Fixed-Oil Excipient Monographs: Development of USP Fixed-Oil Reference Standards - Pharmaceutical Technology

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Fixed-Oil Excipient Monographs: Development of USP Fixed-Oil Reference Standards
This article summarizes the development and modernization of the United States Pharmacopeia-National Formulary (USP-NF) fixed-oil excipient monographs. This article contains bonus online-exclusive material.


Pharmaceutical Technology
Volume 37, Issue 4, pp. 102-108


Table VII: Compendial specifications of Fatty Acid Composition (%) of fixed oils in USP 35−NF 30 (Ref. 1).
Fatty acid composition and melting range or temperature. Ideally, a robust specification should be independent of genetic modifications or seasonal or geographic variations that tend to change the composition of the substance over time and place. The Fatty Acid Composition of a fixed oil, however, varies with natural, seasonal, and geographical changes. Thus, the specification commonly defines the widest possible compositional range that is acceptable from the viewpoints of safety, functionality, and effective extraction. Even so, the specification limits for the Fatty Acid Composition should be narrow enough so that there is reasonable assurance that the substance is pure, not adulterated, not processed improperly or incompletely, and suitable for its intended use. The compendial specifications for Fatty Acid Composition for various fixed oils (not hydrogenated) in USP 35−NF 30 are summarized in Table VII.

Based on current practices employed by the fat and fixed oil industry, Fatty Acid Composition commonly is recommended as an identification test. The use of the Fatty Acid Composition to establish the purity of an oil has been criticized because of large variations permitted for certain fatty acid ranges (see Table VII). Thus, Fatty Acid Composition used alone as an identification test may not be sufficient to guarantee identity because some oils, such as Coconut Oil NF and Palm Kernel Oil NF, may exhibit similar fatty acid composition profiles, and some overlap exists in fatty acid ranges for different fixed oils. These two oils do possess different melting ranges. Therefore, both tests, Fatty Acid Composition and Melting Range or Temperature, used together may be able to achieve adequate differentiation. However, most of the fixed oils are liquids at ambient temperature, so application of Melting Range or Temperature specification is quite limited but can be found in several hydrogenated fixed-oil monographs.

Thin-layer chromatography. In recent years, thin-layer chromatography (TLC), specifically high-performance TLC (HPTLC), increasingly has been used for lipid analysis (21,22). TLC/HPTLC is a specific, reproducible, cost-effective, and routine quality-control analytical tool that provides a fingerprint identity for complex excipients such as fats, oils, and phospholipids. EXC EC has worked with the General Chapters—Chemical Analysis EC and the USP laboratory to propose a new general chapter, Identification of Fixed Oils by Thin-Layer Chromatography <202> (23). USP plans to release 11 USP Fixed-Oil Reference Standards in support of <202> (23). USP’s Dietary Supplements department is planning to add three new fixed oil dietary supplements to <202> to support new monograph development.

Unique patterns of TLC bands from specific triglycerides can distinguish fixed oils of different botanical origins, so it can be used orthogonally in conjunction with Fatty Acid Composition. The procedure is harmonized with European Pharmacopoeia general chapter 2.3.2. Identification of Fixed Oils by Thin-Layer Chromatography (24).


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