Heavily Loaded Columns with Active Supports in Gas Chromatography Jurgen Asshauer and lstvan Halasz Lehrstuhl fur Angewandte Physikalische Chernie der Universitat, Saarbrucken. West Germany
Active supports with a large pore volume can be coated with up to 1 gram liquid/gram support, thus filling the pores completely. Despite these high loadings, the adsorption properties of the support are still important. Depending on the solubility of the sample in the liquid stationary phase, the influence of adsorption on the gas-liquid interface can be negligible or very important. Peak broadening and the potential use of heavily loaded columns in preparative scale gas chromatography are discussed.
In routine gas-liquid chromatography, the support is coated with u p to 20 to 25 grams of liquid stationary phase per 100 grams of support. Higher concentrations of the liquid seemed to be undesirable because the time for the mass transfer in the stationary phase increases with the square of the film thickness. The supports used in this field are more or less inert. Sometimes, it is desirable to increase the concentration of the liquid stationary phases by using supports with large pore volumes (0.6-1.2 ml/g) and small pore diameters (100-400 A). As the pore diameter increases, it is likely that the heavily loaded support will become sticky and the stationary liquid will be removed from the pores by the eluent (especially in liquidliquid chromatography). The consequence of decreasing the pore diameter is that the specific surface area, and therefore the activity of the support, increases. Active supports coated with polar liquids sometimes are advantageous in gas adsorption layer chromatography (2-3) where silica or metal oxides are coated with up to 20% liquid. Although gas chromatographic measurements to be described here were made first, a short communication describing the use of heavily loaded columns in liquid-liquid chromatography was published earlier (4). In this paper, some experimental results in gas chromatography using active supports coated with unusually high amounts of liquids will be described and discussed.
EXPERIMENTAL Apparatus. The apparatus with a flame ionization detector is the same a s t h a t described previously (5). The carrier gas was nitrogen dried over 5 A molecular sieves. In all of t h e measurements described in this paper, the column temperature was held cons t a n t a t 120°C. P r e p a r a t i o n of t h e S t a t i o n a r y P h a s e . The support was a porous silica Porasil A‘ (Waters Associates, Framingham, Mass.) with the following specifications: pore diameter: l o ) , t~ = 0.38 - 0.42 if the support is gas inpenetrable (glass beads, quartz-sand, etc.). In Figure 1, the total porosity t T of some of the columns is shown as a function of the liquid loading on Porasil A. The total porosity of a column packed with 1.05 g liquid/g support is 0.43 f 0.02--i.e., the total pore volume is nearly filled with the liquid stationary phase. Up to this loading, the packing was free flowing and the columns were packed without difficulty (as is usually the case in gas chromatography). The reproducibility of the t~ values shown in Figure 1was better than f 5 % . Speed of Mass Transfer. In Figure 2, the h us. a curves for pentane are shown as a function of the liquid loading (0.13-0.95 g liquid stationary phase/g Porasil A). In Figure 3, the influence of the capacity ratio ( k = 2-40.5) on the h us a curves is shown a t a loading of 0.95 g/g. Because only the ascending branch of the h u s . curves is of interest here, it is possible to describe this linear region with the following simplified van Deemter equation:
h=A+Cii (2) The calculated A and C terms for columns 2 through 5 used with different samples are tabulated in Table 11. When column 5 is heavily loaded (0.95 g/g), the calculat-
(6) A. J. de Vries, M. le Page, R. Beau, and L. Guillemin, Anal. Chern., 39. 940 (19671 -~ ( 7 ) S. J. Gregg, “The Surface Chemistry of Solids,” Chapman and Hall, I~
~
\
London, 1951, p p 137, 235. I
1142
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
Table I. Column Characteristics Column number Length ( c m ) Empty column volume ( c m 3 ) Volume of the mobile phase V, ( c m 3 ) Volume of the liquid phase V l ( c m 3 ) Permeability w ( c m 2 X 10') Liquid loading (g FN l l j g support) Support Bulk density ( g / c m 3 )
1
50 1.57 1.37 1.23
... Por. A 0.41
50
50
5 40
1.57 1.20 0.22 1.60 0.37 Por. A 0.56
1.57 0.95 0.38 1.76 0.63 Por. A 0.67
1.26 0.58 0.47 2.40 0.95 Por. A 0.81
2
4
3
50 1.57 1.26 0.08 1.26
0.13 Por. A 0.46
Figure 1 . Total porosity
6~
1.05 Por. A 0.84
7
50 1.57 1.28 0.10 2.45 0.24 Chrom. W 0.33
-
I
10.
