The HP/Agilent Co-axial sheath-liquid flow CE-MS interface - 3
Important additional aspects of the triple tube CE-MS interface
In the co-axial sheath-liquid flow interface design, sensitivity will become compromised for the following reasons.
- Since the sheath-liquid flow rate is high (1-10 µL/min), the benefits of running the separation in the nanoflow rate regime on ESI (20 - 200nL/min), like sensitivity enhancement and reduction of ion suppression are lost.
- In addition, depending on the ratio of the sheath-liquid flowrate and EOF, the analytes leaving the CE-capillary will become diluted 10-50 times.
In practice, though these putative disadvantages can be offset by
- the better and more stable spray formation benefits by the selection of optimal composition of the sheath solvent
- combining the CE-separation method with electrokinetic focusing methods like field-amplified sample stacking (FASS), capillary isotachophoresis (cITP), in-capillary solid-phase extraction (SPE) or liquid phase microextraction (LPME)
- improvements of the mass analyzer inlet systems (hexabore inlet capillary, ion funnel, Agilent)
Also, it has been reported by several groups that a hydrodynamic flow occurs towards the exit of the CE capillary through a suction effect at the capillary end in the interface ,,,,. The flow of the nebulizing gas draws on the liquid train especially when wider i.d. capillaries (75 and 100 µm) are used. Thereby a parabolic solvent velocity profile becomes established in the CE capillary by which the widths of the moving solutes increase. As a countermeasure, the inlet vial should be under-pressurized.
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