Question

Problem #2 Calculation of percent alcohol (v/v %) in Beer using Internal Standard Chromatography is an important analytical tool because it allows the analyst to separate individual components in a mixture. Gas chromatography is a simple technique with great versatility and is used for the separation and analysis of volatile components in a mixture. The area of a peak in gas chromatography is proportional to the concentration of the analyte in the mixture. For this experiment an internal standard (n-propanol) will be used to minimize random and systematic errors for the analysis of ethanol in various alcoholic beverages. A calibration curve was made using varying amount of ethanol and a constant amount of n-propanol as an internal standard. The data is summarized in Table 1. Table 1: GC peak area response of ethanol and internal standards. (v/v) % Ethanol (v/v) % n- Peak Area Propanol Ethanol Peak Area n-Propanol 0.00 0.50 0.0 301.9 0.13 0.50 45.72 298.6 0.25 0.50 125.4 305.2 0.50 0.50 180.2 260.8 0.75 0.50 380.7 304.7 1.00 0.50 492.4 307.9 Q2. Plot the peak area for ethanol versus the concentration of ethanol a. Include a complete figure with axis titles. (You do not need to include error bars.) b. Perform LINEST for calibration curve and report the slope (m), standard deviation on the slope (sm), intercept (b), standard deviation on the intercept (S), coefficient of determination (r²), and standard deviation of y (s,). Q3. Plot the ratio of ethanol peak area/n-propanol peak area versus concentration of ethanol a. Include a complete figure with axis titles. (You do not need to include error bars.) b. Perform LINEST for calibration curve and report the slope (m), standard deviation on the slope (Sm), intercept (b), standard deviation on the intercept (s), coefficient of determination (r²), and standard deviation of y (sy)./nQ4. A sample of a popular brand of beer (8 ounces) was tested three times. A 10.00 mL sample of beer was diluted into 100.00 mL volumetric flasks with water and enough n-propanol for a final concentration of 0.50%. The following data was obtained for the GC analysis of the diluted beer sample. Table 2: Response of unknown beer sample with internal standard. Sample 1 2 3 %n-propanol 0.50 0.50 0.50 Area ethanol 180 189 182 Area n-propanol 305 298 304 a. Calculate and report the % ethanol in the original beer sample using the signal of ethanol versus concentration calibration curve. b. Calculate and report the % ethanol in the original beer sample using the ratio of signal versus ethanol concentration calibration curve. c. Following your calculations from parts a and b, calculate the % relative and absolute uncertainty for the final concentrations of beer (using average peak areas/ratios) for the two methods. d. Which method gives the best estimate of % ethanol in the beer? Support your answer with results from the analysis.

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