posted on 2022-11-09, 20:20authored byZhongbao Han, Lee Chuin Chen
The small charged droplet generated from the nanoelectrospray
ionization
(nanoESI) source at nL/min flow rate gives its unique feature of high-performance
ionization. A continuous scan of the flow rate in this regime can
trace the effect of droplet size in greater detail for a better understanding
of the ionization process. To date, such practical implementation
is hindered by the lack of a suitable liquid pump and the reproducibility
of microcapillaries-based systems. Here, offline nanoESI mass spectrometry
with a continuously varying flow rate in a dynamic range of several
hundred pL/min to ∼100 nL/min was performed by the precision
scanning of ESI high voltage (HV). The principle is based on the new
paradigm of generating nanoelectrospray from a large Taylor cone with
a known spray current–flow rate relationship. The instantaneous
flow rate controlled by the HV was determined by simultaneous measurement
of the spray current. The system is successfully applied to reveal
the role of nanoflow rate on the average charge state of proteins,
analysis of analyte mixture, and desalting effect. With the use of
a buffer solution with high electric conductivity, a highly controllable
oxidative modification was also observed by tuning the flow rate below
a threshold of ∼5 nL/min, a finding that has potential application
to on-demand oxygen labeling.