Relative standard deviations for a group of 14 elements included in this study have been reported by the author to range between 5 and 20% (4). Precision and accuracy attainable by this technique are dependent to a large extent on the selection of appropriate standards‘ The Of mo17ingplate studies Provide a criterion for the choice of internal standards.
(5) Nelson, R. C., Mines Mag. 47, 68 (1957). (6) Peterson, M. J., Chaney, C. L., U.S. B ~ ~i~~~ ~ . R ~znUest., ~ . 5903 (1961). (7) Tables of Spectral-Line Intensities, NBS Monograph 32, Part 11, U. S. Dept. of Commerce, 1961. RUDOLPH DYCK
LITERATURE CITED
( 1 ) DYck, R., Veleker, T. J., ANAL. CHEM.31, 390 (1959). (2) Gentry, c, H. R., Mitchell, G , p., ~ ~ t ~46, l47 l(1952). ~ ~ ~ i ~ ( 3 ) Laun, D. D., J . Res. Nut. Bur. Std., A . Physics and Chemistry 6% 207 (1964). (4) “Methods for Emission Spectrochemical Analysis, 1964,” p. 565, ASTM, Philadelphia, Pa., 1964.
Chemical and Metallurgical Division Sylvania Electric Products, Inc. Towanda, Pa.
High Sensitivity Determination of Zinc, Cadmium, Mercury, Thallium, Gallium, a n d Indium by Ato mic Fluorescence Flame Spectrometry SIR: By means of optimization of experimental parameters, it is possible to increase considerably the sensitivity for the determination of zinc, cadmium, mercury, thallium, indium, and gallium by atomic fluorescence flame spectrometry.(1-3) Optimization of sources, optics, and oxyhydrogen flames for atomic fluorescence flame spectrometry is discussed and present detection limits and experimental analytical curves are given. EXPERIMENTAL
The instrumental setup developed and used in this study is shown schematically in Figure 1 . This arrangement is similar to the one described by Winefordner and Vickers (3) and the one used by Winefordner and Staab ( I , 2). Several notably different features will be described. These changes were necessary to further increase the sensitivity of analysis. The instrumental setup consists of a Caerny-Turner grating monochromator (No. 4-8400, American Instrument Company, Silver Spring, Md.) equipped with interchangeable 3000 A. blaze and 5000 A. blaze gratings (Aminco Nos. A1 1-61041 and A248-61041), an adjustable slit
mechanism (Aminco No. D42-61041) and an entrance field lens (Aminco No. A83-61041). The entrance aperture was fitted with an aluminum stop tube of 0.9-cm. i.d. and about 1 cm. long. A baffle box, which was effective in excluding room light and other stray radiation from the entrance slit, was built from 1/s2-inch sheet aluminum. The box was 2 inches X 3 inches X 4 inches with 1 inch diameter stops in opposite sides for passing source illumination through and was fitted on the inside with deflectors for directing stray light away from the slit. All of these entrance-optical components were painted flat black and were rigidly mounted in the orientation shown in Figure 1. The optical bench with optical components and the atomizer-burner were similar to those previously ( 2 ) used and were arranged as shown in Figure 1. Figure 2 shows schematically the more significant details of the optical system. The quartz lens in Figure 2 was mounted approximately 3 inches from the burner axis and the sources were mounted on the optical bench 5 inches from the lens. A 1P28 RCA photomultiplier tube, a photomultiplier microphotometer (Aminco No. 10-213) and a 10 millivolt recorder were used to amplify and record signals. Sources employed in this
study were Osram and Philips metal vapor discharge lamps (1, 2 ) and electrodeless discharge tubes (Ophthos Instrument Co., Rockville, Md.). The Osram and Philips lamps were operated from a 220 volt step-up transformer in conjunction with a constant voltage regulator (No. 20-13150, Sola Electric Co., Chicago, Ill.). The lamp operating current was controlled with series-connected power resistors. Electrodeless discharge tubes were operated from the same Rf power supply described by Winefordner and Staab ( 2 ) ) except that an Evenson type Rf cavity (Ophthos Instrument Co.) was found to be superior to the conventional cylindrical upright cavity for the mercury discharge tube. In this study, each lamp was optimized by monitoring an atomic fluorescence signal at a constant metal concentration while varying only the lamp operating current. The optimum lamp currents for the metal discharge tubes are taken as those currents which give the maximum fluorescence signalto-noise ratio. The optimum lamp currents for the Osram and Philips lamps are listed in Table I. The Philips cadmium and zinc lamps mere only slightly less intense than the corresponding Osram lamps-probably be~~
~
Table I.
