Porous Tip Sheathless CE-MS Interface
Moini proposed to etch the end of the CE separation capillary with hydrofluoric acid (HF) which renders the end of the fused-silica capillary porous [1]. The capillary is placed in a stainless steel ESI needle. Co-axially a conductive liquid is delivered to the outside of the porous end which makes a hydraulic electrical contact between the ESI needle and the CE separation BGE. This essential concept has been implemented by Beckman Coulter in a prototype system in 2010 and brought to market very recently (now Sciex Separations, CESI 8000 High Performance Separation-ESI Module, figure taken from the website).
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Porous Emitter CE-MS Interface
Recently, Dick Smith and co-workers reported an approach which may be regarded as propagation of his original work to a CE-MS interface [1]. In the final implementation, they describe a fused-silica emitter with a porous tip (90 µm o.d.) which is shoven into the separation capillary (365x100 µm i.d.) and glued to fix its position [2].
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Microvial Flow Through Interface
In an analogous way as in the original co-axial sheath-liquid CE-MS interface work by the group of Richard Smith [1], the group of David Chen reported an interface with a metal spray needle and a co-axially delivered modifier [2, 3]. In contrast to Smith, these authors sprayed from a 22’ gauge steel needle with optimized exit geometry. The CE separation capillary is moved forward in this needle until it stops after which it is pulled back minimally to allow a hydraulic liquid connection with the BGE in the separation capillary.
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EOF-Driven Sheath-Liquid CE-MS IF
In several publications, Dovichi and co-workers have propagated the use of a co-axial, EOF driven sheath-liquid with a borosilicate spray tube emitter for CE-MS [1-5]. The essential details are given in figures 4 and 5.
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Microchip CE-MS
Since CE is an ultra-low volume separation technique it has been an obvious development to execute CE in narrow conduits on microfluidic chips (microchip capillary electrophoresis, MCE). Since early 2000 MCE has been deployed as an instrumental separation method in dedicated devices such as the Bio-analyzer (Agilent Technologies) or the LabChip (Caliper Technologies, now Perkin Elmer). In contrast though to general purpose CE instrumentation, these devices have highly dedicated tasks like DNA and protein sizing and quantitation. Coupling of such devices with MS is prohibited by their design. Therefore, coupling MCE with MS has remained an area of research since the mid-nineties [1-2].
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Summary CE-MS Interfacing
The coaxial sheath solvent flow interface (triple tube design) is a well-established technology for coupling CE with MS. The method has demonstrated high robustness, excellent spray stability, ease of use and is used in many routine applications of CE-MS. The metal sprayer tube, which is the middle tube, functions as the CE outlet electrode. The sheath liquid serves to establish electric contact between the metal tubing and the BGE in the separation column and so acts as the outlet vial the sheath solvent (mostly acidic alcohol/water mixtures delivered at 1-10 µL/min) optimizes conditions for electrospray ionization by adjusting the pH of the mixture. A more recently modified sheath solvent composition has been used to enhance sensitivity or to act as a terminating electrolyte in CITP.
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