Robust and cost-effective capillary electrophoresis-mass spectrometry interfaces suitable for combination with on-line analyte preconcentration

Abstract

This paper describes several successful cost-effective attempts to couple capillary electrophoresis (CE) and mass spectrometry (MS) without make-up flow or nebulizing gas. An in-depth analysis of several interfaces using conductive spray tips was performed as well as an easy-to-prepare T-junction with direct electrode contact, the latter being the most robust interface. No coating is necessary and the spray voltage is applied through a gold wire positioned at the gap between the separation and spray capillaries. The T-junction interface is made by puncturing a small piece of transparent rubber. The on-line preconcentration CE-MS system allows immunoassay sensitivity, as is demonstrated by a calibration plot in the picomolar range for angiotensin II and gonadorelin. It also shows good reproducibility and has the ability of excellent automation. The secure electrical contact gives a constant spray quality, even with 100% aqueous separation buffers. The described setup has a wide applicability as is demonstrated by the analysis of larger peptides, such as insulin and cytochrome c. Detailed information is given on critical factors in the preparation of the described interfaces.

Recent Developments in Capillary Electrophoresis–Mass Spectrometry of Proteins and Peptides

“…A means to close the CE electrical circuit for analyte separation while simultaneously providing an electrical potential to the spray tip is necessary. The common interfaces can be divided into three categories: sheath-flow 16,[18][19][20]24,27,32,37,38,40,43,47,50,[53][54][55][56]60,66 and sheathless 15,17,25,26,[28][29][30]34,36,44,45,48,49,61 and liquid-junction interfaces. [92][93][94] In the sheath-flow design, a coaxial sheath liquid, usually a hydroorganic mixture, mixes with the capillary eluent at the outlet end of the capillary.…”

“…100 Chang and Her 15 carbon-coated the tapered capillary outlet using a soft pencil, while McComb and Perreault 17 used gold. Waterval et al 29 compared gold and silverpaint-coated conductive spray tips, and reported an easyto-prepare T-junction for direct electrode contact, prepared by positioning a gold wire at the gap between the separation capillary and a pulled capillary tip connected by a small transparent rubber. Wetterhall et al 34 used a conductive polypropylene/graphite mixture for the production of polymeric nanospray needle emitters.…”

“…The drawback, however, is that it often tends to compromise CE performance, resulting in irregular electroosmotic flow (EOF), broader peaks, and poorer resolution. 5 Waterval et al 29 developed an on-line preconcentrator using commercially available extraction disks containing poly(styrenedivinylbenzene) adsorbent particles in a matrix of inert Teflon, permitting loading of as much as 7.5 capillary volumes of a sample. They applied this later to quantitate low concentrations of gonadorelin and angiotensin II in a plasma.…”

On-line sample preconcentration in capillary electrophoresis, focused on the determination of proteins and peptides

“…These chromatographic mechanisms can be of low specificity or of high specificity. Basically, three methods are available using a low-specific chromatographic mechanism: (i) solid adsorptive phase chromatography, including solid-phase extraction (SPE) and membrane preconcentration (mPC) [2,[16][17][18][19][20][21][22][23][24], (ii) liquid distribution phase chromatography, including electroextraction (EE) [2] and supported liquid membranes (SLM) [25,26], and (iii) hollow fibers [27,28]. Low-specific chromatography is probably the most widely used application of chromatographic retention.…”

Capillary electrophoretic bioanalysis of therapeutically active peptides with UV and mass spectrometric detection after on-capillary preconcentration

“…The on-capillary preconcentration has the benefit that sample loading is not limited to the capillary volume and that on-line purification of the applied sample can be accomplished by employing washing steps. The observed capacity of the adsorptive phase allows introduction of typical LC injection volumes, rather than the normally employed nL volumes of CZE [32]. Desorption of the trapped analytes can be accomplished with minimum amounts of elution solvent, compatible with the subsequent CZE separation.…”