obtained when these compounds are volatilized from a Knudsen Cell (23). As a confirmatory test, m / e 81 of the NaCl analysis was peak matched against a perfluorokerosene ion at m / e 81 (80.995215);in the EI/D mode, the peak was found to be 80.94872, which is 0.32 mmass from the theoretical value for (Na2C1)+. Since only positive ions are detected, the ease with which the monohalogen positive ions are formed follows the sequence of the electronegativity of this series. Figure 5 illustrates the scan (temperature) dependent thermal desorption processes for these salts. These outputs, used to determine BET values, are generated in a manner analogous to the time-dependent mass chromatograms of GC-MS (24). The EI/D method, using sample deployment on an activated emitter and thermal desorption prior to electron impact ionization, was found to be a reliable method for the introduction of solid samples into the mass spectrometer. This new method of analysis requires smaller amounts of sample than the conventional direct probe technique. The efficiency of heating and the high temperatures attainable enable this technique to be used to analyze samples that cannot be done by conventional solid sample devices. The volatilization processes appear more uniform in the EI/D mode, a consequence quite naturally of the more precise and efficient thermal parameters attainable when heating a thin film of the sample on the emitter wire with electronic control of temperature. As a result, the uncertainty of the moment of sample desorption is significantly reduced when repeated analyses of similar samples are performed. These features have encouraged the use of EI/D for the analysis of solid compounds in all cases where solubilization can be effected and the emitter adequately charged with the sample. When combined with an on-line data system running in a cyclic scanning mode, mass chromatography and automatic calculation of qualitative and
quantitative parameters can be combined with the EI/D technique.
LITERATURE CITED (1) C. M. Stevens, Rev. Sci. Instrum., 24, 148 (1953). (2) V. J. Caldecourt, Anal. Chem., 27, 1670 (1955). (3) R. H. Roberts and J. V . Walsh, Rev. Sci. Instrum., 26, 890 (1955). (4) P. De. Mayo and R. I. Reed, Chem. Ind. (London), 1481 (1956). (5) M. W. Echo and T. D. Morgan, Anal. Chem., 29, 1593 (1957). (6) Lab guide, Anal. Chem., 48 (lo),101 (1976). (7) R. A. Hites and K. Biemann, Anal. Chem., 39, 965 (1967). (8) C. C. Sweeley, B. D. Ray, W. I.Wood, J. F. Holland, and M. I. Kirchevsky, Anal. Chem., 42, 1505 (1970). (9) H. D. Beckey, J . Mass Spectrom. Ion Phys., 2, 500 (1969). (10)B. Sotmann, C. C. Sweeley, and J. F. Holland, Int. J . Mass Spectrom. Ion Phys., submltted for publlcation.
(11) J. F. Holland, B. Soltmann, and C. C. Sweeley, Biomed. Mass Spectrom., 3, 340 (1976). (12) D. F. Hunt, J. Shabanowitz, F. K. Botz, and D. A. Brent, Anal. Chem., in press.
(13) J. W. Maine, B. Soltmann, J. F. Holland, N. D. Young, J. N. Gerber, and C. C. Sweeley, Anal. Chem., 48, 427 (1976). (14) H. D. Beckey, A. Heindricks, E. Hilt, M. D. Migahed, H. R. Schulten, and H. U. Winkler, Messtecknlk(Braunschwe@), 79, 196 (1971). (15) H. D. Beckey, S. Bloching, M. D. Migahed, E. Ochterbeck, and H. R. Schulten, Int. J . Mass Spectrom. Ion Phys., 8, 169 (1972). (16) H. R. Schulten and H. D. Beckey, Org. Mass Spectrom., 6, 885 (1972). (17) D. Kunimler and H. R. Schulten, Org. Mass Spectrom., I O , 813 (1975). (18) H. D. Beckey, A. Heindricks, and H. U. Winkler, Int. J . Mass Spectrom. Ion Phys., 3 , 9 (1970). (19) H. R. Schulten, H D. Beckey, A. J. H. Boerboom, and H. I.C. Merizeloar, Anal. Chem., 45, 2358 (1973). (20) K. L. Olson, J. C. Cook, and K. L. Rinehart, Biomed. Mass Spectrom., 1, 358 (1974). (21) R. J. Beuhler, E. Flanigan, L. J. (;reen& and L. Friedman, J . Am. Chem. Soc., 96, 3990 (1974). (22) R. J. Beuhier, E. Flanigan, L. J. Greene, and L. Friedman, Biochemistry, 13, 5060 (1974). (23) R. F. Porter, W. A . Chupka, and M. G. Inghram, J . Chem. Phys., 23, 1347 (1955). (24) R. A. Hites and K. Biemann, Anal. Chem., 42, 855 (1970).
