730 Journal of Chemical Education _ Vol. 87 No. 7 July 2010 _ pubs.acs.org/jchemeduc _ r2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100346t Published on Web 05/19/2010 In the Laboratory Iontophoresis and Flame Photometry: A Hybrid Interdisciplinary Experiment Duncan Sharp* Faculty of Health, Leeds Metropolitan University, LS1 3HE, United Kingdom *d.sharp@leedsmet.ac.uk Linzi Cottam, Sarah Bradley, Jeanie Brannigan, and James Davis School of Science and Technology, Nottingham Trent University, NG11 8NS, United Kingdom A new experiment using iontophoresis and reverse ionto- phoresis is described. Iontophoresis (ITP) has traditionally been viewed as a research tool that is at the cutting edge of drug- delivery strategies (1-3) or, in its reverse form, biomarker moni- toring (4-7) as typified by the Glucowatch. In this experiment, students use reverse ITP to extract and effectively pre-concentrate the potassium ions from a tomato, with subsequent analysis by flame photometry (FP). The students also observed ITP visually using the cationic dye methylene blue. Use of AA batteries connec- ted in series (Figure 1) as the voltage source avoids the safety con- cerns associated with large power supplies. Experiment Development In our initial investigations using the computer-controlled potentiostat, a substantial difference in potassium ion concentra- tion from a tomato was found between the anodal and cathodal compartments with means of 1.99 ppm ((0.25) and 39.6 ppm ((15.6), respectively, based on 10 experiments using different tomatoes. The concentration of potassium ions is significantly higher in the cathodal compartment. The ratio for individual experiments ranged from 10 to 35 times higher cathodal potas- sium with the inter-experiment variation ascribed to differences in the tomato specimen used. It is clear, however, that in each case there is a measurable difference between the two chambers. The anode and cathode controls (no current applied) contained 2.16 ppm ((0.67) and 2.14 ppm ((0.54) potassium ion concen- tration, respectively (n = 10). Therefore, the differences observed between anode and cathode samples are solely derived by the current application. The massive influx of potassium ion into the cathode compartment is attributed to the electromigratory processes associated with the negative charge of the cathode increasing the transepithelial efflux. Performing the same experiment using the battery pack as the current source also produced a substantial difference between the two compartments with mean potassium ion concentrations of 1.92 ppm ((1.30) and 18.20 ppm ((8.41) in the anode and cathode chamber, respectively. Individual replicates again show some inter-specimen variation that range between 5 and 50 times higher potassium ion within the cathode chamber, but they effec- tively mirror the results using the computer-controlled system. Thus, the battery pack system is more than capable of performing the ITP, and it possesses numerous advantages over the compu- ter-controlled system in terms of cost, safety, and technical sim- plicity. It has proven to be an exceptionally robust tool for the undergraduate laboratory. Student Experiment In trials with first-year undergraduate students, the experi- mental procedure was completed within 90 min. The experiment was initiated by the preparation of the tomato and the construc- tion of the compartments. The ITP process was conducted over a period of 30 min during which time the standards for the FP experiment were prepared. At the conclusion of the electrolysis, the solutions were removed from the compartments and diluted with deionized water. The flame photometer was calibrated using the standards (2.5 to 20 ppm) and solutions from the two com- partments analyzed. A quality assurance standard (10 ppm) was also run to ensure accuracy of the calibration. Full experimental protocols are available in the supporting information. The results were plotted, yielding a linear relationship between signal inten- sity and potassium ion concentration. The regression details were extracted, and the concentration of potassium ions within the compartment solutions was determined. In normal practice, the remainder of the laboratory would be used for the subsequent data analysis. In this case, however, there is an opportunity for a further experiment. The addition of cationic methylene blue to the solutions in both compartments allows a more visual demonstration of the ITP process. The basic procedure was repeated and then terminated after 30 min. The solutions were removed from both compartments and the tomato skin examined. Images of the latter at various magnifications are shown in Figure 2. The surface of the tomato skin where the anodic compartment was positioned shows substantial methy- lene blue impregnation (Figure 2 A, C, and E). The impregna- tion was not removed by rinsing with water or with gentle wip- ing and confirms that the dye penetrated into the tomato skin. This is in direct contrast with the cathode compartment results Figure 1. Experimental setup for iontophoresis and reverse iontophoresis using a tomato as an in vitro model. Iontophoresis compartments consist of plastic cylinder with carbon fiber electrode attached by glue to copper tape connections. A buffer solution is contained in the compartments.