Chemical Education Today edited by
NSF Highlights
Susan H. Hixson
Projects Supported by the NSF Division of Undergraduate Education
Environmental Analysis in the Instrumental Lab: More Than One Way…
National Science Foundation Arlington, VA 22230
Richard F. Jones Sinclair Community College Dayton, OH 45402-1460
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by M. Sittidech and S. Street*
Environmental problems provide a comprehensive framework for instruction in instrumental methods of analysis. They are practical problems compatible with the methods being taught, sometimes with the addition of just a few new concepts. Typically, the goal is not to teach environmental chemistry, per se, but to use the environmental analysis model to improve student understanding of analytical methods. The pedagogical value of this approach has been recognized; the NSF (DUE) has supported programs using environmental analysis as an organizing theme, and results have been reported in the literature.1 Indeed, undergraduate instruction programs with an environmental focus were recently reviewed in the scientific literature of the field (1). Funds from the NSF-sponsored Course, Curriculum and Laboratory Improvement (CCLI, A&I) program have been used to obtain instruments for our physical chemistry and instrumental methods labs. Instrument acquisitions allowed us to alter our curriculum to include many examples of environmental analysis of samples of both organic and inorganic species. Along with the instrumentation already on hand, the improvement made the most common methods of environmental analysis available to students. Two reports written in a format suitable for publication in a refereed journal are part of the course requirements. The last of these two reports relates the results of a lab practical based on an analysis of one or more environmentally important unknowns. The Problem For the first ten weekly meetings of the lab section of the instrumental methods course students complete relatively straightforward exercises in instrumental analysis using a variety of available spectroscopic, chromatographic, electrochemical and mass spectrometric techniques. The final three to four weeks are used to complete a problem of quantitative analysis drawn from environmental hazards. The students must prepare two procedures for the analysis upon learning of the analyte(s) and matrix, using two different analytical techniques. At least one of the procedures must be suited to our lab using the available instrumentation. A recent example of this approach shows the value of having a variety of instrumental methods available for an open-ended, but guided, lab experience. One team of students was given this problem: Suppose there has been an accident at a manufacturing center located on the Mobile River near Theodore, AL. Given a sample of river water taken near the plant, determine if acrolein and/or acrylonitrile, two chemicals used in the plant’s processes, have escaped containment. Quantify the levels.
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This was the extent of their instructions. We gave them 25 mL of tap water spiked with 2 ppm of both acrolein (2propenal) and acrylonitrile (2-propenenitrile) as the unknown and a standard solution of 10 ppm of both compounds in water.2 What the students quickly discover is that the standard methods of analyses for most environmental hazards are either GC/MS or HPLC, if the analyte is an organic; or AAS, electrochemistry, or perhaps spectrophotometry if the analyte is a metal or inorganic complex (2). Indeed, the students identified a straightforward EPA method3 for quantitative determination of acrolein and acrylonitrile by HPLC-UV detection. They were familiar with this instrumentation from an analysis of caffeine in soda carried out previously. The students encountered problems with the HPLC method, obtaining irreproducible results both in terms of apparent retention times of the analytes and in the signal response as a function of known concentration. Chromatographic resolution was acceptable, but peak identification was difficult. Photodiode array UV–vis detection is usually helpful, but not in this case. Detection at 195 nm is specified in the method and the analytes do not have other distinguishing absorptions. Water is the specified solvent for the method and was used both as the mobile phase and as the diluting solvent in preparing both the standard and sample solutions. Organic impurities in the water were a problem for this trace-level analysis. The team decided to try another approach.4 A Solution The students were familiar with solid-phase microextraction (SPME) sampling used with GC/MS from previous experience in the lab (BTEX analysis: benzene, toluene, ethylbenzene, and xylenes mixture). Sample preparation generally receives little attention in the instrumental lab even though it is often the critical step in analytical methods. In SPME sampling, analytes of interest are allowed to adsorb onto a microfiber made of fused silica coated with a thin film of a stationary solid phase. Analytes from a solution (or from the vapor over a solution in headspace sampling) come into adsorption equilibrium according to their affinity for the solid phase. The microfiber is incorporated into a GC syringe for direct injection onto a GC column. The analytes desorb in the heated GC injection port. SPME sampling in undergraduate quantitative labs was reported in this Journal by the technique’s developer (3). The materials are not expensive and the modification to the standard GC injection port is simple.5 The team completed the analysis using SPME sampling and GC/MS. Surveys of the 1 ppm standard total ion cur-
Journal of Chemical Education • Vol. 80 No. 4 April 2003 • JChemEd.chem.wisc.edu
Chemical Education Today
Hazards Use caution! Both acrolein and acrylonitrile are toxic and carcinogenic. We never let students handle solutions greater than 10 ppm in either compound. Dilutions and other manipulations are performed in a hood. Proper waste disposal is essential. Clearly, this is not a lab for novice students.
