GRE Subject Test: Biology : Understanding Thin Layer Chromatography

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Thin Layer Chromatography

A researcher is trying to identify if caffeine is present in an unknown sample. He chooses to use thin layer chromatography and compare it to a known sample of caffeine. After running the test, he discovers that his unknown sample has the exact same Rf value as the standard caffeine. Which of the following is true about his result?

Possible Answers:

The unknown substance is definitely caffeine

It is likely that the unknown substance is caffeine

It is unlikely that the unknown substance is caffeine

The unknown substance is definitely not caffeine

Correct answer:

It is likely that the unknown substance is caffeine


The fact that the two substances have the same Rf value is strong evidence that they are the same substance; however, it is not conclusive evidence. The researcher would have to verify these results with another test (such as NMR) to conclusively state that caffeine is present in his sample. Rf value is determined by polar and hydrophobic properties that may not be unique to a given compound. Many different compounds are capable of having the same or similar Rf values; therefore, his result does not conclusively show that caffeine is in his sample.

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