adsorbed reactant, double layer charging, or electrode film formation or removal contributes significantly to the current, the total coulombs consumed up to a given potential will be the sum of the coulombs requiting from reaction of diffusing specie‘, n hich depends on the sweep rate, and the coulombs from the other proceq‘es n hich are independent of sweep rate. That is kC &total ylZ f (&adsorbed &double layer &film (7) I n this case a plot of Qtotalus. Y - ~ will / ~ be a straight line with a slope dependent on the concentration of the diffusing species but with a positive intercept corresponding to the quantity of adsorbed reactant and/or the coulombs required to charge the double layer and/or the coulombs due to the electrode film. These three sources of coulombs that are independent of sn eep rate will not in general be separable from each other. Hon ever, experimental conditions can often be adjusted to make two of them negligible, thereby allowing the evaluation of the third. Because the potential rather than the current is controlled in potential sweep experiments, Equation 7 should be obeyed regardless of the order in which the diffusing and adsorbed reactants react. The same total number of coulombs will be observed, for instance, whether the adsorbed and diffusing re. actants react seriatim or concomitantly so long as there is no chemical or electro-
+
+
chemical interaction between the adsorbed and diffusing reactants. This feature of potential sn eep appears t o be a major advantage compared to chronopotentiometric methods for the study of adsorbed reactants. The chronopotentiometric methods which have been discussed (5, 7 , 12) yield transition time-current data which cannot be interpreted without resort to artificial models of the electrode reactions. If the adsorbed species is assumed to react completely before the diffusing species, the data must be treated differently than if the opposite assumption is made (7‘). I n the most likely case of the diffusing and adsorhed species reacting concomitantly, rigorous treatment of the data is denied bj7 the difficulty of the mathematics ( 7 , 1 2 ) . Intcgratiori of current-potential curves avoid‘ thehe drair backs n hich are inherent in the chroiiopotentiometric method. Evperimental work currently in progress appears t o support the above contentions for reversible systems. For example, the extent of adsorption of iodine on platinum electrodes has been determined from the intercepts of Q GS v-l12 plots at various iodine concentrations. A detailed report of this and other applications of the technique n ill appear in the near future.
(2) Delahag, P., ‘‘New Instpmental Methods in Electrochemistry, p. 118, Interscience, New York, 1954. (3) DeMars, R., Shain, I., J . Am. Chem. Soc. 81.2654 (1957). (4) Kats,‘ T. ‘J., Reinmuth, W. H., Smith, D. E., Ibid., 84,802 (1962). (5) Laitinen, H. A, ANAL CHEM. 33, 1458 (1961). 16) Lineane. J. J.. J . Electround. Chem. 1,37g(1960,. (7) Lorenz, K., Z. Electrochem. 5 9 , 737 (1955). (8) Matsuda, H., dyabe, T., Ibid., p. 494. (9) Mizguchi, T., ildams, R.N., J . Am. Chem. Sac. 84,2058 (1962). (10) Mueller, T., Adams, R. N., Anal. Chim. Acta 23, 467 (1960); 25, 482 i 1961). \----,-
(11) Reinmuth, W. H., A S A L . ~ H E M . 32, 1891 (1960). (12) Ibid., 33, 322 (1961). 113) Ross. J. Vir.. DeMars., R.. , Shain., I., , Ibid., 28, 1768 (1956). (14) Snowden, F. C., Page, H. C., Ibid., 22,969 (1950). (15) Valenta, P., “Advance in Polarography,!’ I. Longmuir, Ed., Vol. 111, p. 1004, Perganion, New York (1960). ~
R. A. OSTERYOUSG GEORGE LAUER FREDC. AXSON’
North American Aviation Science Center Canoga Park, Calif. 1 Permanent address, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, Calif.
LITERATURE CITED
(1) Breiter, M., Gilman, S., J . Electrochem. Sac. 107,622 (1962).
RECEIVEDfor review August 17, 1962. Accepted October 16, 1962.
