An open design microfabricated nib-like nanoelectrospray emitter tip on a conducting silicon substrate for the application of the ionization voltage

Abstract

This paper describes a novel emitter tip having the shape of a nib and based on an open structure for nano-electrospray ionization mass spectrometry (nanoESI-MS). The nib structure is fabricated with standard lithography techniques using SU-8, an epoxy-based negative photoresist. The tip is comprised of a reservoir, a capillary slot and a point-like feature, and is fabricated on a silicon wafer. We present here a novel scheme for interfacing such nib tips to MS by applying the ionization voltage directly onto the semi-conductor support. The silicon support is in direct contact with the liquid to be analyzed at the reservoir and microchannel level, thus allowing easy use in ESI-MS. This scheme is especially advantageous for automated analysis as the manual step of positioning a metallic wire into the reservoir is avoided. In addition, the analysis performance was enhanced compared with the former scheme, as demonstrated by the tests of standard peptides (gramicidin S, Glu-fibrinopeptide B). The limit of detection was determined to be lower than 10 Ϫ 2 M. Due to their enhanced performance, these microfabricated sources might be of great interest for analysis requiring very high sensitivity, such as proteomics analysis using nanoESI-MS. Proteomics studies are carried out on small amounts of sample and, thereby, need highly sensitive analysis techniques. ESI-MS matches this sensitivity criterion [2]. It also enables species identification using their precise molecular weights and provides fragment sequences using the MS/MS mode [1,3]. Therefore, ESI-MS is one of the best techniques for use in proteomics.Nanospraying is usually performed using glass or fused-silica-based capillary tips [2], and the spray is obtained by applying the ionization voltage to the electrical conductive tip or through the sample solution. However, capillary sources present a number of weaknesses: (1) a poorly controlled fabrication route resulting in a low reproducibility of the sources, (2) an irregular profile of the aperture after source opening, (3) easy clogging due to the small i.d., (4) signal suppression caused by air bubbles, and (5) deterioration of the conductive coating [4,5] upon operation.Different improvements regarding the quality of the emitter tip are proposed in the literature. Reproducible dimensions can be achieved using an enhanced fabrication route based on laser-micromachining [6,7]. More stable coatings have also been reported [8 -10]. Most of these improvements, however, do not solve the basic problems of clogging and the lack of reproducibility that affect the analysis conditions. Using microtechnology techniques, reproducible and ready-to-use high-quality nanospray tips can be produced in large batches. The tip geometry can be chosen to allow on-chip integration together with other microfluidic components, e.g., a chromatography column, and also to be compatible with the use of standard robots. Microfabrication of ESI emitter tips follows two main approaches: (1) a "needle-like" ap- Our approach is to design an em...

Analytical characterization of microfabricated SU‐8 emitters for electrospray ionization mass spectrometry

“…The linear range for verapamil was 1 n M –10 µ M ( R 2 = 0.9998) and for Glu‐fibrinopeptide B 4 n M –3 µ M ( R 2 = 0.9992). These values are similar to or better than those of other chip‐based ES emitters thus far reported,38, 45, 55, 56 albeit a different type of MS instrumentation was employed. A tip‐to‐tip repeatability for signal intensity was 14% RSD ( n = 3; 5 µ M verapamil) for the optimized emitter design, whereas run‐to‐run repeatabilities were 4–11% RSD ( n = 4; 5 µ M verapamil) for all individual emitters.…”

“…Both patterning and alignment accuracies of the overlaid microstructures rely on lithographic precision. For instance, emitter tips overhanging on a silicon substrate have been successfully implemented on parylene,41, 42 polysilicon,43 and epoxy photoresist SU‐844, 45 by lithography. In addition, we have tentatively demonstrated the fabrication and feasibility of a free‐standing SU‐8 emitter design for ESI/MS 46.…”

Microchip technology in mass spectrometry

“…11a). In more recent work, with optimized tip dimensions detection limits at low nanomolar level for peptide standards were demonstrated with both types of devices (Le Gac, Rolando, & Arscott, 2006;Sikanen et al, 2008). Sharp-pointed emitters have also been patterned on parylene-C by means of UV lithography and etching in oxygen plasma (Licklider et al, 2000;Kameoka et al, 2002).…”

“…Many of the microfabricated emitters discussed above were demonstrated only in direct spray experiments where ESI voltage was introduced via an external electrode wire inserted in the sample inlet. On some occasions, high voltage was applied through the conducting silicon substrate (Le Gac, Rolando, & Arscott, 2006;Nissilä et al, 2007). However, with on-chip separation performed before electrospray, high voltage has to be applied to the ESI (Xie et al, 2005).…”

Microfluidics for multiplexed MS analysis

“…Other technologies, suitable also for commercial production, include direct fabrication of the systems with the electrospray tips prepared in the SU-8 photoresist [55][56][57] or using plasma or laser ablation for creation of the required structures in plastic materials such as polyimide [58][59][60]. The first microfluidic system integrating sample loading loop, chromatographic separation column and electrospray-exit port [61] is now commercially available (www.agilent.com).…”