The product was of a satisfactory purity for our purposes but we repeated the treatment on a 70-ml sample of product using a cold bath temperature of 3" C. The purity of the product from this was 99.99az after 6 freezings. The density was 1.60688 i. 0,00001 grams cc-l at 25.00" C. This represents an increase of 0.00010 gram cc-l over the literature value (5). The purity of the hexafluorobenzene on which the earlier determination was made was 99.97 i 0.01% (5). The result reported now confirms the higher purity of our material and provides a density result for very pure (>99.99x) hexafluorobenzene.
ACKNOWLEDGMENT
We express our appreciation for the practical contributions of George W. Allison during the construction of the apparatus. Thanks are also due to Dennis Gere for making available his GLC apparatus and to Theodore J. Neubert for many useful suggestions.
RECEIVED for review July 20, 1967. Accepted October 20, 1967. Work supported by The Public Health Service Grant NO. GM 14710-01.
Advantage ob Multidiameter Separation Column in Gas ChromatographicAnalysis of Organics J. Q. Walker McDonnell Douglas Corp., St. Louis, Mo. 63/(
1
HIGHRESOLUTION
GAS CHROMATOGRAPHIC TECHNIQUES used in preparative scale and trace analyses have been studied by a number of workers (1-3). Dal Nogare and Juvet (2) mentioned several problems in accurately measuring trace compounds eluting after a major component. Two practical cases frequently encountered in the gas chromatographic analysis of a minor component comprising less than 5% of a sample are shown in Figure 1. In case, 1A the trace compound is eluted before the major compound, and in case lB, it is eluted after the major component. In both chromatograms a large sample is employed to make the trace component evident. Case A permits accurate measurement of the trace compound because it is well resolved from the matrix and is eluted early, therefore presenting a well-defined peak. Case B presents a situation in which accurate measurement of the trace compound is difficult and in which improved resolution is required for satisfactory analysis (4). Rarely can one elute all the minor components from a column p r i v to the major compound. The column liquid phase may - changed and thereby change the component relative elution order. For example, the analysis of high purity ethylene for acetylene and ethane impurities is a difficult problem. The order of elution from a column containing the polar stationary liquid phase benzyl ether would be ethane, ethylene, and acetylene. Acetylene appears as a shoulder on the large ethylene peak, The separation order with a nonpolar stationary liquid phase such as squalane would be acetylene, ethylene, and ethane. Ethane appears as a shoulder on the ethylene peak. An intermediate polarity column would result in no separation. The separation of high purity ethylene is relatively simple with the two-column technique.
(1) J. Q. Walker, Hydrocarbon Process. Petrol. Refiner, 43, 154
(1964).
(2) S. Dal Nogare and R. S. Juvet, "Gas-Liquid Chromatography," Interscience, New York, 1962, p. 300. (3) A. Zlatkis and H. R. Kaufman, Nuture, 184, 2010 (1959). (4) A. Zlatkis and J. Q. Walker, Abstracts, Pittsburgh Conference
on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1962, p. 50. 226
ANALYTICAL CHEMISTRY
A
I I "
0 Time
Figure 1. Typical chromatograms of a trace component in an essentially pure sample
Large sample sizes of the order of 0.2 gram may be separated by increasing the length of an analytical column. However, any increase in column length results in a significant increase in time of analysis. EXPERIMENTAL Apparatus. An F & M Scientific Model 5750 gas chro-
matograph equipped with a dual thermal conductivity detector was used for these experiments. The chromatograms were recorded on a 0- to 1-mV Moseley strip chart recorder. Aluminum columns containing 20 % bis (2-ethyl hexyl) adipate (Distillation Product Industries) on Chromosorb W, 60/80 mesh, were prepared in 5-, lo-, 15-, and 20-foot lengths with diameters of lis, 1/4, 3/8, and inch. Aluminum columns containing Polypak No. 1, (F & M Scientific), 60/80 mesh, were prepared in 4-, 8-, and 12-foot lengths with lip- and 3/s-inch diameters. Stainless steel columns containing 10% Carbowax 20M on Chromosorb W, 60/80 mesh, were prepared in 14- and 3-foot lengths with diameters of 1/4 and 3/8 inch, respectively. Helium was used as the carrier gas. Columns were operated, isothermally at 70" or 80" C. The injection block temperature was maintained at 270" C. The thermal conductivity current was 150 mA. Liquid samples were injected directly with a 2.5-ml syringe (Hamilton No. 1002) equipped with a 6-inch stainless-steel needle for
0 m c
5 W
Column length (feet) Figure 2. Optimum carrier gas flow rates of several column
sizes
- - -. -- -- I
-m-m---m-
-=-
1/2 in. c.d. columns 3'8 in. 0.d. columns 114 in. 0.d. columns 1'8 in. 0.d. columns
on-column injection, and with a IO-pl syringe (Hamilton No. 701). Gas samples were introduced with an Aerograph gas sampling valve No. 57-036 equipped with Viton quad rings. The other column liquid and solid phases were obtained from Varian Aerograph. Procedure. Column supports were prepared by dissolving the liquid phase in acetone, 1:lO by volume; the solution was poured over the solid phase supported in a flask by a sintered-glass disc according to the technique reported by E. C . Horning et al. (5). An aspirator was used to evaporate all the acetone. The coated supports were loaded with a Press-Pak coiled column loader. The columns were preconditioned overnight at 125" C with a helium flow of 1S cc/minute.