Metal Cadmium
Limit of detection (p.p.m.) 0.0002
Zinc
0.0001
Mercury
0 .I
Thallium Indiumb
0.04 10
Galliumb
10
a
b
Optimum Experimental Conditions” and Limits of Detection for Several Elements
Source Philips lamp Osram lamp Philips lamp Osram lamp Ophthos electrodeless discharge tube Philips lamp Electrodeless discharge tube Electrodeless discharge tube
Source current or % power 0.4-0.5 amp. 0 . 8 amp. 0 . 6 - 0 . 7 amp. 1 , 4 amp. 9% power air-cooled
Height in flame above burner tip (cm.) 6-6.5
Slit width (mm.) 2.0
Spectral line (‘4.1 2288
5.5-6.5
2.0
2139
8
1.0
2537
0 . 7 5 amp. 25-5070
64.5 5-6
0.20 1.0
3776 4105
20-4070
5-6
1.0
4172
Power air-cooled
Power air-cooled Optimum H2 flow rate was 7000 cc./min. and optimum 0 2 flow rate was 2500 cc./min. for all analyses. Conditions not optimized, as intensities were not great enough to seem of analytical importance.
VOL. 37, NO. 8, JULY 1965
1049
TRANSFORMER CONSTANT VOLTAQE REQULATOR \
1 Figure 1. Block diagram of atomic fluorescence spectrometry setup
cause the Osram lamps were used with holes cut in their outer glass envelopes, while the Philips lamps were used as supplied by the manufacturer with their outer quartz envelopes left intact. Approximately the same limits of detection could be obtained with either type. The electrodeless discharge tubes were optimized similarly, but two parameters were available in this case% power and rate of air cooling. The optimum conditions for the mercury, indium, and gallium electrodeless discharge tubes are given in Table I. However in this case, the optimum power was quite dependent on the rate of cooling of the Rf cavity and so a range of powers are given in Table I. For each of the elements studied, a fuel-rich Hz/Oz flame was found to give the best signal-to-noise ratio.
Figure 2. Schematic diagram entrance optics (front view)
RESULTS A N D DISCUSSION
The limits of detection are listed in Table I along with the optimum operating conditions. The limit of detection is the lowest concentration which gives a detectable fluorescence signal above the background noise and is defined in the same manner as previously ( I , 2 ) . Table I1 lists relative intensities, I,, standard deviations, u , and signal-to-noise ratios, S/N, for several concentrations of zinc, cadmium, thallium, and mercury. The limits of detection for aqueous solutions of these metals in a Hz/Oz flame may be compared with those obtained by Winefordner and Staab ( 2 ) ,which were: zinc, 0.04 p.p.m.; cadmium, 0.1 p.p.m.; mercury, 0.1 p.p.m.; and thallium, 1.0
Table II. Tabulation of Typical Relative Fluorescence Intensities, IF, Standard Deviations, u, % Standard Deviations, % u, and Signal-to-Noise Ratios, S/N, for Cadmium, Mercury, Thallium, and Zinc
Cd 2288 A. Concn., p.p.m.
