Which blank measures the absorbance of the sample?

• A. Sample blank
• B. Reagent blank
• C. Instrument blank
• D. Zero blank

Answer: (A)

In spectrophotometry, you want the absorbance attributed only to the analyte, so you correct for background from the sample's own matrix. The blank that contains everything in the sample except the analyte measures this background due to the solvent, matrix, and any inherent color of the sample itself. This background is then subtracted from the sample's reading to isolate the analyte's absorbance. The sample blank, therefore, is the one that measures the absorbance of the sample's matrix without the analyte, making it the best choice.