Anal. Chem. 1990, 62, 347-349
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Non-Steady-State Gas Chromatography Using Capillary Columns Anatoly J. Belfer
BP Research, Warrensville Research Center, 4440 Warrensville Center Road, Cleveland, Ohio 44128 David C. Locke* and Isaac Landau Chemistry Department, Queens College and The Graduate School, CUNY, Flushing, New York 11367
Non-steady-state gas chromatography involves the use of a relatively volatile solvent phase that elutes slowly and continuously while solutes are injected. Application of the technlque to the determination of limltlng activlty coefflclents of solutes In the solvent Is shown to provide useful data when the solvent Is condensed onto the walls of an uncoated fused silica caplllary column. Activity coefflclents of n -hexane, n-heptane, and benzene solutes are determined in n-nonane and p-xylene solvents at 40, 50, and 60 OC, and of acrylonitrile and methyl methacrylate In water in the range of 35-60 OC. Compared wlth packed column non-steady-state gas chromatography, Capillary columns require smaller solvent and sample volumes, and measurement times are substantlally reduced.
Non-steady-state gas chromatography (NSGC) has been shown to be an efficacious method for the determination of physicochemical properties of nonelectrolyte solutions (1-3). For this application, NSGC has the particular advantage over normal gas chromatography in that it is useful for determining limiting activity coefficients and other thermodynamic properties of volatile solutes in volatile solvents. The criterion is that the volatility, the product of the vapor pressure and the activity coefficient, of the solute must be greater than the volatility of the solvent. In normal gas chromatography (GC), a stationary solvent of low volatility is used, which limits study to systems that are primarily of theoretical interest. Volatile compounds have served as solvent phases in GC (4-101, but presaturation of the carrier gas with the solvent is required, and the exact weight of the solvent present in the column is always uncertain. In NSGC, the volatile solvent phase is injected into the hot injection port, evaporates, is allowed to equilibrate in the column, and bleeds out at a steady rate dependent on its vapor pressure at the column operating temperature. Solutes more volatile than the solvent are injected repetitively while the solvent is eluting, over the life of the column, and elute with ever-decreasing retention times as the weight of solvent diminishes linearly with time. An analysis of the system (1,2) leads to a remarkably simple equation relating the infinite dilution (limiting) activity coefficient, ylm,of the solute in the volatile solvent to the ratio of their vapor pressures, pl0/p2", and the rate a t which retention time decreases with the time of injection, AtR/At:
Solutes of lower volatility than the solvent will elute at the tail of the solvent band, and cannot be studied from this point of view. *Author to whom correspondence should be addressed. 0003-2700/90/0362-0347$02.50/0
Heretofore all NSGC work (1-3) was done by using a column packed with only a solid support material. We report here the use of a capillary column in which the solvent is condensed onto the walls of the column. The primary advantage of a capillary column in NSGC is the small volume of solvent that is required, which facilitates the study of rare, expensive, and/or toxic compounds as solvents. In addition, the time required to equilibrate the column and to make measurements is reduced substantially compared to that required for packed columns. A more uniform coating of the solvent film and consequently a more constant rate of evaporation of the film will also result. The thin liquid film and the short lateral diffusion distance improve solute masstransfer characteristics. With packed column NSGC, thermal conductivity detector base-line stabilization is assisted by using dual columns, into both of which are injected equal volumes of solvent. For the capillary columns, we used a Hewlett-Packard modulated thermal conductivity detector (11). This detector is quite sensitive, and once solvent equilibration has been achieved, a flat base line results, enabling detection of solute quantities small enough to ensure effectively infinite dilution solutions.
