. Marinkovic and T. J. Vickers, Appl. Spectrosc., 25, 319 (1971) (2) G . W. Dickinson and V . A . Fassel, Anal. Chem. 41, 1021 (1969)
678
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upper arc chamber and a lower stationary electrode holder. The arc chamber consists of two parallel water-cooled brass segments, A1 and A2, with a hollow Pyrex cylinder, B, held between them. Holes for the arc in A1 and A2 are '/z in. in diameter. These parts are held together with four plastic bolts, P, two of which are shown. They are then screwed to a Plexiglas upper plate that is spring loaded on four vertical guides attached to a Plexiglas baseplate. The Pyrex cylinder is 1.3 cm long and has a 2.8-cm i.d. The cylinder is coaxial with the arc. The sample stream is introduced tangentially into this cylinder at its edge by a 5-mm i.d. Pyrex tube located midway between the ends of the cylinder I1 and 12. The pointed electrodes enter the arc chamber through the ?'-in. diameter by 7/8-in. long hollow cylindrical pistons, E l and E2. E l and E2 are machined as part of A1 and A2. These pistons slide up and down within yg-in. cylinders machined into F1 and F2. The tolerance between each piston and its cylinder is only 0.00075 in. to minimize the contamination of the argon atmosphere by air. The electrode holder section consists of four brass segmentstwo electrode holders, H1 and H2, and two cylinder blocks, F1 and F2. Between the electrode holders and the cylinder blocks are cemented with alpha cyanoacrylate (Vigor No. CE-476, Toagosei Chemical Industry Co., Ltd.) two Yls-in. thick Bakelite strips, G1 and G2, which provide electrical insulation betwen the electrode holders and the rest of the device. The electrode holders are screwed to a Plexiglas baseplate with slots that allow easy horizontal alignment of the piston chambers with respect to the pistons. All brass segments except the electrode holders float electrically to prevent arcing to them. About 0.5 l./min cold water is run in a constant stream through ports Wl-W8 in that succession to help remove the up to 600 watts of heat created in the arc chamber. E l and E2 move up and down in the cylinder blocks F1 and F2 to expose the tips of the electrodes for the purpose of igniting the arc. After ignition, the electrode tips are allowed to recede out of the optical path.
Figure 1. Cut-away view of the argon plasma arc. A I . A2. Water-cooled brass segments: 8 , Pyrex cylinder with tangential sidearm: E l , E2, Hollow cylindrical pistons: F1, F2, Cylinder blocks; G1, G2. Bakelite Strips; H I , H2. Eiectrode holders: i l , 12, Thoriated tungsten electrodes; P. Plastic bolts: S. Springs: W1-W8, Water channels
Electrodes. The inverted U-shaped arc burns between the two ?kin. diameter thoriated tungsten electrodes (2% Thor-Tung Electrodes, Air Reduction Company, Inc., New York, N.Y.) in the channel indicated in Figure 1. The space around the electrodes in the cylinders, E l and E2, is 0.031 in. to minimize arc wandering. 'Each electrode is continually' bathed and coaled with a 0.5 1.1 min argon stream introduced tangentially into its piston chamber in F1 or F2. Flow rates less than 0.5 l./min cause the electrodes to heat excessively while flow rates of 2 or more I./min cause distortion of the plasma. The arc can be blown out if flow rates of 5 I./min or larger are used to flush the electrodes. The removable electrodes are sharpened to a pencil-like point having vn included angle of 14". The cathode retains its sharpness while the anode forms a small globule at the apex of the electrode. Although this globule does not distort the arc, it does enlarge with time, and it was our practice to remove it about every eight hours by resharpening. If the electrodes are not kept fairly sharp or if the electrodes are placed too high in cylinders E l and E2, B kind of sputtering occurs. This sputtering consists of hright flashes of light randomly occurring near the anode end of the arc. Because these flashes occur most often just after the electrodes have been resharpened, they are believed to consist of bits of dust and/or tungsten metal momentarily blown off the irregular surface of the electrode. These particles heat rapidly and, as they enter the anode plume, they apparently react violently with the oxygen present causing flashes of light. As the smooth surface of the globule forms, the flashes occur less frequently. The noise spikes from the short flashes were visually discriminated against during quantitative measurements as discussed below. An argon stream which carries the sample aerosol is introduced tangentially into the Pyrex cylinder and assists in the positional stabilization of the I-inch long central core of the arc. The arc has the lowest resistance to current when the flow rate of the sample Stream is 1.8 I./min. However, flow rates of from 1 to 10 l./min have been used. The seals between the Pyrex chamber and the brass end plates are sufficiently air tight and the flow of argon from the electrode chambers is such that once the arc is ignited the arc will continue to burn even without the sample argon Stream. Conceivably, a very low flow rate could be used to introduce the analyte and perhaps increase the residence time of the analyte atoms in the arc. A flow rate of 3.7 I./min was used because the particular sample nebulizer that was used would not work at flow rates as low a8 1.8 l./min. With no sample present, an increase in flow rate from 1.8 to 3.7 I./min causes the resistance of the arc to go from 4.9 to 6.2 ohms. Nebulization of a sample containing 1000-ppm sodium caused the resistance to drop from 6.2 to 6.0 ohms. The arc can he blown out with the sample stream at a flow rate above 10 I./min, above the range of the flaw meter scale.
