In the Laboratory
Quantitative Analysis of Nail Polish Remover Using Nuclear Magnetic Resonance Spectroscopy Revisited Markus M. Hoffmann,* Joshua T. Caccamis, Mark P. Heitz, and Kenneth D. Schlecht Department of Chemistry, State University of New York College at Brockport, Brockport, NY 14420; *
[email protected] As part of an effort to increase exposure to nuclear magnetic resonance spectroscopy (NMR) in the undergraduate chemistry curriculum, we introduced a quantitative NMR experiment in our second-year analytical chemistry course, which currently has three laboratory sections of 10–15 students each. Although many NMR experiments for the undergraduate curriculum are reported in this Journal, we noticed that only a few focus on quantitative analytical determinations (1–10). Of these, Clarke’s report on the quantitative analysis of commercial nail polish remover (1), a common household product students can easily relate to, was appealing to us. We substantially revised the experiment and wish to share our adaptations to the undergraduate analytical chemistry teaching community. With respect to content, we changed the analytical procedure from using an internal standard to the method of standard addition. Description and discussion of experiments using the standard addition method have been presented in this Journal (11–18), including discussions of general systematic approaches (12), application to multi-component analysis (17), estimates on precision (13), and a description of an experiment where students discover how matrix effects necessitate the standard addition method for quantitative analysis (14). We also desired to set aside laboratory time to guide our students through the careful analysis of spectral data. A quantitative NMR experiment lends itself to hone proper spectral data analysis skills because the accurate evaluation of NMR signal intensities requires careful treatment of the spectral data with respect to phasing, baseline subtraction, and setting integration limits. We furthermore recognized the opportunity to provide our students with a group learning experience where they are actively engaged analyzing a fairly large data set as part of a team. For logistical reasons, students cycle through various experiments using modern instruments during the semester. Thus, the typical data set a student collects for any given experiment is only about 5–10 data points. In our adaptation the acquired and analyzed NMR data of all students in a particular section is eventually pooled together, and the students, first as part of a team and subsequently for themselves (for the laboratory report), have to decide which data points are reliable. Finally, from a technical and cost savings perspective, NMR spectra are obtained from aqueous solution samples without any costly deuterated solvents present for a lock signal. The possibility of obtaining well-shimmed spectra, even in routine automation, for samples without deuterated solvents is probably not widely recognized, and this particular aspect of our adaptation might be of much broader interest to the undergraduate education community. The Experiment The formulations of commercially available nail polish remover may vary significantly from vendor to vendor and even from year to year. The typical student data presented here were obtained from brand “Pretty Nails” manufactured by J. Stephen
Scherer Inc. and bought in spring 2005. The analytes were ethyl acetate and 2-propanol. We acquired the proton NMR spectra for the submitted NMR samples in automation outside the laboratory period on a Bruker-Biospin Avance 300 NMR instrument. Further technical and logistical details are provided in the online materials. On day one, students spend 30–45 minutes preparing their NMR samples. The rest of the lab period is devoted to other unrelated laboratory assignments. Each student prepares two aqueous sample solutions in 10 mL volumetric flasks. Analytes are added using micropipets. Each student prepares two compositions from Table 1 for their samples. Thus, each sample composition in Table 1 will be analyzed several times by different students. The addition of acetone is necessary to ensure a homogeneous single phase. Day two of the experiment is spent entirely in a computer lab. The instructor guides the class through the analysis of the unprocessed raw spectral data of their first sample. The steps involved are: accessing the spectral data, Fourier transformation of the free induction decay (FID), familiarization with zoom options for data inspection, phasing the obtained spectra, calibration of the chemical shift axis, application of baseline correction, peak picking (optional), and finally integration of signals. As part of calibrating the chemical shift axis, the spectral assignments based on provided spectral data for the individual components (19) are discussed. Here, the instructor can draw from and reinforce knowledge students should have acquired from the organic chemistry course. Specifically, the spectral assignments in a typical final processed spectrum shown in Figure 1A are as follows. Ethyl acetate shows a triplet, singlet, and quartet at 1.28, 2.04, and 4.17 ppm, respectively. The doublet at 1.20 ppm and the heptet at 4.05 ppm belong to 2-propanol, and acetone shows a singlet at 2.20 ppm, just slightly higher than the singlet of the ethyl acetate methyl group. There is no separate resonance of the hydroxyl group of 2-propanol because of fast chemical exchange with the water solvent, the large signal near 4.9 ppm. Most students are familiar with using tetramethylsilane as the standard chemical shift reference, but are unaware that principally any spectral line could be used as a chemical shift reference as long as its chemical shift value is known, is not significantly solvent dependent, and does not overlap with the Table 1. Composition by Volume of Standard Addition Samples Sample
Nail Polish Remover/mL
Acetone/ mL
Ethyl 2-Propanol/ Acetate/mL mL
1
400
200
0
0
2
400
200
100
100
3
400
200
200
200
4
400
200
300
300
5
400
200
400
400
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 10 October 2008 • Journal of Chemical Education
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In the Laboratory
A 4.2 4.1
1.3
4.0
1.2
of intensity versus analyte concentration. The upper panels in Figure 2 are the “raw” combined student data sets and the lower panels are corrected data. The left panels are for the analysis of ethyl acetate, and the right panels for 2-propanol. The concentrations, C, of added analyte were calculated using
B
C
D
E 8
7
6
5
4
3
2
1
0
Chemical Shift (ppm) Figure 1. Typical proton spectra from a student sample of aqueous nail polish remover and added acetone: (A) fully processed spectrum with insets to better display the coupling patterns present; (B) expanded y scale of same spectral data as in A but not optimally phased and without background subtraction; (C) as in B but optimally phased; (D) as in C but with background subtraction over spectral range of interest; (E) as in D but with additional water background subtraction for spectral region between 3.8 and 4.4 ppm. Spectral assignments are provided in the text.
