Headspace oxygen analysis (HOA)

Headspace oxygen analysis (HOA), also known as Frequency Modulation Spectroscopy (FMS), is a non-invasive technique for the determination of the oxygen content in the headspace of a freeze-dried drug product.

HOA measures the absorption of a laser light with a wavelength of 762 nm that passes through the gaseous headspace of a vial containing the freeze-dried drug product. The oxygen content in the headspace is determined by using a calibration curve from standards of known oxygen concentrations. Since absorption is dependent on the path-length and influenced by the optical properties of the container, container-specific standards are constructed. 

HOA can be used during lyophilization process development, stability studies as well as for quality control testing of freeze-dried products. A consistently low headspace oxygen content is an important parameter for the determination of container closure integrity. Further, changes in headspace oxygen content can indicate product instability of the drug product such as oxidation-related protein degradation. 

A major benefit of HOA is its non-destructive nature, allowing for 100% testing of drug products or a subsequent analysis of the same vial using other techniques. Further, the analysis time is short and its performance straightforward. HOA, however, requires a rather elaborate method development using standards in the respective container, making it less attractive for early-stage lyophilization/formulation development and individual sample testing.