Evaluation of the Basal Plane of Pyrolytic Graphite as an Electrochemical Detector for Liquid Chromatography R. M. Wightman," E. C. Paik, S. Borman, and M. A. Dayton Department of Chemistry, Indiana University, Bioomington, Indiana 4 740 1
Electrochemistry at the basal plane of pressure-annealed pyrolytic graphite is characterized by slow heterogeneous charge-transfer rates for organic compounds In aqueous buffers. Anodic oxidation of the basal plane in citrate-acetate buffer results in long-term changes in the electrochemical properties of this surface. Charge-transfer rates are greatly increased for both reductlon and oxidation. The modified basal plane operates for extended periods of time without further surface modification as a detector for compounds separated by high-performance liquid chromatography. The electrode performance compares very favorably with carbon paste electrodes for the detection of trace amounts of oxidizable organic compounds, and is comparable to reported mercury electrodes for the detection of reduclble organic compounds.
plane of pressure-annealed pyrolytic graphite (PAPG) is an extremely versatile electrode for liquid chromatography detection. This material has a high degree of parallel alignment of the graphite crystallites. The basal plane of PAPG was selected for evaluation in this application because it has an extremely flat surface which is impervious to organic and aqueous solutions. Furthermore, the surface can be easily renewed, and a large potential range is available (14). T o date, we have invest.igated the application of this electrode for the detection of both Oxidizable and reducible organic compounds. Our results demonstrate that this electrode is superior to the commonly used electrodes in LCEC applications. In addition, electrochemical and electron microscopy experiments are reported which give insight into some of the factors which affect electron transfer a t pyrolytic graphite electrodes.
Liquid chromatography with electrochemical detection (LCEC) has become a widely accepted technique, especially in the determination of trace amounts of neuronally important compounds in brain tissue (1,2). For electroactive compounds, the LCEC method is extremely useful since it provides a selective detector and thus can be used in conjunction with very simple separation methods. Although different variations of the 1,CEC technique have been proposed (3, 4 ) , most routine analyses of oxidizable organic compounds employ a carbon paste working electrode. Various types of mercury electrodes have been used for the amperometric detection of reducible compounds (5-7), but the reported detection limits are nowhere near as low as those for oxidizable substances a t carbon paste. Amperometric detection a t carbon paste electrodes is advantageous because the electrodes are stable for months without surface treatment, are extremely sensitive (femtomole limits of detection have been achieved), and the instrumentation and associated electronics are relatively inexpensive ( I ) . Carbon paste electrodes provide low and stable anodic residual current which is an extremely important criterion in their utility in LCEC. However, high cathodic residual currents make them unsuitable for the detection of most reducible compounds, and their dissolution in organic solvents restricts the type of chromatography that can be employed (8). Furthermore, different formulations of carbon paste exhibit variations in sensitivity. T h e difficulties and precautions necessary in preparing carbon paste electrodes for LCEC have been previously documented (9). Improper electrode preparation results in spurious signals because of flaking of the paste from the electrode surface, projection of the paste into the solution cavity, and extension of the paste into microscopic cracks in the electrode holder. In recent years a large number of different forms of carbon have been found to be equally as satisfactory as carbon paste in various electrochemical applications (10, 2 1 ) . Notable among these are glassy carbon (12, 23) and pyrolytic graphite, both of which accommodate rapid heterogeneous charge transfer and exhibit low residual current. In this paper it will be demonstrated that, with surface modification, the basal
EXPERIMENTAL Reagents. All buffer solutions were prepared from reagent grade chemicals without furt,her purification and from water distilled in glass from alkaline permanganate. Dopamine (DA) and 5-hydroxytryptamine (5-HT) were obtained from Sigma Chemical Co. (St. Louis, Mo.). Stock solutions of these compounds were prepared in deoxygenated 0.1 M HCI04 with 0.2 mM NaHS03 and were stored in a refrigerator. Under these conditions, mixtures of DA and 5-HT showed no decrease in concentration for approximately two weeks. Gram quantities of the amino acid derivatives were prepared by reacting equal amounts of 2,4-dinitrobenzenesulfonyl chloride with y-aminobutyric acid and aspartic acid, respectively, at pH 8.5. The solution was acidified after 60 min, extracted with ether, and evaporated to dryness. Mass spectra, NMR spectra, IR spectra, and 'TLC of the isolated products confirmed that pure N-(2,4-dinitrobenzenesulfonyl)4-aminobutyric acid (DNBSG) and N-(2,4-dinitrobenzenesulfony1)asparticacid (DNBSA) were the