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
1051
M y l Chl of rn
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9 .
8
.
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20
18
16
14
12
10
8
RETENTION TIME, MINS.
6
4
2
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Figure 1. Superimposed gas chromatograms for three diluted monomer samples at 133" C.
Flame ionization detector sensitivity settings of 1 X and 1 X ampere were used in addition to the following chromatographic operating conditions: hydrogen gas pressure, 5.5 p s i ; air flowrate, 1.25 c.f.h.; column temperature, 25" and 133" "C.; detector temperature, 280" OC.; injector temperature, 265' "C.;nitrogen carrier gas flowrate, 95.0 cc./min.; and recorder chart speed, 2.0 min./inch. One - microliter Hamilton microsyringe injections of varying allyl chloroformate and diethylene glycol bis(ch1oroformate) concentrations dissolved in chloroform as solvent were made directly onto the coated solid support, thereby eliminating the possibility of catalytic degradation a t metallic surfaces. I n contrast to acetone, chloroform was preferred as solvent since it yields a smaller, narrower peak with the flame detector at a 1 X 10-'0 ampere sensitivity setting for an equivalent amount of material injected. Since flame sensitivity is directly related to carbon content, its response is further inhibited by the presence of chlorine atoms; this decreased response permits better resolution between component and solvent peaks. DISCUSSION OF RESULTS
Using the conditions cited in the experimental section for the flame ionization detector, typical chromato1052
ANALYTICAL CHEMISTRY
grams obtained at 133" C. and a 1 X low8ampere sensitivity setting for several commercial monomer samples (diluted 1 : 4 by volume) are shown in Figure 1; the diethylene glycol bis(chloroformate) and diethylene glycol bis(ally1 carbonate) peaks identified as A and B, respectively. Whereas samples WT-3-11-4 and WT-3-11B have little if any residual chloroformate (Figure l ) , sample WT3 - l l C shows that a considerable amount of this material is present in addition to five other contaminants. When increased amounts of this sample (without dilution) are introduced into the chromatograph, these peaks are enhanced with the appearance of two other trace impurities noted in its chromatograms between peak 5 and that of the monomer. The peak heights of all impurities also increase linearly as the amount of sample injected is increased. To establish the retention time of and the characteristic peak expected for diethylene glycol bis(chloroformate), chromatograms for this material and the diluted sample WT-3-11B used for the data in Figure 1 to which some diethylene glycol bis(ch1oroformate) has been added were obtained. The chloroformate chromatogram showed this so-called pure compound contained two trace components which were also
I
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: *
6
,
4 2 RET. TIME, MINS.
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Figure 2. Superimposed chromatograms of allyl chloroformate at 25" C. Allyl chloroformate (undiluted) a t 1 X ampere 8. Allyl chloroformate, 0 . 2 2 4 pg., a t 1 1 O-'O ampere C. Allyl chloroformate, 0 . 2 2 4 pg., at 1 1 0 - 8 ampere
A.
X
X
observed in the spiked monomer sample. At a sensitivity setting of 1 x 10-10 ampere, a linear calibration curve for diethylene glycol bis(ch1oroformate) relating peak height to chloroformate concentration (0.05 to 0.25 fig.) per 1 pl. injection is obtained. Using this calibration curve, the analysis of a synthetically prepared monomer sample containing 0.1970/, by weight of chloro-
formate showed the chloroformate content to be 0.190%, this value being 96.670 of its theoretical content. For the determination of allyl chloroformate (b.p. = 110' C.), a column temperature of 25' C. is required. I n Figure 2, Curve A is the chromatogram for allyl chloroformate (without solvent dilution) a t 1 X 10-9 ampere whereas Curves B and C represent the chromatographic peaks obtained for 0.224 pg. of allyl chloroforniate a t sensitivity and 1 X settings of 1 X ampere, respectively. At the 1 X ampere setting, the allyl chloroformate calibration curve is also linear over the 0.05 to 0.25 pg. concentration range investigated. As noted in this study, little or no
decomposition was observed for the chloroformates as well as the organic dicarbonate ester monomer which lends itself readily to quantitative analysis a t 133" C. Furthermore, 0.05 pg. of either allyl chloroformate or diethylene glycol bis(ch1oroformate) per 1 p1. of injection yields an easily observed and measurable peak height. Lower detection limits for these materials are possible by increasing the volume of material injected into the chromatograph. ACKNOWLEDGMENT
