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