(13) Martin, R. L., ANAL. CHEM.33, 347 (1961). (14) Meighan, R. M., Cole, R. H., J. Phys. Chem. 68, 503 (1964). (15) Morrow, H. M., Buckley, K. N., Petrol. Refiner 36, 157 (1957). (16) Novhk, J., JanAk, J., Chem. Listy 57, 371 (1963). (17) Podbielniak, W. J., Preston, S. T., Petrol. Refiner 35, 215 (1956).
(18) Preston, S. T., Jr., J. Gas Chromatog. 1, No. 319 (1963). (19) Raupp, G., 2. Anal. Chem. 164, 135 (1958). (20) Stazewski, R., Janhk, J., Collection Czech. Chem. Commun. 27, 532 (1962). (21) Sternberg, J. C.,. Gallaway, W. S., Jones, D. T. L., in "Gas Chroma-
tography," N. Brenner, J. C. Callen, M. D. Weiss, eds., p. 231, Academic Press, New York, 1962.
(22) Tenney, H. M., ANAL. CHEM.30, 2 (1958).
RECEIVEDfor review July 12, 1965. Accepted November 3,1965. Third International Symposium on Advances in Gas Chromatography, Houston, Tex., October 1965.
Support Effects on Retention Volumes in Gas Chromatography PAUL URONE and JON F. PARCHER University of Colorado, Boulder, Colo. Four series of matched columns using deactivated and nondeactivated acid-washed firebrick as support materials and squalane and tri-o-tolyl phosphate as liquid phases were studied to determine the dependence of gas chromatographic retention volumes of polar solutes on surface active supports. Liquid surface effects were absent on both the squalane and tri-otolyl phosphate columns. Retention volume minima at 0.6% and maxima at 2% liquid phase were observed for the solutes on squalane-coated surface active AWFB columns indicating a wide variety of retention effects over a small liquid phase range for low-loaded columns. Zero sample size retention volumes obtained from a series of samples injected on columns being subjected to constant amounts of the same solute in the carrier gas gave straight line Freundlich-type plots for the low-loaded columns. The slopes of the lines, which indicated sample size effects, differed for different liquid loads, being steepest at the lowest liquid phase loads. Adsorption isotherms for acetone on squalane-coated AWFB columns were essentially Type I of the BET classification and paralleled the retention volume dependence upon the amount of liquid phase coating for their relative magnitude.
R
of polar solutes are affected by the type and amount of liquid phase, the type and the treatment of the solid support, the sample size, and the temperature (13). The difficulties of comparing the performance of different column supports have been discussed (1, 12). Martire (6) applied theory of solution techniques to the calculation of activity coefficients of gas chromatographic solutes through their dispersive, orientation, and induction forces. His final expression for the 270
ETENTION VOLUMES
ANALYTICAL CHEMISTRY
activity coefficient includes an empirical residue factor, K , which is probably due to support effects. Martin ( 5 ) , Pecsok et al. (IO), and Martire, Pecsok, and Purnell (7) studied the adsorption of solutes on liquid phase surfaces by gas chromatographic and surface tension measurements. Observed retention volumes were interpreted to be composed of liquid phase and liquid surface adsorption contributions. Previous studies by the author (16) showed that, although the type of solid support and its treatment greatly affected the specific retention volumes of polar compounds, relative retention volumes and activity and partition coefficients were remarkably repetitive from column to column for a given homologous series on a given liquid phase coated on different types of supports. The heats of solution, on the other hand, did not show a strong repetitive pattern from column to column. Further studies of the contribution of the solid support to the retention volumes of polar solutes have been undertaken by comparing the retentive characters of four series of matched columns using deactivated and nondeactivated acid-washed firebrick as support materials. Deactivation of the support was accomplished by intensively treating a portion of the firebrick used in the study with hexamethyldisilazane (HMDS) using a recently developed radiation-induced copolymerization technique called RIC (14). Surface area measurements showed little change in the surface area of the support caused by the treatment. EXPERIMENTAL
Each series of columns included eight to ten columns of the respective firebricks coated with 0.6 to 22% squalane and tri-o-tolyl phosphate (TOTP), respectively. Coating of the columns was accomplished by a special solution