Figure 2. Photographs of the operating plasma arc A. Overhead view with anode plume on the right hand side, B. Side view
Argon flows were regulated by means of a standard two-stage pressure regulator held at 20 psi, and distributed by air flow regulators (types 3PA and 2SA, Kontes, Berkeley, Calif.) controlling flaws to the anode and cathode respectively. A Matheson flow regulator (tube size R-2-15-B) controlled the flow to the Pyrex cylinder. Flow readings of the air flow meters were multiplied by 0.85 toohtain argon flawrates. The power supply used for the arc, model PAK-16, (ElectroMatic Products Co., Chicago, Ill.) has an open circuit voltage of 105 volts. Because the power supply was unregulated, a variable ballast resistor was placed in series with the arc to manually maintain a constant current. For most studies, a current of 9 amperes W B S chosen. The ballast resistor consisted of two, 2-ohm 300-WOhmite variable resistors placed in parallel. Since the arc has a resistance of about 6 ohms, this gave a current control range of 15%. Operating the Arc. To ignite the arc, the cooling water, the argon streams, and the power supply are turned on in that order. Then the spring loaded arc chamber is lowered so that the electrode tips are visible. A short carbon rod with an insulated handle is inserted through the horizontal channel to establish eleetrical contact between the electrodes. When the arc has been struck, the carbon rod is removed and the upper portion is allowed to move back to its original position. The arc now has an inverted U-shaped pattern, Figure 1. Out of both ends of the arc chamber, the hot glowing gases farm luminous plumes. Although bath anode and cathode argon flow rates are the same, the anode plume is ahout two times larger than the cathode plume. This is the case even when the sample argon strevm is off. Figure 2 contains pictures of the arc as seen from overhead and from the anode end of the arc. Nebulizer. Solutions were sprayed into the main argon stream by use of a chamber type nebulizer, Figure 3. The nebulizer was constructed using a Beckman (No. 4020) total consumption burner attached to a Plexiglas tube 4% in. long by 3% in. in diameter with a set of baffles placed 2 in. and 3 in. from the tip of the bumer. The nebulizer consumed 1.9 ml/min of sample solution with an argon flow rate of 3.7 l./min at 20 psi. At these conditions, the ratio of the amount of solution entering the arc to the total amount sprayed into the chamber is 0.06. The nebulizer is connected to the arc chamber by a 9-in. length of 31,s-in. ID S p a n tubing. There is some tendency for the fine aerosol to settle out on the tubing on the way to the plasma. This problem was solved by placing a small piece of heating tape along the tubing and heating moderately (25 watts). Spectrometer Observation System. Emission data were obANALYTICAL CHEMISTRY. VOL. 46. NO. 6, MAY 1974 * 679
IO
-
i\
9-
8.
WFLES
Figure 3. D i a g r a m of t h e nebulizer c h a m b e r
tained using a McKee-Pedersen spectrometer station (McKeePedersen Instruments, Inc., Danville, Calif.). The monochromator was equipped with a 15,000 line-per-inch grating blazed for 400 nm first order (reciprocal linear dispersion of 0.0037 nm/micron first order). In all studies cited in this paper, a spectral band width of 0.1 nm was used. A 1P28 photomultiplier in a MP-1021 housing was operated at -900 volts. The voltage for the photomultiplier was provided by a regulated dc power supply (MP1002). The anodic current from the photomultiplier was converted to a voltage by means of a chopper stabilized operational amplifier (MP-1031) with a high impedance selector (MP-1009) set at lo6 ohms for a feedback resistor. A O.1-qf feedback capacitor was placed in parallel with the feedback resistor to give a time constant of 0.1 sec. The time constant was short enough to allow spikes caused by the sputtering at the anode to be visually discriminated against (ignored) for quantitative measurements. The voltage was recorded using a strip chart recorder (Heath Model EUW 20A). A one-to-one image of the center of the arc was focused with a 50-mm focal length quartz lens onto a 2-mm diameter aperture placed directly in front of the entrance slit of the monochromator. The arc was mounted on a micrometer stage to allow an image of the arc to be moved horizontally and reproducibly across the entrance slit. The center of the optical axis remained in a horizontal plane that passed through the center of the arc. Emission measurements were made on the cathode side of the arc with the optical axis parallel to the horizontal portion of the inverted E-shaped arc. The cathode end of the arc was chosen because significant curvature of calibration curves occurred at high elemental concentration, perhaps due to self absorption, when the anode end was viewed. The electrodes and the vertical arc regions near the electrodes are not viewed by the monochromator. Reagents. Standard dilutions from stock solutions were used for all studies. All solutions were prepared with distilled water and were stored in polyethylene containers.