0 0 0 0 1 10
7,
0.01 0.44 3.14 29,96 331,7 1837
1 0 0 0
0.13 1.72 18.68 179.0
0 04
1 0 0 0
0.20 0.32 3.07 31.18 287.5
0001 010 0 0
0.067 1.11 95.25 693.7
0 1 10 100
0 1 10 100
0 0 1 10
1050
0001 001 01 10 0 0
ANALYTICAL CHEMISTRY
U
0.054 0.064 0.101 0.28 5.72 119 Hg 2537 A. 0.057 0.047 0.089 0.93 T1 3776 A. 0.050 0.088 0 095 0.92 5 4 Zn 2139 A. 0.04 0.075 2.24 35.9
%.
so/% 4.88
73 14.6 3.2 2.8 1.7 6.5
18.1 64.8 67.2 48.7
44 2.7 0.48 0.52
0.92 11.4 58.2 56.2
25 28 3.1 3.0 1.9
1.2 1.5 9.9 23 27
55 6.7 2.3 5.2
0.89 10.4 23 26
of
atomic
fluorescence
p.p.m. The significantly lower detection limits for zinc, 0.0001 p.p.m.; cadmium, 0.0002 p.p.m.; and thallium, 0.04 p.p.m., and the detection of the gallium and indium fluorescence obtained in this study can be attributed to a combination of improved optics and instrumentation, improved flame conditions and optimization of sources. Although the limit of detection for mercury (2537 A , ) remains unchanged, the data for the lower concentrations appear to be more reliable and useful all the way down to the detection limit, as may be seen in Figure 3. Curves of fluorescence signal-to-noise ratio us. monochromator slit width a t various heights were plotted for each spectral line of each element obtained by introducing dilute solutions of the desired element into the flame type of interest. These curves were used in order to obtain the optimum flame type, monochromator slit width and flame height for atomic fluorescence measurements for each element. Optimization of the flame parameters for the T1 3776 A . fluorescence line was less straightforward than for the other
t
I
0 0001 0001
001
ELEMENT
01
I
IO
CONCENTRATION,
100
IO00
P P U
Figure 3. Experimental analytical curves for zinc (Zn 21 39 A.), cadmium (Cd 2 2 8 8 A.), mercury (Hg 2 5 3 7 A.), and thallium (TI 3 7 7 6 A.) Each point, other than those taken from Table 11, represents an average of three readings
elements listed in Table I. The atomic concentration of thallium appeared to fall off very rapidly above the luminous flame tip; and in the lower flame regions the intensity of thermal emission actually became greater than the intensity of fluorescence. Several combinations of flame height and slit width were found for T1 3776 A4.which yielded about the same detection limit. At flame heights between 4 and 51/2cm. in the hydrogen rich Hz/02 flame, the fluorescence intensity was sufficient to obtain the data for thallium listed in Tables I and I1 lT-ithout relying upon a significant intensity contribution from thermal emission. Although the thermal emission became considerably greater in intenbity than fluorescence a t about 1 cm. above the burner tip, the high background noise level which was present prevented rnaking measurements there. This is probably why the detection limit, 0.04 p.p.m. obtained from the relatively weak fluorescence signal in the upper part of the flame compares favorably with the best detection limits u hich have been published for thermal emission. Other lines of thallium which exhibited fluorescence in the order of decreasing intensities are: 5350 A . , 3519 A., 2538 A. > 3530 A., 2768 A, 2580 A. > 2380A.
