Imagine you have a clear liquid that is supposed to be a specific medicine. How can you use light to check if it's the real thing without tasting or touching it?
Think about this:
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The chapter explains that different drugs absorb light at different "fingerprint" wavelengths. If the real medicine should absorb strongly at 260 nm, but your sample does not absorb at 260 nm, what does that tell you?
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The instrument needs a "blank" or reference (often just the solvent) to compare against. Why is this step important? What would happen if you forgot to use the blank?