RECEIVED for review February 24, 1977. Accepted April 15, 1977.
Determination of Polyester Prepolymer Oligomers by High Performance Liquid Chromatography Louis M. Zaborsky I1 Process and Product Development, Firestone Synthetic Fibers Company, P. 0. Box 450, Hope well, Virghla 23860
A hlgh performance llquld chromatographic (HPLC) procedure for the determlnatlon of poly(ethy1ene terephthalate) prepolymer oligomers containlng from one to seven terephthalolyl repeat unlts is descrlbed. Adsorption chromatography uslng a Du Pont Zorbax-SIL column wlth chloroform/alcohol eluent (1OO:l by volume) gave the best results. The bls(2hydroxyethy1)terephthalate content determlned by gas chromatographic analysis Is used as the known standard for the HPLC separatlon. Analytical data for two commercial polyester prepolymer samples are included; preclslon and accuracy are within f1.0% for each compound.
The initial reaction in the production of polyethylene terephthalate (PET) is the esterification of terephthalic acid (TA) with ethylene glycol (EG). The actual esterification product consists of bis(2-hydroxyethyl)terephthalate (BHET), the desired compound, unreacted EG and TA, mono(21166
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
hydroxyethyl) terephthalate (MHET), as well as higher molecular weight oligomeric polyesters. The structures of these compounds are given in appendix A. Analytical methods for determining EG, TA, MHET, and BHET by silylation and gas chromatography (GC) have been described previously (I, 2). However, this procedure is capable of determining only -25% of the total prepolymer sample, as the higher molecular weight oligomers are not resolved by GC. Thin-layer chromatography (TLC) methods have been described for characterizing these high molecular weight oligomers (3, 4). However, quantitation of TLC chromatograms is difficult and hard to reproduce. Steric exclusion chromatography (GPC) has been used to determine monomer (BHET), dimer, and trimer in prepolymer samples with rather poor resolution (5). This paper describes a high performance liquid chromatographic (HPLC) procedure capable of quantitatively determining glycol-ended prepolymer oligomers containing from one to seven terephthaloyl repeat units. Quantitation is
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Table I. Fiber Industries BHET Characterization HPLC result Component GC result 0.2% NAa MHET 91.7% 93.5% BHET Dimer NAa 6.5% NA = Not Analyzed.
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Table 11. Polyester Prepolymer Results of GC Analyses Component Lot No. 1 Lot No. 2 1.33 3.20 EG, % 0.07 0.08 DEG, % TA, % 1.14 1.17 MHET, % 6.69 6.70 BHET, % 10.15 12.95 Total % 19.38 24.10
NUMBER
Figure 1. Polyester oligomer retention times accomplished by using the BHET content determined by GC (1) as the known standard for the HPLC analysis.