Acrylonitrile (2-Propenenitrile)
Peak Area
rent chromatogram showed the separation of the analytes (and the presence of contaminants in the water). Careful pattern matching of the extracted ion-induced fragments allowed specific identification of acrolein and acrylonitrile peaks. To construct the calibration curve and then quantify the concentrations in the sample, the most abundant unique fragment for each compound was followed in extracted ion chromatograms, which allowed both qualitative and quantitative analysis. Time constraints prevented the use of selective ion monitoring, which would have improved the sensitivity even more. The students were then able to compare two sampling procedures: direct immersion of the SPME fiber in the water sample and headspace analysis of the vapor over the solution in a confined volume. The results are identical when equilibrium adsorption is established; quantitative results can still be achieved in a short time if the adsorption sampling time is kept constant (4). The SPME GC/MS method provided several advantages over the HPLC method. Chromatographic resolution was superior. Mass spectral analysis and pattern matching made peak identification much more explicit than UV detection. Baseline drift and contaminant interferences were much less problematic in the GC method compared to the HPLC method, particularly after SPME sampling. Replicate measurements were made and calibration curves demonstrated that the response was linear in the working range and provided data for statistical analysis of the quantitative determinations (see Figure 1). The slightly higher sensitivity for one analyte over the other was reflected in the results (2.18 ± 0.49 ppm and 2.07 ± 0.07 ppm for acrolein and acrylonitrile, respectively, reported at the 95% confidence level). Detection limits using this method were found to be 0.50 ppm for acrolein and 0.14 ppm for acrylonitrile.
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Notes 1. A relatively recent issue of J. Chem. Educ. was devoted to environmental chemistry: J. Chem. Educ. 1997, 74, 1409–1462.
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Concentration (ppm) Figure 1. Calibration curve generated for acrolein and acrylonitrile using a SPME GC/MS method.
Interest in environmental pedagogy goes back a long way in J. Chem. Educ. More than thirty years ago J. W. Day wrote, “It seems that environmental science would also be an especially good vehicle for relating chemistry… and could also offer fresh material for laboratory experiments”! (J. Chem. Educ. 1970, 47, 260). 2. Acrolein and acrylonitrile standard solution obtained from AccuStandard Inc., New Haven, CT 06513 (Cat. # APP-9-007). 3. U.S. EPA Method 8316. Conditions: water mobile phase; C18 reverse phase column; flow rate 2.0 mL/min; injection volume 200 mL; UV-Vis detection (195 nm). 4. An additional problem was sample handling. For the small volumes and low concentrations being used, the vapor pressure of the compounds over water leads to non-trivial loss of the analyte. Hence the subsequent SPME GC/MS headspace analysis. 5. GC Conditions: He carrier gas 1.0 mL/min; HP-5 capillary column (5% phenyl, 30 m ⫻ 0.25 mm); temperature program 40–120 ºC (20 ºC/min). EI detection MS (45–200 m/z scanned). Supelco, Bellefonte, PA, carries SPME supplies: 65 mm CW/DVB fiber (Cat. # 57312); manual sampling SPME fiber holder (Cat. # 5733-U); SPME inlet liner for HP 5890 GC (Cat. # 26375). W
Conclusion Laboratory practicals with environmentally important unknowns are one way to get students to think critically about how a particular analysis is to be done. They will discover standard methods for most analytes, but must consider whether these are appropriate given the sampling methods and instrumentation available to them. Some attention to sampling methods, in particular, can dramatically enhance the lab experience. Here, SPME GC/MS proved to be the solution to a problematic analysis. Our students were gratified to learn that headspace SPME sampling GC/ MS analysis of acrolein appeared in the technical literature several months after (5) they had performed it in an undergraduate lab!
Acrolein (2-Propenal)
Supplementary Material
More information on the acrolein and acrylonitrile lab practical is available in this issue of JCE Online. Literature Cited 1. Wenzel, T.; Austin, R. Environ. Sci. Technol. 2001, 35, 327A– 331A. 2. Patnaik, P. Handbook of Environmental Analysis; CRC Press, Inc.: Boca Raton, FL, 1997. 3. Yang, M.; Orton, M.; Pawliszyn, J. J. Chem. Educ. 1997, 74, 1130–1132. 4. Pawliszyn, J. Solid Phase Microextraction: Theory and Practice; Wiley-VCH: New York, 1997; p 17. 5. Biswas, S.; Staff, C. J. Cereal Sci. 2001, 33, 223–229.
M. Sittidech and S. Street are in the Department of Chemistry, The University of Alabama, Tuscaloosa, AL 354870336;
[email protected].
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