Ana lysis of PoIyethylene Terepht halate PrepoIymer by Trimethylsilylation and Gas Chromatography E. R. Atkinson, Jr., and S. I. Calouche Analytical and Chemical Section, Firestone Synthetic Fibers Company, Box 450, Hopewell, Vu. 23860
THE PRODUCTION of polyethylene terephthalate (PET) from terephthalic acid (TA) and ethylene glycol (EG) involves basically a two-step process. The first step consists of reacting TA with EG (esterification) in the presence of a catalyst to form a prepolymeric mixture which then undergoes polycondensation to the desired product (step two). The esterification product prepared in the first step contains bis-(2-hydroxyethyl)terephthalate (BHET), ethylene glycol (EG), diethylene glycol (DEG) ( I ) , terephthalic acid (TA), mono-(2-hydroxyethyl)terephthalate (MHET), and higher molecular weight oligomeric polyesters. An analytical technique was required for determining compounds such as the above when present together in a complex mixture. Gas liquid chromatography (GLC) best fills this need. Methods for the determination of carboxylic acids (2-4), ethylene and diethylene glycol (5-I2), and polyhydric alcohols (13), have been reported. There is no report, however, demonstrating the use of this technique for the determination of either mono- or bis-(2-hydroxyethyl)terephthalate. This paper proposes a qualitative and quantitative procedure for the determination of MHET and BHET as well as EG, DEG, and TA in polyester prepolymers by conversion to their TMS derivatives followed by GLC. The sample to be analyzed is dissolved in pyridine and treated with bis-(trimethylsily1)trifluoroacetamide (BSTFA). The resulting solution is chromatographed and the TMS derivatives separated and determined using the technique of internal standardization. EXPERIMENTAL
Chromatographic Equipment. All chromatograms were obtained using a Hewlett-Packard 5750 gas chromatograph equipped with a linear temperature programmer and dual flame ionization detectors. Operating Conditions. The flame ionization detector and injection port temperatures were 280 and 270 "C, respectively.
(1) S . G. Hovenkamp and J. P. Munting, J . Polym. Sci., Part A-1, 8, 679 (1970). (2) M. Schnitzer and J. G. Desjardins, J. Gas Chromatogr., 2, 270 (1964). (3) J. F. Klebe, H. Finkbeiner, and D. M. White, J . Amer. Chem. Soc., 88, 3390 (1966). (4) M. L. Kaufman, S. Friedman, and I. Wender, ANAL.CHEM., 39, 1011 (1967). (5) J. R. Lindsay Smith and D. J. Waddington,J. Chromatogr., 36, 145 (1968). (6) D. F. Wisniewski and G. C. Stalker, Petrol. Refiner, 40, 117 (1961). (7) T. Nakagawa, H. Inoue, and K. Kuriyama, ANAL.CHEM.,33, 1524 (1961). 18) L. Ginsburg. ibid.. 31. 1822 (1959). (9) S. Spencer and H.' G.'Nadeau, ibid., 33, 1626 (1961). (10) M. K. Withers, J. Gas Chrornarogr.,6, 242 (1968). (11) B. Smith and 0. Carlasson, Acta. Chem. Scand., 17, 455 (1963). (12) G. G. Esposito, ANAL.CHEM., 40, 1902 (1968). (13) G. G. Esposito and M. H. Swann, ibid., 41, 1118 (1969). 460
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
Helium was used as the carrier gas at a flow rate of 50 ml/min measured at room temperature at the column exit. The column temperature was held at 80 "C for 3 minutes after sample injection and then programmed to 265 "C at 15"/min. A coiled stainless steel column (6 ft, 0.125-in. o.d., 0.085-in. i.d.) packed with 3% OV-101 on 80-100 mesh Chromosorb W was used. The development of the chromatogram takes less than 20 minutes under these conditions. A piece of aluminum foil was placed between the septum and injection port to reduce peak interferences from degradation of the silicone surface by the pyridine/BSTFA reaction mixture. As a precaution, unpacked glass inserts were used in all cases to minimize interaction of the sample with the injection port. Chemicals and Reagents. Bis-(trimethylsily1)trifluoroacetamide (BSTFA) was obtained from Regis Chemical Company. Spectro-quality pyridine from Mallinckrodt Chemical Company was used as the solvent. Ethylene glycol, diethylene glycol, terephthalic acid, and bibenzyl standards were all obtained in high purity from Fisher Chemical Company. Bis-(2-hydroxyethyl)terephthalate was prepared by the transesterification of dimethyl terephthalate with ethylene glycol (mp 110 "C). Mono-(2-hydroxyethyl)terephthalate was obtained by partial saponification of the BHET (mp 180 "C). The column packing, 3% OV-101 on 80-100 mesh Chromosorb W, was obtained from Pierce Chemical Company. Procedure. An internal standard solution of bibenzyl, 2.00 mg/ml, was prepared. Approximately 20 mg of sample, weighed to 0.1 mg accuracy, were placed in a small screw cap septum vial, and 0.50 ml of internal standard solution and approximately 0.50 ml of bis-(trimethylsily1)trifluoroacetamide were added. The screw cap septum was then replaced and the sample heated in an oven at 80 "C for 10 minutes. Two microliters of the reaction mixture were then injected and a chromatogram was obtained, utilizing the previously stated conditions. Quantitation. Peak areas were measured using a HewlettPackard Model 3370A electronic integrator. The relative weight responses for each compound to internal standard were determined from known solutions by measuring the slope of the curves obtained when the weight ratios for each compound were plotted us. their respective area ratios. The per cent of the unknown compound present in a sample is then expressed by the following relationship :
where F is the correction factor (ljrelative weight response) for the compound of interest (see Table I), A , is the area ratio of the unknown peak to that of the internal standard, and W I , and W , are the weights of the internal standard and sample, respectively. Results. Conditions necessary for quantitative conversion to the TMS derivatives were arrived at by noting the variations in the relative peak heights when the reaction conditions were selectively varied. A 1: 1 mixture of pyridine and bis-(trimethylsily1)trifluoroacetamide was used in all cases to ensure the presence of excess reagent. It was found that quantitative conversion was achieved for all of the compounds when the solutions were heated at 80 "C for 3 minutes. Without heating, approximately 75% conversion was achieved.
