198 OXYGEN ANALYZERS

15.Eldridge JDI, Learmonth ID. Component bone interface in cementless hip arthroplasty. In: Learmonth ID, editor. Interfaces in Total Hip Arthroplasty. London: Springer; 1999: 71–80.

16.Sychterz CJ, Claus AM, Eng CA. What we have learned about cementless fixation from long-term autopsy retrievals. Clin Orthop Related Res 2002;405:79–91.

17.Søballe K. Hydroxyapatite ceramic coating for bone-implant fixation. Mechanical and histological studies in dogs. Acta Orthop Scand 65: (Suppl. 255).

18.Prendergast PJ, Huiskes R, Søballe K. Biophysical stimuli on cells during tissue differentiation at implant interfaces. J Biomechan 1997;30:539–548.

19.Lennon AB, McCormack BAO, Prendergast PJ. The relationship between cement fatigue damage and implant surface finish in proximal femoral prostheses. Med Eng Phys 2003; 25:833–841.

20.Dalstra M Biomechanical aspects of the pelvic bone and design criteria for acetablar prostheses. Ph. D. Thesis, University of Nijmegen, 1993.

21.Lee AJC. Rough or polished surface on femoral anchorage stems. In: Buchhorn GH, Willert HG, editors. Technical Principles, Design and Safety of Joint Implants. Seattle: Hogrefe & Huber Publishers; 1994:209–211.

22.Huiskes R. New approaches to cemented hip-prosthetic design. In: Buchhorn GH, Willert HG, editors. Technical Principles, Design and Safety of Joint Implants. Seattle: Hogrefe & Huber Publishers; 1994:227–236.

23.Robinson RP. The early innovators of today’s resurfacing condylar knees. J Arthroplasty 2005;20 (suppl. 1):2–26.

24.Walker PS. Biomechanics of total knee replacement designs. In: Mow VC, Huiskes R, editors. Basic Orthopaedic Biomechanics and Mechanobiology. Philadelphia: Lippincott Williams & Wilkins; 2005:657–702.

25.Lacroix D, Murphy LA, Prendergast PJ. Three-dimensional finite element analysis of glenoid replacement prostheses: A comparison of keeled and pegged anchorage systems. J

Biomech Eng 2000;123:430–436.

26. Szpalski M, Gunzburg R, Mayer M. Spine arthroplasty: A historical review. European Spine J 2002;11 (suppl. 2): S65–S84.

27.Harris WH. Options for the primary femoral fixation in total hip arthroplasty—cemented stems for all. Clin Orthop Related Res 1997;344:118–123.

28.Chang PB, Mann KA, Bartel DL. Cemented femoral stem performance—effects of proximal bonding, geometry, and neck length. Clin Orthop Related Res 1998;355:57–69.

29.Taylor M, Barrett DS. Explicit finite element simulation of eccentric loading in total knee replacement. Clin Orthop Related Res 2003;414:162–171.

30.Stolk J, Maher SA, Verdonschot N, Prendergast PJ, Huiskes R. Can finite element models detect clinically inferior cemented hip implants? Clin Orthop Related Res 2003;409:138–160.

31.Britton JR, Prendergast PJ. Pre-clinical testing of femoral hip components: an experimental investigation with four prostheses. J Biomechan Eng. In press.

32.Viceconti M, Davinelli M, Taddei F, Capello A. Automatic generation of accurate subject-specific bone finite element models to be used in clinical studies. J Biomechanics 2004; 37:1597–1605.

33.DiGioia AM, Blendea S, Jaramaz B. Computer-assisted orthopaedic surgery: minimally invasive hip and knee reconstruction. Orthop Clin North Am 2004;35:183–190.

See also BIOCOMPATIBILITY OF MATERIALS; BONE AND TEETH, PROPERTIES OF; BONE CEMENT, ACRYLIC; HIP JOINTS, ARTIFICIAL; MATERIALS AND DESIGN FOR ORTHOPEDIC DEVICES.

ORTHOTICS. See REHABILITATION, ORTHOTICS IN.

OSTEOPOROSIS. See BONE DENSITY MEASUREMENT.

OVULATION, DETECTION OF. See CONTRACEPTIVE

DEVICES.

OXYGEN ANALYZERS

SUSAN MCGRATH

SUZANNE WENDELKEN

Dartmouth College

Hanover, New Hampshire

INTRODUCTION

Oxygen is essential for all aerobic life on Earth. It is the most abundant element as it comprises a little more than one-fifth of the weight of air, nine-tenths of the weight of water, and almost one-half of the weight of the earth’s crust (1).

Because of its role in supporting and sustaining life, it is often important to monitor the level of oxygen in the atmosphere. Too much oxygen can lead to a toxic atmosphere where as too little oxygen causes asphyxia and eventually death. A relatively constant level of oxygen is required for most aerobic processes.

Oxygen gas monitoring is used for a number of purposes: (1) Medical: anesthesia (drug delivery, airway monitoring), respiratory oxygen content monitoring (inhaled and exhaled), controlled environments, incubators. (2) Physiological: exercise (rate of oxygen consumption), aircraft, spacecraft, scuba diving, fire fighting, mountain climbing, spelunking. (3) Biological: metabolism (oxygen uptake and consumption), fermentation, beverage and food packing. (4) Industrial: combustion control, fuel and pollution management, safe operation of chemical plants, monitoring gas purity.

