Electrospray emitter

There are some variants that have emerged in the wake of DESI. By replacing the electrospray emitter by a metal needle and allowing solvent vapor into the coaxial gas flow desorption APCI (DAPCI) can be performed [106], Other versions are atmospheric-pressure solids analysis probe (ASAP) where a heated gas jet desorbs the analyte, which is subsequently ionized by a corona discharge [107], and electrospray-assisted laser desorption/ionization (ELDI) where a laser ablates the analyte and charged droplets from an electrospray postionizes the desorbed neutrals [108],... 

G.J. Van Berkel, Electrolytic deposition of metals on to the high-voltage contact in an electrospray emitter Implications for gas-phase ion formation, J. Mass Spectrom., 35 (2000) 773-783. 

Barnidge, D. R., Nilsson, S., Markides, K. E., Rapp, H., and Hjort, K. (1999). Metallized sheathless electrospray emitters for use in capillary electrophoresis orthogonal time-of-flight mass spectrometry. Rapid Commun. Mass Spectrom. 13, 994—1002. 

Trapp, O., Pearce, E. W., Kimmel, J. R., Yoon, O. K., Zuleta, I. A., and Zare, R. N. (2005). A soft on-column metal coating procedure for robust sheathless electrospray emitters used in capillary electrophoresis-mass spectrometry. Electrophoresis 26, 1358-1365. 

D. R. Barnidge, S. Nilsson, and K. E. Markides, A design for low-flow sheathless electrospray emitters, Anal. Chem., 71 (1999) 4115-4118. 

Y.-R. Chen and G.-R. Her, A simple method for fabrication of silver-coated sheathless electrospray emitters, Rapid Commun. Mass Spectrom., 17 (2003) 437-441. 

Figure 2.10 Schematic representation of micromachining process used to fabricate microfluidic chips incorporating an electrospray emitter [19].

Miniaturized LC/MS formats based on micromachined chip-based electrospray emitters and ionization sources on silicon (Schultz et al., 2000 Licklider et al., 2000 Ramsey and Ramsey 1997 Xue et al., 1997) and plastic (Vrouwe et al., 2000 Yuan and Shiea, 2001, Tang et al., 2001) microchips is a proactive approach for scale-down platforms. Various micromachining processes are used to fabricate these devices. These microanalytical technologies would create integrated sample preparation and LC/MS applications. The potential benefits of such a system include reduced consumption of sample/reagents, low cost, and disposability. 

Instead of using a planar molding master, a fused silica capillary tube (50 pm i.d. and 192 pm o.d.) was used as a template for casting PDMS channels, and as the fluid inlet/outlet tubes. After PDMS curing, the middle pre-scored section (4 cm) of the capillary was removed to reveal the PDMS channel (192 pm wide and deep) [817]. Similarly, a capillary was used to mold a PDMS channel, and to produce an electrospray emitter. In this case, after PDMS curing, the last 0.5-cm section of the capillary was removed to create a channel [821]. 

The analysis of MPH from human urine was used as a model to demonstrate the direct-infusion quantitative bioanalysis from a polymer-based microfluidic chip electrospray emitter [62]. A calibration curve in the range 0.4-800 ng/ml was acquired. 

Lee and co-workers demonstarted that a micromachined parylene nozzle could serve as an electrospray emitter on a silicon chip. This hollow-needle structure extended more than a millimeter beyond the edge of the silicon substrate. MS/MS analysis of the mixture of peptides obtained from the trypsin digest of cytochrome c was demonstrated with this micromachined emitter, and... 

More recently, porous carbon electrode technology is used to enhance the range of chemical structures that can be ionized by ESI and to reduce background often observed with electrospray. By control of the redox chemistry at the ESI emitter. Van Berkle and co-workers have demonstrated the ability to change the species observed in the full-scan mass spectrum of Reserpine [73,74]. Further understanding of the redox chemistry that occurs at the electrospray emitter is critical to both its use as an analytical tool and the interpretation of ESI-MS data for biotransformahon studies. 