Figure 1. Typical chromatogram of PET prepolymer sample A . Pyridine and mono-(trimethylsily1)tri-
fluoroacetamide
J
B . Bis-(trimethylsily1)trifluoroacetamide
x 64
C . Reagent impurity D. Ethylene glycol (TMSDY E. Diethylene glycol (TMSD) F. Bibenzyl (internal standard) G. Terephthalic acid (TMSD) H. Mono-(2-hydroxyethy1)terephthalate (TMSD) J. Bis-(2-hydroxyethyl)terephthalate (TMSD) K. Unknown a
ti
Trimethylsilyl derivative. 0
TIME, MINUTES
I
Table I.
Retention Times of TMS Derivatives Relative to That of Bibenzyl
Relativea reten- Correction tion time factors 0.28 0.65 0.73 0.91 1.00 1.25 1.00
Compound Ethylene glycol (TMSD) Diethylene glycol (TMSD) Bibenzyl Terephthalic acid (TMSD) Mono-(2-hydroxyethyl)terephthalate (TMSD) Bis-(2-hydroxyethyl)terephthalate (TMSD) a
Values relative to bibenzyl. RT
=
1.45
1.00-
0.90-
0.80
o.70:
0.60-
[
1.55 1.44
1.63
9.4 min. 0.40-
Table 11. Precision for Analysis of Polyester Prepolyrner
N Ethylene glycol Diethylene glycol Terephthalic acid Mono-(2-hydroxyethyl)terephthalate Bis-(2-hydroxyethyl)terephthalate
U
x, Z
0.30-
rel., 0.
z
0.20-
10 4.9 1 0 . 2 4 . 1 10 0.2 0.01 5.1 10 1.0 0.08 8.2 10 3.9 0.3 7.7 10 14.6 0.5 3.4 10
The quantitative aspects of the above reactions were also studied by obtaining the infrared spectra of each compound in pyridine before and after the addition of the TMS reagent. In each case, the -OH stretching bands observed for the pyridine solutions disappeared after the TMS reagent was added and the samples were heated for 3 minutes. In order to ensure quantitative conversion, a ten-minute heating time was selected for use in the procedure. A series of solutions containing from 0.50 mg/ml to 2.00 mg/ml of ethylene glycol, diethylene glycol, terephthalic acid, and both mono- and bis-(2-hydroxyethyl)terephthalate in pyridine were prepared and known amounts of internal standard were added to each. The solutions were treated with the TMS reagent as discussed above, chromatographed, and calibration curves constructed for each compound. A typical chromatogram is shown in Figure 1. The peaks were identified from the retention data obtained with the known solutions above. Table I1 shows the overall precision of the method for the
20
30 40 MINUTES
SO
60
Figure 2. Per cent BHET us. heating time for highly polymerized prepolymer sample
analysis of a prepolymer sample. For each compound, the accuracy was established using known samples and was found to be within the precision of the method. DISCUSSION
Both N,O-bis-(trimethylsily1)acetamide (BSA) and BSTFA were initially used as silylating agents. Quantitative conversion to the TMS derivatives was observed for each. However, chromatographic interferences from reagent and reagent impurities with the determination of ethylene glycol were encountered when using BSA. Consequently BSTFA was selected for use in the procedure. The correction factors used in the quantitations were checked periodically for a month, during which time they reANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
* 461
mained essentially constant. To ensure accuracy, however, response factors were checked weekly and, where necessary, the sample weights for unknown samples were adjusted so that the area ratios for each compound were within those used for calibration. As was mentioned previously, the samples analyzed contained high molecular weight oligomeric polyester compounds and, consequently, exhibited various degrees of solubility in the reaction mixture. The samples normally dissolve completely upon heating. When cooled, the reaction mixture becomes cloudy, apparently because of the precipitation of the high molecular weight compounds. This does not, however, appear to affect theresults. In some instances the samples were highly polymerized and total solution was not possible. For such cases, longer heat-
ing times were required to extract the compounds of interest from the crystal lattice of the polymeric material The time necessary for this varied with the particular sample and was established by plotting the per cent compound cs. heating time (see Figure 2). Results obtained from samples of this type may or may not be accurate because of the uncertainty of effectively removing all the compounds from the insoluble materials. ACKNOWLEDGMENT
The authors thank B. D. Stabley and E. J. Corrigan for their helpful discussions, and C. H. Wilson and R. G. Cozart for their excellent technical assistance. RECEIVED August 19,1970. Accepted November 10,1970.
