458
Anal. Chem. 1989. 61. 458-461
a change in the time scale of mixed-phase formation. It is conceivable that the slowly changing portion of the curve could be mistaken for the equilibrium value, which would appear to be dependent on flow rate. In our treatment of the two-ion problem, a number of complicating factors were excluded from consideration. Sandifer (15) has shown that for the case K X Y >> 1, AgY undergoes ion exchange into the bulk of the AgX membrane as well as at the surface, leaving pores in the membrane. Ion exchange in these pores can taken place at a much slower rate than a t the surface because of poor mass transport between solution in the pores and the bulk. This results in a long nonequilibrium situation that may give rise to false selectivity coefficients. In addition, the area of the membrane surface increases with pore formation, changing the time scale of eq 11. Also, the response of AgI electrodes to I- in solution is not controlled by diffusion a t low I- activities (16,17), which would greatly influence the applicability of any of these models. ACKNOWLEDGMENT We thank Etelka Gr&f and K l h a Tbth for their helpful discussions.
LITERATURE CITED Lindner, E.; Tbth. K.; Pungw, E. Anal. Chem. 1982. 54, 202-207. Lindner, E.; Tbth, K.; Pungor. E. Bunsekl Kagaku 1981, 3 0 , S67. Morf, W. Anal. Chem. 1983, 55, 1165-1168. Hulanicki, A.; Lewenstam, A. Anal. Chem. 1981. 5 3 , 1401-1405. Buck, R. P. Anal. Chem. 1988, 40, 1432-1439. Gfatzi, M.; Lindner. E.; Pungor, E. Anal. Chem. 1985, 57, 1506-151 1. Lewenstam, A.; Hulanicki, A.; Sokalski, T. Anal. Chem. 1987, 59, 1539-1544. Lindner, E.; Tbth, K.; Pungor. E.; Berube, T. R.; Buck, R. P. Anal. Chem. 1987. 59, 2213-2218. Lindner, E.; Tbth, K.; Berube, T. R.; Buck, R. P. Analytiktreffen 1985, 1985, 67-80. Nash, J. C. Numerical Methods for Computers: Llnear Algebra and Function Minimlzatlon; Misted Press: New York, 1979; pp 170-178. Gallant, A. R. Am. Stat. 1975. 29, 73. Schwab, G. M. KollddZ. 1942, 101, 204. Jaenicke, W. Z . Elektrachem. 1953, 57, 843. Jaenicke. W.; b a s e , M. 2.Ebktrochem. 1959, 63, 521. Sandifer, J. R. Anal. Chem. 1981, 53, 312. Lindner. E.; Tbth, K.; Pungor, E. Anal. Chem. 1982, 54, 72. Lindner. E.; Tbth, K.; Pungor. E. Dynamic Response of Ion-Selective Elechodes; CRC Press: B o a Raton, FL, 1988; Chapter 5.
RECEIVED for review April 11,1988. Accepted November 23, 1988. Support from the NSF (under Grants CHE8406976 and INST-8403331) and the Hungarian Academy of Sciences is gratefully acknowledged.
Interface To Eliminate High-Boiling Gel Permeation Chromatographic Solvents On-Line for Polymer Composition Drift Studies A.
H.Dekmezian*
Exxon Chemical Co., Polymers Group, 1900 East Linden Avenue, Linden, New Jersey 07036 Tetsuya Morioka
Tonen Sekiyukagaku Co. Ltd., 4-1 -1 Tsukiji, Chuoh-ku, Tokyo 104, Japan
An on-line interface device is described for collecting solvent-free polymer mlcrofractfons from a high-temperature gel permeation chromatograph. These polymer fractlons, collected on KBr dishes mounted on a fraction cdiector lncorporated into a vacuum oven, are then ready for subsequent off-llne composition analysis, such as by Fourier transform Infrared spectroscopy. The Interface is capable of elhlnaUng even the most demanding chromatographic solvents, e.g. trlchlorobenzene. The technique has been applied to the percent ethylene composition drift analysis of two ethyienepropylene rubbers.
