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Non-invasive Online CO2 Measurement with Chemical Optical CO2 Sensors
Chemical optical CO2 sensors provide a versatile tool for online monitoring of dissolved CO2 in various scale and designs
Athanas Apostolidis
PreSens Precision Sensing GmbH, 93053 Regensburg, Germany
Accurate CO2 measurement in the liquid phase has been a challenge for decades. Although off-gas measurement by means of IR-spectroscopy is a proven technology, the correlation of CO2-levels in gas and liquid phase in biological systems is difficult.
The pCO2 mini transmitter is a precise fiber optic CO2 transmitter. It is used with CO2 sensors based on a 2 mm optical fiber. A PC is connected to run the easy-to-use software. pCO2 mini is additionally equipped with an analogue out and a trigger input. The device is also equipped with a PT 1000 temperature sensor for online temperature measurement and compensation of the measured signal.®
The pCO2 view software is used for controlling the operation of the pCO2 mini transmitter. The data sent from the device are shown in digital display and graphically in a trace chart. The software includes menus for customer specific calibration of sensors as well as the possibility to operate with predefined calibration values. Data can be stored any time within a measurement and exported to MS Excel or plain ASCII-files. The upload of stored measurements is also possible for later evaluation.
CO2 Sensor Performance
The CO2 sensors are optimized for physiological solutions. They are calibrated with gas mixtures of defined CO2 concentration levels and delivered with a final inspection protocol. The customer can use either the factory data for calibration of the pCO2 mini transmitter or perform his / her own calibration. The dynamic range of the CO2 sensor is 1 % to 25 % CO2 in order to fit to typical CO2 levels within cell culture or other biotechnology applications. Sensors can be delivered beta-irradiated or untreated. Autoclavable sensors are also available.
Measurement Principle
The chemical optical carbon dioxide sensor is based on our patented DLR measurement principle[1]. The light of the blue LED excites the sensor to emit luminescence. The luminescence lifetime measured is a superposition of the signals of an analyte sensitive indicator and an inert reference indicator, where both indicators exhibit very different luminescence lifetimes and the luminescence of the CO2 sensitive indicator can be suppressed by CO2 (see (2) in Fig. 4). The measurement signal correlates to the partial pressure of carbon dioxide. This internal referencing compensates intensity fluctuation caused e. g. by changes in excitation intensity.
PreSens CO2 sensors as an alternative to off-line sampling[2]
PreSens CO2 sensors were evaluated for monitoring the cultivation cells in a 3-L bioreactor. As shown in Fig. 5, the sensors show excellent long-term stability over a period of 10 days during cell cultivation of CHO cells with an initial seed density of 0.4 x 106 cells/ml. The readings of the PreSens CO2 sensor were also in good correlation to off-line CO2 measurements. Additionally, the PreSens CO2 sensor offers the possibility to continously monitor the CO2 concentration between two off-line measurements.
References
[1] Patent DLR Nr. EP 1000345 (B1) and Nr. US 6602716 (B1)
[2] Baggio, et al. Physical-Chemical Sensors: Application to Bioreactor Monitoring and Control. ESACT 2009, Dublin, Ireland