Gas-Liquid Chromatography of Some Irritants at Various Concentrations Samuel Sass, Timothy L. Fisher, Michael J. Jascot, and John Herban Chemical Research Laboratory, Research Laboratories, Edgewood Arsenal, Edgewood Arsenal, Md. 21010
THIS REPORT DESCRIBES the application of gas-liquid chromatography (GLC) to the detection and quantitative analysis of irritant compounds, some of which have been used by military and law enforcement agencies. The compounds used as examples in this work were a-bromobenzylnitrile (brombenzylcyanide, CA), o-chlorobenzalmalononitrile (CS), and a-chloroacetophenone (CN). A previous report published by these laboratories employed thin-layer chromatography for the detection of these compounds ( I ) . The GLC procedures discussed here further increase the possibilities for absolute identification of these as well as similar compounds while also allowing their quantitative analysis from the milligram through the nanogram range. Also included here are methods for the assay of individual irritant samples and for the detection of some characteristic impurities that could represent hydrolysis or other residues of these irritant compounds. The procedures are applicable to the estimation of irritants as concentrated and dilute solutions and when sampled directly as vapor or aerosol. EXPERIMENTAL Equipment and Materials. The studies described here were performed on F & M Scientific Corp. (now a division of Hewlett-Packard) Model No. 810 gas chromatograph equipped with thermal conductivity (TC), flame ionization (FI), or electron capture (EC) detector; and a 1-mV MinneapolisHoneywell recorder with a disc integrator. Injections were performed using Hamilton syringes. The column coatings and supports were obtained from Applied Science Laboratories, State College, Pa. All solvents used were of CP grade except for the spectro-quality hexane used in determinations employing the electron capture detector. The 1,lOdibromodecane (used as the internal standard) was obtained through Eastman Organic Chemicals, Rochester, N. Y . Irritant Standards. CA was purified by fractional freezing followed by several recrystallizations from ethyl alcohol and (1) W. D. Ludemann, M. H. Stutz, and S. Sass, ANAL.CHEM., 41, 679 (1969). 462
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
washing with cold petroleum ether. The resultant CA produced near theoretical results by elemental analysis and showed no detectable impurity by thin-layer chromatography ( I ) and only the product peak by GLC. The purity was better than 99.5% and the melting point, 25 "C. CS, originally of 96.5 purity (mp 93-95 "C), was recrystallized from cyclohexane as a white solid melting at 96 "C, with a single GLC peak and no detectable impurity by TLC ( I ) , and with a determined purity of 99.5 %. CN, as recrystallized material, was of better than 98% purity based on melting point (54.5 "C), elemental analysis, ketone determination, GLC peak, and TLC detection. PROCEDURES
SEMIMICRO TO MACRODETERMINATION (MILLIGRAM QUANTITIES). Relative-thermal conductivity-response factor (RTCF) determinations were made for each irritant 1;s. the internal standard (1 ,lo-dibromodecane). Separate benzene solutions containing 50 mg/ml of the individual irritants were prepared along with a 50 mg/ml solution of 1,lO-dibromodecane. Under identical conditions, each solution was injected in 5-111 increments from 5- to 50-pl volumes. The respective slopes of the irritant concentration us. response curves were normalized with respect to that of the 1,lOdibromodecane. Similar solutions containing each irritant in combination with the internal standard were prepared and analyzed. The resultant normalized slopes were identical to those obtained with the separate solutions. Assigning the value of 1.000 to 1,lO-dibromodecane resulted in RTCF values of 1.030 for CA, 1.176 for CN, and 1.111 for CS. To determine purity, a weighed sample of irritant (CA, CN, or CS) and a similar weight of internal standard (1,lOdibromodecane) were placed in a 10-ml volumetric flask and made up to volume with benzene to yield an agent concentration of about 50 mg/ml. Aliquots of sample were injected directly onto a 5.5-ft x 1/4-in,0.d. borosilicate glass column packed with 10% QF-1 on 60/80 mesh Gas-Chrom Q. A helium carrier gas-flow of 90 ml/min was used with temperature-programming from 65-200 OC at 6 "C/min. MICRODETERMINATION (MICROGRAMS). To detect irritants (CA, CN, and CS) in microgram quantities, the chromato-
