Abstract: An unjacketed optical fiber was inserted into a transparent capillary, such that the i.d. of the tube was slightly larger than the o.d. of the fiber cladding; full details are given. A sample was introduced into this annular column at a low flow rate; propagated light from a He-Ne laser source was mode filtered due to partitioning of the chemical species into the polymeric cladding. Light emitted at right angles to the fiber axis was determined with a photomultiplier tube. The sensor exhibited chemical selectivity provided by differences in analyte RI, distribution coefficient and transient response time. The detection limit was 20 ppm for butylbenzene using a polysiloxane-clad fiber and an aqueous 60% methanol mobile phase. The annular column sensor is a potential detector for LC, FIA and GC for process analysis and environmental monitoring. A novel chemical analyzer is described in which an unjacketed optical fiber is inserted into a transparent capillary tube, such that the inner diameter of the tube is slightly larger than the outer diameter of the fiber cladding. This configuration is referred to as an annular column. When a sample volume is introduced to the annular column at a low flow rate, propagated light is mode-filtered due to a change in the critical angle at the core/clad interface, as a result of partitioning chemical species. Conventionally, chemical species partitioning into a fiber-optic cladding are sensed as a change in the transmitted light at the end of the fiber. An alternative approach, measuring this mode-filtered light directly along the side of the fiber, is reported. The new approach has a signal-to-noise advantage over the conventional approach, since the analyte signal is measured against a low background instead of a high background. The result is a low-volume chemical sensor that temporally separates, as well as detects, chemical species that partition into the fiber cladding. The temporal information enhances sensor selectivity. We have examined the modulation of the critical angle by chemical species of interest at steady-state concentrations and as transient concentration profiles that were shifted in time due to chromatographic retention within the sensor. A detection limit of 20 ppm was achieved for butylbenzene using a polysiloxane-clad fiber and a 60/40 methanol to water (by volume) mobile phase. In summary, the sensor has chemical selectivity provided by differences in analyte refractive index, distribution coefficient, and transient response time. The annular column sensor is shown to be a potential detector for liquid chromatography, flow injection analysis, and gas chromatography, in particular for process analysis and environmental monitoring applications. Likewise, solid phase extraction and sensing for online sampling applications are demonstrated. Copyright 1995, American Chemical Society.