Drying of the chromatogram

Paper chromatograms developed with volatile solvents can be completely dried in 

10-30 min if they are hung in a fume hood with an efficient draft. Papers impregnated with reagents such as formamide or developed with poorly volatile solvents such as phenol are better dried in a forced-air heated drying oven, with heated air flow from a hair dryer, or over a hot plate, if the solutes are not heat labile. The final traces of some solvents, such as acetic acid and hydrochloric acid, are particularly difficult to remove. In some cases, traces of impurities may inevitably remain that will interfere with some detection reagents, e.g., ammonium formate on formamide-impregnated paper, which interferes with the use of SbClj. 

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The chromatogram is freed from mobile phase in a stream of warm air, dipped in dipping solution I for 3 s, then heated to 120°C for 5 —10 min, cooled to room temperature and then immersed in dipping solution II for 3 s. The final drying of the chromatogram should take place in a stream of cold air in order to avoid strong background coloration. 

Note The developed chromatogram must be completely freed from nonpolar solvents before derivatization, otherwise an intense fluorescence will be stimulated over the whole plate. The fluorescence intensity of the chromatogram zones remains stable for ca. 40 min it decreases slowly as the layer dries out and can be returned to its original intensity by renewed immersion in the reagent solution or in water. 

Detection and result The chromatogram was freed from mobile phase for 15 min in a stream of cold air and immersed in the dipping solution for 2 s, dried for 5 min in a stream of warm air and then heated to 110-130 °C for 1 -2 min, until the color of the chromatogram zones reached maximum intensity. 

Fig. 1 chromatogram of the thiophosphate insecticides (each ca. SOO ng) after treatment with dipping solutions I and II (A) before and (B) after complete drying of the TLC plate. TVacks 1 and S mixture Track 2 azinphos ethyl Track 3 malathion Track 4 diazinone. 

After drying for 10 min in a stream of cold air 1,4-phenylenediamine (h/Jj 5-10), 2-amino-4-chlorophenol (h/ f 15-20), 4-nitroaniline (h/ f 25-30) and l,4-amino-3-nitro-toluene (h/ f 50-55) appeared as blue-violet chromatogram zones on a blue background. These could be recognized without difficulty for several days from the back of the chromatogram. 

The times of immersion of the chromatogram in the reagent bath are usually short (< 5 s [126]) in order to avoid dissolving the substances out of the stationary phase. This is easily achieved if the Tauch-Fix (Fig. 41) is employed. The chromatogram is then laid horizontally and dried in a stream of air. 

On the other hand concentration detector (DRI) data represent at least 10% of the maximum on the low molecular weight side of the chromatogram (Results), and the Gaussian approximation is probably applicable. 

The iron present as an impurity in silica gel-G affords an appreciable distortion of the chromatogram . Hence, it has become almost a necessary step to purify the adsorbent. The iron-free layers may be achieved by providing the pre-coated and air-dried plates a preliminary development with a mixture of methanol and... 

Analytes are detected upon air-drying of the plate, and subsequent examination under UV light (254 nm). Any secondary spot in the chromatogram obtained with solution (1) must not be more intense than the spot in the chromatogram obtained with solution (3) and not more than one such spot can be more intense than the spot in the chromatogram obtained with solution (4). The test is not valid unless the chromatogram obtained with solution (6) shows two clearly separated principal spots. 

When the developing solvent reaches a few centimeters from the top of the chromatogram, remove the paper from the jar and allow it to dry in a hood. Remove the staples and spray the paper lightly with ninhydrin solution. Allow it to dry in a hood for 10 minutes. The color may be developed by placing the chromatogram in an oven (100°C for 10 min) or at room temperature for 1 to 2 hours. All the spots should be circled with a pencil, as they will fade with time. Describe in your notebook the color of each spot. 

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