Mass spectrometry tests were carried out using a LCQ deca XP + ion trap mass spectrometer (Thermo Finnigan, San Jose, CA, USA). The electrospray emitter tip nib structure was placed on a dedicated holder that is introduced in the inlet of the mass spectrometer. A platinum wire was subsequently inserted in the reservoir feature of the nib, and was connected at its other end on a metallic part on the holder where HV is normally applied for ionization purposes. Solutions to be analyzed were dropped in the reservoir feature using a gel-loader tip typically 5-10 pL of sample were deposited in the reservoir. Liquid was subsequently seen to fuse at the tip of the nib due to capillary action in the slot. HV was then applied, and a spray could be observed with HV values in a... 

Figure 5.9 Novel fabrication process for the second generation of micromachined electrospray emitter tips silicon support wafer (blue), 200 nm thick nickel etch-release layer (white) which is patterned using a HN03-based wet etch, negative photoresist SU-8 which forms the micro-nib support layer and tip which hosts the capillary slot (gold) and single photolithographic masking layer which defines the reservoir and tip (black).

Figure 6.3 (A) Fabrication procedures for parylene electrospray emitter (1) parylene...

Polymeric microfluidic systems coupled to a microfabricated planar polymer tip can be used as a stable ion source for ESI-MS. A parylene tip at the end of the microchannel delivers fluid which easily produces a stable Taylor cone at the tip via an applied voltage. The described device appears to facilitate the formation of a stable spray current for the electrospray process and hence offers an attractive alternative to previously reported electrospray emitters. When this interface was employed for the quantification of methylphenidate in urine extracts via direct infusion MS analysis, this system demonstrated stable electrospray performance, good reproducibility, a wide linear dynamic range, a relatively low limit of quantification, good precision and accuracy, and negligible system carryover. We believe polymeric devices such as described in this report merit further investigation for chip-based sample analysis employing electrospray MS in the future. 

Dahlin, A. R, Bergstroem, S. K., Andren, R E., Markides, K. E., and Bergquist, J., Poly(dimethylsiloxane)-based microchip for two-dimensional solid-phase extraction-capillary electrophoresis with an integrated electrospray emitter tip. Analytical Chemistry, 77, 5356, 2005. 

Bedair, M. F. and Oleschuk, R. D., Fabrication of porous polymer monoliths in polymeric microfluidic chips as an electrospray emitter for direct coupling to mass spectrometry, Anal. Chem., 78, 1130, 2006. 

Liljegren, G., Dahlin, A., Zettersten, C., Bergquist, J., Nyholm, L., On-line coupling of a microelectrode array equipped poly(dimethylsiloxane) microchip with an integrated graphite electrospray emitter for electrospray ionization mass spectrometry, Lab Chip, 5, 1008-1016, 2005. 

Thorslund, S., Lindberg, P., Andren P.E., Nikolajeff, F, Bergquist, J., Electrokinetic-driven microfluidic system in poly(dimethylsiloxane) for mass spectrometry detection integrating sample injection, capillary electrophoresis, and electrospray emitter on-chip. Electrophoresis, 26, 4674-4678, 2005. 

Wang P, Chen Z, Chang H-C (2006) A porous electrospray emitter assisted by an integrated electro-kinetic pump based on silica monoliths. Electrophoresis 27 3964-3970... 

Microfluidic devices can incorporate electrospray emitters by either coupling a conventional capillary emitter to the chip or... 

Chip electrospray mass speclrometiy Electrospray emitter on-chip Electrospray ionization mass spectrometry (ESI-MS) Electrospray mass spectrometry... 

On-Chip Electrospray, Fig. 6 Scanning electron microscopy images of an out-of-plane needle electrospray emitter tip fabricated at the bottom side of a liquid chamber etched into a silicon wafer. The view from the bottom is depicted in image (a) showing the 10 pm diameter needle protruding from the underside of the substrate. The view from the top of the substrate is depicted in image (b) showing the 600 pm liquid chamber (Reprinted with permission from [9])... 

On-Chip Electrospray, Fig. 7 Scanning electron microscopy image (a) and schematic plan and side views (b) of the nib-like in-plane electrospray emitter tip fabricated in SU-8 epoxy using double-exposure lithography [20] (Reprinted with permission from [20])... 

---