INTRODUCTION Hyphenated analytical instruments have provided the analytical chemist with unprecedented power for on-line analysis of complex chemical compositions. Within the last two decades, techniques such as gel permeation chromatography/ultraviolet spectroscopy (GPC/UV) (I),size exclusion chromatography/pyrolysis/mass spectrometry (SEC/pyrolysis/MS) (Z),supercritical fluid chromatography/Fourier transform infrared spectroscopy (SFC/FTIR) (3), liquid chromatography/gas chromatography (4), and high-performance liquid chromatography/Fourier transform infrared 0003-2700/89/0361-0458$01.50/0
spectroscopy (-56)have emerged, enriching the arsenal of the analytical chemist. In general, the success of a technique depends on how well the solutes can be differentiated from the carrier medium. If the carrier medium is an inert gas, the differentiation is easy; if it is a liquid, the carrier medium may seriously interfere with the on-line analysis of a solute, limiting the scope of the technique. In high-performance liquid chromatography (HPLC), this situation has stimulated the design of various flow-through cells (7-10)as well as interfaces to eliminate volatile solvents prior to analysis (11-15). This was the subject of a recent review paper in which applications and limitations of various HF'LC/FTIR designs were discussed (16). However, high boiling solvents, such as 1,2,4-trichlorobenzene (TCB), are difficult to eliminate with the existing solvent-eliminationinterfaces. TCB is a commonly used good solvent in the gel permeation chromatographic (GPC) analysis of polymers (17),but on-line polymer composition analysis in the presence of such solvents by such techniques as flowthrough GPC/FTIR has been difficult, if not treacherous. Ideally, therefore, solvent-solute interference problems can be removed if the high-boiling solvent is eliminated altogether. Eliminating TCB in high-temperature (HT) GPC would enable one to determine composition drifts (CD) in polymers (18-21). CD is the change in comonomer composition of polymer chains as a function of molecular weight (Mw). Most previous polymer composition studies have depended on @ 1989 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 61. NO. 5, MARCH 1, 1989 459 large-scale fractionations (22) or cross-fractionations (23), which yield composition distribution data (i.e. fractionations based on both solubility and molecular weight differences of polymer chains). The solvents (or solvent mixture) from such fractionations are then stripped off to isolate the solutes for subsequent analytical measurements. The solvenestripping step is not always a must with all hydrocarbon polymers. In certain favorable cases empirical relationships can be established that relate solubilities of polymer chains to their chain structure and/or molecular weight, as in the case of semicrystalline copolymers of polyethylene. In contrast, amorphous hydrocarbon polymers do not benefit from such empirical relationships, and therefore most separations must depend on solvent/nonsolvent fractionations (24-26). And even though the fraction collection and/or solvent stripping steps can be automated (27) or robotized, solute isolation for further composition analysis is often the slow step and, therefore, quicker methods are needed. In this context, it must be mentioned that recent advances in flow-through GPC/UV have made on-line composition analysis of some polymers possible, but this technique is limited in its scope, since it requires the presence of W-active polymers; besides, only a very limited number of solvents can be selected. Similarly. a GPC unit has been recently coupled to an on-line flow-through FTIR detector (28) to determine the composition distributions of a variety of polymers. But again, solventaolute interference problems limit the useful range of such flow-through systems and do not permit the acquisition of clean, interference-free, and information-rich spectra of polymer fractions. This paper describes an interface device (29)for the on-line elimination of high-boiling solvents from polymer solutions emerging from an interfaced HT-GPC instrument. The solvent-free polymer fractions are deposited directly onto KBr dishes mounted on a carousel, the FTIR spectra of which are then automatically recorded, in a n off-line manner. And although FTIR is a less sensitive tool than UV, it is a more powerful structural probe because of its superior selectivity for chemical speciation. EXPERIMENTAL SECTION Polymer Samples. Two series-reactor ethylene-propylene rubbers (EPR1 and EPRO of different composition distribution (and drift) were chosen for experimentation. Instrumentation. A Waters 150-C ALC/GPC was used as a fractionator. Four Shodex columns,802,803,804 and 805, were coupled in series. Fractionation was performed at 135 "C with a flow rate of 0.5 mL/min using TCB as a mobile base. A 120-rL portion of 0.6 w t 70sample solution was injected in each run. A Mattson Sirius 100 FTIR spectrometer, equipped with a DTGS detector, was used for the infrared work. Spectral resolution was set to 8 em-', while the number of scans was set at w. No attempts were made to optimize the infrared spectral conditions. A vaccum oven, NAPCO Model 5831, was modified and used as the prototype interface. A pump, capable of evacuating to better than 1 mmHg, is necessary when eliminating highhiling solvents, but it may not be needed for low-boiling solvents. In the latter case, alternate means of evacuation, or even a flow of an inert gas through the chamber, may be all that is needed. In fact, the technology for removing volatile solvents had been developed by Griffiths et al. (14, 15).
