As noted in my Sunday post on the New York Times article on diethylene glycol (DEG) contamination of glycerin used for cough syrup, the US FDA has warned pharmaceutical manufacturers, compounding pharmacies, and repackers to test glycerine stocks for the toxic glycol.
Frequent commenter, Joe, noted that the sad part of the DEG deaths was that testing glycerin purity was a relatively simple chemical analysis. Through their guidance document, FDA has made available a free PDF of a paper from the Journal of AOAC International describing a simple, at-site thin-layer chromatography method for detecting ethylene or diethylene glycol contamination of glycerin-based products. The method is also useful for identifying the safe solvent, propylene glycol. Propylene glycol is often also found in drug and cosmetic products and was the subject of diethylene glycol contamination of acetaminophen syrup in Haiti in 1995-96.
The authors of the analytical method note that while TLC staining helps to visualize the spots, the difference in refractive index under simple white light is good enough for detecting abundant DEG contamination (more than 6% in glycerin and 2% in elixirs). Sounds like a fun but serious lab method for high school or early college chem labs because you only need 20 x 20 cm Merck silica plates (fluorescent indicator preferred), toluene-acetone-5M ammonium hydroxide (5:85:10), and a 20 min run time. (There is better resolution with n-butyl alcohol saturated with 1.5M ammonium hydroxide, but the run time goes up to 75 min). You don’t even need a TLC tank – just a coat hanger and a polyethylene bag to run the plate.
The complete guidance document is here, but FDA also notes a more precise gas chromatographic method from a USP monograph that permits differentiation between glycerin and DEG:
The Agency recommends that:
1. Drug product manufacturers perform a specific identity test that includes a limit test for DEG on all containers of all lots of glycerin before the glycerin is used in the manufacture or preparation of drug products because of the serious hazard associated with DEG contamination.3. The relevant safety limit for DEG is 0.1%, as recognized by the USP monograph for glycerin. The Agency recommends that a manufacturer perform the identity tests, including the limit test for DEG, which appear in the USP monograph for glycerin. Alternatively, a manufacturer may use an equivalent identification procedure that includes a test to detect and quantify DEG provided it meets the relevant safety limit. One alternative procedure is a thin-layer chromatography (TLC) method published in the Journal of AOAC International. 4. [APB: The free article noted earlier]
This identification test is done in accordance with the CGMPs, which require that each lot of a component undergo testing to confirm its identity before use in drug product manufacturing (21 CFR 211.84(d)(1) requires that “[a]t least one test shall be conducted to verify the identity of each component of a drug product. Specific identity tests, if they exist, shall be used”). A specific identity test for glycerin is found in the United States Pharmacopeia (USP) monograph. 5. The USP monograph for glycerin provides a two-part identity test: test A using “Infrared Absorption” and test B using gas chromatography that references the “Limit of Diethylene Glycol and Related Compounds.” The infrared absorption test identifies glycerin and DEG, but does not distinguish between the two. Test B distinguishes glycerin from DEG. 6.
Science Blogs colleague, Molecule of the Day, has a separate and concise post with the structure of diethylene glycol.