isolated products. Stock solutions of these compounds were prepared in pH 4.0 acetate buffer and stored under refrigeration. Apparatus. The liquid chromatographic apparatus was identical to that described previously for the detection of oxidizable species (9). It consisted of a constant flow rate reciprocating pump, Teflon tubing and damping coil, injection port, glass column (2-mm i.d.), and electrochemical detector. Separations of DA and 5-HT mixtures were accomplished with a Zipax cation-exchangeresin (SCX, Du Pont) and a citrate-acetate buffer adjusted to pH 5.2 ( 1 5 ) . Various changes were made in this system to facilitate the separation and detection of the amino acid derivatives. A major problem in the amperometric detection of reducible compounds is the presence of oxygen in the buffer. Very low oxygen levels were achieved by replacing all of the Teflon tubing with stainless steel, eliminating the Teflon damping coil, and bubbling a vigorous stream of nitrogen through the buffer reservoir. Although satisfactorily low residual currents were obtained in this manner, the oscillation in the flow of the mobile phase increased the background noise. Separation of the amino acid derivative was accomplished with Pellamidon (Whatman) using acetate buffer of pH 4.2. Compounds separated by liquid chromatography were amperometrically detected with the basal plane of pressure-annealed pyrolytic graphite (UGAR Oriented Graphites, Grade ZYA, Union Carbide, Parma, Ohio), or with carbon paste electrodes. The PAPG of 0.25-cm thickness was cut into 2.50 cm X 0.95 cm
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Scanning electron micrographs were obtained with an Etec Autoscan U-1. Dynamic focus control and foreshortening were used for all micrographs.
RESULTS AND DISCUSSION Basal Plane of PAPG as an Anode in LCEC. The basal
Flgure 1. Exploded view of the electrochemical cell to evaluate the basal plane of PAPG. RE, reference electrode; AE, auxiliary electrode; WE, working electrode. Lucite forms the top (a)and bottom (b). The spacer (SP)is 125 pm thick. Arrows indicate the direction of solution flow
rectangles. A fresh basal plane was exposed by placing a piece of cellophane adhesive tape in contact with the graphite surface, and then peeling off a layer of graphite with the tape (14). In some experiments the electrode was impregnated with molten ceresin wax. The excess was removed with cotton swabs dipped in methylene chloride until a shiny finish was obtained. The PAPG was then incorporated in the three-electrode cell shown in exploded form in Figure 1. Electrical contact to the PAPG is made by a copper sheet under the graphite (not shown). The entire assembly is clamped together with four screws, with the exception of the platinum auxiliary electrode which is held in place with epoxy cement. The geometric area of the working electrode is approximately 0.3 cm2. The design of the carbon paste electrode was identical to that described previously (9). The carbon paste was prepared from graphite powder (Ultra F purity, Ultracarbon Corp. Bay City, Mich.), and Dow Corning High Vacuum silicone grease (35% silicone grease by weight). The geometrbarea of the portion of the carbon paste electrode exposed to the solution is approximately 0.16 cm2. Amperometric detection following liquid chromatography was achieved either with a commercial potentiostat designed especially for liquid chromatographic applications (LCBA, Bioanalytical Systems, West Lafayette, Ind.) or with a polarographic analyzer (Model l74A, Princeton Applied Research Corporation) operated in the dc mode at constant potential. To prevent potentiostat oscillations, which arise with close electrode proximity as shown in Figure 1, a 1-@F capacitor was connected between the reference and auxiliary electrode. All potentials are referred to a saturated calomel electrode. Cyclic voltammetry at the basal plane of PAPG was also performed in the cell shown in Figure 1 with the polarographic analyzer in the dc mode. For these experiments, the cell was filled with the deoxygenated solution of interest, and the entrance port was sealed. At a scan rate of 100 mV/s and with a 125-pm spacer, the diffusion layer is sufficiently small for semi-infinite linear diffusion conditions to apply. A 100-pF capacitor was placed between the reference and auxiliary electrode in these experiments. As will be shown. anodic oxidation of the basal plane of PAPG is required for satisfactory operation of the electrode as an LC detector. To oxidize the surface, a flowing stream of buffer is passed through the electrode assembly shown in Figure 1, and the oxidation is potentiostatically controlled using the polarographic analyzer. The commercial LC potentiostat has an insufficient current transducer range for this surface treatment. However, the oxidations are possible with this instrument by connecting the working electrode directly to ground. Although problems with uncompensated cell resistance are normally small with the concentrations used in LCEC, they are significant with the currents required for the anodic treatment. Therefore, results comparable to those reported here cannot be obtained with a less efficient electrode geometry (the auxiliary electrode must be parallel to the working electrode).
