From: Soren R. Jensen (oksrj$##$pop.dtu.dk)
Date: Thu Feb 17 2000 - 03:50:01 EST
Jonas,
Obviously, the impurity in your sample can not contain H, N, O or C - or
perhaps a little C? What about co-crystallizing CCl4? I once had college
with this problem. And it is difficult to detect, since you cannot see it in
H NMR.
Before analysis, did you treat your sample(s) with carbontetrachloride?
Another possibility I can think of is a halogen atom in your compound(s)
which has substituted an OH-group during the reaction. Did you get a mass
spectrum? - Soren
At 09:41 16-02-00 +0100, you wrote:
>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
>_______________________________________
>_______________________________________
>_______________________________________
>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 |
>|_____________________| |_____________________|
----------------------------------------------------------------------------
----------
Soren Rosendal Jensen
Dept. of Organic Chemistry Email oksrj$##$pop.dtu.dk
Techn. University of Denmark Fax +45 4593 3968
DK-2800 Lyngby, Denmark Tel +45 4525 2103
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