Advantage of nano-ESI

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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What exactly is the advantage of a nano-ESI source in comparison to a conventional one? Of course it is used with a decreased flow rate, thus less dilution of the sample but why isn't it possible to use a normal ESI source with reduced flow rate? Which different does it make?
Furthermore, does a nano-ESI require a nano-HPLC, is so why?

Thanks for your answers
I've done standard LC with post column split to achieve nano flow for MS coupling. It can be done, but I prefer full nano systems if going that direction.

Nano ESI results in finer initial microdroplets during the electrospray process, and thus relatively more ions make it to the gas phase than standard flow ESI. Whether these are peptides or small organic molecules, both see improved ionization efficiency and therefore improved sensitivity. Thinking of it in terms of the fundamentals makes it most clear. I recommend Electrospray Ionization Mass Spectrometry (1st or 2nd edition--2nd includes MALDI) by Richard B. Cole if you want to get into it, as well as more modern literature.

Why can't one just turn the flows down to do nano scale? Dwell volume is a big part of it. Nano flows on a standard LC system will result in painfully long gradient delay, not to mention the contribution to band broadening from longitudinal diffusion (inversely proportional to time (edit: not inversely! more time = more diffusion)). Flow accuracy is another. Standard LC's are not designed for maximum accuracy of flow rates at the very bottom of their flow capability. What is considered a low dead volume connection at 100uL/min, is like the Atlantic Ocean at 300nL/min. All connections need to be optimized in nano and made correctly or there will be serious band broadening. As for the ESI source, again things are miniaturized for nano. The spray capillary is smaller ID, and the nano source is operated rather close to the heated capillary, so a stage on controllable slides is usually used for fine positional control of the spray capillary, along with cameras to better see the position for alignment.

My personal preference is not quite nano flow and nano ESI, but micro scale. 300um-1000um ID UPLC columns run between 5-100uL/min, with an appropriately sized ESI spray needle. There is a gain in sensitivity at the ESI over the higher flows with 2.1mm+ ID columns, without the clogging headaches that can arise when working with fused silica capillaries used in nano (20um ID). I've learned to deal, but problems still arise, and it's always a steep learning curve for new analysts. My biggest complaint of micro bore is column selection: you can't always find the phase chemistry you want in these smaller bores, especially 300um ID, doubly so if you want to stick with one particular column manufacturer. Example: C30 phases for lipidomics (haven't found one yet) or a HILIC column for polar metabolites from Waters. Not big deals, just something I've encountered.

Hope that helps, albeit rather late (3 weeks after your post). I'm sure I've overlooked something as well, but that's what has come to mind.
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