Oxygen Analyzers

Electrochemical oxygen analyzers are based on electrochemical reduction of O₂ at a negatively polarized electrode. This principle lies in the bases of the Clark-type oxygen-sensitive electrode.  
 

pO₂ (CLARK) ELECTRODE

This electrode is known as "Clark Type" after their inventor, Dr. Leland Clark. The Clark electrode consists of an anode and cathode in contact with an electrolyte solution. It is covered at the tip by a semi-permeable membrane usually polypropylene membrane, which is permeable to gases but not contaminants and reducible ions of the sample (Figure 1). The cathode is in a glass envelope in the body of the electrode. The anode has a larger surface that provides stability and guards against drift due to concentration of the pO₂ electrolyte (usually potassium chloride, 0.1 M). This silver/ silver chloride (Ag/AgCl) anode provides electrons for the cathode reaction. The Clark (pO₂) electrode measures oxygen tension amperometrically. That is the pO₂ electrode produces a current, at a constant polarizing voltage (usually -0.6 V vs. Ag/AgCl) which is directly proportional to the partial pressure of oxygen (pO₂) diffusing to the reactive surface of the electrode. Silver at the anode becomes oxidized.  
 

Reduction of oxygen occurs at the surface cathode which is exposed at the tip of the electrode. Oxygen molecules diffuse through the semi-permeable membrane and combine with the KCl electrolyte solution. The current produced is a result of the following reduction of oxygen at the cathode.

Production of four electrons accompanies each molecule reduced. The pO₂ channel measures this flow of electrons and the resulting microvoltage is displayed as pO₂. Therefore, pO₂ is measured amperometrically; the pO₂ electrode produces a current at a constant polarizing voltage (0.6 V) which is directly proportional to the partial pressure of oxygen diffusing to the reactive surface of the electrode.  
 

Pauling Oxygen Analyzer. The first commercial sample of such analyzers was manufactured by the Beckman Instruments in the beginning of World War II (Figure 2). The military needed an instrument for measuring the amount of oxygen in a sample of mixed gases; this device was needed on submarines and high-flying aircraft to ensure the safety of the servicemen. Linus Pauling contracted with the government to design and produce one in 1940. Pauling’s assistant, Holmes Sturdivant, came to Beckman to ask him to build cases for the one hundred instruments they were manufacturing. Beckman agreed, but soon after the Caltech faculty came back and asked Beckman to manufacture the instruments in their entirety. Apparently they had underestimated the difficulty of mass-producing highly accurate instruments. In March 1942, Beckman agreed to manufacture the Pauling Oxygen Analyzer.  

Leland C. Clark 
 

Leland C. Clark received his Ph.D. in biochemistry and physiology at the University of Rochester School of Medicine. Dr. Clark, one of the century's most prolific biomedical inventors and researchers, is recognized for pioneering several medical milestones credited with saving thousands of lives and advancing the technology of modern medicine. His research accomplishments include the development of the first successful heart-lung machine, the advancement of technology leading to the development of one of the first intensive care units in the world, and pioneering research in biomedical applications of perfluorocarbons and biosensors. He has published more than 400 scientific papers in biomedicine and has generated numerous US and foreign patents, mainly in the field of medical instrumentation and fluorocarbons. He is the recipient of numerous honors and awards including induction into the National Academy of Engineering and the Engineering and Science Hall of Fame.

It is generally agreed that biosensor history started in 19621 and that the progenitor of the biosensor was the American scientist Leland C. Clark. Clark had studied the electrochemistry of oxygen gas reduction at platinum (Pt) metal electrodes, pioneering the use of the later as an oxygen- (and therefore chemi-) sensor. In fact, Pt electrodes used to detect oxygen electrochemically are often referred to generically as "Clark electrodes".

These electrodes have a thin organic membrane covering a layer of electrolyte and two metallic electrodes. Oxygen diffuses through the membrane and is electrochemically reduced at the cathode. There is a carefully fixed voltage between the cathode and an anode so that only oxygen is reduced. The greater the oxygen partial pressure, the more oxygen diffuses through the membrane in a given time. This results in a current that is proportional to the oxygen in the sample. Temperature sensors built into the probe on some advanced measurement systems allow compensation for the membrane and sample temperatures, which affect diffusion speed and solubility. The meter uses cathode current, sample temperature, membrane temperature, barometric pressure and salinity information to calculate the dissolved oxygen content of the sample in either concentration (ppm) or percent saturation t% Sat). The voltage for the reduction can either be supplied electronically by the meter (potentiometric oxygen electrode) or dissimilar metals may be used for the two electrodes, picked so that the correct voltage is generated between them (galvanic electrode).  
 

Clark had the ingenious idea of placing very close to the surface of the platinum electrode (by trapping it physically against the electrode with a piece of dialysis membrane) an enzyme that reacted with oxygen. He reasoned that he could follow the activity of the enzyme by following the changes in the oxygen concentration around it, thus a chemosensor became a biosensor. Based on this experience and addressing his desire to expand the range of analytes that could be measured in the body, he made a landmark address in 1962 at a New York Academy of Sciences symposium in which he described how "to make electrochemical sensors (pH, polarographic, potentiometric or conductometric) more intelligent" by adding "enzyme transducers as membrane enclosed sandwiches". The concept was illustrated by an experiment in which glucose oxidase was entrapped at a Clark oxygen electrode using dialysis membrane. The decrease in measured oxygen concentration was proportional to glucose concentration. In the published paper (Clark, L.C. Jnr. Ann. NY Acad. Sci. 102, 29-45, 1962), Clark and Lyons coined the term enzyme electrode. Clark's ideas became commercial reality in 1975 with the successful re-launch (first launch 1973) of the Yellow Springs Instrument Company (Ohio) glucose analyser based on the amperometric detection of hydrogen peroxide. This was the first of many biosensor-based laboratory analysers to be built by companies around the world.