I090
Anal. Chem. 1988, 60, 1090-1096
Theoretical and Experimental Foundation for Surface-Coverage Programming in Gas-Solid Chromatography with an Adsorbable Carrier Gas L. B. Hung, J. F. Parcher,* J. C. Shores, and E. H.Ward Chemistry Department, University of Mississippi, University, Mississippi 38677
Mass spectrometrk tracer pulse chromatography was used to determine the adrorptlon lsothemw of benzene on a graphltized carbon black adsorbent at temperatures from 10 to 60 O C . Concurrently, the reledon vohmes of krlinlte dllutbn samples of npentsne, n-hexane, acetone, and 2-butanone were measured as a function of surface coverage of the adsorbent with benzene. The retention volume data Indicated that both cooperatlvfty and lnterterence effects canttolled the adsorption and retentlon of molecular probes. The data at temperatures above 20 OC agreed with a retention volume equation previously derived from the two4hmsbnai scaled particle adsorption model. The data Interpretation yielded values for the Interaction energy between like or unlike species adsorbed In a two-dimensional condensed "phase". These values were in the range of 2.5 to 3.0 kcai/moi for pentane-, hexane-, and butanone-benzene Interactions, slightly iower, 1.5-2.0 kcai/mol, for acetone-benzene, and approximately 1 kcal/mol for benzene-benzene interactions. Surface coverage programming was demonstrated to significantly reduce the retention times and Improve the peak shapes for a series of model solutes on GCB. The Investigation showed that chromatographic methods with diverse types of molecular prober can be used to isolate and quanth the relatively weak, but Important, Interactions between adsorbates In a two-dimensional adsorbed phase.
Gas-solid chromatography, GSC, is a well-established separation technique which has been especially useful for the analysis of mixtures of light gases. Yet the method is not as popular as many other chromatographic methods. The primary difficulties encountered in the practical application of GSC are (i) excessively long retention times, (ii) asymmetric elution peaks, and (iii) low chromatographic efficiency. Temperature and flow programming are established methods for diminishing the retention time and peak width of normally late-eluting solutes. Temperature programming is far more commonly used than flow programming in spite of the seeming advantages of flow programming, viz., low temperature, high speed, and simplicity. Unfortunately, there are several critical limitations of this procedure for GSC. The primary difficulty is the limited range of accessible flow rates. With inert carrier gases, the retention volume of any solute is independent of the column flow rate and thus the retention time is only inversely proportional to the flow rate. Consequently, the limited variability of the pressure drop, and hence the flow rate, imposes concomitant restrictions on the reduction of k'for the solutes. It requires an exponential flow increase to effect the same change in k'as a linear temperature program. On the other hand, in supercritical fluid chromatography, SFC, flow or pressure programming effects are enhanced by the dramatic increase in mobile phase density with pressure 0003-2700/88/0360-1090$0 1.50/0
at temperatures close to the critical temperature of the mobile phase. Because the retention volumes decrease with pressure or flow rate, SFC flow programming techniques are significantly more effective in the reduction of solute retention times. The same type of phenomenon has been observed in GSC with an adsorbable component a t subcritical conditions in the carrier gas. Several studies (1-5) have shown that the retention volumes of eluted solutes decrease significantly on graphitized carbon black adsorbents, when the adsorbable component in the carrier gas Ycoatsnthe solid surface with one-half of a monolayer or more. The so-called modifier inhibits the adsorption of the eluates, and the effect observed is exactly analogousto the observed decrease in retention when GCB is coated with a monolayer of nonvolatile stationary liquid phase (6-8). However, the use of volatile, rather than nonvolatile, modifiers provides the flexibility of pressure (flow) programming needed to vary the retention volumes and times continuously over a significant range, Variations in retention volumes of different solutes with surface coverage by volatile modifiers do not follow a simple pattern, although there is usually a sharp drop in the retention volumes at high fractional surface coverage. However, cooperativity effects are often observed in which adsorption of the modifier increases the retention volume of some eluates ( 1 4 ) . Because of the unusual fluctuations of retention volume with surface coverage observed in GSC systems, it is desirable to have a chromatographic theory or model that will allow at least the semiquantitative prediction of retention parameters for different eluates as a function of surface coverage by a polar or nonpolar modifier. One such retention volume equation has been derived ( 5 ) from the scaled particle solution theory by taking the zero pressure limit (Pi = 0) of the ratio of N:d8/P,, where N:ds represents the amount of solute i adsorbed on the solid. The resulting equation is
where Vgs"($ is the specific retention volume (mL/m2)of the infinite dllution solute, i, and q is the reduced surface coverage for the finite concentration modifier, component j . The fi terms are size parameters (A2) for each component, K i is the adsorption coefficient (mmol/(m2Torr)) of component i , and the term aij/flj(kcal/mol) describes the energy for interactions between solute and modifier in the adsorbed phase. This model has been shown to be accurate for simple systems involving propane, butane, and acetone (5);however, the model was inadequate for polar systems or strongly adsorbed modifiers. The principal objective of the current investigation was to further our understanding of the weak molecular interactions 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
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0ubbk
Figure 1. Experimental apparatus.
