Anal. Chem. 1985, 57, 2085-2091
2085
Effect of Temperature and Mobile Phase Composition in Gas-Solid Chromatography with an Adsorbable Vapor in the Carrier Gas Ping J. Lin and Jon F. Parcher*
Chemistry Department, University of Mississippi, University, Mississippi 38677
Two molecular probes, vir., n -pentane and I-propanol, were used to Investigate both attractlve and repulslve molecular Interactions, and the temperature dependence of these Interactlons, In a twodlmenslonal condensed phase of acetone adsorbed on the surface of a graphltlred carbon black adsorbent. The speclflc retention volumes of both solute probes were determlned as a function of the surface coverage of acetone on Carbopack C over a range of temperatures from 0 to 80 O C . Acetone, at a flxed pressure, In helium was used as the carrier gas. The amount of acetone adsorbed at each temperature and pressure was determlned by mass spectrometric tracer pulse chromatography wlth acetone-d,. The retentlon volumes of both pentane and propanol varied dramatlcally wlth temperature, acetone pressure, and surface coverage. The predominant effect of the acetone was steric exclusion or blocking of the alkane solute, pentane. The retention volumes of pentane were hlghest at 40-50 O C and were approximately equal at 0 and 80 OC at the highest acetone pressure. Conversely, a slgnlflcant cooperative effect was observed for propanol wlth acetone; that is, preadsorbed acetone Increased the retentlon volume of propanol. A retention volume equatlon, derived from the two-dlmenslonal scaled particle adsorption theory, was used successfully to descrlbe the effect of temperature, surface coverage, and acetone pressure on the retention volume of both solutes.
One of the major advantages of high-pressure liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) is the continuous control of the solvating power of the mobile phase afforded by the external manipulation of the chemical and physical properties and conditionsof that phase. Gradient elution (solvent programming)methods have proven invaluable for the separation of complex mixtures by HPLC, and pressure programming is the standard protocol for SFC. On the other hand, only temperature programming methods have proven effective for gas-liquid chromatography (GLC). Although these three external control methods are similar in effect, the mechanisms differ significantly. The primary changes in solvent strength occur in the stationary phase for temperature programming, whereas the properties of the mobile phase are altered in the case of solvent or pressure programming in HPLC or SFC. The concepts of gradient elution and pressure or flow programming for gas-liquid and gas-solid chromatography (GSC) have been proposed and discussed for many years (1, 2). This method would involve the use of condensable vapors or “modifiers”in the carrier gas. Such modifiers would dissolve in, or adsorb on, the stationary phase to alter the properties of that phase and, consequently, the selectivity of the system (3-7). Although temperature and pressure programming are both viable procedures, the pressure programming techniques would have some advantages over temperature programming, e.g., milder thermal conditions, symmetric peaks at low temperatures, and shorter cycle time. More importantly, both
techniques would offer a method for continuous, external control of selectivity for GC, as opposed to the discrete, internal control that can only be obtained by changing stationary phases or liquid loadings. The method has more significant potential for gas-solid chromatography because of the greater selectivity range achievable by modification of solid surfaces compared to the polarity range available with liquid mixtures composed of a vapor dissolved in a high molecular weight liquid phase. In the case of GSC, another possible advantage of the use of such surface modifiers would be the ability of these components to block and deactivate the small number of physical and chemical heterogeneities, so-called “active sites”, present on many adsorbents, such as the graphitized carbon blacks (GCBs). An additional advantage would be the possible manipulation of the retention of different solutes by control of the fractional surface coverage. That is, the possible utilization of steric or blocking effects observed on solid adsorbents could be used to alter and control chromatographic selectivity. The major experimental disadvantage of the proposed methods would be the requirement for a selective detection system that would not be adversely affected by the vapor component in the carrier gas. Bruner et al. (7, 8) have used nonvolatile modifiers, Le., common stationary liquid phases, to block and deactivate the active sites on GCB, as well as alter the selectivity of the adsorbent. Significant cooperativity and steric effects were observed, and it was shown that the degree of surface coverage (liquid phase loading) was critical to the selectivity of the system. The measured retention volumes could be made to either increase or decrease with surface coverage. If the solute and liquid phase showed strong interactions in the two-dimensional adsorbed phase, the retention volume of the solute could increase with surface coverage by the liquid. This was observed primarily at low surface coverages. At higher surface coverages, close to a monolayer, the