i am going to purchase gradient hplc system for routine pharmaceutical quality control analysis. Igot confused whether I purchase a low pressure gradient system or high one.I am looking for any guidance. My methods are not specifically asking any type of instument.

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.

It is not correct to say that one is better (or worse) than another. Some brands of high pressue mixing systems out perform thier low pressure mixing counterparts with the oposite being true as well. It is true that generally, high pressure mixing system have lower volumes. This comes at a cost of poorer RT reproducibility and sometimes increased baseline noise (with some detector and mobile phase combinations). Adding mixing to these systems almost always improves the RT performance (and noise) and brings the system volumes to about the same level as LP systems (in some cases higher). This explains the MS folks like of these HP systems as retention time reproducibility is often of little concen when using LC/MS. My personal feeling is you simply get a higher degree of flexability from a 4 solvent (low pressure mixing system). Of course as with all decisions, you have to look at the bigger picture, in this case not just the pump, but the autosampler, detector, software, service, support, cost, ease of use......

Dear Naheed;

I will add some comments to those already expressed. I think the main difference between the systems is the ability of the high pressure one to generate more accurate and reproducible gradients at low flowrates and within narrow ranges of mobile phase variations.

If none of your methods employs narrow bore columns at flowrates below 0.5 ml/min, and difficult gradients (i.e. 0 to 10% B in 30 min), then a low presssure system will do just fine.

The mentioned advantages of the quaternary low pressure as being more versatile, and having lower delay volumes, are true, but they are significantly more expensive, and required more maintenance.

The low pressure system will let you run ternary gradients or quaternary if you wish, and lets you quickly select up to 4 different solvents. However, you can buy a binary high pressure system with a switch valve that will let you use or select up to four solvents (two at a time).

Perhaps all these comments will help you. Good Luck.

Benjamin

Dear Benjamin:

I have to diagree with your statements:

"the ability of the high pressure one to generate more accurate and reproducible gradients at low flowrates and within narrow ranges of mobile phase variations.

If none of your methods employs narrow bore columns at flowrates below 0.5 ml/min, and difficult gradients (i.e. 0 to 10% B in 30 min), then a low presssure system will do just fine. "

I would argue that the exact opposite is closer to the real situation. Imagine 2 pumps doing a high pressure gradient at low flow: At the extreme end of the gradient (lets say 99%A, 1%B at 200 uL/min) pump B is pumping at a flow rate of 2 uL/min. Most pump designs today have a very difficult time accuratly pumping at these low rates. On the other hand, the low pressure pump is pumping at 200 uL/min and depending on a valve opening and closing at the correct rate. I belive (and have seen data to support this) that in these situations you get better results from a low pressure system.

Of course, some HP mixing systems will always outperform some LP mixing system under all conditions with the reverse being true as well.

A.A.

Dear Anonymus;

I am sure your experience is valid. I can only add my observations. Gradient performance will depend on many instrumental factors, valve performance, stroke volume, solvent viscosity, mixing efficiency, etc. In my opinio, all pumping systems will perform somewhat poorly in the conditions you describe (99% A/ 1% B). Some of the new pumps however, would not have problems even in such conditions.

Not too long ago I had a case such as the one this discussion involves. The method required a 10-20% change over 35 min at a flowrate of 0.3 ml/min. My LP mixing PE Series 200 instrument (known to work well generating gradients at 1 ml/min), could not match the performance of an Agilent 1100 (HP mixing). I consulted with manufacturers and we could not come up with a better explanation than limited performance of the LP system in the conditions of the analysis.

Benjamin

One of the things that you need to consider in the QC lab is what the systems are on which your methods were developed, and what the specifications are. Low pressure and high pressure gradients mix differently and have different delay times, which can create occasional difficulties with retention and resolution. If you are free, look for reliability.

I strongly agree with benjamin and anon: for more or less normal flow rates (~1 ml/min) it is not easy do differentiate between HP and LP mixing systems. HP gradient mixing systems are said to have a lower dwell volume, however our HP gradient system in the lab are equipped with mixing chambers and have dwell volumes of 3-4 mL. Thats about the same as most older LP gradient systems I know, and surely more than actual LP gradient pumps. When I looked for gradient accuracy (with 0.1% acetone in water as solvent) I found a LP pump more accurate at 2.5 mL/min.
At the same occasion I compared 2 HP systems (same make S, new and 9 years old, UV detector), 2 LP systems (same make P, 1 and 6 years old, DAD) and a new LP sytem (make D, DAD) in terms of system suitability results: RSD of neak area and height, peak asymmetry, RSD of retention time, number of plates (!), nesolution and RSD of (signal/noise) on a 4 component test mix. All systems were well maintained. The new LP system won resolution, number of plates and RSD of peak area, the 1 year old LP system was best on RSD of (signal/noise) , the old HP system best on RSD of peak height and the new HP system RSD of retention time and asymmetry. All system met our system suitability criteria. So each system had its strong and weak sides.

For a QC lab I would go for the same typ the methods were developed on ( Uwe is right as usually). If that doesnt apply I would look for maintainbility and robustness and possibly go for a high pressure system. If flexibilty is a topic then a low pressure system. Look for the vendors specification (and how it is measured).