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Dissolved O2
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| The PowerLab system can record dissolved oxygen concentrations, both in vivo and in vitro.
Dissolved oxygen (dO2 or pO2) is the amount of oxygen dissolved in water and is usually expressed in milligrams of oxygen per liter of water (mg/L) or parts per million (ppm). It can also be measured as a percentage of the saturation value i.e. the maximum dO2 that water can hold.
Dissolved oxygen can be measured using an oxygen meter. This type of measurement employs a sensor (probe) and a meter. Most sensors are electrochemical cells that have a positive (cathode) and negative (anode) electrode contained within a salt-bridge (i.e. hollow cylindrical body filled with an electrolyte solution). The end of the electrode is covered with a permeable membrane (usually Teflon or polyethylene) across which oxygen molecules can diffuse. The sensor generates an electrical current proportional to the oxygen concentration.
Electrochemical Cells
- Polargraphic or Clark sensors use gold or platinum as the cathode and silver as the anode. Oxygen is reduced within the sensor when a polarizing voltage is applied to the cathode.
These electrodes require a special meter to provide the polarizing voltage (usually between –0.7 V to –0.8 V) that causes a reduction of oxygen. This meter then measures the current flow and converts it into a signal that can be recorded using a PowerLab and LabChart. Most of the research work is done using with Clark electrodes. - Galvanic sensors use silver or platinum as the cathode and lead, iron or zinc as the anode. The reduction of oxygen in the presence of the sensor is spontaneous and no polarizing voltage is necessary. These electrodes have an internal 'shorting resistor' so that the current flow is converted to a voltage signal that can be directly connected to any PowerLab. This type of electrode is ideal for student use but is not recommended for research in which more accurate measurements are required.
Clark and galvanic oxygen electrodes depend on the diffusion of oxygen from the sample solution into the electrode. Consequently, their response times are slow (usually several seconds at best) and only slow sampling rates are required within the data acquisition system (4/s or slower). These slow sampling rates are available within the PowerLab and additional low-pass filters may be applied within LabChart to ensure a noise-free signal.
The meter and electrode should be purchased from the same manufacturer so that they will be matched for sensitivity. Ensure that the meter has an analog output (± 10 V) that is compatible with the PowerLab unit.
Micro-Oxygen Electrodes
ADInstrumnets offer two micro-oxygen elctrode configurations - The MLT1120 Micro-Oxygen Electrode has a 3 mm tip which is best suited to measuring oxygen concentration in small volume samples and is designed for applications that require a fast response time with minimal oxygen consumption. This electrode is supplied with a Membrane Housing Kit (MLT1121). An Analog Adapter (MLT1122) is required to connect the electrode directly to the PowerLab and this is supplied separately.
- The MLT1123 Micro-oxygen Electrode (Flow-Through) features low oxygen consumption, fast response (<20s) requiring less than a drop of solution. This miniature electrode are ideal for monitoring oxygen in a continuous flow such as water, blood, urine, physiological fluids. "T" fitting size is 1/16" (1.5mm). Requires the MLT1122 Analog Adapter to connect to a PowerLab. Each electrode is supplied with an MLT1124 Membrane Housing Kit consisting of six T-membranes and a bottle of electrolyte solution.
Other oxygen measurement systems (i.e. transcutaneous and fiber optic systems) are available from a number of manufacturers. Analog outputs from these devices may be connected to a PowerLab provided that the signal is ± 10 V. The signals may then be recorded, displayed and analyzed using LabChart software. |
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| PowerLab (MacLab) citations: |
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| Roles of oxidative stress and AT1 receptors in renal hemodynamics and oxygenation in the postclipped 2K,1C kidney |
| W J Welch, M Mendonca, S Aslam and C S Wilcox, Hypertension, 41, 692-696, 2003 |
| Renal pO2 in the outer and inner cortex were measured in rats (80 to 100g) by using an ultramicroelectrode. The platinum-iridium electrode was coated with an O2-permeable membrane and connected to an ultra-high-impedence picoammeter (pA6000; World Precision Instruments). The analog output was digitally converted and displayed on a computer with an acquisition system (MacLab 4A; AD Instruments). The electrode provides a rapid, stable, and linear output in response to pO2 from 5 to 500 mmHg. |
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| Primary in vivo oscillations of metabolism in the pancreas |
| P Bergsten, J Westerlund, P Liss and P-O Carlsson, Diabetes, 51, 699-703, 2002 |
| Islet pO2 was measured by impaling superficially located islets (from rats weighing 300–350 g) with a miniaturized Clark electrode. The electrodes were polarized at -0.8 V, which gave a linear response between the oxygen tension and the electrode current. The electrical current was measured by picoamperemeters (University of Aarhus, Aarhus, Denmark). The electrodes were calibrated in water saturated with Na2S2O5 or air at 37°C before and after the experiments. The drift of the microelectrode recordings was less than 0.5% per hour. The data were acquired at 4 Hz and processed by a MacLab Instrument (ADInstruments, Hastings, UK). |
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The material on this page is provided in good faith and believed accurate at the time of writing. No responsibility will be taken, or liability accepted, for damages arising from the use of information herein. Readers are urged to check with respective manufacturers the accuracy of all product related information.
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