EXPERIMENTAL SECTION A Hewlett-Packard 5890A gas chromatograph (GC) equipped with a Hewlett-Packardmodulated thermal conductivity detector and splitless injector was used with a 30 m X 0.53 mm i.d. uncoated Hewlett-Packard fused silica capillary column. The carrier gas was He at flow rates in the range of 2-10 mL/min. Carrier gas flow rate was set to provide a column lifetime of at least 15 min, which is sufficient to allow at least five solute injections. The injector and detector were maintained at 200 and 250 "C, respectively. Measurements were made at column temperatures in the range of 35-60 "C.Without cryocooling, for which the GC used was not equipped, the oven gave unstable temperature control below about 30 "C. Chromatographic data were recorded on a Hewlett-Packard 3392A recording integrator. The attenuator setting was 5 or 6 for injections of 0.2 WL of solute mixtures. To load the column with a presumably uniform and relatively thick film of solvent, the column temperature was set to a value slightly greater than the boiling point of the solvent. Injection of 25-50 WLof solvent into the Grob injector produced a temporary surge in pressure at the head of the column. As the vapors passed into the column, the pressure dropped back to normal. A t this time the column was filled with the saturated vapor of the solvent. Condensation of solvent onto the walls of the capillary column was achieved by opening the door of the GC oven and setting the temperature down to the desired operating value. Liquid nitrogen cryocooling would presumably serve the same function in a more elegant fashion. The base line stabilized in about 5-6 min. If the rapid cooling was started before the inlet pressure had dropped to normal, a liquid plug formed at the column inlet, which led to an excessively long equilibration time and a shorter effective column lifetime. On the other hand, cooling should not be delayed lest elution of solvent vapor leave an amount of solvent insufficient to produce a long-lived column. When the temperature descended to the operating value, the oven door was closed gradually to maintain the temperature. If the door was closed at once, residual 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 4, FEBRUARY 15, 1990
Table I. Activity Coefficients in n-Nonane and p-Xylene solvent
temp, "C
n-nonane
40 50 60 40 50 60 20" 206
p-xylene
Table 11. Activity Coefficients in Water
activity coeff of solute n-hexane n-heptane benzene 0.76 0.80 0.85 1.25 1.20 1.14 1.39 1.44
0.77 0.80 0.84 1.22 1.17 1.10 1.38 1.40
0.97 0.96 0.96 0.91 0.88 0.85 0.98 0.99
" Extrapolated. Reference 7. heat in the oven components caused a sharp rise in temperature followed by a slow equilibration back to the set value. As soon as base-line stabilization was noted, solutes could be injected, neat or in solution or in mixtures if the components were separated. Peak shapes were generally symmetrical, except for the solutions in n-nonane, in which the peaks tended to front, indicative of overload. Injection times were noted on the recorder chart by using the event marker on the integrator. It is not necessary to wait for complete elution of a sample before making a new injection, if elution times allow a separation. Indeed, the more solute injections made over the life of the column, the better the definition of the slope of the retention time/injection time plot. For example, for n-nonane solvent at 40 "C, the column lifetime was about 65 min, which allows time for at least 12 injections of a mixture of n-hexane and n-heptane solutes. The complete evaporation of solvent is noted by a sharp drop in the base line, which signals the need to inject fresh solvent. One need not wait for loss of all solvent; a new solvent injection can be made at any time, and once equilibration has been achieved, solute injections can be resumed.
RESULTS AND DISCUSSION From eq 1, ylm is calculated from the slope of a plot of retention time vs injection time and from the ratio of solute-to-solvent vapor pressures. Vapor pressures for all compounds except HzO were calculated by using the Fortran program of the Design Institute for Physical Property Data (12). The vapor pressure of HzO was calculated from the Chebysev polynomials given by Ambrose and Lawrenson (13). Corrections for vapor-phase nonideality, calculated from the truncated virial equation of state using second virial coefficients calculated according to Reid, Prausnitz, and Poling (141, were insignificant compared with the estimated experimental error (*lo%). Activity coefficients are listed in Table I for n-hexane, n-heptane, and benzene solutes in n-nonane and p-xylene solvents. Values for acrylonitrile and methyl methacrylate solutes in water are given in Table 11. There are few data in the literature for comparison with the values listed in Tables I and 11. For p-xylene solvent, Thomas et al. (7) give ylmvalues for benzene and n-hexane a t 20 "C. Extrapolation of their data for n-pentane and nhexane yields a value for ylmof n-heptane in p-xylene a t 20 "C. Extrapolation of our data to 20 "C yields the values on the next to last line of Table I; on the line below are the values of Thomas et al. (7). The agreement is quite good. The activity coefficient of acrylonitrile in H 2 0 at 25 "C has been calculated by Banerjee using UNIFAC (15). This value, 45.3, is in reasonable agreement with our extrapolated value, 48.4. It is of interest to calculate the average liquid film thickness on the walls of the capillary tubing, assuming a uniform film. This value, dL, can be calculated from the initial solvent volume, VL, which in turn is estimated from the specific retention volume of a solute, VG,the net retention volume per gram of stationary phase where M,is the molecular weight of the solvent. From the
temp, "C 35 40 50 60 25" 25b
activity coeff of solute acrylonitrile methyl methacrylate 44.4 45.2 42.0 38.4 48.4 45.3
" Extrapolated. UNIFAC-calculated, ref
431 388 376 338
15.
