EXPIRED GAS ANALYSIS

Mass Spectrometry and Raman Gas Analysis

Mass spectrometers are devices that measure concentrations of gases largely on the basis of their molecular weight. Gas samples are passed through an ionizer, typically an electron beam, which strips the individual molecules of one or more electrons, giving them a positive charge. The ionizing beam also splits some molecules, creating molecular fragments. After ionization, the gas mixture to be measured is accelerated through a magnetic field and then onto detectors according to charge-mass ratio. The rate at which ions hit the target is proportional to the concentration of that particular gas in the original mixture. These instruments are reliable and relatively rugged, with a delay time typically on the order of 100 to 200 msec.

When a photon from a light source (usually an argon laser) collides with a molecule of gas, it may be re-emitted (i.e., scattered light) with no loss of energy (i.e., Rayleigh scattering). Alternatively, there may be absorption of some of the kinetic energy from the photon, resulting in the scattered photon's having a lower energy level and longer wavelength (i.e., Raman scattering). Spectral analysis of the scattered light can be used to measure simultaneously the individual concentrations of a mixture of gases. Mass spectrometers and Raman gas analyzers in the operating room have been largely supplanted by infrared analyzers.