Chromatography Forum

Many famous LC manufacturer's recently HPLC system give up binary pump system(waters 2695/agielnt 1200/shimadzu 2010).So,as a junior LC operator,I have some questions:1)Is low-pressure mix system the tendency of HPLC? 2)what is the advantage and dis- of high -pressure mix system?
Thanks,everyone.

High pressure mixing systems can reduce outgassing issues if you're forming gradients with poorly degassed eluents. The drawbacks are (generally): higher dwell volume, more consumables to replace/go wrong, more baseline noise at the extremes (more than 1:10 or 10:1 ratios).

As long as you have adequate degassing and pre-pump mixing of eluents, low pressure mixing is the way to go for most applications.

The Agilent 1200 series systems can have either low-pressure mixing or hig-pressure mixing pumps. It depends upon the configuration purchased.

Hi DR,

It’s the other way around. The larger dwell volume is a characteristic of the low pressure mixing configuration.
Regarding the number of consumables – I don’t know if it’s a big deal; you have the GPV in the low pressure configuration. So, it might offset the 2 pump (high pressure) configuration’s somewhat higher maintenance price.
The reduction of the risk of air bubbles release upon mixing aqueous and organic eluents is a very good point. It is also one of my favorites.

Best Regards

I agreed with danko about dwell volume and consumable, and almostly,check valves is the source of higher maintenance price.Also,Iaccept danko's opinion about strongpoint of high-pressure mix system.However,as DR said, "low pressure mixing is the way to go for most applications",why? There must be some reason for this tendency from standpoint of its application.

A low pressure system can typically mix 4 different solvents, which make that kind of system more versatile. I guess that they are somewhat cheaper than the binary high pressure binary systems, since they only contain one pump?

My self, I prefer the high pressure systems. A low-pressure system typically has a dwell volume of 1.0-1.5 ml. That is way too much if you run gradients on a 2.1 mm column at 0.2-0.3 ml/min.

I never thought that Agilent had true quat. pumps. I always thought that they used a double binary pump to get the four channels? I thought this was true at least for their 1100 systems. I am pretty sure it still held for the 1200s too.

Has something changed or am I simply completely wrong?

Yes, Agilent have true quat pumps on the 1100 systems. I'm working on such system

Clarification: In the case of 1100, 1200 systems, having a second pump doesn't really impact the Vd (much).

My initial experiences with high pressure binary systems was with older Shimadzus. With them, you tend to get into some pretty high dwell volumes and gradient delays. Now I'm using Waters Alliance and Agilent 1100s. The Alliance has half the Vd of the old Shimadzus.

I'm sorry if my prejudice based on experience misguided anyone.

Actually, that's a good point. While there are pros and cons to both approaches, I can't make a blanket judgement that either is unquestionably better than the other. I've seen good and bad examples of both.As with so many other things in life, it's the details that matter.

On the other hand,my opinion,in-line degasser effcet application of high-pressure system,because one of its advantage is less bubble produced in high-pressure system.

Why are we worried about bubble formation and degassing in a high-pressure system. There are no bubbles formed before the pump, since nothing is mixed. There are no bubbles formed after mixin on the high-pressure side, since it is high pressure. Bubbles after the column in the detector cell can be avoided by putting a small restrictor capillary behind the detector, and many detectors have such a thing anyway. Or, in MS, you want the entire mobile phase to become a giant bubble...

So bubbles are not a problem in high-pressure mixing systems...

I prefer high pressure systems. The reason is simple.
Compare it to a hotdog with mustard and ketchup on it. You would rather draw a continous line of spices on the hotdog than chopping it and putting the sauce in between the slices.
Obviously this applies especially when you use low flow rates and gradients. The length of the slices is varied during the gradient run. This can not be done as exactly as varying the pump flow on a pump that has typically a resolution of flow rates in the 50 nano liter region.
The weirdest thing that I have read is low pressure mixing for MS. Since your ionisation is influenced by the composition of eluents in the ion source, you would only want the most reproducible gradient system there.

Why are we worried about bubble formation and degassing in a high-pressure system. There are no bubbles formed before the pump, since nothing is mixed. There are no bubbles formed after mixin on the high-pressure side, since it is high pressure. Bubbles after the column in the detector cell can be avoided by putting a small restrictor capillary behind the detector, and many detectors have such a thing anyway. Or, in MS, you want the entire mobile phase to become a giant bubble...

So bubbles are not a problem in high-pressure mixing systems...

I am sorry about my wrong expression of in-line degasser.I want to say that in-line degasser(installed in low-pressue mixed system) have the same ability of reduce bubble formation as high-pressure system.So,in-line degasser effect application of high-pressure system.

One more comment about the relative advantages:

For normal isocratic methods, there is no practical difference. But for gradient separations, there is a significant difference in performance because of the smaller dwell volumes in the binary/high pressure systems.

This becomes important when you are trying to do "fast" LC or "UPLC." If you are running a one minute gradient, then a 10-15 second difference in dwell time can be significant. Also, if you are using a small bore column at a slow flow (0.2 mL/min), a small difference in dwell volume creates a large difference in time. The Agilent 1200 system, for example, has an option that reduces the internal dwell volume for this reason.

But for "normal" LC methods, I think the differences are minor. The best choice for you will depend on your individual needs.