Oxygen Analyzers

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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 pO 2 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 ofthe 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 pO 2. 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.


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Figure 1: A Clark-type oxygen-sensitive 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. Paulings assistant,

Holmes Sturdivant, came to Beckman to ask him to build cases for the one hundred instrumentsthey 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

The feasibility of biosensing was first demonstrated by Leland Clark in the mid-1960s,

when he measured glucose concentration in solution using what has since become known as

the Clark oxygen electrode. Since 1991, Clark has headed the R&D branch of Synthetic

Blood International (SBI) in Kettering, OH, focusing on the development of artificial blood

and the commercialization of an implantable glucose monitor the Holy Grail of the sensor

industry. However, for electrochemists and scientists involved in the biosensor R&D,

Leland Clark is the best known for his Clark type oxygen electrode.

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 inventorsand researchers, is recognized for pioneering several medical milestones credited with saving

thousands of lives and advancing the technology of modern medicine. His research


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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 themembrane 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).

This is a polarographic electrode used for measuring the

concentration of oxygen in liquid medium (e.g. blood) and gases.

The sample is brought into contact with a membrane (usually

polypropylene or Teflon) through which oxygen diffuses into a

measurement chamber containing potassium chloride solution. In the

chamber are two electrodes; one is a reference silver/silver chloride

electrode and the other is a platinum electrode coated with glass to

expose only a tiny area of platinum (e.g. 20 m diameter). The

electric current flow between the two electrodes when polarized with

a potential of -600 mV (vs. Ag/AgCl) determines the oxygenconcentration in the solution. Originally developed for measuring

oxygen gas, it is only a matter of polarity, whether the electrode

senses hydrogen or oxygen gas. For hydrogen measurements +600

mV (vs. Ag/AgCl) are supplied. The reaction is very sensitive to

temperature and to maintain a linear relationship between the oxygen

(or hydrogen) concentration and the current measured the electrode

temperature must be controlled within 0.1 oC. The electrode is

calibrated using two gas mixtures of known oxygen (or hydrogen)

concentration. Such oxygen sensitive electrodes are used in the blood

gas analyser in the clinical chemistry laboratory or in intensive care

areas.


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The Clark-type electrode consists of a Pt- (A) and a reference

Ag/AgCl-electrode (B) covered by a film of half-saturated KCl

electrolyte (C) enclosed within a Teflon membrane (D) which is held

in place by a rubber ring (E). The voltage supply (F) and the

electronic instrument for the measurements of the current output is

shown (G).

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 glucoseoxidase 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.

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Oxygen Analyzers