signals of other displayed resonances. In discussing our chemical shift reference options, the question arises where the center is for a doublet, triplet, quartet, and so forth, and the students are usually able to draw from their organic chemistry experience to resolve this issue. Figures 1B–E illustrate some of the finer points of processing the spectral data to obtain the final processed spectrum shown in Figure 1A. Figures 1B and 1C show examples of unoptimized and optimized phasing, respectively. To properly judge the quality of phasing it is important to significantly expand the vertical scale but still inspect the entire spectral range. Figure 1D shows the spectrum after baseline subtraction. Again, it is necessary to expand the vertical scale significantly for best results. On the expanded vertical scale in Figure 1D, one can see a significant intensity contribution from the large water resonance underneath the quartet and heptet signals at 4.17 and 4.05 ppm. In a second iteration of baseline subtraction for the spectral region between 3.8 and 4.2 ppm, the water (shoulder) background can be successfully removed as shown in Figure 1E. It can easily take 1½ hours of time to guide the students through the complete analysis steps for their first sample spectrum. However, the students were generally able to repeat all steps for their second sample spectrum on their own in only 10–15 minutes, which is a rather positive experience. At this point, individual student results are aggregated into one common spreadsheet for the entire laboratory section so that all the students can see the combined data set. Figure 2 shows plots 1422
C
S Vadded M Vflask
(1)
where ρ is the analyte density, M the analyte molar mass, and Vadded/Vflask is the volume ratio of added analyte and volumetric flask. After about two hours, we observed that students were quite invested in the outcome of the laboratory. Students were somewhat disappointed and wondered “After all the careful NMR data processing, why is there so much scatter? What went wrong?” After some guided questioning and comparison of laboratory notes, several explanations were found. Data points contributing the most to the scatter in the data were identified as coming from the same experimenters. Blatant errors in sample preparation were found as the source of error. Inspection of the acetone intensity, which should be the same for each of the student samples, proved to be a very helpful check in this regard. Some students also discovered that they simply mixed up their two sample labels. Another student noticed mislabeled concentrations in the data spreadsheet. After correcting or removing these errors the bottom graphs in Figure 2 were obtained. This analysis facilitated discussions about statistics, data elimination, and experimental error. A number of possibilities for experimental error were brain stormed such as improper pipet calibration, variations in spectral quality, selection of integration limits, and variations in the NMR tubes. The final decision as to which data points should be included, based on confidence intervals, was left for the individual student as part of a lab report assignment. The students were also asked to report the initial analyte concentrations, Co, which may be either read directly from the bottom graphs in Figure 2, or numerically evaluated by dividing the y intercept by the slope. The necessary background on how the uncertainty of the x intercept may be estimated was provided (20). In the end, it is gratifying that the linear regressions in the corrected data sets yield similar x intercept values, as can be seen in the bottom graphs in Figure 2. The results for the concentrations of ethyl acetate and 2-propanol in Table 2 are thus fairly consistent using the different sets of peaks. The average concentration values from the entries in Table 2 are 0.221 mol L‒1 and 0.145 mol L‒1 for ethyl acetate and 2-propanol, respectively. Additional information on the analysis of the NMR data is provided in the online materials including, for comparison, an alternative analysis of the data using the acetone peak as internal standard, the generation of “per proton” plots from the combined ethyl acetate and 2-propanol data sets, and further details with respect to the statistical data treatment. Hazards Ethyl acetate, 2-propanol, and acetone are flammable solvents and harmful if swallowed or inhaled. Safety glasses are mandatory, and use of hoods is advisable. Waste solutions containing flammable solvents must be collected for waste disposal according to EPA and local guidelines.
