Impact of ISTD Amount on Test Result

[Terry]

Dear all,

I have been trying to validate an in-house residual solvent test method using 1-propanol as internal standard (ISTD), the residual solvents of interest are methanol, ethanol and acetone. The peak area ratio is used to calculate the results.

In the validation protocol, the direction for ISTD diluents preparation goes like this: dilute 250 micro Liter of ISTD to 1000 mL with DI water, use the ISTD diluent to prepare standard solution and test solution.

I accidentally diluted 500 micro liter of ISTD to 1000 mL to make the ISTD diluent. This diluent has been used in the intermediate precision test. This is a deviation. As a corrective action for the deviation as required by QA, I have to make an impact assessment before the tes result is justified.

I know it is common sense that ISTD amount does not have impact on test result, but I have not found any reference yet. Would you please share with me any reference on this case?

Regards,

Terry

[Peter Apps]

Hi Terry

It might actually be quicker, and less painful just to re-do the intermediate precision with the right IS concentration, or show that the peak area is linear with IS concentration from 100 to 1000 ul/l and that the rest is arithmatic.

Peter

[wiclef]

Dear Terry,

My name is Wiclef and I am an analyst also in Rwanda Bureau of Standards.
I read your quote and I had interest in your method.I use to analyse ethanol and methanol in alcoholic beverages and the method that I used is not good for the separation of these alcohol.could you share with me your method?If possible, you could send it to my e-mail: kagisha@yahoo.fr

Thanks

Wiclef

[Alexandre]

Guys just one comment, be careful with ISTDs that are common chemicals, like isopropanol (IPA).

It may be present in your test samples, it is natural fermentation product. Unless you know that it is never more than 0.1% and you quantitate 40% EtOH, then it may be OK.

If IPA % varies, you need to screen sample without ISTD first to check that it does not have IPA. Isotopes are better, but you will need MS detector.
Have you considered CF3(CH2)3OH or something of this kind as ISTD?

[Peter Apps]

Alexandre makes a good point if you are working with natural products. But isotope labelled standards have lower vapour pressures than the normal molecules, and so give smaller peaks in equilibrium headspace analyses.

Peter

[CharapitsaS]

Dear all,

I have been trying to validate an in-house residual solvent test method using 1-propanol as internal standard (ISTD), the residual solvents of interest are methanol, ethanol and acetone. The peak area ratio is used to calculate the results.

In the validation protocol, the direction for ISTD diluents preparation goes like this: dilute 250 micro Liter of ISTD to 1000 mL with DI water, use the ISTD diluent to prepare standard solution and test solution.

I accidentally diluted 500 micro liter of ISTD to 1000 mL to make the ISTD diluent. This diluent has been used in the intermediate precision test. This is a deviation. As a corrective action for the deviation as required by QA, I have to make an impact assessment before the tes result is justified.

I know it is common sense that ISTD amount does not have impact on test result, but I have not found any reference yet. Would you please share with me any reference on this case?

Regards,

Terry

Dear Terry,
there is the following new methodical approach of using ethanol as internal standard in gas chromatographic analysis of volatile compounds in spirit drinks in daily practice of testing laboratories. This method provides determination of volatile compounds concentrations in spirit drinks directly expressed in milligrams per liter (mg/L) of absolute alcohol according to official methods without measuring of alcohol strength of analyzed sample. The experimental demonstration of this method for determination of volatile compounds in spirit drinks by gas chromatography is described here http://www.inp.bsu.by/labs/lar/eis.html and here http://inp.bsu.by/labs/lar/Jerusalem_2011.pdf

Regards,
Siarhei