Three Phases in Gas Liquid Chromatography SIR: For purposes of theoretical interpretation of gas liquid chromatographic results, it is preferable t o utilize a model of at least three phases: gas, liquid, and one or more surface regions. The adoption of this more complex model over the usual two-phase representation is indicated by the recognition that exclusively two-phase systems do not exist, and, in particular, b y the increasing popularity of low-load columns permitted by extremely sensitive detectors. Many low-load columns are of such a composition that their behavior lies in an intermediate range where neither liquid partition nor solid adsorption predominates and their description as a two-phase s p t e m i- incorrect. Our attention was brought to this matter by a recent conimunication of Rogers and Spitzer (24).which suggested that unfused T-ycor alters the nature of Carbowvns so as to render it more 1834
ANALYTICAL CHEMISTRY
efficient as B gn- chroniatogr t~phic.p x titioning agent. An alternati] e, niore physically realistic interpretation of the data is presented, based on a three-phase model which paqses t o two phases under certain limiting conditions. RETENTION BY M O R E T H A N O N E PHASE
The retention time of a d u t e in a chromatographic. column is the sum of its residencc. tii>!tJs in each phase of the column. The situation where the solute is in R near-equilihrium distribution bet iwcn three phases can be easilj. incoiyor:ited in the expression for the R value. where R is the fraction of solute molecules in the mobile phase or the fraction of the time a molecule spends in the mobile phase (10, 20). The equilibrium R value for a solute in a three-phase system is given by (17)
where CY is the ratio of the concentration of solute in the stationary liquid to that in the gas phase, p is the ratio of the concentration of the solute adsorbed on the solid support to that in the gas, A L is the crosa-sectional area of the immobile liquid. A i sis the “cross-sectional area” of the solid surface-i.e., it is some measure of the amount of the support surface-and A M is the cross-sectional area of the mobile gas. The P ( A s ’AM) term is a measure of the extent of participation of the support material. If, as Martin (21) suggested, the liquid gas interface also participates in retention. this Tvould require still another term for a complete description of R . Eupressing the presence of the third phase in this manner implies that we are concerned \+ith adsorption energies of the same order of magnitude as solution energies and that the rates of adsorption and desorption are comparable to rates of solution and dissolution. We
are not concerned with irreversible or extremely strong adsorption which leads to loss of solute along the migration path or t o extreme tailing. At high liquid loads (AL/A~)>>(A,s/A,w)and, depending upon the values of CY and b, CY ( AL I A M>>P(As/Ad, ) so that
and we have gas liquid partition chromatography. Solid adsorption is “swamped out” by the relatively large amount of liquid. It is assumed that the properties of the fixed liquid are essentially identical with those of the same liquid in bulk quantity. As the liquid load i b reduced, the term for solid adsoiption exerts a greater influence on R , so Equation 1 becomes appropi late K e feel that a t low liquid loads, where the influence of the support i i manifest but where there is still complete cowrage, @ ( A s A$I) , is a constant, yo that (3)
For liquitl loads so low that there is incomplete coverage, the amount of solid surface exposed is proportional t o the liquid load-i.e., 4 ~ = k A - a n d (4)
where CY‘
=
CY
+ izp
(5)
and CY‘ represents the retention by both liquid and solid. This corrects a misinterpretation of ours in a n earlier paper (17) u-ith reference t o Martin’s work (21). LIQUID FILM THICKNESS
Rogers and Spitzer seem to have suggested that the nature of the liquid phase is strongly dependent on the supporting medium and the liquid load. The region in which retention is affected by the support material is the transition region from pure adsorption-Le., uncoated support-through a three-phase level of partial and total coverage as described b y Equations 1 and 3, until finally a load is reached where Equation 2 is valid. It seems t o us that interpretations must be governed by a n estimation of the extent and type of coverage involved as it affects Equation 1 rather than b y assigning peculiar properties to an exclusive liquid partitioning medium which ignores other phases active in the retention. Rogers and Spitzer used 0.25 X 30 inch 0.d. copper columns of 60- to 80mesh crushed Vycor glass holding 29.1 mg. of Carbowax 400. Their data are insufficient t o calculate the thickness of the liquid film on the particles, but,
allowing for some approximations, we believe that we can estimate this value by combining i t with other information. Desty, Godfrey, and Harbourn (3) listed packing densities in grams per cubic centimeter of columns of Celite and Johns-Manville C-22 firebrick at various liquid loads for various mesh ranges. We have used their data to find the amount of uncoated firebrick per unit volume of column-Le., the support densities, p-by multiplying their densities by 1- (% liquid load/100) and rearranging the data under particular values of the mesh ranges bearing various liquid loads-e.g., 30- to 44-mesh range with 10, 20, and 3oy0 loadings. When we do this, we find that all of the values for the support densities fall within the mean plus or minus twice the standard deviation [calculated by small-sample theory (16)1. This means that at the 95y0 significance level, the support density, and hence the number of particles per unit volume of column, for any particular mesh range is independent of the liquid load at least up to 3oy0. This is strictly true only for firebrick-Le., we have not tested whether Celite packs the same as firebrick in the same size range. Comparison of the means b y the t-test shows t h a t support densities are different at the significance level, which indicates that packing density is a function of particle size. This is very much what we would expect-Le., that thin films of liquid do not affect packing density but the particle size does. Should the liquid film not give a freely flowing packing, this would not be true. We next presume t h a t the particles are identical perfect spheres and calculate the number of particles per gram of support, pNj and the geometrical surface area per gram of support, A , given by
and A
Table I.
