Oxygen Concentration

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 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.

Both 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). The meter and electrode should be purchased from the same manufacturer so that they will be matched for sensitivity. It should be ensured that the meter has an analog output (± 10 V) that is compatible with the PowerLab unit.

LabChart
LabChart software (for Windows and Macintosh) combines the familiar simplicity of a traditional strip chart recorder with the powerful analysis features of a digital acquisition system. LabChart software and a PowerLab data acquisition unit provide data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Acquisition and analysis capabilities can be further increased with LabChart Extensions and LabChart Modules. LabChart Modules are available as part of LabChart Pro and LabChart Extensions are free for download from the website for existing LabChart users.


GLP and 21 CFR Part 11
For those researchers working within a laboratory requiring GLP and 21 CFR Part 11 compliance the GLP Client and GLP Server are available for use with LabChart (Windows only) and PowerLab data acquisition systems. For more information, visit the Good Laboratory Practice application page or contact your nearest ADInstruments representative.

PowerLab Data Acquisition Systems
The PowerLab are high-performance data acquisition units suitable for a wide range of research applications. Typical applications include human and animal physiology, pharmacology, neurophysiology, biology, zoology, biochemistry, and biomedical engineering. Units are capable of recording at speeds of up to 400 000 samples per second continuously to disk (aggregate), and are compatible with instruments, signal conditioners and transducers supplied by ADInstruments, as well as many other brands. In addition to standard single-ended BNC inputs, 4 differential Pod ports are also available for direct connection of Pod signal conditioners and appropriate transducers. These systems include:

• ML866 PowerLab 4/30 - 4 Channels
  
• ML870 PowerLab 8/30 – 8 Channels
• ML880 PowerLab 16/30 – 16 Channels

Micro-Oxygen Electrodes
ADInstruments offer two micro-oxygen electrode configurations

• MLT1120 Micro-Oxygen Electrode

• MLT1123 Micro-oxygen Electrode (Flow-Through)

Galvanic Oxygen Electrodes
These electrodes are designed for measuring oxygen concentration in aqueous solutions. The output — in air-saturated deionized water — is usually between 20 - 35mV at 25ºC with oxygen consumption of 3.45 x 10-13 mol O₂/s per mV of Signal.

• MLT1115 Galvanic Oxygen Electrode

• MLT1115/D Galvanic Oxygen Electrode (DIN)

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.

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, 692-696, 2003

Rats (80 to 100 g) were used………renal pO2 in the outer and inner cortex were measured in animals 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 mm Hg.

Primary in vivo oscillations of metabolism in the pancreas

P Bergsten, J Westerlund, P Liss and P-O Carlsson, Diabetes, 699-703, 2002

Rrats weighing 300–350 g were anesthetized with an intraperitoneal injection of thiobutabarbital sodium (Inactin; Research Biochemicals International, Natick, MA; 120 mg/kg body wt), placed on a heated operating table maintained at body temperature (38°C), heparinized, and tracheostomized. Polyethylene catheters were inserted into the right femoral artery and the right femoral vein, respectively, to monitor the mean arterial blood pressure with a Statham P 23dB pressure transducer (Statham Laboratories, Los Angeles, CA)………the pancreas of each animal was exposed through a midline abdominal incision and a catheter was placed in the portal vein by inserting a sharpened cannula, connected to the catheter, through the vascular wall and in the same direction as the blood flow. The cannula was loosely placed and no ligatures were used that could affect portal blood flow………The pancreas was immobilized and superficial islets were visualized in situ by intravenous injection of 0.8-ml sterile-filtered 2% (wt/vol) neutral red (Kebo Grave AB, Stockholm, Sweden) (22). The staining procedure made it possible to insert one oxygen tension electrode into a superficially located islet and one into the adjacent exocrine parenchyma by the use of micromanipulators under a stereo microscope. The electrodes were placed 10 mm apart. The injection with neutral red does not affect blood concentrations of glucose or insulin, blood flow of the whole pancreas or islet, or pancreatic oxygen tension measurements. The oxygen tension The oxygen tension electrodes were modified Clark microelectrodes, with an outer tip diameter of 2–6 m and an inner tip diameter of 1–2 m. 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 (AD Instruments, Hastings, UK).