Which instrument is used to measure NADPH in the reference method?

• A. UV spectrophotometer
• B. Infrared spectrometer
• C. Flame photometer
• D. Mass spectrometer

Answer: (A)

NADPH has a strong absorbance in the ultraviolet region, specifically around 340 nm, due to its reduced form. In reference methods for measuring NADPH, you quantify its amount by detecting how much light at 340 nm is absorbed. A UV spectrophotometer directly translates that absorbance into concentration using the Beer-Lambert law, making it the most practical and reliable way to monitor NADPH formation or consumption in enzyme assays. The other instruments aren’t suited for this purpose: infrared spectrometry targets different molecular features and isn’t sensitive to the specific electronic transition of NADPH; flame photometry is used for certain metal ions and relies on visible-range emission; mass spectrometry measures mass/charge ratios and is more complex and expensive for routine NADPH quantification. Thus, the UV spectrophotometer is the appropriate tool for measuring NADPH in the reference method.