The uses of long sampling probes in respiratory mass spectrometry.

We have compared the results of gas analysis by respiratory mass spectrometry using long (30m) sampling probes with those obtained using conventional short (1.3m) probes, examining both static gas mixtures and respired gas at the mouth during manoeuvres designed to make the concentrations of inspired marker gases change in a complex way within a breath. The experiments showed that no important errors were introduced by using the long probes, both for estimates of gas tensions and for derived physiological variables. A slight reduction in signal for a water-soluble component was noted when sampling a moist gas mixture with a long probe, but again this was of no practical significance. Ways in which the use of long sampling probes increase the versatility of the respiratory mass spectrometer are discussed. In essence they enable a greater range of subjects to be studied, they allow simultaneous events to be examined sequentially and they permit a single instrument to be shared between several patients or laboratories.