(4) DeSouza, J. E., Scherbak, M., Analyst 89, 735 (1964). (5) Grimmer, von G., Beitr. TabakForsch. 1. 107 (1961). (6) Hoffm&--D. H., Wynder, E. L., ANAL.CHEM.32, 295 (1960). (7) . . Hoffmann. D. H.. Wvnder, E. L., Cancer 13, 1062 (1960). ” (8) Oliver, B. J., Jr., “Thin-Layer Chro\-
- - I
matoeraDhic SeDaration of Benzo(ab from Cigarette T&,” 19ih Tobacco Chemists Research Conference, Lexington, Ky. (Oct. 26, 1965). (9) Pailer, M., Hubsch, W., Kuhn, H., -
pyrene
1
~
Fachllche Mitt. Oesterr: Tabakregie 1-11; (1965); Abstract #1795, Corresta
Information Bulletin, No. 3, p. 32
(1965). (10) Robb, E. W., Governator, G. C., Edmonds. M. D.. Bavelv. A.. Beitr. Tabak-Fokch. 3, 278 (i966j. ’ (11) Sawicki, E., Hauser, T. R Stanley, T. W., Intern. J. Air Pollut&n 2, 253 (1960): (12) Sawicki, E., Stanley, T. W., Elbert, W. C.. Pfaff. J. D.. ANAL. CHEM.36. 497 (i964). ‘ (13) Schmetz, I., Stedman, R. L., Chamberlain, W. J., Ibid., p. 2499. (14) “Smoking and Health,” report of Advisory C-ommittee to Surgeon Genera1 of Public Health Service, U. S.
Public Health Service Publication No. 1103, U. S. Govt. Print. Office, Washington, D. C., 1964. (15) Van Duuren, B. L., J. Natl. Cancer
Inst. 21, 1 (1958). (16) Ibid., p. 623. (17) Wynder, E. L., Hoffmann, D., Advances in Cancer Research 8, pp. 249453, Academic Press, New York, 1964.
HOWARD J. DAVIS LEONARD A. LEE THOMAS R. DAVIDSON Celanese Research Co. Summit, N. J.
Rapid Determination of Molecular Weight Distribution of Polyoxyethyle ne-Type Nonionic Surfacta nts by Ci rcula r Thin Layer Chromatog ra phy SIR: In the preparation of a polyoxyethylene-type nonionic surfactant by the addition of ethylene oxide to a molecule containing an active hydrogen, the product is always a mixture in terms of the number of moles of ethylene oxide added to a molecule. This phenomenon gives rise to many important problems not only in the study of the physical properties of the material but also in practical applications. Flory (2) suggested that the molecular weight distribution corresponds to a Poisson distribution. Previously, a molecular distribution (3, 5 ) was used to obtain the molecular weight distribution of nonionic surfactants. With materials of a low polymerization degree, the distribution was compared with the Poisson distribution or Weibull’s by Nagase ( 7 ) . Kelly (4) separated a mixed para-, tert-octylphenoxy-polyoqethyleneethanol with 9.7 ethylene oxide units per phenol into its component compounds by elution chromatography and obtained a molecular weight distribution curve which corresponded to a Poisson distribution, Thus, an accurate method has been established for the determination of the molecular weight distribution of polyoxyethylene-type nonionic surfactants, However, this elution chromatographic determination requires more than 15 days for the separation of one sample, Burger (1) reported a rapid separation of polyoxyethylene-type nonionic surfactants into their component compounds by thin layer chromatography. However, a molecular weight distribution curve has not been reported. The object of this work was to develop a method for a rapid determination of the molecular weight distribution of polyoxyethylene-type nonionic surfactants. Polyo.xyethylene-type nonionic surfactants were separated into their component compounds by circular thin
Figure ment
1.