retention volumes of most solutes decreased with surface coverage by the liquid due to the loss of available surface area. The same effects observed for nonvolatile liquids have also been observed with volatile modifiers (9); however, the volatile modifiers offer the significant possibility of continuous, external control of the chromatographic selectivity. The retention mechanisms for GSC with a vapor mobile phase are varied and complex. Interfacial adsorption processes, influenced by cooperativity and steric effects, are the predominant mechanisms, in addition, bulk solubility may operate at high pressures (multilayer adsorption). Strong cooperativity effects have been observed at low surface coverage for many adsorbates on graphitized carbon blacks (8-14). This effect is manifest as an enhancement in the retention of one solute with increasing surface coverage by different volatile or nonvolatile adsorbate. Theoretically, both the composition and pressure of the binary carrier gas could be controlled, changed, or continuously programmed during an experiment. In addition, the temperature could also be programmed to take advantage of the decrease in retention commonly observed with increased temperature. Indeed, Sterkhov et al. (15,16)recently carried
0003-2700/85/0357-2085$01.50/0 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
out this type of dual programming experiment with water vapor on an alumina adsorbent. The method was shown to be successful for the efficient separation of C8-CI8 alkanes in less than 5 min on a normally unsatisfactory adsorbent. This study represents the only published work involving this particular protocol. A major deterrent to the development of this methodology is the lack of a quantitative model to describe the complex effects caused by the concurrent variation of several determinant variables, such as temperature, pressure, and surface coverage, on the retention volumes of different types of solutes. Unfortunately, there are very few accurate physicochemical models or theories for multicomponent phase distribution equilibria. Such a model is necessary for the reasonable prediction and control of the retention and selectivity changes induced by external manipulation of the temperature and carrier gas composition of a chromatographic system with an adsorbable component in the mobile phase. However, a small number of theories for multicomponent adsorption have been proposed (17-1 9) and evaluated (17, 20-22). Several of these theories are two-dimensional versions of nonelectrolyte solution theories. One such model is Steele's two-dimensional analogue (18) of the scaled particle solution theory. Recently, Glanz and Findenegg (21,22)and others (14) have used this theory to accurately describe the adsorption of several multicomponent vapors on GCBs. The model has also been applied to binary systems with one component at infinite dilution (14). Significantly, such systems are exact analogues of those that would be encountered in gradient elution gas-solid chromatography. The scaled particle theory incorporates both the finite interaction energies of the adsorbates, which are responsible for cooperativity, and the "hard core" areas of the adsorbates, which determine the amount of adsorbate required to form a monolayer. Another significant advantage of this model is that it is easily extended to multicomponent systems. A retention volume equation, derived from the scaled particle adsorption model for a binary system, has been used successfully for a variety of systems (14) involving both polar and nonpolar adsorbates on GCB. In the present study the retention parameters of two solutes were measured at different temperatures, pressures, and surface coverages of acetone on Carbopack C. The objectives of this investigation were (i) to evaluatethe potential applications of the scaled particle theory as a model for describing the effect of temperature and pressure on adsorption equilibria in multicomponentgasaolid systems and (ii) to develop a quantitative method for the description of the effecb of vapor phase modifications on the retention characteristics of various systems in order to predict the effect on temperature programming in GSC with an adsorbable carrier gas. EXPERIMENTAL SECTION The experimental procedure used for these investigationswas mass spectrometric tracer pulse chromatography (23, 24) with Hewlett-Packard 5985 and 5995 GC/MS systems. The carrier gases were composed of various mixtures of acetone and helium with a fixed mole fraction of acetone which was always less than 0.05 ( P / P 5 0.6). The binary carrier gas was considered to be ideal although corrections for gas-phase nonideality could be incorporated (25)with an assumed equation of state. Helium was used as a diluent only and did not measurably adsorb on the solid. The amount of acetone adsorbed was determined from the retention time of a plug sample of acetone& which was detected by the mass spectrometer operated in the selected ion monitor mode for mass 64. Eluted samples of n-pentane and 1-propanol were detected in the same manner but with different monitored masses. The experiments were carried out at fixed acetone partial pressures and different temperatures and flow rates. The solid adsorbent used for the isobar and retention volume studies was Carbopack C with a measured surface area of 8.9 m2/g.
