APPENDIX III SAMPLE LAB REPORT. Experiment 1. High Performance Liquid Chromatography. Alfred E. Neuman

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1 APPENDIX III SAMPLE LAB REPORT Experiment 1 High Performance Liquid Chromatography Alfred E. Neuman Chemistry 436

2 September 15, 1999 INTRODUCTION In this experiment, high performance liquid chromatography (HPLC) was used to separate a mixture of three similar alkaloids: caffeine, theobromine, and theophylline. The retention times of the three compounds were measured, and a calibration model for caffeine was constructed based on the chromatographic peak heights produced by a series of six caffeine standard solutions. The derived calibration model was then employed in the prediction of caffeine content in a commercial soft drink. DEVIATIONS FROM WRITTEN PROCEDURES No deviations were made from the procedures described in the laboratory handout. EXPERIMENTAL DATA Retention Time Study Retention Time (sec, sec) Compound Trial 1 Trial 2 Trial 3 Caffeine Theobromine Theophylline Caffeine Calibration Study Aliquot Volume Peak Height (cm, cm) (ml) Trial 1 Trial 2 Trial Unknown CALCULATIONS AND RESULTS Retention Time Study The retention times of caffeine, theobromine, and theophylline were analyzed to compute the mean retention time, standard deviation of retention time, and % relative standard deviation. Means and standard deviations were computed by use of functions resident in Microsoft Excel.

3 Compound Mean Retention Time Standard Deviation % RSD (sec) (sec) Caffeine Theobromine Theophylline Caffeine Calibration Study Calculation of Exact Concentrations of Standards C stock = ( g caffeine/1.0 L) x (1000 mg/g) = mg/l = ppm For a given standard, C std = (A)(C stock )/100 ml where A is an aliquot volume in ml. Example: (Standard #1) C std = (5 ml)(500.0 ppm)/(100 ml) = 25.0 ppm Table I Exact Concentrations of Standards Standard # Concentration (ppm) Calculation of Mean Peak Heights and Confidence Limits The means and standard deviations of the three replicate peak heights for each concentration standard were computed by use of the built-in functions of Microsoft Excel. The 95% confidence limits were computed as Upper Limit = Mean + t*s / n ½ Lower Limit = Mean - t*s / n ½ where Mean is the computed mean peak height for a given standard, s is the corresponding computed standard deviation, n = 3 replicates, and t = for n - 1 = 2 degrees of freedom (Microsoft Excel function tinv )

4 Table II Mean Peak Heights, Standard Deviations, and Confidence Limits (CL) Concentration Mean Peak Height S. Deviation Upper 95% CL Lower 95% CL (ppm) (cm) (cm) (cm) (cm) Calculation of Calibration Model A least-squares calculation was used to compute the slope and intercept of the best calibration model for the above data. The resident least-squares functions of Microsoft Excel were used for this calculation. Peak Height = ( )(Concentration) To evaluate the quality of the calibration model, the correlation coefficient and the standard error of estimate were computed. The built-in functions of Microsoft Excel were used for these calculations. R 2 = s = 1.14 cm Plot of Calibration Model A plot of the calibration model is included as Figure 1. This plot was generated with the commercial software package, Axum (Version 3.0), operating on a Dell 466/L computer. The data values from Table II are plotted along with the computed regression line. Computation of Unknown Caffeine Concentration The mean peak height of the unknown was calculated to be 6.47 cm. This value was used with the computed slope and intercept to determine the concentration of caffeine in the sample of "Pepsi". C unk = ( )/ = 87.9 ppm

5 Figure 1: Mean peak height (cm) vs. caffeine concentration (ppm). Error bars are plotted as the 95% confidence intervals based on three replicate peak height measurements at each concentration. The solid line identifies the computed regression line. Questions 1. Two explanations for constituents not showing up in the chromatograms are (1) their possible co-elution with other peaks in the chromatogram or (2) their being permanently retained on the column. It is also possible that constituents may not absorb light at 254 nm, the wavelength used by the detector employed here. 2. Enhanced qualitative structural information would best be obtained by use of an alternate detector. Mass spectrometric detection would perhaps give the most structural information. DISCUSSION The calibration model produced an excellent fit to the experimental data, as evidenced by the high value of the correlation coefficient. While no formal test of accuracy was performed, the successful calibration model lends confidence to the determined caffeine value. The retention time study indicated clearly that the three compounds could be separated using the C8-column. This study also revealed that the retention times were quite reproducible (relative standard deviations < 1%). No anomalies were encountered during the experimental procedure.