This article gives an overview of the analyzers used to measure gaseous oxygen in medicine, physiology, and biology. Measurement of dissolved or bound oxygen is also important in medicine and is discussed in detail elsewhere in this Encyclopedia.

History and Relevance

Oxygen was not known to exist until the 1770s when it was discovered by French scientist Antoine Lavoisier and English clergyman and scientist Joseph Priestly through experiments on combustion. Previously, air was considered to be an element composed of a single substance. Combustible materials were thought to have a substance called phlogiston, from the Greek word meaning to be set on fire, which escaped as a material was burned. Lavoisier, however, believed that combustion resulted from a combination of fuel and air. He conducted experiments where he burned a candle in a sealed jar and observed that only one-fifth of the air was consumed. He named this unconsumed portion of the air oxygen from the Greek word

meaning acid producing. Although his thoughts about oxygen being the corrosive agent in acidic compounds was wrong, the name stuck and the study of oxygen was born (2).

Oxygen is essential for most life on Earth as it plays a key role in aerobic metabolism as a final electron acceptor due to its high electron affinity. Metabolic rate can be indirectly measured by monitoring oxygen consumption as >95% of energy is produced by reactions of oxygen with other food (3). This method is called indirect calorimetry and is a much more cost effective and timely method for measuring metabolic rate as compared to direct calorimetry (the direct measure of heat energy produced).

Oxygen availability is a function of its partial pressure and the total pressure of the gas mixture in which it resides. At sea level, the partial pressure of oxygen is roughly 21%. With decreasing atmospheric pressure, as accompanies increasing altitude, the total amount of available oxygen decreases (Table 1). For example, at 18,000 ft. (5.48 km) above sea level, although the partial pressure of oxygen is still 21%, there is roughly half the amount of available oxygen. At 29,000 ft. (8.33 km) above sea level on the top of Mt. Everest, there is less than a third the amount of total available oxygen compared to sea level. At such altitudes, most humans require the use of supplemental oxygen. In addition, the body will compensate for the reduced oxygen availability by increasing the heart and respiration rate to keep up with the metabolic demands (3). A climber’s resting heart rate at this altitude is double to triple their normal resting heart rate. Long-term exposure to high altitude prompts the body to produce more red blood cells per unit blood volume thus increasing the number of oxygen carriers and making respiration easier. If the body does not properly adapt to such conditions, altitude sickness, pulmonary and cerebral edema, and potentially death may result (3).

Table 1. Atmospheric Pressure, the Fraction of Available Oxygen Compared to Sea Level, and Temperature All Decrease with Increasing Altitudea.

aSee Ref. 4.

The partial pressure of oxygen remains a fairly constant 21% until very high altitudes [i.e., >50,000 ft. (15.24 km) (5)]. At these altitudes it is necessary to maintain a pressurized, enclosed environment, such as aircraft, spacecraft, or space suite, in order to sustain human life.

Oxygen availability can be decreased by displacement by other gases, such as nitrogen, carbon dioxide, methane, and anesthetics. Oxygen availability is also easily decreased by combustion and oxidation processes. Thus it is necessary to monitor the atmospheric oxygen level in situations where these gases or combustion is present such as in enclosed environments, closed breathing circuits, and fire fighting.

Each year 20 deaths occur as a result of asphyxiation due to displacement of oxygen by another gas in air (6). Accidental asphyxia usually occurs in industry as a result of oxygen depletion by carbon dioxide, CO2, methane (CH4), or a hydrocarbon gas in a confined space, such as a tunnel, laboratory, sewer, mine, grain silo, storage tank, or well

(7). For example, in 1992, a barge operator in Alaska died from asphyxiation and a rescuer lost consciousness during rescue efforts due to a low level of oxygen (6%) in a confined space (8). In anesthesia, accidental asphyxia has resulted from incorrectly connected gas delivery tubes (9).

In choosing an oxygen analyzer for a particular need, it is important to be acquainted with the properties of operation, characteristics, and limitations of these devices. The primary methods for oxygen detection are based on the paramagnetic susceptibility, electrochemical properties, and light absorption properties of oxygen.

PARAMAGNETIC OXYGEN ANALYZERS

All mater exhibits some form of magnetism when placed in a magnetic field. Magnetic susceptibility is the measure of the strength of a material’s magnetic field when placed in an external magnetic field. Diamagnetic substances, such as gold and water, align perpendicularly to an external magnetic field causing them to be repelled slightly. This property arises from the orbital motion of electrons that produces a small magnetic moment. In substances with paired valence electrons, these moments cancel out. However, when an external magnetic field is applied it interferes with the motion of the electrons causing the atoms to internally oppose the field and be slightly repelled by it. Diamagnetism is a property of all materials, but is very weak and disappears as soon as the external magnetic field is withdrawn. In materials with unpaired valence electrons (e.g., nickel and iron) an external magnetic field aligns the small magnetic moments in the direction of the field, which increases the magnetic flux density. Materials with this behavior are weakly attracted to magnetic fields and are classified as paramagnetic. Ferromagnetism is a special case of paramagnetism where materials (e.g., iron and cobalt) are strongly attracted to magnetic fields. Paramagnetic materials have a high susceptibility (10).