Flash-Evaporation Fraction-Collection Interface. A prototype interface device was constructed as shown in Figure 1. The temperature and pressure of the vacuum oven, which incorporates a computer-controlled fraction collector, was regulated so that the solvent flash evaporated as GPC effluentdroplets fell onto KBr dishes mounted on the fraction collector. A Compaq computer was programmed to time an externally mounted stepper motor driving the fraction collector assembly through the stainless
I
lNSULRTlON
Flgure 1. Schematic drawing of solvent-elimhatlon interface. Table I. Vapor Pressure of TCB vapor pressure, "Hg
temperature, "C
760
400
100
40
10
5
1
213.0 187.7 140.0 114.8 01.7 67.3 38.4
steel shaft, while a photologir slotted optical switch (from Electronic Products) was used to sense collector position. The residence time at each collection pmition was preset, depending on the chromatographic response of the polymer studied. After the fractions were collected, the collector. loaded with microgram quantities of fractionated polymer, was removed from the vacuum oven fnr automated FTIR analysis. The design of the frartion collector depends on the goal of the planned experiment. If an off-line bTlR is to be used, the kaction collector in the vacuum oven may be identical to the FTIR autosampler in use, or vice versa. This allows easy swapping of fraction collectors between the vacuum oven and the FTIR autosampler. In one case, for example, a Mattnon autosampler was modified to huild the fraction collector incorporated into the vacuum oven. An aluminum disk 17.8 em in diameter and 0.95 cm thick, with 24 peripheral and equally spaced holes having 11 mm diameter was m a n u f a a d Each opening was further drilled to 13.1 mm diameter and a depth of 0.4 mm. Each opening was then ready to hold various frartion collecting media, such as 13." KBr plates, KBr dishes, or even glass vialn. In the experiments described below, the use of KBr dishes was preferred over KBr plater. KBr dishes were used to help confine the GPC effluent as well as the solute residue left behind (after the solventstripping step) to the center of the dish. If flat crystals are used instead, the residue tends to accumulate at the edges of the crystal, making it difficult to analyze the contents of the collector crystal. The KBr dishes were manufactured in-house from 13 x 2 mm plates by mounting them on a watchmaker's shock on a lathe, setting a 'Ile in. ball end mill at the center of each crystal, and boring 0.006 in. deep craters or dishes, at 500 revolutions per minute. The GPC effluentof interest was d u d into the oven through 0.009-in. stainless steel tubing inserted through a vacuum-tight opening on the sidewall of the oven. Although this tubing may be externally valved for proper flow-ratecontrol. it was not needed for the experimens described in this paper. As mentioned above, the temperature and pressure of the oven must be adjusted to assure efficient solvent flash evaporation. For TCR, the temperaturepressure relationship given in Table I was ronsulted. This relationship established a minimum temperature needed for the vapriirttiun of a single droplet, but actual oven temperatures were maintained at least 40 OC above the theoretical vnlue. This was to compensate fur heat loas as falling droplets eventually drop the surface temperature of the KHr crystal. Another way of compensating for such heat loss is to provide an external heat source to keep the surface of the crystal hot. With TCR as the chromatographic solvent. a stream of hot nitrogen blowing over the collecting dish was found to be beneficial. This stream also effectively eliminated splashing problem. The heated nitrogen was introduced via a 0.09-in. flexible metal tubing. The temperature of the nitrogen was controlled by use of an external cuntrol box and a thermocouple positioned after
