In a thin layer chromatography (TLC)experiment,the TLC plate was pre-developed in the development chamber containing the mobile phase (to remove any contaminants before use),then air dried.

On the same day,the TLC plate was then spotted with the sample & standard and placed in the same development chamber containing the mobile phase. Is this acceptable or should the development chamber be replaced with fresh mobile phase?

I don't know the text book answer but I think the mobile phase should be replaced.

In a thin layer chromatography (TLC) experiment, it is generally acceptable to use the same development chamber containing the mobile phase if it has been pre-developed and air dried. Pre-developing the TLC plate in the development chamber with the mobile phase helps to remove any contaminants that may interfere with the separation process.

However, there are a few factors to consider when deciding whether to replace the mobile phase or use the same one:

1. Contamination: If you suspect that the mobile phase in the development chamber is contaminated or contaminated with previous samples, it is better to replace it with fresh mobile phase. Contamination can affect the accuracy and reproducibility of your results.

2. Repeatability: If you require precise and highly reproducible results, it is recommended to start with fresh mobile phase for each TLC experiment. This ensures consistent separation and minimizes any carryover effects from previous experiments.

3. Cost and availability: If the mobile phase is expensive or not readily available, you may choose to use the same mobile phase for multiple experiments. In this case, make sure to monitor the mobile phase carefully for any signs of degradation or contamination.

Overall, while using the same development chamber with the mobile phase is generally acceptable, factors like contamination, repeatability requirements, and cost considerations should be taken into account when deciding whether to replace the mobile phase or not.