The Mass Spectrum

Mass spectra (El) are routinely obtained at an electron beam energy of 70 eV. The simplest event that occurs is the removal of a single electron from the molecule in the gas phase by an electron of the electron beam to form the molecular ion, which is a radical cation (M +). For example, methanol forms a molecular ion in which the single dot represents the remaining odd electron  

When the charge can be localized on one particular atom, the charge is shown on that atom  

If some of the molecular ions remain intact long enough to reach the detector, we see a molecular ion peak. It is important to recognize the molecular ion peak because this gives the molecular weight of the 

The formation of a simple El mass spectrum from a number (p) of molecules (M) interacting with electrons (ep. Peak 1 represents M , the molecular ion, the ion of greatest mass (abundance q). Peaks 2, 3 represent A+, B. two fragment ions (abundances r, s). Peak 2 is also the largest and, therefore, the base peak. 

In a typical mass spectrometer, an organic compound under high vacuum is bombarded with electrons (of about 70 eV energy). Loss of an electron from the molecule followed by various fission processes gives rise to ions and neutral fragments. The positive ions are expelled from the ionisation chamber and resolved by means of a magnetic or an electric field. 

Instruments vary considerably in the extent to which they can separate ions of closely related m/z values. In the vast majority of routine uses the organic chemist requires only the separation of ions having nominal unit masses of up to molecular weights of about 500-600, which can be achieved using an instrument of low resolution. Occasionally, however, it is of value to determine the precise 

Extensive compilations of such data are available they can easily be modified to include elements other than C, H, O and N. 

One limitation on the use of isotope peak intensities to determine the molecular formula is that the molecular ion must be relatively intense, otherwise the isotope peaks will be too weak to be measured with the necessary accuracy. Difficulty may also arise from spurious contributions to the isotope peak intensities from the protonated molecular ion, from weak background peaks or from impurities in the sample. In any event the method is only reliable for molecules having molecular weights up to about 250-300. 

Members of the last group, and especially chlorine and bromine, are most easily recognised from the characteristic patterns of the peaks, spaced at intervals of two mass units, which they produce in the spectrum. Typical patterns for combinations of bromine and chlorine atoms are shown in Figs 3.77 and 3.78. It may be difficult to estimate the number of oxygen atoms due to the low natural abundance (0.20%) of lsO. 

The most abundant ion formed in the ionization chamber gives rise to the taUest peak in the mass spectrum, called the base peak. In the mass spectrum of dopamine, the base peak is indicated at an mk value of 124. The relative abundances of all the other peaks in the spectrum are reported as percentages of the abundance of the base peak. 

We have seen that the beam of electrons in the ionization chamber can produce the molecular ion. This beam is also sufficiently powerful to break some of the bonds in the molecule, producing a series of molecular fragments. The positively charged fragments are also accelerated in the ionization 

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Knowing and 2 by studying reference components, it becomes possible to calculate from the measurements of ion intensities found on the mass spectrum. 

In the case of mixtures, especially those of petroleum, a variety of compounds can give ions having the same mass the mass spectrum is then the sum of the spectra of each component ... 

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

PA, B2, the reactions leading to B2H can be monitored by the presence of this latter ion in the mass spectrum. The PA of B2 can quickly be narrowed down provided the reference values are well established. 

The amino add analysis of all peptide chains on the resins indicated a ratio of Pro Val 6.6 6.0 (calcd. 6 6). The peptides were then cleaved from the resin with 30% HBr in acetic acid and chromatogra phed on sephadex LH-20 in 0.001 M HCl. 335 mg dodecapeptide was isolated. Hydrolysis followed by quantitative amino acid analysis gave a ratio of Pro Val - 6.0 5.6 (calcd. 6 6). Cycll2ation in DMF with Woodward s reagent K (see scheme below) yielded after purification 138 mg of needles of the desired cyc-lododecapeptide with one equiv of acetic add. The compound yielded a yellow adduct with potassium picrate, and here an analytically more acceptable ratio Pro Val of 1.03 1.00 (calcd. 1 1) was found. The mass spectrum contained a molecular ion peak. No other spectral measurements (lack of ORD, NMR) have been reported. For a thirty-six step synthesis in which each step may cause side-reaaions the characterization of the final product should, of course, be more elaborate. 

During the course of biochemical studies (138). the mass spectrum of 2-acetamidothiazole was recorded its main peaks are the molecular ion (m/e= 142, relative intensity = 26%) and fragments 100 (100), 58 (2. 5), and 43 (39). For 2-acetamido-5-bromothiazole the main peak results again from the loss of C2H2O by the molecular ion. 2-AcetyIacet-amido-4-methylthiazole (2S) exhibits significant loss of from the... 

A protomeric equilibrium favors the acetamido rather than the acetimido form (105, 121). The parent molecular ion has been reported to be absent in the mass spectrum of 2-acylaminothiazoles (130). 

