GRE Subject Test: Biology : Chromatography
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Example Questions
Example Question #1 : Understanding Column Chromatography
Which of the following techniques could help improve the purity of a protein sample while maintaining the protein's native state?
Southern blotting
All of these answers
SDS-PAGE
Column chromatography
Column chromatography
The only choice that would actually be useful in improving the purity of a protein sample would be column chromatography. It is possible to attach a specific ligand to your protein of interest to help separate it from a mixture via this method.
SDS-PAGE results in a separated, but denatured, protein sample. Detergents used in this process damage the protein structure, making it impossible to retain the native state. Southern blotting is a technique used to identify specific sequences of DNA in a sample and has nothing to do with increasing the purity of a protein sample.
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?
It is likely that the unknown substance is caffeine
The unknown substance is definitely not caffeine
The unknown substance is definitely caffeine
It is unlikely that the unknown substance is caffeine
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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