A new high sensitivity microsampling accessory incorporating an all-reflecting microscope for use with FT-IR instruments is described. Measurements of the interstitial oxygen and substitutional carbon concentrations in silicon from sampling areas as small as 100 μm × 100 μm are described. A reflectance version of the microsampling accessory has been used for mapping the epitaxial film thickness on silicon from areas as small as 50 μm × 50 μm.