Mesh size
B.S.S. 30 44 72 100
120 TJS. 30
60
80
(7)
= 6/dp
Packing Properties of Firebrick Columns
.v,
d, em. X lo2
particles/ X 10-3
A , sq. cm./g.
5.00 3.53 2.11 1.52 1.24
6.88 19.6 91.6 245 451
128 178 218
5.90 2.50
4.19 55.0 155
45‘8 108 152
1.77
where d is the diameter of the particle in centimeters and p is the density of the support in grams per cubic centimeter. Scholz and Brandt (66)gave values of p=2.15 for Chromosorb-P and p=2.30 for Chromosorb-W, while Baker, Lee, and Wall ( I ) gave p = 2.26 for Chromosorb-R and p = 2.20 for Chromosorb-W. We take p = 2.22 t o be representative. Our results for various mesh sizes (16) are - shown in Table I with median values, d, p N , and l ,indicated for a range of mesh sizes. Two counts of representative samples of 30- t o 60-mesh bare Chromosorb gave 39,900 j= 1500 particles per gram. Table I indicates 29,600, a difference of 35y0 from the calculated value. We feel that this error results from the fact t h a t the apparent density of the particles is less than the absolute density of firebrick b y virtue of their porosity and that the particles may not be uniformly distributed within the mesh range-i.e., that there are more particles smaller than the average value than there are larger. It is also likely that some mechanical disintegration occurred during the counting process. We can state that actual p~ and are larger than those given in Table I by about 35%. I n Table I we also include the mean packing densities in grams of bare support per cubic centimeter of column, pc. The density of borosilicate glass, as listed b y the Corning Glass Works. is 2.23 grams per cc., which we take as essentially the same as firebrick. If we assume the T’ycor support t o pack with the same particle density as the firebrick, this ~ o u l dmean their packing density was about 0.434 gram per cc. of column as determined from a graph of the data of Table I for a particle diameter of 2.14 X 10-2 em., the median value for 60- t o 80-mesh particles. Rleasurements of 0.25-inch 0.d. copper tubing gave 0.198-inch i.d., which means t h a t their column volume was 15.13 cc. and contained about 6.57 grams of support with a total geometrical area of 828 sq. em.
P., 3, cm. particles/g. X 102 x 10-3 4.26
13.2
2, sq. cm./g.
g./ml.