05.
0
6 64 2.00 0.90 0.80 2.48
[9/93
as a function of liquid loading
Support, Porasil A; liquid stationary phase, "Fractonitril I I"; carrier gas, nitrogen; temperature, 120 "C
h
cm m:
10
/i' 5
4 3 2 1
(
0
Figure 2. h vs. U curves for columns with different amounts of the liquid stationary phase
9/9)
Conditions same as for Figure 1. The figures refer to the column numbers in Table I
Conditions same as for Figure 1 except for the following: A . n-Pentane (k = 2.0); 8.Diethylether (8.3);C. n-Decane (25.2); D. Benzene (40.5)
Figure 3. h vs. 0 curves for different samples on column 5 (0.95
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1143
100
50
0 1 Figure 4. h/D
YS.
2
3
4
capacity ratio k on column 4
(0.63 g/g)
20
10
5
and column 5
Liquid Column loading, No. 919 2
0.13
3
0.37
4
5
0.63
0.95
A (10-3
c(10-3
Sample
k
cm)
sec)
n-Pentane Benzene n-Octane n-Pentane Diethylether Benzene n-Nonane n-Pentane Diethylether n-Nonane Toluene n-Pentane Diethylether n-Decane Benzene
2.95 11.6 19.2 2.35 9.6 14.7 24.7 2.2
28 19 17 27 31 16 15 53 34 9 23 93 68 56 43
6 7
8.0 21.5 39.4 2.0 8.3 25.2 40.5
a 6 10 7 7 15 20 14 9 131 65 29 15
ed C terms of Equation 2 decrease with a n increasing capacity ratio k . It can be calculated from Table I1 that for column 5 (and in a rougher approximation for column 4) the Alii value is negligible compared with C if a = 9 f 1 cm/sec. In this velocity rang h/ii N C. In Figure 4, h / a values are plotted as a function of a n unusually broad range of capacity ratios ( k = 2-100). Such high capacity ratios, however, are not unusual with heavily loaded gas chromatographic columns. 1144
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
40 50
100
k
(0.95 g/g)
Conditions same as for Figure 1 except for the following: X , n-Aliphatics; 0, Aromatics:
Table II. Calculated A and C Terms of Equation 2 for Different Columns and Samples
30
+, Chlorinated aliphatics; W , Oxygenated compounds
The shape of the curves in Figure 4 can be described by the Cs term of the van Deemter equation (8) assuming that the mass transfer term of the mobile phase C m is much smaller than that of the stationary phase Cs. In this case:
where df is the average film thickness and DL is the interdiffusion coefficient of the sample in the stationary liquid film. To calculate the film thickness of columns 4 and 5 in Figure 5 , h / a values are plotted as a function of k / ( 1 k ) 2 . The deviation of the experimental values from the linear interpolation is smaller than *4%. For DL a value of 5 X 10-5 cm2/sec was assumed at 120 "C. This seems to be in good agreement with similar values in the literature and can be calculated using the equation of Stoke assuming t h a t the radius of the diffusing molecule is about 2 A. From the slopes in Figure 5 the following film thicknesses were calculated: dr = 20 p for column 4 (0.63 g/g) and df = 60 p for column 5 (0.95 g/g). The values are in the right order of magnitude for the particle radius of 60-80 p. The pores of the support are filled with the stationary liquid as demonstrated with Figure 1 and with these calculated d f values and the peak broadening is controlled by the mass transfer in the stationary liquid phase. Partition Coefficient K and Capacity Ratio k. The mechanism of peak broadening in heavily loaded columns is controlled by partition. Assuming equilibrium:
+
(8) J. J. van Deemter, F . J . Zuiderweg, and A. Klingenberg, Chem. Eng. Sci., 5, 271 (1956).
