Presented herein are systems and methods for quantifying trace and/or ultra-trace levels of a species—for example, H2S or H2O—in a natural gas line. The systems and methods employ a tunable laser, such as a tunable diode laser, vertical-cavity surface-emitting laser (VCSEL), external cavity diode laser or a vertical external-cavity surface-emitting laser (VECSEL) or a tunable quantum cascade laser (QCL). The laser produces an output beam over a set of one or more relatively narrow, high resolution wavelength bands at a scan rate from about 0.1 Hz to about 1000 Hz. A natural gas sample comprising a trace level of a species of interest passes through a flow cell into which the output beam from the laser is guided. An optical detector receives light from the flow cell, producing a signal indicative of the absorption attenuation from which the concentration of the trace species is determined.