A standard method for creating such as an environment is to dissolve excess sodium sulfite (Na2So3) and a trace of cobalt chloride (CoCI2) in water. Preferably, the water should come from the sample to be measured. These chemicals will remove all oxygen from the sample (see Standard Methods for the Examination of Water&Wastewater,method 4500-OG, 19th edition). Alternatively, you may place the probe in 100% nitrogen gas.