Vapor pressure by DSC: extending ASTM E 1782 below 5 kPa

In 1996, American Society for Testing and Materials (ASTM) Method E 1782–96, Standard Test Method for Determining Vapor Pressure by Thermal Analysis received final approval and was published as a standard method. For vapor pressure measurements by DSC, the method requires use of hermetic-type sealable pans with a single pinhole ≤125 μm (50–75 μm recommended) in the center of the lid. Pinholes of this size produce generally acceptable results over the 5 kPa to 2 MPa operational pressure range of the method. However, as the minimum recommended pressure is approached, boiling endotherms become increasingly broad. Recent efforts using larger pinholes (175–375 μm) have shown improved peak sharpness and elimination of leading edge curvature when pinhole size is increased as pressure is reduced. This need for increased pinhole diameter at lower pressures is consistent with orifice selections used in effusion studies. Optimization of pinhole size as a function of pressure may extend the practical operational range of ASTM E 1782 to permit rapid vapor pressure determination to at least 0.2 kPa.