Issues

Overloading plates

If excessive material is loaded on a plate, then the compounds are unable to interact properly with the stationary phase. This results in streaking occurring below the spot on the plate with the spot looking like it has a tail behind it. There is a compromise to be found between loading enought to visualise and excessive compound. Visualisation can be aided with the use of stais where UV light is insufficient.

Solvent fronts not running in a straight line

When running plates, it is important that the mobile phase rises up the plate in a steady and even fashion. There are a number of causes for plates not running properly that should be considered

• Plate not resting level on the base of the tank

• Baseline not straight

• Edge of plate in contact with the filter paper or sides of the TLC tank

The effect of an imperfect solvent front can be minimised through the use of co-spots, but efforts to ensure the solvent front runs properly are more important.

Compound not stable on the stationary phase

Some compounds may prove to be unstable on the stationary phase. For unmodified silica, this is usually due to the acidic nature of the stationary phase. There is an easy method to determine if compounds are stable on the TLC stationary phase, and this is known as 2-dimenstional chromatography. In 2D TLC, two baselines are drawn on two adjacent sides and the compound is spotted onto the intersection of the baselines. The plate is then run in one dimension as normal, the plate removed, turned 90° and developed a second time. When the TLC is visualised, if a compound is stable then spots should only appear on the diagonal across the plate. If a compound is unstable, then spots that have run in the first dimension should produce additional spots when run in the second dimension, leading to spots appearing off the diagonal. If compounds are unstable, consideration should be given to the compound and stationary phase and whether an alternative material can be utilised.

Compound dissolving into the mobile phase

If the baseline sits below the mobile phase level in the TLC tank, compounds will dissolve into the mobile phase and can run anywhere on the plate. The mobile phase will need replacing as it will have been contaminated, and either less mobile phase can be added to the tank or a higher baseline used on the plate.