From: Jonas Nilsson (jonni$##$ifm.liu.se)
Date: Wed Feb 16 2000 - 03:41:17 EST
Thankyou for your ideas. I will post them here below. However, some
clarifying of my problem seem to be in place.
1. Yes. I have analysed for oxygen, as an effort to isolate the problem.
2. There is no known impurity that would make up for the large error. I've
tried H2O/MeOH/CO2/CH3COOH. I have a program which checks for this.
3. I do have the acetate salt to some extent. This however will not make up
for the impurities. 16% of the content should be something else but C,H,N,O.
4. I have eluated 700 ml of solvent from the HPLC-system, evaporated it and
it gives no weight.
5. The sample is combusted at 1800 degrees centigrade. This is not an
ajustable parameter. Maybe the oxygen flux can be raised. I will check.
Here is my first letter:
_______________________________________
Jonas Nilsson wrote:
> Hello
>
> I'm having some trouble with my elemental analysis of some of my
> compounds. The ones which gives me trouble are all aromatic amidines
> Ar-C(=NH)-NH2. This is an example of how it works out:
>
> Theory:Given
> C:65.3%:54.0%
> H:6.4%:5.6%
> N:14.2%:12.0%
> O:14.2%:12.4%
>
> Total:100%:84.0%
>
> The compound is purified on HPLC (C18-column, MeOH/H2O/CH3COOH), there
> should be no other elements than C,H,N and O. The isolated compound is
> an acetate salt of the amidine. The NMR looks very pure. The sample is
> dried overnight at 80 deg. centigrade, 5 mTorr (0,005 mm Hg) before
> elemental analysis is run.
>
> The other functionalities in the compound are another aromatic system,
> and a diamide:
> Ar-NH-CO-C-C-CO-NH-CH2-Ar
>
> As you all can see all percentages are much to low.
> Now, what is the 16% which is not C,H,N,O? And how can the C analysis be
> 11% low?! I have 8 other samples all containing the amidine which gives
> me similar results.
>
> Please advice. I cannot publish this until i get it right, and I know
> the compounds are pure. (There are at least not 10-20% impurities).
>
_______________________________________
_______________________________________
_______________________________________
Here are the replys:
_______________________________________
From: "Dr. Ulrich Jordis" <ujordis$##$pop.tuwien.ac.at>
Did you really analyze for oxygen? Oxygen is not analyzed routinely but
calculated as the difference of all others to 100%
Furthermore you can try out my JASPER program to find solvents, impurities:
see http://perdix.tuwien.ac.at/ANALYSE2.HTM.
_______________________________________
From: "Yuehui Zhou" <yuehuizhou$##$hotmail.com>
Hi, the problem may lie in the acetic acid....amidine is very strong
base...it may form salt with acetic acid....try to recalculate your result
as salt....good luck!
_______________________________________
From: "Juan F. Miravet Celades" <miravet$##$qio.uji.es>
Regarding the problem with your elemental analysis of amidines posted in
orglist, I find that the found relations C/H, C/N and C/O are quite good.
It seems that you have your compound and something that lowers all the
percentages, probably inorganic. Could it be possible that some derivative
of silica is coming from the column (if I am right C18 is silica-gel
modified with C18-alkyl chains) (methanol-H20 mixture is able to solubilize
to some extent silica-gel). You could try to take the same amount of
eluent (that has passed through the column) that you have when you collect
your sample and add a quantity (similar to that isolated of your stuff) of
pure, reference compound and evaporate and process the sample as you use
to. Check if it gives you wrong numbers again.
_______________________________________
From: Hormuzd Mulla <mulla$##$caribe.chem.uky.edu>
Hi There
I think the culprit is water or methanol. Try including integral
numbers of water molecules and try calculating again. I'm sure you will
come up with a requisite number of water/methanol molecules that might
have been adsorbed on the surface of your compound. I had a similar
problem with some pyridinium salts I'd made but we circumvented that when
we considered water in the crystal. I am pretty sure you will iron out the
kinks by doing this.
_______________________________________
From: Kotohiro Nomura <nomurak$##$ms.aist-nara.ac.jp>
I 'm afraid that this might be the problem of combustion of your sample in
the analyzer if your sample is pure enough. You may ask to raise the
temperature of combustion tube or add oxygen more to complete the reaction.
If not, take the analysis with smaller amount.
I hope this would be fine for you.
Kotohiro Nomura
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_______________________________________
_______________________________________
Thats all!
/jN
_____________________ _____________________
| Jonas Nilsson | | |
|Linkoping University | | Telephone |
| IFM | | --------- |
| Dept. of Chemistry | | work: +46-13-285690 |
| 581 83 Linkoping | | fax: +46-13-281399 |
| Sweden | | home: +46-13-130294 |
|_____________________| |_____________________|
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