Chromatography Forum

Danko, you said:

“So, now you need to explain why the fatter column needs to be loaded with 2X the amount needed for the thinner in order to achieve the same peak height? The peak height should be proportional to the concentration – unless there are differences in the peak width. But the peak widths are the same!â€

Dancho, the original scaling example involved a change in the flow rate to get the peak on the fat column to the same retention time as the peak on the thin column. I answered to this very specifically.

Your statement that "people said that flow rate ... did not matter" is incorrect.

As long as you have a concentration-proportional detector, what counts is the concentration at the column exit, which has been discussed in detail.

Dancho,

here is the hopefully final exercise: we inject the sample directly into the detector at the same flow rate as it was used for the thinner column. I get a peak area and a peak height. The peak area is exactly the same as it was at the same flow rate on the thinner column. Since the peak was not diluted by the column, the peak is higher and narrower, but the area is the same, with or without column (unless you get out of the linear range of the detector). Now I am doubling the flow rate, keeping the injection volumn constant. The peak area will be half, and the peak height will be about the same as in the former case. Now I am doubling the amount injected. The peak area is double, and if I used the same injection volume (doubling the concentration), the peak height will be double.

Can you tell the difference between this example and the example with the two columns? Or rather what is the same? Clue: mass is concentration times volume.

On the importance of this discussion: The only importance lies in ending the confusion created by it. Some 20 years ago I bought a 2mm diameter column to get into the intensity advantage, the theoretical background for this had long since been cleared by that time. (Sadly I couldn´t use it as my apparatus could´t handle the high pressure, + + +).
Dancho seems to be carrying out a phantom argument: He implies that someone (the phantom) said there is no concentration difference and then argues that there is. Also, he is hooked mentally on the idea that changing flow rate within the same system is identical to changing flow rate in two different systems (I detailed this above).

Hi Hans et. al

He implies that someone (the phantom) said there is no concentration difference and then argues that there is.

I summarized it clearly in my last post (Posted: Wed May 21, 2008 2:43 pm).
Which - very shortly - states: Given the same amount is loaded on 2 different column diameters, the amount will be diluted to 2 different concentrations corresponding to the column diameters.
Larger diameter columns will cause more dilution, or in other words lower analyte concentration.
This translates directly in larger bands, which in turn means broader peaks – i.e. lower column performance in terms of plate count (regardless of what the detector sees). One of the most important consequences of this is the higher sensitivity obtainable using smaller diameter columns.
That is also what I posted right from the start. So, if someone here has misunderstood something, it could impossibly be me – the one that had the right answer.
If you believe you can drag me in a name-game, I can assure you; it won’t be easy!

Who said what, when and why, doesn’t really matter to me. I’m here (and I believe the most of us are) to share knowledge, experience and inspiration!

Best Regards

You are getting closer, but there is still no difference in the plate count between the fat and the thin column.

Hi Uwe,

but there is still no difference in the plate count between the fat and the thin column............

……as “seenâ€

Danko, it is highly appreciated here that you will refrain from name calling.
Here probably my last attempt to show where the error lies. You are apparently using the reasoning that goes into the system where the flow rate is doubled, the retention time is halfed, and peak width is halfed, area as well, for a system in which the flow rate is doubled, the retention time is not changed so the peak width is not changed either. These are two entirely different systems. I already pointed out above, any flow rate after the column can not influence the peak width. Another possibility I didn´t mention above: You can pump in a liquid after the column so that your flow rate is doubled (4x that of the "thin" column), your peak width will remain unchanged.

Hi Hans,

I already pointed out above, any flow rate after the column can not influence the peak width. Another possibility I didn´t mention above: You can pump in a liquid after the column so that your flow rate is doubled (4x that of the "thin" column), your peak width will remain unchanged.

That is exactly where you are making a mistake!
And here is my attempt to facilitate your understanding of the issue: If you stop the flow the moment the analyte/band enters the flow cell (thus achieving the lowest possible flow rate), the resultant peak will last forever, or until you start the flow again. This would be the opposite of what we are discussing right now.
Got it?

Best Regards

I suspect the basic problem here is one of semantics. Rather than argue in public, I think it would be better to continue the discussion privately via e-mail.

You can contact me for the e-mail addresses (if you would prefer that I *not* give out your e-mail address, let me know).

In the meantime, I'm locking the thread.