Chromatography Forum

Continuation

Dear Peter,

verification of method stability against dilution of testing samples was been done by the next way. Three reference ethanol-water solutions were analyzed after dilution with water in ratio 1:1 and 1:3. The obtained results are presented below in the Table.

Compound___Known_conc._of_i-th____Measured._c.___Relatively____Measured._c._____Relatively_
__________compound_in_standard__after_dilution_1:1_discrepancy_after_dilution_1:3__discrepancy
_____________(mg/L)_______________(mg/L)___________%__________(mg/L)__________%_____

acetaldehyde____10.11______________10.34___________+2.2__________10.50_________3.8____
_______________99.64______________97.28___________-2.4___________97.40________-7.3___
_______________497.6______________483.3___________-2.9___________473.1________-4.9___
methyl_acetate__10.00______________10.25___________+2.5___________9.78_________-2.2___
_______________100.0______________92.76___________-7.2___________89.17________-10.8__
_______________500.0______________463.7___________-7.3___________452.4________-9.5___
ethyl_acetate____10.47_____________10.18___________-2.8___________10.63________+1.6___
_______________104.7______________100.0___________-4.5___________95.46________-8.8___
_______________523.4______________489.6___________-6.5___________477.3________-8.8___
methanol________103.2_____________97.99___________-5.0___________95.18________-7.8___
_______________1005______________921.9___________-8.3___________904.1________-10.0__
_______________5013______________4654____________-7.2___________4514________-9.9___
2-propanol______11.80______________11.63___________-1.2___________10.56_______-10.4__
_______________103.2______________97.86___________-5.2___________93.13_______-9.7___
_______________509.4______________479.5___________-5.9___________463.7_______-9.0___
1-propanol______10.21______________10.36___________1.5____________10.01_______-1.9___
_______________102.1______________98.00___________-4.0___________96.23_______-5.7___
_______________523.4______________482.9___________-5.8___________483.2_______-7.7___
isobutyl_alcohol__10.00______________10.42___________+4.2__________10.35_______+3.5__
_______________100.0______________96.87___________-3.1___________94.32_______-5.7__
_______________500.0______________480.1___________-4.0___________471.5_______-5.7__
n-butanol_______10.00______________10.17___________+1.7___________9.98________-0.2__
_______________100.0______________97.02___________-3.0___________95.21_______-4.8__
_______________500.0______________482.9___________-3.4___________475.2_______-5.0__
isoamyl_alcohol__10.37______________11.28___________+8.8___________10.35_______-0.2__
_______________103.7______________103.0___________-0.6___________99.52_______-4.0__
_______________518.2______________509.0___________-1.8___________500.6_______-3.4__

Estimated volume of relative accuracy for proposed method followed from analysis of the obtained experimental in accordance with "ISO 5725-1994. Accuracy (trueness and precision) of measurement methods and results" does not exceed 11 %.

Thousands of testing laboratories over the world every day carry out gas chromatographic analysis of volatile compounds in spirit drinks. They may test this approach in their real practice. It is important to note that there is no need to perform any additional measurements. This method could be tested while performing current measurements with existing instrumentation and calculations could be done in parallel according to two different methods: using traditional ES method (GOST R 51698-2001. Vodka and ethanol. Gazochromatography express-method for determination of toxic microadmixtures content, 2001) or according to (Commission Regulation (EC) No 2870/2000 of 19 December 2000 laying down Community reference methods for the analysis of spirits drinks, 2000) with pental-3-ol as ISTD and using ethanol as ISTD.

I promise my prompt help on e-mail: chere@inp.bsu.by

Best Regards,
Siarhei

In reference primary methods the requirement to conc. of ISTD is that it has to match as much as possible the concentration of the analyte. In the reference labs the procedure is - screen, estimation, adding of ISTD as 1:1 and calculation without calibration graph. In routine labs it has to be justified by validation and requirements to accuracy and precision set by your regulator. If it is +/- X% around 0.001%-0.01%, just tell us that X and show the data. You also need to provide a comparison of the data obtained by proposed method and the official reference method.

On the examples provided in the link the EtOH peak was tailing badly. It was also integrated as B to X% tail, not as Baseline to Baseline. Did the EtOH peak pass requirement for tailing and symmetry? Can a better column be used?

With this integration the amount of the EtOH was obviously under estimated, and if integrated properly the result of impurities to EtOH could be almost 2 times lower. I see a problem with this integration in settling disputes with customers, who work hard to make good booze? In their labs they may integrated B to B.

Dear Alexandre,

I agree with your message. The proposed new methodical approach should help “customers, who work hard to make good booze”.

Analyst before measurement should do procedure of addition of internal standard in the analysed sample in accordance with official reference methods, for example, the following:
1) AOAC Official Methods 972.10. Alcohol (higher) and ethyl acetate in distilled liquors. Alternative gas chromatographic method, 1975;
2) AOAC Official Methods 972.11. Methanol in distilled liquors. Gas chromatographic method, 1973;
3) Commission Regulation (EC) No 2870/2000 of 19 December 2000 laying down Community reference methods for the analysis of spirits drinks, 2000.

To simplify the whole method, to facilitate work of analyst and to increase the reliability of the data method "Ethanol as ISTD in Spirit Drinks" proposed.

Best regards,
Siarhei