Liquid Chromatographic Sensing in Water on a Thin-Clad Optical Fiber by Mode-Filtered Light Detection

• Published in: Analytical chemistry (Washington) Vol. 68; no. 8; pp. 1456 - 1463
• Main Authors: Foster, Marc D, Synovec, Robert E
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.04.1996
• Subjects: Analytical chemistry; Chemistry; Chromatographic methods and physical methods associated with chromatography; Exact sciences and technology; Fiber optics; Hydrocarbons; Liquid chromatography; Organic compounds; Other chromatographic methods; Q1; Sensors; Water; Theoretical study; Liquid chromatography; Chemical sensor; Experimental study; Detection limit; Optical fiber
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac950937w

An improved annular column sensor is reported. The sensor is made by applying a thin polymeric film cladding of trifluoropropylpolysiloxane (TFPS), nominally 0.1 μm thick, to a 200 μm diameter bare glass fiber core and inserting the fiber into a short section of 250 μm i.d. fused silica capillary tubing, producing an annular column. A He/Ne laser is directed into the end of the fiber optic. When a small sample volume is injected into a mobile phase flowing along the sensor, selective partitioning of an analyte into the thin polymeric film, i.e., the stationary phase, causes a local change in the cladding refractive index, and thus the aperture of the fiber, and a consequent change in the amount of light emerging from the side of the optical fiber is sensitively detected. Temporal selectivity is simultaneously obtained as there is some chromatographic separation of analytes flowing through the annular column sensor. The sensor is both theoretically and experimentally characterized in terms of the sensitivity dependencies upon the capacity factor and flow rate. Reversed-phase liquid chromatography with low capacity factors for hydrophobic organic species using pure water as the mobile phase is demonstrated with the annular column sensor. The sensor is used to characterize the selectivity of the TFPS stationary phase. When the sensor signal is corrected for refractive index dependence, the revised signal is proportional to the partition coefficient into the polymeric cladding and is found to correlate well with the octanol/water partition coefficient for the analyte. The sensor can be used for the simultaneous separation and detection of organics in water and yields a detection limit of 500 ppb for cumene at 1 μL/min flow rate. This detection limit corresponds to an effective refractive index change in the mobile phase of 8 × 10-8 for a detection volume of 17 nL. The sensing mechanism does not require a chromophore, so the sensor also works well for chlorinated hydrocarbons and provides unique retention times and sensitivities, such as a 1.0 ppm detection limit for trichloroethylene in water.