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Liquid core optical fiber waveguides are capillaries that contain a liquid core -liquid sample for spectroscopic analysis. The sensitivity of absorbance spectroscopy can be improved by extending optical pathlength. Liquid core waveguides provide for long optical pathlengths by constraining light propagation within a liquid medium which has a higher refractive index than the surrounding solid tubing. We present a simple optic method that allows to extend the sensitivity of conventional spectroscopic measurement. For long pathlength absorbance spectroscopy measurement the Teflon AF waveguide capillary cell for low refractive index liquids was designed. For ultraviolet, visible and near infrared region this type of fiber optic sensor with liquid core was designed. Benefit of this solution is possibility to use of small volume samples, long pathlengths by constraining light propagation within a liquid medium which has a higher refractive index than the Teflon AF tubing, sensitivity increase of conventional absorbance spectroscopy by two or more orders of magnitude. No preconcentration is required in chemical analysis. The analytical procedures employed in long pathlength absorbance spectroscopy are amenable to miniaturization and autonomous operation.

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