65.2
0,412
1.81
1.38
168 348
153 198
4.20
29.6 105
130
2.14
pc,
0.444 0.506
76.9
VOL. 34, NO. 13, DECEMBER 1962
1835
Taking the density of the partitioner to be 1.1 gramspercc. [1.10Oto1.140(23)], the calculated liquid film thickness is 3195 A. This film may be even thinner, since our areas in Table I are probably small. The liquid load would be 0.4y0 for their amount of liquid. These numbers ought to be of a t least a reasonable order of magnitude. If one takes the molecular weight of liquid to be that of H(OCHzCH*)gOH, and its density, the diameter of the molecule calculated from the molar volume is about 11 A. and the liquid layer will be 290 molecular diameters thick. This calculation presumes that the true surface of the support equals the geometrical surface of perfect spheres of uniform diameter. Scholz and Brandt (25) reported 5 t o 6 sq. meters per gram for the surface area of Chromosorb-P and 3 to 4 sq. meters per gram for Chromosorb-W, as determined by B E T liquid nitrogen adsorption for particles of 60- to 80mesh, while Baker, Lee, and Wall (1) reported 4.8 sq. meters per gram for Chroniosorb-R and 1.2 sq. meters per gram for Chromosorb-JT. The ratio of real surface to geometrical surface areas ranges from 462 t o 92. A ratio of 280 would reduce the liquid film thickness to about 11 A. or one molecular diameter. The large surface of firebrick is due to its high porosity (1) arising from each particle being an agglomerate of other particles. The actual surface areas compensate for the error in calculating areas from the absolute density of the support, etc. Celite has an area of 0.45 sq. meter per gram and Teflon of 0.64 sq. meter per gram (1). Harris and Emmett (14) obtained surface areas of 19.0 and 24.4 sq. meters per gram, respectively, for fresh and slightly etched glass beads in the 3- to 5-micron size range. Assuming a diameter of 2 microns for ideal spheres and using the value quoted for the density of glass, one obtains a geometrical surface of 1.34 sq. meters per gram for these beads. The ratio of measured area to geometrical area is 14.2 in the first case and 18.2 in the second, which would indicate a film thickness of about 197 il. or 18 molecular diameters for 60- t o 80-mesh beads, which assumes the same ratio of real surface t o geonietrical surface for these larger particles. These same authors reported measured areas of 120 t o 134 (average of 126) sq. meters per gram for 100- to 200-mesh (1.49 to 0.75 x 10-2 cm.) porous acid-leached highsilica glass. Assuming perfect spheres and aparticledianieter of 1.12 X cm. the average ratio of measured to geometrical area is 5.23 X lo3. t o give a film thickness of 0.6 A , which is less than one molecular diameter-Le., an incompletely covered surface for 60to 80-mesh crushed glass. If we presume that the surface of the glass used b y Rogers and Spitzer was 1836
ANALYTICAL CHEMISTRY
between that of glass beads and that of porous glass, their partitioning liquid was somewhere between 200 molecules deep t o incomplete coverage. The latter case can hardly be termed liquid partition chromatography and is best described by Cremer’s (2) term of gasadsorption chromatography or identified as a n active adsorbent carrying a small amount of liquid as a tailing reducer (4). Recognizing the range of liquid coverage. it is difficult t o understand the remark of Rogers and Spitzer that the liquid was probably being used more efficiently on the porous Vycor. This would seem to imply that the phase responsible for retention could still be considered as a liquid even a t this low load, where there is a good possibility of incomplete coverage. For incomplete coverage, retention can best be described by a combination of solid and liquid effects, as given by Equation 4. It is likely that Equation 3 cannot be applied. Whether the properties of the solid are to be considered as altered by adsorption of liquid on active sites or liquid altered by virtue of its adsorption as a very thin or even discontinuous film on a solid seems to be an open question a t this time. We prefer the former viewpoint, since it seems more realistic and capable of extension. Harkins and Jura ( I S ) considered the problem of determining the distance to which the attractive field of a solid has an effect upon the energy of molecular interaction in an adjacent adsorbed film. They chose a model of a multilayered adsorbed film in preference to a monomolecular film accompanied by capillary condensation, which they considered to be untenable. For titanium diouide, they arrived a t a film thickness of five molecular layers for water and ten molecular layers for nitrogen. The interaction fell off by a factor of about one fourth (an evponential effect) for each successive layer, as demonstrated by the energy of vaporization of water from these layers. We might suppose that within an upper limit of ten molecular layers, the properties of the adsorbed liquid are different from those of the bulk liquid by virtue of the adsorption and that the propertie.: chow a gradient in the direction normal to the surface. Some such difference is a cornerstone of the liquid-gel model of paper chroniatography as proposed by Martin (19,22). Depending on the qurface of the glaqs. Rogers and Spitzer could have worked anywhere above or beIoiy this region. Awuming a ten-layered adsorbed film, a simple calculation for polyethylene glycol on firebrick. using a ratio of real surface t o geometrical surface of 280. gives a liquid load of 3.0y0 for 60- to 80-mesh firebrick and 2.5% for 30- to 60-mesh. It has been reported that the effect of the solid support is noticeable up to liquid loads of 10 to 20% (5. 6 ) ,
depending upon the nature of the support, partitioner, and solutes. Obviously there is adsorption of the solute onto the support from the solution of the solute in the partitioner, since the properties of the liquid could not possibly be altered b y adsorption with this degree of loading. It must be that of Equation l is large enough so that the term for adsorption by the support is not rendered insignificant, even though AL>> 4s. Such might be the case for the gas chromatography of polar solutes on a polar support holding a nonpolar liquid. It is possible that at around 3 to 0.5% a of Equation 1 might change because the liquid now exists as an adsorbed film with properties conceivably different from those of the bulk liquid. We do not feel that the solution properties of this adsorbed liquid are much different from those of the bulk liquid. The liquid-gel stationary phase proposed by Martin (19, 22) for paper chromatography often approximates the partitioning behavior of pure water. This argues against any great change in the properties of the liquid as compared t o the bulk by virtue of existing in an adsorbed state. Rogers and Spitzer wrote of correcting retention volumes for liquid-impregnated Vycor by using retention volumes measured on unimpregnated support. For near-equilibrium operation in the absence of severe pressure gradients, the retention volume, V,. of the i t h solute may be w i t t e n (I?)
