A method and instrument for determining optical density of a solution is disclosed. A flow cell 1 having at least three light paths (4a, 4b, 4c) is provided (100), wherein each light path has a respective predetermined path length, l. Absorbance readings are taken (400), A, of the solution at the at least three light paths (4a, 4b, 4c). For each pair of light paths, a slope, αc, is calculated (500) by dividing a difference in absorbance reading, ΔA, with a difference in path length, Δl. The calculated slopes, αc, are compared (600), and a) if the calculated slopes, αc, are the same, the slope is used for determining (700) optical density of the solution, or b) if he calculated slopes, αc, are not the same, the steepest slope of the calculated slopes is used for determining (701a) optical density of the solution, or the slope of the calculated slopes being in the range of an absorbance reading of 0.01 to 2 is used for determining (701b) optical density of the solution

A purpose of the present invention is to provide a sensor with high sensitivity or high target substance selectivity.

A sensor that includes a structure having an internal space into which a detection target is capable of flowing, a light-emitting element, and a photo-receptor element is provided. The sensor is disposed so that light emitted from the light-emitting element passes through the internal space to reach the photo-receptor element. A wavelength of the light emitted from the light-emitting element falls within a range from 2.5 m to 15 m inclusive. A length of the internal space in a direction perpendicular to an extension direction of the structure should preferably be less than or equal to 1000 m.

A method and instrument for determining optical density of a solution is disclosed. A flow cell 1 having at least three light paths (4a, 4b, 4c) is provided (100), wherein each light path has a respective predetermined path length, l. Absorbance readings are taken (400), A, of the solution at the at least three light paths (4a, 4b, 4c). For each pair of light paths, a slope, c, is calculated (500) by dividing a difference in absorbance reading, A, with a difference in path length, l. The calculated slopes, c, are compared (600), and a) if the calculated slopes, c, are the same, the slope is used for determining (700) optical density of the solution, or b) if the calculated slopes, c, are not the same, the steepest slope of the calculated slopes is used for determining (701a) optical density of the solution, or the slope of the calculated slopes being in the range of an absorbance reading of 0.01 to 2 is used for determining (701b) optical density of the solution.