Monitoring of gas delivery: oxygen concentration analysers Flashcards
Technologies used in oxygen concentration analysers
Electrochemical (2), physical (2), brief description of mechanism/uses
Electrochemical:
* The (galvanic) fuel cell: fuel cells produce an electric current proportional to the partial pressure of oxygen to which the sensor is exposed. Most frequently used for measuring inspired oxygen as part of ‘machine monitoring’
* The polarographic cell: Produces a current proportional to the oxygen concentration. Less freqeuently used than fuel cells for gaseous oxygen analysis. Common in blood gas analysers.
Note: slow response time (~20s) of electrochemical methods mean not suitable for measuring changes between inspired and expired gas, but can be used to measure oxygen concentraiton in inspiratory limb of breathing system.
Physical
* Paramagnetic analyser: use indirect measurement of a magnetic force upon oxygen molecules in the gas sample. Found in modern multi-gas analysers. In these units, other gases are analysed by infrared absorption spectroscopy.
* Mass spectrometer (not commonly used clinically): detects different molecular species in a gas mixture by the deflection of ionized molecules by a strong magnetic field. Degree of ion deflection depends on mass:charge ratio and intensity of magnetic field.
Simple electrical cell
Circuit
Cathode and anode
Fuel cell oxygen analyser: components
Galvanic fuel sensor, typically consisting of:
* gold plated cathode
* lead anode (oxidation of anode surface eventially leads to limitation of reaction rate)
* postassium hydroxide electrode
* permeable membrane (teflon or similar) through which oxygen can diffuse to gain access to the cathode (where undergoes a chemical reaction)
AND a suitable amplification circuit and digital display
Fuel cell sensor.
Gaseous oxygen diffuses through the gas-permeable membrane to undergo a chemical reaction at the cathode.
Fuel cell oxygen analyser: reaction at the cathode
Formula, how converted to displayed oxygen concentration
At the cathode, oxygen is reduced to hydroxyl ions:
O2 + 2H20 + 4e- -> 4OH-
Note that the gold-plating on the cathode remains chemically unchanged by the reduction of oxygen.
The rate of reaction (and hence the current flow) is proportional to the partial pressure of oxygen in the gas to which the sensor is exposed.
The current is amplified by the analyzer circuitry, and the oxygen concentration is displayed.
Fuel cell oxygen analyser: reaction at the anode
Formula, lifespan
At the anode, lead is oxidised to lead (II) oxide by hydroxyl ions:
Pb + 2OH- -> PbO + H20 + 2e-
The ‘fuel’ in the cell’s name is the lead of the anode: the overall cell reaction can be considered as involving the lead being ‘burnt’ by the gaseous oxygen.
Oxidation of anode surface eventually leads to limitation of the reaction rate -> sensor fails ot provide a current flow proportional to the oxygen partial pressure acting at the cathode.
Fuel cell oxygen analyser: response time, position within breathing system, frequency of calibration
Typically 10-20s to register 90% of any change in oxygen
-> too slow to monitor the changing oxygen concentration between inspired and expired gas
-> are commonly used to measure the oxygen concentration in the inspiratory limb of a breathing system
As current output from the sensor varies during the life of the cell, display readings against 21% O2 and 100% O2 should be checked daily, recalibrating as necessary
Fuel cell oxygen analyser: working life
Determinants, sign that beginning to fail
Working life depends on the oxygen concentration to which the sensor is exposed
* May last several months if removed from the breathing circuit and exposed to room air when not in use
* If left exposed to a high oxygen concentration, will become depleted within a few weeks
* Supplied in sealed impermeable packages containing nitrogen to prevent deterioration in storage
Commonest sign that sensor is beginning to fail is failure of the sensor to span range 21% -> 100% oxygen –> should be replaced.