.,
Acetone Pressure (torr)
Flgure 1. Adsorption isotherms of acetone on Carbopack C: 0, 20 O C ; 0 , 30 O C ; A, 45 O C ; 0, 60 O C ; 75 O C .
A different batch of Carbopack C was used in a previous investigation (11)of the isotherms of acetone and the retention volume of several solutes, including pentane and propanol. The data from that study were used for comparison in the present work. The specific retention volume and isotherm data cited in the original study (11)were reported for a Carbopack C adsorbent with a nominal surface area of 12 m2/g. The surface area of the adsorbent was later measured accurately (26)and determined to be 6.3 m2/g. The specific retention volumes and acetone isotherms referenced in the current study were corrected for the error in the reported surface area of the original adsorbent. RESULTS AND DISCUSSION Single-Component Isotherms and Isobars. The single-component isotherm equation for the two-dimensional scaled particle theory (18) is
where Pi is the partial pressure of component i and K irepresents the adsorption coefficient (reciprocal of the Henry's law constant) of component i on the GCB surface. The parameter, q = Pin?ds,is a reduced surface concentration; ai is the interaction parameter representing the strength of the interactions between the adsorbate molecules in the two-dimensional adsorbed phase. The size term, pi, represents the hard-core area of the adsorbate which should be about half (18) of the specific areas determined from the BET equation. The previously measured acetone isotherms (11)are shown in Figure 1. These were fit to eq 1by nonlinear least squares. The calculated regression values for a and p were ai= 5.8 (f0.4) X lo8 (J m2)/mo12and pi = 10.3 A2. This size parameter is ca. one-third of the value for the size term measured by Avgul and Kiselev (27) for acetone fit to the BET equation, Le., 34 A2. The interaction and size terms did not show any systematic variation with temperature over the range studied. The adsorption constants did vary with temperature, and the measured values are given in Table I. In addition, Ki values
ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
Table I. Adsorption Coefficients, Xi, of Acetone on Carbopack C Obtained from Single-Component Isotherm Data Fit to the Scaled Particle Theory and from Specific Retention Volumes (11 ) (Elution Chromatography) adsorption coefficient X102, pmol/ (m2torr) calcd from specific calcd from singlecomponent isotherm data retention volume data
temp, O C 20 30 45 60 75
16.6 8.87 5.87 3.15 2.06
Table 11. Adsorption Isobars of Acetone and Specific Retention Volumes of n -Pentane and 1-Propanolon Carbopack C
Pressure of Acetone = 40.2 torr
0.10
0.26 0.38 0.67 1.38 2.13 3.02 3.74 4.57 5.66
80 70 60 50 40 30 20 10 0
-
E,
Y
.-s0 .-c
0 c
.-c
:0.02 . I
0.05 0.08 0.10 0.20 0.28 0.40 0.56 0.97 1.66
80 70 60 50 40 30 20 10 0
r 0)
2.6
0.39 0.67 1.38 4.50 7.52 12.0 17.5 28.3 39.0
1.12 1.69 2.56 4.67 6.21 7.98 7.74 6.25 4.58
Pressure Acetone = 3.91 torr
0.05 .