h -
0
[rnsec]
100
50
0.05
0 Figure 5. h/L. vs. k / ( l
+ k ) 2 on columns 4 and 5
Conditions same as for Figure 1
h
=
K(V,/Vm)
(4)
where Vl is the volume of the liquid stationary phase. K the partition coefficient is independent of VLif the support is inert. Silanized Chromosorb W (with a specific surface area less than 1 m2/g) is regarded as a more or less inert support. In Table 111, the partition coefficients K i measured on this support a t 120 "C and with a liquid loading of 0.24 g/g are shown. When an active support is used, the experimentally determined partition coefficient K is a function of the average film thickness. This is demonstrated in Figure 6 for different samples. Even a t the highest possible loading ( i e . , more than 1 g FN II/g Porasil A ) the influence of the active support is not negligible. However, the slopes of the curves in Figure 6 in this region depend on the polarity of the sample. To explain the influence of the different partition mechanisms, various models have been proposed (9-1 1). One possible approach is that given in reference (12):
or dividing by the phase ratio VL"/ Vm":
K
=
K,
+-K.AIAIO vi0
+
K~KA~A~O vi0
(6)
where K is the experimentally determined partition coefficient as plotted in Figure 6. The contribution of the ad(9) R . L. Martin, Anal. Chem., 35, 116 (1963). (10) V. G . Berezkin and V . M. Fateeva, d Chromatogr 5 8 , 73 (1971). (11) B. L. Karger, R . C. Castells, P. A. Sewell. and A. Hartkopf, J. Phys. Chem.. 75,3870 (1971). (12) J. R . Conder, D. C. Locke, and J. H. Purnell, J. Phys. Chem.. 73, 700, 708 (1969).
Table 111. Partition Coefficients KL (cm3/cm3) for Fractonitril II at 120 OC Ki Extrapolated On Chromosorb W
n-Hexane &Heptane n-Octane n-Nonane Benzene Toluene Methylene chloride Chloroform Ether Acetone
from Figure 7
1.5 f 0.5
...
3.0 f 0.5
...
5.0 f 0.5 8.0 f 0.6 26. f 2
, . .
43. f 3 18. f 2. 24. f 2.
... 30. f 5. 35. f 5. 18. f 4. 24. f 5.
4. f 1 . 24. f 2.
15. f 5.
...
sorption a t the gas-liquid interface is K A ~ A L and ' KAJ~O is due to adsorption a t the liquid-solid interface. For convenience all parameters with the superscript "'"in Equations 4 and 5 are related to the unit volume of the column. The K values of Figure 6 are plotted in Figure 7 us. 1/ VL".As indicated by Equation 5 , the slope of these curves is proportional with the amount of sample adsorbed (KAIALO+ K L K A J ~ "and ) the intercept K L is the partition coefficient. The results with the columns where 1 / V l o was higher than 8 were neglected because of tailing and poor reproducibility. The data plotted in Figure 7 can be approached with a linear function. The intercepts of some samples (ether, acetone, and n-aliphatics) are, however, negative; consequently, they have no physical meaning as will be discussed later. ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1145
K
1000
a
K
loa
100
10
1
Figure 6. Partition coefficients K determined on different columns Conditions same as for Figure 1 except for the following: a . n-Pentane: b. n-Hexane; c. n-Heptane; d . n-Octane: e. n-Nonane; 1. Benzene: g, Toluene; h. rn-Xylene; i. 1,3,5-Trirnethylbenzene; k . Diethylether; 1. Dichloromethane; m. Chloroform; n. Acetone
/"
Y' ,,
,9
Figure 7. Partition coefficients K vs. l / V I " Conditions same as for Figure 6
The extrapolated positive Kl values of the other samples are shown in the last column of Table 111. Both Kl values for benzene, toluene, methylene chloride, and chloroform in Table I11 show acceptable agreement. The solubility of these compounds in the stationary liquid phase is relatively high compared with the compounds with negative intercepts in Figure 7. As can be seen from Figure 6 for the more polar compounds ( i e . , for those with high Kl values), the influence of the active support is noticeable even with very high liquid loadings. Consequently, the first and the third terms on the right side of Equation 5 are always important. If the second term of this equation is negligible, the Kas values calculated with this assumption will be independent of the loading. By neglecting the gas-liquid interface adsorption term, Equation 5 can be rearranged as follows:
K,, 1146
=
V,"(K - Ki)/(KiA,")
ANALYTICAL CHEMISTRY, VOL. 45,
(7) NO. 7,
JUNE 1973