Oxygen has a relatively high susceptibility when compared to other gases (see Table 2). This property is the key principle behind paramagnetic oxygen analyzers.

200 OXYGEN ANALYZERS

Table 2. Relative magnetic susceptibility values on a scale

The three main types of paramagnetic oxygen analyzers are (1) thermomagnetic (magnetic wind); (2) magnetodynamic (dumbbell or autobalance); (3) magnetopneumatic (differential pressure).

Paramagnetic analyzers are typically used for monitoring the quality of breathing air in open and enclosed environment, biological laboratory measurements, and in industrial combustion analysis (2,14).

Limitations of Paramagnetic Analyzers. Because the magnetic susceptibility of oxygen depends on temperature, it is necessary to operate at a constant temperature or to have some temperature compensation ability (1,15). The output of the sensor is also proportional to the absolute atmospheric pressure and thus pressure compensation is sometimes necessary (1,15).

Paramagnetic devices are typically delicate instruments with moving parts and are thus adversely influenced by vibrations. They generally are not used as portable devices (13).

Paramagnetic sensors work well for percent oxygen measurement, but are not recommended for trace oxygen measurements. In addition, these sensors should not be used when interference effects cannot be compensated for (i.e., sample gas containing other paramagnetic or diamagnetic gases, or varying background gas composition) (14). The effects of background gases used in anesthesia are small but not always negligible. These effects are summarized in Table 3.

Thermomagnetic (Magnetic Wind)

Thermomagnetic analyzers are based on the fact that magnetic susceptibility decreases inversely with the square of temperature.

Principles of Operation. A schematic diagram of a thermomagnetic analyzer can be seen in Fig. 1. A gas sample is admitted into the inlet that branches into equal

Table 3. Errors in Paramagnetic Analyzer measurements due to anesthesia gasesa.

aSee Ref. 15.

segments and converges again at the outlet. These tubes are connected by another tube halfway between the inlet and outlet. This cross-tube is heated by a platinum coil that is separated in the center by a thermal resistance bridge. These two heater coil segments form two arms of a Wheatstone bridge with the third arm being the output of the sensor. A magnetic field is applied to one-half of the coil. Any oxygen in the gas is attracted to the magnetic field in the cross-section. These oxygen molecules are subsequently heated by the heater coil and immediately begin to lose their magnetic susceptibility. They are then displaced by cooler oxygen molecules with higher magnetic susceptibility. This flow of gas through the cross-tube, referred to as the magnetic wind, cools the magnetized heating coil and heats the unmagnetized coil causing an imbalance in the bridge resulting from the difference in resistance between the two coils. The bridge output is then calibrated by passing a gas with known oxygen concentration through the chamber (1,11,13).

Limitations of Thermomagnetic Analyzers. Measurement by themomagnetic oxygen analyzers is affected by the magnetic susceptibility and thermal conductivity of the

EMF

Figure 1. Schematic diagram of a thermomagnetic oxygen analyzer (1).

carrier gas, the sample gas temperature, ambient temperature, tilt, sample flow, and pressure (1,16).

Magnetodynamic (Dumbbell or Autobalance)

Developed by Faraday in 1884, this is the most popular method of paramagnetic oxygen analyzers and the earliest developed oxygen analyzer (13). Magnetodynamic oxygen analyzers are based on property that oxygen will be drawn into a magnetic field because it is paramagnetic. These analyzers essentially measure the magnetic susceptibility of sample gas.

Principles of Operation. A simple form of this device consists of small, dumbbell shaped body made of quartz and filled with nitrogen or some gas with small or negative magnetic susceptibility, an optical lever system, and a nonuniform magnetic field (Fig. 2). The dumbbell is suspended in a closed chamber by a quartz or platinum wire between two permanent magnets that are specially shaped to have a nonuniform magnetic field. The dumbbell is free to rotate. Since the dumbbell is slightly diamagnetic, it will naturally rotate away from the highest magnetic field intensity. Oxygen in a sample gas will be attracted to the region of maximum field intensity and displace the dumbbell even further. This deflection is measured by an optical lever system in which a light source outside the test chamber shines a beam onto a mirror which is mounted in the center of the dumbbell. The beam is then reflected onto a scale outside the chamber. The amount of defection is directly proportional to the partial pressure of oxygen in the sample (1,17).

Modern designs of this sensor are self-nulling and have temperature compensation capabilities. A single turn coil is wound around the dumbbell. The coil produces a magnetic field when current flows through it which will in turn cause the dumbbell to rotate in the external magnetic field. The deflection of the dumbbell is measured by an optical lever system that uses photocells to detect the light reflected from the mirror.

Specially shaped magnets

Dumbbell filled with

nitrogen

Torque

due to paramagnetic gas

Mirror

Specially shaped magnets

Figure 2. Schematic diagram of paramagnetic ‘‘dumbbell’’ sensor. Adapted from (1).

Sensor

output 21%

Feedback current

Differential

photocells

Amplifier

Figure 3. Self-nulling paramagnetic analyzer. Adapted from Refs. 1 and 17.

