A method for producing data for a correction curve used for adjusting measured sample responses for effects of varying concentrations of solvent or compound of an assay device, for example a surface plasmon resonance device comprising a first flow cell with a reference surface and a second flow cell with an active surface wherein the active surface comprises a quantity of ligands and the reference surface comprises a lesser quantity of ligands or no ligands, comprises the steps of: a) providing a source of a liquid comprising a first concentration of the solvent or compound, b) providing a source of a liquid comprising a second concentration of the solvent or compound, c) providing a fluid mixer for mixing the liquid comprising a first concentration of the solvent or compound with the liquid comprising a second concentration of the solvent or compound, wherein said fluid mixer comprises a mixer chamber and/or a mixer pathway arranged to cause mixing of the two liquids, wherein the fluid mixer is adapted to ensure that the liquids are mixed before leaving the fluid mixer, d) providing a first pump for providing a flow of the first concentration of the solvent or compound through said fluid mixer to the active surface and the reference surface and a second pump for providing a flow of the second concentration of the solvent or compound through said fluid mixer to the active surface and the reference surface, e) controlling the pumping rate of one or more of said first and second pumps (77, 79) to produce a flow of liquid from the fluid mixer wherein the liquid from the fluid mixer comprises a first mixed concentration of the solvent or compound over said active and reference surfaces (67, 63), f) measuring and recording the relative response of said active surface and said reference surface to said first mixed concentration of solvent or compound, g) controlling the pumping rate of one or more of said first and second pumps (77, 79) to produce a flow of liquid comprising a further mixed concentration of the solvent or compound over said active and reference surfaces (67, 63), wherein said concentration of the solvent or compound is changed by changing the relative flow rate of the first and second pumps (77, 79), h) measuring and recording the relative response of said active surface and said reference surface to said further mixed concentration of solvent or compound, and i) repeating steps g) and h) N times where N is greater than or equal to zero, hereby allowing multiple different concentration of the solvent or compound to be quickly prepared and subsequently injected into the assay device. a) providing a source of a liquid comprising a first concentration of the solvent or compound, b) providing a source of a liquid comprising a second concentration of the solvent or compound, c) providing a fluid mixer for mixing the liquid comprising a first concentration of the solvent or compound with the liquid comprising a second concentration of the solvent or compound, wherein said fluid mixer comprises a mixer chamber and/or a mixer pathway arranged to cause mixing of the two liquids, wherein the fluid mixer is adapted to ensure that the liquids are mixed before leaving the fluid mixer, d) providing a first pump for providing a flow of the first concentration of the solvent or compound through said fluid mixer to the active surface and the reference surface and a second pump for providing a flow of the second concentration of the solvent or compound through said fluid mixer to the active surface and the reference surface, e) controlling the pumping rate of one or more of said first and second pumps (77, 79) to produce a flow of liquid from the fluid mixer wherein the liquid from the fluid mixer comprises a first mixed concentration of the solvent or compound over said active and reference surfaces (67, 63), f) measuring and recording the relative response of said active surface and said reference surface to said first mixed concentration of solvent or compound, g) controlling the pumping rate of one or more of said first and second pumps (77, 79) to produce a flow of liquid comprising a further mixed concentration of the solvent or compound over said active and reference surfaces (67, 63), wherein said concentration of the solvent or compound is changed by changing the relative flow rate of the first and second pumps (77, 79), h) measuring and recording the relative response of said active surface and said reference surface to said further mixed concentration of solvent or compound, and i) repeating steps g) and h) N times where N is greater than or equal to zero,