460
ANALYTICAL CHEMISTRY, VOL. 61, NO. I
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0.021
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2900
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2800
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the heating cartridge. The tip of the nitrogen source was equipped with a baffle to control the size of the heated area. This flow of inert gas also helped remove the flashed-off solvent from the vacuum chamber and directed it into a cold trap system. Also of importance was the temperature of the transfer line tubing, the one connecting the GPC to the vacuum oven. It was found that for the experiments described below, the temperature of this line must be maintained between 95 and 105 "C. Insufficient heating led to slow rates of solvent elimination, while excessive temperatures led to premature evaporation at the tip of the effluent tube. Detection Limit of Transmission FTIR. This was evaluated by using thin polymer films. A solution of 83 pg/mL of ethylenepropylene rubber (EPR) in CCl, was prepared, then 8 pL of it was deposited on a KBr dish and dried to form a thin film. The amount of EPR deposited was about 660 ng. The FTIR spectrum of this film was then obtained by using 500 scans. "Thicker" films were also obtained by depositing more of the abovementioned solution on separate KBr dishes. Experimental Details. The carousel movements were timed based on an analysis of the elution time profile from a previous chromatogram. During the actual fraction collection, the DRI detector was bypassed. In two separate experiments, seven fractions were collected from each of the two rubbers. The interface temperature was maintained at 120 "C, oven pressure was in the Vicinity of 5-10 mmHg, and the nitrogen flow rate was about 3 cm3/min. Percent Ethylene Analysis of EPR Fractions. To calibrate for percent ethylene in EPR fractions,six ASTM EPR standards, ranging in ethylene content between 35% and 80%, were used. A linear least-squares regression analysis of the data, using absorbance (A) values at 2922, 2850, and 2953 cm-l, gave the following equation: [(A2922
+ Am)/A29531
3000
1
, 2800
I 2000
, 2700
Wavenumber (cm-1)
Figwe 2. Partial spectrum of 660 ng of EPR deposited on a KBr plate. Spectrum measured with 8 cm-' resolution and 500 scans.
% ethylene = 37.74 In
I
I
3100
- 3.77
RESULTS AND DISCUSSION Detection Limit of Transmission FTIR. Two spectral regions can be used to detect EPR down to very low concm-', centrations. One is the C-H stretching region, 2-3000 the other is the C-H bending region, 1350-1500 cm-'. Since the intensities of the stretching signals are 5 to 6 times stronger than the latter, it was decided to limit our initial efforts to the stretching region. The C-H stretching region of the FTIR spectrum of 660 ng of EPR sample deposited on a KBr dish is shown in Figure 2. The absorbance at 2962 cm-' (vCHs) was 2 x IO4 and signal to noise ratio was better than 2. Therefore microgram quantities of EPR fractions are sufficient for quantitative analysis. The typical concentration of low and high ends in the GPC effluent is of the order of lo4 g/mL and if 10 fractions are collected per sample, then the collection period of each fraction is about 2 min and the quantity of deposited sample at high and low ends will be about 1 pg. Since the total quantity of sample injected into the GPC column is typically 720 pg, and
Figure 3. Partial spectrum of 6.6 pg of EPR deposited on a KBr plate. Spectrum measured as in Figure 2. loot
lo-' lo-?
