Abstract
Voltage-driven electrospray of highly conductive solutions under high pressure without gaseous breakdown enabled a low flow rate nanoESI using an emitter of large inner diameters such as a commercial micropipette tip. This feature was previously exploited to observe unique ionization responses at flow rates close to their minimum values (Chem. Sci.14, 4506–4515 &Anal. Chem.94, 16,015–16022). Here, we studied the properties of such pipet tip sprayers at a higher spray current up to >2 μA with flow rates approaching their maximum values. Normal pipet tips with 0.37 mm i.d. and tips packed with chromatographic particles (ZipTip, i.d. 0.44 mm) were used. The nanoflow rate was tuned using emitter voltage with spray current as the monitoring signal for flow rate. The spray current I and flow rate Q relationship followed the I∝Q1/2 law for I < 1 μA. Above that point, the Taylor cone was still stable but the spray current increased at a higher rate with the flow rate. The packed pipet tips have a larger flow resistivity, required a higher onset voltage, and interestingly, supported a higher stable spray current and flow rate. HP-nanoESI-MS of protein and oligosaccharide showed variation in charge state and ion intensity with the spray current. The direct nanoESI of extraction solution from the ZipTip right after the desalting and concentrating procedures were also demonstrated.
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