In the Laboratory edited by
Topics in Chemical Instrumentation
David Treichel Nebraska Wesleyan University Lincoln, NE 68504
A New Start to Advanced Analytical Laboratory: Comparison of the Performance Characteristics of Various Instruments Paul L. Edmiston Department of Chemistry, The College of Wooster, Wooster, OH 44691-2363;
[email protected] One of the emerging educational goals in analytical chemistry is teaching students how to systematically solve problems as opposed to simply acquiring knowledge (1–4). There has also been an increased effort by instructors and funding agencies to expose students to the most modern and sophisticated chemical instruments, such as GC–MS and NMR (5). The hands-on use of such instruments is important; however, there is a tendency to perform a single “off-the-shelf ” experiment using a particular technique, which gives students experience but neglects how the technique compares to other methods. This approach also promotes the impression that one technique can be used to solve any analytical problem. When performing a traditional series of one- or two-day experiments on different instruments it is often the instructor who does most of the problem solving (i.e., choosing what technique is best suited for a chemical measurement); this leaves the student to follow instructions. Little critical thinking about the comparative virtues and limitations of instrumental techniques can take place with such an approach. An alternative to this paradigm, developed by Fitch et al., uses many instrumental techniques to measure a single analyte (6 ). In this case, elemental lead was tested using a variety of techniques ranging from UV–vis to NMR to electrochemical methods. Students are afforded an opportunity to compare the results obtained with different methods and discover that an instrumental technique can only be used effectively with certain types of chemistries. Overview of the Laboratory Exercise To foster this type of comparative-based learning, a laboratory in which students critically assess the strengths and limitations of a set of instruments was developed for an upper-level analytical chemistry class. In the first semester of analytical chemistry, students are exposed to the analytical method and the more common techniques of spectroscopy, chromatography, and electrochemistry. The laboratory that accompanies the first semester focuses on problem solving. The second semester of analytical chemistry, entitled “Instrumental Analysis”, focuses on more sophisticated methods of chemical analysis with emphasis on laboratory work. This paper describes the 2–3-week laboratory that begins the second semester of the course. In it students are required to do four things: (i) choose an instrument from the departmental offerings that they would like to learn how to use, (ii) develop at least 20 specific questions they would ask about the instrument before operating it and use literature resources answer the questions, (iii) choose an appropriate compound to test their instrument, and (iv) determine the limit of
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detection of the instrument for their compound. After these tasks have been accomplished, the class meets as a group and students identify their test compound, report the limit of detection they found, and detail other aspects of the experiment such as how long an analysis took, what type of compounds they were limited to when deciding on a sample, and any difficulties they encountered. Students perform simple experiments to add a piece of data to the the overall set of instrumental performance characteristics that is compiled in the final discussion. The group compares the strengths of the instruments, noting, for instance, which have the lowest limits of detection or which can analyze mixtures. The exercise also provides a vehicle to give them exposure to instrumentation that may be new to them. The main purpose of this exercise is to have students think critically about the limitations of chemical instruments in a way they may have not done before. Students up to this point will have had hands-on experience with the analytical method, but will not have investigated the limitations of a particular technique compared to a suite of instrumental methods. The laboratory is not designed to be a central feature of the semester, but instead a beginning exercise to establish a mind-set that encourages students to compare and contrast instrumental methods as they learn about the theory governing instrument design. A comparative approach to instrumental/chemical analysis is used in various texts (7, 8), and this laboratory is designed to be a starting point for this pedagogy. Prelab Assignment Before coming to the laboratory students choose (or are assigned) an instrument they will operate. Each student works on different a instrument. The students are then asked to develop a list of questions one would need to answer before using the instrument. These questions are based on textbook readings and examining the instrument. Their purpose is to allow the students to acquire some background on the technique they will use. This part of the exercise forces students to ask the questions instead of being given information, an important skill they need to practice. To exemplify the type of information that is covered, the list of questions written by one student who was to perform atomic absorbance (AA) spectroscopy are given in the box. For specific questions such as “How does one control the fuel/ oxidant ratio?”, it is appropriate for the instructor to help the student. The questions are answered, collected, corrected, and returned to the students before they begin the experimental portion of the laboratory.
Journal of Chemical Education • Vol. 79 No. 5 May 2002 • JChemEd.chem.wisc.edu
In the Laboratory
Student-Derived Questions for Atomic Absorbance Spectroscopy Why do an AA analysis? What information is obtained? What analytes can be measured by AA? What types of samples are ideal? How large is the sample size? How is a sample atomized? Is calibration necessary? If so, how is it achieved? If not, why? Is flame height important? Where is the hottest part of the flame? How does a flame become more stable? Why is a stable flame important? Can any gas be used for the flame? What are some common fuel/oxidant ratios? How does one control the fuel/oxidant ratio? What region of the flame is used most often in flame spectroscopy? What is the difference between flame atomizers and electrothermal atomizers? How long does each method take for analysis? Are there other atomization methods? What are the key parts to an AA instrument? What is the most common radiation source? Why use a monochromator? What two types of interference are most common?
work with each student for approximately one hour to explain the basic operating principles of each instrument. In this situation, two weeks are required to complete the laboratory. However, the task is less daunting, since it is typical for several students to have some expertise with certain instruments from previous classes (organic and analytical chemistry), their research, or teaching apprenticeships. Students who have no previous hands-on experience with their instrument are considered first. The students’ background research in developing and answering their prelab questions is invaluable, since they should already understand the basic principles of the technique. After they are familiar with the operation of their instrument, students are instructed to run the analyte several times for practice and to measure the precision of the technique. They should also note the length of time it takes to measure a sample, including sample preparation time. Finally, they are required to determine the limit of detection for the species. In this case, the limit of detection is defined as the concentration or amount of the analyte that gives a response three times the peak-to-peak noise level. (A thorough discussion of the limit of detection and how it can be measured can be found elsewhere [9, 10].) During the most recent implementation of this laboratory students examined the following instruments: fluorescence spectrometer, GC–MS, differential scanning calorimeter, HPLC (UV detection), low-pressure LC (UV detection), UV–vis, FTIR, AA, and NMR.