plane of PAPG was evaluated for the amperometric detection (E,,, = +0.5 V) of mixtures of DA and 5-HT separated on a cation-exchange liquid chromatographic column. T h e heights of the chromatographic peaks were compared with those obtained a t conventional carbon paste electrodes. For the initial injection, peak heights at a freshly exposed basal plane were comparable to those obtained a t carbon paste electrodes. However, the peak height for subsequent identical injections rapidly diminished, and within 24 h peak heights were approximately 50% of those obtained initially. After mechanical polishing of the basal plane, identical results were obtained. PAPG that had been wax-treated gave uniform chromatographic peak heights; however, they were approximately 40 times smaller than carbon paste for DA and 10 times smaller than carbon paste for 5-HT. Obviously, in all of these cases PAPG is inferior to carbon paste as an LCEC detector electrode. Preoxidized Basal Plane of PAPG as an Anode in LCEC. Numerous investigators have observed that the results obtained at carbon electrodes are greatly influenced by surface pretreatment. For example, radiofrequency etching of pyrolytic graphite electrodes in an oxygen plasma increases heterogeneous charge-transfer rates for certain compounds, apparently by altering the types of functional groups a t the surface of the carbon electrode (16, 17). Pretreatment of carbon electrode surfaces by cycling the potential between anodic and cathodic limits alters the surface electrochemical waves of carbon electrodes (18, 19),and also increases heterogeneous charge-transfer rates (20,21). Therefore, the effect of anodic oxidation of the basal plane of PAPG was evaluated. The working electrode was potentiostated a t 1.4 V for 20 min (current density of 8 mA/cm2) in a flowing stream of citrate-acetate buffer (0.018 F acetic acid, 0.050 F sodium acetate, 0.025 F citric acid, adjusted to p H 5.2). Subsequently, the detector was potentiostated a t 0.5 V, and the chromatographic peak heights were observed for mixtures of DA and 5-HT. The peak heights observed were larger ( 2 5 4 0 % )than those obtained at a freshly exposed basal plane. The electrode response faded approximately 10% in the first 24 h of operation, but peak heights for identical injections remained constant for extensive periods of time subsequent to this initial break-in period (Figure 2). In fact, in a three-month comparison of the preoxidized basal plane PAPG electrode with carbon paste, the longevity of the PAPG electrode exceeded that of carbon paste without further detector treatment. Noise levels were approximately three times smaller than with carbon paste electrodes (Figure 3). Like carbon paste electrodes, the electrochemically modified basal plane of PAPG exhibits linear current response to concentration over five orders of magnitude (22). These results are not improved by longer electrolysis times. Identical results are obtained with wax-treated electrodes after anodic oxidation. We have used these electrodes in routine analysis of brain samples for nine months with only occasional electrode replacement. The major source of electrode failure is solution leakage around the Teflon spacer. Surprisingly, preoxidation of the basal plane of PAPG in acetate buffer alone at similar p H and ionic strength, and identical current density, does not result in a stable LCEC detector. As shown in Figure 2 an acetate-electrolyzed electrode gives a rapidly decreasing peak height for DA and becomes very insensitive after 36 h. In both acetate and citrate-acetate buffers, the applied potential was selected as
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Figure 4. Cyclic voltammograms at the basal plane of PAPG, (a) and (c) before surface modification, (b) and (d) after surface modification. buffer; (c), (d) 0.085 mM DNBSG in pH 4.0 acetate buffer. Scan rate, 100 m V / s
(a),(b) 0.1 mM CIA in pH 4.0 acetate
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Figure 2. LCEC response of the basal plane of PAPG to 20 pmoi of DA following anodic oxidation in different solvents. ( 0 )Prior oxidation in citrate-acetate buffer. (0)Prior oxidation in acetate buffer
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Flgure 3. Chromatograms of a mixture of 1 pmol each of DA and 5-HT. (a) Detection at the basal plane of PAPG following oxidation in citrate-acetate buffer. (b) Detection at a carbon paste electrode. The chromatograms were obtained under identical column conditions, and the noise levels are authentic traces of the recorded data
