Question

1. A pigment solution contains compound A with 4 polar groups and compound B with 2 polar groups. You plan to separate these compounds using paper chromatography with a nonpolar solvent. Predict the location of the two bands relative to the solvent front. Explain your answer.

2. Paper chromatography is used to separate pigments extracted in acetone from fresh spinach. Explain how differences in polarity are used to separate the pigments in this chromatography experiment.

Answer 1

1. Paper chromatography is an analytical separation technique which uses paper as a stationary phase while non- polar solvent are utilized as mobile phase which carry the mixture of compounds. Here paper used as a stationary phase is polar in nature as it is composed of sugar molecules which contains hydroxyl group and give it polarity, therefore paper is more polar than the mobile phase. This means the affinity of polar molecules will be stronger towards the stationary phase and will take more time to elite out as compared to non- polar or less polar analytes.

As nin the question it is mentioned that compound A contains 4 polar groups while compound B contains only 2 which means that A is more polar in nature than B. So the affinity of A will be more as compared to B and so the speed of A will be slower than B. This means that B will elite out quickly as compared to A.

So the bands of B will be at a greater distance than A

2. Spinach leaves contain 4 pigments which are:

Chlorophyll A	Dark green
Chlorophyll B	Yellowish-green
Xanthophylls	Yellow
Carotenoids	Orange

These pigments differ in their polarity. Beta carotene is a hydrocarbon and hence is non polar in natur. While on the other hand chlorophylls contain several polar C-O and C-N bonds and hence are more polar than the carotene. Chlorophyll contains a methyl group wheras chlorophyll b contains an aldehyde functional group. This makes chlorophyll b more polar than chlorophyll a. Xanthophyll also a type of carotenoids contain oxygen atom as functional group and hence is more polar than carotene while less polar than chlorophylls.

According to the decreasing polarity the pigments can be observed as

Chl b > chl a >. Xanthophyll > carotene

This means that the chlorophylls will have a greater affinity towards the stationary phase (paper) as compared to the other pigments. This means that the distance travelled by the less polar pigments will be more as compared to the Chlorophylls.

According to the decreasing Rf values or distance of bands on the paper the pigments are seen as:

Carotene > xanthophyll > chl a > chl b