Top issues in HPLC solvent mixing and flow splitting

[Mindy]

Hello All, I am doing research on some of the most common problems HPLC users face when it comes to solvent mixing and flow splitting.

We know that poor solvent mixing or high delay volumes can cause significant noise, tailing, and slow response. What are you doing to combat that with the mixers that come standard on your pumps?

I'm also interested in common issues when it comes to flow/stream splitting. I've heard that clogging can be a problem in systems that use capillaries. Are there other problems the market isn't yet solving or are there minimal problems?

Thanks for your help!

[Andy Alpert]

We recently became interested in this subject when we perceived that we were getting inadequate mixing in HIC (hydrophobic interaction chromatography). That involves mixing solvents of very different salt concentrations and viscosities. After some systematic experimentation, we ended up using an empty HPLC column packed with stainless steel ball bearings as a static mixer. It's a cheap, effective solution to the problem.

[Mindy]

Interesting - What kind of flow rates were you using? Were you worried at all about increasing the dead volume?

What was the original pump/mixer model you were using prior to your work-around? Thanks!

[Andy Alpert]

Flow rates: Either 1 ml/min or 0.2 ml/min; we made mixers suitable for each, with volume to match. As for any concern about increasing the dead volume: not really. The mixer was located between the pumps and the injection valve, so it didn't affect the peak shapes. It merely introduced a modest amount of delay in the effective start of the gradient on the column.

The system we were using merely had a tee for mixing the two mobile phases. I might note that we compared our mixer with commercial mixers from ASI. Both had comparable dead volume (which is something to consider regarding your apprehension about introducing additional system volume; everyone else finds this solution to be acceptable). The ASI mixers were slightly more effective than ours. That said, putting two of our cheap mixers in series made the baseline flat as a pancake.

[lmh]

flow-splitting: Yes. The regular problem we encounter is that you cannot buy a flow splitter with a guaranteed split ratio. They are all merely a couple of resistances which may have been set up in carefully controlled circumstances to give a particular split ratio, but the moment you attach a further resistance to one of the outlets, the ratio changes, and annoyingly, you almost always will.

For example, we can set up a flow-splitter feeding a mass spec and a fraction collector. We measure the flow on the mass spec side by taking the tubing off the MS and checking the volume that comes out over a few minutes. Knowing the volume of the system (MS spray needle and all associated tubing), we calculate the delay time appropriately, but in practice the delay time is potentially much longer because the extra resistance of the mass spec's spray needle reduces the flow hugely.

With current technology, the effect can only be minimised by using flow-splitters with very large resistances so that the added resistance of a detector is a small fractional change, but this leads to running systems at huge back-pressures which isn't ideal.

Obviously it's possible to make a flow-splitter that actively guarantees a particular ratio - very simple example: fit a flow-measuring device (thermal flow-through type, as used on nano-flow controllers) to the two outlets, and make it turn the control knob of a variable splitter - but this is a horrendously expensive and high-tech way to do it. If anyone can devise a simpler, more cost-effective way to create a guaranteed split ratio, I'd be interested.

[Peter Apps]

The viscosity of a liquid goes down with increasing temperature. Flow increases with decreasing viscosity. If the liquid flow has a fixed wattage of heat flowing into it the rise in its temperature will be inverse with flow. So put a fixed wattage heater on each leg of the splitter - whichever side flows slower will get hotter, and less viscous and so will flow faster and get cooler until it all comes into a stable equilibrium.

Seemples !

Peter