the onset of gas evolution (+1.6 V for acetate, +1.4 V for citrate-acetate). These results indicate that the mechanism of oxidation of citrate plays an important role in the long-term stability observed with these electrodes but the exact chemical intermediates formed in the anodic oxidation have not been determined. Acetate oxidation at carbon electrodes is known to generate carbon dioxide and possibly carbonium ion intermediates (23). With citrate, one would expect to generate larger amounts of C O P ,but whether this is the determining factor in the stable surface modification cannot be determined from these experiments. Anodic oxidation of carbon paste electrodes in citrateacetate buffer was also investigated. Following oxidation of the electrode surface, peak heights were approximately 50% larger; however, the sensitivity decreased to previous levels within 24 h. Characterization
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PAPG. Cyclic voltammetry of DA in p H 4.0 acetate buffer
demonstrates that, for this compound, heterogeneous charge-transfer rates have been significantly increased by anodic preoxidation in citrate-acetate buffer. Figure 4a is a cyclic voltammogram obtained a t a wax-treated basal plane of PAPG, whereas Figure 4b was obtained with the same solution after the basal plane had been oxidized. T h e voltammogram recorded at a nonelectrolyzed electrode with DA present has a larger current than that of the buffer alone, but no obvious oxidation waves are apparent. T h e cyclic voltammogram in Figure 4b exhibits a 120-mV separation of the anodic and cathodic waves, whereas 200-mV separation is obtained at carbon paste under identical solution conditions. T h e increase in heterogeneous charge-transfer rate is also observed in the reduction of DNBSG as seen in Figures 4c and 4d. The nonelectrolyzed electrode exhibits virtually no current for the reduction of DNBSG, but two chemically irreversible waves can be obtained a t an electrode that has been preoxidized. In this case, a comparison with carbon paste electrodes is not feasible since oxygen incorporated in the carbon paste gives electrochemical waves of sufficient magnitude at this concentration to obliterate all meaningful data. With both oxidations and reductions, an electrode that is neither wax-coated nor electrolyzed exhibits behavior intermediate to these cases, that is, the waves are more prominent than in Figures 4a and 4c, but far less reversible than observed in Figures 4b and 4d. Scanning electron micrographs of the surface of the basal plane of PAPG electrochemically modified in citrate-acetate and acetate buffers were obtained with the electron beam a t a 70' angle to the surface to examine the roughness of the In each electron micrograph, the surface (Figure 5 ) . right-hand side of the carbon surface was covered by the Teflon spacer, and was not oxidized. T h e left-hand side of each of the electron micrographs, the portion that was oxidized, shows that extensive roughening of the electrode surface arises from anodic oxidation of the surface. It is also apparent that the P U G oxidized in a citrate-acetate buffer has a much rougher surface than that oxidized in a pure acetate buffer. The electron micrographs and electrochemical data presented here and reported in the literature give several insights into the observed performance of the modified basal plane of PAPG as a liquid chromatographic detector. Surface roughening alters the crystalline arrangement of the basal surface and exposes more edge orientation (the plane perpendicular to the basal plane). T h e rate of electrochemical reduction or oxidation of organic compounds greatly increases a t this modified surface in aqueous buffers. Similarly, the rate of oxygen reduction is much greater at the authentic edge
\IALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
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Figure 6. Chromatograms of mixtures of amino acid derivatives. Arrow indicates injection time, the first peak is solvent front, the second is DNBSG, and the third is DNBSA. (a)8.5 pmol DNBSG, 38 pmoi DNBSA; (b) 17 pmol DNBSG, 76 pmol DNBSA
Figure 5. Electron micrographs of the basal plane of PAPG. (A) Prior oxidation in citrate-acetate buffer. (B) Prior oxidation in acetate buffer
