The mass spectrometer is the ideal gas analyser for many applications in respiratory research. A number of companies which manufactured mass spectrometers suitable for use in respiratory research have long since ceased production. Two modifications in the presently manufactured MAX300-LG (Extrel, Pittsburgh, PA, USA) have made it suitable for studying respiratory physiology. The initial step was to bypass the four-port valve and connect the sampling capillary directly to the roughing pump. The second step was to replace the original capillary, which had an internal diameter of 0.4mm, by a 2-m long capillary having an internal diameter of 0.15 mm. The 90% response time was shortened to 85 ms, which is suitable for respiratory research.
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