In order to obtain good, evaluable and reproducible results in thin-layer chromatography (TLC), the application of the sample is a very crucial step. A frequently asked question here is whether a dot- or a brush-like application of samples in the μl range leads to better results.
In the case of a manual application of the samples, the dot-like application with fixed volume capillaries is relatively simple. In contrast, a special instrumental technique is necessary for a brush-like application. These dosing syringes can also be controlled without software systems, but they must still be filled manually and put into the system. This is no longer necessary for fully automated machines. The substances are sprayed onto the layer as aerosol in nl or μl volumes through a carrier gas. The system can move both the syringe and the layer to achieve the desired line length. The carrier gas supports the evaporation of the solvent during the spraying process, so that a homogeneous distribution of the substances on the layer occurs.
The technical equipment for a brush-like application can certainly be considered a disadvantage compared to the simple dot-like application with a pipette. However, this technique also offers a number of advantages: Separations in the mg range on TLC plates with a layer thickness of up to 2 mm are possible. Due to the large volume of the application, spraying can also be used simultaneously as an enrichment step where the volume dosing error is significantly minimized. The all-automatic application results in highly symmetric patterns with homogeneously distributed substances, which allows subsequent measurements in the TLC scanner and increases both the reproducibility and the determination limit.