The Measurement Of Pharmaceutical Lubricants
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•Application Note: The Measurement Of Pharmaceutical Lubricants
By Micromeritics Instrument Corporation
A major advantage for using the TriStar II with the Krypton Option is the ability to measure low surface areas. Although the TriStar II standard nitrogen system can measure surface areas as low as 0.01 m2/g, the Krypton Option enables you to measure areas as low as 0.001 m2/g and provides increased accuracy for materials under 1.0 m2/g. Krypton is also useful for analyzing materials that are difficult to sample or have very low density; pharmaceutical lubricants, for example.
Isotherms are collected by measuring the amount of gas adsorbed by a material over a range of pressures at a constant temperature. The quantity of gas adsorbed by a material is determined by taking the original quantity of gas dosed into a tube (Vi) and subtracting the amount of gas remaining in the tube after equilibrium is reached (Ve). For materials with low surface areas, the difference between the original amount of gas and the amount remaining after equilibrium (Vi – Ve) can be substantially small and difficult to measure, resulting in increased error. Normally for materials with low surface areas, the amount of material is maximized in order to increase this difference (Vi – Ve). Unfortunately this approach has an upper limit depending on the size of the sample tube and the physical properties of the material. Another approach is to use an alternate analysis gas.
Krypton is an excellent choice for low surface area measurements. Nitrogen at 77 K has a saturation pressure of 760 torr, whereas krypton has a saturation pressure of only 2.5 torr. Since pressure is proportional to the number of moles or molecules in a set volume, there are ~ 300 molecules of nitrogen for every 1 molecule of krypton. When the quantity adsorbed is significantly small, lowering the amount of molecules present by a factor of 300 substantially reduces the amount of error.
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•Application Note: The Measurement Of Pharmaceutical Lubricants