When diluting the samples, materials freely soluble in water required only dilution in Milli-Q water, however some water-soluble polymeric excipients (e.g. povidone) were shown to stick to the analytical column, which did effect the overall sensitivity of the method.
Fat-soluble excipients (e.g. magnesium stearate) were assessed using both a direct aqueous extraction, but also using an organic/aqueous back extraction. Results were shown to be comparable, but it was noted that the process of back-extraction increases the chances of low level contamination from the environment. As such, a process blank was undertaken alongside all sample extractions.
Materials submitted by the client containing significant levels of chloride impacted the accuracy and sensitivity of the method due to chloride eluting immediately prior to nitrite. Unfortunately, the gradient cannot be further optimised, however, to remedy this, a switch from conductivity to UV/vis detection allowed for the interference from the chloride peak to be removed. RSSL are further investing in a new instrument in 2023 which will allow for this approach to become routine for samples containing chloride.
Figure 2: Overlaid chromatogram of nitrite standard (black), spiked (pink) and unspiked (blue) sample containing high level of chloride.
RSSL were able to develop a highly sensitive method of nitrite/nitrate analysis which was applied to a range of pharmaceutical API and excipient materials using high pressure ion chromatography with conductivity detection. The method has since shown to be suitable for a range of materials and going forward, can be validated as per individual client requirements.