The photocells are connected to a feedback loop that controls the amount of current through the coil, keeping the dumbbell centered with respect to the photocell detectors. As the paramagnetic components of a gas sample move into the strongest part of the magnetic field, the dumbbell is displaced and begins to rotate. The photocells detect this motion and drive an amplifier to produce the necessary current in the coil to keep the dumbbell in the original zero position. The system is zeroed using a sample of pure nitrogen. In this case, the dumbbell is at an equilibrium position and there is no current flowing through the coil. The current is directly proportional to the magnetic susceptibility of the sample. The system is calibrated using a sample of known oxygen content. See Fig. 3 for a diagram of this design.

Limitations of Dumbbell Analyzers. The main limitation of the dumbbell design is its slow response time ( 10 s) (15). Thus, the dumbbell analyzer is not recommended for uses where real-time oxygen analysis is needed. These analyzers also have moving parts and are extremely sensitive to tilt and vibrations.

Magnetopneumatic (Differential Pressure)

This sensor operates on the principle that a differential pressure will be generated when a sample containing oxygen is drawn into a nonuniform magnetic field with a reference gas of different oxygen content. Differential pressure sensors directly measure the magnetic susceptibility of sample gas and are thus not influenced by thermal properties of background gas.

Principles of Operation. A reference gas is admitted at a constant rate into a chamber like the one seen in Fig. 4. The

202 OXYGEN ANALYZERS

Reference gas inlet

Figure 4. Schematic diagram of a differential pressure sensor. (Adapted from Ref. 1.)

reference gas is split into two paths with a flow equalizer to ensure equal flow in each path. Each path is joined at the midpoint by a connector pipe containing a differential pressure sensor (e.g., a capacitive differential pressure sensor or a microflow sensor). The two paths reconnect at an outlet where the sample gas is admitted. There is a strong nonuniform magnetic field placed over one of the reference gas outlets. The reference gas and the sample gas combine in the outlet. Oxygen or other paramagnetic gases in the sample gas will be drawn into the nonuniform magnetic field and cause a pressure build up on that side of the reference gas path.

The differential pressure is proportional to the magnetic susceptibility of the sample gas only. This imbalance is sensed by the differential pressure sensor in the cross-tube. The output of this sensor is calibrated in terms of oxygen

Sample inlet

Sample outlet

Gas permeable membrane

Cathode

Depleting anode

Figure 5. Schematic diagram of a galvanic sensor. (Adapted from Ref. (14).)

concentration by using a reference and sample gas of known oxygen content. The output is zeroed by using a sample gas that is the same as the reference gas. In this case, the output of the differential pressure sensor will be zero (1,15).

Limitations of Magnetopneumatic Analyzers. Differential pressure sensors are sensitive to tilt and vibrations. An alternating magnetic field reduces the effects of background flow and tilt on the sensor (1).

ELECTROCHEMICAL OXYGEN ANALYZERS

There are two main types of electrochemical oxygen analyzers: those with aqueous electrolytes, and those with solid electrolytes. These sensors use the chemical reactivity of oxygen to produce a measurable current that is proportional to the partial pressure of oxygen in a sample gas.

Aqueous Electrolyte Sensors

Galvanic Oxygen Analyzer. Galvanic oxygen analyzers are commonly called a Hersch cell after the inventor. They are essentially a battery that produces energy when it is exposed to oxygen. Fig. 5. Galvanic sensors are typically insensitive to vibration and tilt. They are usually packaged small and made out of inexpensive and sometimes disposable materials (14). Disposable capsules containing galvanic cells are fairly inexpensive ( $85) and typically last 1–5 years (18). Recently, small, portable galvanic sensors have been manufactured and approved for medical breath analysis purposes (Fig. 6) (19).

Figure 6. A small, handheld galvanic oxygen sensor (model AII 2000, Analytical industries Inc.) (19).

Galvanic sensors are typically used for industrial purposes, such as validating the quality of semiconductor grade gases and for environmental monitoring (e.g., monitoring the quality of breathing air or monitoring the oxygen content in potentially hazardous or explosive environments) (14).

Principles of Operation. Basic cells consist of a cathode made of a precious metal (platinum, gold, silver, etc.) and an anode made of a base metal (lead, cadmium, antimony). These electrodes are in contact with a liquid or semisolid electrolyte, such as potassium hydroxide. A gas sample is admitted into the cell and diffuses through a membrane made of a thin material, such as Teflon or silicone, which is permeable to oxygen but not to the electrolyte. The oxygen in the solution is chemically reduced at the cathode to form hydroxyl ions that flow to the anode where an oxidation reaction occurs. This oxidation–reduction reaction results in an electromotive force (EMF), which is proportional to the oxygen concentration in the solution and the partial pressure of oxygen in the sample gas. The electron flow is measured by an external galvanometer connected to the electrodes.

Reactions at the cathode and anode are as follows 1, 20–22):

1. Cathode reaction:

O2 þ 2 H2O þ 4 e ! 4 OH

2. Anode reaction:

2 Pb þ 6 OH ! 2 PbO2H þ 2 H2O þ 4 e

3. Overall reaction:

O2 þ 2 Pb þ 2 OH ! 2 PbO2H

Designs that allow for every oxygen molecule passing through the cell to react are called coulometers and are suitable for trace (parts per million, ppm) measurements (1,20–22).

Limitations of Galvanic Analyzers. Because the anode is consumed by oxidation, the cell has a limited life. These devices tend to lose sensitivity as they age resulting in falsely low readings (14).

