SURFACE CHROMATOGRAPHY JAMES E. MEINHARD
AND
NORRIS F. HALL
University of Wisconsin, Madison 6, Wis.
A powdered adsorbent, fixed rigidly to a microscope slide by means of a suitable binder, provides a convenient system for microchromatography. By means of a flexible technique and sundry variations in method, the system demonstrates broad applicability. Its use is illustrated in the analysis of a simple mixture of iron (111) and zinc salts.
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ZMAILOV and Shraiber (6)chromatographed organic mixtures on thin layers of adsorbent powders, and observed the resulting concentric circular zones under ultraviolet light. Williams (IO) developed the idea further by forming a sandwich of adsorbent between glass plates, the upper plate containing a small hole through which solutions are introduced These methods suffer from the disadvantages inherent in working with loose powders. Hopf ( 5 ) demonstrated the use of filter papers impregnated with complexing agents for the chromatography of metallic ions Brown (a) employed a sandwich of blotting paper, a device like that of Williams, in the analysis of organic mixtures. These methods suffer from the limited choice of adsorbent compositions, and from discontinuities and variations in the adsorbing medium. The system described below was developed with the hope of eliminating the faults and combining the virtues of the foregoing methods. In the chromatography of inorganic compounds the adsorbent of choice appears to be alumina, but considerable variations in average particle size and in particle size distribution exist among commercial products. It is advisable, therefore, to hold to a single product and, where possible, a single batch. In this research Merck's reagent aluminum oxide was successfully employed and was used directly without activation treatment. A number of preliminary tests showed that average particle size was a much more important factor than any type of activation treatment.
warmed, with agitation, on a water bath until coagulation is complete. The coagulum is allowed to cool, then triturated with an additional portion of xater (2.5 ml.) until a thick cream is formed. The product is applied to microscope slides by means of a spatula and spread evenly to a depth of 2 mm. Gentle tapping of the slide ensures even distribution and a smooth surface. One end of the slide is left untreated in order to facilitate handling. The microscope slides require no previous treatment except cleaning to prevent the presence of an oil film. The freshly prepared slides are dried in a dust-free atmosphere, either a t room temperature or a t 90" C. A somewhat more even surface is obtained with slow drying. The quantities given above are sufficient for treating 12 to 13 slides. The surface thus obtained possesses mechanical stability and a fine, even texture. It exhibits no tendency toward deformation when wetted with aqueous or organic liquids. METHOD
The chromatographic process may be divided into four general parts: (1) application of samqle; (2) development with solvent or reagent; (3) immobilization of adsorbate; and (4) introduction of a color reaction, or a physical process, which differentiates between the zones formed. In their work on the chromatography of inorganic ions Schwab and Jockers have, unfortunately, applied the term development to part 4, which is at variance with general usage and introduces an ambiguity (9). In this report the second phase of the process is called development, while the fourth phase is referfed to as ripening. The sample is prepared according to the type of separation desired. This will depend on the specific equilibria involved. Inasmuch as this procedure involves a dilution of tlie solute, fractionation may be based on the relative rates of decomposition (or solvolysis) of metal complexes to an insoluble, or less mobile, species. I n other cases fractionation may depend on the precipitation of hydrous metal oxides, or oxy- salts, under the influence of a gradual increase in pH. Here it is required merely that the original sample be sufficiently acid to hold all ions in solution.
In order to fix the adsorbent in a continuous, rigid surface, a binder must be used. Among several adhesives tested, ordinary cornstarch was found to be one of the best. Most resinous materials--e.@;., glyptal, phenol-formadehyde, etc.--nwe disqualified on the basis of their color-forming and hydrophobic properties. A brief survey of various starches led to the adoption of Amioca starch (National Starch Products Co.). Amioca, consisting mainly of amylopectin, contributes to the ease of compoundng through its comparatively -;htr gelatinizing range (70" to 80 The tendency torrard skin formation and the occurrence of lumps observed in formulations employing ordinary starches are considerably reduced when Amioca is subst ituted. . i n a n a l y t i c a l filter aid, Cclite (Johns-Manville), is .added to improve rigidity and to prevent fissuring in the final surface. The ingredients are slurried with water, heat coagulated, triturated to a sirupy consistency, plated on microscope slides, and dried.
8.).
The sample is introduced into the surface by means of a capillary pipet of 0.005- to 0.008-ml. capacity. The pipet is a simplification of one originally described by Harkins and Anderson ( 4 ) and consists of a section of drawn capillary tubing 5 cm long sealed into the ehd of a section of ordinary glass tubing by means of a thermoplastic resin. The pipet is filled by capillary action, or by gentle suction, and is drained into the adsorbent surface by capillary action. Development is carried out with water, or other reagent, using a pipet designed according to Figure 1. I t is a modified medicine dropper with a support of glass rod sealed to its side. Instead of the usual rubber bulb, a short section of rubber tubing is used for filling, and its upper end is stoppered TTith a short column of firmly packed alumina. This arrangement allows drainage of the liquid a t a dom steady rate. The delivery end of the pipet is drawn to capillary dimensions. The filled pipet is applied to the slide a t the point of introduction of the sample. Development is carried out until the wet circular area approaches the edges of the slide. A slight further spreading takes place after removal of the pipet.
PROCEDURE
Three and one half grams of Celite (%yo dry basis), 0.3 grams of Amioca, and 6.2 g r a m s of a l u m i n a a r e thoroughly mixed (dry), then slurried with 18 ml. of distilled water. The mixture is
Figure
Ripening may now be carried out on the still moist latent pattern by the application of gaseous reagents-e.g., ammonia, hydrogen sulfide, iodine. When iodine vapor is used, reaction
1. Developing Pipet 185
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ANALYTICAL CHEMISTRY
ocours with the starch binder. The blue color is profoundly influenced by the presence of certain ions, and thus reveals their location. The pattern formed, however, is not stable, because zone boundaries gradually diffuse and became indistinct. For this reason an immobilising or fixing operation is introduced (part 3). The hydrous oxides of the metals may be precipitated with ammonia gas, or the slide may be dried. The most even patterns result from slow drying. An oven temperature of 50" C. is recommended. Typical chromatographic slides are presented in Figure 2. These examples were taken a t random from a large number of tests and are included here primarily to demonstrate the type of patterns obtained rather than for any special significance they may have. All these slides were ripened with iodine vapm.
This type of ripening is an example of a general method which employs a color resetion not directly involving the chromatographed ions. Although the ions themselves form no colored product. their presence influences the course of the reaction and becomes manifest in B change of shade or intensity of color. This type is further exemplified in the work of Brockmann and Volpers ( 1 ) and of Sehse (8), who chromatographed organic mixtures on fluorescent adsorbents-e.g., alumina treated with small amounts of heavy metal, etc.-md located the sdsorbetes by differences in fluorescence of the irradiated column. A second type of ripening is adapted from the differentialstaining techniques employed in bacteriology. The freshly developed chromatogram is treated with ammonia gas, or other suitable reagent, to form an insoluble, highly adsorbent structure with the metal salts present. Indeed, the original developing process itself may he sufficient to form this kind of structure. The slide i s then treated with a dye solution, followed by gentle washing with an aqueous detergent solution. The detergent differentially desorbs the dye. Obviously, a too highly activated alumina, ar % too readily adsorbed dye, will give rise to irreversible adsorption throughout the whole system. As the alumina used in this work has been ignited to a t leest 900' C. during manufacture (according to density measurements), its adsorbing characteristics are suffioiently different from the hydrous metal oxides to allow differential staining.
ras. failowed hv a solution of methylene blue, then rentlv agitated Tn a warm solu