I I I l l 1
I I I I I I I I
0 1 2 3 4 5
0 1 2 3 4 5 6 7
ia) Figure 3.
Elution time (min) (b)
(c)
Sample: reagent grade ethanol
(a) Volume: 100 pl Column: 12-ft X '/&-inch20% 2-ethylhexyl adipate Conditions: 80" C; 45 cc/min helium (b) Volume: 200 pl Column: 12-ft X l/r-inch 20% 2-ethylhexyl adipate Conditions: 80" C; 45 cc/min helium (c) Volume: 200 p1 Column: 3-ft X 3/s-inch and 8-ft X '/&-inch2097, 2-ethylhexyl adipate Conditions: 80" C; 50 cc/min helium
RESULTS AND DISCUSSION
The efficiency for each column is calculated with the aid of the van Deemter (6)equation from the observed peak width and elution time of normal hexane which was injected into the column. This equation where N is the number of theoretical plates is as follows:
N = 16
retention time peak width
The efficiency for every column is determined as a function of carrier gas flow rate in the range of 5 to 300 cc/minute. These column efficiencies as a function of carrier velocity and length for different diameters are illustrated in Figure 2. The results indicate that the smaller the diameter, the lower the optimum flow rate; also, the shorter the column, the lower the optimum flow rate. From Figure 2, several column diameters and lengths can be matched at their most efficient flow rates. For example, a 3.5-ft, 3/8-in~h column and an 8-ft, '/(-inch column have the same optimum velocity of 30 ml minute. However, when these columns are connected in series, by a reducer tubing union with the larger diameter ( 5 ) E. C . Horning, E. A. Moscatelli, and C. C. Sweeley, Chem.
Ind. (London), 1959,751, ( 6 ) J. J. van Deemter, F. J. Zuilderweg, and A. Klinkenberg, Chem. Eng. Sci., 5, 271 (1956).
column first, the most efficient flow rate is experimentally determined to be 50 ml/minute. To take another example, a 3-ft, 1/4-inchcolumn and a 14-ft, l/s-inch column have the same optimum flow of 12 cc/minute from Figure 2. When these columns were connected in series, a new optimun flow of 15 cc/minute is determined. These multidiameter columns-at their most efficient gas velocities-can be used for both preparative scale and analytical scale analyses. A typical preparative application is the separation of methanol and 2-propanol from ethanol. High-purity ethanol is isolated from a 100-pl sample of reagent grade ethanol using a 12-ft, l/r-inch column containing the bis-adipate phase at the optimum column flow rate of 45 cc/minute, (values taken from Figure 2). This chromatogram is shown in Figure 3n. When the sample size is increased from 100 to 200 pl, the methanol and 2-propanol are poorly separated from ethanol as shown in Figure 36. Figure 2 shows that a 3-ft, 3/8-inch column and a 8-ft, '/(-inch column have the same optimum flow rate of 30 cc/minute. When these two columns are connected in series, the optimum flow rate is found experimentally to be 45 cc/minute. The chromatogram from a 200-4 sample of reagent grade ethanol with the 3/s-in~h-1/4inch tandem column is shown in Figure 3c. VOL 40, NO. 1, JANUARY 1968
227
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N
X aJ m c
L n CD
5
2
LL1
W W c
d
x
x
4
X
c W W X W r 0
-
3h V
0
M W m c
W
A
I
I I 1 1 l 1 1 1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 101112
0 1 2 3 4 5 6 7 8 9101112
Time (min) (a)
T i m e (minl (b)
(a) Column: 12 ft X
