In the Fiske Subbarow phosphate assay, the UV measurement is performed at which wavelength?

• A. 340 nm
• B. 420 nm
• C. 520 nm
• D. 600 nm

Answer: (A)

In the Fiske-Subbarow phosphate assay, the measurement relies on a chromophore formed during the reaction that absorbs light most strongly in the near-UV region. The color development from the molybdate-based reaction, after reduction, yields a species whose absorbance peaks around 340 nm. Reading at this wavelength captures the greatest, most reliable signal corresponding to the amount of inorganic phosphate present, providing good sensitivity and minimizing interference from other components in the sample.

Wavelengths in the visible range (such as those around 420, 520, or 600 nm) would not align with the chromophore’s absorption, leading to weak signal and poorer accuracy. Thus, 340 nm is chosen because it matches the chromophore’s absorption characteristics and yields a dependable standard curve for quantitation.