An approach to compensate for both refractive index and turbidity effects is described. A light emitting diode-based dual-wavelength, double-beam (fully referenced), dual-flow-cell photometric detection system is used. The only requirement is that the analyte yields a product (that can be detected by absorbance measurement) as a result of a reaction which does not affect the optical properties of the suspended matter. The principle is mathematically established and experimentally demonstrated by determining micromolar concentrations of bromthymol blue in samples containing up to 1.5 % whole milk and 60% ethanol with an absolute error of <3.7%.

Figure 3. Dual wavelength LED-based photometric detection system, shown schematically. See text for a detailed description.