In the Laboratory
A Strategy for Incorporating Hands-On GC–MS into the General Chemistry Lecture and Laboratory Courses
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Perry C. Reeves* and Kim L. Pamplin Department of Chemistry, Abilene Christian University, Abilene, TX 79699; *
[email protected] Many colleges and universities, including numerous junior colleges, have acquired GC–MS systems (gas chromatograph coupled to a mass spectrometer) during the past few years. The attractiveness of the GC–MS is partially related to its relatively low purchase price, its ease of use and maintenance, and its versatility. While several laboratory experiments describing its use in the organic chemistry course have appeared in this Journal (1), the major use of the instrument continues to be in upper-level analytical chemistry courses (2). Thus a large percentage of the students who enroll in college chemistry courses have little or no contact with this instrument. This is unfortunate, since its use is becoming routine in environmental, chemical, biomedical, and forensic laboratories. This paper describes strategies that we have used during the past two years to introduce students in the first-year chemistry course to the GC–MS and to provide them with hands-on experiences in its use. The Approach The atomic structure of matter is one of the earliest topics presented in the general chemistry course. Atomic number and mass number of the elements are defined in terms of the numbers of protons and neutrons present. Often instructors find that students can define the term “isotope”; however, when confronted with the use of the periodic table and atomic masses, they demonstrate a fundamental lack of understanding of the underlying concepts. Asked for the atomic mass of a single chlorine atom many students consult the periodic table and answer “35.453 amu”. Most general chemistry textbooks introduce the mass spectrometer to demonstrate that elements often exist as more than one isotope. One classroom (or laboratory lecture) period should be devoted to describing the essential components of a GC–MS, particular emphasis being given to the parts of the mass spectrometer. Gas chromatography will be explained in more detail in the organic chemistry course. Although most common GC–MS instruments utilize a quadrupole mass analyzer, we have found that the magnetic sector instrument is more appropriately described in the general chemistry course. Its method of separating ions by mass is more easily described and understood by a beginning student. Basic terminology and a discussion of the use of molecular ion peaks and fragmentation patterns in the identification of compounds are presented in this lecture. In addition, describing uses of GC–MS in biomedical, forensic, and environmental fields heightens student interest. Discussions of the quadrupole mass analyzer are best deferred until later analytical chemistry courses. Following the lecture presentation, teams of 2–3 students conduct the laboratory experiments. We have found it advantageous to prepare a videotape that illustrates the parts of the instrument and procedures for sample preparation, injection, and data collection. The students must view this videotape before they are permitted to use the instrument. The obvious 368
advantage of the videotape is that it details the operation of our particular instrument and students can view it repeatedly. An introductory experiment consists of the preparation and injection of a sample containing fluoro-, chloro-, bromo-, and iodobenzene. These compounds are chosen because each produces an intense molecular ion peak. At the conclusion of the chromatographic separation, a mass spectrum is obtained for each compound. Returning to the laboratory lecture room, students identify the molecular ion of each compound. They then are asked why fluorobenzene and iodobenzene produce only one peak in the molecular ion region whereas chlorobenzene and bromobenzene produce two peaks. This leads to a discussion of isotopic abundance and its relationship to the atomic mass of an element. Even though the mass spectra of the halobenzenes do not exhibit fragmentation peaks corresponding to the cation of the halogens (with the exception of iodine), the isotopic distribution can be inferred