0.01
0.52 0.96 2.00 5.25 7.14 9.43 13.0 16.9 27.8
1.09 1.69 2.46 3.04 2.98 2.76 2.38 1.65 1.07
Pressure of Acetone = 19.3 torr
i5 c
0.58 0.91 1.49 2.76 3.63 4.55 5.46 6.31 7.91
80 70 60 50 40 30 20 10 0
9.04 4.87 3.00 1.76
h
8
specific retention vol, mL/m2 1-propanol n-pentane
amt of acetone adsorbed, pmol/m2
temp, "C
0.20 -
c
2087
I
I
I
I
2.8
3.0
3.2
3.4
1000IT
0.34 0.60 0.88 2.46 3.94 5.84 9.93 22.1 51.6
1.06 1.66 2.48 5.75 8.13 11.8 13.9 22.3 32.64
lo.w -
3.6 5.w
-
-1
E
2.w
-
a
?MI-
P
0.w
(OK-')
Figure 2. Limklng adsorptlon coefficient of acetone on Carbopack C as a function of 1 / T : 0, calculated from the isotherms by eq 1; 0, calculated from specific retention volumes ( 7 7).
determined from the specific retention volumes of infinitely dilute samples of acetone on GCB with pure helium as the carrier gas obtained in the same study (11)are also given in Table I for comparison. The results of these two independent measurements, viz., isotherms and retention volumes, were in good agreement as shown in Figure 2. The limiting heat of adsorption of acetone calculated from the temperature dependence of the combined set of K, values was -32.2 kJ/mol which is close to the value of -34.7 kJ/mol reported by Avgul and Kiselev (27). The temperature dependence of the adsorption coefficients was described by an equation of the form In Ki = ASi/R - AHi/RT, with AS, = -63.2 J/(K mol) and AHi = -32.2 kJ/mol for acetone on Carbopack C. In the present study, the effect of temperature on the adsorption of acetone a t constant pressure was determined by measuring the isobars of acetone on Carbopack C at three pressures and nine temperatures, and the results of these studies are given in Table 11. The data are also presented in Figure 3 in the form of van't Hoff plots of In niadavs. 1/T although there would be no significance to any linearity observed in such a plot because the surface coverage, temperature, and pressure all varied concurrently. The measured amount of acetone adsorbed at each temperature agreed with
5.
t51
-
6
3
2
om-
1 P
0.10
On*
-
t 26
?.a
an
az
34
a.4
100O/T
a8
40
41
44
4.a
.a
(W')
Figure 3. Isobars of acetone on Carbopack C at three acetone pressures: solM symbols, from singlecomponent isotherms ( 7 7); open symbols, this work.
the previously determined isotherm data ( I I ) , except for a systematic deviation at 20 OC. The measured adsorption at 20 O C in the present work was consistently less than that observed previously. The cause of this deviation is unknown. The amount of acetone adsorbed at each temperature and pressure was calculated from the constants, cy,, &, ASi, and
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
Mi,obtained from the acetone isotherms. There is no analytical solution for the scaled particle isotherm equation of the form of reduced surface coverage as a function of temperature; however, numerical solutions of eq 1 were obtained using the NONLIN subroutine of the International Mathematical and Statistical Library (IMSL). The calculated results for the three experimental pressures are shown as the lines in Figure 3. The closed data points in Figure 3 are interpolated values from the isotherms shown in Figure 1. The calculated results agreed within experimentalerror with the experimental isotherms and isobars. The theoretical lines were extended beyond the experimental domain to show the convergence caused by the formation of a monolayer at low temperatures. Thus, although it is not a commonly used theory, the scaled particle adsorption theory provided a practical mathematical model for the single-component adsorption equilibria of acetone on Carbopack C over a range of temperatures. One of the major advantages of the scaled particle model is that it is easily extended to the description of multicomponent systems. However, there is very little data for binary adsorption isotherms available to test such adsorption models. Chromatographic data for retention volumes as a function of surface coverage, such as that given in Table 11, are valuable for testing at least a limiting form of such a model. Thus, one of the primary goals of the investigation was to utilize this model along with the measured parameters ai, Pi, AS,,and Mi for pure acetone to model the adsorption of binary mixtures with acetone as one component. Chromatographic Elution Experiments. The scaled particle isotherm equation for adsorption of component i in a mixture of components i and j is
The size parameter, P,, is an averaged value, xi is the mole fraction of i in the condensed phase, a,, is the cross-interaction term, and K,* is the adsorption coefficient of i on a surface partially covered with a coadsorbed component. The chromatographic experiments were always carried out with one component at infinite dilution. If this component is designated to be component i, then xi approaches zero and P, approaches 0,in the limit. A retention volume equation can be derived from the limiting form of eq 2 by using the relation that the retention volume is directly proportional to the slope of the isotherm of component i on a surface of coadsorbed component j . The retention volume equation derived from the scaled particle theory is (14) vg,io(7) = Ki*273R(1 - t) exp
