Differential refractometer

A differential refractometer comprising:

• a light source that generates measurement light;
• a flow cell that is provided with a sample cell through which a sample solution flows, a reference cell for a reference solution and a partition wall provided between the sample cell and the reference cell, and refracts the light from the light source in a case where a refractive index of the sample cell and a refractive index of the reference cell are different from each other;
• an optical system that irradiates the flow cell with the measurement light from the light source, causes the measurement light to be transmitted through the sample cell and the reference cell of the flow cell, and causes a slit image of the measurement light that has been transmitted through the flow cell to be formed;
• and
• a detector that includes a light-receiving surface at a position at which the slit image of the measurement light that has been transmitted through the flow cell is formed, wherein
• the differential refractometer is configured such that the slit image is displaced in a certain direction on the light-receiving surface according to a change in refractive index of the sample cell,
• the detector includes two light-receiving element rows in which a plurality of light-receiving elements are consecutively arranged in a displacement direction of the slit image on the light-receiving surface,
• a width of the light-receiving element in the displacement direction is larger than a width of the slit image and smaller than two times of the slit width,
• a boundary between adjacent light-receiving elements in one light-receiving element row is deviated from a boundary between adjacent light-receiving elements in another light-receiving element row and closest to the boundary by a distance smaller than the width of the slit image in the displacement direction,
• the slit image is set to be formed and extend to both of light-receiving element rows, and
• the differential refractometer includes
• a measurement light-receiving element pair selector configured to select a pair of light-receiving elements, which are adjacent to each other in any one of the light-receiving element rows and has a boundary between the pair of light-receiving elements and on which a slit image is formed, out of the light-receiving elements of the detector, and
• a calculator configured to acquire detection signals of the measurement light-receiving element pair and obtain a displacement amount of the slit image based on a difference between the detection signals of these light-receiving elements.