Twenty years of interface development for capillary electrophoresis–electrospray ionization–mass spectrometry

Abstract

Capillary electrophoresis–electrospray ionization–mass spectrometry has the potential to become a preferred tool for the analysis of biological mixtures and other complex samples. The development of improved interfaces in the past twenty years has been critical in demonstrating the feasibility of this technique. However, a compromise still exists between interfaces that give optimal performance and those that are practical for commercial applications. The first section of this review focuses on the technological advances in CE–ESI–MS as they relate to the key interface features for both sheath-flow and sheathless systems: delivery of the sheath liquid, shaping of the emitter tip, formation of electrical contact, and practicality in terms of ease of use and lifetime. In the second section, we review the fundamental processes that affect interface performance. Because of the complex natures of both capillary electrophoresis and electrospray ionization, flow rate, arrangement of the electrical circuit, electrochemistry, tip geometry and location of electrical contact must all be carefully managed in the design of a successful interface.

Introduction

When first introduced in 1987, the combination of capillary electrophoresis with mass spectrometry through an electrospray ionization interface provided an intriguing solution for combining two highly powerful analytical techniques. Capillary electrophoresis (CE) had already been shown to give excellent separation efficiencies, however, short optical path lengths provided by the small capillary inner diameters resulted in a concentration sensitivity that fell short of those achieved by other separation techniques. On the other hand, electrospray ionization mass spectrometry (ESI–MS) had previously been shown to offer low detection limits and fast response times as a detector for liquid chromatographic separations [1], as well as providing valuable structural information on the analytes. Smith and co-workers performed the first demonstration of online electrospray ionization of CE effluent in 1987, using a metal sheath around the capillary terminus to replace the terminal electrode of a traditional CE setup, providing gaseous analyte ions for mass spectrometric analysis [2]. Unfortunately, the publications that followed on the subject seemed to highlight the limitations of the technique. The use of electrospray ionization placed limitations on the composition of the background electrolyte that could be used in the CE separation, and the flow rates required to maintain a stable electrospray could only be supplied under conditions giving a maximized electroosmotic flow (EOF) [3], [4]. The use of a flowing sheath liquid was offered as a solution to these problems; however, it came at the price of reduced sensitivity, as the sheath liquid diluted the CE effluent.

Over the past twenty years a wide variety of interface designs have been proposed in order to maximize the potential of this hyphenated technique. These have been summarized in a number of excellent technology-based reviews on the subject [5], [6], [7], [8], [9], [10], [11], [12], [13]. However, commercially available CE–MS platforms continue to use interfaces that are far from optimal in so far as sensitivity is concerned, limiting them to applications where concentration is not a limiting factor [14], [15]. This is in large part due to the fact that those interfaces offering the best sensitivity and lowest detection limits have also been among the most fragile, making them unattractive for commercialization. The compromise of robustness over sensitivity among commercial CE–MS platforms is one of the major reasons that the technique has still not been widely adopted as a routine method of analysis.

As capillary electrophoresis continues to gain popularity as an alternative to liquid chromatography for performing separations in proteomic, metabolomic and pharmaceutical research, the importance of being able to reliably interface CE separations with mass spectral analysis grows. Unfortunately, the nature of CE lends some particular challenges when it comes to online MS detection. Both the CE and ESI processes require stable electrical contact of the solution with an electrode at the capillary outlet without interruption of the electroosmotic flow from the CE separation. In addition, the low volumetric flow rates used in CE place particular restrictions on the geometry of the tip if a stable electrospray is to be maintained [16]. Finally, compatibility of the background electrolyte with the electrospray process and on the resulting mass spectra must be considered.

In this review, we aim to summarize the techniques currently employed in constructing interfaces for CE–ESI–MS, followed by a discussion of the fundamental aspects of the CE–ESI–MS process that pose challenges in the design of a robust and efficient interface.