Electrochemically Assisted Injection in Combination with Capillary Electrophoresis‐Mass Spectrometry (EAI‐CE‐MS) – Mechanistic and Quantitative Studies of the Reduction of 4‐Nitrotoluene at Various Carbon‐Based Screen‐Printed Electrodes

Abstract

Electrochemically assisted injection (EAI) is a recently introduced injection concept for capillary electrophoresis (CE) enabling the separation of neutral analytes by means of electrochemical generation of charged species during the injection process. EAI‐CE is particularly attractive in combination with mass spectrometry (MS) leading to enhanced performance of MS detection. In this work, a fully automated EAI injection device was developed and applied to mechanistic and quantitative studies of the reduction of 4‐nitrotoluene (4‐NT). Three different carbon‐based screen‐printed electrodes (SPEs) were used including unmodified carbon, carbon nanofiber and reduced graphene oxide SPEs. Under acidic conditions the main products of 4‐NT reduction were 4‐hydroxylaminotoluene (4‐HAT) and 4‐aminotoluene (4‐AT). The EAI‐CE‐MS approach enabled the separation and selective determination of both compounds. On the basis of this methodical concept it was possible to study the formation of reduction products of 4‐NT in dependence on electrode potential and electrode material. In contrast to conventional electrochemical techniques like cyclic voltammetry EAI‐CE‐MS provides detailed information regarding changes of the product composition in dependence on various experimental conditions. Over the complete potential range studied the ratio of 4‐HAT/4‐AT was clearly different for the reduced graphene oxide SPE compared to the unmodified carbon and carbon fiber SPE. The construction of mass voltammograms added considerably to the information content of voltammetric experiments. Another aspect of this work was the reliable quantitative determination of 4‐NT by EAI‐CE‐MS using isotopically labeled 4‐NT as an internal standard. In this way the “classical” problem of changing response characteristics at solid electrodes could be eliminated based on the assumption that the target analyte and its isotopically labeled form behave in the same way. A corresponding protocol for quantitative EAI‐CE‐MS determinations of 4‐NT was elaborated and applied to standard solutions and spiked soil samples.

Sensitive Determination of Nitrofurantoin by Flow Injection Analysis Using Carbon Nanofiber Screen Printed Electrodes

“…In addition, the CNF material is 100 times lower in cost than single‐wall carbon nanotubes 10, making the production of low cost SPEs possible. However, few reports appear in literature about CNF‐SPE, among them one in which three different carbon‐based SPEs (unmodified carbon, CNF and reduced graphene oxide SPEs) were used to mechanistic and quantitative studies of the reduction of 4‐nitrotoluene, finding that reduced graphene oxide SPEs led to clearly different product compositions than unmodified carbon or carbon nanofiber SPEs 11. In other work, to study the effect of the carbon nanostructures on the electrode surface, SPE were nanostructured with CNF and characterized by using methylene blue.…”

Hyphenation of Electrochemistry with Mass Spectrometry for Bioanalytical Studies

“…Additional offline and especially online combination with a separation system is From [77], with permission from Wiley a promising, fully instrumental approach which has a good potential to surpass time-consumable in vitro and in vivo experiments. However, based on numerous studies in this field, it can be concluded that there is no universal approach which can completely simulate complex physiological conditions.…”

“…The formation of corresponding hydroxylamines and amines paved the way for selective and sensitive CE-MS determinations without the need of adding surfactants to the electrophoresis buffer. Further on a fully automated EAI injection device was developed and applied to mechanistic and quantitative studies of the reduction of 4-nitrotoluene [77] (Fig. 10).…”

Characterization of the Oxidative Behavior of Cyclic Nucleotides Using Electrochemistry–Mass Spectrometry

“…Electrochemically assisted injectioncapillary electrophoresismass spectrometry (EAI‐CE‐MS) represents an online approach which enables electrochemical conversion of analyte species during or prior to injection into the separation capillary. A fully automated setup for EAI‐CE‐MS was recently developed and characterized by our group 16 and the experimental approach is described in detail in 10. The oxidation of 1 mM cGMP on SPCE was done as in case of LCMS analysis following automated injection for 10 seconds into the fused silica capillary (50 µm×50 cm).…”