Ascending chromatography

Geraniol [106-24-1] M 154.3, b 230 , d 0.879, n 1.4766. Purified by ascending chromatography or by thin layer chromatography on plates of kieselguhr G with acetone/water/liquid paraffin (130 70 1) as solvent system. Hexane/ethyl acetate (1 4) is also suitable. Also purified by GLC on a silicone-treated column of Carbowax 20M (10%) on Chromosorb W (60-80 mesh). [Porter Pure Appl Chem 20 499 7969.] Stored in full, tightly sealed containers in the cool, protected from light. 

Method. The TLC plates (silica gel) are activated for 30 min at 10S °C, cooled to room temperature and then spotted with a few microlitres of the sample. The chromatograms are developed by ascending chromatography, dried at 80 °C for IS min, cooled to room temperature and exposed to bromine vapour for a period of time ranging from 15 sec to several min. Excess of bromine is removed by means of a stream of cool air passed over the surface of the adsorbent. The plates are then sprayed with DDQ (a fresh 0.1% solution in benzene) and dried for 2 min with a stream of warm air. Visual observations are made with a UV lamp at long wavelength. Spots may be quantitated with a chromatogram scanner in the fluorescence mode. 

Harman and co-workers described a series of solvent systems for ascending chromatography on Whatman 3MM paper(45). The following table compares the values of indomethacin (I), N-deschlorobenzoy]indomethacin (II) and O-desmethylindomethacin (III). 

Fig. 7. Water content of paper in ascending chromatography. Paper, Schleicher and Schiill 598 G (data according to Griine, G23) B, water level H, height above water level Q, water content of the strip expressed in percentage of the paper (w/w).

For spots no. 1 and no. 5, use your isolated methyl esters obtained from com oil. For spot no. 2, use methyl oleate for spot no. 3, methyl linoleate and for spot no. 4, methyl palmitate. For each sample use a separate capillary tube. In spotting, apply each sample in the capillary to the plate until it spreads to a spot of 1 mm diameter. Dry the spots with a heat lamp. Pour about 15 mL of solvent (hexane diethyl ether 4 1) into a 500-mL beaker. Place the spotted TLC plate diagonally for ascending chromatography. Make certain that the spots applied are above the surface of the eluting solvent. Cover the beaker lightly with aluminum foil to avoid excessive solvent evaporation. 

In paper chromatography, 5-anilino-2-mercapto-l,3,4-thiadiazole is used as a reagent in the ascending chromatography of various cations (68RRC909), while 2-amino-1,3,4-... 

Basic bismuth gallate and free tribromophenol can be detected in small samples of bismuth tribromophenoxide by the simultaneous ascending chromatography of a neutral ethanolic suspension of one portion of the sample and an acidified ethanolic solution of... 

Labeled compounds were applied as spots on STLC plates and developed with distilled water by ascending chromatography. The developed plates were exposed to Kodak Royal Blue X-ray film for 2 to 3 weeks. The Rf value (relative frontal movement) for each compound was measured as the front of the spot or streak in the resultant autoradiogram (30). 

What is one advantage of descending over ascending chromatography ... 

A. Ascending Ascending chromatography is the most frequent development mode in TLC. Following sample application, a plate is placed in an appropriate chamber so that the solvent is below the point of sample application. The solvent is allowed to rise by capillary action usually from 10 to 18 cm above the origin on a 20 X 20 cm TLC plate and from 3 to 7 cm on a smaller, high-performance TLC plate. 

B. Sandwich chambers (S-chambers) The sandwich chamber is very thin to accommodate a single 20 X 20 cm plate and minimal solvent for ascending chromatography. S-chambers with a plain glass cover plate are considered unsaturated, whereas saturated conditions are achieved by employing a counter plate coated with sorbent (often cellulose) and soaked with solvent. S-chambers offer advantages over rectangular tanks, that is, faster gas-phase equilibration and excellent reproducibility both due to the much smaller internal volume when compared with N-tanks [51]. 

Fig. 5.9. Shandon Universal Apparatus assembled for ascending chromatography. The frame, holding up to 12 papers, sits in the solvent in the bottom of the tray.

Paper chromatography is only applicable on a micro scale and several attempts have been made to extend it to a larger scale. Mitchell and Haskins (1949) have described a chromatopile for such fractionations. This consists of a pile of filterpapers which are used as the column. Jones (1949) suggests the use of thick paper which may best be run by ascending chromatography. Recently there has become available preparations of powdered cellulose which are suitable for use in columns. 

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