experimentally determined ylmvalue for a solute of vapor pressure pl", VG can be calculated. For an injection of this solute early in the column life, the retention time and flow rate are measured. The total column volume is measured a t the end of the column life from the retention time of air and the flow rate. From these values can be estimated the initial weight and volume of solvent present in the column, and assuming a uniform film on the walls of the capillary tube, dL can be calculated. For example, for the acrylonitrile/water system a t 50 "C, V , = 24 pL and dL = 1.2 X cm. VL is consistent with the volume of water injected, which was 25 PL.
Film thicknesses of this magnitude lead one to raise the obvious question as to whether the solutions are sufficiently dilute to give effectively infinite dilution activity coefficients. An estimate of the most dilute concentration of the solute in the chromatographic solution can be made by using the maximum permissible sample size calculation of Conder and Young (5),assuming the solvent film is uniformly distributed. According to this calculation, the column plate number, N , is related to the total column length, L, and to the length of the column occupied by the solute, d,, when it begins to elute:
N = 5.545(L/~i,)~ For example, for acrylonitrile in H20, N = 16070, from which d, = 930 mm. The 24 pL of solvent initially present in the column is 1.3 mmol; i.e. the solute is dissolved in 1300 pmol X (d,/L) = 25 pmol of HzO. Since 0.05 p L of acrylonitrile (1pmol) is injected, the mole fraction of solute in solvent at the end of the column is about 0.03. For p-xylene solvent, the solute mole fraction is about 0.05, and for n-nonane, 0.1. The precise upper limit of solute mole fraction consistent with effectively infinite dilution solutions is somewhat arbitrary (5). In any case, the concentration of the solution in n-nonane is rather high, all the more so when one considers that this calculation refers to the most dilute conditions, i.e. just before the solute elutes from the column. The bandwidth on injection should be quite narrow, producing a substantially higher concentration, and that concentration will decrease with the square root of the length of column traveled; the bandwidth will increase in the same way, finally reaching the value d, a t the outlet of the column (16). The apparently low values of the activity coefficients of n-hexane and n-heptane in nnonane could be accounted for in terms of their solutions being at finite concentrations; the fact that the peaks fronted substantiates this. In current work we inject solutions containing only 1-10 pg of each solute, 0.01-0.1 times the quantity injected to obtain the data reported here. Although the theory of NSGC does not require a uniform liquid film as long as the rate of evaporation is constant over time, a second question can be raised regarding whether the solvent wets the capillary tube. If a solvent does not wet the surface, presumably it forms globules and droplets on the surface of the silica (17). According to Grob (181, untreated glass capillary tubes are wet by nonpolar solvents but not by polar solvents, presumably including water. Fused silica
Anal. Chem. 1990, 62, 349-353
should behave in a similar fashion (19).One way to compare the uniformity of liquid films would be through the measured plate numbers. Uniform films should produce more efficient chromatographic behavior. However, although more plates were generated in the hydrocarbon solvent systems, since k’ values and flow rates were different, a meaningful comparison cannot be made. It is not obvious either what the effect of nonwetting would be on the derived activity coefficients, or how to circumvent the problem if the effect is detrimental to their accuracy. One could start with a fused silica column containing a cross-linked polar phase such as a polyethylene glycol or a propylcyanosiloxane, which should be wet by polar solvents such as water, but these phases would of course change solute retention behavior and obfuscate interpretation of the data. This problem is currently under investigation by us.
ACKNOWLEDGMENT We thank C. Wysocki of B P Research for technical assistance.