coating technique which helped to give some assurance that columns coated with small amounts of liquid phase had a uniform distribution of the liquid (9). Retention data were obtained a t different liquid loads, sample sizes, under various conditions of column sweep time, and in the presence of known amounts of solute in the helium carrier gas. Each column was made of copper tubing 1 meter by 4-mm. i.d. All retention data were taken a t 75" C., and all retention volumes were corrected for pressure and air peak volumes. All columns were properly conditioned before use. Columns with no liquid phase were conditioned by heating to 200" C. over a half-hour period. Coated columns were conditioned overnieht a t 75" C. Sample injections for Figures 1 and 2 were obtained by using helium saturated a t 30" C. with the respective vapors and a 0.922-m1. gas sampling loop. Liquid sample injections of acetone less than 1.0 pl, were obtained by diluting the acetone with an inert solvent such as hexane. A 1000-cubic-inch (16.4 liter) stainless steel tank containing helium a t 80to 100-p.s.i.g. pressure and acetone a t approximately 50% of saturation pressure (100 to 160 mm.) was used as a source for a constant amount of acetone (Figure 3). The helium-acetone mixture was bled into the carrier gas stream with a capillary tube and controlled by a fine-control needle valve. The concentration of the acetone was determined by absorbing the acetone in a 1.ON NaOH solution, adding excess iodine, and back-titrating with thiosulfate (3, 8). RESULTS AND DISCUSSION
Figures 1 and 2 show the variation of the net retention volume per gram of coated support with the per cent of liquid phase. Reproducible values obtained from consecutive sample loop injections of the individual solutes at 5-minute intervals were used because the first injections in the low-loaded nondeactivated (AWFB) columns either
PERCENT LIQUID PHASE
PERCENT
Figure 1 . Retention volumes as function of per cent squalane on deactivated (RIC) and nondeactivated (AWFB) supports at 75' C. ReDroducible valuer. 5-minute intervals. SamDle sizes: ' MI.; methanol, 0.40 cyclohexane, 0.3 1 PI. - Squalane on acid-washed firebrick Squalane on RIC firebrick
&.;
acetone, 0.70
-.----
could not be detected or were not reproducible. The differences between the five-minute interval and the first injection values became increasingly greater as the liquid load became less. For all the RIC columns, the 5-minute interval values were essentially the same as the first injection values. In all instances, the net retention volumes for the RIC columns were lower than the retention volumes on the corresponding nondeactivated AWFB columns. The greatest difference was shown by the most polar solute, methanol, while only a slight difference was shown by the least polar solute, cyclohexane. Because the surface areas of the two types of supports were essentially the same, the differences in the data obtained indicated that the surface of the liquid phase, as such, did not significantly contribute to the retention of the solute. The retention of methanol, for example, was much higher on the nondeactivated, acid-washed firebrick than on the RIC columns with both squalane and TOTP and a t all liquid phase loads studied including the 22% columns. The above findings do not necessarily preclude the possibility of adsorption of nonpolar solutes on highly polar liquid phase surfaces as pointed out by Martin and others (5, 7, IO) where independent measurements of surface tension and
LIQUID PHASE
Figure 2. Retention volumes as function of per cent TOTP on deactivated (RIC) and nondeactivated (AWFB) supports at 75" C. Reproducible values, 5-minute intervals. Sample slzer: ketone, 0.31 MI.,methanol, 0.40 MI.; cyclohexane, 0.31 PI. - TOTP on acid-washed firebrick TOTP on RIC firebrick
methyl ethyl
------
partition coefficients combined with the use of the Gibbs adsorption equation were used to check the partition coefficients obtained by gas chromatography. The experiments of this study, however, indicate a more complicated mechanism than that of simple liquid surface adsorption. At low liquid phase loads (2Q/,), the contribution of
Acetone Sampler
Thermal Conductivity Detect or
Figure 3. acetone
Schematic representation of apparatus for obtaining constant loads of
VOL 38, NO. 2, FEBRUARY 1966
271
0
,
I
,
02
04
06
Sample size
I
I
OB
(JI
1.0
12
the support surface, and KL and K. are partition coefficients. K , is a complex function of the sample size, polarity of the liquid phase, liquid phase loading, and the types of surfaces present in the support materials. On the assumption that little, if any, solid surface effects were present in the RIC column data, the partition coefficients for the squalane and the TOTP were calculated and used to account for the liquid phase contributions to the retention volumes on the surface active supports. In this manner, an overall partition ratio, IC*, was obtained from the net retention volume beyond that attributable to the liquid phase, V,V*, where :
V.V* = V I Z- (Vm
Figure 4. Dependence of retention volumes on sample size and partial pressure of acetone in carrier gas Acetone on 5.39% rqualane/AWFB at 75' C. 1. 0 . 0 0 m m . 2. 0 . 3 5 mm. 3. 0.78 mm. 4. 1 . 2 3 mm.
