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Andrea Twiss-Brooks John Crerar Library University of Chicago Chicago, IL 60637
Introducing Students to the Scientific Literature An Integrative Exercise in Quantitative Analysis Lee Roecker Department of Chemistry, Berea College, Berea, KY 40404;
[email protected] Several suggestions to help students read and break down the complexities of a scientific article have been presented in this Journal and elsewhere (1–7). These papers illustrate a variety of ways in which the scientific literature can be used. In addition to presenting an outlining strategy to assist students in reading scientific articles (1), these papers illustrate the use of the primary literature as the basis of student papers that are used to satisfy college-wide writing requirements (2, 3), as a way to enhance laboratory reports (4), and as a springboard for classroom discussion (5–7). Many of the activities characterized in refs 2–7 involve student selection of articles and extensive outlining. This paper describes another use of the primary literature in the classroom through assignments in quantitative analysis. In these assignments, students are provided articles that have some relevance to analytical chemistry. They use the articles to answer a series of questions designed to help students understand the main point of the paper and of the figures within it. In addition, the exercise provides students with practice in interpreting statistical data, using spreadsheets, and critically evaluating scientific evidence, as well as gaining insight into the variety of work that analytical chemists do. Extensive outlining of the article is not required and the production of a student paper is not an immediate goal of these assignments. Two examples of these article assignments are provided in this paper. Article Selection Criteria Over the course of the semester in a quantitative analysis class students are asked to answer questions concerning four, pre-selected articles that relate to analytical chemistry. Articles are selected based on the three criteria described below. 1. Articles should be fairly short, well written, and concerning a topic that has the potential to readily engage students. For many students, these papers will be the first scientific papers that they have read. If the article is long and does not focus on a single, welldefined topic, inexperienced students are easily overwhelmed and resort to copious highlighting in a futile attempt to make sense of what they are reading. Care must be taken to ensure that students can comprehend a significant portion of the article given their limited chemistry background. Holliday has summarized useful guidelines for effectively selecting appropriate reading material (8).
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2. Articles must contain statistical information. These assignments are used to provide students with a semesterlong review on the use and understanding of statistical data in analytical chemistry. The “technical papers” section in Analytical Chemistry is a good source of articles. These articles often focus on a single technique and demonstrate its effectiveness by statistical comparison to standard samples or alternate methods of analysis. 3. Articles should have a theme not usually addressed elsewhere in the curriculum. For example, through articles based on the primary literature students can be introduced to challenges in the environmental, nuclear, forensic, or archeological fields. Papers that address areas of chemistry that students are not familiar with help to illustrate the variety and breadth of work performed by analytical chemists.
Sample Assignments Assignments are designed with several goals in mind. In a general way, the assignments assist students to become acquainted with the structure of a scientific article: the relationship between figures and text, the statistical information that can be found in tables and in the experimental section, the use of equations and how previous work is referenced. The questions in the assignment help guide students to understand the main point of the paper. Regarding quantitative analysis, these exercises provide additional practice at interpreting statistical information and using spreadsheets to generate and model statistical data. When given the assignments, students are instructed to write no more than can be contained in the space after the question. If they need more room, students are told that they are not getting to the point directly enough. Students use Excel or similar programs to plot normal error curves generated from means and standard deviations provided in the article. Along with the written responses to the assignment questions, students also turn in well-labeled plots of normal error curves, the data generated to ±4 s about the mean and formula sheets—each scaled to fit onto a single sheet of paper. The article assignments comprise a total of 10% of the course grade. The first article, “Greenland Ice Evidence of Hemispheric Lead Pollution Two Millennia Ago by Greek and Roman Civilizations”, presents a lead analysis of an ice core sample from Greenland (9). The authors describe decontamination of the ice cores by a mechanical chiseling process to remove
