mentum transfer by molecular mechanism vf = kinematic viscosity for momentum transfer by turbulent convective mechanism p = density 7 = analysis time parameter +(r) = function defined by Equation 7 #(r) = function defined by Equation 7
SUBSCRIPTS ill = mobile phase S = stationary phase I = first eluted solute I1 = second eluted solute LITERATURE CITED
(1) Aris, R., Proc. Roy. SOC.A235, 67 (1956). (2) Zbid., A252, 538 (1959). (3) Bird, R. B., Steward, W. E., Light-
foot, E. N., “Transport Phenomena,” Chap. 5, 6, Wiley, New York, 1960. (4) Zbid., p. 629. (5) Desty, D. H., Goldup, A., “Gas Chromatography 1960,” p. 162, Butterworths, London, 1960. (6) Desty, D. H., Goldup, D. G. F., Whyman, J., J . Znst. Petrol. 45, 287 (1959). (7) Giddings, J. C., ANAL.CHEM.35, 439 (1963). (8) Giddings, J. C., Seager, S. L., Stucki, L. R., Stewart, G. H., Zbid., 32, 867 (1960). (9) Golay, M. J. E., “Gas Chromatography,” D. H. Desty, ed., p. 36, Butterworths, London, 1958. (10) Hofmann, H., Chem. Eng. Sci. 14, 193 (1961). (11) Levenspiel, O., Ibid., 17, 575 (1962). (12) Purnell, J. H., Ann. N . Y . Acad. SCZ. 12, 592 (1959). (13) Purnell, J. H., “Gas Chromatog-.
raphy,” p. 152, Wiley, New York, London, 1962. (14) Purnell, J. H., J . Chem. SOC.1960, 1268. (1:) Scott, R. P. W., Hazeldean, G. S. F., Gas Chromatography 1960,” p. 144, Butterworths, London, 1960. (16) Stewart, G. H., Seager, S. L., Giddings, J. C., ANAL. CHEM.31, 1738 (1959). (17) Taylor, Geoffrey, Proc. Roy. SOC. 219A, 186 (1953). (18) Zbid., 233A, 446 (1954). (19) Tichacek, L. J., Barkelew, C. H., Baron, T., A.Z.Ch.E. J . 3, 439 (1957). (20) Zlatkis, A., Walker, J. Q., ANAL. CHEM.35, 1359 (1963).
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I
RECEIVEDfor review July 8, 1965. Accepted November 29, 1965. Third International Symposium on Advances in Gas Chromatography, Houston, Texas, October 1965.
Aerogel Columns in Gas Chromatography ISTVA” HALASZ and HANS-OTTO GERLACH lnstitut fh Physikalische Chemie der Universitat, Frankfurt am Main, Germany
b Aerogel columns were produced by drawing out to 0.4-mm. i.d. glass tubes loosely packed with highly dispersed silica having a particle size of about 1 micrometer. Although the particles appear to be closely packed, the packing is highly dispersed and only 2 to 470 of the total volume is occupied by the stationary phase. The gas permeability of the aerogel columns was somewhat lower than that of the conventional packed columns, but minimum values of 0.01 2 cm. for h and 0.035 cm. for H were achieved with high carrier gas velocities. Fast analyses within seconds were carried out. The aerogel columns are compared with other column types and some analytical applications for the separation of C1 to Ce paraffins and olefins are demonstrated.
where E is the interparticle porosityLe., the fraction of column cross section available to moving gases :
P
Since in classical packed columns the interparticle porosity, E , is practically constant, the permeability, K , is proportional to the square of the particle size, the pressure drop, A p , is limited to 4 atm. on account of technical reasons, and the relative peak broadening decreases with decreasing particle size, compromise must be made in choosing the particle size. Values of d, N 0.10 to 0.15 mm. have proved to be most favorable. Equation 1 shows that a smaller particle size might be chosen without lowering the permeability, if the decrease in d, could be compensated for by increasing the interparticle porosity, E . Column efficiency could be further improved by increasing the velocity of
broadening in gas chromatography is usually characterized by the h or H values, where h = L w2/16 1 2 and H = L w2/16 ( t t R ) 2 . Previous experiments have shown that in very rough approximation the relative peak broadening ( h or H ) in packed columns decreases linearly with decreasing particle size, d,, of the support (1, 7 , 8, 11,18-20). But greater efficiency of the column with decreasing particle size must be paid for by increasing pressure drop of the carrier gas. The Kozeny-Carman equation (2, 5 ) shows that specific gas permeability, K ( l d ) , is proportional to d,Z: EAK
w
e = l - L
VCP
(2)
where T1’, is the total weight of the support in the column, V , is the total volume of the column, and p is the apparent density of the support measured by mercury displacement. Equation 1 shows and experiments on regularly packed gas chromatographic columns have proved ($1) that permeability is independent of the inner diameter of the column. I n classical packed columns the interparticle porosity is practically independent ( E = 0.40 to 0.41) of the mesh size (163 to 545) of the support, although the total porosity increases from 0.76 to 0.80 (6) *
obtaining equilibrium between the moving and the stationary phases. Equilibrium can be the more rapidly attained, the closer the contact between the stationary and gas phase- ke., the better the stationary phase is “dispersed” in the gas phase. ITsually particles as spherical as possible are employed in the preparation of packed columns, in order to achieve uniform packing and gas flow through the column. But spherical particles-especially if they are poreless-are unfavorable on account of their minimum specific surface area a t maximum packing. On the other hand (15, 16) under certain circumstances irregularly packed columns*.g., packed capillary columns-are more efficient than regularly packed columns. I n view of these facts we tried to find a fibrous material which combines maximum surface area with minimum packing volume. EXPERIMENTAL
Chromatographic Apparatus. The gas chromatographic apparatus was similar to that described (3). The flame ionization detector, FID, was combined with a recorder (full scale time 0.3 second) or with a high speed ultraviolet galvanometer recorder. It was possible to record undistorted peaks as narrow as 0.2 second. The outlet pressure, p,, of the column was atmospheric in all experiments. The gas holdup values, to, were determined with methane. Methane, instead of helium, could be used for this purpose, sipce by means of a microthermal conductivity cell it was found that methane a t 25’ C. behaves VOL. 38, NO. 2, FEBRUARY 1966
281
like an inert gas on all the stationary phases described below. Support. A highly dispersed silica, Aerosil 2491/380 (manufactured by Degussa, Frankfurt am Main), was used and proved to be a suitable support. Made by flame hydrolysis of S i c & , this SiOs is a loose, bluish white, flaky, and Rontgen-amorphous powder, which forms a colloidal solution with water. Relevant properties ( 4 ) of the support are listed in Table I. The primary particles (diameter 30 to 150 A.) are spherical and free of pores. They agglomerate to straight and branched “secondary” chains, which form a “tertiary” structure consisting of irregular three-dimensional networks.
Table 1.
Properties of Aerosil
2491 /380 SiOs in moisture-free sub>99.8 stance, wt. yo 2.36 True density, g./cc. ca. 60 Bulk density, g./liter Effective particle diameter,
30-150
A.
Specific surface area (BET), 380 sq. meters/gram Ignition loss at 1000° C., wt.
=k 40
%
