Gas chromatography combined with matrix isolation infrared spectroscopy (GC/MI-IR) enables identification and quantification of components of complex mixtures by infrared spectroscopy at levels of a few nanograms. These levels are several orders of magnitude lower than those achieved by light pipe-based infrared measurements. Researchers used GC/MI-IR to identify polynuclear aromatic hydrocarbons in extracts of several types of air samples, including ambient air particulate matter, diesel engine exhaust, and wood smoke. Included are examples of these analyses and a discussion of the on-going research using MI-IR for identification of unknown compounds associated with the highly mutagenic fractions of air samples.