Table II.
Mole Fractions of Components in Polyester-Based Polyurethane Component NMR Hydrolysis AA 0.37 0.35 ED 0.44 0.46 MDI 0.14 0.15 HEHQ 0.05 0.04
The aromatic region can also be employed in the analysis, if the N H resonances hilve been located. The composition of the polymer of Figure 4-D calculated from the four equations above, was [AA] = 13.43, [ED] = 0.39, [PD] = 0.10, and [TDI] = 0.08. The NMR analyses on polyurethanes prepared in our lab-
oratories agreed with the known compositions, within the experimental accuracy. We have obtained a verification of the reliability of the NMR method on one “unknown” commercial polyurethane by an independent analysis, based on hydrolysis of the polymer and quantitative isolation of the hydrolysis products. The composition of that polyurethane, as found by the two methods, is given in Table 11. ACKNOWLEDGMENT
The authors are indebted to Drs. M. Brown, J. W. Crary, and R. J. Athey for supplying the polyurethanes, and for helpful discussions. J. W. Crary and M. DeBrunner performed independent hydrolysis analysis on some of the polyurethanes including the sample reported in Table 11. V. A. Brown did most of the experimental work.
RECEIVED for review November 23,1966. Accepted February 6,1967.
Gas Chromlatographic Analysis of Dilute Aqueous Systems W. G. Jennings and H. E. Nursten Department of Food Science and Technology, University of California, Davis, Calif.
WHILEGAS CHROMATOGRAPHY has contributed significantly to the analysis of volatiles in many systems, this degree of success has not been achieved with dilute aqueous systems, particularly those such as milk, which contain solventextractable nonvolatiles. Many attempts have been made to surmount these difficulties (Id), but they have not been too successful. Methods involving the injection of head-space vapors ( I , 3, 6, 7) are limited by the fact that in order to obtain detectable quantities of dilute vapor components, one must use such a massive quantity of vapor that the injection requires a considerable period of time, and the separated components are in consequence so diluted by carrier gas that they may escape detection. The use of low temperature precolumns has been suggested (3, 5, 7, 8) to permit the concentration of volatiles from head-space vapors prior to their chromatographic sepacltion, but these suffer from obvious disadvantages. While a degree of concentration can be achieved, sufficient diffusion occurs in the pre-column to limit the quantity of gas distillate 01- head-space vapor that can be used, and in dilute aqueous systems the major volatile in the pre(1) R. Bassette, S. Ozeris, and C. H. Whitnah, J. Food Sci., 28, 84 (1963). (2) I. Hornstein and P. F. Crowe, ANAL. CHEM., 34, 1354 (1962). (3) . , W. G. Jenninas. S. Vilihalmsson, and W. L. Dunkley, J. Food Sci., 27, 306 (1962). (4) L. M. Libbey, D. D. :Bills, and E. A. Day, Zbid.,28, 329 (1963). (5) M. E. Morgan and E. A. Day, J. Dairy Sci., 38,1382 (1965). (6) R. Teranishi, R. G. ELuttery, and R. E. Lundin, ANAL. CHEM., 34, 1033 (1962). (7) J. M. Mendelsohn, Pd. A. Steinberg, and C . Merritt, Jr., J. Food Sci., 31, 389 (1966). (8) D. E. Heinz, M. R. Sevenants, and W. G. Jennings, Zbid., 31, 63 (1966).
