On-line coupling of a microelectrode array equipped poly(dimethylsiloxane) microchip with an integrated graphite electrospray emitter for electrospray ionisation mass spectrometry

Abstract

A novel method for the manufacturing of microchips for on-chip combinations of electrochemistry (EC) and sheathless electrospray ionisation mass spectrometry (ESI-MS) is described. The technique, which does not require access to clean-room facilities, is based on the incorporation of an array of gold microcoil electrodes into a poly(dimethylsiloxane)(PDMS) microflow channel equipped with an integrated graphite based sheathless ESI emitter. Electrochemical measurements, which were employed to determine the electroactive area of the electrodes and to test the microchips, show that the manufacturing process was reproducible and that the important interelectrode distance in the electrochemical cell could to be adequately controlled. The EC-ESI-MS device was evaluated based on the ESI-MS detection of the oxidation products of dopamine. The results demonstrate that the present on-chip approach enables full potentiostatic control of the electrochemical cell and the attainment of very short transfer times between the electrochemical cell and the electrospray emitter. The transfer times were 0.6 and 1.2 s for flow rates of 1.0 and 0.5 microL min(-1), respectively, while the electrochemical conversion efficiency of the electrochemical cell was found to be 30% at a flow rate of 0.5 microL min(-1). To decouple the electrochemical cell from the ESI-MS high voltage and to increase the user-friendliness, the on-line electrochemistry-ESI-MS experiments were performed using a wireless Bluetooth battery-powered instrument with the chip floating at the potential induced by the ESI high voltage. The described on-chip EC-ESI-MS device can be used for fundamental electrochemical investigations as well as for applications based on the use of electrochemically controlled sample pretreatment, preconcentration and ionisation steps prior to ESI-MS.

The influence of the thin-layer flow cell design on the mass spectra when coupling electrochemistry to electrospray ionisation mass spectrometry

“…The latter can be accomplished using sufficiently long transfer lines ($30 cm) between the electrochemical cell and the mass spectrometer [6,17], the insertion of a ground point prior to the ESI-MS system [20][21][22][23][40][41][42][43][44] or by allowing the whole EC-ESI-MS system to float on the potential induced by the ESI high voltage, as described by Cole and co-workers [7,8], Nyholm and co-workers [27,28,[45][46][47], and Van Berkel et al [48]. The main disadvantage with the use of a long transfer line is the increased transfer time between the flow cell and the ion source which makes the detection of unstable electrochemical reaction products difficult.…”

“…Although it has been demonstrated that the electrochemical reactions in the electrospray can be used to oxidise or reduce species in the solution or the electrospray capillary itself [7,41,42,50,51], it is generally difficult to utilise this approach for the present purposes as the electrochemical potential of the spray capillary is determined by the magnitude of the electrospray current, the capillary material and the concentrations of the available electroactive species in the solution [26]. The most appropriate way to decouple the electrochemical cell from the ESI high voltage is consequently to allow the EC system to float on the potential induced by the ESI high voltage either by using a battery-operated potentiostat [7,8,47] or an isolation transformer [27,28,46,48]. The main draw back with this approach is the hazard associated with the use of a (high voltage) floating system, which is why particular care needs to be taken when employing such EC-ESI-MS systems.…”

“…As pointed out by several authors [3,9,14,27,28,[46][47][48][49][50], the flow cell configuration needs to be considered when coupling electrochemical flow cells to ESI-MS. This is particularly true when flow cells originally designed for electrochemical detection purposes are utilised in EC-ESI-MS.…”

Microfluidics for multiplexed MS analysis

“…Many different approaches have been published in the past 7 years with complete references available in the review articles. Some of the latest attempts include systems integrating the ESI ionization nozzle in devices fabricated in polydimethylsiloxane (PDMS) [53,54]. The use of casting technologies is quite popular for rapid laboratory replications and prototyping.…”

Sample pretreatment on a microchip with an integrated electrospray emitter

“…ESI has been obtained directly from the PDMS bulk, either by applying the potential upstream [16][17][18][19][20][21], or by using a sheathless design [15]. The use of sheathless electrospray enables electrophoretic separation before electrospray without any interaction between the electrode and the eluting analytes.…”