Chromatography Forum

Hi all,

I've just started work with a new Dionex Ultimate 3000 LC system, which comes with Variable Wavelength Detector (VWD3100) and an analytical flow cell (PEEK, 11uL volumne, 10mm pathlength). I'm planning to use the system for separations for metabolite idfentification with an MS attched to the rear end of the VWD.

I can run at UHPLC pressures, and am using a 2.1 x 100mm column with 1.7um particle size at 0.5mL per minute.

Looking at the various options, then there's a choice of moving to a smaller flow cell, either a semi-micro-cell (2.5uL cell volume. 7mm path length, PEEK or stainless steel) or an Ultra-low dispersion UV monitor flow cell (45nL volume, 10mm path length). The blurb from Dionex/Thermo suggests that "using [the ultra-low dispersion] cell provides the best resolution of ultra-narrow peaks for both the absorption detection and a subsequent mass spectrometer".

If I believe the blurb, I should go straight for the Ultra-low dispersion cell, however, I'm aware that flow cells for other UHPLC/UPLC systems do not tend to be as small. What effect will I have moving to either cell?

Sorry if this is LC-101, I'm coming from the MS/Structural elucidation side rather than chromatography!

Thanks for your answers!

Someone correct me if I'm wrong here, but with greater path length comes greater sensitivity (but a smaller upper range of usability) and a lower volume cell will distort peaks less than a higher volume cell. So, my suggestion is to go straight to the Ultra-low dispersion cell. My concern would be having it in-line with the MS - how much backpressure will be created by the tubing from the cell to the source? What pressures does Thermo claim the cell can withstand?

Thanks, and yes, that's what I've been reading - path length is needed for sensitivity whilst lower volume is needed to reduce the peak broadening. Just wanted to check.

The Ultra-low dispersion can handle 30MPa pressure, whilst the semi-micro is either 12MPa (steel) or 5MPa (PEEK). The current PEEK flow cell is holding up to the back pressure from the MS, so I've no doubt any of the above can.

2.5uL cell volume. 7mm path length
As long as you are not using capillary columns that is ok.

Use simple math: with 0.5 mL/min and 2.5 µL flow cell, you will be able to resolve 80 data points/min (contents of the flow cell will be renewed 80 times per minute). If you need 15 data points per peak, your minimum peak witdh (at baseline) is 11 – 12 seconds. If that is enough for you, go for the 2.5 µL cell. If not, go for the ultra low dispersion. If I remember correctly, the max pressure for this cell is 200 bar, that should be fine for an inline detector before MS.