Interface sheathless

In this design, on-column sample enrichment is incorporated into the sheathless interface (Janini et al., 2003). A miniaturized solid-phase extraction (mSPE) cartridge, made of reversed-phase material, was attached to the CE capillary near the injection end as shown in Fig. 16.1. 

Cao R and Moini M. (1997), A novel sheathless interface for capillary electro-phoresis/electrospray ionization mass spectrometry using an in-capillary electrode, J. Am. Soc. Mass Spectrom. 8, 561-564. 

Several reports concerning the development of stable and rugged sheathless interfaces were proposed. The first sheathless interface was developed by Olivares et al., and two types of sheathless interfaces are currently distinguished. The first one consists of a nanospray needle, which is inserted with a connection unit to the CE capillary. This setup allows changing the spray needle alone independently on the capillary exchange.The second approach involves the use of the end of capillary tip as an emitter with the help of a capillary-outlet conductive coating " or by inserting a conductive wire into the capillary outlet. 

Petersson, M. A., Hulthe, G., and Fogelqvist, E. (1999). New sheathless interface for coupling capillary-electrophoresis to electrospray mass-spectrometry evaluated by the analysis of fatty acids and prostaglandins./. Chromatogr. A 854, 141 — 154. 

Ramsey, R. S., and Mcluckey, S. A. (1995). Capillary electrophoresis electrospray ionization ion trap mass spectrometry using a sheathless interface.. Microcolumn Sep. 7, 461—469. 

Samskog, J., Wetterhall, M., Jacobsson, S., and Markides, K. (2000). Optimization of capillary-electrophoresis conditions for coupling to a mass-spectrometer via a sheathless interface. ]. Mass Spectrom. 35, 919—924. 

The sheathless interface (Fig. 2c) is known since the first CE/MS attempt by Olivares et al. (10). In this system the CE capillary was sleeved in a metal tube, whereas in modem sheathless interfaces the capillary exit is carefully sharpened or pulled to a fine point (14). The outer surface of the capillary tip is coated with metal, usually gold, which is readily accessible for electrical contact. This setup enables the maintenance of both electrical circuits from CE and ESI as well. The advantage of the sheathless approach over the coaxial sheath flow interface is that the eluting CE zone is not diluted by makeup flow and therefore the obtainable sensitivity can be quite high, especially when small-ID capillaries (e.g., 10 yarn) are used. Detection limits in the low fmol range have been demonstrated (13). A... 

The sensitivity of CE/ESI-MS has proven significantly better if the sheathless interface is coupled to a micro- or nanoelectrospray source designed for low flow rates. 

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. 

Some reviews [5-7] have appeared on NCE-electrospray ionization-mass spectrometry (NCE-ESI-MS) discussing various factors responsible for detection. Recently, Zamfir [8] reviewed sheathless interfacing in NCE-ESI-MS in which the authors discussed several issues related to sheathless interfaces. Feustel et al. [9] attempted to couple mass spectrometry with microfluidic devices in 1994. Other developments in mass spectroscopy have been made by different workers. McGruer and Karger [10] successfully interfaced a microchip with an electrospray mass spectrometer and achieved detection limits lower than 6x 10-8 mole for myoglobin. Ramsey and Ramsey [11] developed electrospray from small channels etched on glass planar substrates and tested its successful application in an ion trap mass spectrometer for tetrabutylammonium iodide as model compound. Desai et al. [12] reported an electrospray microdevice with an integrated particle filter on silicon nitride. 

In the sheath liquid approach the column effluent is diluted by the sheath liquid and hence it results in a lower sensitivity compared to the sheathless interface. Moreover, ions in the sheath liquid compete with the analytes for the transport of charge during the electrospray process, again resulting in a loss of sensitivity [62], Finally, because the sheath liquid technique involves the input of additional solvents and other chemicals, the potential to add significantly to the background noise exists. However, the use of a sheathless CEC—ESI-MS interface implies the need for specialized capillary and electrospray hardware, and thus is not as popular as the sheath liquid interface. 

Because there is no ionizable groups of the coating in the neutral capillary, the interaction between charged molecules with ionic capillary surface is eliminated. Also, the electro-osmotic flow (EOF) of a neutral capillary is eliminated. However, a continuous and adequate flow of the buffer solution toward the CE capillary outlet is an important factor for routine and reproducible CE-ESI-MS analysis in order to maintain a stable ESI operation, some low pressure applied to the CE capillary inlet is usually needed, especially when the sheathless interface is employed. The disadvantage of the pressure-assisted CE-ESI-MS is the loss of some resolution because the flat flow profile of the EOF is partially replaced by the laminar flow profile of the pressure-driven system. A typical neutral capillary is a LPA (linear polyacrylamide)-treated capillary. Karger and co-workers [6] used mixtures of model proteins, a coaxial sheath flow ESI interface. 

Several research groups have presented work on the development of CE-ESI-MS interfaces. The interfaces developed can be categorized into three main groups coaxial sheath flow, liquid junction, and sheathless interfaces. A schematic of the sheath-flow interface first developed for CE-ESI-MS by Smith et al. [6] is illustrated in Fig. la. A sheath liquid, with... 

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. 

Unlike spectroscopic detectors which are performed on capillary, the outlet end of the CE must be removed from the vial and positioned in front of the mass spectrometer inlet. This process must be performed without sacrificing separation efficiency and ensuring electrical continuity. Therefore, the interface between CE and ESI-MS is one of the keys to the success of this technique. Three modes have been reported, including liquid-liquid junction, sheath-liquid and sheathless interfaces... 

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... 

In the sheathless interface the tip of the capillary column is the spray point. The tip is pulled out or sharpened to a fine point with an orifice of about 5-20 p,m. The outer... 

Construct the sheathless interface by attaching the CE separation capillary to the commercial distal coated nanospray needle through a teflon joint. The length of the CE capillary is to be set to 100-130 cm. 

There are many advantages of sheathless interfaces compared to those that require a sheath flow. The main difference between sheath and sheathless interface designs is that sheathless interface does not require the external flow of a coaxial sheath liquid to establish electrical contact with the CE... 

The same group followed up on this initial design by adding the capability for on-column sample enrichment into the sheathless interface." A small solid-phase extraction cartridge, made of RP material, was attached to the CE capillary near the point of injection. Optimization of the proper capillary diameters showed that they could achieve a mass limit of detection of 500 amol for CE-MS/MS analysis of a standard peptide using a 20-p.m i.d. capillary. This design brings to reality a true zero dead-volume sheathless CE— MS interface with the ability to preconcentrate the sample within the same capillary. 

---