There are some designs that lessen the rate of anode consumption by using a third inert electrode made of platinum that is kept at a constant positive potential. This results in the majority of the current to be conducted through this electrode instead of the consumable anode (1,4).

Acidic gases in the sample (i.e., SO2 CO2, Cl2) react with the electrolyte and must be scrubbed out. There are some coulometric cell designs that overcome this problem by the use of additional electrodes (1,23).

Exposure to very high oxygen concentration can lead to oxygen shock where the sensor is saturated and does not recover for hours (14).

Polarographic Oxygen Analyzers. This sensor responds to changes in the partial pressure of oxygen in a sample

Anode

Porous

membrane

Electrolyte

Conducting

metal film

Conducting ring

Figure 7. Schematic diagram of a polarographic sensor. (Adapted from Refs. 1,24,25.)

gas. Polarographic sensors can be used for measuring oxygen in dissolved liquid (14) or in a gas sample. They are often used in anesthesia gas delivery systems. Polarographic sensors are insensitive to shock, vibration, and tilt. The effects of flow are minimal because diffusion is controlled by a membrane (14).

Principles of Operation. A polarographic cell, as seen in Fig. 7, consists of two electrodes, usually a silver anode and a gold cathode, immersed in an electrolyte, such as potassium chloride (1). An EMF is applied across the electrodes inducing oxidation–reduction reactions when a sample containing oxygen is admitted into the cell. Like the galvanic cell, oxygen diffuses through a thin membrane that is preferentially permeable to oxygen and not to the electrolyte. This membrane is usually made from poly(tetrafluoroethylene) (PTFE) and controls the rate of oxygen flux to the cathode. The current flow in the cell is proportional to the applied EMF and the partial pressure of oxygen in the sample. As seen in Fig. 8, there are four main regions in the EMF–current curve of importance (1):

Polarizing EMF

Figure 8. Diagram showing the operating regions of a polarographic sensor. (Adapted from Ref. 10.)

204 OXYGEN ANALYZERS

Region i: If the applied EMF is very low, then the presence of oxygen hardly has any effect on the current. There are very little reactions occurring at the electrodes.

Region ii: Oxygen molecules begin to react at the electrodes causing a measurable increase in current. For a given level of oxygen partial pressure, an increase in the EMF produces a sharp increase in the current.

Region iii: This region is the polarized or working region for the polarographic sensor. Here, the current plateaus and an increase in the EMF does not alter the current. In this region, all the oxygen molecules are reduced immediately at the cathode. A calibration curve is used in this region relating oxygen concentration to sensor current.

Region iv: In this region, an increase in EMF leads to a sharp, nonlinear increase in current as a result of the breakdown of the electrolyte solution.

The reactions at the cathode and anode are as follows (1,15):

1. Cathode reaction:

O2 þ 2 H2O þ 4 e ! 4 OH

2. Anode reaction:

4 Ag þ 4 Cl ! 4 AgCl þ 4 e

Limitations of Polarographic Analyzers. Polarographic sensors generally have slow response because oxygen must diffuse through membrane. They are also sensitive to pressure and temperature and compensation for these factors is sometimes required. In addition, these sensors lose sensitivity over time due to degradation of anode and electrolyte solution giving falsely low readings. Polarographic sensors that consume all the injected oxygen change the content of the sample gas and are not good for closed-loop systems, such as closed-circuit anesthesia systems (26).

Capacitive Coulometry. Capacitive coulometry (also referred to as coulometric microrespiometry) is another aqueous electrolytic method for oxygen analysis. This method is based on the replacement of oxygen consumed by an organism in a closed system with electrolytic oxygen produced by discharging a capacitor through a solution of CuSO4 (27). Such analyzers can be used to monitor oxygen consumption and metabolism rate of tissues or microorganisms. However, this type of sensor is not used as frequently as other more common types of aqueous electrolyte sensors.

Solid Electrolyte Cells

Some ceramics conduct electricity at very high temperatures. This conductivity is largely a result of the oxygen

Figure 9. Schematic diagram of a solid electrolyte concentration cell. (Adapted from Refs. 1,15.)

mobility in this solid solution. Oxygen mobility is the principle behind solid electrolyte or concentration cells.

There exists a family of solid electrolytes, such as MgO or CaO in ZrO2 (ceramic), whose electrical conductivity is mostly due to the mobility of O2 as opposed to electrons in the solid. At room temperature, the conductivity is low, but at high temperatures (>6008 C) the conductivity is comparable to that of an aqueous electrolyte and electron mobility can be neglected (1).

A solid electrolyte sensor is commonly referred to as a concentration cell. Solid electrolyte oxygen sensors are commonly used in anesthesia and patient monitoring for breath to breath analysis (15). Solid electrolyte cells typically have a fast response (<150 ms) (15) and are good for real-time oxygen analysis.

Principles of Operation. A concentration cell is made by separating a test chamber and a reference chamber by a solid, oxygen conducting electrolyte, such as ZrO2 or Y2O3 with a porous electrode on either side (Fig. 9). When the temperature is increased by an external heater, the solid electrolyte begins to conduct O2 and an EMF is established between the electrodes. The EMF is related to the partial pressure of oxygen in the test chamber by the Nernst equation:

where R is the universal gas constant (8.314 J K 1 mol 1), T is the operating temperature in kelvin, F is Faraday’s constant (9.6485 104 C mol 1), P0O is the reference partial pressure of oxygen, and P00O2 is the2 sample partial pressure of oxygen.

