Question of the month - February 2019
How do we determine assay values for Mikromol impurity reference standards?
For our impurity reference standards, whenever possible, we use high-performance liquid chromatography with photodiode array (HPLC-PDA) or another chromatographic technique, plus volatile content (water and residual solvent) to calculate the assay value using the equation below. This is also known as the 100%-method:
HPLC is the most commonly used technique to assess organic purity. Gas chromatography (GC) is used only where HPLC is not applicable.
Water is determined by Karl Fischer Titration. Residual solvents are estimated by the use of proton nuclear magnetic resonance (1H-NMR), which is a powerful technique, and almost as accurate as determination by GC/headspace techniques. Loss on drying (LOD) experiments are not carried out, as the technique is not as precise and accurate as the combined determination of water and residual solvents.
For more information, we recommend downloading our white paper “Characterisation of non-compendial impurity reference standards: How good is good enough?”.