A method for measuring the absorbance of UV light by a protein in a solution in a flow cell , said method comprising the steps of: - transmitting (S1) a first light beam from an ultraviolet, UV, light source of a wavelength of 180 nm to 300 nm towards a beam splitter ; - dividing (S3) the first light beam into a signal light ray and a reference light ray by the beam splitter ; - modulating (S5) the signal light ray ; - modulating (S7) the reference light ray ; - providing (S9) the measuring flow cell such that the signal light ray passes through the measuring flow cell ; - detecting (S11) a signal in a detector , which signal is the combined signal intensity of the signal light ray and the reference light ray detected by the detector ; - performing synchronous detection (S15) of the detected signal in order to reconstruct the intensities of the signal light ray and the reference light ray from the combined signal detected by the detector , said synchronous detection being based on the modulation performed to the signal light ray and the reference light ray, wherein said dividing of the first light beam into signal and light rays is performed by an asymmetrical beam splitter such that a larger portion of the first light beam is directed to the signal light ray than to the reference light ray; and - wherein the step of modulating (S5) the signal light ray comprises modulating the signal light ray at a first frequency and the step of modulating (S7) the reference light ray comprises modulating the reference light ray at a second frequency which is different from the first frequency, and wherein the reference light ray and the signal light ray are detected in the same detector simultaneously. - transmitting (S1) a first light beam from an ultraviolet, UV, light source of a wavelength of 180 nm to 300 nm towards a beam splitter ; - dividing (S3) the first light beam into a signal light ray and a reference light ray by the beam splitter ; - modulating (S5) the signal light ray ; - modulating (S7) the reference light ray ; - providing (S9) the measuring flow cell such that the signal light ray passes through the measuring flow cell ; - detecting (S11) a signal in a detector , which signal is the combined signal intensity of the signal light ray and the reference light ray detected by the detector ; - performing synchronous detection (S15) of the detected signal in order to reconstruct the intensities of the signal light ray and the reference light ray from the combined signal detected by the detector , said synchronous detection being based on the modulation performed to the signal light ray and the reference light ray, wherein said dividing of the first light beam into signal and light rays is performed by an asymmetrical beam splitter such that a larger portion of the first light beam is directed to the signal light ray than to the reference light ray; and - wherein the step of modulating (S5) the signal light ray comprises modulating the signal light ray at a first frequency and the step of modulating (S7) the reference light ray comprises modulating the reference light ray at a second frequency which is different from the first frequency, and wherein the reference light ray and the signal light ray are detected in the same detector simultaneously. <EOS>