In spectrophotometry, what is the purpose of a reagent blank?

• A. Reagent blank — zero instrument before procedure
• B. Sample blank — zero instrument during procedure
• C. Instrument blank — baseline during measurement
• D. Environmental blank — accounts for ambient light

Answer: (A)

In spectrophotometry, you want the measured absorbance to come from the analyte alone, not from the other components in the mixture. The reagent blank contains all reagents and solvent used in the assay except the analyte, so measuring it captures just the background absorbance from those reagents. By zeroing the instrument with this blank before you run the real samples, you subtract that background and ensure the subsequent readings reflect only the analyte’s absorbance. The instrument blank and environmental blank serve different background corrections, and the sample blank accounts for the sample’s own background, but the reagent blank specifically removes the contribution from the reagents themselves.