Analysis of Soft Drinks: UV Spectrophotometry, Liquid Chromatography, and Capillary Electrophoresis 1

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1 Analysis of Soft Drinks: UV Spectrophotometry, Liquid Chromatography, and Capillary Electrophoresis 1 Valerie L. McDevitt, Alejandra Rodríguez, and Kathryn R. Williams* Department of Chemistry, University of Florida, P.O. Box , Gainesville, FL An experiment for the undergraduate instrumental analysis laboratory should accomplish several instructional goals. First, it must demonstrate the capabilities and limitations of the method, as well as the proper procedures for data acquisition and computation of results. Students must also be reminded of the importance of the chemistry of the sample and how this relates to the analysis. Application of the method to a commercial product always helps stimulate student interest and teaches the extra considerations necessary in a realworld analysis. It is also instructive to analyze the same product by more than one instrumental method. Analyses of regular and diet soft drinks fulfill all these objectives. The samples are common everyday products, and they may be analyzed by a variety of means. Soft drink components have been determined by HPLC with UV detection (15) for a number of years, and methods utilizing capillary electrophoresis to determine caffeine (6, 7) and other components (8) have been developed. Although certainly not as useful as LC or CE, multicomponent UV analysis can also be used if the product does not contain too many absorbing components. This paper describes a series of undergraduate experiments using these three instrumental methods for the analysis of components of public interest in commercial soft drinks: caffeine, a central nervous system stimulant; sodium benzoate (determined as benzoic acid), which serves as a preservative; and the artificial sweetener aspartame. In addition to teaching the physical bases and practical applications of the three instruments, the experiments stress the chemical nature of the sample, especially the acid/base character of the three compounds and the importance of ph in the design of LC and CE separations. As part of the data acquisition and analysis, students also determine the method detection limits (MDL) for the three compounds by LC and CE. The concepts of MDL, false positives, and false negatives are especially relevant, considering the current interest in natural foods. Multicomponent UV Analysis Multicomponent spectral analysis is described in standard analytical texts (9, 10) and is a common experiment in the undergraduate curriculum (11). In addition to accessing the software in the Hewlett-Packard 8450A Spectrophotometer, University of Florida students are required to analyze the data manually using simultaneous equations, as described in the references. Because the number of equations must equal or exceed the number of components, manual data reduction can be unwieldy for a three-component system. To simplify the math, the analysis is limited to two of the compounds, caffeine and benzoic acid. *Corresponding author. Figure 1. Structures and UV spectra of 7.83 mg/l caffeine ( ), 6.55 mg/l benzoic acid ( ), and 18.7 mg/l aspartame (- - -) in 0.01 M HCl. The structures represent the predominant protonated forms at ph 2.0. Spectra show little variation with ph. Figure 2. Spectrum of a 1:25 dilution of Mello Yello in 0.01 M HCl. Figure 1 shows the structures and spectra of all three compounds in 0.01 M HCl. The absorption profiles of caffeine and benzoic acid are quite different, an indication that this pair is well suited to the method. In some instructional settings it may be feasible to use a diet drink and analyze the aspartame separately by a suitable solid-phase extraction technique. However, to save time and limit the focus of the data analysis, a nondiet beverage is used. Of the several caffeinated drinks that were tested, the best choice was Mello Yello. Colas do not give acceptable results because an appreciable colorant band extends into the UV. The spectrum of Mello Yello is shown in Figure 2. Although there is a small absorption at 300 nm, due probably to colorant, the effect on the results is not detrimental to the overall goals of the experiment. An essential part of the laboratory experience is the group prelab quiz. Questions for this experiment focus on the funda- JChemEd.chem.wisc.edu Vol. 75 No. 5 May 1998 Journal of Chemical Education 625

