Helpful TLC Techniques The major problem in coating one's own preparative (thick) layer chromatography plates is the difficulty in ohtaining a uniformly thick adsorbent layer. This problem can he overcome easily by vigorously rocking the coated plate an the edge of the bench top for about 15 sec as soon as the adsorbent has been applied. This procedure provides a smoother, more uniform layer and will also help to cover any uncoated corners or edges of the plate with adsorbent. After the plate has been so treated, it is left to air dry several hours before activating at llO°C in an oven as usual.
IODINE Location of zones on thin layer chromatography plates is often best done by exposure to iodine vapor. However, this method is sometimes destructive of the sample, and is therefore not suited for preparative layer chromatography. The usual technique of masking off all hut a narrow band of the preparative plate and exposure to the location reagent fails with iodine vapor. Instead, however, an iodine-vapor spray device (see figure), simply constructed from a disposable pasteur pipet, glass wool, crystals of iodine and a pipet bulb, can he used effectively in conjunction with a clean glass plate used to mask off the bulk of the plate. Zones can he located by spraying a narrow (