and the atomic masses of bromine and chlorine calculated from the relative abundances of the M (molecular ion) and M+2 peaks (3). Table 1 contains one set of typical data collected by our students. For a recent lab section (8 teams), the average calculated atomic mass of Cl was 35.44 (s = 0.02, error = ᎑0.03%) and Br was 79.91 (s = 0.02, error = 0.01%). If the mass spectrometer is fitted with a solids injection probe, students can determine the isotopic distributions and atomic masses of various transition metals. The carbonyl compounds of several transition metals are commercially available as somewhat volatile solids [Cr(CO)6, Mo(CO)6, W(CO)6]. Despite their toxicity, the minute quantities used in the experiment minimize laboratory safety concerns. Introduction of these solids into the mass spectrometer yields spectra in which groups of peaks arising from the various isotopes of the metal atoms are clearly visible. These groups are characterized by successive loss of carbon monoxide ligands until a group of peaks representing the cations of the various metal isotopes is observed. Signal intensities of the peaks in the group of metal ions are used to calculate percent abundance of each isotope. The data obtained from our instrument for molybdenum compares favorably with the accepted values (Table 2). If a solids injection probe is not available, students can use the GC–MS to observe the isotopic distribution of iron by injecting a solution of iron pentacarbonyl (liquid, bp 103 °C) in dichloromethane. The iron pentacarbonyl passes through the gas chromatographic column and into the mass spectrometer, where it fragments in the same manner as the metal hexacarbonyl compounds (Table 3). Attempts to volatilize the solid metal carbonyls similarly were unsuccessful. Table 4 lists the calculated atomic masses of the metals studied in this experiment together with the accepted values. Data from iron pentacarbonyl from a recent lab section of five teams resulted in an average atomic mass for iron of 55.86 (s = 0.001, error = 0.03%)
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
In the Laboratory Table 1. Isotopic Distribution and Atomic Mass of Chlorine and Bromine Abundance (%) Atomic Mass/amu Isotope Halogen No. Exptl Accepted Calcd Accepted Chlorine
Bromine
35
75.8
75.5
37
24.2
24.5
79
50.7
50.5
81
49.3
49.5
Table 2. Isotopic Abundance of Molybdenum Abundance (%)
35.47
35.45
79.98
79.90
Table 3. Isotopic Abundance of Iron Abundance (%)
Isotope No.
Exptl
Accepted
Isotope No.
92
15.2
14.8
54
5.4
5.8
94
9.4
9.3
56
92.7
91.7
95
16.1
15.9
57
1.7
2.2
96
16.6
16.7
58
0.2
0.3
97
9.8
9.6
98
23.5
24.1
100
9.4
9.6
Exptl
Accepted
Table 4. Atomic Masses of Some Metals from Mass Spectrometry Data Metal Chromium
Atomic Mass/amu Exptl
Accepted
Error (%)
52.15
52.01
0.27
Iron
55.81
55.85
0.06
Molybdenum
95.88
95.94
0.06
183.80
183.85
0.03
Tungsten
In summary, this approach introduces students in the general chemistry course to an important piece of scientific equipment, the GC–MS, and utilizes it to clarify the concept of isotopic distribution and its relationship to the atomic mass of an element. Since some institutions may not have instruments equipped with a solids injection probe, it should be noted that the mass spectra of these compounds are available from the NIST library that is installed on many GC–MS systems. For those institutions not having access to a GC–MS system, the mass spectra are available via the Internet (5). The Laboratory Experiment
The Instrument A Shimadzu QP-5000 GC-MS equipped with an autosampler for liquid samples and a solids injection probe is used. The GC column (0.25 mm i.d.) is a 30-m Restek XTI-5 (poly 5% diphenyl/95% dimethylsiloxane) column with a film thickness of 0.25 µm. Electron impact ionization (70 eV) is used to generate ions for the quadrupole mass analyzer. The Conditions Chromatographic runs are conducted isothermally (oven temperature 100 °C for halobenzenes, 50 °C for iron
