finding Void time(To)

[rc_12321]

Is there any ideal substance available for this purpose ? The answer ,perhaps,is NO.Hence with in the available resources ,we need to verify the substance that best suits our needs.

What ,in your view, is the best suitable substance for this purpose?

[Uwe Neue]

Uracil and thiourea are good general choices.

Hans: what is ideal? I can measure the retention difference between benzene and toluene under conditions where neither is adsorbed. The effect is due to size exclusion between the chains. So why am I going to worry at great length about the validity of an unretained peak marker? We used to call this a "Streit um Kaisers Bart", i.e. an argument over the emperor's beard, i.e. something worth doing over some beer if you got plenty of spare time...

[Einar Ponten]

What is wrong with nitrate (NO3 -)?

No retention.
UV absorbance.

On a HILIC column, completely different story, toluene or similar.

[Uwe Neue]

NO3- is one of the inferior choices. How do you know it is not excluded from the pores via ion-exclusion?

[HW Mueller]

How did I get into the "corner" of propagating an ideal marker, when I point out the contrary, namely that this whole thing isn´t simple?
Uwe, I am not at all worried about using an unretained (+ nonexcluded) marker, I am worried about not knowing whether a marker is retained or not under the conditions used. Take toluene on a HILIC, I don´t believe that it wouldn´t be retained under some conditions which someone might use
I am just looking at ZIC-HILIC. It clearly and strongly excludes several negative ions near ~pH 4 and above but not at a pH of 2. (Incidentally, its behavior in this is surprisingly close to the Atlantis HILIC, or any other silica based column, including RP, which I have checked). So one could now believe that NO3- at a pH of around pH=2 or below is fine for any silica based column. I would say: careful! Even here. I have injected some anions dissolved in water with a high salt content, moving the anion up away from where it appeared in "pure" water (whre it came at the tm). (Was it Tracy who mentioned salting out in this regard?).

Therefore my reply to rc_12321: Use something you think should be at tm but report it, don´t say the tm is ____, rather indicate that compound X was at t which is probably the tm. Use the approximation calcs for tm, if available, as an indicator of how close your t must be to tm. Maybe live with the fact that your reported k possibly includes a systematic error. If you are careful, as I advised, you have an idea of how far off you may be. Usually the error is too small to bother (Kaiser´s Bart comes in here), it´s just licentious, in my opinion, to use any old stuff someone recommended, which might have even worked for him, and apply it at totally different conditions.

[rc_12321]

I think i have hit the wrong nerve at the wrong time.Thanks,But the discussion is on the track only.

I am not insisting one thing for all.I will put my question in rather different way.
How to know that a particular compound will give accurate and reliable void volume values for a particular column at our disposition ? This is what i supposed to ask .

[tom jupille]

How to know that a particular compound will give accurate and reliable void volume values for a particular column at our disposition ?

If your purpose is to track performance of a given system over time, then an accurate measurement of void volume is not really required; what you need is reliability. For that purpose, I think many of the compounds suggested here would work (uracil seems to be generally accepted as a reliable choice for reversed-phase).

If your purpose is to study column chemistry, retention mechanisms, distribution coefficients, etc., then you do need accuracy, and many of the complications discussed in this thread become relevant.