An apparatus for optical analysis of a sample eluted from a sample substrate to an incubation buffer contained in a sample well of a sample holder loadable into the apparatus, the apparatus comprising; - a plurality of microtiter plates, each microtiter plate comprising an array of sample wells, the microtiter plates serving as said sample holder, and - an optical measurement unit, characterized in that; - the apparatus comprises a sample substrate contained in one sample well of said array of sample wells, which sample well contains an incubation buffer, - the optical measurement unit comprises: ∘ a light source which is configured to direct light through the sample well containing the sample substrate and the incubation buffer at a predefined wavelength or wavelength range absorbed by the sample eluted from the sample substrate but transmitted by the sample substrate, and ∘ a detector which is configured to detect light transmitted through the sample well, - the optical measurement unit is adapted to measure an amount of predefined analyte in the sample eluted from the sample substrate contained in the sample well by means of the light source and the detector, and in that - the apparatus comprises a computing unit adapted to calculate, based on an amount of light transmitted through the sample well and detected by the detector, an absorbance value indicative of a degree of elution of the sample in the sample well containing the sample substrate and the incubation buffer, said amount of light transmitted through the sample well by said optical measurement unit before or after the optical measurement of the amount of the predefined analyte in the sample well. - a plurality of microtiter plates, each microtiter plate comprising an array of sample wells, the microtiter plates serving as said sample holder, and - an optical measurement unit, characterized in that; - the apparatus comprises a sample substrate contained in one sample well of said array of sample wells, which sample well contains an incubation buffer, - the optical measurement unit comprises: ∘ a light source which is configured to direct light through the sample well containing the sample substrate and the incubation buffer at a predefined wavelength or wavelength range absorbed by the sample eluted from the sample substrate but transmitted by the sample substrate, and ∘ a detector which is configured to detect light transmitted through the sample well, ∘ a light source which is configured to direct light through the sample well containing the sample substrate and the incubation buffer at a predefined wavelength or wavelength range absorbed by the sample eluted from the sample substrate but transmitted by the sample substrate, and ∘ a detector which is configured to detect light transmitted through the sample well, - the optical measurement unit is adapted to measure an amount of predefined analyte in the sample eluted from the sample substrate contained in the sample well by means of the light source and the detector, and in that - the apparatus comprises a computing unit adapted to calculate, based on an amount of light transmitted through the sample well and detected by the detector, an absorbance value indicative of a degree of elution of the sample in the sample well containing the sample substrate and the incubation buffer, said amount of light transmitted through the sample well by said optical measurement unit before or after the optical measurement of the amount of the predefined analyte in the sample well.