There is probably a simple answer to this and I think i know what it is, but I wanted to get some additional input. We have an API3000 LC/MS with ESI, I was wondering how to calculate how long the split tubing needs to be to achieve a desired split flow ratio. Assuming that the ID of the tubing going to the ESI and the waste are the same, will the split ratio just be a function of the ratio of the tubing lengths (eg a 3:1 split ratio is just a 3:1 ratio of the tubing lengths after the splitter)?

If you need an accurate split I would buy one of the active splitting devices. Otherwise just use the length ratio as a rough guide and install an extra 50-100% and measure the split with a vol. flask and trim as necessary.

There is no clean calcualtion because it will be based on head loss which will also be dependent on any unions, frits, tubing condition, how cleanly the tubing is cut, etc. Also with a static splitter your split ratio will not be constant over variations in mobile phase viscosity. However, ESI usually has a range of acceptable input and a static splitter is good enough. Good luck

This is the first time I've replied to a post but here goes. This works very approximately because you drop a whole bunch of the variables from the equation.

You can get a ratio of the flow going through a split by doing the following:

(difficult to write math like this)

F1/F2 = (L2.r1^4)/(L1.r2^4)

Where F1 = flow rate going through tubing 1
F2 = flow rate going through tubing 2
L2 = length of tubing 2
L1 = length of tubing 1
r1 = radius of tubing 1
r2 = radius of tubing 2

As its a ratio, just keep the units top and bottom the same (whatever they are).

So, if you have ~10cm of 0.005" tubing on the source side (including the length of the electrospray capillary) and ~10cm 0.010" tubing on the waste side you get a ratio of 1:16.

Therefore, @300ulmin-1 a split of ~18ulmin-1 into the source and ~282ulmin-1 into the waste (or fractionated).

Give it a try.