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lead introduced during sample acquisition and they provide details of sample analysis by graphite furnace atomic absorption spectroscopy. Core samples dating from 500 B.C.E. to 300 C.E. showed elevated lead concentrations that the authors attributed to tropospheric pollution caused by mining and smelting operations of Greek and Roman civilizations. The assignment for this article (Figure 1) is introduced early in the term in the context of examining the textbooks’ presentation of statistics in chemistry. Students find this article to be engaging for several reasons. The environmental theme attracts some students, while others are curious about the archeological thread and its relevance to ancient civilizations that they have studied in other courses. Being short and well written, it is a good introduction to the primary literature. Relative to quantitative analysis, it provides a practical example of detecting and eliminating determinate error from an analysis. The next sample assignment based on the article “δ 13C Analysis of Cholesterol Preserved in Archaeological Bones and Teeth” introduces students to gas chromatography and mass spectrometry (10). (See Figure 2.) In this article, a semiautomated procedure for the analysis of cholesterol extracted from human femurs is described. Femurs of 50 individuals recovered from old cemeteries along the coastline of the United Kingdom were analyzed in hopes of obtaining paleodietary information. The isolated cholesterol was derivatized to a trimethylsilyl ester and purified by gas chromatography. After oxidation of the purified cholesterol to carbon dioxide and analysis by mass spectrometry, it was found that the samples were enriched in 13C. Enrichment of 13C suggests the population had a preference for a marine diet, since marine diets are enriched in 13C in contrast to terrestrial diets, which are depleted in 13C. The article is longer and more detailed than the first example and it might not be good to use as the first literature assignment in quantitative analysis. Additional guidance is provided in a single-page handout that presents an overview of the experiment. Similar to the other literature-based assignments used during the term, this assignment provides students with a good review of statistics. Student Benefits from the Assignments
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1. What is the major source of determinate error that the authors attempted to correct in their determination of Pb in the ice cores? Describe how they accounted for it. 2. The authors use a concentration unit of pg/g (picogram/gram). How many ppm (parts per million) is 1 pg/g? Show your work. 3. The open circles in the figure below reproduce Figure 1C from the paper. Suppose that the plot of the data actually appeared like that illustrated by the solid squares in the figure below. How would this affect the interpretation of Figure 1B? Another way to think about this would be to ask why the authors thought it was important to include Figure 1C in the paper.
4. (a) What is the relative standard deviation of the results in Table 1? What is the absolute error of each result for the inner core? Provide your answers below for the following depths.
129.5
3.90
349.25
0.66
399.3
1.17
• Illustrating the variety of work in which an analytical chemist might be engaged
For instance, the two example assignments outlined here introduce students to analytical chemistry in environmental and archeological contexts. As detailed next, undertaking these assignments can benefit students in four main ways. www.JCE.DivCHED.org
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Hong, S.; Candelone, J.; Patterson, C. C.; Boutron, C. F. Greenland Ice Evidence of Hemispheric Lead Pollution Two Millennia Ago by Greek and Roman Civilizations. Science 1994, 265, 1841–1843.
• Introducing students to technical articles within the primary literature
• Allowing students to see specific applications of analytical chemistry
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Refer to the article by Hong et al. to answer these questions.
Depth, m
• Providing students with opportunities to become more proficient using statistics and computers
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Article Analysis Exercise 1
This series of assignments is designed to address several educational goals, including:
• Enhancing their ability to understand and explain the main point of a paper or specific figures within a paper
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Pb, pg/g
Relative Error
Absolute Error
(b) Prepare and overlay normal error curves for the data from 470 years B.P. and 1775 years B.P. Calculate and plot points to ±4 s for each, with a point every 0.10 pg/g. (c) Do the results for 470 and 1775 years B.P. differ at a 90% confidence level? Be sure to show your work. Usually when reporting a result from atomic absorption spectroscopy an average of at least five readings is reported. Assume n ⫽ 5 in your calculation. Figure 1. Sample assignment with adapted article figure, guided questions, and data table. N.B.: spacing condensed from the original.
