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Physics & Monitoring. 28.7.03 Dr Rishi Mehra. Explain the principles involved in the analysis of gases using ultraviolet or infra-red absorption, paramagnetic analysis, gas chromatography, mass spectrometry and raman scattering. Infrared analysers.

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physics monitoring

Physics & Monitoring

28.7.03

Dr Rishi Mehra

slide2

Explain the principles involved in the analysis of gases using ultraviolet or infra-red absorption, paramagnetic analysis, gas chromatography, mass spectrometry and raman scattering

infrared analysers
Infrared analysers
  • Infra-red wavelength – between 4.2 and 4.4 micrometres
  • Used for analysis of molecules with dissimilar atoms
  • E.G. can analyse CO2, N2O, volatiles etc
  • Cannot analyse O2 or N2
infrared analysers1
Infrared analysers
  • Two basic types- dispersive- non-dispersive
  • Concept of dispersion – use of a prism or optical filter
  • Light broken down into different wavelengths chosen in relation to absorption peaks of gases
infrared analysers2
Infrared analysers
  • Absorbance of light follows Beer-Lambert Law:

It = transmitted lightIi = incident lightd = distance travelledc = concentration of compoundt = molar absorbtivity with units L/mol/cm

infrared analysers3
Infrared analysers
  • Non-dispersive- used for a single gas e.g. ETCO2- Disadvantages – overlap with gases of similar absorption wavelengths- e.g. CO2 absorbs strongly between 4.2 and 4.4 micrometers, while N2O absorbs strongly between 4.4 and 4.6 micrometers- Hence may introduce inaccuracies in measurement of these agents
infrared analysers5
Infrared analysers
  • Advantages- Rapid response time, suitable for breath by breath analysis
ultraviolet analysis
Ultraviolet Analysis
  • Principle similar to infrared
  • Light from mercury lamp is filtered and passed through gas to a photoreceptor
  • O2, H2, N2 all absorb in ultraviolet wavelength range
  • Originally developed for halothane, but later found to cause decomposition of halothane
paramagnetic analysis
Paramagnetic analysis
  • Commonly used for O2 analysis
  • O2 is attracted to a magnetic field more than other gases
  • O2 electrons in outer shell are unpaired
  • Traditional setup involved: A gas chamber containing two spheres connected in a ‘dumbell’ like fashion
  • Spheres contain N2
  • Chamber surrounded by magnetic field
paramagnetic analysis1
Paramagnetic analysis
  • Spin of dumbell related to O2 concentration.
  • Modern setup different: Sample gas passed through rapidly changing magnetic field. Resultant pressure changes are measured by very sensitive pressure transducer
gas chromatography
Gas chromatography
  • Chromatography = “Write in colour”
  • Gas sample introduced to 2m long steel pipe
  • Inert carrier gas (nitrogen or helium) flows through pipe
  • Temperature kept constant
  • Detector at end of pipe
gas chromatography1
Gas chromatography
  • Most volatile agents move fastest hence reach detector first
  • Can measure very low concentrations of gases
  • Disadvantage – cannot be used for continuous analysis
mass spectrometer1
Mass Spectrometer
  • Once ionised, degree of travel varies with:- Charge- Molecular weight
  • Hence can be used to detect several different gases simultaneously
raman scattering
Raman Scattering
  • Sample gas enters a measurement chamber- Struck by high intensity monochromatic light, produced by an argon laser- wavelength = 448 nm
  • Most energy is re-emitted at same wavelength (Raleigh scattering)- Scattered light is re-emitted at longer wavelength (Raman scattering)
  • Specific photodetectors for specific gases