Apparatus for
develop-
1, separatory funnel; 2, developing solvent; 3, stopper; 4, glass tube (35-mm. diameter, 80-mm. length); 5, capillary; 6, circular glass plate (200-mm. diameter) with a hole (20-mm. diameter); 7, filter paper containing deveioping solvent; 8, adhesive; 9, ring made of 5-mm. acryl-resin plate; 10, chromatoplate ( 2 0 0 X 2 0 0 mm.); 1 1 , stand
layer chromatography (CTLC) and the visualized chromatograms were photographed. The transmittance of the film was measured and the molecular weight distribution curve could be constructed readily by a simple calibration of the density distribution curve. The resultant molecular weight distribution curve is compared with the Poisson distribution curve. Successful results were obtained with polyoxyethylene nonylphenolether (p = 9). EXPERIMENTAL
Apparatus. A Stahl-type apparatus (Desaga) for thin layer chromatography and 20- X 20-cm. glass plates were used. An apparatus for circular thin layer development was made in our laboratory according to bhe procedures described by Musha (6),and is shown in Figure 1. A micro-
photometer (Shimazu Seisakusho Ltd., %type) was used for the transmittance measurements. A vapor pressure osmometer (Mechrolah Inc. Model 301A) was used for the molecular weight determinations. Reagents and Materials. A 1% iodine solution was made by dissolving 2.5 grams of iodine in 250 ml. of methanol. Silicic acid (Wako Gel B-0 suitable for thin layer chromatography) was used as an adsorbent with the addition of 0.5% carboxymethylcellulose (CMC). Special grade methyl ethyl ketone saturated with distilled water a t room temperature was used as a developing agent. Polyoxyethylene nonylphenolether (p = 9) was prepared in our laboratory. Polyoxyethylene glycols which are present as by-products were removed by a counter-current distribution extraction (8) using a n-butanol-water system. Five hundred milligrams of the refined polyoxyethylene-type nonionic surfactants were dissolved in 5 ml. of methanol and used as the sample solution. Polyoxyethylene nonylphenolethers, with 8, 9, and 10 mole ethylene oxide units, were prepared by the elution chromatographic separations and these were used as standard reagents. Minicopy film (Fuji Film Co., Ltd. ASA32) was used to take photographs of the chromatograms. Preparation of Chromatoplates. Thin layer chromatoplates (20 x 20 em.) were prepared from silicic acid by mixing 30 grams of dry powder and 150 mg. of CMC with 60 ml. of water and applying this onto the glass plates with a ~piwtderset a t a thickness of 250 microns. The plates were air-dried for 30 minutes and heated in an oven at 105’ C. for 2 hours. After drying, the layers were stored in a dry box. Sample Application and Circular Development. Fifty microliters of the
sample solution were applied through a micropipet to the center of a layer in a ring (about 20 mm. in diameter). The chromatoplate and the developing V O L 38, NO. 12, NOVEMBER 1 9 6 6
1755
Circular thin layer chro. matogram of polyoxyethylene nonyl. phenolether (p = 9 ) Figure 2.