where t, is the retention time of the ith solute, F is the volume flow rate, and F is the average linear gas velocity. The three integrals are the residence times in the gas, liquid, and solid phases, respectively. They are independent only in the equilibrium appro~imation. Tailing. as observed by these authors, can be produced by kinetic effects. and in such a case, the liquid phase integral may not be evaluated by subtracting the values of the gas and solid surface integrals presumably evaluated in the absence of the liquid phase. Furthermore, the solid surface term for uncoated support will be different from the solid surface term for impregnated support. The reported increase in the retention time of the air peak on increasing the support area implies that air is chromatographed, and its residence time is not a valid measure of the first integral on the right-hand-side of Equation 8
(which includes extracolumn contributions t o ti). This retardation of air, coupled with the calculations presented earlier, strengthens our conviction that the low-load Vycor column was a case of partial coverage where there were both bare surface and regions of thin liquid films. DIFFUSION IN LIQUID FILM
Equation 1 applies strictly t o the situation where solute adsorbed on the support. in solution in the partitioning liquid. and in the gas phase are all a t equilibrium. Chromatography, however, is a dynamic process and it may be argued that, a t high liquid loads. the solute has insufficient time t o diffuse through the liquid t o the solid where it may be adsorbed. A broad simplification is to treat diffusion as a random walk problem (16) and presume that all of the diffusing substance starts out a t the gas liquid interface a t time t = 0, which here mould correspond t o when the zone first enters the column segment. The time interval for diffusion t o the support is the period between t = 0 and the instant the peak maximum enters the segment. The root-mean-square displacement is
Experimental diffusion coefficients of representative members of classes of compounds through some of the kinds of liquids employed as partitioners-e.g., stopcock grease-are not readily available in the literature. The diffusion coefficients of benzene and n-heptane in n-octadecane at 25’ C. are 8.6 X and 9.2 X sq. cm. per second, respectively (16). Keulemans (18) assumed a diffusivity of 0.4 X sq. em. per second. Giddings, Nallik, and Eikelberger (11) measured a diffusion coefficient of 3.3 x sq. cm. per second for both benzene and toluene in tri-o-tolyl phosphate a t 50’ C. We feel safe in assuming a diffusion coeffisq. cm. per second to cient of 1 X be representative. This gives a rootmean-square velocity of 1.4 X em. per secoiid or 1.4 X l o 5 A. per second. A 30% liquid load on firebrick of ratio of real surface to geometrical surface of 280 has a film thickness of about lOi0 A,, which means that, on the average, a molecule reaches the solid surface in 7.6 X 10-3 second. It would seem that, under the conditions popularly employed in gas chromatography, the solute has ample time to reach the support and there be adsorbed. The statement that diffusion in the liquid phase is the slowest or ratedetermining step does not mean that it is slow per se. One may also claim that the actual situation is very close t o true equilibrium, which can lie called the near-
equilibrium state. We do not feel that the support effects are not seen at high liquid loads because the molecules do not reach the support but because the partitioning effect overwhelms the absorption effect as demonstrated b y Equation l. Giddings (8, 9) has been much concerned with such diffusional processes. I n our discussion, we have assumed that the liquid is uniformly distributed over the total available surface regardless of its roughness or the details of the size and distribution of pores (1). This is a severe oversimplification. A more realistic, but still idealized, model has been given by Giddings (7), who substitutes a network of adjacent conical cavities for the complex system of tortuous and perhaps even interconnecting channels t h a t probably constitute t h e real situation. Presuming t h a t the liquid wets the surface, Giddings proposes that the liquid is first adsorbed but t h a t a t even very low liquid loads the capillary cavities begin to fill, so that one has a series of connected filled or partially filled cavities. For example, for a liquid load of 30j0, one third of the liquid is in capillaries. A uniformly thick liquid film becomes a poor model for certain percentages. The distance that a molecule must migrate t o reach the surface will then depend upon its location. Giddings uses the concept of a mean-square-depth (8) in his equilibrium departure term for this situation. Even with this complicating feature, our fraction of a second is very small compared t o the total length of time that a solute zone spends in any selected section of the column, so we do not feel that a nonuniformly thick distribution of liquid changes our conclusions.
