An optical spectral sensing device for determining at least one property of a fluid. The device has an elongated porous body, a first end and a second end, a solid-state optical emitter at the first end of the body oriented to emit radiation toward the second end of the body, and a solid-state optical detector at the second end of the body oriented to detect radiation emitted by the optical emitter. A package for detecting properties of a fluid includes a body defining a cavity, with a movable and biased carrier for an optical detector or emitter mounted in the cavity for increased reliability. A system for determining relative concentrations of fluids in a sample includes emitter/detector pairs operating at reference wavelength and wavelengths corresponding to absorption peaks of at least two fluids, and a processor for determining concentration based on measured data and calibration data.