Article pubs.acs.org/ac
Study of Electrochemical Reactions Using Nanospray Desorption Electrospray Ionization Mass Spectrometry Pengyuan Liu,† Ingela T. Lanekoff,‡ Julia Laskin,*,‡ Howard D. Dewald,† and Hao Chen*,† †
Center for Intelligent Chemical Instrumentation, Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio University, Athens, Ohio 45701, United States ‡ Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999 K8-88, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: The combination of electrochemistry (EC) and mass spectrometry (MS) is a powerful analytical tool for studying mechanisms of redox reactions, identification of products and intermediates, and online derivatization/recognition of analytes. This work reports a new coupling interface for EC/MS by employing nanospray desorption electrospray ionization, a recently developed ambient ionization method. We demonstrate online coupling of nanospray desorption electrospray ionization MS with a traditional electrochemical flow cell, in which the electrolyzed solution emanating from the cell is ionized by nanospray desorption electrospray ionization for MS analysis. Furthermore, we show first coupling of nanospray desorption electrospray ionization MS with an interdigitated array (IDA) electrode enabling chemical analysis of electrolyzed samples directly from electrode surfaces. Because of its inherent sensitivity, nanospray desorption electrospray ionization enables chemical analysis of small volumes and concentrations of sample solution. Specifically, goodquality signal of dopamine and its oxidized form, dopamine o-quinone, was obtained using 10 μL of 1 μM solution of dopamine on the IDA. Oxidation of dopamine, reduction of benzodiazepines, and electrochemical derivatization of thiol groups were used to demonstrate the performance of the technique. Our results show the potential of nanospray desorption electrospray ionization as a novel interface for electrochemical mass spectrometry research.
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In this study we demonstrate coupling of EC with MS using nanospray desorption electrospray ionization.32−36 Nanospray desorption electrospray ionization is a new ambient surface ionization technique that enables sensitive analysis of both solid32−34 and liquid samples35 on surfaces. The nanospray desorption electrospray ionization source comprises two capillaries, a primary capillary that supplies solvent to a surface, to dissolve or extract analyte, and a second nanospray capillary that removes the resulting analyte solution and generates charged droplets at the MS inlet by nanoelectrospray. Nanospray desorption electrospray ionization MS has been used for the analysis of organic and biological molecules deposited onto solid supports. Examples of nanospray desorption electrospray ionization MS applications include analysis of ambient organic aerosol samples,33,36 imaging of biological tissues,34 and analysis of liquid petroleum samples.35 It has been demonstrated that nanospray desorption electrospray ionization is a soft ionization technique that can be used to study labile molecules. Similar to DESI, chemically labile compounds are preserved during nanospray desorption electro-
oth electrochemistry (EC) and mass spectrometry (MS) play an important role in analytical chemistry. The combination of these two techniques (EC/MS) enables identification and structural characterization of products and intermediates of electrochemical reactions.1−4 Furthermore, EC is often used for online analyte pretreatment or derivatization for improved detection5,6 and MS/MS characterization.7 EC/ MS has been also used for mechanistic studies of redox reactions of interest to biology,8−12 environmental science,13 and drug discovery.4,14 EC has been interfaced with MS using a variety of ionization techniques including electron ionization (EI),15 thermospray (TS),16 fast atom bombardment (FAB),17 and electrospray ionization (ESI).18−22 In particular, EC/ESI-MS has found extensive applications because ESI is a soft ionization method and can be used to detect both labile compounds and large biomolecules. The most recent coupling method is EC/liquid sample desorption electrospray ionization (DESI)MS,11,12,23−26 which was developed by us in 2009. In DESIMS,27−30 an electrosonic spray ionization (ESSI) probe31 is used to desorb and ionize molecules in the electrolyzed sample solution. EC/DESI-MS coupling24 has the advantage of simple instrumentation, fast signal response, and flexibility in the selection of solvent systems for electrolysis. © 2012 American Chemical Society
Received: April 6, 2012 Accepted: June 8, 2012 Published: June 8, 2012 5737
dx.doi.org/10.1021/ac300916k | Anal. Chem. 2012, 84, 5737−5743
Analytical Chemistry
Article
Figure 1. Online coupling of an electrochemical flow cell with nanospray desorption electrospray ionization MS. (a) Scheme showing the configuration of the EC/nanospray desorption electrospray ionization MS with an electrochemical flow cell. Nanospray desorption electrospray ionization MS spectra acquired when the dopamine solution flowed through the thin-layer electrochemical cell with an applied potential of (b) 0.0 and (c) 1.5 V. (d) EIC of m/z 152 acquired when the dopamine solution flowed through the thin-layer electrochemical cell.
electrode is 2 mm ×10 μm (length × width), and the interval between two adjacent electrodes is 5 μm (Figure 2a and Figure S1a in the Supporting Information). The whole IDA electrode was mounted in a USB-like connecting cable provided by the manufacturer (ALS, Tokyo, Japan). A potentiostat (CV-27 model, BAS, West Lafayette, IN) was used to apply potentials to the electrochemical flow cell and the IDA electrode. Samples were analyzed using either a Finnigan LTQ/ Orbitrap mass spectrometer (Thermo Electron, Bremen, Germany) or a Bruker HCTultra ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) both equipped with a custom-designed nanospray desorption electrospray ionization source.32 The electrochemical flow cell or IDA electrode was mounted onto a nanospray desorption electrospray ionization custom-built sample holder. Fused-silica capillary (50 μm i.d., 183 μm o.d., Polymicro Technologies, L.L.C., Phoenix) was used to make both the primary and the nanospray capillaries. The capillaries were independently positioned using two XYZ stages. The solvent injected through the primary capillary for extraction was a mixture of MeOH/H2O/HOAc (50:50:1 by volume). The typical flow rate of this solvent was 2 μL/min. The solvent flow rate was maintained by carefully positioning one end of the primary and the nanospray capillaries relative to each other forming a solvent bridge (Figures 1a and 2a). The other end of the nanospray capillary was pointed to the heated capillary of the mass spectrometer. A high voltage of 2−3 kV was applied to the primary capillary, when using the LTQ/ Orbitrap, or to the MS inlet, when using the ion trap, for nanospray ionization.
spray ionization analysis,33,37 which is important for detection of labile intermediates in EC reactions. In addition, its high sensitivity32 and the ability to analyze small volumes (
