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THE TLC PROCESS

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p144-145

2026-07-08

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THE TLC PROCESS

TLC is carried out in a chamber. Its dimensions depend upon the size of the coated plastic or glass plate. The plate must fit so that it does not touch the sides of the chamber. The plate must also fit entirely within the chamber. A typical arrangement may use a 5 × 10^-cm plate and a 400-ml beaker. A piece of filter paper is put inside the chamber up against the side. This will absorb some of the mobile phase so that the entire inside of the chamber is saturated with the mobile phase. The top of the chamber is tightly covered. In many cases, Parafilm® works well as a sealer. An apparatus used for performing TLC is shown in Figure 1. The analyte is dissolved in a small amount of a suitable solvent. The solvent must be volatile. Chloroform or methanol is often used. A well of a spot plate can be used for this. A spot of the dissolved analyte is put onto the TLC plate about 1 cm up from the bottom. This spot is kept as tiny as possible. A very narrow capillary can be used to make the spot. If necessary, the spot can be over spotted, but the original spot should be dried first by blowing on it. This will keep the spot as small as possible. One of the advantages of TLC over other forms of chromatography is that more than one sample can be run at the same time. There is generally room for several spots along the bottom of the TLC plate even with the need to keep some space between each spot. For example, in a drug case containing three exhibits where cocaine is suspected, one TLC plate can hold spots of each of the three exhibits as well as a sample of known cocaine and perhaps lidocaine, a common cutting agent for cocaine. After the plate is loaded with samples, a small amount of the mobile phase is put in the chamber. There must be enough to travel up the plate but not enough to cover the spots. The chamber is covered and the mobile phase travels up through the stationary phase, carrying with it the analyte spots. Using the same principles that govern all chromatographic separations, the components of the analyte will be differentially retarded by the stationary phase. When the mobile phase front as nearly reached the top of the plate, the process is stopped by removing and drying the plate.

FIGURE 1 A thin-layer chromatography (TLC) apparatus. Note the piece of filter paper in the chamber (beaker). A small amount of mobile phase will be put in the chamber. The filter paper helps to saturate the space in the chamber. After the TLC plate is added, the chamber is tightly sealed.