pentacarbonyl) with injection and interface temperatures of 280 °C. Column flow is 0.8 mL/min with a split ratio of 138 and a total flow of 112 mL/min. The detector is turned off until the solvent has eluted from the column (1.65 min). Hazards All solutions should be prepared in an operating hood. The iron pentacarbonyl solution should be prepared by the lab instructor or teaching assistant. Other metal carbonyls should be loaded onto the direct-insertion probe in the hood and transferred directly from the hood to the spectrometer in a well-ventilated area. Disposable pipets used to handle solutions should be considered hazardous waste and disposed properly. Laboratory Management Plan At our university, laboratory sections in the first semester of general chemistry usually have 20–25 students (the total enrollment is 120–130). With this number of students and a single instrument, a plan is essential for orderly, efficient use of the instrument and the students’ time (4 ). Before the experiment is conducted, students receive a handout containing a detailed description of the theory and procedure of the experiment. Students sign up for a 15-minute time slot during the following week’s lab period in which to conduct the experiment. They arrive before their scheduled time to view the short videotape (discussed earlier) and to prepare their sample. The sample is prepared by adding, by disposable pipet, one drop of each halobenzene to approximately 2 mL of dichloromethane (CH2Cl2), which is contained in a sample vial compatible with the GC’s autosampler. The instructor should prepare a sample of iron pentacarbonyl by injecting 15 µL of the carbonyl into 1 mL of CH2Cl2 in one of the sample vials. A teaching assistant is present to guide the students through the data entry and acquisition process. The chromatogram is acquired isothermally and requires 5 min to complete. After the chromatographic run, the computer identifies chromatographic peaks and prints the chromatogram, the mass spectrum for each peak, and the peak report identifying the relative signal intensity attributed to each ion. A worksheet in the handout guides students through calculations leading to the atomic mass of the halides. Students’ results are then compared to accepted values. Mass spectra of the solid metal hexacarbonyls can be obtained during a portion of the next laboratory period. Once again students reserve a 15-min time slot. When they arrive at the instrument, the laboratory instructor injects a sample of one of the metal carbonyls using the solids insertion probe. Since the metal carbonyls are volatile, data can be collected rapidly. After the experimental data are processed, students calculate the experimental value of the atomic mass of the metal and the percent error. This leaves ample time for conducting another experiment during the same laboratory period. W
Supplemental Material
Mass spectral data and equations necessary for calculation of atomic masses are available as supplemental material in this issue of JCE Online.
JChemEd.chem.wisc.edu • Vol. 78 No. 3 March 2001 • Journal of Chemical Education
369
In the Laboratory
Acknowledgments Support for this project was provided by the Abilene Christian University Research Council and the National Science Foundation ILI Program (Grant Number DUE9750833). Literature Cited 1. Some representative organic chemistry experiments utilizing GC–MS: Novak, M.; Heinrich, J. J. Chem. Educ. 1993, 70, A150. Rowland, A. T. J. Chem. Educ. 1995, 72, A160. McGoran, E. C.; Melton, C.; Taitch, D. J. Chem. Educ. 1996, 73, 88. Pelter, M. W.; Macudzinski, R. M. J. Chem. Educ. 1999, 76, 826. 2. Some representative analytical experiments utilizing GC–MS:
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Mabbott, G. A. J. Chem. Educ. 1990, 67, 441. Hamann, C. S.; Myers, D. P.; Rittle, K. J.; Wirth, E. F.; Moe, O. A. J. Chem. Educ. 1991, 68, 438. Quach, D. T.; Ciszkowski, N. A.; Finlayson-Pitts, B. J. J. Chem. Educ. 1998, 75, 1595. 3. A related approach was suggested previously: O’Malley, R. M. J. Chem. Educ. 1982, 59, 1073. O’Malley, R. M.; Lin, H. C. J. Chem. Educ. 1999, 76, 1547. 4. Related approaches for managing GC–MS hands-on laboratories: Asleson, G. L.; Doig, M. T.; Heldrich, F. J. J. Chem. Educ. 1993, 70, A290. Illies, A.; Shevlin, P. B.; Childers, G.; Peschke, M.; Tsai, J. J. Chem. Educ. 1995, 72, 717. 5. National Institute of Standards and Technology. Chemistry WebBook; National Institute of Standards and Technology: Gaithersburg, MD; http://webbook.nist.gov/chemistry/name-ser.htm (accessed Oct 2000).
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
