The shortcut USP comes from United States Pharmacopeial convention. Every kind of material or phase has a USP code. It is a general grouping for materials and the differences between materials with similar modifications are excluded. For example all C18 phases have the same USP column code. Some examples: C18 (USP L1), C8 (USP L7), Si-phases (USP L3), Phenyl-phases (USP L11), Amino-phases (USP L8), Ion exchange phases like Eurokat H (USP L17).
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On the KNAUER website you can search columns via the USP column code using the interactive column finder.
The USP codes of KNAUER columns and phases is also given in the document below.