Abstract
Malachite green oxalate (MG oxalate) and leucomalachite green (LMG) have been prepared and certified as pure reference materials. The purities of MG oxalate and LMG were assessed by high-performance liquid chromatography–diode array detection (HPLC–DAD), nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), Karl Fischer titration, ashing and thermogravimetric analysis (TGA). MG oxalate was purified by supercritical fluid extraction (SFE). Prior to purification, commercial MG oxalate purity was estimated to be about 90%. The main impurities present in SFE-purified MG oxalate were identified and quantified using HPLC–DAD. The main impurities were found to be monode-MG (monodemethylated MG oxalate synthesis impurity), 4-(dimethylamino)benzophenone (4-DMABP), MG-carbinol and LMG. The homogeneity of both reference materials was also determined. Issues associated with the stability of LMG and MG oxalate in solution forced an extensive study investigating different parameters i.e. solvent, acid, analyte concentration and temperature. MG oxalate (100 μg/mL) was found to be stable in acetonitrile containing 1% v/v glacial acetic acid for at least 155 days and LMG (100 μg/mL) was stable in acetonitrile for at least 133 days. The final purity value for MG oxalate was 94.3 ± 1.4% m/m at the 95% confidence interval (or 67% m/m if MG cation is reported). For LMG, the certified purity was found to be 98.8 ± 0.8% m/m at the 95% confidence interval.
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Acknowledgements
The authors would like to thank the Chemical and Biological Metrology Knowledge Base Programme, which is funded by the UK Department for Innovation, Universities and Skills. The authors would also like to thank Nick Bosley, Mike Ludlow and Ben Joseph of LGC for their work on respectively TGA, NMR and moisture, and Andrew Busby of QinetiQ (Sevenoaks, Kent, UK) for his work on the SFE extraction.
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Le Goff, T., Wood, S. Production of malachite green oxalate and leucomalachite green reference materials certified for purity. Anal Bioanal Chem 391, 2035–2045 (2008). https://doi.org/10.1007/s00216-008-2048-2
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DOI: https://doi.org/10.1007/s00216-008-2048-2