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.

Standard capillaries can be used for the CE separation. As a manufacturer, Agilent Technologies provides sources for additional MS ionization techniques, seamless coupling of these sources to the whole Agilent 6000 series MS instruments and integrated data acquisition, data processing and automation software in one comprehensive system. This allows a CE-MS user to grow with progress in new Agilent MS instrumentation technology (e.g. the new ion mobility Q-ToF or 6495 triple quad MS).

The perceived disadvantage of reduced sensitivity since the separation electrolyte carrying the analytes is diluted 20-50 times by the sheath solvent is the modest price that the user pays with this interface. Still, 1 µM in sample analyte concentration is routinely achieved. Electrokinetic pre-concentration methods like CITP have been described that may offset this disadvantage. 

Over the past few years, considerable progress has been made in the design of new interfacing techniques for CE-MS.

In this context, the porous tip sheathless interface described by Moini emerged as a very promising interfacing technique for sensitive proteomics and metabolomics studies at different laboratories. The recent introduction to the market of this CE-MS interface by Sciex Separations will allow global access to the method but the long-term stability, robustness, and ease of use in practice need to be demonstrated. Moreover, this is a proprietary solution requiring the CE instrument, adapters for MS, capillaries, and supplies from the vendor.

On the other hand, the microvial flow-through interface has the potential to become a viable alternative since standard CE capillaries can be used. However, at present, it is still in the preliminary stage. Moreover, commercial availability is not clear.

The microvial flow-through interface and the recent porous tip emitter interface of the Smith group have only been shown in the field of proteomics so far.  These CE-MS interfaces reported by Dovichi and Smith, are at this stage expert approaches lacking commercial support.

Microchip CE-MS remains a promising approach, however, without connection to a MS manufacturer a dream for some time.

Despite progress and potential in CE-MS interfacing, the overruling factor in practical application of CE-MS will be the selection of the MS system. From there, the user will become directed into a (commercial) solution. Like in LC-MS it is not the separation technique that drives the choice of MS but the requirements of application and therefore the interface that is used.