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12.6 Introduction to Mass Spectrometry

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12.6 Introduction to Mass Spectrometry 12_BRCLoudon_pgs4-4.qxd 11/26/08 9:01 AM Page 558 558 CHAPTER 12 • INTRODUCTION TO SPECTROSCOPY. INFRARED SPECTROSCOPY AND MASS SPECTROMETRY place to mount the sample in the infrared beam, and the optics and electronics necessary to measure the intensity of light absorbed or transmitted as a function of wavelength or wavenum- ber. Modern IR spectrometers, called Fourier-transform infrared spectrometers (FTIR spec- trometers), can provide an IR spectrum in a few seconds. (See Further Exploration 12.2.) The Further Exploration 12.2 FTIR Spectroscopy IR spectra in this text were obtained with an FTIR spectrometer. The sample containers (“sample cells”) used in IR spectroscopy must be made of an in- frared-transparent material. Because glass absorbs infrared radiation, it cannot be used. The conventional material used for sample cells is sodium chloride. The IR spectrum of an undi- luted (“neat”) liquid can be obtained by compressing the liquid between two optically pol- ished salt plates. If the sample is a solid, the finely ground solid can be dispersed (“mulled”) in a mineral oil and the dispersion compressed between salt plates. Alternatively, a solid can be co-fused (melted) with KBr, another IR-transparent material, to form a clear pellet. Simple presses are available to prepare KBr pellets. IR spectra can also be taken in solution cells, which consist of sodium chloride plates in appropriate holders equipped with syringe fittings for injecting the solution. When mineral-oil dispersions or solvents are used, we have to be aware of the regions in which the oil or the solvents themselves absorb IR radiation, because these absorptions inter- fere with those of the sample. A number of solvents are commonly used; chloroform (CHCl3), its isotopic analog (CDCl3), and methylene chloride (CH2Cl2) are among them. As a few stu- dents learn the hard way, solvents that dissolve sodium chloride, such as water and alcohols, cannot be used. 12.6 INTRODUCTION TO MASS SPECTROMETRY In contrast to other spectroscopic techniques, mass spectrometry does not involve the absorption of electromagnetic radiation, but operates on a completely different principle. As the name implies, mass spectrometry is used to determine molecular masses, and it is the most important technique used for this purpose. It also has some use in determining molecular structure. A. Electron-Impact Mass Spectra The instrument used to obtain a mass spectrum is called a mass spectrometer. In one type of instrument, an electron-impact mass spectrometer,acompoundisvaporizedinavacuum and bombarded with an electron beam of high energy—typically, 70 eV (electron-volts) (more 1 than 6700 kJ mol_ ). Because this energy is much greater than the bond energies of chemical bonds, some fairly drastic things happen when a molecule is subjected to such conditions. One thing that happens is that an electron is ejected from the molecule. For example, if methane is treated in this manner, it loses an electron from one of the C H bonds. 1 1 L H H HHC e_ HHC| 2e_ (12.15) 3 H1 3 + 3 H1 8 + The product of this reaction is sometimes abbreviated as follows: H1 HC| H abbreviated as CH4| 3 H1 8 8 The symbol | means that the molecule is both a radical (a species with an unpaired electron) and a cation—a8 radical cation. The species CH4| is called the methane radical cation. 8 12_BRCLoudon_pgs4-4.qxd 11/26/08 9:01 AM Page 559 12.6 INTRODUCTION TO MASS SPECTROMETRY 559 Following its formation, the methane radical cation decomposes in a series of reactions called fragmentation reactions. In a fragmentation reaction, a radical cation literally comes apart. The ionic product of the fragmentation (whether it is a cation or a radical cation) is called a fragment ion. For example, in one fragmentation reaction, it loses a hydrogen atom (the radical) to generate the methyl cation, a carbocation. (12.16) CH4| |CH3 H mass 8 16 + 8 = methyl cation mass 15 = Notice that the hydrogen atom carries the unpaired electron, and the methyl cation carries the charge; consequently, the methyl cation is the fragment ion in this case. The process can be represented with the free-radical (fishhook) arrow formalism as follows: H H HH"C|H H "C| (12.17) L 8 L + 8 "H "H Alternatively, the unpaired electron may remain associated with the carbon atom; in this case, the products of the fragmentation are a methyl radical and a proton. (12.18) CH4| CH3 H| mass 8 168 + mass 1 = methyl radical = In this case the proton is the fragment ion. Further decomposition reactions give fragments