2 mental principles of multicomponent analysis, the requirements of the absorbing system, the choice of wavelengths, and the special features of the HP 8450A diode-array instrument. The procedure calls for the preparation of a series of caffeine and benzoic acid standards in 0.01 M HCl, with concentrations in the ranges 420 and 210 mg/l, respectively. Students measure the UV spectrum of each standard, a synthetic unknown prepared by the instructor, and a to mL dilution of filtered Mello Yello. In consultation with the teaching assistant, they choose two wavelengths and obtain the absorbance data for the simultaneous equations method. To verify that the two spectral profiles are additive, a mixed standard containing known concentrations of both components is also tested, and the spectrum is plotted on the same sheet as the spectral sum of the two contributing single-component standards. In preparation for the subsequent LC and CE experiments, students obtain spectra of all components in the buffer systems to be used. While in the laboratory, the students obtain the concentrations of the two analytes in the synthetic unknown and Mello Yello from the HP software. For their laboratory reports, they prepare Beer s law plots for each component at the two wavelengths and use simultaneous equations to determine the concentrations manually. Liquid Chromatography and Capillary Electrophoresis Initially, the LC and CE analyses were combined into a single experiment. Although this was feasible in terms of laboratory time, the average student failed to grasp all the instrumental and chemical concepts, and the experiment was split. The three-week soft drink module is scheduled in the order UV, LC, CE. In addition to the fundamental concepts of LC and CE, the oral quizzes and written reports stress the chemistry of the system, especially the importance of ph in the two separation processes. The laboratory manual gives students the literature values for the pk a s of benzoic acid (4.202 at zero ionic strength [12]) and aspartame (2.96 and 7.37 at 0.15 M ionic strength [13]). The actual dissociation constant for caffeine is not available in standard references. According to The Merck Index, the ph of a 1% solution is 6.9 (14). A 0.01 M solution in highly purified water was prepared in this laboratory. The readings for the solution and the water were both 7 within experimental error. Thus, the students are told that caffeine s pk b is ca. 14. An eluent mixture of 45% methanol/55% M aqueous phosphate, ph 3.0, gives the best separation of the three components without exposing the bonded octadecyl (C18) stationary phase to excessive acidity. To understand the relationship of the mobile phase composition to the separation, students must first recognize that components should be neutral to interact with the octadecyl stationary phase and that compounds are expected to elute in order of decreasing polarity. As shown above, caffeine () is such a weak base that it is neutral at ph s higher than about 1. In the ph 3.0 mobile phase, the benzoic acid is also in its neutral protonated form (H). On the basis of the pk a data, aspartame exists as an equimolar mixture of the fully protonated H + and zwitterionic H forms. In the oral quiz, students predict the elution order to be partially ionic aspartame first, followed by very polar caffeine and less polar benzoic acid. In actuality, the aspartame elutes shortly after the caffeine, as shown in Figure 3. The most logical explanation is the presence of 45% methanol, which may increase the fraction of aspartame in the H form (i.e., pk a1 may decrease). Also, benzoic acid s long retention time is indicative of considerable interaction of the phenyl group with the stationary phase, and this effect may also occur with aspartame. The CE experiment is performed on a Hewlett-Packard 3D Capillary Electrophoresis system, which has an automatic sample changer and a diode-array detector, using an applied potential of 20 kv with the cathode at the outlet. With this configuration, the migration order is cations first in order of decreasing electrophoretic mobility (µ ep ), neutrals next as a group, and anions last in order of increasing µ ep. To separate the three components, a M borate buffer, ph 9.4, is used. At this high ph only is neutral; aspartame and benzoate are both