Because: That is the official" Waters SOP for measurement of lamp energy on many of their diode-array detectors (with D2 lamps) using Methanol and the known energy peak (apex) observed and measured at 230.68 nm (max output).

• Other fully acceptable PDA lamp energy test methods used to determine lamp energy evaluate values at multiple wavelengths (across the usable range 190-600 nm) are also run using more transparent solutions such as ACN which may provide more detailed and practical information (because lamp energy usually shows more noise in the low UV range as they age), but some vendors use a more simplified method. It is a choice.