Addition of reagents to the sheath liquid: A novel concept in capillary electrophoresis-mass spectrometry

• Concept of sheath-flow chemistry for CE-MS is described and demonstrated.
• Fundamental effects of capillary diameter investigated using example of hydrogen/deuterium exchange.
• Antioxidant assay using DPPH with CE-MS developed using these concepts.

Conventional coupling of capillary electrophoresis with electrospray ionisation mass spectrometry typically relies on the use of a triaxial sheath-flow liquid interface to facilitate electrical contact and provide a stable electrospray. In this type of analysis, the use of additives in the sheath liquid itself can also be used to improve ionisation of analytes and even facilitate reactions between separation and detection steps (which we broadly term “sheath-flow chemistry”). In the present work, this concept is demonstrated using two types of sheath-flow reactions for CE coupled with quadrupole time-of-flight (Q-TOF) MS detection. Sheath liquid compositions containing deuterated solvents or DPPH (2,2-diphenyl-1-dipicrylhydrazyl) stable free-radicals yield useful additional structural information for separated analytes. Investigations of fundamental physical and chemical characteristics of the sheath liquid coupling show their direct influence on the efficiency and some of the products of the respective reactions. For example, reducing the capillary internal diameter from 75 to 25 μm increased the relative abundance of fully deuterated ions detected by 63–65% (5 exchangeable hydrogens) using constant sheath-flow conditions. Addition of 0.05–0.2 mM DPPH to the sheath liquid reduced the peak total ion count obtained for typical antioxidant species by 20 to >95% allowing strongly antioxidant species from mixtures to be readily identified and further studied. The presented approach allows a rapid and information-rich analysis to be performed with minimal reagent and sample consumption.