Spherosil in Modified Gas-Solid Chromatography C. L. Guillemin, Michel Deleuil, Simone Cirendini, and Jean Vermont Produits Chimiques, Pechiney-Saint Gobain, 93 Auberuiiiiers, France
A systematic study of Spherosil (Pechiney-Saint Gobain) coated with small amounts of stationary phases, has allowed u s to give a definition of modified gas-solid chromatography and to draw the essential characteristics of the support-selectivity and analysis time. These properties depend on two fundamental parameters only: the thickness e; of the layer of stationary phase and the specific surface area S of Spherosil. It has been possible to establish some simple and straightforward rules which govern any separation by modified adsorption chromatography on Spherosil: At constant S and with variable ei, selectivity is variable. With variable S but constant el, selectivity is constant. Analysis t i m e also depends on the thickness e,: of the layer. As a matter of fact, the relationship between specific retention volume and thickness of the layer shows a characteristic m i n i m u m dependent on t h e nature of the stationary phase. Because of the spherical shape, Spherosil beads give very efficient columns. Up to 1400 theoretical plates per meter can be reproducibly obtained i n analytical columns with diameters between 1 and 4 mm, for a ratio p of bead diameter t o column diameter between 0.03 and 0.18. Numerous examples of separation, taken f r o m different groups according to Kiselev’s classification, are given to illustrate the versatility of this chromatographic technique which has been applied with success to a large number of various analytical problems.
IN A PREVIOUS article (1) Spherosil (product of Pechiney-Saint Gobain) used as a n adsorbent in gas chromatography is particularly recommended for separation of molecules of groups A and B according t o Kiselev’s classification ( 2 , 3); these include saturated and unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons in general. In the case of polar molecules belonging t o groups C and D, modified gas-solid chromatography is more useful and is not hampered by excessive peak tailing and long retention times. Modified gas-solid chromatography is not a new method (4-8) one can assert, like M. Jourdain in “Le Bourgeois Gentilhomme” by Moliere, that every chromatographer has practiced or still practices modified gas-solid chromatography, without knowing it! Basically the technique consists in modifying the properties of an adsorbent by adding a limited amount of stationary phase resulting in a n important decrease in analysis time, suppression of peak tailing, and increase in selectivity. (1) C. L. Guillemin, Melle Le Page, and A. J. De Vries, J . Chromarugr. Sci., 9, 470 (1971). (2) A. V. Kiselev, Adcan. Chromatogr., 4, 113 (1967). (3) A. V. Kiselev and Y. I. Yashin, “Gas-Absorption Chromatography,” Plenum Press, New York, London, 1969. (4) I. Halasz and E. Heine, Adoan. Chromatogr., 4, 239 (1967). (5) I. Halasz and C. Horvath, ANAL.CHEM.,36,2226 (1964). (6) G. C. Scott, “Gas Chromatography 1962,” M. Van Swaay, Ed., Butterworths, London, 1962, p 36. (7) C. Vidal-Madjar and G. Guiochon, Separ. Sci., 2, 155 (1967). (8) C . Vidal-Madjar and G. Guiochon, Bull. Soc. Chim. Fr., 1966, 1096.
Physical Characteristics of Spherosil Nominal Porous specific surface area, Av pore volume, Type of Spherosil m%“ diameter, A cm3/g 400 80 1 Spherosil XOA 400 Spherosil XOA 200 200 150 1 Spherosil XOB 075 100 300 1 Spherosil XOB 030 50 600 1 Spherosil XOB 015 25 1250 1 Spherosil XOC 005 10 3000 1 0 Measured by B.E.T. method (15). Table I.