anionic ( and ). Because of its larger size, should have a lower µ ep than. Therefore, stu- Figure 3. Liquid chromatogram of a caffeine/benzoic acid/aspartame mixture obtained on a 15 cm 4.5 mm Higgins Analytical column packed with 5 µm Hiasil C18, using a flow rate of 1.0 ml/min and a mobile phase composition of 45% methanol/55% M aqueous phosphate, ph 3.0. Other instrument components included a TSP P200 pump, a Valco injector with a 20-µL loop, a TSP UV 100 detector set to 218 nm, and a SpectraPhysics SP4270 integrator. Figure 4. Electropherogram of a caffeine/benzoic acid/aspartame mixture obtained on a Hewlett-Packard 3D CE system using a M borate buffer, ph 9.4. The 33 cm 50 µm capillary was operated at 20 kv. The detection wavelengths were 272 nm for caffeine, 229 nm for benzoate, and 210 nm for aspartame. Before the laboratory period, the capillary was flushed with 0.10 M NaOH, water, and the borate buffer. Buffer flushes were included after every third sample. 626 Journal of Chemical Education Vol. 75 No. 5 May 1998 JChemEd.chem.wisc.edu

3 dents expect to migrate fastest, followed by, with last. As shown in Figure 4, this order is indeed observed. In addition to explaining the ph effects and predicting the elution/migration orders, students are asked to use the UV spectra to choose wavelengths for the LC and CE analyses during the prelab quiz. For the LC analysis, the chosen wavelength is 218 nm. As Figure 1 shows, although the components all absorb appreciably at 218 nm, this wavelength does not correspond to λ max for any of the three compounds. The diode-array capability of the CE system allows an optimum wavelength to be used for each component (210 for, 272 for, and 229 for ). In the laboratory, students analyze the same solutions on both instruments. First, individual solutions of each component in water are injected to determine the elution and migration times. On the CE, students also verify the peak assignments from the spectra obtained by the diode-array detector. They next inject a series of mixed standards, each containing known concentrations of all three components, for preparation of the three calibration plots. The standards, which are prepared by the instructor before the laboratory period, have concentrations in the ranges mg/l for caffeine, mg/l for benzoic acid, and mg/l for aspartame in water. The remaining samples are unknowns: filtered soft drinks, including Mello Yello and several diet drinks, a synthetic unknown, and a solution of one packet of Equal (aspartame) in 100 ml of water. As described further below, students determine the method detection limits for the three compounds. To obtain the necessary data, they prepare seven replicate dilutions of one of the standards and obtain the peak areas using the same separation conditions as the drink samples. Data analysis for both experiments starts with preparation of six calibration plots of peak area versus concentration for each component by each method. The least-squares equations are used to evaluate the concentrations of the components in the unknowns and the aspartame content of a packet of Equal. To emphasize the relationship of the detection systems to the UV spectrophotometer, students are also asked to calculate the ratios (: and H:) of the slopes of the LC calibration plots. They observe that these ratios are close to the corresponding ratios of the absorptivities at the LC wavelength (218 nm) obtained from the UV spectra. The CE report includes comparisons of the LC and CE results for the same samples and results for Mello Yello by all three methods. Analysis of the same compounds by more than one method provides an excellent opportunity to evaluate detection limits and compare the values for different instruments. For this exercise, students are asked to determine the method detection limit according to the method specified by the Environmental Protection Agency (15, 16) for caffeine, aspartame, and benzoic acid by both LC and CE. As explained further in the Appendix to this paper, the EPA method takes into consideration the statistical fluctuations of the entire analytical Sample Table 2. UV Syn Unk Mello Yello LC/CE Syn Unk Diet Coke Diet Pepsi Table 1. Spectral Data for feine, oic Acid, and artame λ a ε Compound (nm) (L/g cm) ( 103L /mol cm) feine oic Acid artame method (sample pretreatment, injection, peak quantitation). This is considered to be a