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Absorption spectroscopy represents one highly relevant approach in current analytics to non-invasively characterize liquid analytes. Here we present the concept of waveguide-integrated absorption spectroscopy via nanobore optical fibers. Using a liquid-filled nanochannel inside the core of a microstructured step-index fiber, the spectroscopic characteristics of liquid analytes located inside the nanochannel are imprinted onto the propagating mode via its evanescent fields, allowing to conduct absorption spectroscopic experiments at extremely small sample volume levels. We reveal the limits of this spectroscopic approach by analyzing the dependence of the power fraction inside the bore on the fiber parameters and experimentally demonstrate its capabilities by 1) using the cut-back technique using an optofluidic mount and 2) fully encapsulating a highly doped dye solution inside the nanobore fiber. Based on its high degree of integration and the straightforward handling capabilities, application of the nanobore fiber based absorption spectroscopy concept can be anticipated in fields, such as bioanalytics, analytical chemistry, and environmental science. [ABSTRACT FROM AUTHOR]