EXPERIMENTAL Chromatographic Equipment. All chromatograms were obtained using a Du Pont Model 830 liquid chromatograph equipped with a 1O-kL injection valve (No. 204590) and an ultraviolet detector (wavelength = 254 nm). The column was a 2.1-mm i.d. ('/*-in. 0.d.) X 25 cm long Du Pont Zorbax-SIL. A Spectra-Physics System I computing integrator was used to measure peak areas. Operating Conditions. All chromatograms were run at room temperature. The solvent pump pressure was maintained at 2000 psi. The solvent system was chloroform (1250 parts by volume) and reagent alcohol (12 parts by volume). The reagent alcohol (Fisher Scientific Co. No. A-962) consists of 90 parts ethanol and 5 parts each methanol and isopropanol. Chemicals and Reagents. All solvents were reagent grade and were purchased from Fisher Scientific Company. BHET was obtained in high purity from Fiber Industries Inc., Charlotte, N.C. Procedure. Approximately 15 mg of sample, weighed to 0.1 mg accuracy, were placed in a 3-dram snap-cap vial, and 5 mL chloroform/alcohol (9/1 by volume) were added. The sample was stirred on a magnetic stirrer until in solution. If complete solution was not attained, the solution was filtered through a 0.5-pm Fiberglas filter using a Swinney filter holder. Valve injections were made and chromatograms were obtained using the previously stated conditions. The GC analytical method used was that reported by Atkinson and Calouche ( I ) with some modifications: Between 25 and 40 mg of prepolymer sample, weighed to 0.1 mg, are placed in a 5-mL screw cap septum vial, and 1mL of a 2.0% bibenzyl in pyridine solution (internal standard) and 0.5 mL bis(trimethylsily1)trifluoroacetamide are added. The vial was heated at 100 "C for 10 min, then allowed to cool to room temperature. On-column injections were made onto a Varian 3700 GC equipped with dual flame ionization detectors. The injector and detector temperatures were 280 "C. The column was 0.25-in. 0.d. glass (2-mm i.d.), 2 m in length, and packed with 10% Dexsil300 on high performance Chromosorb W, 80/100 mesh. The column temperature was held at 80 "C for 3 min, then programmed at 20°/min to 275 "C. The helium flow rate was 30 mL/min. The GC analysis time using these conditions was approximately 15 min. Calculations. Because oligomer standards were not available, an equal ultraviolet detector weight response at 254 nm was assumed for all species, since they are a homologous series. As the BHET content of prepolymer samples can be quantitatively determined from the GC analysis, the higher oligomers are calculated from the following equation: Weight 5% Oligomer, - Area Oligomer,, X ?6 BHETGc -
Area BHET,,,,
Figure 2. Typical chromatogram of PET prepolymer sample. (A) Solvent front, (E) Heptamer, (C)Hemmer, (D) Pentamer, (E) Tetramer, (F) Trimer, (G) Dimer, (H) Monomer (BHET)
where % BHETGc= BHET content by GC, and Area BHETH~Lc = BHET peak area by HPLC. Mori et al. (3)reported that chromophoric complexes of polyester monomer (BHET), dimer, and trimer have approximately the same absorbance at a given wavelength per terephthaloyl equivalent. Since the oligomer equivalent weights per terephthaloyl group are also approximately the same, the equal UV weight response assumption appears to be valid.
RESULTS AND DISCUSSION Chloroform modified with methanol (1250 CHC13:15 MeOH) and reagent alcohol (1250 CHC13:12 Alc.) were the solvents used during this investigation. A plot of log retention time vs. oligomer number is shown in Figure 1,and illustrates that reagent alcohol is a more effective modifier for resolving the higher oligomers ( n = 4-7). A standard BHET solution (0.3 mg/mL in 9:l CHC13:Alc.) was run daily to monitor the column and solvent performance. The methanol modifier had a tendency to evaporate from the solvent tank, resulting in longer BHET retention times. The alcohol-modified solvent was much more stable over a longer period of time. The solvent polarity was adjusted with small amounts of alcohol to yield a BHET retention time of -40 min in order to obtain better resolution of the higher oligomers. This BHET sample was also assayed by GC to determine the total BHET content by weight; Table I lists the results. The HPLC analysis data are also in Table I; these results are based on area normalization. Thus, for relatively pure BHET, the two methods yield the same results. For this report, two representative P E T prepolymer samples from our production facilities were analyzed by the GC and ANALYTICAL CHEMISTRY, VOL. 49, NO. 8. JULY 1977
