Dear all, three questions, each other related:

1. What's the effect of the different spray-needle voltage (in ESI) on the charged droplet formation in the ion source?

2. Regular ESI (~200ul-1ml/min flow rate) normally prefers a higher needle voltage than lower flow electrospray, like micro-spray, why is that?

3. Higher flow rate, higher needle voltage? why, mechanistically, why so?

thank you!

I don't have very good answers, but I will tell you what I do know (or a best guess).

1. I don't know many details about this one, but I can tell you that the voltage appears to have a major effect on the spray above and beyond any pneumatic assistance. As you turn down (or off) your needle voltage, you'll see your nice fine droplet spray will turn into a coarser spray or even a stream of liquid. If the voltage is set too high, then corona discharge or arcing start to occur.

2& 3 appear to be the same question. This is only a guess based on some assumptions. In ESI, you are generating charged droplets, so you must be doing a redox reaction before the droplet leaves the needle, and electric current must be flowing. (The redox reactions have been discussed in the literature.) Assuming the droplet size is the same, with the same amount of charge per droplet at the different flow rates, then more current is needed at the higher flow rates. If the electrical resistance is the same, or if it does not decrease enough with flow rate increases, then more current requires a higher voltage. In reality, I think that droplet size is smaller in nanospray, but whether it's enough to invalidate my explanation, I don't know. Also, I have no idea what really happens to the electrical resistance with changing flow rates. So the above answer is pure speculation.

thanks, MG. very insightful. I remember on Thermo instrument, it allows you to read the spray current, wondering what the spray current would change with voltage changes. Unfortunately, I only have Sciex instruments now, don't know how to read the spray current on sciex instrument.

So, what do you think? what's the optimum spray voltages for different flwo rates?

Yes, having used Agilent and Bruker instruments with ESI current readback, I miss it on the Sciex instruments. ESI current can tell you if you aren't spraying, or if you have a corona discharge that isn't visible to the naked eye.

At times I have gone to the trouble to optimize the spray voltage, only to find that the optimum is often very broad, and you can vary it +/- 500V with little change in the signal. I do mostly higher flow work with 2.1 mm columns, so I don't have a good feel for what happens at lower flow rates. I have found that the optimum spray voltage is more important for negative ion work. For positive ion (on the Sciex instruments), I use 5000V, but 4500 or 5500 produces about the same result. For negative ion, I use anywhere from -2500 to -3600 V, and I usually try to optimize it for a given mobile phase and analyte. Default value is -4200, which I think is too high in many cases. Too high of a voltage in negative mode will cause a corona discharge that may or may not be visible to the naked eye, so you will see it as a reduction in signal quality. (Once I produced a corona discharge in *positive* mode with a certain mobile phase, and it was brilliant and readily visible!)