I
to-6 1 0 - ~ Polymer Mass (Grams)
Figure 4. Linearity of absorbances at 2962 and 2926 cm-' as a function of mass of ethylene-propylene rubber deposied on KBr plates. if 16 fractions are collected, then we can estimate that each fraction will contain about 45 pg of sample. Therefore, the sensitivity of the transmission FTIR technique, using the C-H stretching region, is more than adquate for reliable ethylene CD analyis in an EPR using GPC as the fractionating medium. A residue of 6.6 pg would still have an excellent signal-tonoise ratio, as shown in Figure 3. Figure 4 shows the correlation of peak intensities and quantity of sample deposited. A linear relationship is observed in the 10-6-10-3 g range. Also, peak intensity ratios in the spectra are constant in this concentration range. Therefore, we can expect to get quantitative information by using these peaks, provided that the fractions are no less than 500 ng in mass. Higher sensitivities could be achieved with a different choice of infrared hardware. Effect of Vacuum on GPC Performance. The GPC system pressure was not affected by pressure drop as the vacuum chamber was evacuated (actual pressure was in the range of 14-16 bar at 0.5 mL/min flow rate). An analog-signal record of the system pres'sure showed a minor decrease (because of reduction of back pressure), but had no effect on the elution time. Effect of Deposition Method. From the pressure-temperature relationship of TCB shown in the Table I, it was expected that at 5 mmHg pressure, this solvent would flashevaporate at 67 "C. However, under the prevailing conditions, the evaporation was rather slow, particularly after a few drops of the solvent had flashed off during the early part of the collection period. The temperature of the oven had to be increased up to 120 "C to achieve smooth flash-evaporation. This was ascribed to the fact that the heat of vaporization is not negligible and that the temperature of the KBr surface drops quickly far below that of the oven as more and more droplets impinge upon it. The geometry of the collecting KBr crystal was found to be important. If a flat KBr plate is used, the solution tends
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1,
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(Minutms)
Elutlon Tlm.
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Flgure 5. Percent ethylene composition distribution (top trace) in
Figure 6. Percent ethylene composition distribution (top trace)
EPR-1. The chromatographic cuts are shown on the GPC chromatogram (bottom trace).
gram (bottom trace).
to flow to the edges of the plate before it evaporates. Consequently, the polymer tends to deposit along the edges and may result in poor-quality spectra. Therefore, the use of KBr dishes was preferred; a KBr dish acts like a small "vial" and helps position the solute a t the center of the plate. If, on the other hand, flat KBr plates are to be used, then the GPC effluent must be applied to the plate as a fine mist or microdroplets, using such devices as nonelectrostatic ultrasonic atomizing devices (30). These are preferred over pneumatic devices because they can deliver low velocity soft sprays that eliminates "bounce back". Moreover, the amount of nitrogen entering the chamber can be controlled independent of the atomizing device. The 10-20-pm particles thus generated tremendously help the flash-evaporation process, depositing a thin layer of fractionated material on the collection plate. The nozzle geometry and air current flow patterns within the oven must be carefully adjusted to direct the atomized particles to the center of the collecting plate. More will be said about this subject in a future paper. Composition Drift in Polymers. Ethylene-propylene rubber (EPR) is typically analyzed by using high-temperature GPC. The rubber is dissolved in a "good" solvent such as TCB to ensure dissolution of the high ethylene components. It is very difficult, if not impossible, to analyze the composition of the rubber fractions in the overpowering presence of the solvent. But by use of the interface described here, that interference was eliminated. The two EPR samples, EPR-1 and EPR-2, were fractionated by using the current setup. From the spectra of the fractions, the percent ethylene of each fraction was calculated. The retention time vs percent ethylene profile is shown in Figure 5 for EPR-1. The vertical lines on the GPC curve show the timing of the fractionation scheme. The percent ethylene is lower at the extremes of the molecular weight distribution, consistent with the particular series-reactor process used. Ethylene composition distribution in EPR-2 is shown in Figure 6. In this case the percent ethylene decreases with decreasing molecular weight. The presence of a low ethylene, low molecular weight fraction is evident in the figure. Although these profiles were obtained from a single GPC fractionation, the extension to more complex cross-fractionations or solvent/nonsolvent fractionations is obvious. The ease of obtaining such composition distribution information is unprecedented.
