An apparatus for performing spectroscopy includes an optical waveguide comprising a fluidic channel to receive a fluid sample, in which the optical waveguide is to propagate lightwaves at a set of frequencies. The apparatus also includes a wavelength selective device coupled to the optical waveguide, in which the wavelength selective device comprises a predetermined bandwidth and is to capture frequencies of light within the predetermined bandwidth. The apparatus further includes a detector coupled to the wavelength selective device to generate signals that identify the frequencies captured by the wavelength selective device.