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Learning the Role of Statistical Analysis in Chemistry Students see that what they learn during the first weeks of studying quantitative analysis has a “real life” application— chemists use statistical analysis in the course of their work. By having each article assignment refer to and use statistical information, students are required to return to statistics throughout the semester. This is particularly beneficial to students who are not clear on the importance of statistics or who are having difficulty understanding statistical concepts. While they also receive repeated exposure to statistics in the course of analyzing laboratory data, the laboratory applications tend to be more narrowly focused compared to the article assignments.
Using Computerized Models and Spreadsheets The assignments provide repeated experiences for students to become more adept at using the computer to model data and to analyze results. Students plot normal error curves using data that they generate based on averages and standard deviations that are reported in the articles. While students are often accomplished at using technology to locate data, they are much less accomplished at generating and evaluating data on their own. Many students report that producing the normal error curves on the first assignment takes them several hours. This is in spite of having competed two prior computer-based homework exercises where they generated and plotted data from textbook problems. At this stage, students are particularly poor at troubleshooting problems in their spreadsheets. The fundamentals of using the computer to the students’ advantage are stressed throughout the assignment—work the essence of the problem out on paper first, double-check the results of the first spreadsheet calculations using a calculator, and if difficulties arise, break down the problem into a series of smaller calculations. By the time they have completed the second article assignment, most students have prepared a template that generates normal error curves after the input of a mean and standard deviation. While similar templates are readily available from many sources, students are not guided towards them. Troubleshooting problems in calculations and spreadsheet layout is preferable as a developmental tool.
Understanding the Breadth of Analytical Chemistry Students can be exposed to a variety of analytical chemistry topics through the literature. Articles can be chosen to illustrate the range of work that analytical chemists do, introduce areas of analytical chemistry not usually covered in the curriculum, strengthen areas of the curriculum already introduced, explore instrumental techniques, strengthen reading skills, or examine the writing conventions between different journals (7) or within a single journal. The bulk of the assignments described here are designed to strengthen student use and understanding of statistics in analytical chemistry. In addition, students are asked to draw upon their past experience (11). In the sample assignment that relates to the analysis of cholesterol in bones and teeth, for example, students are asked to sketch the structure of cholesterol in question 1a (Figure 2). Each time this article has been used, several students come for assistance because they 1382
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Article Analysis Exercise 2 Refer to Stott and Evershed’s article to answer these questions. Stott, A. W.; Evershed, R. P. δ13C Analysis of Cholesterol Preserved in Archaeological Bones and Teeth. Analytical Chemistry 1996, 68, 4402–4408. 1. (a) Sketch the structure of cholesterol. (b) Cholesterol was derivatized to a trimethylsilylester to make the molecule more volatile and hence easier to isolate by gas chromatography. Indicate on your sketch where this derivatization occurs. 2. The experimental section describes the use of “analytical blanks” every 10th tube during the analyses. What is the purpose of these “blanks”? 3. Figure 2b shows that the δ13C value was constant for a single femur as a function of the distance along the femur. If the plot appeared as follows, why would the differentiation between a marine and terrestrial diet have been more difficult to make? Another way to think about this might be to ask why the experiment summarized in Figure 2b was necessary.
4. Suppose we measure a sample and find 30 δOTMS to be ᎑692.2. Calculate the δ13C value for the underivatized cholesterol sample. Does this δ13C value indicate a terrestrial or a marine diet? 5. List two reasons why femurs and teeth are preferred sources of δ13C information. 6. Answer these questions based on the data in Table 1. (a) Section FB has a mean value of ᎑22.4. How many replicate measurements was this based on? How do you know? (b) Calculate the 50 and 95% confidence level for section FB. 7. Answer these questions based on the data in Table 2. (a) Using the mean δ13C values for SK1674 and SK1773 and their standard deviations: •
Prepare a normal error curve to ±4 s generating a point every 0.25 s. Overlay the two plots.
