Analysis of 4-bromo-3-fluorobenzaldehyde and separation of its regioisomers by one-dimensional and two-dimensional gas chromatography

• Two methods were developed for regioisomer impurity control in API.
• Very unique resolution of chiral GC column for the 10 regioisomers was utilized.
• 2D-GC method was developed for the separation of the ten regioisomers.
• μ-ECD detector achieved the best LOQ of the ten regioisomers.
• The methods were validated for p-GTI control in trace level in API.

A starting material, 4-bromo-3-fluorobenzaldehyde, was used for active drug substance (API) AMG 369 production. The presence of the regioisomer impurities in the starting material 4-bromo-3-fluorobenzaldehyde presented significant challenges for the API synthetic route development due to the physical-chemical similarities of the impurities. These impurities significantly impact on the purity of the starting-material and final drug substance. Control of these impurities is important due to the potential genotoxicity of these impurities (p-GTI). Analytical development was carried out to develop GC methods with high resolving power and high sensitivity to quantify the regioisomers presented in starting material and therefore to control the purity of the starting material and the final drug substance.In the study, complete resolution of the ten regioisomers by 1D-GC and heart-cutting two-dimensional GC (2D-GC) was achieved. A sensitive GC/micro electron capture detection (μ-ECD) method with high resolving power and sensitivity to fully resolve all the ten regioisomers of 4-bromo-3-fluorobenzaldehyde was obtained by using a CHIRALDEX GC column (1D- GC). To facilitate the systematic GC method development, heart-cutting two-dimensional gas chromatography (2D-GC) using a Deans switch was exploited for the separation of the ten regioisomers. The resulting heart-cutting 2D-GC method successfully separated all the ten regioisomers with better sensitivity and resolution. Regioisomer impurities in the starting material were identified and quantified by these GC methods. The sensitivity for the methods is in the range of 0.004 ng to 0.02 ng for the regioisomers. Linearity for the methods is: R2 = 0.999 to 1.000. The methods were suitable for control of the regioisomer impurities, p-GTIs, in the starting material and final drug substance.