Fuel cell: An alternative configuration of a concentration cell is a fuel cell. If a fuel gas such as hydrogen is admitted into one of the chambers, the cell converts

Sample gas in Outlet

Heater coil

Sensor ouput

Figure 10. Schematic diagram of a flow through tube sensor. (Adapted from Refs. (1,28,29).)

the chemical energy of the fuel into electrical energy which may be delivered to a load placed across the electrodes (1,15).

Oxygen pump: A concentration cell may also be used as an oxygen pump. An EMF applied across the electrodes will pump oxygen from one chamber to another, the rate and direction depending on the strength and polarity of the applied EMF (1,15).

Sensor design: There are a number of different solid electrolyte sensor designs typically used: a flow through tube sensor, a test tube sensor, and a disk sensor.

Flow-through tube and test tube sensor: The flowthrough tube sensor is made from a solid electrolyte tube with a porous platinum electrode on the outside and inside of the tube (Fig. 10). An external heater is used to raise the temperature of the solid electrolyte to its operating temperature where it conducts oxygen. Ambient air outside the tube is used as the reference gas. The sample gas is admitted into the central part of the tube. This simple design was one of the first forms used (15,30,31).

A common variation on this design is the miniature test tube sensor (Fig. 11), which uses a sealed tube containing a reference gas with known oxygen partial pressure (1,32).

Sample inlet

Heater

Reference chamber

Sample outlet

Outer electrode

Sample

inlet

Inner electrode

Figure 11. Schematic diagram of a test tube sensor. (Adapted from Refs. 1,33.)

Figure 12. Schematic diagram of a miniature disk sensor. (Adapted from Refs. 1,34,35.)

Like the flow-through sensor, the test tube sensor is made of a solid electrolyte tube with porous electrodes on the inside and outside of the tube, but it is hermetically sealed at one end. The closed end of the tube is place in an external heater. The inside of the tube is used as the reference chamber and ambient air is used for the reference gas. The sample gas is admitted into a chamber surrounding the tube and flows around the outer electrodes. Because of its simple design, it is the most common solid electrolyte cell (1).

Disk sensor: The disk sensor is made with a solid electrolyte disk with porous electrodes on each side. The disk is attached to a metal tube of equal thermal expansion coefficient. Sample gas is admitted into a chamber on one side of the disk and a reference gas on the other. A heater is placed inside the tube on the reference chamber side (1,15,34).

A miniature design of the disk sensor (Fig. 12) has symmetrically placed porous platinum electrodes on either side of a thin solid electrolyte disk. The disk divides a small ceramic cylinder into two hermetically sealed chambers: one for the reference gas and one for the sample gas. An electrical heater brings the cell to the operating temperature. A temperature sensor and feedback loop attached to the heater ensures that the electrodes are also heated to the same temperature as each other and to the disk. If there is a difference in oxygen concentration between the reference and sample chambers, a voltage is generated between the two electrodes. A thin metallic holder is used to suspend the ceramic cylinder and ensures that the ceramic will not crack due to sudden thermal expansion.

Limitations of Ceramic Analyzers. Because the electrode is catalytic, any combustible gas will react with oxygen on the electrode causing it to age and the sensor to give a falsely low reading.

206 OXYGEN ANALYZERS

The high operating temperature of these sensors precludes its use around combustible gases.

Ceramics fracture due to thermal shock. Miniaturization helps considerably. These sensors usually require a warm up time (1,15,34).

Pressure on both sides of the disk must be the same or the reading will be inaccurate. This can be accomplished by venting both sides to the ambient atmosphere (15).

OPTICAL SENSORS

Fluorescence Quenching

One of the more recent oxygen analyzer designs uses a technique known as florescence quenching. Fluorescence dyes, such as perylene dibutyrate fluoresce for a certain amount of time in an atmosphere without oxygen. The presence of oxygen quenches this fluorescence. The fluorescence time is thus inversely proportional to the partial pressure of oxygen. In addition to oxygen sensing, fluorescence sensing can be used to detect glucose, lactate, and pH in the laboratory setting (36).

Fiber optic sensors: Fiber optic sensors use florescence quenching to measure the partial pressure of oxygen. A fiber optic strand delivers an optical pulse (usually blue light) to the fluorescent dye. The dye molecules are held in place by small beads of clear plastic. The beads are enclosed by a porous polypropylene membrane that is gas permeable and hydrophobic. The fluorescence is sensed by a photodetector at the end of a second fiber optic strand. The configuration of this sensor can be seen in Fig. 13. The time of fluorescence is calibrated for oxygen concentration. Because the fiber optic is only used to deliver the optical pulse and the fluorescence quenching is used to sense oxygen, the term fiber optic sensor is somewhat of a misnomer.

Light detector

Gas permeable membrane

Fiber optic bundles

Beads with fluorescent dye

Light source

Figure 13. Schematic diagram of a fiber optic sensor. (Adapted from Refs. 1,37.)

Figure 14. Schematic diagram of an integrated optic oxygen sensor chip. (From Ref. (38).)

