Zero calibration

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Zero calibration


Zero calibration is together with air calibration one of the two steps of the OroboPOS calibration. It is performed in the closed chamber after all the oxygen has been removed by the addition of dithionite or by respiration of imt or cells. Any incubation medium can be used for zero calibration with dithionite or sample. Unlike air calibration it is not necessary to perform a zero calibration each day.

Abbreviation: R0

Reference: MiPNet06.03 POS-calibration-SOP, O2 calibration - DatLab

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A zero calibration should be performed
A zero calibration has to be performed
  • When measurements are performed at low oxygen (<10 µM).
  • Bracketing zero calibrations are required when measurements are performed near the limit of detection at oxygen concentrations close to 5 nM (0.005 µM).
  • When a new or serviced OroboPOS is used for the first time.
  • When a new membrane was applied to the sensor.
  • When the gain is changed.
  • When a new OroboPOS-Connector is used.

Incubation medium for zero O2 calibration with dithionite

Any incubation medium can be used for zero calibration with dithionite. MiR05 or a highly concentrated phosphate buffer are needed for measurement of instrumental background oxygen flux when using dithionite, to avoid secondary oxygen consuming reactions after a step titration of dithionite. For zero calibration, however, an excess dithionite concentration is applied which consumes all dissolved oxygen in the medium, and secondary reactions are not an issue.

Zero current

Each polarographic oxygen sensor has a zero current, i.e., a current [µA] that flows even at an oxygen concentration of zero, which is converted into a voltage [V] as the raw signal. This zero oxygen signal is taken into account by the zero calibration and can be expressed as % of the raw signal at air saturation. New OroboPOS have a zero signal below 2 % of the signal at air saturation. For accurate calibration, the zero signal must be stable, which may be the case for zero currents up to c. 5 % of the signal at air saturation. The stability of the zero current can be assessed by keeping track of calibration values using the "O2k-Calibration-list.xls" template.
A high or unstable zero current is caused either by the OroboPOS, by the OroboPOS-Connector or by a bad connection between those two parts. Cleaning and drying OroboPOS-Connector electrical connections frequently solves the problem. Otherwise, the problem has to be localized by switching components.
If the problem is located on the POS head, an intensive sensor service should be applied at least two times. If this does not help the sensor may have to be replaced.
If the problem is located at the POS connector cleaning the connection as described above should be tried again. If the problem was decreased but not solved by cleaning and applying contact oil, the cleaning process may have to be repeated several times. If cleaning the connector does not help the POS connector may have to be replaced.


» MiPNet06.03 POS-calibration-SOP
» Previous version: MiPNet19.18D O2k-Series G and DatLab 6: Calibration

Keywords: Oxygen signal


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