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Questions about HPLC Lab

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Hey I am about to do a lab to determine the amount of caffeine and sodium benzoate in soft drinks via HPLC. To prepare I had to answer a series of questions before the lab. I am confused on some of them and request your help. The questions are:

Why is it necessary to perform the HPLC separation at a pH of 3? How does the pH affect the separation of caffeine and sodium benzoate?

Predict the elution order for caffeine and sodium benzoate in a reversed phase HPLC used in the experiment?

Why is reverse phase HPLC is being used in this experiment?

At pH=4 what will be the charge if any of caffeine and benzoate?

Can you use an ion exchange HPLC column. If so, what ion exchange would be best?

What is the detection limit for a UV-vis detector?

Why aren't you concerned that the soda is colored or not, unlike the situation for the diode array experiment?

What would happen if I double the length of the HPLC?

When looking at the calibration curve, would I use peak heights pr peak area for accurate results?

Any help would be greatly appreciated.

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