High-pressure nanoESI of highly conductive volatile and non-volatile buffer solutions from a large Taylor cone: Effect of spray current on charge state distribution

• Published in: International journal of mass spectrometry 2022-06, Vol.476, p.116845, Article 116845
• Main Authors: Han, Zhongbao, Chen, Lee Chuin
• Format: Article
• Language: English
• Subjects: Charge state distribution; Desalting; High-pressure electrospray; Nanoelectrospray; Scaling laws; Taylor cone
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2022.116845

Direct ESI-MS of proteins in nonvolatile buffer requires the generation of very fine initial droplets to resolve the peak for meaningful analysis. This is usually achieved using nanoESI capillary with an inner diameter of ∼1 μm or smaller. Recently we reported the generation of “true nanoelectrospray” from a micropipette tip with a large inner diameter under the super-atmospheric pressure condition (J. Am. Soc. Mass Spectrom.2022, 33, 491–498). The flow rate in the nL/min regime can be reproducibly adjusted using high voltage with the spray current as the indicator for flow rate. Here two further works are done on the same ion source. First, the validation of the spray current-flow rate relationship has been extended to 0.5 MPa, and the achievable minimum spray current is found to reduce with the ion source pressure. Second, the ion source is applied to study the flow rate effect on the charge state distribution in the nanoflow regime for proteins and peptides in volatile (formic acid and ammonium acetate) and the nonvolatile (phosphate buffer saline (PBS) and 1 M Tris-HCl) buffers. For the nonvolatile buffers, clean nanoESI mass spectra are only obtained at a low flow rate of ∼1 nL/min. The average charge state of the tested proteins is primarily determined by the type of buffer. For acidic solutions, the charge state shifts to a higher value at a lower flow rate. For non-denaturing solutions, the average charge number remained relatively constant but the variation with the flow rate appeared to be analyte and buffer dependent. [Display omitted] •Ultra-low-flow rate nanoESI operation using a disposable micropipette tip.•Q1/2 scaling law validated down to nL/min flow regimes.•Direct detection of proteins from high-salt buffers.•Fine-tuning of flow rate to study CSD without changing capillary.