Using Oxidation–Reduction Chemistry in Sample Preparation for Sub-PPM Quantitation by LC/MS

Abstract

Low level quantitation by liquid chromatography/mass spectrometry (LC/MS) has become very popular due to the superior selectivity and sensitivity of mass spectrometry. Despite the tremendous power of LC/MS, the application of this technique can be limited if the analyte of interest lacks certain properties such as stability or ionizability at the MS interface. Consequently, derivatization prior to LC/MS analysis to increase stability, sensitivity, and selectivity is often required. In this study, we report the case of oxidation–reduction chemistry to aid the LC/MS analysis. In one case study, reduction of the free radical 2,2,6,6-tetramethylpiperidine-1-oxyl to its reduced form by sodium ascorbate was performed to increase stability and allow more accurate quantitation, permitting the development of a reversed-phase LC/MS method with a 0.1 ppm limit of detection. In another scenario, the oxidation of nitrite (MW 46) to nitrate (MW 62) via potassium permanganate under acidic conditions was conducted to increase the molecular weight of nitrite above the lower mass cutoff of the MS detector used. The reversed-phase LC/MS method developed can easily detect 10 ppb nitrate in solution.

Keywords:

• derivatization
• liquid chromatography
• mass spectrometry
• nitrate
• nitrite
• TEMPO

Acknowledgments

The authors would like to thank Feng Xu, Ryan Cohen, and Justin Pennington for the helpful discussions and suggestions.