Chromatography Forum

Just wanted to throw in my $0.02 worth. To maximize sensitivity for EITHER column you will want to inject the max volume without causing an unacceptable loss of column performance. Roughly, 25 uL and 1.5 uL for the 4.6 and 1 mm respectively.

When this is done you won't see any difference in sensitivity, theoretically.

Practically speaking you will almost certainly see better sensitivty with the 4.6 mm column since extracolumn broadening due to system volume and flow cell will be significant with most HPLC systems. Also, packing 1 mm columns is much more difficult than 4.6 mm columns of that lenght and the plate count is likely to be considerably worse.

The above post nailed it.

And by the way, this entire discussion assumed a UV or concentration sensitive detector. If using a mass sensitive detector, the sensitivity will increase with injection volume and, therefore, will be greater for the larger diameter column: assuming the injection volume is being scaled properly.

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If I inject a larger volume, I will get a higher detector response for any detector, concentration proportional or not...

Under most practical circumstances, MS detectors work in a nearly concentration proportional regime. If you want to get into a true mass proportional regime, you need to work at much LOWER flow rates.

And in addition to Uwe’s remarks, It’s important to focus on what’s important here. The injection volume is mentioned several times as if it’s the only way of loading more mass. Well, if I want to load more stuff on the column I’d prefer to (prepare) load more concentrated solution – and in that way it’s even possible to reduce the volume while loading the same amount – or even more.

Best Regards

Uwe: In my post, notice the phrase "if the injection volume is scaled properly".

If injection volume is scaled in proportion to the column diameter squared, a mass sensitive detector will give better sensitivity when you go to a larger ID column; whereas a concentration sensitive detector will not (simply becuase mass is increasing but concentration is staying constant).

I will stay out of the discussion of what category Mass Spec systems fall into.

Adam: if you do your experiment with an ESI-MS instrument in the common LC flow rate range, the response and the signal-to-noise ratio will remain surprisingly constant.