OptiMelt Automated Melting Point System - Stanford Research ...

Chapter 3 Melting Point Determination 55Tips• The meniscus point is often the temperature listed in European Melting Point tablesand the preferred value of the British Pharmacopeia methodology.• In an attempt to remove subjectivity from its detection, the Laboratory of theGovernment Chemist (LGC) defines the meniscus point as “the point where adefinite meniscus is visible and there is equal volumes of solid and liquid in thecapillary”.• The meniscus point is not specifically mentioned by the US Pharmacopeia MeltingPoint methods (Method of USP25-NF20). The clear point (described below) isidentified as the “melting point” of a substance instead. Notice that this is asignificant difference in interpretation between the British and US Pharmacopeias.• The SRS OptiMelt can automatically detect and record the meniscus point of asample. A user adjustable threshold (Single%) is available to match the visual andautomatic records of the meniscus point.• The use of a packing wire during sample preparation can lead to excessive bubbleformation and trapping for some samples. Bubbles can interfere with the automateddetermination. Do not use a packing wire if bubbles interfere with the detection of themeniscus point.Clear or Liquefaction PointThe clear point corresponds to the stage of the melt at which the substance becomescompletely liquid – no more solid is left (i.e. the last crystals are melted).Figure 33. Clear Point.The clear point is more dependent on the ramping rate than the onset point. In general,the clear point increases with increasing ramping rates (see Table 1).OptiMelt Automated Melting Point System