Adsorbents for TLC

Silica gel (Fig. 13.3) is the most commonly used adsorbant for TLC. The rate at which compounds migrate up a silica gel plate depends on their polarity. In a given length of time, the most polar compounds move the least distance up the plate while the least polar move the furthest. 

Cellulose. This organic material consists of polymerized glucose fibers (400-500 molecules) in nature and also as a synthetic product (40-200 glucose molecules). Cellulose interacts with water and serves as a stationary phase for the separation of polar substances by paper chromatography. As a powder, it is used as an adsorbent for TLC. Separation of polar substances by paper chromatography is described in the European Pharmacopeia for identification of Tc-pertechnetate (Council of Europe 1982). 

Physicochemical properties, that is the hardness and polarity of silica gel, are related to particle size (pm), surface area (m /g), pore size (A), and presence or absence of additives. The surface area of silica gel adsorbent for TLC is typically 300-400 mVg. and pore sizes range from 20 to 150 A. Adsorbent with larger surface area (smaller particle size) will generally give better resolution, but a slower... 

The solid adsorbent in TLC is usually alumina (AI2O3) or silica gel (silicic acid, Si02 X H2O), both of which are polar. Alumina is the more polar of the two and is commercially available in three forms neutral, acidic, and basic. Acidic and basic alumina are sometimes used to separate basic and acidic compounds, respectively, but neutral alumina is the most common form of this adsorbent for TLC. Silica gel, which is slightly acidic, is the adsorbent used in the experimental procedures described in this section. 

Starch is another polysaccharide which is suitable as an adsorbent for TLC. As a result of the linear combination of maltose units up to a molecular weight of at least a million, it contains many hydroxyl groups and is consequently strongly hydrophilic. Like cellulose, it is suitable for separating hydrophilic substances. 

Applied Science Laboratories, Inc., P. O. Box 440, State College, Pennsylvania, USA (Adsorbents for TLC, spreaders and accessories). 

Fluka AG., Buchs SG, Switzerland (Adsorbents for TLC pure chemicals). 

Mallinckrodt Chemical Works, 2nd Mallinckrodt Sts., St. Louis 7, Minnesota 63160, USA (Adsorbents for TLC). 

Riedel-de Haen AG, 3016 Seelze, Hannover, W. Germany (Adsorbents for TLC fluorescence indicators). 

Serva Entwicklimgslabor, 6900 Heidelberg, Rdmerstr. 118, W. Germany (Spreading bar for TLC adsorbents for TLC reagents). 

Column Chromatography. The substances to be purified are usually placed on the top of the column and the solvent is run down the column. Fractions are collected and checked for compounds using TLC (UV and/or other means of visualisation). The adsorbent for chromatography can be packed dry and solvents to be used for chromatography are used to equilibrate the adsorbent by flushing the column several times until equilibration is achieved. Alternatively, the column containing the adsorbent is packed wet (slurry method) and pressure is applied at the top of the column until the column is well packed (i.e. the adsorbent is settled). 

TYPICAL PROPERTIES OF INORGANIC OXIDE ADSORBENTS USED FOR TLC... 

Also, NIRS detection in reflectance mode can be used for TLC [762]. The main reason for studying the applicability of NIRS as an in situ detection tool in TLC is that adsorbents such as silica gel have no strong absorption in the NIR region background... 

Principles and Characteristics The prospects of Raman analysis for structural information depend upon many factors, including sample scattering strength, concentration, stability, fluorescence and background scattering/fluorescence from the TLC substrate. Conventional dispersive Raman spectroscopy has been considered as a tool for in situ analysis of TLC spots, since most adsorbents give weak Raman spectra and minimal interference with the spectra of the adsorbed species. Usually both silica and cellulose plates yield good-quality conventional Raman spectra, as opposed to polyamide plates. Detection limits for TLC fractions... 

The use of ionisation techniques such as El and Cl for TLC stationary phases has generally been limited to relatively nonpolar and thermally stable molecules. Polar involatile compounds, separated on silica gel, generally strongly adsorb on to the matrix, and decompose when heat is applied for volatilisation [817]. Use of less-adsorbent phases, such as polyamide, is particularly useful for TLC-EIMS work, because the analytes are not as strongly adsorbed to this phase and do not require high probe temperatures [818,819]. For compounds that are not suitable candidates for TLC-EIMS, FAB can be employed. Chemical ionisation, although suitable for TLC-MS, appears to have been little used. 

Reversed-phase chromatography is the term commonly applied to a system where a nonpolar liquid phase is coated on the solid support and elution carried out with an immiscible polar phase. Such systems are often necessary for separations which cannot be carried out by normal partition or adsorption chromatography. For TLC, the stationary phase is normally a liquid of high boiling point which does not readily evaporate from the adsorbent. Paraffin oil, silicone oil or n-tetradecane coated on silica gel or Kieselguhr are frequently used with water-based mobile phases such as acetone—water (3 2) or acetic acid-water (3 1). Reversed-phase chromatography is very useful for the TLC analysis of lipids and related compounds. 

In the 1950s and 1960s a major impetus was given to the use of thin-layer chromatography by Stahl [ 15-17) who constructed a spreader for the preparation of thin-layer plates, suggested the silica adsorbent ( silica gel nach Stahl ), and edited excellent text books, etc. Simultaneously, equipment for developments, chemicals, and laboratory supply for TLC became commercially available. 

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. 

---