Gas Chromatography of Solid Organic Compounds SIR: Although gas chromatography has rapidly become a significant tool in all areas of organic-analytical chemistry, a major gap still exists in the general applicability of the method. Solid organic compounds generally have such high boiling points t h a t thermal decomposition of both t h e sample and the column liquid becomes a problem at the high temperatures usually necessary for operation. To minimize these difficulties, conditions have been adjusted so t h a t gas chromatograms may be obtained at temperatures as much as 250" C. below the boiling point of a component. The method depends on the fact that even a t these low temperatures many compounds have a vapor pressure sufficient for column operation. Retention times have been greatly decreased by reducing the amount of liquid substrate to 0.05 t o 0.2%, instead of the 20 to 30% usually used. Such small amounts of liquid phase could not be used with conventional solid supports such as Celite and firebrick, because of strong surface absorption effects TI hich Fere eliminated by the use of glass microbeads (200-micron diameter) as support for the liquid phase. It was also necessary, because of the low vapor pressure of the compounds, to reduce the sample size to a few tenths of a
milligram t o prevent saturation of the gas phase. A flash vaporizer was used for introduction of the sample and B-as operated a t 375" C. Samples were injected as solutions in a volatile solvent. A chromatogram of n-octacosane (m.p. 61" C., b.p. 412" C.) was obtained using a 2-meter, 0.05% siliconoil column. The retention time was 21 minutes at 200" C. Anthracene (m.p. 216" C., b.p. 354" C.) was eluted from a similar column a t a temperature of 100" C. The method seems to be applicable to a wide variety of organic molecules, as evidenced by the fact that chromatograms have been obtained with retention times less than 20 minutes for the following compounds a t the column temperatures noted: chrysene (m.p. 250" C., b.p. 448" C.) a t 190" C.; anthraquinone (m.p. 286" C., b.p. 380" C.) a t 175" C.; p-dibromobenzene (m.p. 87" C., b.p. 218" C.) a t 103" C.; octadecanol (m.p. 59" C., b.p. 210" C. a t 15 mm.) at 200" C.; 1,3diphenylbenzene (m.p. 86" C., b.p. 363" C.) a t 200" C.; methyl oleate (b.p. 216" C. a t 20 mm.) a t 148" C. The number of theoretical plates in such columns is below those reported for conventional chromatographic columns. However, other factors, especially temperature, are important and greatly
enhance the separation of similar compounds. For example, a mixture of isomeric eicosanes (n-eicosane, m.p. 37" C., b.p. 205" C. a t 15 mm.) could be resolved a t a temperature of 127" C., while n-octadecane was eluted with the air peak, and a-dodecane was not eluted after 90 minutes. Operation of gas chromatography columns a t low temperatures has obvious advantages. It is possible to n ork ITith higher boiling materials without thermal decomposition; thermal conductivity detectors have higher sensitivity; and a much wider choice of liquid substrate is possible. Over 500 chromatograms have been obtained using this technique, with results which would be difficult, if not impossible by conventional methods.
Bristol Laboratories Syracuse, N. Y.
C. HISHTA J. P. MESSERLY R. F. RESCHEB
Cornel1 University D. H. FREDERICKS Baker Laboratory W. D . COOKE Ithaca, K.Y. RECEIVEDfor review March 18, 1960. Accepted April 5, 1960,
Theory of Plate Height in Gas Chromatography SIR: The van Deemter equation (1) has been generally accepted as an expression for the relation between the height of an equivalent theoretical plate (HETP) and the physical parameters in a packed gas chromatographic column. Jones (3) developed an equation which differed from that of van Deemter primarily in the relative importance assigned to the roles of liquid and gaseous diffusion processes. Recent work has shown that neither of these equations is correct and t h a t the experimental data are accurately represented by another equation, also developed by Jones ($), which reconciles the differences b e h e e n the two earlier equations. This new equation clarifies much of the confusing and often contradictory information previously published on this subject. The new equation, integrated to allow for column pressure drop, is: 880
ANALYTICAL CHEMISTRY
The first three terms of Equation 1 are those of the familiar van Deemter equation. If Sa, is zero, as for a n inert sample, or very large, the C term becomes insignificant. If the van Deemter equation were correct, HETP nould approach A as a limit with increasing gas velocity in those cases. However, HETP goes through a minimum for all samples when the data are appropriately corrected for the effects of volumes external to the column and of the detector time constant. The coefficients in Equation 1 were evaluated by the method of least squares
from data obtained a-ith samples of air (Sa== 0), butane = 2), and cyclohexane (Sa== 31) on a 1-meter column of 10% Don. Corning 200 silicone oil, 50-centistoke viscosity, on 100- to 120mesh Chromosorb. Table I presents the results. This set of coefficients s h o w d a n excellent fit to the data from all three samples. The contribution of each term to the total plate height at optimum velocity is also shown in Table I. None of the terms can be neglected and the contribution of each of the last three terms mill become dominant for some particular value of Saz. Additional data and a full discussion of this work will be submitted to this journal for future publication. NOMENCLATURE
A
=
velocity-independent constant, of random flow pattern in a packed column, cm.