Integrated optic oxygen sensor chip: Some sensors that can detect and identify multiple gases in a sample are called multianalyte sensors. One such sensor is an integrated optic multianalyte sensor. This recent development is a miniature optical sensor that has both biomedical and commercial applications. It is based on fluorescence quenching by oxygen.

This sensor, shown in Fig. 14, consists of a multimode ridge waveguide deposited on a dielectric substrate of a higher refractive index than the ridge. Spots of solgel, doped with an oxygen sensitive fluorescent dichlororuthenium dye complex, are deposited at the end of the waveguide and directly excited by a blue LED. Optical detectors at the other end of the waveguide detect emissions from the fluorescent spots. The fluorescence is efficiently coupled to the waveguide as the fluorescent spots are oriented to preferentially emits photons at an angle exceeding the critical angle defined by the two mediums. The theory of fluorescence emission at a dielectric interface is discussed further in (39).

The main limitation of this type of device is the response time. Typically, a 10 s integration period is used for each partial pressure oxygen measurement.

The main advantage of this device is its size and relative ease of fabrication (38). These chips have a very small foot print (<1 cm2). They can be quickly manufactured using soft lithography.

Polarization-based oxygen sensor: Another sensor based on fluorescence quenching is the polarization based oxygen sensor. This sensor uses an oxygensensitive film (Ru(dpp)3Cl2) and an oxygen-insensi- tive film (Styrl7). A diagonally polarized source illuminates these films that fluoresce in different ways. The oxygen-insensitive film is stretched so that the molecules preferentially emit vertically polarized photons. The Ru(dpp)3Cl2 film emits mostly unpolarized photons. Orthogonally oriented polarizers select for the vertical and horizontal components of the combined emitted light. The overall polarization of

0.6

Polarization - based oxygen sensor

Exc. 514 nm

Em. 625 nm

0.4

–0.2

–0.4

Figure 15. The polarization of the combined film photon emission is proportional to the partial pressure of oxygen. (From Ref. (40).)

the combined emission is sensitive to the partial pressure of oxygen in a sample (Fig. 15). The theory of polarization sensing is discussed further in (40).

UV Absorption

Spectroscopy can also be used to detect oxygen. Oxygen has a maximum absorption coefficient around a wavelength of 0.147 mm, which is in the ultraviolet (UV) range. Most other gases have a much smaller absorption coefficient at this wavelength (1).

A simple device uses an ultraviolet light source, such as a discharge lamp or UV laser as the light source. The beam is split into two paths by a vibrating mirror and directed into either a reference cell filled with nitrogen or a sample cell filled with the gas in question. A photomultiplier tube is used for detection at the end of each path. The ratio of energy received through the sample and reference cell is related to the partial pressure of oxygen in the sample.

Raman Spectroscopy

Raman spectroscopy can be used to monitor multiple gases in a sample. Raman spectroscopy is commonly used in medicine for real-time breath-to-breath analysis or for monitoring respiratory gas mixtures during anesthesia (41–44).

This technique uses the inelastic or Raman scattering, of monochromatic light. The frequencies of the returned

light give information about the vibrational, rotational, or other low frequency modes of atoms or molecules in a sample. This information is specific to given elements and compounds and can thus be used to identify and distinguish different gases in a mixture.

In Raman spectroscopy, a sample is illuminated with a laser beam, usually in the visible, near-IR, or near-UV range. Most of the light is scattered elastically or by Rayleigh scattering and is of the same frequency as the incident light. However, a small portion of the light is scattered in-elastically. Phonons, which are quanta of vibrational energy, are absorbed or emitted by the atom or molecule causing the energy of the incident photons to be shifted up or down. This shift in energy corresponds to a shift of frequency. Frequencies close to the laser line are filtered out and frequencies in a certain spectral window are dispersed on to a photomultiplier tube or CCD (charged couple device) camera.

OTHER GASEOUS OXYGEN SENSORS

Gas Chromatography–Mass Spectrometer

A gas chromatography–mass spectrometer (GCMS) combines GC and MS to identify substances in a gas sample. It can be used to detect a variety of compounds in a mixture or it can simply be used to detect the presence of oxygen. The GCMS analyzers are commonly used to measure gas composition in respiratory circuits (42).

Principles of Operation. The gas chromatograph separates compounds into the molecular constituents by retaining some molecules longer than others. These molecules are broken up into ionized fragments that are identified by the mass spectrometer based on the molecules’ mass/charge ratio (m/z).

The GC consists of an injector port, an oven, a carrier gas supply, a separation column, and a detector. The injected sample is vaporized in a heated chamber and pushed through the separation column by an inert carrier gas. The separation column is typically a capillary column made of a long, small diameter (usually 1–10 m in length and 0.5 mm in diameter) tube of fused silica (high quality drawn glass) or stainless steel formed into a coil. The components of the sample are separated by two different mediums inside the column that control the speed of travel. These mediums are either coated on the inner surface of the column or packed in the column. Part of the media, known as the stationary phase, absorbs molecules for a certain amount of time before releasing them. The amount of time depends on the chemical properties of the molecule and thus certain molecules are detained longer than others. This, in effect, separates the molecules in the mixture in time. The molecules then travel to the detector. The output of the detector is processed by an integrator. The response of the detector over time is the chromatograph (Fig. 16).

