This behavior is typical of hygroscopic lipids. The hygroscopic lipids are generally those with short fatty acid chains (£C10) or double bonds in the fatty acid chain. This would include the short chain synthetic glycero-phospholipids (e.g., C6 PC, C8 PC, etc.), the unsaturated synthetic glycero-phospholipids (e.g., oleic, linoleic, arachidonic, etc.) and the non-hydrogenated natural glycero-phospholipids (e.g., Egg PC, Brain PS, etc.). These products should be weighed under dry conditions (i.e., in a dry box) or dissolved in an organic solvent and aliquoted as a solution (handling procedures for organic solutions of lipids apply).