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Suppose we sample SK1674 a hundred times. How many times out of 100 would you expect a result between ᎑21.8 and ᎑23.7? How many times out of 100 would you expect a result more negative than ᎑23.2?
(b) Over what δ13C range are we 95% confident that the true value for SK1674 lies? Figure 2. Sample assignment with adapted article figure and questions about article’s tables. N.B.: spacing condensed from the original.
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do not know what cholesterol looks like. They are directed to look in their organic text or are introduced to the Merck Index (12). More students request help on question 1b, which asks for the site of derivatization on cholesterol. Students who claim that they have “no idea” where this reaction occurs are led through a series of questions: How does the experimental section describe this process? What kind of reaction is this? What is the general structure of an ester? Where can you make an ester attached to cholesterol? These types of questions give students a reason to draw on their previous chemistry knowledge and make connections among their various courses. Students seem surprised and pleased to discover that what they learned in organic chemistry might be important to analytical chemists.
Improving Science Writing Skills This series of assignments provides another way to help students improve their ability to read and understand technical material and subsequently become better at writing about science. By presenting them with relatively straightforward articles and guided questions that have a relationship to classroom topics, students gain confidence in reading primary literature and develop strategies that they can use to break down complicated material. Both sample assignments contain questions that help strengthen reading skills. In order to calculate the absolute error in the lead concentrations at various ice core depths (question 4a in the first sample assignment) required students to notice the statement in the paper: “Lead contents were determined with a precision of about ±10% by graphite furnace atomic absorption spectrometry”(9). To answer statistical questions in the second assignment students needed to find the number of replicate measurements in the data table where it was stated for the means reported for each femoral section that “n ⫽ 3” (10). While they work on the first assignment, many students ask questions such as, “How do I know what n is?” or “How do I find out what the standard deviation is?”. Even though they have completed similar problems from their textbook, students often do not know where to find this information in a paper and miss details like this on their first reading of the article. These simple questions decrease dramatically on the subsequent assignments as students discover where to look for experimental or statistical details. Not surprisingly, the questions that students have the most difficult time addressing are those that attempt to evaluate their understanding of the material. The third questions in both sample assignments ask students to consider the consequences if key figures in the papers were to appear differently. In the first sample assignment, evidence was offered in the paper that the increase in lead levels 2000 years B.P. in Greenland ice was due to increases in atmospheric lead. Since lead in the atmosphere would have fallen out of the atmosphere with precipitation and been trapped in the ice, the crustal enrichment factor (a ratio of the lead found in ice to the lead found in the soil) peaks 2000 years ago at the height of smelting operations in the Mediterranean region of Europe. If the crustal enrichment factor did not change over time, as suggested in the alternate version of the figure provided in the assignment, then the increased lead levels in the ice were merely a reflection of lead levels in the local soil. www.JCE.DivCHED.org
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Students have trouble clearly stating “the plot provided by the authors is needed to demonstrate that changing levels of lead in the ice that occurred during this time period were not due to crustal variations in lead levels” or “if the increased lead levels in the ice were mirrored by increased lead levels in the soil, the authors’ hypothesis that the lead in the Greenland ice originated from Greek or Roman civilizations would not be supported.” In the second sample assignment, the authors demonstrated that cholesterol content (as measured by δ13C) was consistent throughout the length of a human femur. In response to the hypothetical situation where the δ13C values changed throughout the length of a femur, many students don’t see that “if the δ13C values were not consistent along the length of a femur, then the researchers would need to take care that samples were removed from the same section of femur in each skeleton” or “if the authors did not demonstrate that the δ13C values were consistent along the length of a femur, then data collected from femur fragments would not be of value because it would be difficult to determine from which part of the femur they originated.” Student responses are