t o this case. In the absence of an intimate knowledge of the structure of such a system, we suggest t h a t the p term he retained in Equation 1 with an appropriate reduction in the area factor ;%\ representative of solute adsorption in regions uncovered b y partitioning liquid While the liquid areas cannot be considered bulk phases, any solute molecule adsorbed in this region mill have a n apparently reduced adsorption coefficient, as i t must compete with partitioner molecules for adsorption siteq. Effectively, we have a case of two site adsorption where the ratio of the respective areas are proportional to the liquid load. The LY term, then, is a of diminished value and will be the appropriate p term in the case of full coverage. Light-load, complete-coverage columns are those in the range of about 0.5 to 30j0 liquid loading. With increased loading of the support, material will add largely by capillary condensation. The solid surface area term will remain a constant in Equation 1 through the further loading of bulk material. As the loading is increased, all of the surface capillaries become interconnected by bulk partitioning liquid. As the depth of the bulk phase increases. the solid surface phase becomes less important. giving us a final classification of the the high-load column where the solid surface area term becomes negligible. I n all cases, the support can participate in retention in the sense that solute molecules can reach the support. LITERATURE CITED
(1) Baker, W. J., Lee, E. H., Wall, R. F.,
in “Gas Chromatography,” H. J. Noebels, R. F. Wall, X. Brenner, eds., p. 21, Academic Press, Kew York,
CONCLUSIONS
The present procedure of describing the packing of a gas chromatographic column in terms of percentage liquid load is adequate for reproducing euperimental conditions, but is insufficient for understanding the physical situation and can lead to questionable hypotheses, as demonstrated here for a particular case. Authors should be urged to report liquid film thickness or surface areas and mesh sizes, in order that systems can be compared a t equal values of the film thickness. It is this parameter which is important. Specifically, glass beads and firebrick with the same percentage liquid load have very different film thicknesses because of the difference in porosity. The merits of powdered Teflon ought t o be examined at the same film thickness as borne b y other supports and not at the same liquid load. I n the range of 0 to 0.5% liquid load on firebrick, the surface is only partially covered. There are several approaches
1961.
( 2 ) Cremer, E., Arch. Biochem. Biophys. 83, 345 (1959).
(3) Desty, D. H., Godfrey, F. M., Har-
bourn, C. L. A . , in “Gas Chromatography, 1958,” D . H. Desty, ed., p. 200, Butterworths, London, 1958. (4) Eggertsen, F. T., Knight, H. S.. A N A L . CHEV. 30, 15 (1958). (5) Fukuda, T., Japan Analyst 8 , 627
(1959). (6) Fukuda, T., Omori, T.. Zbzd., 8, 630 (1959). ( 7 ) Giddings, J. C., ANAL.CHEV.34, 458 (1962). (8) Giddings, J. C., J . Chromatog. 5 , 46 11961). ( s i Ibid., p. 61. (10) Giddings, J. C., Keller, R. A , , in
“Chromatography,” E . Heftmann, eci., p. 92. Reinhold, Sew York, 1961. (11) Giddings, J. C., Mallik, K. L., Eikelberger, M., ASAL. C H m . 34, 1026 (1962). (121 “Handbook of Chemistrv and Phvsics,” 36th ed., p. 3081, Chemical RubGer Publishing Co., Cleveland, Ohio, 19545.5. (13) Harkins, W. D., Jura, G., J . Am. Chem. Soc. 66, 919 (1944). (14) Harris, B. L., Emmett, P. H., J . Phys. Colloid Chem. 53, 811 (1949).
