Anecdotally I have been told that high pressure mixing is more effective than low pressure mixing in HPLC. However, I would be interested to hear from anyone who has definite views in weather they have observed a "High Pressure" binary pump being better at mixing than a "Low Pressure" Quat. Pump. Or is there more to the whole argument than just mixing?

regards
mark

If you are talking about precision, you can't beat a 2 pump high pressure mixing setup. Since the solvent is mixed after the pump heads, you also get the benefit of lower delay volume in the system. On the other hand, if you are looking at a little more versatility, and want to do some development work, a 4 solvent low pressure mixing system is probably the way to go. One of the problems with LP mixing, is the formation of bubbles(outgassing). Degassing is a must when working with LP mixing. Although it is a great idea for all LC work too. Alot of the mass spec people here swear by high pressure mixing because of its' low dead volume. With an in line static mixer, the mixing is not a problem. They come in very low volumes, a bunch of 75ul ones around here.

Once you have established the analytical requirements. (I have never needed to use more than binary gradient system ... only once have I seen an application requiring a three eluent mix. Why would you need four I do not know!!!)
Only if you are lazy in switching bottles between applications could I see anybody wanting a 4 solvent delivery system. (sorry clean up and regen excluding).
I prefer to invest my money on 2 X high quality high presssure pumps. So go for the 2 pump high pressure system. For the arguments above.
Look at initial purchasing and servicing costs.
I have had bad experience with servicing/reliablity when using these very fast switching valves that cycle very quickly to produce a 4 solvent LP mix. These are susceptable to failure / breakdown and poor gradient reproducility.

You will see the quality difference by comparing the reproducility of a gradient baselines. Perform a subtraction using two High Pressure baselines. You should get a striaght line.
Repeat using the Low Pressure systems.
Now compare the two subtractioned baselines.
The High pressure baselines will tend to be more flat than the low pressure ones.
I hope the above makes sense!

I'll also reccomend high pressure mixing, no degassing required and high precision available with the variable volume impeller mixing chamber type like the Gilson 321 pumps we use.