Abstract
The SPB/SFX instrument at the European XFEL provides unique conditions for single-particle imaging (SPI) experiments due to its high brilliance, nano-focus and unique pulse structure. Promising initial results provided by the international LCLS (Linac Coherent Light Source) SPI initiative highlight the potential of SPI. Current available injection methods generally have high sample consumption and do not provide any options for pulsing, selection or orientation of particles, which poses a problem for data evaluation. Aerosol-injector-based sample delivery is the current method of choice for SPI experiments, although, to a lesser extent, electrospray and electrospinning are used. Single particles scatter only a limited number of photons providing a single orientation for data evaluation, hence large datasets are required from particles in multiple orientations in order to reconstruct a structure. Here, a feasibility study demonstrates that nano-electrospray ionization, usually employed in biomolecular mass spectrometry, provides enough ion flux for SPI experiments. A novel instrument setup at the SPB/SFX instrument is proposed, which has the benefit of extremely low background while delivering mass over charge and conformation-selected ions for SPI.
Highlights
The first experiments at the European X-ray free-electron laser (XFEL) instrument SPB/ SFX have shown new scientific possibilities especially for the life science community with successful serial femtosecond crystallography (SFX) experiments (Grunbein et al, 2018)
The sample is generally introduced via pressure through a capillary with flow rates of several microlitres to millilitres per minute in a gas dynamic virtual nozzle (GDVN) at a concentration of 1012 particles mlÀ1 followed by a focusing aerodynamic lens
At a similar concentration of particles, electrospray ionization (ESI) uses flow rates of 1–2 ml hÀ1 leading to a theoretically higher number of hits per sample volume. Both nano-ESI and GDVNs can be operated at lower flow rates in favourable cases; volumes consumed per minute in a GDVN last for an hour in nano-ESI
Summary
The first experiments at the European XFEL instrument SPB/ SFX have shown new scientific possibilities especially for the life science community with successful serial femtosecond crystallography (SFX) experiments (Grunbein et al, 2018). The sample is generally introduced via pressure through a capillary with flow rates of several microlitres to millilitres per minute in a gas dynamic virtual nozzle (GDVN) at a concentration of 1012 particles mlÀ1 followed by a focusing aerodynamic lens. This approach has a relatively low sample efficiency resulting from a combination of high sample consumption and low hit rates of around 1% (Daurer et al, 2017). At a similar concentration of particles, ESI uses flow rates of 1–2 ml hÀ1 leading to a theoretically higher number of hits per sample volume. Both nano-ESI and GDVNs can be operated at lower flow rates in favourable cases; volumes consumed per minute in a GDVN last for an hour in nano-ESI
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