Abstract: An apparatus for determining a parameter of substance includes a cell defining an inner space, an inlet associated with the cell for introducing a sample of the substance to the inner space, an outlet associated with the cell for removing the sample from the inner space, an optic couple for passing NIR radiation through the sample in the inner space along an optic path so as to provide an NIR spectrum indicative of a parameter of the sample, the optic couple defining the optic path, and structure for directing flow in the inner space, whereby vortices within the inner space are inhibited and the NIR spectrum is rapidly provided upon introduction of the sample to the inner space.