Where V,,,"(q) is the specific retention volume of the small plug of component i at a fixed reduced surface coverage, 11, of componentj . K,* is the adsorption coefficient of component i on a GCB surface where the active sites have been blocked by preadsorption of component j . The specific retention volumes of small samples of pentane and propanol were determined at different temperatures and surface coverages in the same experiment in which the isobars were determined, and the results are also given in Table 11. The elution experiments were designed to investigate the
50.0
c
E
j
z
a
10.0
-
5.0
-
1.0
-
c
E > E
.9 C I
U
.o e m
*
0.5 2.6
I
1
1
I
I
I
2.8
3.0
3.2
3.4
3.8
3.8
10001T
4
(OK-').
Flgure 4. Specific retention volume of n-pentane at fixed acetone pressures: *, 0 torr (helium carrier gas) (73);A, 4 torr; 0,19 torr; 0. 40 torr. to00
1.0
7
1
i
01 26
,
26
28
30
~
2 ;
i2
31
1 32
31
100O/T
1
38
36 (OK-')
Figure 5. Specific retention volume of 1-propanol at fixed acetone
pressures. The abscissa scale is offset for clarity. Legend is the same as that given in Figure 4.
effects of solute-adsorbate (acetone) molecular interactions in the two-dimensional condensed phase and the effects of the formation of a monolayer of acetone on the retention of the infinitely dilute elution samples. The retention volumes of the two solutes varied dramatically with temperature at a fixed pressure of acetone as shown in Figures 4 and 5. The retention volume data for pentane clearly illustrate the effect of monolayer formation at the lower temperatures and higher pressures. This influence was so great that the retention volumes of the pentane samples were essentially equal at the highest (80 "C) and lowest (0 "C) temperatures studied when the pressure of acetone in the column was 40 torr. This temperature dependence is anomalous;however, it illustrates the potential for control of retention and selectivity in GSC by manipulation of the column temperature and carrier gas composition. A t the higher temperatures (60,70, and 80 " C ) ,the plots were normal, Le., linear, and the heat of adsorption of pentane,
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
calculated from only thebe three points, was -40.2 kJ/mol which is slightly higher than the published values which range from -36.0 to -38.9 (27-29). At these higher temperatures, the acetone modifier had little or no differentiating effect on the retention volume of pentane. The abnormal reduction in the retention volume at low temperatures and high pressures is caused by the loss of available surface area due to the preferential adsorption of acetone. The maxima in the retention volume plots at different acetone pressures occur ut the same surface coverage of ca. 3.5-4 fimolfm' which corresponds to a fractional surface coverage of 0.6-0.7. The temperature at which the critical surface coverage was achieved varied from 40 "C at 40 torr of acetone to less than 0 "C with only 4 torr of acetone. No maximum was observed in the experimental data at the lowest pressure because the amount of acetone adsorbed was less than 3 pmol/m2 even at 0 "C. The data points shown as asterisks (*) in Figures 4 and 5 represent previously published retention volume data (11)for the same solutes on Carbopack C with pure helium as a carrier gas. The specific retention volumes of pentane with a carrier gas doped with acetone were always less than the same data with helium carrier. This decrease was caused by the deactivation of the active sites on the surface by preferential adsorption of acetone even at low pressures and high temperatures. Completely different results were obtained for the polar solute, propanol. The retention volume of propanol increased monotonically, but nonlinearly, with decreased temperature at all three acetone pressures. This is shown in Figure 5 where the data sets for different acetone pressures are offset for clarity. The acetone modifier affected the retention volume of propanol far less than the retention volume of pentane. Acetone enhanced the retention of propanol due to cooperativity effects caused by the strong lateral interactions of acetone and propanol in the condensed phase. Similar strong cooperativity effects have previously been observed for the retention of propanol on GCB with ethanol coadsorbed (13). In sharp contrast to the pentane data, cooperativity effects offset the site-blocking effects for propanol, and the retention volumes of propanol were usually greater with acetone in the carrier than with pure helium (asterisks in Figure 5). Some steric effects were evident at the lowest temperatures and highest pressure; however, no maximum was observed for the retention volume of