From the specific surface area and from the bulk density of the Porasil A support, it is calculable that A," = 1.9 X 106 cm2/cm3. The calculated K A values ~ for different samples are given in Table IV. The Ki values for these calculations are those determined with silanized Chromosorb W support (Table 111). As to be seen in Table IV, the Kas values are in good approximation, independent of the liquid loading for benzene, toluene, and chloroform (the calculated values for methylene chloride are similar to those of chloroform). It seems t o be, that for these samples of good solubility the gas-liquid interfacial adsorption is negligible indeed. The samples with negative intercepts in Figure 7 are represented in Table IV by n-heptane and n-octane. The solubility of these compounds in the liquid stationary phase is relatively small as indicated by the Kl values in Table 111. On the other hand, the gas-liquid interfacial adsorption is not negligible. It seems to be that for the aliphatic hydrocarbons, this term is the most important one. The slope of the K us. l / V l plot for these compounds can be constant only if Al" is not a function of the loading Vi. However, if the amount of the stationary liquid phase is increased, the pores of the support will be filled as shown in Figure 1. Consequently, the surface of the liquid Ai" decreases if l / V L o < 2.5 and the slope becomes smaller and has to result in a positive intercept Ki. An extrapolation of the curves in Figure 7 resulting in a negative intercepts cannot be made. Heat of Sorption. In the region of 100-140 "C, the temperature dependence of the capacity ratios was measured on columns with different loadings. The heats of sorption were calculated with Equation 8: dln k A H = -R--RT d'l T As demonstrated in Figure 8b, the heat of the sorption for aromatic compounds is independent of the loading in the first approximation. The contrary is true for the nparaffins (Figure 8 a ) . This is in good agreement with the mechanism of sorption as discussed above. Heavily Loaded Columns in Prepscale GC. The potential advantages in this field are: The amount of the liquid stationary phase is larger than in conventional columns by a factor of 5 or more (Table I, column numbers 6 and 7). Consequently, the sample size can be increased
-cH
-AH
I%[
[ S I IO
i
10
h 9
0
8
f
k
6
6
Figure 8. Heat of sorption - A H as a function of liquid loading Conditions same as for Figure 6
Table V. Maximum Sample Size (smax) for Column 4a Sample k h / u , msec ,,s mg
Table I V . Calculated Adsorption Coefficients K A ~ KAs, (cm3/cm*) X l o 7 No.
Liquid loading, g/g
2 3 4
0.13 0.37 0.63
5
0.95
6
1.05
Column
Benzene
Toluene
Chloroform
nHeptane
n-Octane
1.6 1.6 1.6 1.9 1.3
2.1 1.9 1.7 1.7 1.2
1.3 1.5 1.5 1.7 1.0
14.3 10.1 6.3 2.7 1.4
16.4 11.0 6.7 2.6 1.2
without increasing column diameter. The concentration of the sample a t the outlet of the column is higher than with conventional columns a t a given carrier gas velocity. Therefore less energy is required for the separation of the sample from the carrier gas. For demonstration in Table V, the maximum sample size (Smax) is tabulated for different samples. In plotting h us. sample size s, a sharp increase of h can be seen a t a certain s value ( S m a x ) . In our experiments, column 4 seemed to be a fair compromise between efficiency and maximum sample size. It should be pointed out t h a t the column length was only 50 cm. When heavily loaded columns are used in prepscale gas chromatography, the high capacity ratios can be disadvantageous because of the decrease in the speed of separation. To find the optimum structure of the support for heavily loaded columns, the following considerations are necessary. With increasing surface area, the amount of liquid stationary phase can be increased because of the convenient pore structure. Unfortunately, the support becomes more active and the pore diameter decreases, thus resulting in increased asymmetry of the peaks. In our opinion, a support with a specific surface area about 30 m2/g, with a pore volume of 0.8 ml/g and with pore diameters between 300-600 A could be near to the optimum. To our best knowledge, none of the commercially available supports fits these requirements.
