Mass Spectrometry for the Masses

In the Laboratory

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Mass Spectrometry for the Masses Jared D. Persinger, Geoffrey C. Hoops, and Michael J. Samide* Department of Chemistry, Butler University, Indianapolis, IN 46208; *[email protected]

The importance of gas chromatography–mass spectrometry (GC–MS) to chemists cannot be denied; in fact, many universities now incorporate GC–MS into their undergraduate curriculum. However, existing laboratory experiments that involve GC–MS tend to be designed specifically for general and analytical chemistry courses. In these cases, students are expected to already have a minimal understanding of the instrumentation as well as the theory behind mass spectrometry and other advanced topics. Examples of published GC–MS experiments for use in the undergraduate analytical laboratory can be found that explore theoretical manipulation (1) and environmental testing (2–5). For example Gross and coworkers (1) illustrated the theory behind both gas chromatography and mass spectrometry through an analysis of a series of ketones. GuistoNorkus and coworkers (2) employed mass spectrometry to study plasticizers while Fleurat-Lessard (3) determined the quantity of polycyclic aromatic hydrocarbons in diesel exhaust. All of these laboratory experiments are beneficial to advanced students, but are not appropriate for those enrolled in lower-level, introductory courses. There are experiments geared towards introductory chemistry classes (6–10), but often times these still remain overly complex. Sadoski, Shipp, and Durham (6) examined metal complexes that form upon addition of trifluoropentanedione. Other published experiments incorporate mass spectrometry into the general chemistry curriculum or a course for nonscience majors by simplifying the procedure or by working with “real-world” samples. Schildcrout’s work (7) centers on the explanation of isotopes through a spectrometric study of bromobenzene. This procedure allows students to employ the GC–MS as a learning tool. O’Hara and

coworkers’ (8) examination of pesticides in drinking water incorporates the testing of samples that are of popular concern. These experiments are better suited for general chemistry courses, but they still involve complex analytes not familiar to nonscience majors. We have developed a simple, qualitative experiment for implementation into the laboratory curriculum of a chemistry course designed for nonscience majors. Though the GC– MS plays an integral role in this experiment, we feel that the average student in a nonmajor course is not interested in details related to instrumentation, but is interested in what information the tool can provide. In this experiment, various samples of air are collected and analyzed by GC–MS techniques. Before analysis, however, students use information from lecture, along with a common understanding of the environment, to formulate a hypothesis on the outcome of a chemical process. For example, a student might expect that the quantity of carbon dioxide in a person’s exhaled breath will be larger than that found in room air. After predictions have been made, students test their hypotheses by performing the reaction and examining the reaction products with a GC–MS. In all cases the students are looking for changes in the quantities of common components found in air (N2, O2, CO2, H2O, Ar). On the basis of these results, students are able to determine the validity of their hypotheses. Procedure Students perform reactions or collect air samples in sealable plastic bags, such as Ziploc-brand bags, which are then brought to the GC–MS for sampling and analysis. The chemical reactions used to produce these air samples are detailed in Table 1.

Table 1. Chemical Processes To Be Examined via GC–MS Analysis of Air Samples Process

Sample Preparation

Expected Outcome

Oxidation

Reaction of steel wool with oxygen in the presence of acetic acid to produce rust

Consumption of oxygen

Decomposition

Hydrogen peroxide catalyzed by yeast

Production of oxygen

Respiration

Collection of exhaled breath

Production of carbon dioxide and water and consumption of oxygen

Photosynthesis

Collection of air from a bag containing a live plant

Consumption of carbon dioxide and production of oxygen

Combustion

Collection from the tailpipe of a running automobile

Production of carbon dioxide and water and consumption of oxygen

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Vol. 81 No. 8 August 2004

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Journal of Chemical Education

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Relative Abundance (%)

In the Laboratory

28 100 75 50 32

25 14 18

40 44

0 10

20

30

40

50

60

70

80

m /z Figure 1. Typical GC–MS background (room) air analysis. Massto-charge ratios of interest are: 18 (H2O), 28 (N2), 32 (O2), 40 (Ar), and 44 (CO2).

To demonstrate proper use of the instrument, the instructor may perform an analysis of room air, which will be used as the background, or standard, condition. These results may be shared with the class to save time. Students, in small groups, then test their own samples and collect the data. Hazards The mass spectrometer makes use of high voltages. In addition, the gas chromatograph injector port is 200 ⬚C. Glacial acetic acid (17.5 M) is corrosive and any spills should be neutralized with a solution of sodium bicarbonate.

graphic conditions employed in this analysis. For certain samples, however, students may see two distinct peaks, depending on the chromatographic parameters used during the experiment. An example of this involves the elution of acetic acid vapors upon analysis of the oxidized steel wool sample. Students should be aware that the information they seek is contained with the first chromatographic peak that elutes from the instrument. For each sample analyzed by the GC–MS, data similar to that shown in Figure 1 is produced. Interpretation of the data is accomplished by dividing the peak intensity for the analyte of interest (H2O, O2, or CO2) by the peak intensity for a component of air that should remain unchanged (N2 or Ar). Typical intensity ratios are listed in Table 2. Once calculated, these ratios are compared to the corresponding intensity ratio from the room air and a change in composition is determined. The percent change along with the hypothesized results are listed in Table 3. percent change =

sample ratio − background ratio background ratio

× 100

In every case, the results from the mass spectrometric analysis support the hypotheses made for each process studied. Furthermore, some observations can be made for which no hypothesis was generated. For example, the high levels of carbon dioxide in the H2O2 decomposition sample (an increase of 1200%) come from the fermentation of the yeast. Summary

