Physics & Monitoring

1. Physics & Monitoring 28.7.03 Dr Rishi Mehra

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

3. 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

4. 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

5. 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

6. 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

7. Infrared analysers

8. Infrared analysers • Advantages- Rapid response time, suitable for breath by breath analysis

9. 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

10. 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

11. 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

12. Paramagnetic analysis

13. 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

14. 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

15. Mass Spectrometer

16. Mass Spectrometer • Once ionised, degree of travel varies with:- Charge- Molecular weight • Hence can be used to detect several different gases simultaneously

17. 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