32 February / March 2021 A
2-butynoic acid and propiolic acid, were not resolved in isocratic or gradient IC methods but were easily detected, resolved and quantified using extracted ion chromatograms. When faced with co- eluting impurities, rather than relying on the information obtained by their elution profiles, using information provided by measuring their masses, and extracting the corresponding ion chromatograms, allows resolving these impurities even if that is chromatographically not possible. MS hyphenation, therefore, becomes particularly important when quantifying closely eluting or overlapping impurities.
B
Without the need for derivatisation, IC- MS is significantly more straightforward and faster to perform, saving valuable time during method development and follow-up routine analysis. It offers advantages over reversed-phase HPLC and GC to yield sensitive and reproducible measurements of polar organic acid impurities, that can also be applied to study other polar ionic analytes such as amines and glyphosate [12].
References
Figure 5. Overlay of IC-sCD chromatogram with extracted ion chromatogram (EIC) for each component of the organic acid mix at (A) 1 ppb and (B) 10 ppb with a retention order of acetic (black), propionic (blue), formic (pink), butanoic (brown), crotonic (lime green), pentanoic (cyan), butynoic (grey), and propiolic acid (emerald green) [2].
industry needs to be reliable and reproducible. The reproducibility of the IC-MS response was evaluated across ten injections of both the 50 ppb mixed organic acid samples and the 0.1%(w/w) impurity level organic acid mix in 25 mg/L of 2-butynoic acid. The relative standard deviation (RSD) was calculated from the obtained peak area values from ten injections. Using IC-sCD, all components, except for pentynoic acid, produced a peak area RSD of less than 2%, indicating excellent reproducibility. MS detection produced a peak area RSD in the range of 4-10% for the two sample types (Table 3). This MS response variability is a direct result of the underlying ionisation process but fell within the acceptable RSD range for trace analysis. These results show that even with little optimisation of MS conditions, IC-MS is reproducible across all components for routine and trace analysis.
Conclusion
By coupling IC with MS, it was possible to accurately quantify impurities of small organic acids such as 2-butynoic acid without derivatisation but with the benefit of absolute identification enabled by the mass spectrometric data. With linearity, LOQ, and repeatability measurements within the pharmaceutically acceptable range, this method can now be applied for routine analysis of impurities (using IC-sCD after the appropriate method development) or trace analysis (using IC-MS to leverage component identification). Additive choice for make-up solvent, a key variable in IC-MS methods, as expected, directly impacted the MS response. Optimising the column temperature allowed improvement in peak shapes.
The additional selectivity provided by MS allowed peak purity tracking measurements that were otherwise challenging with IC- sCD only. For instance, co-eluted analytes,
1. Global organic acid market: Development in pharmaceutical industry stokes growth, Transparency Market Research (2018)
2. T.A. Corry, B.A. Jackson, A.D. Ray, Impurity analysis of 2-butynoic acid by ion chromatography-mass spectrometry. Journal of Chromatography. A. (2019) 460470
3. J. Št’ávová, J. Beránek, E.P. Nelson, B.A. Diep, A. Kubátová, Limits of detections for the determination of mono- and dicarboxylic acids using gas and liquid chromatographic methods coupled with mass spectrometry, J. Chromatogr. B 879 (2011) 1429–1438
4. Y. Lu, D. Yao, Chi Chen, C 2-hydrazinoquinoline as a derivatization agent for LC–MS-based metabolomic investigation of diabetic ketoacidosis, Metabolites 3 (2013) 993–1010
5. I. Molnár-Perl, Role of chromatography in the analysis of sugars, carboxylic acids and amino acids in food, J. Chromatogr. A 891 (2000) 1–32
6. M.A. Adams, Z.L. Chen, P. Landman, T.D. Colmer, Simultaneous determination by capillary gas chromatography of
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