Alternative to the dual-beam method for absorption measurement that uses a compensation flow cell to eliminate the sources of error introduced by the flow cell, the carrier medium and window contamination.
In the case of the simplest optical configuration for absorption measurement with the dual-beam method (Photometer), the flow cell through which the sample passes is inserted in the measurement beam, while the reference beam path is empty. Besides the (sought) absorption of light by the sample, passage of the light beam through the flow cell also causes reflection losses at the windows and possibly absorption by the carrier medium; the latter have to be compensated by means of a calibration correction.
Dual-beam method with compensation cell in the reference beam
Insertion of a compensation cell "KV" in the reference beam path eliminates these effects automatically. It has the same length as the flow cell and is filled with the solvent (water, acetone, etc.). Formation of the quotient with the dual-beam method compensates both reflections at the windows and solvent effects.
If there is a danger of cell window contamination, e.g.
because one substance crystallizes or finely dispersed particles are deposited, the method
described above is not sufficient. Any deposits on the windows will increase the
absorption, but it remains constant in the reference cell with its pure carrier medium.
The solution to this problem is to pass the same sample through the compensation cell so
that both beams will suffer equally from window contamination.
Differential measurement method
But in this case the path length of the compensation cell has to be shorter than that of the flow cell proper. The effective photometric path is the difference between the two path lengths.