Polarographic oxygen analyser: components
Aka ‘Clark electrode’
* Sensor consisting of a platinum cathode (A) and silver anode (B)
* Potassium electrolyte (C)
* Semi-permeable membrane (polythene, Teflon or similar) through which oxygen can diffuse to gain access to the cathode (D)
* ‘O’ ring to hold the membrane in place (E)
* Battery to supply the polarizing voltage (F) of 0.6V (600mV). Potential difference between the platinum cathode and silver anode is too small to drive the electrode reactions therefore a battery or equivalent is required.
* Amplificaiton circuit and digital display (shown as a galvenometer G for simplification)
Polarographic oxygen analyser (‘Clark electrode’) components
Polarographic oxygen analyser: reaction at the cathode
At the cathode, oxygen is reduced to hydroxyl ions:
O2 + 2H20 + 4e- –> 4OH-
Note that the gold plating of the cathode remains chemically unchanged by the reduction of oxygen.
The rate of reaction (and hence current flow) is proportional to the partial pressure of oxygen in the gas to which the sensor is exposed. Current is amplified by the analyser circuitry, and oxygen concentration is displayed.
At the anode, silver is oxidised to silver chloride:
Ag + Cl- —> AgCl + e-
Silver chloride is soluble in the potassium chloride electrolyte. Therefore the anode is not corrupted by a layer of oxidation product.
The depletion of the anode is extremely slow and does not affect the life of the sensor.
Polarographic oxygen analyser: response time, uses, calibration requirements
- Can be used to measure oxygen in gases or liquids (e.g. in a blood gas analyser)
- Response time is similar to fuel cell: ~20s to register 90% of any change in oxygen -> too slow to monitor the changing oxygen concentration between inspired and expired gas, but commonly used to measure the oxygen concentration in the inspiratory limb of a breathing system
- Like the fuel cell, the polarographic electrode drifts during use and must be calibrated frequently
Polarographic oxygen analyser: working life
The semi-permeable membrane deteriorates with time and must be replaced.
Note requires a battery or other source of electromotive force
Null displacement paramagnetic oxygen analyser analyser and the dumbbell principle
Mechanism: how detects concentration of oxygen
Paramagnetic property of oxygen: oxygen molecules are attracted into a strong magnetic field.
- Focused magnetic field is created between the poles of two magnets within the sample chamber of the analyser.
- Two glass spheres containing nitrogen (non-magnetic) are arranged like a dumbbel on a rotating vertical suspension
- Oxygen molecules are attracted into the field and displace the dumbbell (because there is no magnetic force acting on the nitrogen in the dumbbell)
- Displacement of the dumbbell is detected: a small mirror is mounted centrally on the dumbbell suspension, and movement of the mirror is sensed by a pair of photocells sensing light reflected by the mirror
- Signal from the photocells is amplified and used to control a feedback current passed through a coil surrounding the spheres. The current produces a force on the coil (motor effect), maintaining the dumbbell in its original position (‘null displacement’
- Current varies according to the force on the dumbbell, which is proportional to the concentration of oxygen in the gas surrounding the dumbbell
- Current (i.e. the current required to keep the dumbbell in position) is measured -> indicates oxygen concentration
Differential pressure paramagnetic oxygen analyser: Components and function
Components
* Draws sample gas and a reference gas (room air) through the arms of a T-shaped tube. The T is arranged with the junction of the tube in the air gap of a strong electromagnet.
* Flow is restricted in both arms by coils that act as fixed resistances
* A differential pressure transducer is used to measure the pressure in the arms between the flow restrictors and the T-junction
Function
* Electromagnet provides a high-frequency pulsed magnetic field in the air gap.
* Oxygen molecules in the gases within the T-arms are attracted towards the field when the magnet is turned on, reducing the pressure within both arms. The pressures equalize when the magnet is turned off.
* Reduction of pressure in each arm during magnet activation is directly proportional to the concentration of oxygen in the gas within the arm
* Differential pressure between the sample gas arm and reference arm -> can be used to calculate the oxygen concentration in the sample gas.