often vague. In response to the question just discussed, for example, students have written short statements such as “The above graph has a greater difference in concentration, so this procedure couldn’t work”, and, “If this figure were obtained the data would tell nothing about dietary habits”. Many students will mention that the original experiment demonstrated the homogeneity of cholesterol in the femur, but never clearly describe why lack of homogeneity would be a problem—“The effect is that there is less homogeneity across the femur. This shows various amounts of cholesterol within the bone tested.” To assist students in writing concise and accurate responses it might be better to present this type of question in the early assignments in a multiple-choice format to illustrate appropriate responses. Providing students with this focused exposure to the primary literature will increase their confidence as they read other articles. The articles read over the course of the assignments also provide examples of how scientific papers are constructed. Specific questions in the assignment that relate to figures lead students back to the text and, hopefully, they begin to see that inclusion of figures in a paper is not arbitrary. Students observe how figures are incorporated into a manuscript and can emulate these examples in their own writing. The same can be said regarding experimental sections and the use of data tables in scientific papers. Course evaluations, exit interviews, and performance on examinations suggest that this multifaceted assignment is useful to student development. In the spring of 2005, article assignments were the only graded homework for the course. When responding to a prompt on the standard campus course evaluation form that stated, “The instructors’ assignments were helpful to my learning”, eight students “strongly agreed”, three students “agreed”, one student was “neutral” and one student “strongly disagreed”. Written comments regarding the article assignments included statements such as “[A]ssignments are extremely helpful when preparing for exams”, and “[T]aught me something practical for the ‘real world’ of chemistry”. One year, students were surveyed regarding their previous reading of the scientific literature. A large majority
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of students responded that they had read fewer than four scientific articles. The article assignments had often more than doubled their previous exposure to the literature. Finally, 20% of the points on the final course examination relate to statistics. Prior to using the article assignments, students often performed poorly on this section and these questions were often the ones missed most frequently. Now, because of the repetition provided by the article assignments, students rarely lose points on these questions.
Providing students with pre-screened articles that relate to a classroom topic helps them to see the relevance of classroom material to the scientific literature. In the series of assignments described here, the topic of statistics provides students with practice in manipulating and analyzing data throughout the semester. As side benefits of these assignments, students see the variety of subjects that one can investigate as an analytical chemist and begin the difficult process of learning to read and reflect upon the scientific literature. Placing this exercise in a mid-level course like quantitative analysis makes it a useful first component of a larger curricular package designed to strengthen reading and writing skills of students majoring in chemistry. Repetition of this assignment throughout all upper-level chemistry courses would create graduates who are more aware of the scientific literature and of the opportunities available to them after graduation.
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The author thanks Berea College colleagues Dawn Anderson in the Department of Biology and Paul Smithson in the Department of Chemistry for helpful discussions and comments in the preparation of this paper. In addition, interaction with many colleagues through the College-sponsored professional development program, Communication across the College, is gratefully acknowledged. Literature Cited
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1. Drake, B. D.; Acosta, G. M.; Smith, R. L. Jr. J. Chem. Educ. 1997, 74, 186–188. 2. Paulson, D. R. J. Chem. Educ. 2001, 78, 1047–1049. 3. Whelan, R. J.; Zare, R. N. J. Chem. Educ. 2003, 80, 904–906. 4. Tilstra. L. J. Chem. Educ. 2001, 78, 762–764. 5. Herman, C. J. Coll. Sci. Teach. 1999, 28, 252–253. 6. Levine, E. J. Coll. Sci. Teach. 2001, 31, 122–125. 7. Kuldell, N. J. Coll. Sci. Teach. 2003, 33, 32–35. 8. Holliday, W. G. J. Coll. Sci. Teach. 1992, 22, 58–60. 9. Hong, S.; Candelone, J.; Patterson, C. C.; Boutron, C. F. Science 1994, 265, 1841–1843. 10. Stott, A. W.; Evershed, R. P. Anal. Chem. 1996, 68, 4402–4408. 11. Rieck, D. F. J. Chem. Educ. 1998, 75, 850. 12. Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 14th ed. O’Neil, M. J., Heckelman, P. E., Koch, C. B., Roman, K. J., Eds.; Merck Publishing: Whitehouse Station, NJ, 2006.
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