The mass spectrometer separates ions from the gas chromatograph by their charge to mass ratio (m/z) by using an electric or magnetic field. Mass spectrometers usually consist of an ion source, a mass analyzer and a detector.

Figure 16. An example gas chromatograph and corresponding mass spectrograms.

The ion source ionizes the sample gas usually with an electron gun. The charged particles are accelerated with an electric field and are steered by the mass analyzer on to the detector by means of a varying electric or magnetic field. The speed and deflection of the particle depends on its mass an charge. When a charged particle comes near or strikes the surface of the detector, a current is induced and recorded. This current is typically amplified by an electron multiplier or Faraday cup. The resulting ionic current plot is the mass spectrum of the ions.

Limitations of Gas Chromatography–Mass Spectrometry. The main limitation of the GCMS is its extremely high price. Mass spectrometers alone run $40 k. However, a GCMS may be used for any time of gas measurement.

The Warburg Apparatus

A much older method of oxygen analysis was pioneered by German biochemist Otto Heinrich Warburg (45). The Warburg apparatus was used for measuring cellular respiration and fermentation. This method is based on Boyle’s law, which relates the pressure and volume of a gas, and Charle’s law, which relates the pressure and temperature of a gas. Combining these laws yields the ideal gas law (PV ¼ nRT, where P ¼ pressure, V ¼ volume, n ¼ number moles, R ¼ universal gas constant, T ¼ temperature). At a constant temperature and volume, any change in the amount of gas can be measured as a change in pressure. A typical Warburg apparatus consists of a detachable flask, a waterbath, and a barometer. The sample is placed in a flask and immersed in a bath of water held at a constant temperature. Pressure is measured periodically to determine the amount of gas produced or absorbed by the sample. A variation of this device, used to measure gas production in plants directly from the stem, is the Scholander–Hemmel pressure bomb (46).

CONCLUDING REMARKS

There are three primary types of oxygen sensors available: paramagnetic, electrochemical and spectrographic. Each type of oxygen sensor has limitations and uses dependent on their design, cost and operational environment. Other

available oxygen sensors include semiconductor sensors, fluidic sensors, and electron capture oxygen sensors

(1). These devices are typically not used in medicine or biology.

BIBLIOGRAPHY

Cited References

1.Kocache R. The measurement of oxygen in gas-mixtures. J Phys E Sci Instrum 1986;19:401–412.

2.O2 Guide. Electronic Source, Delta-f Corporation. Available at http://www.delta-f.com/O2Guide/o2guide.htm; Accessed 2005.

3.Guyton A, Hall J. Medical Physiology. New York: W.B. Saunders C. 2000.

4.Allsopp PJ. Electrical Cell. UK patent, 969,608. 1964.

5.Johnson T, Rock P. Acute Mountain Sickness. N Eng J Med 1988;319:841–845.

6.Safety_Advisory_Group. Campaign Against Asphyxia. European Industrial Gases Association (EIGA) Safety Newsletter (Special ed.). Brussels.

7.Watanabe T, Morita M. Asphyxia due to oxygen deficiency by gaseous substances. Forensic Sci Int 1998;96:47–59.

8.Operator Dies in Oxygen Defficient Compartment of Ice-Making Barge. Alaska Fatality Assessment & Control Evaluation.

9.Bonsu AK, Stead AL. Accidental cross-connection of oxygen and nitrous oxide in an anesthetic machine. Anaesthesia 1983; 38:767–769.

10.Cheng DK. Field & Wave Electromagnetics. New York: Addison-Wesley; 1984.

11.Instruction Manual: Paramagnetic Oxygen Analyzer: Fuji Electric Co. Ltd.

12.Havens G. The magnetic suceptibilities of some common gases. Phys Rev 1933;43:992–1000.

13.Servomex. Paramagnetic oxygen analysis. Electronic Source, Servomex Corporation. Available at http://www.servomex. com/Servomex.nsf/GB/technology_paramagnetic.html.

Accessed 2005.

14.Corporation D-f. O2 Guide. Electronic Source, Delta-f Corporation. Available at http://www.delta-f.com/O2Guide/o2guide. htm; Accessed 2005.

15.Kocache R. Oxygen analyzers. Encyclopedia of Medical Devices and Instrumentation, Webster JG. editor. New York: Wiley & Sons; 1988. p. 2154–2161.

16.Medlock RS. Oxygen Analysis. Inst Eng 1952;1:3–10.

17.Pauling L. Apparatus for determining the partial pressure of oxygen in a mixture of gases. US patent, 2,416,344, 1947.

18.Meyer RM. Oxygen analyzers: failure rates and life spans of galvanic cells. J Clin Monitoring 1990;6:196–202.

19.Analytical_Industries_Inc. Oxygen analysis pocketed and ABS protected. MD Med. Design, 2001.

20.Hersch P. Electrode assembly for oxygen analysis. UK patent, 913,473. 1962.

21.Hersch P. Improvements relating to the analysis of gases. UK patent, 707,323. 1954.

22.Hersch P. Improvements relating to the analysis of gases. UK patent, 750254. 1956.

23.Gallagher JP. Apparatus and method for maintaining a stable electrode in oxygen analysis. US patent, 3,929,587. 1975.

24.Bergman I. Improvements in or relating to electrical cells. UK patent, 1,344,616. 1974.