8. The mobile phase in the above analysis is 50% methanol / 50% aqueous 1% aceti

Question

8.     The mobile phase in the above analysis is 50% methanol / 50% aqueous 1% acetic acid. Firstly, it is changed to 40% methanol / 60% water (with 1% acetic acid) and then, secondly, to 50% acetonitrile / 50% water (with 1% acetic acid) instead of the original mobile phase in a series of additional chromatography experiments. All other experimental conditions remain the same. Explain (3-6 sentences using key words to describe the chemistry) the effect that each change will have on:

1-the chemical nature of the mobile phase

40% methanol / 60% water (with 1% acetic acid)) is more polarity than (50% acetonitrile / 50% water (with 1% acetic acid))

2-interaction of the different solute types with the stationary phase

3-the retention times of the two peaks due to drugs A & B in each instance

Explanation / Answer

1.Methanol and acetic acid react to formethyl acetate.At the give n percenatges of the solvents in water,the primary function of organic modifier is to weaken hydrogen bonds ,which acetonitrile will do better than methanol and becaus4e the hydroxyl group of methanol can participate in hydrogen bonding so it is more polar than acetonitrile solvent mixture.

3.As methanol is more polar than acetonitrile,it increases the retention of hydrophobic compouns in reversed phase.Acetonitrle based solution has a higher elution strength.In some cases,where the mixture ratio i low,the same retention time can be acheived using acetonitrile with less than half the ratioof methanol.

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