Method and apparatus for determining optical density of a solution

An instrument for determining optical density of a solution, the instrument comprising:

• a flow cell having a solution inlet and a solution outlet, the flow cell comprising at least three fixed light paths, each light path being arranged such that a direction of flow of a solution from the solution inlet towards the solution outlet passes through each light path,
• each light path having a respective predetermined invariable path length, l, and
• a light source arranged to irradiate the at least three light paths,
• a detector located at an opposite side of the light paths from the light source, the detector arranged to detect electromagnetic radiation going through the light paths from the light source,
• a data processing means for determining optical density of the solution, wherein the data processing means is arranged to calculate the absorbance, A, of the solution at the at least three light paths, and to calculate a slope, αc, for each pair of light paths by dividing a difference in absorbance reading, ΔA, with a difference in path length, Δl, wherein Δl is a difference between each respective predetermined path length, l, of the at least three fixed light paths, and comparing the calculated slopes, αc, and
• a) if the calculated slopes, αc, are the same, the slope is used for determining optical density of the solution, or
• b) if the calculated slopes, ac, are not the same, the steepest slope of the calculated slopes is used for determining 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 optical density of the solution.