the bulk liquid phase becomes more dominant, but the effect of the support is never completely removed (Figures 1 and 2 ) . Also, the retention of a solute cannot unequivocally be assumed to be due to a simple addition of the contributions of the support (or liquid) surface and the liquid phase. The retention may as easily be explained by a model using a modified liquid layer whose partitioning properties are affected by the support to varying degrees depending upon the surface activity of the support and the amount of liquid phase used. Such a model may explain some of the results obtained by illartin et al. (5, 7 , 10). The downward dip in the retention volumes of the low-loaded, surface active squalane columns (Figure 1) resulted from the fact that repetitive injections were used to obtain the reproducible retention volumes plotted. The retention volumes under these conditions depended on the build-up of a steady-state type roating conditioned by retained portions of previously injected samples. The effect was not as noticeable for the TOTP columns (Figure 2) because the TOTP competes more effectively for the more active adsorption sites of the support. The overall adsorption effects, nevertheless, are apparent through comparison of the AWFB and RIC curves in Figure 2. If the additivity principle of retention is assumed (4, 5 , ?'), the retention volume may be represented by : where V , and V L are the mobile and liquid phase volumes, A , is the area of
272
ANALYTICAL CHEMISTRY
+ KLVI,)
KA/V,
(4)
Because dependable net retention volumes on surface active supports are difficult to obtain, a method of injecting a sequence of samples increasing from 0.01 pl. to 5.0 pl. on a column being subjected to a known, constant amount of the same solute was developed (Figure 3). Acetone was used as the solute because it had an intermediate polarity and was relatively easy to handle. Retention volumes obtained in this manner were plotted us. sample size for different partial pressures of acetone in the carrier gas (Figure 4). For a given column, the retention volumes tended to become the same as the sample size and the partial pressure of the acetone in the carrier gas became greater. Sample sizes of 5 11. and larger gave retention volumes essentially independent of the amount of acetone in the carrier gas. Retention volumes for zero sample sizes per gram of coated support, ( V ~ ~ o * ) s =were o l obtained by extrapolation from log retention volumes us.
101' -08
3
3
-06 - 0 4
1
-02
"
0 Log c m
02
"
04
06
2
4
6
8 10 I2 14 PERCENT SQUALANE
IS
I0
20
22
Figure 6. Variation of zero sample size retention volumes with per cent squalane on AWFB supports Constant load of acetone ml./ml.
= 1.6 X
(2)
(3) =
0
' 1
08
Figure 5. Dependence of zero sample size retention volumes on partial pressure of acetone and per cent of squalane on nondeactivated (AWFB) supports at 75" C.
sample size plots. These retention volumes gave the limit of retention of a peak pulse a t the known partial pressure of vapor in the gas phase. They are directly related to the slope of the adsorption isotherm a t the given partial pressure. They can be used to indicate the rate of change of the overall partition ratio
(5) where kc,,,* is the partition ratio of the solute a t a given partial pressure of the acetone in the carrier gas, C,. Figure 5 shows the log-log relationship of the (VN,*)s values, which are equal to k*V,, with the partial pressures of acetone in the carrier gas, C,. The curves show that retention volumesLe., partition ratios-vary considerably with the concentration of the acetone in the carrier gas and with the amount of liquid load on the column. The relationships are linear for the low-loaded columns. At 2y0 liquid phase the retention volumes become erratic, while a t higher liquid phase loads, the retention volumes drop rapidly with increases in the amount of acetone in the carrier gas. The linear portions of the curves in Figure 5 indicate a Freundlich type of dependence which can be identified with a partition ratio as follows:
-
where a is the amount of solute adsorbed per gram of adsorbent, P is the pressure of the solute, k , and n are constants. The term a / P is a form of partition coefficient (11). Hence, log k , = log k ,
+ (+)
log P
(8)
Cl
W
m
z
g
v)
W
U U W
n a
8 W
U
Figure 9. Dependence of amount of acetone adsorbed at constant partial pressure upon per cent of squalane on AWFB supports
c VOLUME Figure 7. Schematic adsorption-desorption chromatogram for constant amount of solute in carrier gas
where k , is a partition coefficient and k , differs from k, by a proportionality factor. Equation 8 indicates a linear relationship between the log of the partition coefficient and the pressure of the solute with a slope of (1 - n)/n. Slopes of log-log plots may thus be used to calculate TZ values which may then be used to evaluate sample size retention effects by the association of sample size to the partial pressure of the solute in the carrier gas over the linear range of the C, relationship. The slopes and vertical positions of the lines in Figure 5 change in a complicated manner with the amount of liquid phase on the support. The variation can be shown more clearly by taking the (VAr,*), values a t a given partial pressure for all the columns and plotting them us. the per cent of liquid load on the column. Figure 6 shows the data presented in this manner. The striking
feature of this figure is that there is a strong resemblance to Figure 1. Again there is a minimum a t approximately 0.6% liquid phase and a maximum a t about 2Tcliquid phase. The results shown by Figures 1 and 6 were reproduced in a third manner: one of using adsorption and desorption chromatograms to calculate the amount of solute adsorbed a t a given concentration of acetone in the carrier gas. Figure 7 shows a typical adsorption and desorption chromatogram. The amount of acetone remaining adsorbed on the support a t any point of the desorption curve can be calculated from the following extension of Gluckauf's equation
(9:
a
where
;If Q
=
C
=
0.00% '2s Ob
V Ap0
p, '0
2
I
4
I
I
L T ' ,
I
Y
B W