Journal of Chemical Education • Vol. 85 No. 10 October 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
Intensity / (106 arb unit)
In the Laboratory
triplet singlet quartet
60
doublet heptet
40
20
Intensity / (106 arb unit)
0
60
40
20
0 0.2
0.1
0.0
0.1
0.2
0.3
C / (mol/L)
0.4
0.1
0.0
0.1
0.2
0.3
0.4
0.5
C / (mol/L)
Figure 2. Combined student standard addition data of ethyl acetate (left) and 2-propanol (right) in commercial nail polish remover obtained from integrated NMR signal intensities in arbitrary units. The two top graphs show the initial combined student data. The two bottom graphs were obtained after students corrected several sample label mistakes and eliminated the most outlying data points.
Table 2. Results from Figure 2 for Analyte Concentrations in Nail Polish Remover Analyte/Group
Slope/ (107 L mol–1)
y Intercept/ 106
Ethyl acetate/Triplet
3.89 ± 0.08 8.43 ± 0.21 0.217 ± 0.009
Ethyl acetate/Singlet
3.76 ± 0.09 8.12 ± 0.21 0.216 ± 0.010
Co/(mol L–1)
Ethyl acetate/Quartet 2.51 ± 0.09 5.77 ± 0.22 0.230 ± 0.015 2-Propanol/Doublet
7.93 ± 0.15 11.05 ± 0.46 0.139 ± 0.006
2-Propanol/Heptet
1.32 ± 0.07 2.01 ± 0.21 0.151 ± 0.022
Acknowledgments We acknowledge financial support from the National Science Foundation (DUE-0408617) for instrument purchase with matching funds from SUNY College at Brockport, and undergraduate summer stipend ( JTC). Literature Cited 1. Clarke, D. W. J. Chem. Educ. 1997, 74, 1464–1465. 2. LeFevre, J. W.; Silveira, Augustine, Jr. J. Chem. Educ. 2000, 77, 83–85. 3. Phillips, J. S.; Leary, J. J. J. Chem. Educ. 1986, 63, 545–546. 4. Schmedake, T. A.; Welch, L. E. J. Chem. Educ. 1996, 73, 1045–1048. 5. Wallace, T. J. Chem. Educ. 1984, 61, 1074. 6. Peterson, J. J. Chem. Educ. 1992, 69, 843–845. 7. Peterson, T. H.; Bryan, J. H.; Keevil, T. A. J. Chem. Educ. 1993, 70, A96–A98. 8. Anderson, S. E.; Saiki, D.; Eckert, H.; Meise-Gresch, K. J. Chem. Educ. 2004, 81, 1034–1037.
9. Woodworth, J. K.; Terrance, J. C. J. Chem. Educ. 2006, 83, 1065–1066. 10. Markow, P. G.; Cramer, J. A. J. Chem. Educ. 1983, 60, 1078–1079. 11. Van Atta, R. E.; Van Atta, R. L. J. Chem. Educ. 1980, 57, 230–231. 12. Bader, M. J. Chem. Educ. 1980, 57, 703–707. 13. Bruce, G. R.; Gill, P. S. J. Chem. Educ. 1999, 76, 805–807. 14. Harvey, D. J. Chem. Educ. 2002, 79, 613–615. 15. Ophardt, C. E. J. Chem. Educ. 1985, 62, 257–258. 16. Pandey, S.; Powell, J. R.; McHale, M. E. R.; Acree, W. E., Jr. J. Chem. Educ. 1997, 74, 848–850. 17. Raymond, M.; Jochum, C.; Kowalski, B. R. J. Chem. Educ. 1983, 60, 1072–1073. 18. Strohl, A. N. J. Chem. Educ. 1985, 62, 343. 19. National Institute of Advanced Industrial Science and Technology. Spectral Database for Organic Compounds, SDBS. http:// riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng (accessed Jun 2008). 20. Harris, D. C. Quantitative Chemical Analysis, 7th ed.; Freeman: New York, 2007
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Oct/abs1421.html Abstract and keywords Full text (PDF) Links to cited URLs and JCE articles Supplement Laboratory handout Instructor notes
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