column is ineviti-.; water, which saturates the pre-column and again limits the quantity of vapor that can be used. Although the flame ionization detector is relatively insensitive to water vapor, massive slugs of water cause major deflections and effectively blank out at least the first portion of any chromatogram. Scott and Phillips (9) proposed the use of gas-solid chromatography to achieve a concentration of volatiles from dilute systems, and their subsequent elution to a gas-liquid chromatography column for analysis. Several investigators have used activated charcoal for adsorbing volatiles for subsequent analysis--e.g., Walls (IO); the work of Dhont and Weurman (11)on model systems indicated that recovery efficiencies were quite high. A major advantage of charcoal for this purpose lies in its relatively low affinity for water, and Heinz et al. (8) employed it to adsorb volatiles from several dilute aqueous systems for subsequent elution and analysis. It appeared logical that a significant advantage might be gained by substituting charcoal in the gas-solid chromatography step of the method proposed by Scott and Phillips (9). This work was directed toward the development of a method that would permit the concentration of volatiles from very large vapor samples and their elution for chromatographic analysis. METHODS AND PROCEDURES Gas Chromatography, Gas chromatographic separations
utilized a modified Aerograph 600B with flame ionization detection. The output impedance was increased to 20 kohms (9) C. G. Scott and C . S. G. Phillips, “Gas Chromatography,” A. Goldup, Ed., Brighton, 1964, p. 273. (10) L. P. Walls, J. Pomol. and Hort. Sci., 20, 59 (1942/3). (11) J. H. Dhont and C. Weurman, Analyst, 85,419 (1960). VOL. 39, NO. 4, APRIL 1967
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Figure 1. Distillation-adsorption apparatus from the standard 1 or 10 kohms to increase sensitivity. The column, 10 feet X 0.125-inch 0.d. stainless-steel, was packed with 10 Carbowax 20 M on 45-60 mesh Gas Pak F, and operated isothermally at 150" C with flow rates of about 25 cc N2and 30 cc H2/minute. Charcoal Adsorbent. To 20 grams of activated charcoal (MCB,8-12 mesh ground to 20-40 mesh) was added 100 ml of spectrograde CSz. Considerable heat is evolved. The
CS, was decanted after 15 minutes of stirring, and the charcoal was washed twice more with 50-ml portions of CS2, which remained in contact with the adsorbent for 15 and 30 minutes, respectively. The charcoal was then sucked dry on a Buchner funnel, transferred to a vacuum oven, and held under 29-inch vacuum overnight at room temperature. When an oil pump was used to supply the vacuum, back-diffusion of vapors from the oil pump contaminated the charcoal. A water pump gave satisfactory results. The temperature of the oven was then increased to 50" C and held overnight, and then to 120' C and held overnight, maintaining the vacuum. (Unless the CS2 has been thoroughly evacuated, the hot vapors may explode when air is allowed to enter the vacuum oven. A tank of water-pumped nitrogen, connected to the oven through a capillary leak, aids in sweeping most of the vapors from the oven and permits using nitrogen to dissipate the vacuum at the end of the operation.) Distillation-Adsorption. The apparatus is sufficiently described in Figure l. The volatiles in l liter of aqueous medium were distilled under water-pump vacuum for 1 hour from a waterbath at about 40" C and the distillate passed through 0.25-gram charcoal, purified as above. Desorption. The charcoal was blotted between sheets of filter paper and placed in a 5-cm stainless-steel tube, 0.125inch o.d., 0.008-inch wall, to which had been silver-soldered male and female Luer joints. After considerable experimentation, CS2 was selected as the eluting solvent. It is more effective than other common solvents at displacing the materials adsorbed on charcoal, and it gives minimum response with flame ionization detection. Two 250-p1 lots of CS, were injected at 10-minute intervals from a syringe into a 2-cc screw-top vial through this tube. Almost all of the first lot was held within the charcoal. The vial was closed with a center-drilled screw cap containing a silicone rubber septum, which was able to contain the sample adequately for several hours, although not overnight; 9.0-pl portions were used for gas chromatography. When the above process was carried out on 1 liter of double-distilled water, the chromatogram exhibited only peaks due to air, to CS2(approximately 5 mV response), and to minor impurities (retention relative to 2-octanone: 1.58 with a response of 0.06 mV; relative retention 2.17,0.18 mV). Model Aqueous System. To 4000 cc of double-distilled water was added 4.0 pl of each of the following: acetone, 2-butanone, 2-octanone, diacetyl, dimethyl disulfide, npentyl acetate, methional, and butyrolactone.
