Complicated Mixture

[syx]

Dear members,
I find very hard condition in my lab. We want to develop determination method for tablets that contains 3 main substances, let us name them: A, B, and C. Those substances are polar and water soluble, thus need high concentration of aqueous portion in the mobile phase.

C is very unstable in aqueous solution, it will be hydrolized. Then we had tried to add acids and antioxidants, but the degradation was still occured. We decided to use pure methanol as the solvent for compound C (smallest injection is needed to reduce peak distortion).

Next problem, when we mix A, B, and C in one solution... C is degraded by the present of A and B!

I do not know what should I do...

Does anyone have solution for this solution?

Regards,
Siswanto

[vicentexxx]

do you have the use of a refrigerated sampler (4°C)?

what type of compounds are you dealing with?
you may observe the chemistry of your compounds and try to find a solution...

[HW Mueller]

Whom are you feeding those pills?

[Mark Tracy]

You just have to work very, very quickly! You may not have the luxury to extract a batch of samples, queue up the autosampler and go home. Instead, do the extractions one-by-one and analyze immediately. It's a real chore.

I have seen something rather similar with water-soluble vitamins. Vitamin C oxidises rapidly, and also attacks Vitamin B12.

[Einar Ponten]

Metal catalysed oxidation is a quite common source to stability problems.

Maybe degassing and addition of EDTA may stabilise your compounds?

[syx]

substance C:


the peak will be decreaced in time and two more additional peaks increased (degradant peaks).

substance A:


substance B:

[ym3142]

syx,
I can not see your pasted picture. do you mind informing us the molecule information about C?

[ym3142]

Can you derivatize and stablize C before any analysis?

[syx]

The pictures are OK on mine.
Excell, C is metamizole sodium (dipyrone). The references are very rare and difficult to find. Metamizole was banned in Sweden in 1974, in the United States in 1977; more than 30 countries, including Japan, Australia, and most of the European Union, have followed suit. [1]. But, in Indonesia it is still used.
Other substances are thiamine (B) and pyridoxine (A).
I have tried to use mobile phase that contains 0.3% monothioglycerol as antioxidant, but it does not work.

1. http://en.wikipedia.org/wiki/Dipyrone

[HW Mueller]

I am relieved, but have compassion with Endonesian people.
Can´t you find out how the manufacturer handles these compounds? Maybe they will at least tell you a possible stabilizing pH (buffering most likely required if pH reaction dependent). Maybe by mixing the compounds you inadvertendly changed the pH, causing the reaction?
(Even the oxydation propensity of many compounds, including ascorbic acid, are strongly dependent on pH, not to speak of all kind of acid/base catalyzed reactions)

[Alex Buske]

Hans,

Why are you relieved? Its sold in germany under a number brand names.

alex

[vicentexxx]

In italy too...

[HW Mueller]

Darn and da__ .... . Didn´t register the "most" (which of course means not all) in the EU of syx´s statement above. Pretty soon I will have to go to a headshrinker.

[ym3142]

Let's see if we can find a condition which can stabilize the C a bit:
If it were me, I will try one or combinations of the following:
1) neutral aqueous
2) dark or/and cold
3) protic organic such as MeOH, EtOH;
4) aprotic organic such as DMSO, DMF, dioxane, acetone, or ethyl acetate(try to inject less than 20ul or better 10ul if straight organic solvents were used)
6) some amines as solvent e.g. pyridine, triethylamine; piperidine and so on including some other amines which have similar structure to C;

By the way, go to library , check out all available articles about the N-N bond species if possible. I am sorry I am not familar about this. It is interesting though.

[promochrom]

You may try a HILIC column. It uses 80-90% acetonitrile for separation of polar compounds. Your compounds should be more stable in such media as well.