inch 20% 2-ethylhexyl adipate Conditions: 70' @; 50 cc/min helium (b) Column: 3.5 ft X 3/8 inch and 8 ft X l/d inch 2 0 z 2-ethylhexyl adipate Conditions: 70" C; 50 cc/min helium
1
1
1
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1
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1
1
1
1
1
1
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Column: 3 ft X l/4 inch and 14 ft X 1/8 inch 10% CW 20 M Conditions: 70" C; 15 cc/min helium 4
1
5 6 7 8 91011121314151617 Elution time ( m i n )
Figure 6. Sample: 5 pl of butyl acrylate
228
ANALYTICAL CHEMISTRY
I
I
(a)
1
1
1
1
1
1
1
1
I
0 1 2 3 4 5 6 7 8 E l u t i o n t i m e imin)
ib)
Figure 5. Sample: 2.0 cc of natural gas at 1 atm pressure l/n inch Poly-Pak No. 1 60/80 Mesh Conditions: 70" C; 50 cc/min helium (b) Column : 4 ft X inch and 8 ft X l / p inch Poly-Pak No. 1 60/80 Mesh Conditions: 70" C; 50 cc/min helium (a) Column: 12 ft X
Figure 4. Sample: 200 pi of spectro grade cyclohexene
0 1 2 3 4
I
X
0 1 2 3 4 5 6 7 8 Elution time (min)
Y U
l l 1 l l I I l l I I I l
l
N
The preparative separation of 98% cyclohexene can be performed with the bis-adipate column. A lZ-ft, 1/&1ch column resolves quite well the impurities prior to resolving cyclohexene in a 2 0 0 4 sample; however, the benzene eluting after cyclohexene is not resolved (See Figure 4a). If 200 jd of cyclohexene is injected into a 3.5-ft, 3/8-inch column connected to an 8-ft, l/c-inch column, much better resolution is obtained, as shown in Figure 4b. An important impurity in natural gas is carbon dioxide. Most of the reported gas chromatographic separations resolve carbon dioxide after the large methane peak. An analysis of natural gas with a lZ-ft, 1/4-inchcolumn containing Polypak No. 1 is shown in Figure 5a. The poor resolution of carbon dioxide appearing as a shoulder on the side of the methane peak should be noted. If the same size sample is introduced into a 4-ft, 3/s-inch Polypak column connected to an 8-ft, l/r-inch column, carbon dioxide is separated from methane (Figure 5b). Carbon dioxide concentrations as low as 5 ppm have been determined using this technique. Another typical analytical application is the separation of 11 impurities in butyl acrylate. A 14-ft, l/s-inch column has the same optimum flow rate as a 3-ft, l/r-inch column (See Figure 2). When the 1/4-inch and 1j8-inch columns are connected in series, their optimum carrier gas flow rate is found experimentally to be 15 cc/minute. This two-diameter column resolves the trace impurities in butyl acrylate as shown in Figure 6.
CONCLUSION
Data presented show that the two-diameter column gives enhanced resolution of normally difficult separations for typical preparative and trace analyses. The two-diameter technique can be used for large samples without significantly increasing the time of analysis for both gases and liquids. This method may be applied to a combination gas chromatograph connected in series to a mass spectrometer. Frequently a trace component is too small to be identified by the mass spectrometer. The two-diameter column allows a much
larger sample to be introduced into the gas chromatograph for a separation, without column overloading, and allows a satisfactory determination by the mass spectrometer, ACKNOWLEDGMENT
The author gratefully acknowledges the technical assistance of T. C. Harby in preparing the column used in these experiments. RECEIVED for review July 10, 1967. Accepted October 23, 1967.
