Impurity Isolation at a glance

Impurity isolation has always had a place in the Reach purification lab. However, over recent years the frequency of requests has steadily increased. This has given us plenty of opportunity to fine-tune our approach, as well as broaden our scope to include characterisation.

Why isolate in the first place?

During synthesis and subsequent manufacture, an API is unlikely to be 100% pure. Impurities that are present over given ICH thresholds must be identified and often tested. Chromatography provides a simple route to obtaining a small amount of pure material for characterisation. Scale up of the same techniques can be employed to generate material for toxicological studies or for use as analytical standards.

What’s the approach?

As with all purification work, the story starts in analysis. Typically, a client will supply primary data identifying the target impurity, by molecular mass and/or by retention time. Once we can replicate this method or detect the molecular ion, the impurity can be tracked across new methods as we screen for an optimal separation.

HPLC and SFC are perfect partners when screening for a method, due to the orthogonality of the techniques. SFC is advantageous at preparative scale as throughput is likely to be higher and fractions are collected in a reduced volume of solvent for evaporation.

A perfect scenario is to perform a high throughput ‘first pass’ using SFC to efficiently produce fractions enriched in the target impurity. This often involves processing large amount of material which has its own challenges (see our blog on large-scale purification). Analytical spiking studies can be used to confirm the correct impurity is being targeted.

The enriched material will next undergo a second pass to reach the desired purity. HPLC is often the answer as it allows for easy fractionation as well as orthogonality to the first SFC pass. As there is now much less material post-enrichment, aqueous HPLC fractions are at manageable volumes for freeze-drying.

The end result?

An impurity that began at perhaps 0.1% in the crude material is now enriched to >95% purity and can be returned to the client for testing.

A few years ago this is where the story ended; impurity isolation and return. Now, due to numerous requests, we’ve teamed up with MS-MS and NMR specialists to offer a full package of isolation and elucidation. All this information in a single report provides clarity and saves the chemist time in-house. More on our external partners here.

You can download our White Paper on Impurity Isolation here.