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.
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).
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.
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.