A robotic milking system includes a milking control, a milk line, and a sampling and analysis device for the milk, wherein the milking control controls the robotic milking device based on the analysis of the milk sample. The sampling and analysis device includes a control unit, a tape reel with a tape with reagent pads that detect the presence of a substance in the sample, a tape mover, a dosing device to provide a sample onto a reagent pad, a camera to image of the reagent pad with said sample, and to analyse the images to indicate a presence or concentration of said substance. The lengthwise consecutive substantially rectangular reagent pads are separated by a hydrophobic line, and the control unit recognises said line in the image and determines a position thereof. The control unit controls the tape mover on the basis of the obtained image and line position.