Use of a mass spectrometer for direct respiratory gas sampling from the hyperbaric chamber

Background. When conducting respiratory gas measurements during hyperbaric chamber research, it is preferable to carry out gas concentration analysis by mass spectrometry. Gas samples for the mass spectrometer are normally ta ken from a bypass flow exiting the high pressure chamber to the ambient atm osphere. Under these conditions, mixing in the sampling line smoothes the c oncentration profile, and much of the advantage of low sampling flow is los t. We propose to use a direct sampling method by mass spectrometer that ove rcomes these deficiencies. Methods: In the present study, the original high resistance capillary of a QP 9000 mass spectrometer was inserted through t he wall of a hyperbaric chamber. Series A: Air and pure nitrogen flowed alt ernately (1 s each) via the sampling tip of the mass spectrometer. Series B : End expired CO2 from 15 immersed, professional divers exercising at 405 k Pa was measured in a screening test for CO2 retention for nitrox diving. Re sults: There was no difference in the recorded rise time, fall time and pla teau reached in the concentration of oxygen at pressures of 101, 202, 303, 405 and 506 kPa. The new sampling method functioned correctly throughout th e full-scale experiment, and the recording of end tidal CO2 was more precis e than in the conventional method. Conclusions: Direct sampling of gases fr om a hyperbaric chamber by the QP 9000 mass spectrometer has many advantage s over sampling of the same gases once they are outside the chamber.