Oxygen referencing is a technique that is used to ensure that toxic or regulated emissions measurements are presented in a standardised format.
Assume local regulations require that a process can only emit a maximum of say 75 ppm of NO. The first measurement of the emissions indicates that the oxygen reading is 6.5% and the NO reading is 80 ppm. Simply by adjusting the process controls to increase the oxygen level, a pass can be achieved. Effectively the sample is diluted by increasing the excess air. At an oxygen reading of 8% the NO reading would be diluted to 72ppm.
To overcome this problem regulatory authorities require that some reading are referenced to a pre-defined oxygen level. If the regulations require referencing to say 5% then in the above situation the reported NO reading would become:
NO ref = 80 x ((20.9-5.0)/(20.9-6.5)) = 98.6 ppm