CONCLUSIONS On coating porous active supports with up to 1 g liquid stationary phase/g support, the porosity of the column decreases by a factor of up to 2 as the pores are filled. Consequently, for a given linear velocity, a smaller volume flow rate is required. This may be advantageous in preparative work. The adsorptive properties of the active
n-Pentane Diethylether n-Nonane Toluene
2.2
23.0 1.2 26.0 1.6 21.5 15.4 4.4 39.4 12.8 4.5 Liquid loading, 0.63 g FN l l / g Porasil A ; time averaged velocity, 7.5 cm/sec; and temperature, 120 “C.
8.0
support are still noticeable even at maximum liquid loading. The peak broadening is, of course, controlled predominantly by the mass transfer in the stationary phase. The film thickness of the liquid calculated from the experimental data is in agreement with the average radius of the support particles. For samples with high solubility in the liquid phase, the partition mechanism is controlled by absorption in the liquid and adsorption on the surface of the support. For compounds with very low solubility in the liquid phase, the adsorption on the gas-liquid interface is the main contributor to the partition mechanism. This is confirmed by the calculated heats of sorption as a function of the liquid loading. Here it is assumed t h a t both liquid and solid support are “polar” or “apolar.” Advantages and disadvantages of the use of heavily loaded columns in preparative gas chromatography seem to be in balance.
NOMENCLATURE A = constant of Equation 1 (cm) Al = surface area of the liquid stationary phase (cmz) A s = surface area of the solid stationary phase (cmz) C = constant of Equation 2 (sec) C, = mass transfer term in the mobile phase (sec) Cs = mass transfer term in the stationary phase (sec) Dl = diffusion coefficient in the liquid phase (cm2/sec) F , = flow rate a t the column outlet (crnS/sec) AH = isosteric heat of sorption (kcal/mole) K = experimentally determined sorption coefficient (cm3/ cm3) KAL = gas-liquid interfacial adsorption coefficient (cm3/ cmz) Kas = gas-active support adsorption coefficient (cm3/ cm2) K L = gas-liquid partition coefficient (cm3/cm3) L = columnlength(cm) ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1147
R = gasconstant T = temperature ( " K J C'K = empty column volume icms)
Li
cp
= volume ofthe liquid stationary phase (cm3)
I',
= volume ofthe mobile phase icm3)
d, = inner diameter of the column ( c m ) d f = average film thickness ofthe liquid stationary phase d p = particle diameter ( c m ) h = height equivalent to a theoretical plate icm) j = pressure correction factor of .James and Martin fi = ( t -~t,)/t,,capacityratio [ - ] I p = pressure drop ( a t m ) Y = sample size (gram) t o = elution time of the inert compound (sec) t R = elution time o f a retarded compound (sec) u = L , ' t , time averaged linear velocity icm sec) c~ = t o c p total porosity of a column
+
1148
ANALYTICAL CHEMISTRY, VOL
= interparticle porosity = porosity caused by the pore volume q = ( u ~ l L ) / ( I p jpermeability ') (cm*) q = viscosity (poise) All parameters with the superscript are related to the unit volume o f the column. to
45
NO 7. JUNE 19-
ACKNOWLEDGMENT LVe agpreciate helpful discussions with B. L. Karger, Northeastern Lniversity, Boston, Mass.
Received for review November 22, 1972. Accepted January 15. 197;;. This is a part of the Ph.D. Thesis of J. Asshauer University of Frankfurtlhlain 1971.The authors thank the Deutsche Forschunpgemeinschaft (Sonderforschungsbereich -52. .Analytik. Saarbrucken) for financial furtherance of this research work.