Distance f r o m Arc Center (mm) Figure 4. E f f e c t of arc c u r r e n t upon intensity of the 324 7 - n m
-.,
C u ( l ) line for 50 pprn Cu
-8 amperes, signal, . - 0 , 9 amperes, signal, --A, signal,- 0 , 9 amperes, background 7r
.-2!
RESULTS AND DISCUSSION A horizontal section across the center of the arc was studied to determine t h e effect of arc current on an analytical signal. Figure 4 shows how the emission signal a t 324.75 n m (corrected for background) varies with arc current and position for a 50-ppm copper sample. T h e background was observed with a water blank being nebulized a t the same rate as t h e copper solution. T h e background for the 9-ampere blank is plotted for the purpose of comparison. Figure 4 shows t h a t increasing the arc current causes a decrease in the neutral atom analyte signal in t h e center of the arc. Several causes for this effect might be considered such as gas expansion and greater ionization a t higher currents, b u t further study would be necessary to eliminate assumptions and t o determine which causes are truly most important. T h e decrease on the emission profiles toward the center of the arc may in part be due to t h e fact t h a t the analyte is introduced around the periphery of t h e arc and diffuses into the center under the influence of a concentration gradient. The concentration is therefore expected to be higher around the periphery t h a n in the center. Figure 4 shows t h a t t h e arc is approximately symmetrical and t h a t the positions from 6.35 t o 0.00 m m have a higher signal-to-background ratio t h a n t h e positions from 680
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 6, M A Y 1974
10 amperes,
2
I/ ' 1
00
1.27
2.54
3.81
5.W
6.35
Distance from Arc Center (mm) Figure 5. Variation of the relative intensity of the C u ( l ) 324.7Cu along a cross section of n m line for 50 p p m 0 and 5 pprn the a r c
0.00 to -6.35 m m . Because of this higher signal-to-background ratio, most of the studies were done in the region from 6.35 to 0.00 m m . Figure 5 shows an emission profile study of 50 ppm copper and 5 ppm copper. T h e 5-ppm scan has a sharper emission peak t h a n does the 50-ppm scan. The non-similar peak shapes show that the linearity or shape of a n a n alytical curve may depend upon the position where the measurements are made. Figure 6 shows emission profiles for a neutral atom line and a n ion line each for the elements of hafnium a n d cal-
Table I. Detection Limits
( p g ! ml)
Element
Line in nmcL
Position in nm
Argon plasmab
CalciumUI) (1) Copper (11 (1) Hafnium (11) (1)
393 .37 422 ,67 324.75 327 .40 339.98 368.22
1.90 3.81 2.86 ... 2.54 2.54
0.5 0.01 0.06
..
. . .
0 . O'.;
1.5 6
198
io.ooo
. - ,.
-0
&l
a
,'
,
..
0.0009~
0.0005 0,001 ...
...
a These lines may not have been the ones for the flame work. Using a 0.1-sec time constant as discussed in the text. (91, using a 10-secintegration time. 0 Integration times or time constants unspecified.
7r
Atomic ahsorptionf
Flame emission*
...
2 Ref. (6).
'
Ref. (7). e Ref. ( 8 ) . Ref.
