Direct nitrite analysis simplifies tests of pharmaceutical excipients for risk checks

A liquid chromatography method with ultraviolet detection has quantified nitrite in strongly coloured excipients without derivatisation, which could make a demanding measurement more accessible to routine laboratories

A research team from Sumitomo Pharma in Osaka, Japan has developed a high-performance liquid chromatography method with ultraviolet detection to quantify nitrite in pharmaceutical excipients that produce strong matrix effects. The workflow avoids derivatisation and removal of the strongly coloured organic matrix which has addressed two potential sources of complication in an analysis relevant to pharmaceutical nitrosamine-risk assessment.

The study examined the colour additive ‘Sunset Yellow FCF’, its aluminium lake and five grades of microcrystalline cellulose. The suffix FCF denotes ‘for colouring food’ and forms part of the additive’s established name. The researchers developed an extraction procedure suited to these challenging samples and used ion chromatography with conductivity detection as a reference platform.

Excipients perform roles such as support, bulk or colour in a formulation but their composition can complicate trace analysis. A strongly coloured extract may absorb light in regions relevant to optical detection while other matrix components can affect separation or recovery. A method must distinguish the analyte from those contributions and show that sample preparation retains a representative amount of nitrite.

Derivatisation converts an analyte into a chemical form that may be easier to detect or separate. Although useful, it introduces additional reactions, reagents and timing requirements. If the conversion is incomplete or the analyte changes during preparation, the final signal can misrepresent its original concentration. A direct method removes those particular steps, although it still requires careful control of extraction and detection.

The reported calibration coefficients exceeded 0.9999. This indicated a very close relationship between concentration and instrumental response within the calibration conditions. However, a high coefficient alone cannot demonstrate that a method is free from matrix interference or accurate at every concentration. Independent checks of recovery and repeatability are therefore important companions to the calibration result.

Repeatability ranged from 0.8 to 3.1 per cent relative standard deviation. Relative standard deviation expresses the spread of repeated measurements as a proportion of their mean, which helps compare precision across concentration levels. These figures described consistency under the study’s repeat conditions. 

Spike recoveries in the two colour additives were 96.9 and 105.2 per cent. A spike-recovery experiment adds a known quantity of analyte and tests how much the method measures after the relevant preparation. Values around 100 per cent support the ability to recover and quantify the added material. A result above 100 per cent reflects measurement variation or bias rather than the creation of additional analyte.

One aluminium-lake product contained approximately 88 micrograms of nitrite per gram. An aluminium lake is a pigment form of the colour additive, and its analytical behaviour can differ from that of the soluble dye. 

The tested microcrystalline celluloses were below the method’s quantification threshold meaning that nitrite could not be quantified reliably at the relevant level with this workflow but did not establish complete absence. For a risk assessment, the threshold itself and the amount of excipient in a formulation determine how informative such a result is.

The reference analysis by ion chromatography offered a useful comparison because conductivity detection relies on a different measurement principle from ultraviolet absorption. Agreement between complementary methods can strengthen confidence in a difficult matrix, particularly where interference might affect one detector more than another. It remains necessary to consider whether both methods share any preparation-related source of bias.

The attraction for routine laboratories is the use of widely available liquid chromatography equipment. Avoidance of derivatisation and aggressive matrix removal may simplify implementation and reduce opportunities for nitrite loss. 

The study has supplied an accessible analytical route for matrices that can frustrate direct nitrite measurement. Its contribution to pharmaceutical quality work is a better means to characterise a potential precursor, with numerical performance data in selected difficult excipients. The results support further method transfer and validation, while the significance for any individual medicinal product must remain tied to its complete formulation and manufacturing context.

For further reading please visit: 10.1002/sscp.70300