To estimate the weight percents of certain nonmetal elements in organic compounds found in petroleum reservoirs, the Bureau of Mines designed and constructed a controlled-atmosphere plasma arc which effectively excludes air from the plasma. Selected organic compounds that contained nonmetal elements were mixed with a solvent essentially free of those elements. The weight percents of the nonmetal elements were estimated from the produced diatomic spectral bands; results were reasonably accurate. Analytical working curves were prepared for nitrogen in the range from 15 to 450 ag for 100 mg samples using the analytical carbon-nitrogen band at 3,883 oa and the internal standard carbon-carbon band at 3,607 oa. Curves also were prepared for hydrogen, sulfur, and oxygen that had limited sensitivity to detection. Another indicated application of this plasma arc is for characterization of high-boiling petroleum fractions in liquid chromatographic effluents. The rates at which diatomic species are produced and destroyed directly influence the intensity of each corresponding band spectrum.