With electrodeless discharge tubes for sources, approximately 10 p.p.m. of both gallium and indium could be detected at the 4172 A. and 4105 A. wavelengths, respectively. Lines of gallium which exhibited fluorescence are in the following order of relative intensities:4172A. > 4033A. > 2874A. Likewise, for the case of indium, the lines are: 4105 A. > 3256 A. > 3259 A. > 3039 A. Further work is presently being done in this laboratory to produce intense, unreversed sources for these and other elements. Analytical curves for zinc, cadmium, mercury, and thallium are given in Figure 3. I t should be noted that the atomic fluorescence analytical curves are almost linear from the limits of detection to concentrations about lo5 times greater than the limits of detectione.g., the analytical curves are nearly linear over the range 0.0001 to 10 p.p.m. for zinc, 0.0002 to 10 p.p.m. for cadmium, 0.1 to over 1000 p.p.m. for mercury and 0.04 to over 1000 p.p.m. for thallium. It would certainly seem that atomic fluorescence flame spectrometry has promise as a n analytical tool. The great range of linearity and the very low limiting detectable concentrations for zinc, cadmium, mercury, and thallium are certainly two of the most out-
standing features of this new method of analysis. Few methods can claim such sensitivities along with such ranges of near linearity. I n addition, simple and inexpensive experimental equipment was used to obtain the data. At the present time, instrumental modifications are being made, new electrodeless lamps are being made, and new flame types are being studied in order to determine the possibilities for the atomic fluorescence flame spectrometric analysis of some of the transition metals, the alkaline earths and some of the soft metals (lead, copper, silver, gold, etc.), LITERATURE CITED
(1) Winefordner, J. D., Staab, R. A,,
ANAL.CHEM.36, 165 (1964). (2) Zbid., p. 1367. (3) Winefordner, J. D., Vickers, T. J., ANAL.CHEM.36, 161 (1964). J. M. MANSFIELD J. D. WINEFORDNER Department of Chemistry rciversity of Florida Gainesville, Fla. CLAUDE VEILLON National Bureau of Standards Washington, D. C. WORKsupported in part by the Xational Science Foundation, Grant No. NSFGP2481. One of the authors, (J.M.M.) was supported by a S A S A traineeship.
Determination of Allyl Chloroformate and Diethylene Glycol Bis(Ch1oroformate) Impurities in Diethylene Glycol Bi~(AllylCarbonate) Monomer by Gas Chromatography SIR: I n establishing the purity of diethylene glycol bis(ally1 carbonate) monomer, the determination of some organo-halogenated impurities is extremely important since subsequent color formation in its normally transparent, colorless polymerization products has been directly attributed to residual contaminants a t 0.10 to O . O O l ~ o concentration levels ( 3 ) . Since the monomer can be prepared by either of two organic synthesis routes (4, 5 ) , the two most probable chloroformate materials present as impurities are either allyl chloroformate or diethylene glycol bis(ch1oroformate). I n one process, the formation of allyl chloroformate results from passing gaseous phosgene under prescribed operating conditions through allyl alcohol. After the reaction mixture has been water-washed to remove excess alcohol and dried, the unsaturated chloroformate is reacted with diethylene glycol in the presence of an alkaline reagent, yielding the desired alkyl dicarbonate ester. Alternately, diethylene
glycol bis(ch1oroformate) may be initially prepared and subsequently added to the unsaturated alcohol to form diethylene glycol bis(ally1 carbonate). Whereas Gudzinowicz and Driscoll ( 1 ) showed in 1961 that multicomponent dialkyl carbonate mixtures could be separated and analyzed quantitatively by gas chromatography without decomposition, Hishta and Bomstein ( 2 ) developed a n indirect analytical method for benzyl chloroformate and other acid chlorides in which the diethyl amide derivatives of these compounds were formed prior to analysis with a thermal conductivity detector. Although no mention had been made relative to the thermal stability and direct gas chromatographic behavior of benzyl chloroformate, they noted that the direct determination of the acid chlorides was not adopted since these decomposed during passage through the copper column. The purpose of this paper, therefore, is to show that the high molecular weight alkyl dicarbonate ester (not to be
confused with previously chromatographed dialkyl carbonates), allyl chloroformate and diethylene glycol bis(ch1oroformate) , can be analyzed by gas chromatography without decomposition with glass columns; the allyl and diethylene glycol chloroformates, separated at column temperatures of 25' and 133' C., respectively, yielding linear peak height/concentration relationships for 0.05- to 0.25-pg. chloroformate injections. EXPERIMENTAL
The Jarrell-Ash Model 28-710 gas chromatograph equipped with a flame ionization detector and a Bristol Dynamaster Model 1P12H560, 0- to 10-mv., 11-inch strip-chart recorder was used for these investigations. The chromatographic column was made of borosilicate glass (6-ft. by 3/16-in~h0.d.) packed with 5y0 by weight of SE-30 silicone gum rubber on 80/90 mesh Anakrom AS (Analabs, Inc., Hamden, Conn.). Prior to use the column was conditioned at 250' C. for 24 hours. VOL. 37, NO. 8, JULY 1965
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