Methods for photometric analysis

A method for compensating for the effects of scattered light in a photometric fluid analyzer having a light source, a sample rotor having a plurality of cuvettes for holding a quantity of the fluid to be analyzed and an opaque body disposed at the position of one of the cuvettes, and a light detector disposed to detect light directed through the cuvettes, the method comprising:

• moving the rotor to a position wherein the opaque body disposed at the position of one of the cuvettes is aligned with the light source and the light detector so that light from the light source will be prevented from directly impinging on the light detector by the opaque body while allowing scattered light to strike the detector;
• flashing the light source while the rotor is positioned to prevent the light from directly impinging on the light detector while allowing scattered light to strike the detector;
• measuring a signal from the light detector while the light is prevented from directly impinging on the light detector by the rotor and while scattered light strikes the detector;
• moving the rotor to a position wherein one of the cuvettes holding the fluid is aligned with the light source and the light detector so that light from the light source can be directed through the fluid and onto the light detector;
• flashing the light source while the rotor is positioned to pass light through fluid in a cuvette;
• measuring a signal from the light detector while the light is directed through the fluid;
• and
• modifying the signal measured when the light was directed through the fluid based on the signal measured when the scattered light struck the detector.

A method for determining the concentration of a substance in a fluid sample in a photometric fluid analyzer having a light source, a sample rotor having a plurality of cuvettes for holding a quantity of the fluid to be analyzed, and a light detector disposed to detect light directed through the cuvettes, the method comprising:

• mixing a first quantity of the sample in an assay cuvette with a reagent selected to produce a chemical reaction between the reagent and the substance whose concentration is to be measured, the reaction producing a known reaction product;
• moving the rotor to a position in which light is directed from the light source through the sample in the assay cuvette;
• simultaneously measuring light transmitted through the sample in the assay cuvette at a first wavelength which is selectively absorbed by the reaction product and at a second wavelength which is not absorbed by the reaction product;
• storing a first value corresponding to the amount of light transmitted through the assay cuvette at the first wavelength;
• storing a second value corresponding to the amount of light transmitted through the assay cuvette at the second wavelength;
• moving the rotor to a position in which light is directed through a reference cuvette not containing the reaction product;
• simultaneously measuring light transmitted through the reference cuvette at the first wavelength and at the second wavelength;
• storing a third value corresponding to the amount of light transmitted through the reference cuvette at the first wavelength;
• storing a fourth value corresponding to the amount of light transmitted through the reference cuvette at the second wavelength;
• comparing the first and second values to produce a sample value;
• comparing the third and fourth values to produce a reference value;
• and
• comparing the sample and reference values to determine the concentration of the substance in the fluid sample.