David Smith has carried out most successful and comprehensive studies of a variety of gas-phase ionic reaction processes at thermal energies, including ion–molecule interactions, ion–ion and ion–electron recombination and electron attachment. In variety, depth and range of application, this effort probably surpasses any other in the world. To achieve this he conceived, developed and exploited a unique apparatus, the selected-ion flow tube (SIFT).
The SIFT technique has now been adopted by many leading research groups around the world. His work has very wide application as is illustrated by the important contributions which he has made to aeronomy, astrophysics and astrochemistry, laser physics, plasma physics and plasma chemistry, and also to the fundamental understanding of ionic reactions in the gas phase.
A change of research direction resulted in the development of selected-ion flow tube mass spectrometry, SIFT-MS, used for the direct analysis of trace gases in ambient air, exhaled breath and the headspace of biological fluids in the search for volatile biomarkers that can be exploited for non-invasive clinical diagnosis and therapeutic monitoring.