Why can't paper chromatography, for many times, allow complex mixtures to separate? Also, why are there limitations placed on the quantity of samples to be resolved, in paper chromatography? If...
Why can't paper chromatography, for many times, allow complex mixtures to separate?
Also, why are there limitations placed on the quantity of samples to be resolved, in paper chromatography?
If you can't answer the 2nd question, then it's fine! The first one is the one i really need to know about.
The principle behind paper chromatography is simple - polarity. The mobile phase and stationary phase have different polarities - usually a more polar stationary phase (e.g. silica) and a less polar stationary phase. Paper chromatography separates the components/constituent molecules of a sample based on the chance that its components will have slightly different polarities. These molecules will be resolved as spots along the plate. The distances by which the spots travelled will depend on their affinity to the mobile or stationary phase - that is, if they are more polar or less polar. If they are more polar, they will have higher affinity to the stationary phase (in this case), and hence stay closer to the bottom of the plate.
The resolution will depend on a lot of factors, one of which would be the combination of mobile and stationary phase - the difference between their polarities is critical.
Most of the time, paper chromatography can't resolve more complex mixtures/solutions. Note that the main basis of separation is polarity. Hence, the resolution will not be as great when a complex solution is run versus when a relatively sample is used. It might be hard to separate a complex sample into its components especially if most of them have a very small difference in polarities.
In such cases, we can increase the resolution by using 2-D paper chromatography. This is same as the one-dimensional chromatography - except that after the first run, the paper is turned 90 degrees and run again - thus increasing resolution. The sample is also spotted at the corner so only one sample can be run at a time. For 1D paper chromatography, the number of samples really depend on the size of the paper. We limit the number of samples to avoid smudging of the points in case the samples do not move along a straight path, or in case the resolved points are smudged by the solution.