Metallized sheathless electrospray emitters for use in capillary electrophoresis orthogonal time-of-flight mass spectrometry

Abstract

Chromium-gold coated fused silica emitters for electrospray ionization have been fabricated with lifetimes upwards of 100 hours. The coating on these emitters is extremely durable withstanding electrical discharges, abrasion and mechanical stress. Usable flow rates range from 30 nL/min to 1 mL/min using both standard electrospray solutions and aqueous buffers. Coated capillary electrophoresis columns incorporating the emitter provide excellent separation, low limits of detection (70-700 attomoles for peptides), and are easy to use. Performance of the emitters in continuous infusion experiments and capillary electrophoresis/ electrospray ionization mass spectrometry applications are presented. Copyright # 1999 John Wiley & Sons, Ltd. Received 27 February 1999; Revised 30 March 1999; Accepted 31 March 1999 Fused silica capillaries have played an important role in the development of electrospray ionization mass spectrometry (ESI-MS) since they provide a durable, non-reactive, and inexpensive way to couple various flow streams to ESI sources. Although fused silica capillaries are ideally suited for transporting solutions to an ESI source, they cannot be used to provide an electrical connection at the ESI source without modification since fused silica is a nonconductive material. Despite this fact, a variety of distinct approaches have been taken to try and get around the problem of applying a high potential to a solution flowing in a nonconductive fused silica capillary in order to generate an electrospray. Niessen 1 presents a good review of this topic in relation to atmospheric pressure ionization (API) sources in a recent article.One solution to this problem is to coat the fused silica capillary with a conductive metal. Metallized fused silica capillaries have been used extensively as emitters for ESI, with silver and gold being the most commonly used metals for coating. Silver can be deposited onto a fused silica ESI tip using a variety of means ranging from liquid silver solder to vapor deposition. However, the silver layer is unstable under the red-ox conditions at the ESI tip, resulting in the degradation of the electrical contact between the silver and the capillary electrophoresis (CE) buffer. Gold on the other hand is most often sputtered onto a fused silica ESI tip, but it can also be electroplated. Although gold is more electrochemically stable than silver, it is physically unstable on fused silica and has a finite lifetime before electrical contact is lost and the gold must be reapplied. Nonetheless, coating capillaries with a conductive metal continues to be a viable alternative for creating an electrical contact since this type of design (a.k.a. sheathless design) allows for low flow rates to be used. This is of particular importance when coupling capillary electrophoresis to electrospray ionization mass spectrometry (CE/ESI-MS) since flow rates are dictated by the electroosmotic flow (EOF) that is generated, which can range from 10-200 nl/min depending on the CE conditions used. ESI emitters workin...

Capillary electrophoresis – mass spectrometry: 15 years of developments and applications

“…, and (D) adopted from [89], [367], and [124], respectively, and reprinted with permission. (B) adopted from [64], with permission.…”

Robust and cost-effective capillary electrophoresis-mass spectrometry interfaces suitable for combination with on-line analyte preconcentration

“…The dilution effect is not necessary if other ways of applying electrical contact can be employed. For example, capillaries with a conductive-coated tip [10][11][12][13], a stainless steel tip [14], a microdialysis junction interface [15], direct electrode contact combinations [16][17][18][19], or even electrode-less systems [20] can be used. In these cases, the diluting effect of the make-up flow is avoided.…”

Effects of modulation defects on hadamard transform time-of-flight mass spectrometry (HT-TOFMS)

“…fused-silica capillary (Polymicro Technologies, Tucson, AZ). One end of the capillary was converted to a gold-coated sheathless electrospray emitter following the procedure described by Barnidge et al [17,18]. Solutions were nebulized by applying 35 hPa (0.5 psi) to a sealed sample vial connected to the ESI emitter, which was held at 2.5 kV.…”