Other Gas And Vapour Analysis Techniques

Question 1

What is chromatography

Answer 1

The general term used for analytical procedures that separate a mixture into its components as the mixture passes through the column (think mix of dies passing through calcium carbonate column)

Different components are held for different periods of time as the mixture passes through the column hence separating out the components

Question 2

what are the components of a gas chromatograph

Answer 2

1. Carrier gas with flow control
2. Sample injection port
3. Coiled column containing silica - alumina coated with polyethylene glycol OR silica oil.
4. Coiled column within an oven for temp control
5. Flame ionization detector
6. Display

Question 3

How does a gas chromatograph work

Answer 3

Carrier gas flows (e.g. N2). Sample gas introduced. Flow through column within oven at set temp. The components pass through column at speeds which depend on their differential solubility between the two phases (stationary and mobile).

In the flame ionization detector H2 and Air are added to the sample to produce a flame at the end of the column. Constituents of flame are ionized particles. If a polarising voltage is applied across the flame by two electrodes, a current is produced –> the magnitude of this current depends on the particular charged particles in the flame. If a organic vapour is present in the carrier gas, it is ionized in the flame and increases current flow –> size of current = amount of vapour present

Each substance in a mixture has a characteristic retention time in the column. If this time is known, the peak current at this time is measured which corresponds to the concentration of that substance.

Question 4

What are alternatives to the flame ionization detector

Answer 4

1. Thermal conductivity detector (katharometer)

2. Electron capture detector

Question 5

What is gas chromatography used for clinically

Answer 5

Measure volatile anaesthetic agents

Measure
- Barbiturates
- Phenothiazines
- Benzodiazepines
- Steroids
- Catecholamines
(these are converted into volatile compounds)

Question 6

What are the advantages of gas chromatography

Answer 6

Allows for identification of very low concentrations of drugs

Both liquids and gas samples can be analyzed

Question 7

What are the disadvantages of gas chromatography

Answer 7

Continuous analysis is impossible

Details of sample constituents must be known

Question 8

How does a mass spectrometer work?

Answer 8

Sample is drawn through a tube into the sample chamber of a mass spectrometer by a pump. A molecular leak permits a few molecules of the sample to diffuse into an ionization chamber where they are bombarded by a beam of electrons passing from the hot cathode to the anode.

When the molecules of gas are hit by electrons, some of them become charged ions which are accelerated out of the chamber in a narrow beam by means of acceleration and focusing plates. The stream of ions then passes through a strong magnetic field which deflects the streams according to their MOLECULAR MASS. Various detectors are sited at the end point of these deflected beams and the quantity arriving at the receptor determines the magnitude of the signal and hence the concentration of a particular substance in a mixture.

Question 9

Where are Gas chromatographs and mass spectrometers used

Answer 9

In the laboratory - they are bulky

Question 10

what is the piezo-electric effect

Answer 10

When an electric potential is applied across a crystal of quartz it contracts slightly. A crystal can be made to oscillate at its resonance frequency by a suitable alternating potential.

Question 11

What techniques are used clinically for the measurement of anaesthetic volatile agents in theatre

Answer 11

1. Infrared analyser
2. The vapour analyser (piezo-electric effect)
3. Refractometer
4. Raman Effect

Question 12

How can the piezo-electric effect be applied to measure volatile anaesthetic agents

Answer 12

Quartz crystals are coated with specific materials that absorb anaesthetic agents.

The absorption of volatile that occurs on the surface of the quartz crystals changes the resonance frequency of these crystals. From Henry’s law the higher the partial pressure of volatile the more anaesthetic agent will be absorb and hence the more change in the resonant frequency of the crystal which can be measured electronically