Chemically etched open tubular and monolithic emitters for nanoelectrospray ionization mass spectrometry - PubMed (original) (raw)

Chemically etched open tubular and monolithic emitters for nanoelectrospray ionization mass spectrometry

Ryan T Kelly et al. Anal Chem. 2006.

Abstract

We have developed a new procedure for fabricating fused-silica emitters for electrospray ionization-mass spectrometry (ESI-MS) in which the end of a bare fused-silica capillary is immersed into aqueous hydrofluoric acid, and water is pumped through the capillary to prevent etching of the interior. Surface tension causes the etchant to climb the capillary exterior, and the etch rate in the resulting meniscus decreases as a function of distance from the bulk solution. Etching continues until the silica touching the hydrofluoric acid reservoir is completely removed, essentially stopping the etch process. The resulting emitters have no internal taper, making them much less prone to clogging compared to, e.g., pulled emitters. The high aspect ratios and extremely thin walls at the orifice facilitate very low flow rate operation; stable ESI-MS signals were obtained for model analytes from 5-microm-diameter emitters at a flow rate of 5 nL/min with a high degree of interemitter reproducibility. In extensive evaluation, the etched emitters were found to enable approximately four times as many LC-MS analyses of proteomic samples before failing compared with conventional pulled emitters. The fabrication procedure was also employed to taper the ends of polymer monolith-containing silica capillaries for use as ESI emitters. In contrast to previous work, the monolithic material protrudes beyond the fused-silica capillaries, improving the monolith-assisted electrospray process.

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Figures

Figure 1

Figure 1

Schematic depiction of the emitter etching procedure. (A) Surface tension causes a concave meniscus to form on the capillary. (B) The etch rate in the meniscus decreases as a function of distance from the bulk solution. Horizontal arrows are vectors indicating etch rate. (C) Completed emitter after the capillary has etched through and separated from the etchant solution.

Figure 2

Figure 2

Photomicrographs of etched silica emitters. Additional description is in the text.

Figure 3

Figure 3

ESI performance for 5 μM reserpine at 5 nL/min (top) and 50 nL/min (bottom). Open circles refer to peak height (arbitrary units, left axes), and closed circles correspond to %RSD (right axes). The solvent was 99% water, 1% HAc.

Figure 4

Figure 4

ESI-MS reproducibility for 5 μM reserpine with three 5-μm-i.d. emitters. Open shapes indicate reserpine intensity vs. ESI voltage for the different emitters (left axis). Closed circles refer to %RSD of peak height for the replicates. The flow rate was 50 nL/min and the solvent was 99% water, 1% HAc.

Figure 5

Figure 5

Photomicrographs of etched ESI emitters containing (A) silica and (B) polymer monoliths. The capillaries in both (A) and (B) were 360 lm o.d. and 50 lm i.d.

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