Monitoring of gas delivery: N20 and anaesthetic agent monitoring, IR and mass spectrometry

Question 1

What type of molecules absorb IR radiation

Answer 1

Molecules containing 2 or more different elements e.g. N20, CO2, water vapour, volatile anaesthetics.

Molecules comprising only one element e.g. O2, N2 do not absorb IR radiation. Oxygen may be measured using paramagnetic analyzers or fuel cells.

Question 2

Range of wavelengths of IR spectrum

Answer 2

Wavelengths greater than 0.75 micrometres and below 300 micrometres (in the invisible part of the electomagnetic spectrum.

Question 3

Range of wavelengths of visible light spectrum

Answer 3

0.4 - 0.75 micrometres in the electromagnetic spectrum

Question 4

What wavelengths of IR radiation are absorbed by
* H20
* CO2
* N20

Answer 4

H2O absorbs wavelengths @ 1.4-1.9 µm
CO2 absorbs wavelengths @ 4.3 µm
N2O absorbs wavelengths @ 4.6 µm

Question 5

What wavelengths of IR radiation are absorbed by anaesthetic agents?

Which is the lowest

Answer 5

Anaesthetic agents (desflurane, enflurance, halothane) absorb * 8-9 micrometres

Note desflurance also absorbs around 3.3 micrometres (low end)

Question 6

What determines the wavelengths and amount of radiation absorbed by a molecule

Answer 6

The amount of radiation absorbed is proportional to the concentration of substance present in the gas sample.

Wavelengths of energy absorbed are characteristic for the bonds between constituent atoms of molecules (because these bonds differ in strength and character, depending on the atoms involved)

Question 7

How do bonds between molecules react when they absorb radiation

Answer 7

When radiation is absorbed the bonds vibrate or oscillate in a variety of ways.
Examples: rocking, scissoring, stretching asymmetric, stretching symmetric, twisting, wagging
i.e. bonds between atoms can vibrate in different planes

Question 9

IR spectrometer: function

How does it identify different anaesthetic agents

Answer 9

Several sensors are used to calculate the IR absorption at different wavelengths.
The absorption patterns at these wavelengths produce a pattern or ‘spectral’ shape which is characteristic for a particular agent present in the sample. This spectral shale is compared with others stored in the memory of the sensor, and used to identify the agent(s) present in the sample

Question 10

What is the maximum IR absorption of:
Oxygen
Desflurane
Water
Nitrous oxide
Carbon dioxide

Answer 10

Oxygen: does not absorb IR radiation
Desflurane: 8.5 micrometres
Water: 1.4-1.9 micrometres
Nitrous oxide: 4.6 micrometres
Carbon dioxide: 4.3 micrometres

Question 11

IR spectrometer: components

Answer 11

• Sample of gas from the anaesthetic breathing system is drawn towards the analyzer via the gas sample inlet
• Heated element produces the IR source
• The generated radiation passes through the sample chamber -> and then through a filter that allowes only discrete wavelengths to fall onto the IR detectors
• Reference beam allows the detector software to calculate how much energy has been absorbed by the sample at each wavelength and therefore the concentration of agent in the sample. Otherwise, variations in IR output could be mistaken for sample absorption.

Question 12

IR spectrometer: advantages (4), disadvantages (4)

Answer 12

Advantages:
* Alarm limits can be set for inspired and expired agents
* Multiple agents can be analyzed simultaneously
* Relatively inexpensive compared with other methods
* Modern analyzers are compact

Disadvantages:
* Lag time for gas to travel down sampling line to sample chamber
* Risks of disconnection, leakage or occlusion of sample line
* Older spectrometers have to be ‘told’ what agent they are measuring
* Variations in IR output could be mistaken for sample absorption – reference beam used

Question 13

Mass spectrometer: principles and mechanism

How are different chemicals identified

Answer 13

• When a volatile chemical is bombarded with electrons it is reduced to a collection of fragments with different masses and charges
• These charged particles can be separated by accelerating them through a magnetic field.
• The higher the mass and lower the charge of a particle, the less its path of travel is altered by the magnetic field.
• Therefore different masses and charges and their relative abundance can be measured
• Different chemicals produce a characteristic pattern of abundences of mass:charge ratios - so each has its own ‘fingerprint’

Question 14

Mass spectrometer: components

Answer 14

• Gas sample is introduced into a vacuumed chamber (to prevent contamination of the sample by air)
• Sample is bombarded with electrons from the cathode
• Generated positive ion stream is accelerated away from the anode and focused by the negatively charged plates into the magnetic field which lies perpendicular to the plane of the ion stream
• The magnetic field deflects the ions from their straight path into a trajectory whose radius of curvature depends on their mass:charge ratio
• The ions with the ‘correct’ mass:charge ratio fall onto the detectors at the far side of the spectrometer.

Question 15

Mass spectrometry: What is the impact of increasing/ reducing the mass or speed of the ion stream on the amount of deflection

Answer 15

Increasing the mass or speed of the ion stream -> less deflection.
Reducing the mass or speed of the ion stream -> more deflection

Question 16

Mass spectrometers: disadvantages (3)

Answer 16

• Expensive compared with other methods
• Equipment is very bulky
• Nearly always shared between several patients but it can only analyze one patient at a time. Therefore frequency with which any individual patient is monitored depends on the number of patients being sampled and the length of time each is analyzed

Note: mass spectrometer output is affected by the presence of other gases, but the chamber is vacuumed to eliminate this potential problem

Question 17

Non-standard monitoring methods: ultraviolet absorption

Uses

Answer 17

Has been used in the past for measuring halothane concentration. Not in mainstream use.
Similar principle to IR absorption but utilizes ultraviolet absorption instead.

Question 18

Non-standard monitoring methods: piezoelectric quartz crystal oscillation

Mechanism, disadvantages

Answer 18

• A piezoelectric quartz crystal exhibits a natural resonant frequency when stimulated.
• When one of two crystals is given a lipid coating, lipid-soluble inhalation agents dissolve into the coating and alter its natural resonance
• The change in resonant frequency from that of the non-coated crystal is directly proportional to the partial pressure of the anaesthetic agent present

Disadvantages:
* Results are affects by water vapour
* Anaesthetic agent specificity is poor

Question 19

Non-standard monitoring methods: raman scattering

Mechanism

Answer 19

All molecules in the gas/volatile phase can be identified by their characteristic spectrum of Raman scattering.
* Anaesthetic gas sample is illuminated by an intense argon-laser
* Some light energy is reflected but some stimulates the sample molecules, causing them to scatter light of a different wavelength from that of the incident light energy (Raman scattering)
* This scattered light is detected at right-angles to the laser beam
* The difference in energy level between the incident and reflected light is measured

Question 20

What methods are used for oxygen measurement

Answer 20

Oxygen may be measured using paramagnetic analyzers or fuel cells.

Note oxygen does not absorb IR radiation as contains only one type of molecule