carried out. The inside'of the developing chamber was filled with the solvent vapor before development by placing a filter paper impregnated with the solvent in the chamber. The rate of solvent must be controlled so that the solvent may penetrate naturally. When the front of the solvent arrived at about 90 mm. in radius of a circle, the development was stopped. The time required for the development was about 150 minutes. Visualization and Photographing of Chromatogram. The developed chromatoplates were dried a t room temperature and the visualization was carried out by spraying with 1% solution of iodine in methanol. The colored chromatogram, however, fades in a short time as the iodine is only weakly fixed on the layer by Van der Waals' force. For this reason, the visualized chromatogram must he photographed immediately after color development, within 2 to 3 minutes. The colored chromatogram consists of many concentric rings which resemble the annual rings of a tree. Each ring has a differentintensity of visualization
in proportion to thr concentration of tl wmponenf srparawd and the intensii i viaiinliz8iion is recorded direct on the film. Determination of Molecular Weigl Distribution bv CTLC. For the nu pose of obtaihning a molecular 'di trihution curve, a densitometric ana ysis of the photographed chromr togram was carried out in the follov ing way: the transmittance of visih light through the film was measure by a microphotometer from the cente outward along a given radius of tt circular chromatogram. The filn placed on a stand, was moved in sui cessive 0.1-mm. steps with fine contr, of the handle rotation and the tran mittance was measured with a O.OEmn slit width and a 5-mm. shutter widtl The resulting transmittance was plotte against the radius of each 'concentr ring from the center (density distribt t,ion mirve>~ T h e m o n r m t r a t i n n nf th GO me aistance transported, so the density distribution curve must he calibrated to obtain the molecular weight distribution curve. The peak areas in the density distribution curve were measured by triangulation after drawing a base line from the lefehand peak base to the righehand peak base. Then, these areas were multiplied by the distance transported. The resulting peak areas were divided by the largest resulting peak area among themand then the molecular weight distribution curve was constructed by plotting these values us. ethylene oxide units. RESULTS AND DISCUSSION
The circular thin layer chromatogram of polyoxyethylene nonylphenolether (? = 9) is shown in Figure 2. About 12 components which have different numbers of ethylene oxide in a molecule are completely separated. From this result, it is evident that CTLC is a more effective technique for the separation than ordinary one-
-. .
dimensional development. The ethylene oxide number of the chromatogram was assigned by developing, along with a ample, a standard sample of which the ethylene oxide number was known. The standard sample solution (150 mg./ ml.) contained three components, polyoxyethylene nonylphenolether (n = 8, 9,10), applied in a ring so that every neighboring quarter of the circle consisted of the sample to be separated and the standard alternately. The number of ethylene oxide units was assigned as shown in Figure 3. The density distribution (transmittance us. radius) curve of polyoxyethylene nonylphenolether is shown in Figure 4. This curve was calibrated according to the pr+ cedure described previously and the resulting molecular weight distribution curve is compared with what is given hy elution chromatography and the
1.o
0.0
0
20.6
. -
-" 0
.
'0.4
0.2
. ,,' I
01
P
4 6 0 No. of 0xr.thyl.n.
10 Unlb
19
I4
Figure 5. Molecular weight distribution curve of polyoxyethylene nonylphenolether (F = 9) Figure 3. Circular thin loyer chramatogram showing the identification of components
1756
ANALYTICAL CHEMISTRY
0
---
. .
of- polyoxyethyleie nonylphenolether (P = 9)
Clrculor thln layer chromatography Elution chromatogrephy Theoretical
theoretical Poisson distribution. These three curves are shown in Figure 5. Two curves obtained by CTLC and elution chromatography are in fair agreement with the Poisson distribution curve. The agreement is good enough for this CTLC method to be useful in analytical applications. As the elution chromatographic procedure requires about fifty-fold as much time as the CTLC method, the determination of molecular weight distributions of polyoxyethylene-type nonionic surfactants can be carried out very rapidly by the CTLC method. This method has special merit in the routine
analysis in which many samples must be analyzed in a short time. -4similar result was obtained with a polyoxyethylene laurylether (P = 6). In this case, the molecular weight distribution curves obtained by the CTLC method and elution chromatography showed relatively good agreement, but they gave wider distributions than Weibull-Nycander distribution. LITERATURE CITED
(1) Burger, K., 2. Anal. Chem. 196(4), 259 (1963). (2) Flory, P. J., J . Am. Chem. SOC.62, 1561 (1940).
(3) Karabinos, J. Y.,Quinn, E. J., J . Am.
Oil Chemists' SOC.33,223 (1956). (4) Kelly, J., Greenwald, H. L., J . Phys. them. 62, 1096 (195g). (5) &layhew, R. L., Hyatt, R.C., J . A m . Oil Chemists' SOC.29, 357 (1952). (6) Musha, S., Ochi, H., Japan Analyst 14,202 (1965). ( 7 ) Nagase, K., Sakaguchi, K., Kogyo Kagaku . h s h i 63, 588 (1960). (8) 64, 635 (lg61).
