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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.
[pic]
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.
[pic]
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.


|[pic] |
|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. 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

| |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 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).


|[pic] |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 oxygen concentration 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. |
| |[pic] |
| |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
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.