Measurements of dissolved nonmethane hydrocarbons in sea water

Summary

An automated stripping technique for the measurement of dissolved hydrocarbons in sea water is presented together with some results obtained during a ship cruise from Europe to Brazil. The sea water concentrations of NMHC were determined in a three step process: degassing, preconcentration, and gas chromatographic analysis. In a stripping chamber the dissolved gases were purged from sea water with helium. The stripped hydrocarbons were cryogenically concentrated, and after thermal desorption they were injected into the gas chromatograph. The light fraction (C₂–C₄) was separated on a packed and the heavy fraction (C₅–C₁₀) on a capillary column. All valves were microprocessor controlled in order to achieve an automated process. For the C₂–C₄ hydrocarbons the stripping efficiencies exceeded 90% except for acetylene (80%), the lower limit of detection was 1 to 4.5 pmol hydrocarbon per liter of sea water and the accuracy of the method was better than 25%, depending on the individual hydrocarbons. Typical oceanic concentrations were in the 10 and 100 pmol/l range. Alkenes were generally more abundant than the corresponding alkanes and within the homologous series the concentrations decreased with increasing number of carbon atoms.