The reading shown by Sigrist fluorescence instruments is based on a quinine sulfate calibration in the 0..10 ppm range. The associated measurement unit is referred to within the Sigrist organization as FLU (Fluorescence Unit).
For certain applications, it is desirable to display directly the concentration of the substance being measured. Once the respective product is known, there is no reason why the instrument cannot be calibrated to do so. By way of illustration, the following Figure shows the fluorescence of a crude oil as a function of concentration.
The range of interest for oil trace detection is located in the linear portion of the curve (concentrations up to about 100 mg/l). The shape of the above curve is typical for many petroleum products, but the absolute figures differ sharply from case to case.
An important practical aspect of the calibration procedure is the mixing of the insoluble oil with water. What is needed is a reproducible, homogeneous suspension of finely dispersed oil droplets. The method used by Sigrist consists of dissolving the oil in a p quantity of acetone and then adding the solution to water. The acetone mixes completely with the water, leaving the required suspension of very fine oil droplets.