Picoelectrospray Ionization Mass Spectrometry Using Narrow-Bore Chemically Etched Emitters
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Electrospray ionization mass spectrometry (ESI-MS) at flow rates below ~10 nL/min has been only sporadically explored because of difficulty in reproducibly fabricating emitters that can operate at lower flow rates. Here we demonstrate narrow orifice chemically etched emitters for stable electrospray at flow rates as low as 400 pL/min. Depending on the analyte concentration, we observe two types of MS signal response as a function of flow rate. At low concentrations, an optimum flow rate is observed slightly above 1 nL/min, whereas the signal decreases monotonically with decreasing flow rates at higher concentrations. For example, consumption of 500 zmol of sample yielded signal-to-noise ratios ~10 for some peptides. In spite of lower MS signal, the ion utilization efficiency increases exponentially with decreasing flow rate in all cases. Significant variations in ionization efficiency were observed within this flow rate range for an equimolar mixture of peptide, indicating that ionization efficiency is an analyte-dependent characteristic for the present experimental conditions. Mass-limited samples benefit strongly from the use of low flow rates and avoiding unnecessary sample dilution. These findings have important implications for the analysis of trace biological samples.
Key wordsNanoelectrospray nano-ESI Quantitation Mass-limited analysis
The authors thank William F. Danielson for writing the syringe pump control software, and Sarah Rausch, Allison Sheen, and Levi Broeske for assistance with data processing. This research was supported by the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) intramural program and grants from National Institutes of Health: the National Institute of General Medical Sciences (grant 8 P41 GM103493-10) and the National Cancer Institute (1R33CA155252). The EMSL is a national scientific user facility sponsored by US DOE’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL) in Richland, WA. PNNL is a multiprogram national laboratory operated by Battelle for the DOE under contract no. DE-AC05-76RLO 1830.
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