Table I. Recovery and Apparent Sensitivity for Various Substances Per cent recovery after Retention" Detector response, distillation and Injected injected neat desorption neat Desorbed V per 11 1 pl/liter 0.1 &liter Substance 0.33 3.6 Acetone 0.29b 14 0 0.35 11.4 2-Butanone 55 0.39 56 12 0.37 Diacetyl 71 46 0.59 0.57 21 Dimethyl disulfide 0.72 0.68 81 54 26 n-Pentyl acetate 32 72 56 1 .oo 1 .oo 2-Octanone 2.17 2.16 11 0.9 Methional 4.6 0.2 4.77 0.2 4.66 9.6 Butyrolactone 0.55 32 4 0.56 n-Butanold 0.82 32 27 0.79 n-Pentanold 32 29 1.21 1.18 n-Hexanold 37 1.83 23 1.78 n-Heptanold 45 15 2.77 2.73 n-Octanold Relative to 2-octanone (retention 3.28 minutes). b Shoulder only. Based on 9-11 injection of desorbate. d Injected as a separate series.
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Apparent sensitivity of method,c ppb 1 &liter 0.1 pl/liter
0.084 0.037 0.026 0,024 1.1 26 0.39 0.063 0.058 0.065
0.081
0.083 0.058 0.039 0.030 12 25
Figure 2. Chromatogram from the 1.0 pliliter model system
RESULTS AND DISCUSSION Figure 1 shows a diagram of the apparatus used to sweep volatiles from dilute aqueous systems onto a small charge of activated carbon; Figure 2 illustrates the type of chromatogram obtained from 1 liter of the 1.0 &liter model system by means of the apparatus. The additional minor peaks on the chromatogram are due to impurities in the chemicals used to make up the model system. A series of charcoal traps revealed that even though the adsorptive capacity of the first trap is more than sufficient to accommodate all of the volatiles from 1 liter of the model system, a minor portion penetrates to the second trap, and a trace to the third. There is relatively little change in the proportions of the individual volatiles in each trap, implying that displacement or preferential adsorption is riot important under these conditions. When the model system is slowly percolated over the charcoal charge, the adsorption is not as complete as when the same quantity is distilled through the apparatus as shown. The distillation was repeated with 100 ml of the model system after dilution with water to 1 liter, and with 1 liter of an aqueous solution conraining 1.0 pl each of the n-alcohols, Ca-C8. Aliquots, 5.0 pl, of 0.05% solutions of the pure liquid substances in CS2 were also chromatographed directly to permit the calculation of approximate yields. The combined results are presented in Table I. The table indicates that in favorable cases the sensitivity of the method appear!; to be about 30 parts per trillion. It must be emphasized thzit this is without modification to achieve higher sensitivities. It would be easy to increase the sensitivity by using larger volumes of liquid, both for the distillation and for injection into the gas chromatograph. The method gives lower recoveries for the low- and highboiling compounds investigated. This is to be expected,
because the conditions were chosen with concern for cornpounds of intermediate boiling point. A proportion of low boilers escape because of their volatility, but their determination is complicated further by the presence of a somewhat variable peak due to the large excess of carbon disulfide. If the lower boilers are a focus of interest, then more appropriate conditions of chromatography should be selected, in particular, a lower temperature. That recovery drops off also with the high-boilers is not surprising, because a distillation step is involved. In spite of this relatively lower recovery, the technique is noteworthy for the fact that it does permit the determination of comparatively large compounds by headspace analysis; in related work (12) even y-decalactone from apricots gave a definite well-shaped peak. These authors also established that if the distillation is continued to a point where there is competition for the adsorptive sites on the charcoal, the relative amounts of the individual volatiles is changed ; the resultant chromatogram exhibits larger amounts of more strongly adsorbed materials, such as alcohols. ACKNOWLEDGMENT
The authors are particularly grateful for the extremely capable technical assistance of W. J. Murray. RECEIVEDOctober 4, 1966. Accepted January 6, 1967. H. E. Nursten is a Visiting Professor supported by a grant from the Dairy Council of California. This project was supported in part by Research Grant E F 0092 of the Division of Environmental Engineering and Food Protection, USPHS. (12) C . S. Tang and W. G. Jennings, J. Agr. Food Chem., 15, 42
(1967).
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