Spherosil has been tried with success in modified GSC by several authors ( I , 9-13), but definite rules for obtaining optimal results are still lacking. Therefore, following Kiselev’s studies (2, 3) and more recently (14, we have systematically studied the relation between the physical characteristics of modified Spherosil, coated with various stationary phases and its chromatographic properties. Spherosil is composed of pure silica, and is available in the form of spherical beads with diameters which may range from 20 to 300 microns. Each type of Spherosil is characterized by total pore volume, specific surface area, and mean pore diameter, independent of the particle size. Spherosil beads are perfectly rigid and incompressible despite their considerable pososity ; they are resistant t o attrition and do not swell in any liquid. The different grades of Spherosil may be subjected to temperatures up to 600 “Cwithout any change in texture. Table I gives the commercially available grades in the particle size range of 40-100 microns and 100-200 microns. The characteristics indicated are nominal and may vary slightly from one batch to another. EXPERIMENTAL
Coating Procedure. Coating of Spherosil with liquid phases is similar to the coating of classical supports like diatomaceous earth. It may be useful to distinguish between hydrophilic and hydrophobic phases. In the case of hydrophilic coatings like P,P’-oxydipropionitrile or Carbowax 20 M, no special precautions have t o be taken and, in particular, preliminary dehydration of Spherosil is not necessary. Dehydration is however required if hydrophobic coatings, such as silicone oils are used. Packing Procedure. Spherosil, a spherical material, fills the columns relatively easily. A reservoir containing Sphero(9) C. L. Guillemin, Melle Le Page, R. Beau, and A . J. De Vries, ANAL.CHEM.,39, 941 (1967). (IO) D. J. Brookman and D. T. Sawyer, ibid., 40, 1368 (1968). (1 1) A. F. Isbell, Jr., and D. T. Sawyer, ibid., 41, 1381 (1969). (12) D. F. Cadogan and D. T. Sawyer, ibid., 42, 190 (1970). (13) B. L. Karger, P. A . Sewell, R. C. Castells, and A. Hartkoff, J . Colloid Ititerface Sci., 35, 328 (1971). (14) A V. Kiselev, N. V. Kovaleva, and Y . U. S. Nikitin, J . Chromulogr., 58, 19(1971). (15) D. M. Young and A. D. Crowell, “Physical Adsorption of Gases,” Butterworths, London, 1962.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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Figure 2. Asymmetry of a peak us. layer thickness of fl,p’oxydipropionitrile on different types of Spherosil. Asymmetry is measured on peak of methyl isobutyl ketone (Figure 4)
+=
Figure 1. Characteristic peak shape in adsorption chromatography si1 continuously feeds the column, which is fixed on a vibrating system. Vacuum is applied at the exit of the column to make the packing easier. This procedure is valid for analytical column diameters, between 1 and 4 mm. The final column conditioning at temperature will be done by heating in situ in the chromatograph. CHARACTERISTICS AND PERFORMANCES OF GSC COLUMNS PACKED WITH COATED SPHEROSIL Influence of the Stationary Phase Layer Thickness on Chromatographic Quantities. A systematic study was undertaken to show the influence of the stationary phase layer thickness on peak asymmetry and specific retention volume. Assuming the layer to be homogeneously distributed, its apparent average thickness is given by Equation 1.
where w z is the weight of the stationary phase of density p ; p stands for the weight of Spherosil, which specific surface area is S. It should be kept in mind that Equation 1 represents a purely formal definition. In fact, calculated el values are fictitious if surface coverage is incomplete. The latter situation will arise when the amount of liquid phase is insufficient to form a continuous monolayer over the entire surface area. The experimental study was carried out with Spherosil of specific surface area of 3, 7,20, and 64 mz/g coated with p,p‘oxydipropionitrile as a stationary phase at 1, 2, 5, 10, 20, and 30% by weight. A synthetic mixture of ethyl ether, acetone, methyl ethyl ketone and methyl isobutyl ketone was used under the following conditions: column 4 mm i d . ; particle size 150-200 microns; length 1 m, filling weight 9 grams; carrier gas, nitrogen, flow rate 3 L/h; F.I.D. detector; hydrogen 2 l./h; and air 15 l./h; column temperature, 110 “C; injection of 5 pl of the mixture. INFLUENCE ON THE ASYMMETRY OF A PEAK. Adsorbents generally give rise to distorted peaks, with considerable tailing. Dal Nogare and Chiu (16), have given an expression discussed by Ettre (17), that measures asymmetry As. (16) Dal Nogare and T. Chiu, ANAL.CHEM., 34, 890 (1962). (17) L. S. Ettre, J . Gas Chrornatogr., 3, 100 (1965).
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3 mz/g 0 = 7 mz/g A = 20 m2/g X = 64 m2/g
AS
=
wb
w b - AW
~
+
where wb = b f; b and f a r e the segments determined by the tangents at the inflection points and the perpendicular from the maximum of the peak to the base line (see Figure 1); Aw is the absolute value of the difference jb - f[. Figure 2 shows that the asymmetry of the methyl isobutyl ketone peak decreases rapidly with increasing thickness of P,P‘-oxydipropionitrile layer on the various types of Spherosil. The curves are close to each other and similar for all types of Spherosil, except for the smallest specific surface area (3 m*/g), which may be due to nonuniform coating in the case of a very small amount of liquid phase (