The base peak in the mass spectrum of the LM free metal-ligand ion and the fragmentation patterns of this parent ion are of particuliar significance since they illustrate the effect of coordination upon the properties of the thiazole ligand. The free thiazole fragments upon electron impact by two major routes (Scheme 86 also cf. Section II. 6). 

Modern mass spectrometers are interfaced with computerized data handling sys terns capable of displaying the mass spectrum according to a number of different for mats Bar graphs on which relative intensity is plotted versus m z are the most common Figure 13 40 shows the mass spectrum of benzene m bar graph form... 

FIGURE 13 40 The mass spectrum of benzene The peak at miz = 78 corresponds to the CgHg molecular ion... 

The mass spectrum of benzene is relatively simple and illustrates some of the mfor matron that mass spectrometry provides The most intense peak m the mass spectrum is called the base peak and is assigned a relative intensity of 100 Ion abundances are pro portional to peak intensities and are reported as intensities relative to the base peak The base peak m the mass spectrum of benzene corresponds to the molecular ion (M" ) at miz = 78... 

There is a small peak one mass unit higher than M m the mass spectrum of ben zene What is the origin of this peak d What we see m Figure 13 40 as a single mass spectrum is actually a superposition of the spectra of three isotopically distinct benzenes Most of the benzene molecules contain only and H and have a molecular mass of 78 Smaller proportions of benzene molecules contain m place of one of the atoms or m place of one of the protons Both these species have a molecular mass of 79... 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

The peak at m/z 77 m the mass spectrum of chlorobenzene m Figure 13 41 is attributed to this fragmentation Because there is no peak of significant intensity two atomic mass units higher we know that the cation responsible for the peak at m/z 77 cannot contain chlorine... 

Some classes of compounds are so prone to fragmentation that the molecular ion peak IS very weak The base peak m most unbranched alkanes for example is m/z 43 which IS followed by peaks of decreasing intensity at m/z values of 57 71 85 and so on These peaks correspond to cleavage of each possible carbon-carbon bond m the mol ecule This pattern is evident m the mass spectrum of decane depicted m Figure 13 42 The points of cleavage are indicated m the following diagram... 

FIGURE 13 42 The mass spectrum of decane The peak for the molecular ion is extremely small The most prominent peaks arise by fragmentation... 

FIGURE 13 43 The mass spectrum of propylbenzene The most intense peak is... 

Deduce the structure of a compound having the mass spectrum and H NMR spectrum pre sented in Figure 13 48... 

Three of the most intense peaks in the mass spectrum of ] 2 methyl 2 butanol appear at m/z 59 70 and 73 Explain the origin of these peaks J... 

The most intense peaks in the mass spectrum of an alcohol correspond to the ton formed according to carbon-carbon cleavage of the type shown ... 

Mass Spectrometry Ethers like alcohols lose an alkyl radical from their molecular ion to give an oxygen stabilized cation Thus m/z 73 and m/z 87 are both more abun dant than the molecular ion m the mass spectrum of sec butyl ethyl ether... 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular ion peak m their mass spectra Aldehydes also exhibit an M— 1 peak A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acylium ions) by cleavage of an alkyl group from the carbonyl The most intense peak m the mass spectrum of diethyl ketone for example is m z 57 corresponding to loss of ethyl radi cal from the molecular ion... 

Compound A and compound B are isomers having the molecular formula C10H12O The mass spectrum of each compound contains an abundant peak at m/z 105 The NMR spectra of com pound A (Figure 17 23) and compound B (Figure 17 24) are shown Identify these two isomers... 

DDT resistant insects have the ability to convert DDT to a less toxic substance called DDE The mass spectrum of DDE shows a cluster of peaks for the molecular ion at m/z 316 318 320 322 and 324 Suggest a reasonable structure for DDE... 

The mass spectrum is a fingerprint for each compound because no two molecules are fragmented and ionized in exactly the same manner on electron-impact ionization. In reporting mass spectra the data are normalized by assigning the most intense peak (denoted as base peak) a value of 100. Other peaks are reported as percentages of the base peak. 

If the substrate (M) is more basic than NHj, then proton transfer occurs, but if it is less basic, then addition of NH4 occurs. Sometimes the basicity of M is such that both reactions occur, and the mass spectrum contains ions corresponding to both [M + H]+ and [M + NH4]. Sometimes the reagent gas ions can form quasi-molecular ions in which a proton has been removed from, rather than added to, the molecule (M), as shown in Figure 1.5c. In these cases, the quasi-molecular ions have one mass unit less than the true molecular mass. 

The mass spectrum is characteristic for different substances and can be used like a fingerprint to identify a substance, either by comparison with an already known spectrum or through skilled interpretation of the spectrum itself (Figure 3.2). 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

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