truer representation of the detection limit than the value obtained according to IUPAC recommendations (17, 18), which utilizes only the fluctuations in the instrument signal. The full EPA protocol is slightly modified (see Appendix) to meet the time constraints of the student laboratory. Results The wavelengths chosen for the UV analysis are the λ max s for the two components: 229 nm (benzoic acid) and 272 nm (caffeine). Both compounds obey Beer s law over the concentration ranges used in the analysis. Table 1 summarizes the absorptivity data, including values at 218 nm and 210 nm (aspartame only), which are used in the HPLC and CE experiments. Typical analytical results are presented in Table 2. Results for the synthetic unknown show excellent agreement with the instructor s values, although the caffeine result for Mello Yello is somewhat high. As stated above, this is probably due to a small absorbance for Mello Yello at 300 nm, where the individual components do not absorb. This undoubtedly extends into the UV and accounts for the high caffeine result. In the conclusion section of their report, students are expected Results of feine/oic Acid/artame Analyses Compound a Value (mg/l) Experimental Result (mg/l) UV b 11.09/ / / /219 d Equal a Instructor s value for synthetic unknowns; manufacturer s value for commercial products, if available. b Results by HP software/simultaneous equations. c Average of 2 determinations. d Solution prepared by dissolving 1 packet in 100 ml of water LC CE c JChemEd.chem.wisc.edu Vol. 75 No. 5 May 1998 Journal of Chemical Education 627

4 to comment on how well the Mello Yello system meets the requirements for multicomponent analysis, and the extra absorption is an obvious point to address. Thus, this analytical interference is actually very useful instructionally. There is also some concern about possible interactions between the two components, because the solubility of caffeine in pharmaceutical preparations is known to increase in the presence of sodium benzoate (19). However, the additivity test described above produces spectra that match almost perfectly, and a published UV analysis for pharmaceutical mixtures of caffeine, sodium benzoate, phenacetin, and Pyramidon showed good agreement, with relative errors of less than 3% (14). The LC and CE calibration plots are also very linear, and the results for the synthetic unknown are generally quite good, with relative errors less than 10% (less than 4%, if aspartame by LC and caffeine by CE are excluded). For the most part, LC and CE analyses of the drinks agree with each other, the major exception being caffeine in Mello Yello, which produces a significantly higher value by CE. Of the Mello Yello analyses, the LC result is closest to the manufacturer s value. The high CE result may be due to another neutral comigrating with the caffeine, but the spectral profile shows no obvious deviation from that of pure caffeine. The LC and CE values for caffeine in Diet Coke may also be compared to those reported previously in this Journal: 92 mg/l (3) and 134 mg/l (5) by LC; 124 (7) by CE. Agreement with the latter two values is very satisfactory. The MDLs (mg/l, µm) by LC are 1.2, 6.1 for caffeine; 0.57, 4.7 for benzoic acid; and 1.4, 4.9 for aspartame. The MDLs by CE are 1.7, 8.5 for caffeine; 0.29, 2.4 for benzoic acid; and 2.2, 7.5 for aspartame. The LC and CE values are quite close to each other, and all six values are within 2 and 8.5 on a micromolar basis. The two caffeine MDLs are also remarkably close to the previously reported value of 1.9 mg/l by CE (7 ). Conclusions In the conclusion to the CE report (i.e., after all three experiments have been performed), students are asked to compare the advantages and limitations of the three methods. Factors such as the number of analyzable components, MDLs, sample size, ease of performance, and accuracy (compared to manufacturer s values) are discussed. The most notable consideration is the number of components, which is clearly a limitation in UV multicomponent analysis. Good students note that sample size is another important factor. Although the MDLs are about equal for LC versus CE, the 20-µL injection loop on the LC can be rinsed and filled with about 50 µl of filtered drink, whereas the standard CE autosampler vials hold about 500 µl. Thus, even though most of the sample can be recovered from the autosampler vial, the CE