Chromatography Forum

Hi,
Do anybody knows how to determinate the assay of 3-cyclohexylpropionic acid by GC?
I use it as an internal standard in 2-ethylhexyl acid analysis on HP-FFAP(10x0,53x1,0) column. My basic standard of 3-cyclohexylpropionic acid expired and I have to evaluate it as the working one.
Maybe there is a general procedure of determination of assay of organic acids by GC without derivatization?
Thx for help.

Presumably you need to know the purity of the substance that you intend to use as a calibration standard ?

How much effort you need to put into this depends on how much of the uncertainty of the results of your anlyses is contributed by the purity of the standard.

GC is a poor choice because the most likely impurity (water) is chromatographically intractable and cannot be detected by the detector with the widest dynamic range; the FID. Also there are a host of other possible impurities that would not be detected by GC - anything that is not sufficiently volatile for instance.

As a minimum I would suggest that you determine moisture by Karl Fischer or loss on drying, residual solvents by headspace GC, and involatile residue.

If you still have some of the expired standard, and it has been properly stored and is not long past its expiry date you might be able to justify comparing the new batch with the old one.

Peter

Presumably you need to know the purity of the substance that you intend to use as a calibration standard ?

Only as an internal standard - all standard solutions (to calibration curve) are prepared on the basis of ISS (internal standard solution) - it stays const.

How much effort you need to put into this depends on how much of the uncertainty of the results of your anlyses is contributed by the purity of the standard.

Because it's ONLY the internal standard it's not so important for the analysis to have EXACT assay (that's why I can use expired standard), but I need to do this as proper as it's possible.

GC is a poor choice because the most likely impurity (water) is chromatographically intractable and cannot be detected by the detector with the widest dynamic range; the FID. Also there are a host of other possible impurities that would not be detected by GC - anything that is not sufficiently volatile for instance.

I know that. Water will be determine by Karl Fischer.
Anyway I use this standard in GC analysis (EurPh procedure), that's why I need to determine the GC-assay.

I'm just wondering whether there is a procedure of determination of assays of such compounds on GC. I can adapt the procedure of other organic acid if You have one or know where to find it.
...or eventually I will inject it on that column in the same system I use in the analysis, where this acid is IS... and just check the purity.
I would prefer to apply known method...

Thanks,
Sylwia

Hi Sylwia

If you are using the cyclohexylpropionic acid as an internal standard, and calibrating by plotting analyte peak area / standard peak area against analyte quantity then the purity of the internal standard makes hardly any difference.

Of course the i.s. must be free of anything that elutes at the analyte's retention time. You can determine this by simply injecting a solution under your usual anlytical conditions.

Doing a thing as proper as is possible can waste a lot of time and money. Ask yourself how good does it have to be ?, and do it as well as that.

Peter

Anyway I use this standard in GC analysis (EurPh procedure), that's why I need to determine the GC-assay.

If you are following, and reporting to, a standard method, such as EP, I suspect use of an expired standard would be unacceptable. Unless the manufacturer is prepared to issue a new certificate ( with a later expiry ) for that batch, you should just purchase a later batch..

Playing about with expired chemicals for assays that are reported to compendial methods would create paradise for auditors.

It doesn't matter whether it's an Internal Standard or not, it's expired. It could be fine for a research method, but not for any regulatory method.

How can you simply show that the expired product doesn't affect your assay in any way without performing a range of tests?.

Bruce Hamilton

Hi Sylwia
Reading Bruce's comments makes me think that I may not have correctly interpreted what the problem is. That you have a standard that is expired is clear but I am now not certain whether you want to continure using that expired standard, and need to validate that by testing its purity, or whether you have a new batch of standard whose purity you need to test.

I agree with Bruce that in a regulated procedure using an expired material is bound to lead to problems with auditors (no matter whether it is technically defensible or not).

On the basis of doing a thing only as well as it needs to be done the most cost effective way of getting a validated standard is to buy one (apart from the cost of on in-house purity determination, how are you gong to assign an expiry date ?). Is there a reason why you cannot do this ? Of course, if you cannot source a standard material with the necessary purity and expiry date you are going to have to do some in-house work on it.

Presumably you have a standard of the 2-ethylhexyl acid that you use for calibration ? You could show that for given known concentrations of the 2-ethylhexyl and the i.s. the relative peak areas have not changed (significantly) over the lifetime of the i.s. and argue that you can extrapolate that into the future, but this will need historical data that you may not have, and you are still going to have to justify it to the auditors. You could also try demonstrating to the auditors with simple arithmatic that an internal standard calibration is robust to i.s. purity, but that would not adress other potential problems such as interferences.

Peter