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Dry-Column Chromatography

Published in Encyclopedia of Chromatography, page 262-263

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Introduction

Dry-column chromatography (DCC) is a modern chromatographic technique that allows easy and rapid transfer of the operating parameters of analytical thinlayer chromatography (TLC) to preparative column chromatography (CC). The dry-column technique bridges the gap between preparative column chromatography and analytical thin-layer chromatography.

Discussion

Thin-layer chromatography has become an important technique in laboratory work, because it permits the rapid determination of the composition of complex mixtures. TLC allows the isolation of substances in micro amounts. If, however, milligrams or even grams of substance are required, column chromatography (CC) has to be applied, as TLC would involve a high cost and excessive time. In many cases, even the so-called thick layer or prep layer is but a poor choice because of time, cost, and sometimes inadequate transferability of the parameters of the analytical technique. In addition, the transfer from TLC to column chromatography, however, often proves to be difficult because the column chromatographic adsorbent is not usually analogous to the TLC adsorbent.

It is imperative that when transferring conditions of TLC separations to preparative columns, the conditions responsible for the TLC separation be meticulously transferred. Both CC and TLC use the same principle of separation. For normal operating conditions, a TLC layer has a chromatographic activity of II–III of the Brockmann and Schodder scale. Therefore, the sorbent used for DCC has to be brought to the same grade of activity. TLC layers often contain a fluorescent indicator in which case the DCC sorbent has to contain the same phosphor.

In TLC, the silica or alumina layer is “dry” before it is used and contacts the solvent only after it has been placed into the developing chamber. This is why, in DCC, the dry column is charged with the sample. Contrary to the normal CC, DCC is a nonelution technique. Therefore, only a limited amount of eluent is used in DCC to merely fill the interstitial volume between the adsorbent particles.

Scientific Adsorbents, Inc. DCC adsorbents, which are commercially available from Scientific Adsorbents, Inc. (Atlanta, GA, U.S.A.) are adjusted to meet the physical-chemical properties of TLC as closely as possible. These adjustments are made during the manufacturing cycle, and the material is packaged ready to use. With similar physical-chemical properties, the values obtained for the substances under investigation from TLC are practically identical to those obtained with dry-column chromatography.

Using these especially adjusted adsorbents for DCC, one can use the same sorbent and the same solvent for the column work and can transfer the TLC results to a preparative scale column operation rapidly, saving time and money. DCC materials are available corresponding with the most common thin layers: silica DCC and alumina DCC.

These DCC sorbents have found wide use when it is necessary to scale up TLC separations in order to prepare sufficient quantities of compounds for further chemical reactions and/or analytical processes. DCC can be practically used for every separation achievable by TLC (Fig.1).

Simplified procedure

Preparation

  1. Use the same solvent system that was developed on a TLC plate.
  2. Cut a Nylon tube to the desired length. To isolate 1 g of material, use approximately 300 g of sorbent in a 1-m × 40-mm tube (Fig.2).
  3. Close the tube by rolling one end and securing it by a seal or a clip/staple.
  4. Insert a small pad or wad of glass wool at the bottom of the column; pierce holes at the bottom with a needle.
  5. Dry fill the column to three-fourths of its length (Fig.3).
  6. The sample to be separated should be combined with at least 10 times its weight of the same sorbent in a conical test tube.
  7. Add an additional centimeter of sorbent on top of the sample, followed by a small pad of glass wool (Fig.4).
  8. Fasten the tube to a clamp on a stand.
  9. Open the stopcock of the solvent reservoir and add solvent until it reaches the bottom of the column. Stop. Elapsed time: approximately 30 min. (Fig.5).
  10. Find the locations of the separated bands by visible, ultraviolet (UV), or UV quenching.Alternatively, cut a 1/16-in. vertical slice off the tube. Spray the exposed area with an appropriate visualization reagent and align with the untreated column to identify (mark) the separated bands.
  11. Mark the location of the bands on the Nylon tube.
  12. Remove the column from the clamp.
  13. Slice the column into the desired sections (Fig.6).
  14. Elute the pure compounds from the sliced sections with polar solvents.

Suggested Further Readings

  1. Love B. , Goodman M. M., Chem. Ind. (London), (1967) 2026.
  2. Love B. , Snyder K. M., Chem. Ind. (London), (1965) 15.