Residual Solvent Analysis Information

Residual solvents in pharmaceuticals are defined here as organic volatile chemicals that are used or produced in the manufacture of drug substances or excipients, or in the preparation of drug products. The solvents are not completely removed by practical manufacturing techniques. Appropriate selection of the solvent for the synthesis of drug substance may enhance the yield, or determine characteristics such as crystal form, purity, and solubility. Therefore, the solvent may sometimes be a critical parameter in the synthetic process.

In 1988, the United States Pharmacopoeia (USP) provided control limits and testing criteria for seven organic volatile impurities (OVIs) under official monograph USP <467> Residual Solvents. The compounds were chosen based on relative toxicity and only applied to drug substances and some excipients. They have since extended the compound list in General Chapter <467> Residual Solvents, and harmonized their efforts by aligning limits with the International Council for Harmonization (ICH) guidelines ICH Q3C Guideline for Residual Solvents.

Allowable limits or Permitted Daily Exposure (PDE) for residual solvents are defined in ICH Q3C. The PDE is based on the toxicity of the solvent. Solvents are defined into three class:

• Class 1 solvents: Solvents to be avoided—known human carcinogens, strongly suspected human carcinogens, and environmental hazards.
• Class 2 solvents: Solvents to be limited—on-genotoxic animal carcinogens or possible causative agents of other irreversible toxicity such as neurotoxicity or teratogenicity. Solvents suspected of other significant but reversible toxicities.
• Class 3 solvents: Solvents with low toxic potential—solvents with low toxic potential to man; no health-based exposure limit is needed. Class 3 solvents have PDEs of 50 mg or more per day.

ICH Q3C and USP <467> are harmonized in their approach with a salient exception: whereas ICH Q3C applies only to new drug products, USP <467> applies the same requirements to all new and existing drug products.

To meet low levels in regulated analytical protocols, many modern methods employ valve and loop style headspace auto-samplers, gas chromatographs with flame ionization or mass spectrometric detection and compliant chromatography data software.

For more information on these technologies, please visit our Volatile Organic Impurities section of web content.

A key stage in many pharmaceutical processes is the complete or partial removal of a solvent or solvents from a product or intermediate. This drying process can occur in a variety of process vessels, including vacuum dryers, tray dryers and rotary dryers. Many API drying processes involve the removal of two or more solvents from a potential list of over thirty compounds. This requires complex chemometric modelling to turn the spectroscopic data into process friendly concentration data. Gas analysis by MS offers significant advantages of simplicity in both sampling and data manipulation for the solvent drying application.