Data Analysis After injection of a sample into the GC, students may expect to see several peaks, which represent the various gases in their sample. This, however, is not the case; students see only one peak that consists of the components of interest. Molecules such as water, nitrogen, oxygen, and carbon dioxide along with elemental argon co-elute under the chromato-

This laboratory is well suited for an introductory chemistry course for many reasons. Concepts such as molecular weight calculations and the scientific method are reinforced, and the overall theme of the experiment is easily related to environmental issues. Furthermore, it incorporates the use of advanced instrumentation and gives students an idea of how “real-world” samples are analyzed.

Table 2. Experimentally Determined Intensity Ratios for Air Samples Collected Under a Variety of Conditions O2/N2

Sample

a

O2/Ar

CO2/N2

CO2/Ar

H2O/N2

H2O/Ar

Room

0.214 ± 0.003

14.4 ± 0.8

0.003 ± 0.001

0.23 ± 0.06

0.047 ± 0.003

3.2 ± 0.3

Oxidation of Iron

0.179 ± 0.001

12.1 ± 0.6

0.005 ± 0.001

0.350± 0.03

0.053 ± 0.001

3.6 ± 0.1

Decomposition of H2O2

1.97 ± 0.04

130 ± 20

0.045 ± 0.003

2.90± 0.3

0.078 ± 0.003

5.1 ± 0.5

Respiration

0.181 ± 0.001

11.6 ± 0.3

0.191 ± 0.006

12.2 ± 0.3

b

0.059 ± 0.008

3.7 ± 0.5

b

Photosynthesis

0.226 ± 0.001

16.1 ± 0.2

ND

ND

0.052 ± 0.001

3.7 ± 0.3

Combustion

0.054 ± 0.002

3.3 ± 0.1

0.57 ± 0.01

35 ± 1

0.087 ± 0.001

5.35 ± 0.07

a

Standard deviations are provided for each ratio (n = 3).

b

ND signifies that CO2 was not detected by the mass spectrometer. No peak was seen at a m/z of 44 for this sample.

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In the Laboratory

The experiment was also popular with students; not only did they appreciate being trusted with the use of the instrumentation, they found the sample preparation and collection to be a welcome relief in their day. This laboratory experiment is ideal for an introductory chemistry course as it provides the background desired while using ideas applicable to those outside of the research setting. W

Supplemental Material

Instructions for the students, a material and equipment list, and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Gross, Michael L.; Olsen, Virgil K.; Forcé, Ken R. J. Chem. Educ. 1975, 52, 535–537.

2. Guisto-Norkus, Rebecca; Gounili, Giv; Wisniecki, Peter; Hubball, John A.; Smith, S. Ruven; Stuart, James D. J. Chem. Educ. 1996, 73, 1176–1178. 3. Fleurat-Lessard, Paul; Pointet, Karine; Renou-Gonnord, MarieFrance. J. Chem. Educ. 1999, 76, 962–965. 4. Quach, Dinh T.; Ciszkowski, Nancy A.; Finlayson-Potts, Barbara J. J. Chem. Educ. 1998, 75, 1595–1598. 5. Wilson, Ruth I.; Mathers, Dan T.; Mabury, Scott A. J. Chem. Educ. 2000, 77, 1619–1620. 6. Sadoski, Robert C.; Shipp, David; Durham, Bill. J. Chem. Educ. 2001, 78, 665–666. 7. Schildcrout, Steven M. J. Chem. Educ. 2000, 77, 1433– 1434. 8. O’Hara, Patricia B.; Sanborn, Jon A. J. Chem. Educ. 1999, 76, 1673–1677. 9. Eichstadt, Karen E. J. Chem. Educ. 1992, 69, 48–51. 10. Reeves, Perry C.; Pamplin, Kim L. J. Chem. Educ. 2001, 78, 368–370.

Table 3. Percentage Change in Sample Air versus Room Air Sample

Hypotheses

Oxidation of iron

Consumption of O2

O2/N2 O2/Ar

᎑16.4 ᎑16.0

Consistent with hypothesis

Decomposition of H2O2

Production of O2

O2/N2 O2/Ar

821 800

Consistent with hypothesis

Respiration

Consumption of O2

O2/N2 O2/Ar

᎑15.4 ᎑19.4

All consistent with hypotheses

Production of H2O

H2O/N2 H2O/Ar

25 16

Production of CO2

CO2/N2 CO2/Ar

6000 5140

Production of O2

O2/N2 O2/Ar

Consumption of CO2

CO2/N2 CO2/Ar

᎑100 ᎑100

Production of CO2

CO2/N2 CO2/Ar

20,000 15,000

Consumption of O2

O2/N2 O2/Ar

Production of H2O

H2O/N2 H2O/Ar

Photosynthesis

Combustion

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Ratio Examined

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Change (%)

5.61 11.80

Remarks



᎑75 ᎑77 85 67

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Mass Spectrometry for the Masses

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