orientation of PAPG than a t the basal plane (21). These data indicate that electrochemical reactions which involve protonand electron-transfer are facilitated a t the edge orientation. Other factors which may play an important role in the observed electrochemistry have also been considered. A large increase in microscopic area would lead to much larger charging current as well as enhanced Faradaic current. However, the increase in charging current following surface oxidation is small, and this charging current is much less than t h a t observed a t the authentic edge orientation ( 2 4 ) . Cyclic voltammograms indicate t h a t the electrochemistry is diffusion-controlled and, thus, irreversible adsorption of depolarizer is not a major effect. Simple exposure of the edge orientation by mechanical abrasion or anodic oxidation in acetate buffer does not result in an electrode with the long-term stability exhibited by electrodes oxidized in citrate-acetate buffer. Thus, modifications in the surface functional groups may well be important. Obviously, further experimentation is needed to clarify these alternatives. Basal Plane of PAPG as a Cathode in LCEC. Mixtures of the two amino acid derivatives, DNBSG and DNBSA, were analyzed by LCEC. T h e amino acids are of interest because of their role as putative neurotransmitters in mammalian brain. T h e derivatizing agent, 2,4-dinitrobenzenesulfonyl chloride, was selected because it is easily reduced in a multielectron process (2-four-electron waves (2511, and because the chemical derivatization of amino acids with a similar chloride) reagent (N,N-dimethylaminonaphthalenesulfonyl
is well documented (26). With an unmodified basal plane (Eapp = -0.8 V), peak heights were very small as would be expected from the cyclic voltammetry (Figure 4c). However, entirely satisfactory chromatographic measurements were obtained with an oxidized electrode surface as shown in Figure 6 (Eapp = -0.5 V). Peak heights were doubled a t the potential of the = 4 . 8 V), but oxygen reduction contributed second wave (Eapp to a less stable baseline. Even though the separation is relatively inefficient, and the noise from the undamped solution flow is considerable, the amounts of compound determined are lower than those previously reported at mercury electrodes in LCEC applications. Efforts are currently underway to improve this separation and to dampen oscillations in the mobile phase.
CONCLUSIONS T h e basal plane of PAPG provides an extremely useful electrode for use in LCEC when pretreated by anodic oxidation in a citrate-acetate buffer. T h e electrode is superior to carbon paste, the most commonly used electrochemical detector, in many respects. The electrode is easier to prepare than carbon paste, and exhibits slightly superior sensitivity. Like carbon paste, it is stable for months without surface treatment and exhibits a wide dynamic range. Unlike carbon paste, it is suitable for the detection of reducible compounds at negative potentials. The long-term electrochemical surface modification is successful in citrate-acetate buffers, but not in acetate buffer. Electron micrographs show that the oxidation of the surface results in increased surface roughness. Future experiments will be directed a t examining the chemical and physical processes important in this modification, the use of the electrode in nonaqueous solvents, and the application of the detector in the detection of reducible compounds derived from brain samples.
ACKNOWLEDGMENT T h e recommendation of PAPG for LCEC detection by R. J. Brodd, and discussions of charge transfer with G. M. Brown are gratefully acknowledged.
LITERATURE CITED (1) P. T. Kissinger. Anal. Chern., 49, 447A (1977). (2) R. N. Adams, Anal. Chern.. 48, 1126A (1976).
1414 (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978 Y. Takata and G. Muto, Anal. Chem., 45, 1864 (1973). D. G. Swartzfager, Anal. Chem,, 48, 2189 (1976). D. L. Rabenstein and R . Saetre, Anal. Chem., 49, 1036 (1977). R. C. Buchta and L. J. Papa, J . Chromatogr. Sci., 14, 213 (1977). T. Wasa and S. Musha, Bull. Chem. Soc. Jpn., 48, 2176 (1975). R. N. Adams, "Electrochemistty at Soli Electrodes", Marcel Dekker, New York, N.Y., 1969. R. Keller, A. Oke, I. Mefford, and R . N. Adams, Life Sci., 19. 995 (1976). R. E. Panzer and P. J. Elving, J, Nectrochem. Soc., 119, 864. (1972). R. E. Panzer and P. J. Elving, Electrochim. Acta. 20, 635 (1975). 8.Fleet and C. J. Little, J. Chromatogr. Sci., 12, 747 (1974). J. Lankelma and H. Poppe, J. Chromatogr., 125, 375 (1976). J. Randin and E . Yeager, J. €/ectroana/. Chem., 38, 257 (1972). S. Sasa and C . L. Blank, Anal. Chem., 49, 354 (1977). J. F. Evans, T. Kuwana, M. T. Henne, and G. P. Royer, J . Electroanal. Chem., 80, 409 (1977). J. F. Evans and T. Kuwana, Anal. Chem., 49, 1632 (1977). B. D. Epstein, E. Dalle-Molle, and J. S. Mattson, Carbon, 9, 609 (1971). G. Mamantov. D. B. Freeman, F. J. Miller. and H. E. Zittel, J Nectroanal. Chem., 9, 305 (1965). W. J. Biaedel and R. A. Jenkins, Anal. Chem., 47, 1337 (1975).