propanol in the range of temperatures studied. Scaled Particle Adsorption Theory. For both solutes, the retention volumes reflected the concurrent influence of steric (geometrical)and interaction (electronic) effects on the adsorption mechanisms. The scaled particle theory (eq 3) provides a relatively simple mathematical model for the complex variations observed in the retention volume studies. The size parameters of the solute and adsorbate can be determined from isotherm data. Theoretically, the adsorption coefficient could also be determined from elution data with no adsorbable vapor in the carrier gas. Unfortunately, this is not true in practice because the adsorption coefficients, K,*, obtained from binary adsorption studies are subtly different from the adsorption coefficients, K,,obtained from singlecomponent isotherm data such as those given in Table I. The latter coefficients may be significantly influenced by preferential adsorption on the active sites of the adsorbent, whereas, the adsorption coefficients in eq 2 and 3 refer to the adsorption of the solute on a GCB surface with the active sites blocked by a coadsorbate. Thus, measured K, and Ki*values may differ significantly (14) depending upon the homogeneity of the adsorbent as well as the polarity of the solutes and adsorbate.
Table 111. Scaled Particle Theory Parameters of Pentane and Propanol Coadsorbed with Acetone on Carbopack C at Several Temperatures and Pressures 10-8a,,,
infinite dilution solute
PI, A2
(J m2)/ mol2
AS,*, J/(K mol)
n-pentane
15.8
3.8
-63.5
-37.0
1-propanol 10.0
6.2
-73.6
-41.6
MI*, kJ/ mol
9st9
kJ/mol (ref) -36.0 -37.2 -38.9 -33.9
(28) (27) (29) (27)
If the active sites were blocked by dopant, the remaining surface would be more homogeneous than the original surface, and the isosteric heat of adsorption of the solutes would be constant. Thus, eq 3 can be altered to explicitly express the temperature dependence of the retention volumes In Vg,,"(a)= In (273R(1 - 7))
I+-_--a 2PL) Pj (1 - a )
% P,R T - (
mi*
+ ASi* + + RT Pi
a'
Pi
(I - a)'
(4)
ASi* is the adsorption entropy change for component i and AHi* is the limiting isosteric heat of adsorption of i on a surface with the active sites blocked by a coadsorbate. The two-dimensional interaction parameter, aLi, is not available directly; however, it can be approximated by the geometric mean of the individual interaction parameters, CY, and CY]. The experimental Vg,+" values at each temperature and pressure (27 data points) were fit to eq 4 to determine the "best" values of aL,, AS,*,and AH;*for each solute. The size parameters were fixed for acetone (10.3 A') and both solutes. The fixed 0 values of the solutes and the calculated regression values of the other parameters are given in Table 111, along with literature values for the limiting isosteric heats of adsorption of pentane and propanol on GCB. The specific retention volumes calculated from eq 4 with the parameters given in Table I11 are shown as the theoretical lines in Figures 4 and 5. One set of only three parameters for each solute provided an accurate fit for the entire data set at three pressures and nine temperatures. The observed maxima in the retention volumes of pentane were accurately predicted. The scaled particle theory described the dominance of steric effects for the pentane solute, A theoretical maximum is shown for the low pressure curve although the experimental measurement did not extend to low enough temperatures for this observation. The experimental data for the retention volumes of propanol were less precise; however, there was adequate agreement between the experimental and calculated results. The general trends are obvious and the dominance of the cooperativityeffects over steric effects is shown clearly. The theory is adequate for even this polar solute-dopant combination. Thus, this model provides a simple, yet accurate, method for predicting the effects of changes in temperature and/or carrier gas composition on the retention characteristics of elution solutes for GSC with an adsorbable vapor in the carrier gas. Elution Peak Shape. The elution peaks for pentane were symmetrical for all of the temperatures and pressures investigated. The main effect of the acetone dopant was to diminish the retention time and volume of pentane at low temperatures. Thus, the elution peaks were sharper at the lowest temperature than at intermediate temperatures (20-40 "C). The effect is illustrated in Figure 6, which is a threedimensional plot of the chromatograms of pentane at different temperatures. The loss of availabie adsorbent surface due to the preferential adsorption of acetone caused rapid elution of pentane and a significant improvement in the symmetry
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
(I"
80
/ A V /
1h
V / / / /
0
0
0
1
2
3
5
4
Retention Time (min )
Flgue 6. chromatograms of npentane on carbopeck C with acetone (19 torr) in the carrier gas over a range of temperatures.