of progressively smaller mass. (Show how these occur by using the fishhook notation.) |CH3 |CH2 H (12.19a) mass8 14+ 8 = |CH2 |CH H (12.19b) 8 mass 13+ 8 = |CH C| H (12.19c) mass 8 12+ 8 = The ions formed in Eqs. 12.16 and 12.19a–c are very unstable species. They are not the sorts of species that would be involved as reactive intermediates in a solution reaction. Recall from Sec. 9.6, for example, that methyl and primary carbocations are never formed as intermediates in SN1 reac- tions. These ions are formed in the mass spectrometer only because of the immense energy imparted to the methane molecules by the bombarding electron beam. Thus, methane undergoes fragmentation in the mass spectrometer to give several positively , charged fragment ions of differing mass: CH4| |CH3, |CH2, |CH, C|, and H|. In the mass spectrometer, the fragment ions are separated 8according8 to their mass-to-charge8 ratio, mÜz (m mass, z the charge of the fragment). Because most ions formed in the electron-impact mass= spectrometer= have unit charge, the mÜz value can generally be taken as the mass of the ion. A mass spectrum is a graph of the relative amount of each ion (called the relative abun- dance) as a function of the ionic mass (or mÜz). When the ions are produced by electron im- pact, the mass spectrum is called an EI mass spectrum.The EI mass spectrum of methane is shown in Fig. 12.14 on p. 560. Note that only ions are detected by the mass spectrometer— neutral molecules and radicals do not appear as peaks in the mass spectrum. The mass spec- trum of methane shows peaks at mÜz 16, 15, 14, 13, 12, and 1, corresponding to the various = 12_BRCLoudon_pgs4-4.qxd 11/26/08 9:01 AM Page 560 560 CHAPTER 12 • INTRODUCTION TO SPECTROSCOPY. INFRARED SPECTROSCOPY AND MASS SPECTROMETRY 100 m/z = 16 relative m/z abundance 80 1 3.36 12 2.80 60 13 8.09 14 16.10 40 15 85.90 16 100.00 (base peak) 20 relative abundance 17 1.17 0 0 10 20 mass-to-charge ratio m/z Figure 12.14 The EI mass spectrum of methane. Can you explain why there is an ion at mÜz 17? (See Sec. 12.6B for the answer.) = ionic species that are produced from methane by electron ejection and fragmentation, as shown in Eqs. 12.16–12.19. The mass spectrum can be determined for any molecule that can be vaporized in a high vac- uum, and this includes most organic compounds. (Other techniques for vaporizing less volatile molecules have been developed and are discussed briefly in Sec. 12.6E.) Mass spec- trometry is used for three purposes: (a) to determine the molecular mass of an unknown com- pound, (b) to determine the structure (or a partial structure) of an unknown compound by an analysis of the fragment ions in the spectrum, and (c) to confirm the structures of compounds with known or suspected structures. The ion derived from electron ejection before any fragmentation takes place is known as the molecular ion, abbreviated M. The molecular ion occurs at an mÜzvalueequaltothemolecular mass of the sample molecule.Thus,inthemassspectrumofmethane,themolecularionoccursat mÜz 16. In the mass spectrum of decane (see Fig. 12.15), the molecular ion occurs at mÜz 142.= Except for peaks due to isotopes, discussed in the next section, the molecular ion peak is the= peak of highest mÜz in any ordinary mass spectrum. The base peak is the ion of greatest relative abundance in the mass spectrum—that is, the ion with the largest peak. The base peak is arbitrarily assigned a relative abundance of 100%, and the other peaks in the mass spectrum are scaled relative to it. In the mass spectrum of methane, the base peak is the same as the molecular ion, but in the mass spectrum of decane, the base peak occurs at mÜz 43. In the decane spectrum and in most others, the molecular ion and the base peak are different.= B. Isotopic Peaks Look again at the mass spectrum of methane in Fig. 12.14. This mass spectrum shows a small but real peak at mÜz 17, a mass that is one unit higher than the molecular mass. This peak is called an M 1 peak,= because it occurs one mass unit higher than the molecular ion (M). This ion occurs+ because chemically pure methane is really a mixture of compounds contain- ing the various isotopes of carbon and hydrogen. 12 13 12 methane CH4, CH4, CDH3, and so on = mÜz 16 17 17 = The isotopes of several elements and their natural abundances are given in Table 12.3 on p. 562. 12_BRCLoudon_pgs4-4.qxd 11/26/08 9:01 AM Page 561 12.6 INTRODUCTION TO MASS SPECTROMETRY 561 100 base peak 43 80 57 CH3(CH2)8CH3 60 decane 40 20 71 relative abundance molecular ion (M) 85 142 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 mass-to-charge ratio m/z Figure 12.15 The EI mass spectrum of decane.
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