Tatiana C. Rohner

Microfabricated polymer injector for direct mass spectrometry coupling

This paper demonstrates the coupling of a plasma etched polymer microfluidic system with an electrospray mass spectrometer by generation of a nanospray. Taking advantage of the microtechnology processes and polymer properties, high volume production with good reproducibility of hydrophobic interfaces could be obtained. The nanospray was directly produced from the outlet of the plastic microfabricated chip positioned in front of the capillary entrance of the mass spectrometer. No chemical background due to the polymer has been observed under standard nanospray conditions. The performances of the spray as well as its efficiency have been demonstrated by flow measurements, stability establishment and tandem mass spectrometry experiment on angiotensin II. The spray was actuated without additional flow in methanol: water:acetic acid (50:49:1%) solution. A 40 fmol/νL detection limit could be reached.

Electrochemical and theoretical aspects of electrospray ionisation

An overview of the electrospray processes is given and discussed, from the formation and the onset of the spray to the generation of gas-phase ions. This soft ionisation method for mass spectrometry is indeed characterized by non-equilibrium conditions, and the striking features relative to its use are difficult to be analytically or numerically modelled or even observed. In particular, the ionisation model known as the “ion evaporation model” will be largely discussed and its relevance with respect to the other model for gas-phase ions generation, the “charge residue model” will be highlighted. Lastly, the influence of the species nature and respective concentrations in the sprayed solution will be reviewed.

On-line electrochemical tagging of cysteines in proteins during nanospray

An electrochemical modification of free cysteine residues is studied and characterized by means of quinone addition. Taking advantage of the electrolytic nature of electrospray interfaces (ESI), an electrochemical tagging is performed prior to mass spectrometry (MS) analyses. The tagging has been studied by MS and different mechanisms, involving electrochemical and/or chemical steps, could be characterized. It is demonstrated that the present nanospray is a very efficient tool to obtain cysteine modification. Using the high voltage electrode of the nanospray interface to perform protein specific tagging is a novel method that can be associated to analytical or preparative techniques, such as digestion of proteins or capillary electrophoresis, for post-column modifications.

MALDI MS imaging of amyloid.

Label-free molecular imaging by mass spectrometry allows simultaneous mapping of multiple analytes in biological tissue sections. In this chapter, the application of this new technology to the detection Abeta peptides in mouse brain sections is discussed.

On-line electrochemical tagging of free cysteines during nanospray ionisation for mass spectrometry analysis

A microinterface for ElectroSpray Mass Spectrometry (ESI-MS), acting as an electrochemical cell, was developed for the on-chip electrochemical tagging of cysteines in peptides and proteins during electrospray ionisation which occurs via a 1,4-Michael addition. Benzoquinones are electrogenerated from the corresponding hydroquinones to react specifically with cysteine residues and the production of the resulting adducts is followed by MS. The electrotagging efficiency was tested on L-cysteine using several hydroquinones bearing either electrodonating or withdrawing groups. The reaction kinetics was determined by electrochemical techniques such as Cyclic Voltammetry (CV) and was checked by MS experiments. In both experiments, carboxymethylhydroquinone was found to be the best tagging reagent. The electrochemical tagging was successfully applied to the identification of target proteins through the labelling of peptides coming from their proteolytic digestion. The information on the number of cysteine residues in proteolytic peptides was found to enhance the protein identification confidence.

Protein Analyses with Electrochemical and Nanoelectrospray Detection on Disposable Plastic Microchips

The development of µ-total analysis systems for proteomics faces features that are specific to the diversity in protein properties such as hydrophobicity and heterogeneity. Particular problems such as non specific adsorption on the capillary walls (glass or plastics) or the use of surfactants or solvent mixtures render the use of electrokinetic pumping and free flow electrophoresis more problematic than in DNA analyses. Therefore, we developed concepts where disposable chips are fabricated with various properties and where the chips are functionalised for affinity or chromatography purposes with electrochemical or mass spectrometry detection. Finally, a novel sub-nL injection method without electrokinetic means has been developed in order to enable the integration of injector, separation channel and detector on a single µ-chip.