1
I I
0.1
I
1
10
100
1000
Concentration (pprn) Figure 7. Log-log analytical curves f o r hafnium, 368.2 n m , and copper, 324.7 n m . The relative intensities between the curves d o not correspond
Distance from Arc Center
(mm)
Figure 6. Variation of the relative intensity of severai emission lines along a c r o s s section of the arc - - - B , H f ( l ) 368.2 nm. 1000 ppm Hf: - - - O , Hf(l1) 339.9 nm. 1000 ppm C a ( l ) 422.7 nm. 10 ppm Ca; ----A, C a ( l i ) 393.4 nm. 10 Hf; -X,
P P Ca ~
cium. The concentration of calcium for both lines is 10 ppm. T h e concentration of hafnium for both lines is 1000 ppm. Both concentrations were on the linear part of an analytical curve. T h e peak locations of maximum emission for both hafnium lines occur a t about 2.54 m m while the peak locations for the neutral atom line and the ion line for calcium are separated. The relative locations of the peaks for the calcium ion line and the neutral atom line might be partially explained by the competition of the calcium ion and neutral atom for the total available calcium in the arc that has a large temperature gradient from its central 2- to 4-mm diameter core to its outer regions. As the central region of the arc is approached, a larger fraction of calcium atoms becomes ionized causing a decrease in the emission from the atoms and a n increase in emission from the singly charged calcium ions. As the central region is approached more closely, the singly charged ion loses another electron resulting in a decrease in its emission signal. Other factors such as self absorption, molecule and radical formation, and changes in diffusion coefficients with temperature and charge undoubtedly complicate the situation. For instance, in contrast to the separated calcium atom and ion peaks, the coincidence of the positions of the hafnium ion line emission peak with the neutral atom line emission peak might be due primarily to oxide formation rather t h a n ionization. Hafnium forms a refractive
oxide HfO with a boiling point over 5000 "C (3) and a dissociation potential greater t h a n 7 eV (2). By comparison, the monoxides of calcium and copper have dissociation potentials of 3.9 and 4.9 eV, respectively ( 4 ) . In the arc, there is a n appreciable amount of oxygen introduced with the water. Calculations based on flow rates and ideal gas law expansion suggest that water vapor introduced in the form of the sample aerosol makes u p an average of about 3% of the plasma atmosphere. Two different processes involving HfO can be imagined. One would be an equilibrium situation where the concentration of oxygen in the arc would be important in determining the fraction of hafnium that is tied up as oxide. On the other hand, particles of hafnium oxide that result upon droplet desolvation in the arc may be readily vaporized by the arc. A large fraction of the hafnium might be tied up in such particles until they had time to vaporize or until they reached the more energetic regions of the arc. As an example of a problem caused by slow particle vaporization, the interference of phosphate a t 40 ppm was studied and found to suppress the emission of the 422.67nm line of calcium a t 10 ppm in the cooler regions of the arc. A t a position of 6.35 m m from the center of the arc, phosphate reduced the signal by 25% while a t a nearby position of 5.08 m m , the reduction of the calcium line was only 2%. Apparently the arc is energetic enough to dissociate calcium phosphate. once t h e sample moves in from the very outer regions where it is initially introduced. Figure 7 shows analytical curves for hafnium, 339.98 nm, and for copper, 324.75 nm. Least squares fits gave slopes of 1.00 f 0.02 and 1.04 f 0.01, respectively, for the log-log plots of concentration us. photocurrent. A slope of (3) R. C. Weast. Editor-in-Chief, "Handbook of Chemistry and Physics.' 51st ed , The Chemical Rubber Co., Cleveland, Ohio, 1970. p 894. ( 4 ) P. W. J. M . Boumans, "Theory of Spectrochemical Excitation." Plenum Press, New York. N . Y . , 1966, p 383.
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 6 , M A Y 1 9 7 4
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1.00 indicates t h a t the relationship between the emission signal and concentration is linear and passes through the origin. The detection limits for calcium, copper, and hafnium are shown in Table I. The detection limits are defined as the concentrations of the element in solution which when aspirated into the plasma gives emission signals equal to twice the standard deviation in the background measurements ( 5 ) . The standard deviation was obtained as Y5 of the peak-to-peak noise level determined over a period of time equal to many time constants. Flame atomic absorption and emission detection limits are included in Table I for rough comparison purposes. (5) T. J. Vickers and J. D . Wlnefordner, "Flame Spectrometry," in "Analytical Emission Spectroscopy," Part I / , E. L. Grove, Ed., Marcei Dekker, Inc , New York, N.Y , 1972. o, 333 (6) 0. Menis and T. C. Rains. in "Analytical Flame Spectroscopy," R. Mavrodineanu, Ed., Macmilian and Co. Ltd., London, New York, 1970, p 60. (7) R . L. Warren, Analyst (London), 90, 549 (1965). (8) G. D. Christian and F. J. Feldman, "Atomic Absorption Spectroscopy." Wiiey-Interscience, New York, N.Y.. 1970, p 174. (9) S. Slavin, W. B. Barnett, and H. L. Kahn, At. Absorption Newslett.. 11, (2) 38 (1972). ~~~
There are at least two important factors other t h a n the intensity of a line which affect the detection limits. The emission from the center of the arc is very sensitive to minor current fluctuations. Therefore, as the position for observation of a n analyte line moves toward the center of the arc, these fluctuations in the background emission can become a limiting factor. Thus, although Figure 6 shows the peak intensities for Ca(1) a t 3.8 mm and Ca(I1) at 1.9 m m to be about equal, the detection limit for the Ca(I1) line a t 1.9 mm is 50 times poorer than the detection limit for the Ca(1) line. A second important factor is the effect of the spectral lines neighboring the line of interest. Although, for a concentration of 1000 ppm, the hafnium ion line is over six times as intense as the neutral atom line, Figure 6, its detection limit is only four times better than the detection limit for the neutral atom line. The ion line is in the region of OH band emission while the neutral atom line is found in a "spectrally clean" region of the arc. Received for review June 18, 1973. Accepted December 18, 1973.