1167
Table 111. Polyester Prepolymer HPLC Analytical Results Lot No. 2 Component Lot No. 1 10.15 12.95 BHET‘ 14.52 16.55 Dimer X i0.35 i0.71 U 2.4% 4.3% RS D 13.30 14.46 Trimer X t0.63 i0.79 U 4.7% 5.5% RSD 10.42 9.14 Tetramer X t0.76 i1.02 U 7.3% 11.1% RSD 6.90 5.19 Peritamer X r0.59 t0.39 U 8.6% 7.5% RSD 5.00 3.47 Hexamer X i0.25 i0.15 U 5.0% 4.3% RSD Heptamer X 3.51 U i0.51 RSD 14.3% HPLC total (less BHET) 53.71 48.81 19.38 24.10 GC total 73.09 72.91 Total assay Number of determinations 5 4 a Results from GC analysis. HPLC methods. The GC results are listed in Table I1 and show less than 25% of the sample is determined; over 75% is unaccounted for. Table I11 lists the HPLC results with precision data for the two prepolymer samples. These results show the greatly increased assay of the prepolymer possible by using the HPLC method. The prepolymer chromatogram given as Figure 2 does show the presence of oligomers higher than heptamer which are not well enough resolved for quantitation. The HPLC procedure, like the TLC methods reported in the literature ( 3 , 4 ) ,is based on adsorption chromatography. The oligomers assayed by this HPLC method are of the G(TG), series, where G = ethylene glycol and T = terephthalic acid. Attempts to analyze MHET were unsuccessful, as no
peak eluted. Because of their higher polarity, carboxyl-ended oligomers based on MHET are retained on the column. Cyclic oligomers, if present, would elute rapidly because of their lack of free end groups. No cyclic oligomers were detected in any prepolymer samples; however, chloroform extracts of ground polyester resin did show a component which eluted shortly after the HPLC solvent front. Since commercial polyester contains approximately 1-2% cyclic trimer, the observed HPLC peak is most probably the cyclic trimer. Du Pont (5) has separated prepolymer oligomers through tetramer by steric exclusion chromatography (GPC), which is based solely upon molecular size separation. However, the resolution was poor and no oligomers above tetramer were even detected. The procedure described in this paper clearly resolves oligomers through heptamer with octamer and nonamer visible as shoulder peaks which are not quantifiable.
APPENDIX A The structures of the compounds discussed in this paper are as follows: 1. Ethylene glycol (EG): HOCH2CH20H. 2. Diethylene glycol (DEG): HOCH2CH20CH2CH20H. 3. Terephthalic acid (TA): 1,4-C6H4(COOH)2. 4. Mono(2-hydroxyethyl) terephthalate (MHET): 1,4CGH~(COOH)(COOCH~CH~OH). 5. For bis(2-hydroxyethyl) terephthalate and the higher oligomers, the following structure can be assigned: HO(CHzCHzOCO-1,4-C6H4COo),cH~CH2OH,where for n = 1, the compound is BHET; for n = 2, the compound is dimer; for n = 3, the compound is trimer, etc. LITERATURE CITED (1) E. R. Atkinson and S. I. Caiouche, Anal. Chern., 43, 460 (1971). (2) J. Yamanis, R. Vilenchlch, and M. Adelman, J . Chromatogr., 108, 79 (1975). (3) S.Mori, S. Iwasaki, M. Furusawa, and T. Takeuchi, J. Chromatogr.,62, 109 (1971). (4) K. Dimov and E. Terlemezyan, J . folym. Sci., 10, 3133 (1972). (5) R . C. Williams, Du Pont Instruments Applications Laboratory, private communication, July 9, 1971, “Analysis of Polyester Oligomers”.
RECEIVED for review March 9,1977. Accepted April 22,1977.
Experimentally Measured Mass Absorption Coefficients in Quantitative X-ray Diffraction Analysis Stefan0 Battaglia and Leonard0 Leoni” Institute of Mineralogy and Petrology, University of Pisa, 56 100 Pisa, Italy
Experimental mass absorption coefflclents, determlned from the measured lntensltles of Ag K a Compton scattered radlatlon, were used to carry out a quantitative x-ray diffraction analysls of a crystalline powder. The analysls was performed on three series of binary mixtures: quatlz-hematlte, quartr-calclte, and hematlte-calcite.
One of the most powerful techniques to obtain a quantitative analysis of a crystalline mixture is the x-ray powder diffraction technique. The basic relation between the dif1168
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
fraction intensity I,(hkl) of a crystalline component (i) contained in a sample and its concentration (Wt %) is given by:
x,
X. 1. = h 1.I 1 P
(1)
where is the total mass absorption coefficient of the sample at the diffracted wavelength and k , is a constant which depends on the characteristics of the apparatus and on the structure of component i. To correct for matrix effects (absorption), a constant and known quantity of a crystalline
