The present invention creates a spectrometer (1; 1′) for measuring the concentration of at least one analyte in a fluid sample (2; 2′), with a light source (3; 3′) to generate a light beam (4; 4′), with a photosensor (5; 5′) to receive the light beam (4; 4′), and with a measurement length (6; 6′), in which the fluid sample (2; 2′) can be placed, in the beam path of the light beam (4; 4′), the measurement length (6; 6′) being provided in changeable form.