Non-Severinghaus Potentiometric Dissolved CO₂ Sensor with Improved Characteristics

Abstract

A new type of carbon dioxide sensor comprising a pH glass electrode measured against a carbonate-selective membrane electrode based on a tweezer type carbonate ionophore is presented here for the first time. No cumbersome liquid junction based reference element is utilized in this measurement. The sensor shows an expected nernstian divalent response slope to dissolved CO(2) over a wide range covering the routine environmental and physiological PCO(2) levels. Unlike the conventional Severinghaus CO(2) probe for which the response is substantially delayed to up to 10 min due to diffusion of carbon dioxide into the internal compartment, the ion-selective CO(2) sensor proposed here shows a response time (t(95%)) of 5 s. When used together with a traditional reference electrode, the sensor system is confirmed to also monitor sample pH and carbonate along with carbon dioxide. A selectivity analysis suggests that Cl(-) does not interfere even at high concentrations, allowing one to explore this type of sensor probe for use in seawater or undiluted blood samples. The CO(2) probe has been used in an aquarium to monitor the CO(2) levels caused by the diurnal cycles caused by the metabolism of the aquatic plants and shows stable and reproducible results.

Extruded colour-based plastic film for the measurement of dissolved CO2

“…These response and recovery times are obviously completely inadequate for the monitoring of rapid changes in dissolved CO2, as required for blood gas monitoring for example, where the %CO2 is typically 5%. A Severinghaus electrode for example is often able to respond in a minute, although this can be considerably longer if it possesses a large volume internal bulk electrolyte, or if the concentrations of CO2 are low [22,42,43]. For example, Xi and Bakker [22] report that, for a shift in CO2 saturation from 0.04% to 0.66%, the Severinghaus probe takes at least 5 min to stabilise to its new potential.…”

“…A Severinghaus electrode for example is often able to respond in a minute, although this can be considerably longer if it possesses a large volume internal bulk electrolyte, or if the concentrations of CO2 are low [22,42,43]. For example, Xi and Bakker [22] report that, for a shift in CO2 saturation from 0.04% to 0.66%, the Severinghaus probe takes at least 5 min to stabilise to its new potential. Interestingly, this is the probe's 'response time', as the concentration of CO2 has been increased, and it follows, from the work of others [42,43], that the associated recovery time for the probe will be longer, possibly by a factor of 4, which implies a recovery time of ca.…”

“…20 min. Note that the CO2 saturation values used by Xi and Bakker [22] (0.04% to 0.66%) in their work on a Severinghaus electrode are not too dissimilar to those used here for the TB film (0.1% to 1%) in Fig. 11, which suggests that the 47.5 μm, TB plastic film has a similar response time, and probable recovery time, very similar to that of the Severinghaus electrode used by Xi and Bakker [22], when responding to these very low levels of CO2.…”

“…Obviously, any methods that require reagents, regular sample taking and/or pumps are inappropriate for long-term, in-situ deployment, and under such circumstances solid state sensors, such as optodes [20] and potentiometric devices [6,15,16,21,22] appear more appropriate.…”

Trends in Electrochemical Sensing of Blood Gases

“…An interesting novel approach to carbon dioxide sensing was reported by Xie and Bakker [46]. This concept is based on the measurement of the potential difference between a pH glass electrode and a carbonate-selective electrode.…”