A membrane-inlet mass spectrometer miniprobe for the direct simultaneous measurement of multiple gas species with spatial resolution of 1 mm

The design and construction of a membrane-covered gas miniprobe inlet using a quadrupole mass spectrometer is described. The probe is a metre-long steel capillary tube (o.d. 1.6 mm) terminating in a 4 cm 300 mu m o.d. hypodermic needle. Response times for H-2, CH4, N-2, O-2 and CO2 in gaseous or liquid phases are of the order of minutes (t(100%)). Gas concentrations of the order of 1 mu M can be measured with a spatial resolution of the order of 1 mm. Examples are given of the vertical distributions of O-2 and CO2 in a freshwater sediment, and of O-2, CO2, and CH4 in a peat core. The characteristics of this system are discussed in detail in terms of the usefulness of the device for localized ecological gas measurements.