Kazuo KONISHI YAMAGUCHI SHINICHIRO Kao Soap Co., Ltd. Industrial Research Laboratories 1334Minatoyakushubata Wakayama-shi, Japan
Low Pressure Emission Spectrometric Determination of Part-Per-Billion Residue Levels of Organophosphorus Insecticides SIR: The emission spectrometric detector developed by Cooke (6, 6) has been used for analysis of organophosphorus ( 1 ) and iodoorganic (2) pesticides. In this detection system, compounds eluting from a gas chromatographic column were fragmented and excited in an intense microwave-powered argon discharge. Spectrometric analysis of the resultant desired line or band emission followed. The system was maintained at atmospheric pressure for pesticide analysis. Cooke (3) suggested that low pressure operation of the discharge might result in enhanced emission response by increasing the mean free path of bombarding electrons and diminishing recombination effects. In the present work, this detection system has been accordingly reevaluated a t reduced pressure and found to be more sensitive and selective by about an order of magnitude for analysis of organophosphorus insecticides. EXPERIMENTAL
Apparatus. The equipment used was identical to that reported previously ( I ) except that a Cenco Pressovac single stage mechanical vacuum pump with a 500-ml. Erlenmeyer flask as a ballast tank was connected to the exit of the discharge tube, A simple Utube manometer was used to measure pressure. Use of the vacuum pump made it necessary to increase the column length to 6 feet to eliminate chromatographic peak distortion and to improve column resolution. The column substrate was 10% DC-200 on 80- to 100-mesh Gas Chrom Q. It was not possible to measure carrier gas flow rates at reduced pressures. However, the 10-pound argon tank pressure used in this study corresponded to a carrier gas flow rate of 25 cc. per minute at atmospheric pressure. Isothermal column temperatures were adjusted to yield insecticide retention times of 10 to 20 minutes. These temperatures
DIMETHOATE
'-
bureEffect
pressure On detector response '0 organophosphorus insecticides and on base line Of
were all in the range of 180' to 210' C. The microwave power setting was 70 to 80% of the total 90 watts available. Up to 100 grams of sample were extracted by blending with acetone. Insecticides in the extracts were isolated from interferences by acetonitrile partitioning and chromatography on Florisi1 (4). The eluted chloroform fraction was evaporated, redissolved in 1 ml. of ethyl acetate, and the latter solution was partitioned with saturated sodium chloride. Up to 30 pl. of the ethyl acetate layer were injected for analysis. The 2535.65 A. atomic phosphorus emission was measured for insecticide residue analysis. RESULTS AND DISCUSSION
Figure 1 shows the effect of pressure on the response of Dimethoate [O,Odimethyl - S - ( N - methylcarbamoyl-
[O,O - diethyl - S - (ethylthiomethy1)-
studied previously ( 2 , 6) and on base line shift. Response is enhanced to a much smaller extent when measuring the atomic iodine emission. Base line shift is much greater, however, than that observed with the measurement of the 2535.65-A. atomic phosphorus line. The retention time of TIBA increased from 32.5 minutes a t a pressure of 0.8 cm. to 73 minutes a t atmospheric pressure. Since the peak was broad, its area (rather than height) was measured by cutting out and weighing the paper included under it. The selectivity ratio of the detector a t 2535.65 A. and a pressure of 20 cm. was 1000 to 1, based on a comparison of peak area for Thimet us. phenanthrene or lindane (hexachlorocyclohexane gamma isomer). The ratio found previously ( 1 , 6) at atmospheric pressure was 100 to 1. This improved selectivity permitted injection of much larger equivalent portions of each sample (and of injection solvent) than was possible at atmospheric pressure. The strong VOL. 38, NO. 12, NOVEMBER 1966
0
1757