Gas Chromatographic Determination of Diethylene Glycol in Poly(Ethy1ene Terephthalate) Leonard H. Ponder American Enka Corp., Enka, N . C . PRODUCERS OF POLY(ETHYLENE TEREPHTHALATE) yarns maintain that ether linkages resulting from incorporation of diethylene glycol in the polymer chain adversely affect light and oxidative stability, wash-and-wear properties, and dyeing properties ( I , 2). Ethylene glycol used in production of the polymer contains small quantities (approximately 0.04x) of diethylene glycol and more is formed during polymerization (2). A method suitable for routine use by technicians was required for quality control of polymerization, for evaluation of polymer quality, and for additional studies of the effects of ether linkages on fiber properties, Consequently, the method should have a minimum of separate steps and be capable of handling a large number of samples in minimum time. The methods of Janssen and coworkers, and Mifune and Ishida for determination of diethylene glycol in poly(ethy1ene terephthalate) have been reviewed by Kirby, et a/. ( 2 ) in a published modification of the latter procedure. While the former method requires a long reaction time (16 hours), the Kirby method is too cumbersome for handling a large number of samples. A method presented by Esposito and Swann (3) for characterization of polyhydric alcohols in synthetic resins has been extended to include poly(ethy1ene terephthalate). In each of these procedures, as in the procedure reported here, the polymer is first decomposed. Gaskill, et a / . ( 4 ) have proposed that it is not essential to liberate all the diethylene glycol from the sample because the diethylene glycol-ethylene glycol ratio is constant after a 0.5-hour saponification. The procedure is not suitable for handling a large number of samples because of the time and bench space required. Further, the required use of glass columns is a serious disadvantage in routine gas chromatographic analysis. More recently Kalal and Hornof (5) reported a complete (1) R. Sakurai and K. Kazarna (to Teikoku Jinzo Kenshi Kabushiki Kaisha), British Patent 960,460 (June 10, 1964). (2) J. R. Kirby. A. J. Baldwin. and R. H. Heidner. ANAL.CHEM.. 37, 1306 (1965). (3) G. G. Esoosito and M. H. Swann. Zbid.. 33. 1854 (1961). (4 D. R. Gaskill, A. G . Chasar, and C . A. Luchesi, ibid.,39, 107 (1967). (5) J. Kalal and V. Hornof, Man-Made Textiles, February, 1967, pp. 2 6 7 . .
I
hydrolysis requiring 48 hours with the final determination by titration. In the following procedure up to 22 samples are hydrolyzed simultaneously in 4 hours with quantitation of diethylene glycol by gas chromatography directly on the hydrolyzate. EXPERIMENTAL Apparatus. Hydrolysis of samples was accomplished using a Paar Item 4501 pressure-reaction vessel equipped with a stainless-steel wire basket to prevent direct contact between the sample containers and the walls of the vessel's chamber. For construction of sample containers j/*-inch o.d., thick-walled borosilicate glass tubing was used. The analysis was performed on two commercially available gas chromatographs: a Micro-Tek Model DSS-172 and a Varian Aerograph Model 1520. Both thermal conductivity and hydrogen flame detectors on the Micro-Tek unit and a hydrogen flame detector on the Varian Aerograph instrument were used. A Sargent Model SR recorder equipped with a 1-mV range plug was used for recording detector responses. Chromatograph inlets were maintained at 260" C and detectors at 270" C. Helium was used as the carrier gas at a flow rate of 50 ml per minute through a column maintained at 180" C. Small changes in column temperature and/or flow rate have been made on occasions in changing from one column t o another or as a column aged. Columns. Packings have been made from various silanetreated diatomaceous earths using a 10 (by weight) coating of Carbowax 20M ; a perfluorocarbon coated diatomite (Gas Pack F) similarly prepared with Carbowax 20M was obtained (Chemical Research Services, Addison, Ill.). The silane treated products used were Chromosorb W-HMDS, 60-80 mesh, Gas Chrom Z , 60-80 mesh (Applied Science Laboratories, State College, Pa.) and Aeropak Number 30, 100-120 mesh (Varian Aerograph, Walnut Creek, Calif.). Ten-foot columns of l/s-inch X 0.055-inch id., Type 316 stainless steel (Stainless Piping Supply, Charleston, W. Va.) were made in each case. Columns were conditioned for 24 hours at 200" C before connecting to the detector. Matched columns were usually used with the Micro-Tek instrument only, but columns were sufficiently stable so that differentia! analysis was not necessary. Reagents. Standards employed were polymer grade ethylene glycol (Dow Chemical Co.), polymer grade diethylene glycol (Jefferson Chemical Co.), and Fisher certified grade VOL 40, NO. 1, JANUARY 1968
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