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(4) Locke, D. C. Adv. Chromatogr. 1976, 74, 87. (5) Conder, J. R.; Young, C. L. Physicochemical Measurements by Gas Chromatography; Wiley: New York, 1979. (6) Laub, R. J.; Pecsok, R. L. Physicochemical Applications of Gas Chromatography; Wiley: New York, 1978. (7) Thomas, E. R.; Newman, B. A.; Long, T. C.; Wood, D. A.; Eckert, C. A. J. Chem. Eng. Data 1982, 27, 399. (8) Eckert, C. A.; Newman, B. A.; Nicolaides, G. L.; Long, T. C. AIChE J. 1981, 27, 33. (9) Terasawa, S.; Itsuki, H.; Yamaki, H. Anal. Chem. 1988, 58, 3021. (10) Itsuki, H.; Terasawa, S.; Yamana, N.; Ohotaka, S. Anal. Chem. 1987, 59, 2916. (11) Craven, J. S.; Clauser. D. E. Analusis 1980, 8(1), 1. (12) American Instiiute of Chemical Engineers, Design Instiiute for Physical Property Data. Project 807, DCAP I I Users Guide; The Pennsylvania State University: University Park, PA, 1983. (13) Ambrose, D.; Lawrenson, I.J. J. Chem. Thermodyn. 1972, 4 , 755. (14) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Prope&s of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. (15) Banerjee, S. Environ. Sci. Technoi. 1985, 79, 369. (16) Littlewood, A. B. Gas Chromatography; Academic Press: New York, 1967; p 127. (17) Jennings, W. Gas Chromatography with Glass Capillary Columns, 2nd ed.; Academic Press: New York. 1980; p 21. (18) Grob, K. Helv. Chim. Acta 1965. 48, 1362. (19) Jennings, W. Gas Chromatography with Glass Capillary Columns, 2nd ed.; Academic Press: New York, 1980; p 33.
LITERATURE CITED (1) Belfer, A. I.Neftekhimiya 1972, 72, 435; Chem. Abstr. 1973, 78, 20591. (2) Belfer, A. J.; Locke, D. C. Anal. Chem. 1984, 56, 2465. (3) Yang, Y.; Xiao, S.; Li, H.; Fu, Y. J. Chengdu Univ. Sci. Technol. 1988, No. 1, 35; Chem. Abstr. 1988, 709, 157451.
RECEIVED for review September 1,1989. Accepted November 22, 1989. This work was supported in part a t QC by grants from the National Science Foundation (CHE-8420326) and the PSC-CUNY FRAP Program.
Pulse Voltammetric Techniques at Microelectrodes in Pure Solvents Malgorzata Ciszkowska and Zbigniew Stojek Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warsaw, Poland
Janet Osteryoung* Department of Chemistry, State University of New York, University at Buffalo, Buffalo, New York 14214
Experlmentai conditions are described for application of pulse voltammetric techniques with microelectrodes in solvents Containing no deliberately added supporting electrolyte. Elimination of Supporting electrolyte was found to be advantageous In the case of alkyl Iodides, for which supportlng electrolyte strongly Influences the voltammetric curves. The primary reductbn waves obtained at mercury mkroelectrodes were free from phenomena due to adsorption and following chemical reaction. The heights of both linear scan reduction and reverse pulse oxidation curves were linearly dependent on ethyl, butyl, and decyl iodide concentration.
Research on microelectrodes is focused on the ways of preparation, properties, theory, and application of these electrodes (I, 2). One of the important properties of microelectrodes is a very low level of current flowing through the electrochemical cell. Consequently, the ohmic dorp (iR) may be very small despite large resistance of the solution. Therefore it is possible, with the proper instrumentation, to monitor electroactive species in solvents containing virtually no added supporting electrolyte (3-8). These conditions can be interesting, since any salt used as the supporting electrolyte can be a source of unwanted impurities in trace analysis and
a source of unwanted water in nonaqueous solvents. The first published papers on this subject proved that obtaining a curve in a pure solvent and measuring its height are possible. However, many bothersome questions remain. First of all, how does ion migration current contribute to the total current? For large-area electrodes it is well-known that at low supporting electrolyte concentrations the reduction signal of positively charged species can be appreciably higher than that at high concentrations. An experimental study done with microelectrodes showed minor changes of the limiting reduction current for Fe(CN)6-3with a change in supporting electrolyte concentration in water (6). This result is linked to substantially enhanced diffusional transport to electrodes of very small area. On the other hand, it is not surprising that uncharged molecules give wave heights virtually independent of the electrolyte level ( 3 , 4 ,6). A recent paper by Oldham deals with theory of microelectrode steady-state voltammetry with various ratios of reactant to supporting electrolyte (9). At a hemispherical electrode, for reactions that engender an increase in ionic strength a t the microelectrode interface, the faradaic redistribution of ions is predicted to diminish the ohmic overvoltage. Simultaneously the counterions will be brought into the neighborhood of the microelectrode so effectively that a very low electrolyte concentration can behave as “excess” supporting electrolyte. In the present case of no
0003-2700/90/0362-0349$02.50/00 1990 American Chemical Society
