Method for measuring absorption that uses alternate detection of the internal transmission density at two different wavelengths to eliminate sources of error.
The Sigrist instrument range incorporates several features designed to eliminate the sources of error inherent in the measurement of absorption in continuous flow: the differential measurement method compensates the effects of reflection losses at the flow cell windows, of the absorption of the solvent, and of window contamination; the free-fall flow cell solves the problem by eliminating all contact of the liquid with windows. The dichromatic method goes even further by compensating changes in the measurement liquid itself. For example, changing turbidity, air bubbles and other absorbing substances will have no effect on the instrument's reading.
This is achieved by measuring the same sample alternately with light of two different
wavelengths. The "measurement wavelength" must be at a point within the spectrum over which the sought substance absorbs, and the
"reference wavelength" must be at a point at which it does not absorb. In the
example shown in the following Figure, the water's color is measured at 390 nm while the
reference measurement takes place at 650 nm. The first wavelength detects the sum of the
absorptions (Total) of the color and the turbidity,
the second that of the turbidity alone (30 ppm Sio2). An electronic evaluation
unit calculates the coloration (50 HAZEN) by subtraction. The
turbidity value can also be stated as second measurement result.