Hazards
In-Class Discussion Period
Most serious chemical hazards can be minimized by the appropriate choice of test materials. There are important safety concerns when operating particular instruments and the instructions for these instruments should be well understood by the instructor and students. MSD sheets for all chemicals used should be read before experimentation.
The most important aspect of this exercise is the way in which the information gathered in the laboratory is disseminated to the rest of the class. Students compile all of their data in one setting and immediately begin to draw conclusions. This is done during the lecture portion of the course, where each student explains what he or she did and reports the limit of detection for the analyte measured. Students are prompted by the instructor to compare the limits of detection by constructing a table of values from the reported data on the board and in their notes. They are also asked, for example, to determine whether their instrumental technique could be used to run complex mixtures without prior separation or to analyze a large number of samples in a short period of time. Data such as this are added to the table to formulate a more complete picture. During this period students begin to compare the applicability of the various techniques with an eye toward “real-life” variables such as time and sampling limitations. The discussion takes about a hour of lecture time. A final report turned in by each student details his or her experiment and contains all of the data and observations.
Laboratory Procedure Students must choose a sample to test before operating their instrument. An attractive feature of this experiment is that when students pick their analytes they take ownership of the results to a greater extent than in a typical laboratory experiment. They are instructed to choose a chemical that will be simple to measure, pure, readily available, and safe to use. All test analytes are subject to final approval by the instructor. Examples of compounds that have been analyzed by students in this lab and the corresponding instrumental technique are ethylbenzene (NMR), decane (GC–MS), methylene blue (UV–vis), polystyrene (FTIR), and fluorescein (fluorescence). If a chromatographic technique is used, the students are instructed to look up a method for the separation and prepare mobile phase solutions if necessary. It is best if students do not perform a separation by running pure samples or a sample comprising a minimal number of components that can be quickly and easily separated. After samples are prepared, each student is assigned a time to work individually with the instructor to learn how to correctly and safely operate the designated instrument. The class in which this approach has been used has two 3-hour laboratory sections of 5–7 students each. The instructor must
Practical Aspects Since the laboratory takes place in the beginning weeks of the course, the students do not have detailed knowledge about the theory behind most instrumental methods. This is not problematic because as a part of their previous classes and labs they will have learned about or used IR, NMR, UV– vis, or GC for practical reasons. With the data in hand, things such as an in-depth analysis of why the detection limits for various instruments are vastly different can be discussed later
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in the course within a framework already established by the exercise. Much of this exercise depends on the cooperation of the students. An instructor has only some control over the quality of effort they put forth, and this aspect must be considered when implementing the lab.
If there are too few students then only a minimal number of instruments could be tested each year; if too many, students would be forced to work in groups. Neither condition would necessarily preclude the implementation of the experiment.
Variations
A new approach in beginning a laboratory for an upperlevel analytical chemistry class is presented. In it, students perform relatively simple experiments to learn general information about several techniques. The exercise is exploratory. Students first derive questions and answers about the instrument they will use and then perform an experiment to measure the limit of detection. A key aspect is that students meet as a class after the experiment so that the strengths and weaknesses of each instrument can be compared as a starting point for the rest of the course.
There are several ways in which this laboratory can be modified to serve different learning purposes or overcome practical limitations. One is to have a student compare two instruments of the same type but different design. A good example would be to compare a scanning dual-beam UV–vis spectrophotometer to a diode array instrument in terms of speed, precision, and limit of detection. This type of experiment was carried out by one student in the class, and the results were discussed in context with the rest of the experiments. Another aspect that can be studied is the trade-offs between resolution, bandwidth, and measurement time on a particular instrument. This type of experiment is ideal for spectroscopic instruments such as scanning UV–vis, FTIR, and fluorescence spectrometers. For instance, a student measures the amount of time it takes to acquire spectra at different resolution settings on an FTIR. Another student could examine the signal-to-noise ratio for each resolution. A final variation that has been attempted is to use different compounds. Comparisons between results obtained by a single instrument measuring different compounds would be interesting and could be expanded year to year as students continue to test different species. A large number of complex instruments is not necessary, since the experiment can be carried out using virtually any instrument in the alternative ways described above. The size of the class may require some modification of the procedure.
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Conclusions
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Mabrouk, P. A. J. Chem. Educ. 1998, 75, 527–529. Hope, W. W.; Johnson, L. P. Anal. Chem. 2000, 72, 460A–467A. Thorpe, T. M.; Ulman, A. H. Anal. Chem. 1996, 68, 477A. Hughes, K. D. Anal. Chem. 1993, 65, 883A–889A. NSF Instrumentation and Laboratory Improvement Grants in Chemistry; J. Chem. Educ. 1997, 74, 29–32. Fitch, A.; Yunlong, W.; Mellican, S.; Macha, S. Anal. Chem. 1996, 68, 727A–731A. Settle, F. A. Handbook of Instrumental Techniques for Analytical Chemistry; Prentice Hall: Upper Saddle River, NJ, 1997. Enke, C. G. The Art and Science of Chemical Analysis; Wiley: New York, 2001. Ingle, J. D. J. Chem. Educ. 1974, 51, 100–105. Rubinson, K. A.; Rubinson, J. F. Contemporary Instrumental Analysis; Prentice Hall: Upper Saddle River, NJ, 2000.
Journal of Chemical Education • Vol. 79 No. 5 May 2002 • JChemEd.chem.wisc.edu