\‘OL
34, NO. 13, DECEMBER 1962
1837
(21) Martin, R. L., ANAL. CHEX. 33, 347 (1961). (22) Moore, S., Stein, W. H., Ann. Rev. Biochem.21, 521 (1952). (23) Pharmacopeia of the United States of America, 16th rev., p. 554, Mack Printing Co., Easton, Pa., 1960. (24) Rogers, L. B., Spitaer, J. C., ANAL. CHEM.33, 1959 (1961). (25) Schola, R. G., Brandt, W. W., 1961 International Gas Chromatography Symposium, Michigan State University, East Lansing, Mch., 1961.
(15) Hoel, P. G., “Introduction to Mathematical Statistics,” Wiley, New York, 1947. (16) Jost, W., “Diffusion,” p. 25, Academic Press, New York, 1960. (17) Keller, R. A., Stewart, G. H., J. Chromatog.9, 1 (1962). (18) Keulemans, A. I. M., “Gas Chromatography,” 2nd ed., p. 192, Reinhold, New York, 1959. (19) Martin, A. J. P., Ann. Rev. Biochem. 19, 517 (1950). (20) Martin, A. J. P., Endeavour 6, 21 (1947).
ROY-4. KELLER Department of Chemietry University of Arizona Tucson, Aria. GEORGE H. STEWART Department of Chemistry Gonaaga University Spokane, Wash. RECEIVEDfor review Bugust 10, 1962. Accepted September 27, 1962. Work supported in part by the National Institutes of Health, RG 7046 Bio (Cl).
Inverse Temperature Programming in Gas Chromatography SIR: Gas chromatography has been widely used for the analysis of hydrocarbon mixtures, and a variety of column packing materials and substrates have been applied to this purpose. Typical of the column substrates used in analyses of such compounds are long-chain saturated paraffins such as n-hexadecane ( 3 ) . I n this laboratory primary interest centers on analysis of hydrocarbons in the CZ through C6 range, and extensive use has been made of n-hexadecane columns. However, n-hexadecane has serious drawbacks as a partitioning agent, in that compounds beyond the C i s are retained for an excessive length of time, with the result that peaks for the C i s and C i s are broad and unsymmetrical. Moreover, the column cannot be heated, since hexadecane bleeds severely from the column above room temperature. Eicosane has been previously reported to be effective for hydrocarbon determinations @), and since this compound can be heated to some extent without destroying the column, it was felt that such a substrate might be useful for analyses of Cz to C6 hydrocarbons. Therefore an eicosane column consisting of 25y0 by weight eicosane on 30- to 60-
t s
mesh firebrick was prepared and evaluated, and some rather novel results have been obtained.
A mixture containing saturated and unsaturated Ca and Cs hydrocarbons, butane, isobutane, and n-hexane was used as a test mixture. The analyses were run on a Burrell K-2 gas chromatograph using a 2.5-meter column, with a helium carrier a t a flow rate of 45 cc. per minute. The thermal conductivity detector was operated a t a current of 180 ma. and maintained a t a temperature of 150” C. Resolution of the components in the mixture was extremely poor when the column was operated a t room temperature. Figure 1 shows a typical run, in which the column was heated after elution of the C4components in an attempt to reduce retention time for the nhexane. It is evident that this type of operation does not give a useful analysis. I n view of the resolution obtained for the lower hydrocarbons with n-hexadecane, the performance of the eicosane column was surprising. The only apparent difference between the two materials which might conceivably account for the large divergence in their per-
formance as substrates is their physical state a t room temperature, eicosane being a solid while n-hexadecane is a liquid. With this thought in mind, we heated the eicosane column to a point just above the melting point of the substrate (- 40’ C.). The sample mixture previously mentioned was injected and the components through the C1’s were nicely resolved, the longest retention time being of the order of 10 minutes. The hexane, however, remained on the column for a period comparable to that observed in the initial programmed run, and the peak shape was the same. I n a repeat run, the sample was again injected while the column temperature was about 40’ C., but after elution of the C4’s, the column was allowed to cool to room temperature. This resulted in elution of the hexane (plus some Csand c6 impurities) in less than 30 minutes, as shown in Figure 2. This procedure, which we have chosen to term ‘‘inverse programming,” clearly produces the most rapid analysis and the most satisfactory peak shapes. I n attempting to understand the mechanism by which the column functions, the possible significance of various terms included in the van Deemter H E T P equation has been considered for
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Figure 1.
1838
Conventional program
ANALYTICAL CHEMISTRY
Figure 2.
Inverse program
