The selected ion flow tube (SIFT)--a novel technique for biological monitoring.

We describe the use of our selected ion flow tube (SIFT) technique for the rapid detection and quantification of trace gases in atmospheric air, with special reference to the analysis of human breath. It is based on the chemical ionization of the breath trace gases to the exclusion of the major breath gases, using 'soft' proton transfer from H3O+ ions. Breath samples can either be introduced into the SIFT from bags or by direct breathing into the apparatus, the advantage of the latter approach being that surface active gases such as ammonia and many organic vapours which adsorb onto bag surfaces can be more accurately quantified. We present examples of the analysis of laboratory air, the breath of a non-smoker and of a smoker taken from bag samples, and illustrate the rapid time response of the technique by showing the time profile of acetone on breath during direct breathing into the apparatus. The current partial pressure sensitivity of our SIFT method is within the range 30 ppb to in excess of 100 ppm, but with further development the device could be made more sensitive, 1 ppb being well within reach. A transportable SIFT device is under development which will have applications in environmental, medical and biological research, health and safety monitoring, and in clinical diagnosis.