Paper-Based Electrochemical Cell Coupled to Mass Spectrometry.

On-line coupling of electrochemistry (EC) to mass spectrometry (MS) is a powerful approach for identifying intermediates and products of EC reactions in situ. In addition, EC transformations have been used to increase ionization efficiency and derivatize analytes prior to MS, improving sensitivity and chemical specificity. Recently, there has been significant interest in developing paper-based electroanalytical devices as they offer convenience, low cost, versatility, and simplicity. This report describes the development of tubular and planar paper-based electrochemical cells (P-EC) coupled to sonic spray ionization (SSI) mass spectrometry (P-EC/SSI-MS). The EC cells are composed of paper sandwiched between two mesh stainless steel electrodes. Analytes and reagents can be added directly to the paper substrate along with electrolyte, or delivered via the SSI microdroplet spray. The EC cells are decoupled from the SSI source, allowing independent control of electrical and chemical parameters. We utilized P-EC/SSI-MS to characterize various EC reactions such as oxidations of cysteine, dopamine, polycyclic aromatic hydrocarbons, and diphenyl sulfide. Our results show that P-EC/SSI-MS has the ability to increase ionization efficiency, to perform online EC transformations, and to capture intermediates of EC reactions with a response time on the order of hundreds of milliseconds. The short response time allowed detection of a deprotonated diphenyl sulfide intermediate, which experimentally confirms a previously proposed mechanism for EC oxidation of diphenyl sulfide to pseudodimer sulfonium ion. This report introduces paper-based EC/MS via development of two device configurations (tubular and planar electrodes), as well as discusses the capabilities, performance, and limitations of the technique.