(21) (22) (23) (24) (25) (26)
I. Morcos and E. Yeager, Electfochim. Acta, 15, 953 (1970). C. L. Blank, J . Chromatogr.. 117, 35 (1976). W J. Koehl, J . Am. Chem. Soc., 88, 4686 (1964). J Randin and E. Yeager, J. Electroanal. Chem., 58, 313 (1975). G. W. Jackson and J. S. Dereska, J. Electrochem. Sac., 112, 1218 (1965). J. P. Zanetta, 0.Vincendon, P. Mandel, and G. Gombos, J. Chromarcgr.. 51, 441 (1970).
RErElVED for review May 1, 1978. Accepted June 16, 1978. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Department of Chemistry, Indiana University, for support of this research. E.C.P. was a participant in the 22nd annual High School Summer Science Institute, Indiana University, Bloomington, Ind., 1977. M.A.D. was a combined Medical-Ph.D. candidate, Indiana University, Rloomington. Ind.
Evaporative Analyzer as a Mass Detector for Liquid Chromatography John M. Charlesworth' Department of Industrial Science, University of Meibourne, Victoria, Australia 3052
A study of the variables which influence the response of the evaporative analyzer has been undertaken. The instrument appears to function adequately as a mass detector In liquid chromatographic applications, provided the solute is considerably less volatile than the solvent at the operating temperature. Furthermore at the normal atomization air pressure of 1.4 X 10' kPa, the calibration curve is very nearly linear for concentrations of solute in the range 1 X to 1.5 X g ~ m - ~Evidence . has been found indicating that in the thlrd zone of the instrument, light is deflected predominantly by reflection and refraction although in the case of solute droplets less than approximately 0.9 pm in radius, Mie scattering is the most likely mechanism. For a model system, calculations have shown that the sum of the intensities of the light reflected and refracted at an angle of 135' to the incident beam is almost Independent of the refractive index of the solute, which in turn explains the experimentally observed approximate independence of response and chemical composition.
During the course of an investigation into the distribution of low molecular weight species produced in the early stages of the formation of diamine-diepoxide network polymers, the need to separate mixtures of structurally complex compounds became apparent. Gel permeation chromatography (GPC) has been used as a highly effective tool for separating moderately low molecular weight material ( 1 ) including compounds similar to those anticipated in the first stages of the reaction ( 2 ) ,but accurate quantitative detection of the separated components presents a problem. The majority of commercially available liquid chromatographic instruments are equipped with one or more of the following detectors ( 3 ) : differential refractometers, conductance bridges. transport 'Current address, Department of Chemical Engineering. University of Melbourne, Victoria, Australia, 3052, 0003-2700/78/0350-14 14$01. O O / O
detectors, fluorimeters, and UV, visible, and IR photometers. Apart from the need to use a noninterfering solvent in several of these, difficulties also arise through the necessity to calibrate the instruments with each of the compounds which are to be rmined. This requirement arises because the sensitivity vary w i t h changes in the chemical structure of each of the eluted components and the response may not always vary linearly with changes in concentration. Bearing these limitations in mind, none of the above detectors could be considered as ideally suited to the task a t hand, because of the considerable difficulties associated with isolating sufficient amounts of each polycondensation product for calibration purposes. However, preliminary observations by Ford and Keniiard (1)indicate that the evaporative analyzer (EA) (5) could provide an acceptable solutioii to this problem. These workers have shown that a variety of low molecular weight polymers produce almost equivalent responses when passed through the instrument, irrespective of their chemical composition. This suggests that, under certain operating conditions, the EA instrument may function as a pure mass detector and as such it may be very suitable for determining the concentrations of compounds which are difficult to isolate for calibration purposes. The aim of this paper is therefore to report the results of some systematic investigations into the factors which might influence the response, with particular reference to the polycondensation products formed by the reaction between diepoxide and diamine monomers. Furthermore, since the mechanism by which the EA detector functions has not been well defined ( 4 ) ,sufficient analysis of the data is presented to enable a working explanation to be postulated for most of the phenomena observed.
EXPERIMENTAL Instrument Description. A diagram illustrating the main features of the EA detector as used in this study is shown in Figure 1. The instrument consists of three zones, the first of which, 3, is a continuous sampler constructed from a stainless steel capillary tube, 1, (0.45-mm i.d, 0 81-mm o.d) carrying the effluent from the GPC columns. This is surrounded by a larger tube, 2, (1.49-mm i d) through which filtered, dried, and pressurized air 'C 1978 American Chemical Society
