Sheath Flow Focusing in Supersonic Jet Spectroscopy

Abstract

The mechanism of cooling in sheath-flow-focused supersonic jet expansions is examined. Cooling is found to be strongly influenced by the sheath gas flow properties but independent of the carrier gas in the sample stream. These results indicate that considerable turbulence and mixing between the sheath and sample gases occur downstream from the orifice. However, mixing cannot be complete, since, relative to results with a conventional jet expansion, a substantial enhancement of analyte is obtained along the centerline of a sheath-flow-focused jet expansion. Spectral broadening at “high” analyte mass flow rates within the sample stream is found to arise from inefficient cooling. There are limits to both how large and how small the nozzle orifice can be. Small orifices result in spectral broadening, even at very low analyte mass flow rates. Large orifices may have Reynolds numbers sufficient to cause turbulent flow, which degrades the focusing effect. The optimum nozzle geometry and gas flow conditions for sheath-flow-focused jet expansions are discussed.