Methods and systems for real time, in situ monitoring of fluids, and particularly the determination of both the energy content and contaminants in a gas or oil transmission facility, are provided. The system may include two separate scanning sources to scan two different, but overlapping, NIR ranges, or may involve two separate scans from a single scanning spectroscopy source. The first scan ranges from approximately 1550 nm up through 1800 nm and a second scan concurrently scans at a high resolution across a band from approximately 1560-1610 nm, the wavelength of interest for hydrogen sulfide (though similar scans are contemplated in alternative wavelength ranges for alternative contaminants). The second scan may provide very narrow (0.005 nm) step resolution over just the wavelength of interest for the contaminant and may scan at a substantially higher power level. The spectroscopic optical data from the two scans, however obtained, must then be combined into an analytical processing module containing models that analyze the multi-scan data and yield both energy content and contaminant quantitative data.