A Versatile and Easy Method to Calibrate a Two-Compartment Flow Cell for Differential Electrochemical Mass Spectrometry Measurements

Online techniques for the quantitative analysis of reactionproductshave many advantages over offline methods. However, owing to the lowproduct formation rates in electrochemical reactions, few of thesetechniques can be coupled to electrochemistry. An exception is differentialelectrochemical mass spectrometry (DEMS), which gains increasing popularitynot least because of its high time resolution in the sub-second regime.DEMS is often combined with a dual thin-layer cell (a two-compartmentflow cell), which helps to mitigate a number of problems that arisedue to the existence of a vacuum|electrolyte interface. However, theefficiency with which this cell transfers volatile reaction productsinto the vacuum of the mass spectrometer is far below 100%. Therefore,a calibration constant that considers not only the sensitivity ofthe DEMS setup but also the transfer efficiency of the dual thin-layercell is needed to translate the signals observed in the mass spectrometerinto electrochemical product formation rates. However, it can be challengingor impossible to design an experiment that yields such a calibrationconstant. Here, we show that the transfer efficiency of the dual thin-layercell depends on the diffusion coefficient of the analyte. Based onthis observation, we suggest a two-point calibration method. Thatis, a plot of the logarithm of the transfer efficiencies determinedfor H-2 and O-2 versus the logarithm of theirdiffusion coefficients defines a straight line. Extrapolation of thisline to the diffusion coefficient of another analyte yields a goodestimate of its transfer efficiency. This is a versatile and easycalibration method, because the transfer efficiencies of H-2 and O-2 are readily accessible for a large range of electrode-electrolytecombinations.

Automated summary

Online techniques for quantitative analysis of reaction products have limitations in coupling with electrochemistry due to the low product formation rates. However, differential electrochemical mass spectrometry (DEMS), combined with a dual thin-layer cell, provides high time resolution and addresses some challenges arising from the vacuum-electrolyte interface. To accurately translate signals observed in the mass spectrometer into electrochemical product formation rates, a calibration constant considering both DEMS sensitivity and dual thin-layer cell transfer efficiency is required. We propose a two-point calibration method based on the dependence of transfer efficiency on analyte diffusion coefficient, which offers a versatile and easy approach for a wide range of electrode-electrolyte combinations.