In assaying caffeine concentrations via HPLC, theophylline, a structurally relat

Question

In assaying caffeine concentrations via HPLC, theophylline, a structurally related compound, is often used as an internal standard. In a calibration experiment, a solution containing 99.314 g/mL caffeine and 113.2 g/mL theophylline produces signals of 0.395 and 0.268, respectively. You then assay coffee beans. You first grind up 0.78889 g of coffee beans and dissolve the beans in solvent containing 65.4 g/mL theophylline such that the total volume is 10.0 mL. You repeat your HPLC experiment and observe signal intensities of 0.534 for caffeine and 0.146 for theophylline. What is the mass percent of caffeine in the coffee beans?

Explanation / Answer

a solution containing 99.314 g/mL caffeine and 113.2 g/mL theophylline produces signals of 0.395 and 0.268.

Ratio of concentration is 99.314/113.2 = 0.877

Ratio of signal is 0.395/0.268 = 1.47

65.4 g/mL theophylline such that the total volume is 10.0 mL

signal intensities of 0.534 for caffeine and 0.146 for theophylline

Ratio of signal 0.534/0.146 = 3.657

When the ratio of signal is 1.47 ratio of concentration is 0.877

So when the ratio of signal is 3.657 ratio of concentration is 2.18

caffeine/theophylline = 2.18

caffeine/65.4 = 2.18

caffeine = 142.7 ug/ml

So in 10 mL we have 1427.1 ug of caffeine

Mass percent is 1427.1 x 10-6 g/0.78889 g x 100 = 0.1809% of caffeine

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