High Resolution Atomic Absorption Spectrometry Using an Echelle Grating Monochromator Peter N. Keliher and Charles C. Wohlers Chemistry Department, Villanova University, Villanova, Pa., 19085
A direct comparison is made, using a high resolution echelle spectrometer, between atomic absorption using a line source (hollow cathode lamps), and atomic absorption using a continuum source (150-W xenon lamp). Sensitivities using the continuum source tend to be less than those using a line source, but not to any great extent. The effective spectral bandwidth using a continuum source with the echelle spectrometer is wider than for a line source but much narrower than that used in most previous studies of atomic absorption with continuum sources. Linear calibration curves, having slopes of approximately one, were obtained with the continuum source when the echelle system was used, but when medium resolution instrumentation was employed, in conjunction with the continuum source, much lower slopes and sensitivities were observed.
Since the introduction of atomic absorption spectrometry as an analytical technique by Walsh ( I ) in 1955, it has been generally assumed t h a t a sharp line spectral source, usually a hollow cathode lamp (HCL), was necessary for the practical utilization of atomic absorption spectrometry. The objections raised by Walsh ( I ) to the use of a continuum source were, first, that the very narrow width of the absorption profiles would require a monochromator with a resolution of a t least 500,000 (which would be much larger and more inconvenient to use than conventional spectrometers), and, second, that it was questionable whether the continuum source used would provide
enough radiation over so small a wavelength range (approximately 0.01 A) to give a favorable signal-to-noise ratio. With the introduction of high resolution, high luminosity monochromators using the echelle grating ( 2 - 7 ) . however. it is possible that the objections mentioned by Walsh ( 1 ) have been overcome. It is the objective of this paper to discover to what extent this may be true. The use of a continuum source in atomic absorption spectrometry enjoys certain potential advantages over spectral line sources, certainly the most obvious of which is the advantage of using only one source for a variety of elements, thus saving considerable expense. Also, a continuum source would be highly useful in qualitative analysis and would make background correction simpler. In order to exploit some of these advantages, a number of workers have employed continuum sources with medium resolution instrumentation (8-12). Although the spectral bandpass employed was always much greater than the (2) (3) (4) (5)
(6) (7)
(8) (9) (10) (11) (12)
( 1 ) A Walsh, Specirochm Acta 7, 108 (1955)
682
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G. R . Harrison, J . Opt. SOC.Amer.. 39, 522 (1949). D. Richardson, Specfrochim. Acfa, 6, 61 (1953) W. G . Elliott, Amer. Lab., 2 (3). 67 (1970) M. S. Cresser, P. N. Keliher, and C . C. Wohlers. Spectrosc. Lett.. 3, 179 (1970). F. L. Corcoran, Jr., P. N. Keliher, and C. C. Wohlers, Amer. Lab., 4 ( 3 ) ,51 (1972). M S Cresser, P. N . Keliher, and C C. Wohlers, Anal. Chem . 45, 111 (1973). J. H . Gibson, W. E. L Grossman, and W. D. Cooke, Appi. Spectrosc., 16, 47 (1962). N. P. lvanovand N. A . Kozireva, Zh. Anal. Khim., 19, 1266 (1964). V . A Fassel, V. G Mossotti, W E. L. Grossman, and R . N Kniseley, Spectrochim. Acta. 22, 347 (1966) W. W . McGee and J. D . Winefordner, Anal. Chim. Acfa. 37, 429 (1967). C W. Frank, W G Schrenk, and C. E. Meloan, Anal. Chem. 39, 534 (1967).
