Liquid junction interface

Jussila, M., Sinervo, K., Porras, S. P, and Riekkola, M. L. (2000). Modified liquid-junction interface for nonaqueous capillary electrophoresis-mass spectrometry. Electrophoresis 21, 3311—3317. 

In the sheathless interface, the electrical contact is obtained by coating with either a metal [85, 88-90] or a conductive polymer [91] the separation capillary outlet, which is shaped as sharp tip. Also employed are sheathless interfaces in which the electrical contact is established using a metal electrode or a conductive wire inserted into the outlet of the separation capillary [92], A small gap between the separation capillary and the needle of the ionization device filled by a liquid is the approach made to establish the electrical contact in the liquid junction interface [86,87], This arrangement is also realized by making porous through chemical etching the tip [93] or a small section of the wall [94] of the separation capillary at its outlet. 

Electrolyte Electroreflectance (EER) is a sensitive optical technique in which an applied electric field at the surface of a semiconductor modulates the reflectivity, and the detected signals are analyzed using a lock-in amplifier. EER is a powerful method for studying the optical properties of semiconductors, and considerable experimental detail is available in the literature. ( H, J 2, H, 14 JL5) The EER spectrum is automatically normalized with respect to field-independent optical properties of surface films (for example, sulfides), electrolytes, and other experimental particulars. Significantly, the EER spectrum may contain features which are sensitive to both the AC and the DC applied electric fields, and can be used to monitor in situ the potential distribution at the liquid junction interface. (14, 15, 16, 17, 18)... 

Fig.l Schematic illustration of CE-MS interfaces to an ESI source (a) a coaxial sheath-flow interface (b) a liquid-junction interface (c) a sheathless interface. 

A liquid-junction interface has also been suggested and applied for CE-ESI-MS [8]. Electrical contact with this interface is established through the liquid reservoir which surrounds the junction of the separation capillary and a transfer capillary, as shown in Fig. lb. The gap between the two capillaries is approximately 10-20 fim, allowing sufficient makeup liquid from the reservoir to be drawn into the transfer capillary while avoiding analyte loss. The flow of makeup liquid into the transfer capillary is induced by a combination of gravity and the Venturi effect of the nebulizing gas at the capillary tip [8]. 

Reinhoud, N. J., Niessen, W. M. A., Tjaden, U. R., Gramberg, L. G., Verheij, E. R. and van der Greef, J. Performance of a liquid-junction interface for capillary electrophoresis mass spectrometry using continuous-flow fast-atom bombardment Rapid Commun. Mass Spectrom. 3 348-351, 1989. 

Either a sheathless, coaxial sheath-flow, or liquid-junction interface is commonly used for CE-MS, Figure 9.11. The reproducible and straightforward construction of the coaxial sheath-flow interface has resulted in its general use, although the sheathless interface provides higher sensitivity. The difficulty in making low-dead... 

The liquid-junction interface consists of a metal tee with an electrical connection and outlet buffer reservoir. The ends of the separation column and electrospray capillary are positioned opposite each other at the center of the tee with a gap between them of 10-25 p,m. The outlet reservoir provides additional makeup flow to the separation buffer through the third branch of the tee. The interface decouples many aspects of the separation system from the electrospray source and can be used with a wide range of column flow rates. On the other hand, it is rather cumbersome to use, particularly the alignment of the capillaries. In addition, it is difficult to avoid band broadening at the open connection and the makeup liquid dilutes the sample concentration at the sprayer. 

Capillary Electrophoresis Chips. Some of the first microchip-CE separations have been performed on glass chips that used on-chip miniaturized pneumatic nebulizers, or on-chip or off-chip liquid junction interfaces coupled to a subatmospheric ESI ion trap instrument. - High-efficiency ( 31,000 plates/chip or 300,000 plates/m) separations on 11 cm long channels, or fast separations on 4.5 cm channels, of peptide/protein samples were achieved (Figure 53.17). Some of these chips were also tested for performing transient isotachophoretic sample preconcentration. ... 

A free-standing liquid junction interface was coupled to a flat edge glass CE microchip for the analysis of small molecules (drugs, metabolites). It was demonstrated for the detection of recovered carnitine, acylcarnitines, imipramine, and desipramine spiked into urine or plasma at 5-500 p,g/mL level. Separations were typically performed in < 1 min and intra-assay precisions ranged from 4.1% to 7.3% Relative Standard Deviation RSD. A similar device, but fabricated in polymeric Zeonor material, was demonstrated for the analysis of carnitine standards. ... 

Liquid-junction interface. In this type of interface, a stainless-steel tee is used to introduce the makeup liquid (Figure 5.13c) [81,82]. The cathode end of the CE capillary and the ESI needle are introduced from opposite ends of the tee, with a narrow gap (10 to 25 p,m) between the two terminals. The electrical contact between the CE capillary and the ESI needle is established through the makeup liquid that surrounds the junction of the two capillaries. The liquid-junction interface is easy to assemble and operate. The additional hquid flow often degrades the detection sensitivity. CE separation is also compromised at the liquid junction. 

CE provides a complementary approach to HPLC separation. It is performed in several different formats, including capillary zone electrophoresis, miceller electrokinetic chromatography, capillary gel electrophoresis, capillary isoelectric focusing, isotachophoresis, and capillary electrochromatography. Of these formats, capillary zone electrophoresis is the most popular separation technique. The most successful coupling of CE with mass spectrometry is achieved via an ESI interface. The three most practical designs are sheathless interface, sheath-flow interface, and liquid-junction interface. 

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