max pressure

[sunnysideup]

when you are developing a method, should the max back pressure be about half of what the instrument is capable of?

[Kostas Petritis]

It will depend on the viscosity of your solvents during gradient elution. I would avoid going higher than 70-80% of the intruments high limit...

[sunnysideup]

i'm using agilent 1100 instrument with a 3um column. i'm injecting about 10uL of solution containing about 50:50 acn:water. mobile phase is acn / water as well. the initial pressure is about 180-200 and then drops as organic mobile phase is introduced. max instrument pressure is 400. i was just wondering if this is pushing limits. column temp is 40C..

[Kostas Petritis]

Your HPLC is perfectly happy at those pressures... Pushing the limits would be to operate at >350 bars...

[sunnysideup]

Great! thanks for the advice!

[JGK]

With older stationary phases, I generally prefer to keep the pressure <2000 psi where possible.

Newer generation stationary phases <3000 psi.

If in doubt, consult the maufacturers literature.

[Mattias]

After the evolution of high-pressure systems as UPLC - it has become more clear that the system pressure is an efficiency parameter.

With the flat Van-Deemter curves of new column materials - it is a waste of time and resources to work at low pressures. To work at 75-80% of max pressure you will save time (higher flow) or increase separation (longer column) - or a combination of both. That is true not only for the UPLC's.

This is just a general comment on pressure - and common knowledge to you. I just get frustrated by seeing new LC systems run at 60 bars using newly developed methods...

[Uwe Neue]

I can't speak for all manufacturers, but most Waters HPLC columns, even the older ones, are packed at 9000 psi (600 bar) and are good for the entire pressure range of an HPLC instrument. No need to go to low pressures with older columns.

There are exceptions that depend on the strength of the particle, but the column C/U will tell you, if this is the case.

[tom jupille]

With the flat Van-Deemter curves of new column materials - it is a waste of time and resources to work at low pressures. To work at 75-80% of max pressure you will save time (higher flow) or increase separation (longer column) - or a combination of both. That is true not only for the UPLC's.

I think you're right, and I agree with you in principle, but in practice, back pressure tends to increase as columns age. In many cases, it's not the fault of the column, but rather garbage from the samples building up, but regardless, I prefer to have some "wiggle room" and start off at something like 40% of the system's rated pressure. There's nothing sacred about that value, it just provides a comfortable margin for me.

[HW Mueller]

Mattias, it seems to be difficult to determine whether there is a difference in the use of resources between high and low pressure systems.
Also, it might be interesting to see what an affinity chromatography at 10000 bar will do.

[Hfranz]

I think 100,000 bar will be even more interesting!