High pressure system with LCMS

[Javier.Ricardo]

Hi all.

I´d like to know the reason why is it important to have a high pressure HPLC system coupled with LCMS.

As a technique, I can run it with a Low pressure gradient?

Thanks a lot for your attention.

[dblux_]

Hi all.

I´d like to know the reason why is it important to have a high pressure HPLC system coupled with LCMS.

As a technique, I can run it with a Low pressure gradient?

Thanks a lot for your attention.

And what if they are isocratic runs ? Will you call them "high pressure HPLC" ?

[Javier.Ricardo]

ohhh Yeah sure. I mean High pressure techniques (isocratic or gradient) those more than 40MPa of pressure.

But ok anyway. Is it possible to have an LCMS with a working at a pressure less than 40MPa, with regular 5um columns?

[dblux_]

As LCMS systems are in use before UHPLC was introduced then it is possible.

[tom jupille]

Is it possible to have an LCMS with a working at a pressure less than 40MPa, with regular 5um columns

Yes it is possible, but less than ideal. It all depends on how high a plate number you need to get your desired chromatographic resolution. If your samples are relatively clean, with few interferences, you can get by with very short (3 cm or so) narrow columns packed with 5 micron particles and probably only run a few hundred psi (maybe 5 MPa ?) at reasonable flow rates. The price you will pay is broad peaks, poor resolution, and diminished sensitivity.

The reason for small particles is to get high plate numbers (narrow peaks) which improve resolution and sensitivity. The price you pay is higher pressure and more stringent demands on instrument dead volume.

[mckrause]

Most MS systems that are compatible with lower pressure HPLC systems have restricted flow rates; typically somewhere around 0.3-0.4 mL/minute. This means that for optimum flow you need to use a narrow bore (2.1 mm) column. 5 uM particles just don't behave the same in a 2.1 mm column with low flow as they do in a 4.6 mm column with higher flows (experience talking here). You're way, way better off going to a 3 uM particle, or even a solid core 2.7 uM particle. This demands higher pressures, but it allows you to take full advantage of the MS.

[lmh]

You can always split the flow post-column if you've got too much for your MS. Of course you can use whatever pressure system you like. The reason for choosing a high-pressure system is mostly economic:

Viewed very, very crudely, a good high-pressure UHPLC might cost you twice as much as a basic low-pressure HPLC, and a very basic single quad might cost you a similar price to a basic HPLC. Let's call this "X".

If you decide to get a low-pressure HPLC and use traditional 250*4.6mm 5u columns, you will probably be using 30-40min runs. You have paid 2X for a maximum of 48 runs per day.

If you get the high-pressure UHPLC, and use 1.7u columns, or a solid-core equivalent, then you will probably find that you can easily get similar results in only 5min. You are now paying 3X for a maximum of 250 runs per day, so the cost per run is now between 3 and 4 times lower than it would be with the lower-pressure system.

If, of course, you're using the HPLC with a more expensive mass spec, which is highly likely, then the cost benefit grows further. The benefit does, of course, assume you have enough work to fill available time on the instrument.

[James_Ball]

The ABI3200 we have can take a maximum flow of 1ml/min from the column so it can handle the low pressure flow rates. However at those flow rates it requires very high flows of desolvation gas to make sure you don't collect liquids in the source. You do get much better performance with lower flow rates from narrow bore small particle sized columns which require more pressure.

Whether or not you can make good use of the lower pressure systems will be how the source is designed.