The author thanks J. P. R. Levesque, American Optical Company, Southbridge, Mass., for the commercial monomer samples and the allyl chloro-
Microboiling Point Determination at Thermal Analysis SIR: The use of differential thermal analysis (DTA) for determining boiling points a t atmospheric pressure is well known; see for example the work of Vassallo and Harden (6). Extension of the technique to subatmospheric pressures was suggested by Krawetz and Tovrog, who cited data on toluene in the 65-760 torr range (6). Their boiling temperature was the algebraic sum of the cell temperature a t the endothermal boiling peak and the peak height in degrees centigrade, usually 2-10'C. This work demonstrates the use of DTA, employing two different calculation techniques to determine boiling points, over a pressure range of 30-760 torr for hydrocarbon samples with atmospheric boiling points of 175'-325' C. Sample sizes are such that fractions from analytical gas chromatographs may be used. EXPERIMENTAL
The differential thermograph and auxiliary equipment used in this study have been described previously (8-4). The heating rate used in all cases was 8' C. per minute. Purified carborundum (4) was used as a diluting agent and as the inert DTA reference material. Using a calibrated medicine dropper, a 0.02-ml. sample was placed in a DTA sample tube containing 0.15 gram of 500-mesh carborundum. The powder was mixed intimately with the liquid using a quartz rod 1 mm. in diameter. A semidry powder with the liquid spread evenly over the granules was obtained after a few moments mixing. The ceramic thermocouple probe was introduced and the sample compacted gently. A 0.15-gram carborundum reference was prepared by the same com-
LITERATURE CITED
( 1 ) Gudzinowicz, B. J., Driscoll, J. L., ANAL.CHEM.33, 1508 (1961). (2) Hishta, C., Bomstein, J., Ibid., 35, 65 (1963). (3) Levesque, J. P. R., American Optical Go., Southbridge, Mass., private communication, January 1965. (4) Muskat, I. E., Strain, F. (to Pittsburgh Plate Glass Co.), U. S. Patent 2,370,571 (Feb. 27, 1945). (5) Zbid., U. S. Patent 2,384,115 (Sept. 4, 1945).
BENJAMIN J. GUDZINOWICZ Research Department Jarrell-Ash Co. Waltham, Mass.
30 to 760 Torr by Differential
paction technique. The thermograms were run in nitrogen using a mercury manometer to determine the system pressure within the bell jar containing the DTA cell (4). American Petroleum Institute Project 44 n-Clo, n-Cll, n-C1*, n-Cla, n-C14, n-Clb, n-decylbenzene, and Eastman Kodak White Label 1-phenyldodecane were used. The normal boiling point values for these compounds are shown in Table I. Boiling points by DTA were measured by extending the base line, prior to the boiling endotherm, into the boiling point region. Another straight, line was drawn on the chart through the steepest (and most linear portion) of the boiling endotherm. The intercept of these two straight lines was taken as the boiling point. The temperature a t the intercept was determined by measuring its distance from the recorder zero with a vernier rule. The distance was translated into millivolts using a recorder factor plus the known bucking potential. Thermocoudes had been calibrated as described previously (4).
Table I.
formate and diethylene glycol bis(chloroformate) compounds used as standards.
The effect of sample size on apparent boiling point by DTA was evaluated using n-decylbenzene in the concentration range 0.01-0.02 mlJ0.15 gram carborundum. The locat'ion of the boiling point was unaffected from 0.010.07 m1./0.15 gram sample on the carborundum. Duplicate runs on 0.01, 0.02, 0.04, and 0.06 m1./0.15 gram samples were repeatable to =t0.5' C. The extremely small sample size permits measurements of boiling points on fractions collected from a conventional analytical gas chromatograph. Use of such fractions provides a rapid method of collecting high purity samples not readily available from other sources. RESULTS AND DISCUSSION
The DTA data were examined both by reading the chart directly and using only thermocouple calibration factors and by bracketing with API hydrocarbons. The latter treatment removes any possible error arising from thermocoude calibration. The bracketed
Comparison of Normal Boiling Points Determined by Differential Thermal Analysis with Literature Values
Compound Literature ( 1 ) n-Decane 174 12 n-Undecane 195 89 n-Dodecane 216 278 n-Tridecane 235 434 n-Tetradecane" 253 515 n-Decylbenzene 297 88 n-Octadecane 316 33 1-Phenyldodecane 327 611 a Phillips 99 mole per cent. * Bracketed by DTA.
Normal boiling point, "C. Found by DTA 173 93 195 90 216 65 235 48 252 92 297 82 316.23 -f: 0 09* 327 70
Error -0
+o +o fO -0 -0 -0
+o
VOL. 37, NO. 8, JULY 1965
19 01 37 05 59 16 10 09
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