between adsorbates in a two-dimensional layer adsorbed on the surface of a solid in order to provide a foundation for the development of a useful strategy of surface-coverage programming in GSC. The interactions between adsorbed molecules are an order of magnitude weaker than the interaction of the molecules with the solid surface, so it is difficult to investigate such interactions experimentally. The use of molecules, rather than energetic particles, as probes for the investigation of solid surfaces and adsorbates located on such surfaces is a unique method for the study of interactional phenomena at the molecular level. In this investigation a variety of molecular probes were used to study the thermodynamics of "two-dimensional solutions" on the surface of a graphitized carbon black adsorbent. An additional objective was to illustrate the potential utility of flow or pressure programming in GSC systems, in which the controlling factor is surface coverage, rather than mobile phase density or velocity. Such a surface coverage programming technique would have many of the advantages of temperature programming without the need for high temperatures or lengthy cycle times.
EXPERIMENTAL SECTION The adsorption isotherms of benzene on Carbopack C and the specific retention volume of four eluates were measured at six temperatures over the range of 10-60 "C. The experiment procedure used was mass spectrometrictracer pulse chromatogrpahy (9, IO). This method is a variation of the classical tracer pulse chromatographic procedure in which nonradioactive tracers are employed with detection by mass spectrometry, MS, rather than counting methods. In this work, the labeled isotope was [2Hs]benzene (Columbia Organic Chemicals). The mass spectrometer used as the mass-specific GC detedor was a Hewlett-Packard 5995 benchtop MS equipped with a membrane separator. The spectrometer was operated in the selected ion monitor mode in order to detect the labeled benzene isotope and the other probe solutes in the presence of a significant background of natural benzene in the carrier gas. Two independent sources of carrier gas were used to provide a range of benzene pressures in the column. The instrumentation is shown in Figure 1where S1 and S2are flow transducers (Model 8102, Matheson Gas Products), S3and S4are pressure transducers (Model 204, Serta Systems, Inc.), R1and & are flow control valves (Model 8203, Matheson Gas Products), and GSV represents the gas sampling valve used for injection of the gaseous solutes. The mole fraction of benzene in the mixed gas tank was 0.0215; thus the mole fraction of benzene in the carrier gas could be varied from 0 to 0.0215 by control of the proportional flows from each source by means of the controllers R1and R2.The entire flow and injection system was monitored and controlled by a microcomputer via the analog-to-digital converter, ADC, digital-toanalog converter, DAC, and the digital output of an Adalab-PC board (Interactive Microware, Inc.). The components marked with a down arrow in Figure 1were monitored through the ADC, and the components marked with an up arrow were controlled by the computer through the DAC (R, and R2)and the digital output (pneumatic valves on the GSV). The precise flow and pressure control provided by this instrumentation was necessary in order to accurately determine the large variations in retention
0 0
0.1
0.2
1 0'. 3
Relative Pressure of Benzene
Flgure 2. Adsorption Isotherm of benzene on Carbopack C fit to the 20 O C ; 0 , 30 "C; A,40 O C ; BET equation (solid line): 0,10 O C ; X, 50 "C;
V, 60 "C.
+,
often observed for very small changes in pressure, especially at low surface coverages. The columns were prepared from in 0.d. stainless steel tubing of relatively short length (