0
2
I
4
8
8
10
12
Retention Time ( m l n )
I
1
e
8
10
12
Retention Time ( m l n )
Flgue 7. Chromatograms of 1-propand on Carbopack C with helium carrier gas.
of the elution peaks a t very low temperatures. The Carbopack adsorbents are very popular and useful adsorbents for the separation of polar samples. However, the GCB must be "deactivated" with a small amount of a nonvolatile liquid. The uncoated adsorbents are unsatisfactory for polar solutes, such as propanol, because of the peak distortion caused by preferential adsorption on the active sites of the GCBs. This effect is shown in Figure 7 which illustrates the chromatograms of 1-propanol on untreated Carbopack C with helium as the carrier gas. The system showed significant deterioration at temperatures below ambient. The peak tailing was caused by preferential adsorption on the active sites and the wide energy distribution of the active sites on the untreated adsorbent. GCB supports coated with a small amount of a nonvolatile liquid usually give sharp symmetrical elution peaks for most samples, especially at high temperatures. Blocking the active sites with a nonvolatile liquid is an effective but inflexible deactivationmethod. In addition, the liquid loading may have a critical effect on the selectivity and resolution of a system. For example, Bruner (8) recently showed that there was a remarkable difference between the chromatographic properties of Carbopack F coated with 0.15%, which represented a fractional surface coverage of ca. 0.8, liquid compared to an identical support coated with 0.19% of the same stationary liquid phase. A blocking effect, similar to that observed with nonvolatile adsorbates, can be achieved with volatile adsorbates, such as acetone. Adsorption of acetone significantly reduced the retention time and peak width of the propanol samples as shown in Figure 8. The chromatograms of propanol on the acetone-modified Carbopack C are very similar to those ob-
W e 8. chromatograms of l-propand on Cattopack C with acetone (40ton) in the carrier gas. The solkl peak is the profile for l-propand at 0 O C on Carbopack C coated with 0.2% Carbowax-1500.
tained on Carbopack C coated with 0.2% Carbowax-1500, which is a very common and efficient stationary phase. This chromatogram is displayed as the solid peak in Figure 8. The chromatograms in the figures are the results for only one prmure of acetone. The peak shapes varied significantly with pressure and temperature; however, the effect of the acetone dopant on the peak shapes and retention volumes persisted even a t the lowest pressures investigated. The retention times of the elution peaks in Figures 6-8 do not correlate directly with the specific retention volumes in Table II because the experiments were carried out at constant acetone pressure, not constant flow rate. The flow rate varied from 35 to 25 mL/min from the lowest temperature to the highest. In addition, the specific retention volumes were all corrected to 0 OC. The elution peaks in the chromatograms were also far broader than would be expected for an efficiently packed column. The peaks were broad because of the very short columns used for this study. The columns were only 25-30 cm long and packed with less than 1 g of adsorbent in order to facilitate the measurement of the very high retention volumes encountered at low temperatures in a reasonable time (
