Spectroscopic apparatus and methods incorporating a gas sensor configured to detect low concentration gases, including gases that are hazardous volatiles are provided. Low concentration gases can comprise gases where detection of concentrations on the order of parts-per-million (ppm), and in many embodiments part-per-billion (ppb) is required. The gas may be a species, such as, for example hydrogen sulfide (H2S) that may be produced in drilling and/or volcanic eruptions. The spectroscopic apparatus and methods are configured to operate in particular atmospheres where gas detection can be challenging, such as in ambient air and/or in space where various contaminants may be present. The spectroscopic apparatus and methods may incorporate a long path length detector, such as, for example, a cavity-enhanced absorption spectrometer. The methods and apparatus further incorporate a wavelength modulation technique to improve the signal-to-noise ratio.