Stereoselective Determination of Thyroxine Enantiomers on Chiral Crown Ether Column by UPLC‐ESI‐Tandem Mass Spectrometry

Liquid Chromatographic Enantioseparations Utilizing Chiral Stationary Phases Based on Crown Ethers and Cyclofructans

Natural compounds can exist in different forms, where molecules possessing chirality play an essential role in living organisms. Currently, one of the most important tasks of modern analytical chemistry is the enantioseparation of chiral compounds, in particular, the enantiomers of compounds having biological and/or pharmaceutical activity. Whether the task is to analyze environmental or food samples or to develop an assay for drug control, well-reproducible, highly sensitive, stereoselective, and robust methods are required. High-performance liquid chromatography best meets these conditions. Nevertheless, in many cases, gas chromatography, supercritical fluid chromatography, or capillary electrophoresis can also offer a suitable solution. Amino acids, proteins, cyclodextrins, derivatized polysaccharides, macrocyclic glycopeptides, and ion exchangers can serve as efficient selectors in liquid chromatography, and they are quite frequently applied and reviewed. Crown ethers and cyclofructans possessing similar structural characteristics and selectivity in the enantiodiscrimination of different amine compounds are discussed less frequently. This review collects information on enantioseparations achieved recently with the use of chiral stationary phases based on crown ethers or cyclofructans, focusing on liquid chromatographic applications.

Simultaneous enantioselective separation method for thyroid hormones using liquid chromatography-tandem mass spectrometry and its applications

An analytical method for the simultaneous determination of chiral thyroxine and the related iodinated chiral compounds using LC-MS/MS is introduced in this study. D-Thyroid hormones (THs), which are not commercially available, were produced through the racemization reaction of the L-THs in acetic acid solution containing salicylaldehyde. The solution containing D- and L-THs after the reaction was used for optimizing the chiral separation. The D- and L-THs were well separated enantiomerically under isocratic conditions in 70 % acetonitrile containing 0.1 % formic acid on a CROWNPAK® CR-I (+) column, but some peaks, such as those of diiodo-D-tyrosine (D-DIT)/monoiodo-L-tyrosine, diiodo-D-thyronine/diiodo-L-tyrosine and D-thyroxine/triiodo-L-thyronine, overlapped chromatographically, causing misinterpretation in impurity analysis. This was overcome by using the gradient condition providing the best chiral selectivity (α) and resolution (Rs) ranging from 1.14 to 1.37 and from 2.39 to 4.52, respectively. The linearity was above 0.999 and the detection limits ranged from 8.2 to 57.7 ng/mL by the separation method. This method was applied to identify and quantify chiral impurities in authentic standards and pharmaceuticals. As a result, D-enantiomers corresponding to the L-THs standards as well as L-DIT were commonly observed as impurities. In the stability test of DL-thyroxine under acidic conditions for identifying the distribution of chiral products, it was observed that the formation of DIT by hydrolysis increased over time. Additional products formed through esterification, including thyroxine methyl ester and diiodo-tyrosine methyl ester, were newly separated and identified using a C18 column.