Inorganic thin-layer chromatography on microcrystalline cellulose'
The rapid growth of thin-layer chromatography (TLC) in the past decade since STAHL first standardized the basic equipment and procedures has been phenomenal", 09 1% 20. Strangely, although much of the first work done in this field was inorganic in nature, the greatest development has been in the realm of organic chemistry, Recently, however, the number of inorganic publications has begun to increase, with SEILER'O-~~ in Switzerland and (the late) Dr. POLLARDO~' in England among the leaders, The latter, with MCOMIE, had published the leading book in the field of inorganic paper chromatography8, but had turned to TLC from paper procedures in the last year or two.
The application of paper chromatographic methods to TLC on cellulose plates appeared to be alogical step, but a number of factors made this only partially possible. The solvents, many of which contained large proportions of water, loosened and even washed off the cellulose layers. Various binders were tried, but most of them affected the results obtained in one way or another. Calcium sulfate could not be used where either of its ions might interfere -or where insoluble compounds might be formed with the ions to be separated. Starch was sometimes successful, but affected both the Rp values and many of the visualization reactions. The fibrous nature of the cellulose layers reduced resolution due to the greater diffusion taking place. In spite of all these disadvantages, however, some work has been done on cellulose plates, but more research has apparently gone into silica gel as an adsorbent. Most of SEILER'S work has been done on this medium, but its ion exchange properties have necessitated the development of entirely new solvent systems.
In our laboratory silica gel was first used for inorganic separations, but the availability of so many well detailed paper chromatographic procedures led us to try cellulose layers. Results, for the reasons outlined above, were not as satisfactory as desired. A number of grades of cellulose were tried and finally, following the work of WOLFROM and his co-workers21r 22 microcrystalline cellulose ("Avicel" -a product of the American Viscose Division of FMC) was found to meet the necessary requirements. This material forms a remarkably stable layer, which stands up in water solutions, and can be written upon with a pen or pencil like paper. Initial separations made with this adsorbent, using paper chromatographic solvents and visualization reagents, gave results almost identical with those obtained on paper. Possible application to water analysis was investigated and reported previously*.
Work with most of the'same group of ions (Ca2+, Mgs+, Fe3+, A13+, SO,+, Pods-, and C032-has continued in an effort to learn more about the use of this new adsorbent, and to increase the sensitivity of the visualization and simplicity of the separations.
Ex$whnentaZ
Since this phase of the work was qualitative in nature, solutions of the ions were made up on the basis of 1.00 oh of the anhydrous salt in water (Table I). In the case of a mixture of equal volumes of all the solutions, however, it was necessary to add a few drops of cont. HCl to keep insoluble compounds from precipitating.