Tell me about the determination of caffeine in beverages by liquid chromatography?

Answer 1

Caffeine is a fairly easy chromatographic separation to perform. You would need an HPLC system of course. A generic C18 HPLC column would do. You would need a mobile phase, let's try 50 % water and 50 % methanol. Of course, you would need pure caffeine to prepare as your reference standard.
You can visit Agilent technologies' web site for some very specific information. They will require that you register on their site.

Answer 2

Separation science can quantify caffeine a number of ways. With liquid chromatography, specifically HPLC, the analyte is introduced at the head of the column. The mobile phase carries the caffeine down the column, other compounds move down the column at a slower or faster rate. The amount of caffeine is determined at the specific retention time for caffeine. The detector is typically an IR photodiode array. Of course standards with known caffeine concentration are also tested, to establish a calibration curve.

Liquid chromatography works best when your sample matrix is water-based. If I was doing the determination on dry material, I might select organic solvent extraction, followed by gas chromatography. Caffeine does have solubility issues in water.