A word about pumping…

We get asked a lot about pumping requirements for our electrospray sources, obviously.

The UHV4i series has three differential pumping stages inside the source:

Stage 1: This stage bares the brunt of the gas load. The entrance capillary brings in the mixture of air and your electrospray plume into the system, so the first stage basically has a massive leak into it, so it can only reach a pressure of around 1 mbar. In our lab we use a dry scroll pump to pump this stage. It happens to be an Edwards nXDS6i that can pump about 6 cubic metres an hour, but until recently it was an Agilent SH-110 which has roughly the same pumping capacity. No particular reason for one or the other, it’s just how it happened. On Stage 1 pumping speed is more important than ultimate pressure since you have loads of gas to pump but you’re only ever going to get to around 1 mbar.

Stage 2: This stage accepts the plume from Stage 1 along with the residual gas load, but due to the very small skimmer aperture, this stage can reach about 0.1 mbar. Again, on our system we’re pumping Stage 2 with another Edwards nXDS6i (and yes, again until recently it was an Agilent SH-110). By the way, 0.1 mbar is a good enough pressure to back a small turbo pump…

Stage 3: This stage accepts the plume from Stage 2 along with any residual gas load, but now the physics of differential pumping means that this stage can reach into the 10-4 mbar range. So Stage 3 needs a turbo molecular pump. On our system – again no affiliation or particular preference – ours is pumped by a Pfeiffer HiPace 80 as you can see in some of the photos. We get this stage down to about 1 x 10-4 mbar. Because these turbos are quite happy being backed by a pressure of 0.1 mbar, we couple the backing line to the Stage 2 scroll pump using our Coupling Tee & Bellows, which you can purchase as an accessory.

Stage 4: This stage isn’t usually part of the UHV4i source, but does form part of the whole assembly because you need to deposit the molecules and nanoparticles somewhere. This stage is the preparation chamber of your UHV system, or a dedicated deposition chamber. This is where we can’t predict what the pressure is going to be, since every system is different. In our system this is the analysis chamber of an XPS instrument. Our chamber is pumped by a Pfeiffer HiPace 800, which gets Stage 4 down to 5 x 10-8 mbar with the gate valve open to the UHV4i system. Again, we’re not saying that you need this level of pump to get you where you want to be – this is just what our XPS is using to pump its massive analysis chamber and x-ray source before we attached the electrospray to it – your system is going to be different.

Stage 4: This stage could actually be part of your electrospray source if you incorporated one our UHVX additional differential pumping stages. If this is the case then it needs the same pumping as Stage 3. In our lab this would be a second Pfeiffer HiPace 80, but in your lab just grab that small turbo off the shelf over there and that will probably be fine.