analysis requires a total sample volume about ten times that for LC. The analysis of soft drink components stimulates student interest and is instructionally useful. The experiments demonstrate the analytical use of the three instruments, as well as their advantages and limitations. The MDL determinations teach an important procedure frequently used in the workplace but not included in most texts. The experiments also reinforce students knowledge of acid/base behavior and stress the importance of fundamental chemistry in the design of an analytical method. Acknowledgment Purchase of the capillary electrophoresis system was facilitated by grant #DUE from the National Science Foundation. Note 1. Presented at the annual meeting of the Southeast Association of Analytical Chemists, Athens, GA, October 1996, and the Annual Meeting of the Florida Sections of the American Chemical Society, Orlando, FL, May Literature Cited 1. Smyly, D. S.; Woodward, B. B.; Conrad, E. C. J.A.O.A.C. 1976, 59, Gillyon, E. C. P. Chromatogr. Newslett. 1980, 8, Delaney, M. F.; Pasko, K. M.; Mauro, D. M.; Gsell, D. S.; Korologos, P. C.; Morawski, J.; Krolikowski, L. J.; Warren, F. V., Jr. J. Chem. Educ. 1985, 62, DiNunzio, J. E. J. Chem. Educ. 1985, 62, Strohl, A. N. J. Chem. Educ. 1985, 62, Mabrouk, P. A.; Marzilli, L. A. Presented at the 212th National ACS Meeting, Orlando, August 1996; Paper No. 276; see CHED Newslett. 1996, Fall. 7. Conte, E. D.; Barry, E. F.; Rubinstein, H. J. Chem. Educ. 1996, 73, Schuster, R.; Gratzfeld-Hüsgen, A. Hewlett-Packard Publ. # E; Hewlett-Packard: Palo Alto, Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis, 4th ed.; Saunders: Fort Worth, 1992; p Harris, D. C. Quantitative Chemical Analysis, 4th ed.; Freeman: New York, 1995; p Williams, K. R.; Cole, S. R.; Boyette, S. E.; Schulman, S. G. J. Chem. Educ. 1990, 67, Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1974; Vol. 3, p Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1974; Vol 5, p The Merck Index, 11th ed; Budavari, S., Ed.; Merck: Rahway, NJ, 1989; p U.S. Environmental Protection Agency. In Code of Federal Regulations; Part 136, Title 40, Appendix B, Revision 1.11, U.S. Government Printing Office: Washington, DC, 1990; pp Harris, D. C. Quantitative Chemical Analysis, 4th ed.; Freeman: New York, 1995; p Winefordner, J. D.; Long, G. L. Anal. Chem. 1983, 55, 712A 724A. 18. Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis, 4th ed.; Saunders: Fort Worth, 1992; pp Machek, G.; Lorenz, F. Scientia Pharmaceutica 1966, 34, Appendix The EPA defines the MDL as the minimum concentration that can be measured and reported with 99% confidence that the analyte concentration is greater than zero and is determined from an analysis of a sample in a given matrix containing the analyte (15). The latter part of the definition indicates that the MDL must be determined by an actual analysis and is strictly valid only for the particular sample conditions. The code also gives the complete protocol, but, for the reader s convenience, a brief explanation is included here. First, the analyst must have an estimate of the MDL. The code lists several types of estimates, but the two most applicable to these analyses are (i) the concentration giving a signal roughly 2.5 to 5 times the baseline noise (method usually used by the students), and (ii) the lowest concentration in the linear response range (i.e., the concentration at which the calibration plot shows a noticeable change in slope). Next, a standard containing 1 to 5 times the approxi- 628 Journal of Chemical Education Vol. 75 No. 5 May 1998 JChemEd.chem.wisc.edu

5 mate MDL is prepared, and seven or more aliquots are analyzed by the usual laboratory procedure. Because of the time required to dissolve the solid compounds, the procedure is modified to have students prepare and inject seven replicate dilutions of one of the mixed calibration standards. The analyte concentration is evaluated from the least-squares response equation (and suitable additional calculations, if applicable). The MDL is given by: MDL = t 99 s where s is the standard deviation of the replicate concentration measurements and t 99 is Student s t (one-sided) at the 99% confidence level (98% confidence level if a table of two-sided t s is used) for N 1 degrees of freedom (N = the number of replicates). JChemEd.chem.wisc.edu Vol. 75 No. 5 May 1998 Journal of Chemical Education 629