Mass spectrometry
Principle
Amount of deflection of charged ions in magnetic field depends on their mass/charge ratio.
Setup
- Gas molecules are bombarded with electrons, some of which become ionized.
- Ions are accelerated and focused in a narrow beam and pass through a magnetic field.
- The deflected ion streams are filtered and detected by a detector.
- Most spectrometers have 7-8 collectors and can analyse 7-8 gases.
Use
Measures fractional composition of the gas mixture (not partial pressure)
Fragmentation
- Some molecules become fragmented during the ionization process.
- For gases like CO2 and N2O which have the same molecular weights, the fragments CO+ and NO+ are used to identify the gases.
- Similarly, enflurane and isoflurane are also identified by the fragments
Advantages
- Rapid response time
* But may require long sampling line if servicing multiple theatres at the same time.
- Multiple sites
- Very sensitive
Disadvantages
- Machine is large and expensive
* Usually only one machine per theatre complex
- Only measures the gas proportions, not the partial pressure
- Periodical calibration necessary
- Tolerate very little movement
- Presence of another molecule in significant quantities will give erroneous readings
- Gases in the chamber have to be pumped out or aspirated
Quadrupole mass spectrometer
Use of 4 electrically charge rods instead of magnetic field.
- Machine smaller and more lightweight
- Rapid
- Lower sampling rates