Chromatography Forum

Hi,

I am trying to develop a GC direct injection method to analyze residual Acetic acid and Ethylene glycol in an API. The API is a carboxylic acid but also a hydrochloride salt because it has amine group on it. The diluent is DMSO; this API has good solubility in DMSO. The column is JW-624.

My injection sequence is blank, followed by six std injections, then sample, the wrap up std injection. The peak area of Ethylene glycol in first six injection looks good, the sample doesn't seem to have any detectable Ethylene glycol. However, the peak area of Ethylene glycol is significantly smaller in my wrap up standard. If I waited long enough (say at next morning) to inject std again, the peak area of Ethylene glycol is normal again.

I have tried to increase injector temp from 150 C to 220 C; added one equiv. of sodium hydroxide to neutralize the sample, increased concentration of Ethylene glycol in my STD, nothing seems work so far.
Just wonder if anyone with experience on this can give me some suggestions, I am very desperate at the moment.

Thanks, Meadowlane

Have you tried a split liner the type the USP details in its glycerin monograph with that same column type for residual ethylene glycol ?

Your sample injection changes the inertness level of your injection port. It is no wonder that the following std appears lower then the initial.

This is no surprise but an expected outcome.

I would be pleased that you do not see any carryover levels of EG in your sample injection. Count your blessings.

best wishes,

Rod

Rod:

Any way to go around the problem, say don't include wrap std injection, if the probelm can not be easily
solved?

Thanks,

The only 'solution', and I would not call it that, is to replace the components of the injector with clean inert parts after each sample injection.

Imagine having a clean floor, and you measure the shine on the floor, and then you walk over the floor with wet muddy boots, then measure the shine again.

Would you really expect to get the same measurement?

How would you 'solve' the problem? wash the floor of course.

Putting salts and acids and bases into an injection port is like walking over a clean floor with dirty boots.

You do it only because you have to do it.

And if you want a nice shiny floor you put forth a lot of cleaning effort.

But if the analysis is that important, then you do what is necessary.

As CPG as often mentioned, derivitization of EG as a TMS ether is highly regarded in GC analysis. I have no idea if it would work for your matrix.

HPLC analysis was used when I needed EG and HoAc measured in drug matrices 'back in the day'.

100% water mobile phase was used. Again this might not be possible for your matrix.

We had analyses that required a complete replacement of the injector port. It is the cost of doing business and doing good analytical work.

GOOD LUCK,

Rod