A method of determining a shift in a background signal comprised in optical measurement signals of an analyzer, the method being characterized in comprising: obtaining a plurality of sets of optical measurements (2a-2e, 4a-4f) of a sample taken by the analyzer at different points in time, wherein each set (2a-2e, 4a -4f) includes at least one measurement taken in a first channel (λ 1) and another measurement taken in a second channel (λ 2) different from the first channel, splitting the plurality of sets of optical measurements (2a-2e, 4a-4f) into one group of sets (group A) and another group of sets (group B), wherein the measurements in the one group of sets (group A) are taken during a different period of time than the measurements in the other group of sets (group B), and wherein the measurements in the one group of sets (group A) show a higher signal change in time than the measurements in the other group (group B); determining a first background signal (bA) based on a relation of measurements in the first and second channels (λ 1, λ 2) in the one group of sets (group A), wherein determining the first background signal (bA) based on the relation of measurements in the first and second channels in the one group of sets consists of determining a model relation between the measurements in the first and second channels, the model relation being parametrized by a slope and an intercept and calculating a first intercept (bA) and a first slope (m) of the model relation for between the measurements in the first and second channels (λ 1, λ 2) of the one group of sets (group A); determining a second background signal (bB) by applying the model relation to measurements in the first and second channels in both the one group of sets (group A) and the other group of sets (group B), wherein the determining the second background signal (bB) includes constraining the slope to be equal to the first slope (m) as determined using the measurements in the first and second channels (λ 1, λ 2) of the one group of sets (group A); and calculating the second intercept subject to the constraint on the slope; and setting the second background signal equal to the second intercept; and determining a background shift based on the first and second background signals (bA, bB). obtaining a plurality of sets of optical measurements (2a-2e, 4a-4f) of a sample taken by the analyzer at different points in time, wherein each set (2a-2e, 4a -4f) includes at least one measurement taken in a first channel (λ 1) and another measurement taken in a second channel (λ 2) different from the first channel, splitting the plurality of sets of optical measurements (2a-2e, 4a-4f) into one group of sets (group A) and another group of sets (group B), wherein the measurements in the one group of sets (group A) are taken during a different period of time than the measurements in the other group of sets (group B), and wherein the measurements in the one group of sets (group A) show a higher signal change in time than the measurements in the other group (group B); determining a first background signal (bA) based on a relation of measurements in the first and second channels (λ 1, λ 2) in the one group of sets (group A), wherein determining the first background signal (bA) based on the relation of measurements in the first and second channels in the one group of sets consists of determining a model relation between the measurements in the first and second channels, the model relation being parametrized by a slope and an intercept and calculating a first intercept (bA) and a first slope (m) of the model relation for between the measurements in the first and second channels (λ 1, λ 2) of the one group of sets (group A); determining a second background signal (bB) by applying the model relation to measurements in the first and second channels in both the one group of sets (group A) and the other group of sets (group B), wherein the determining the second background signal (bB) includes constraining the slope to be equal to the first slope (m) as determined using the measurements in the first and second channels (λ 1, λ 2) of the one group of sets (group A); and calculating the second intercept subject to the constraint on the slope; and setting the second background signal equal to the second intercept; and determining a background shift based on the first and second background signals (bA, bB).