Comparison of Different Mass Analyzersa Characteristic Mass range, up to (Da) Resolution in .NET

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TABLE 15.3. Comparison of Different Mass Analyzersa Characteristic Mass range, up to (Da) Resolution
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Figure 15.7. Typical con gurations used in biological MS. In MALDI TOF, the ions produced by a short laser pulse travel across a ight tube, arriving at different times at the detector. In ESI triple quadrupole, the rst quadrupole (Q1) is used to separate the sprayed ions, in the second (Q2, also called the fragmentation cell) argon atoms collide with the ions; the resulting ions (daughter ions) are analyzed in Q3, and subsequently detected.
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several quadrupole, magnetic, and electrostatic analyzers. A triple quadrupole mass analyzer is composed of two scanning quadrupoles separated by one rf-only quadrupole, where a gas collides with analyte ions in order to produce fragmentation (CID, collision-induced dissociation). Figure 15.7 shows two common combinations of ionization source and mass analyzer. Relatively new mass analyzers with very high resolution include the quadrupole ion trap (QIT) and the Fourier-transform ion cyclotron resonance (FTICR) instruments. In both analyzers, ions are trapped in a 3D eld, and are analyzed once trapped. In Table 15.3, the mass analyzers described here are compared. The combination of MS with other analytical techniques is also very common; MS has been widely used following chromatographic separations, for mass analysis. 15.2.3. Detectors The ions separated by the mass analyzer are detected through their production of a current signal; this is generally achieved using an electron multiplier or a scintillation counter.
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MASS SPECTROMETRY OF BIOMOLECULES
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An electron multiplier consists of a series of dynodes held at increasing potential; the ions arrive at the rst dynode, producing an emission of secondary electrons. These are accelerated in the electric eld and strike a second dynode that produces more electrons. Successive dynodes continue to amplify the current, and the total ampli cation is high; values of 106 are normally obtained. The operation and design are very similar to a photomultiplier tube, but the electron multiplier is not shielded by a glass bulb; for this reason contamination, mainly at the rst dynode, is important and the lifetime of the electron multiplier is relatively short. In the scintillation counter, ions strike a dynode, from which electrons are emitted. These electrons impact a phosphorous-coated screen, causing light emission that is detected by a photomultiplier tube. 15.3. INTERPRETATION OF MASS SPECTRA High-resolution MS of low molecular weight compounds results in a series of isotopic peaks for the parent ion and each detectable fragment. The intensity of each peak in the series depends on the relative abundance of a given isotope as well as the number of atoms of a given identity in the detected fragment. For example, a mass spectrum of CO2 shows two main peaks that correspond to 12C16O2 and 13 16 C O2 (with the latter showing $1.1% of the intensity of the former) as well as a number of less intense peaks that result from other isotope combinations. Table 15.4 shows the main isotopes of several elements as well as their relative abundances; from these data, it is clear that molecular and fragment ions containing C and S atoms are expected to produce isotopic peaks with signi cant intensities.
TABLE 15.4. Isotopic Abundances of Selected Elements Isotope C C 1 H 2 H 14 N 15 N 16 O 17 O 18 O 32 S 33 S 34 S 36 S 31 P
13 12
Mass (Da) 12.0000 13.003354 1.007825 2.014102 14.003074 15.000109 15.994915 16.999131 17.999160 31.972071 32.971457 33.967867 35.967081 30.973762
Abundance (%) 98.89 1.11 99.986 0.015 99.63 0.37 99.762 0.038 0.200 95.02 0.75 4.21 0.02 100
INTERPRETATION OF MASS SPECTRA
Figure 15.8. Mass spectra for a peptide of nominal mass 2537. Note that at resolution 2000 (R 2000) 6 isotope peaks are clearly observed. The gure represents spectra for a unique molecular ion state, for example (M H) , a protonated molecule without fragmentation.
The resolution of the mass spectrometer, R, and the charge state of the detected ion, z, are the most important factors that determine whether individual isotopic peaks will be observed at a given m/z value. At low resolution, these individual species contribute to the observed coalescent envelope peak , as shown in Figure 15.8. It can be seen in this gure that the position of the envelope peak is offset with respect to that of the most abundant isotope; this offset is predictable based in the natural isotopic abundances of the elements. The mass of a given chemical species may be calculated in three different ways. The simplest of these uses the integer mass of the most abundant isotope of each element, and results in the nominal mass of the species; this value is not particularly useful in MS. The monoisotopic mass, calculated using the exact mass of the most abundant isotope of each element, is more useful, especially for high-resolution MS of low molecular weight species. For biomolecules and their fragment ions, the most useful value is the average mass. The average mass is calculated using elemental atomic weight values that are averaged over all isotopes; this value allows the position of the envelope peak to be predicted.13 Table 15.5 shows examples of nominal, monoisotopic, and average mass values. Although the differences between nominal and average values may appear small, especially for elemental species, they become larger as molecular complexity increases and are important in establishing the identities of proteins. In particular, site-directed mutagenesis and posttranslational modi cation studies require accurate average mass values to con rm the identities of product proteins. For puri ed samples of small molecules studied on low-resolution mass spectrometers, and for singly charged larger molecules, fragment ions are readily identi ed by straightforward calculations using average mass values. The presence or absence of observable fragments (including the parent ion) are known to depend on the type