I conducted a TLC experiment on ink samples using 5 different solvent systems. 2 of the systems produced TLC plates where the solvent front moved up the plate but the samples were left on the origin and did not separate. Any suggestions as to why this happens?
When performing your TLC experiment, the components that you are separating have different polarities. If the Rf value is 0, you need to increase your solvent polarity because the sample is not moving and sticking to the stationary phase. If the Rf value is 1, you need to decrease your solvent polarity because the compound does not separate. Your choice of solvent and the ratio of solvent mixture are critical. The stationary phase is equally important. The following are common problems with TLC.
- Over-large Spots: Spotting sizes of your sample should be not be larger than 1-2 mm in diameter. The component spots will never be larger than or smaller than your sample origin spot. If you have an over-large spot, this could cause overlapping of other component spots with similar Rf values on your TLC plate. If overlapping occurs, it would prove difficult to resolve the different components.
- Uneven Advance of Solvent Front: Uneven advance of the mobile phase is a common problem encountered in TLC. Consequences would be inaccurate Rf values due to the uneven advance of sample origin spots. This uneven advance can be caused by a few factors listed below.
Rarely, water is used as a solvent because it produces an uneven curve front which is mainly accounted for by its surface tension.
- Streaking: If the sample spot is too concentrated, the substance will travel up the stationary phase as a streak rather than a single separated spot. In other words, the solvent can not handle the concentrated sample and in result, moves as much of the substance as it can up the stationary phase. The substance that it can not move is left behind. This can be eliminated by diluting the sample solution. To ensure that you have enough solution, use a short-wave UV light to see if the spot is visible (normally purple in color), as stated earlier.
- Spotting: The sample should be above the solvent level. If the solvent level covers the sample, the sample spot will be washed off into the solvent before it travels up the TLC plate. An example is shown below.
When the ink sample spots did not move it means that the solvent system polarity was not compatible with the ink sample. You have to increase the polarity of the solvent system so that there is less difference between the polarity of the solvent system and the ink sample. Hope it helped.
The polarity of the solvent should be increased. For example, water drops must be added or even acidic solution also should be added in gradual elation manner.
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