CONCLUSION The concept of on-line flash evaporation of high boiling solvents such as TCB has been successfully demonstrated.
in
EPR-2. The chromatographic cuts are shown on the GPC chromato-
The device described here can form the basis of a GPC/FTIR interface for on-line monitoring of chemical compositions of GPC effluents. A continuous on-line system would also be a useful extension to achieve process control. Although the examples given here are based on fractionations using size exclusion chromatography, the interface may be adapted, with proper modifications and valvings, to other fractionations based on crystallinity differences (temperature rise elution fractionation), field flow effects (31,32),adsorption chromatography, or others which may involve emulsions or colloidal solutions.
LITERATURE CITED Mori, S.;Suruki, T. J. Liq. Chromatogr. 1981, 4(10), 1685. Mori, S . J. Chromatogr. 1980, 194, 163. French, S . B.; Novotny, M. Anal. Chem. 1986, 58, 164. Davies, I. L.; Bartie, K. D.; Williams, P. T.; Andrews, G. E. Anal. Chem. 1988, 60, 204. Kuehl, D. T.; Griffiths, P. R. Anal. Chem. 1980, 52, 1394. Kalaslnsky, K. S.; Smith, J. A. S.;Kalasinsky, V. F. Anal. Chem. 1985, 5 7 , 1969. Vidrine, D. W.; Mattson, D. R. Appl. Spectrosc. 1978, 32, 502. Viirine, D. W. J. Chromatogr. Sci. 1979, 17, 477. Wang, C. P.; Sparks, D. T.; Williams, S. S.; Isenhour, T. L. Anal. Chem. 1964, 56, 1268. Johnson, C. C.; Taylor, L. T. Anal. Chem. 1984, 56, 2642. Conroy, C. M.; G r i f f ~ s P. , R.; Duff, P. J.; Azarraga, L. V. Anal. Chem. 1984, 56, 2636. Conroy, C. M.; Griffiths, P. R.; Jinno, K. Anal. Chem. 1985, 5 7 , 822. Griffiths, P. R.; Conroy, C. M. A&. Chromatogr. 1986, 25, 105-138. Griffiths, P. R.; Pentoney, S. L., Jr.; Giorgetti, A,; Shafer, K. H. Anal. Chem. 1988, 58, 1349A-1366A. Hellgeth, J. W.; Taylor, L. T. J. Chromatogr. Sci. 1986, 24, 519-528. Jlnno, K.: Fujimoto, C.; Ishii, D. J. Chromatogr. 1982, 239. 625. Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Slze Exclusion Liquid Chromatography; Wiley: New York, 1979; Chapter 8. Melster, J. J.; Nicholson, J. C.; Patil, D. R.; Field, L. R. Macromohscules 1986, 19, 803. Ogawa, T. J. Appl. Polym. Sci. 1979, 23, 3515. Wild, L.; Ryle, T. R.; Knobeloch, D. C.; Peat, I. R. J. Polym. Sci.: Polym. Phys. Ed. 1982, 20, 441. Mori, S. J. Chromatogr. 1960, 194, 163. Kamide, K.; Mizayaki, Y.; Abe, T. E r . Polym. J. 1981. 13, 168. Nakano, S.:Goto. Y. J. Appl. Polym. Sci. 1981, 26, 4217. Sato, H.; Takeuchi, H.;Tanaka, Y. Macromolecules 1986, 19, 2613. Teramachi, S . ; Fukao, T. Polym. J. 1974, 6, 532. Termachi, S.; Nagasawa, M. J. Macromol. Sci., Chem. 1968, A2(6), 1169. Newhouse, D. L.; Wheeler, R. G.; Waitz, R. H. U S . Patent 4,604,363, 1986. Marketed by Misubishi Petrochemical Company Limited, Mle, Tokyo, Japan. Dekmezian, A. H.; Morioka, T. U. S. Patent Applied (June 1988). Sono-Tek Corp., Patent Nos. 3,861,852; 4,655,393; and others. Schlmpf, M. E.; Giddlngs, J. C. Macromolecules 1987, 2 0 , 1561. Wahlund, K. G.; Winegarner, H. S.; Caidwell, K. D.; Glddlngs, J. C. Anal. Chem. 1986, 58, 573.
RECEIVED for review June 20,1988. Accepted November 28, 1988.
