Molecular ion envelope

Figure 6. Molecular ion envelope of native lysozyme (M.V. 14,305) obtained using FAB ionization on a high field mass spectrometer.

Figure 7. Molecular ion envelope of glycosylated lysozyme Amadori products (top) and cesium iodide reference mass cluster ions (bottom). The centroided masses of the tiro labeled peaks are 16,265 and 16,427 corresponding to lysozyme Amadori adducts containing 12 (G 12) and 13 (G=13) glucose residues respectively.

The latter expression of specific activity is used almost always for high specific activity tritiated compounds, and often for carbon-14-labeled compounds. Measures of radioactivity per molar unit can be calculated from mass spectrometry data ". In this analysis, the distribution of isotopic species (e.g., Hq, Hj,. ..) is determined by measuring the relevant peak intensities in the molecular ion envelope and correcting them for naturally occurring isotopes (e.g., C, present in the molecule this can be done manually or by use of readily available computer algorithms. The contribution of each isotopic species to the total can be used to calculate the average number of isotopic atoms per molecule and thence, from the molar specific activity of the pure isotope, the molar specific activity of the compound. 

A more complicated task faces an ESI-LC/MS designed to separate and determine the molecular weights of proteins. The proteins have to be separated by the column, the multiply charged ion envelope must be measured, and deconvolution calculations made to determine the molecular weights of the separated protein(s). Often, there are partially resolved proteins mixtures and the overlapping peak envelopes must be resolved to determine molecular weight for both components present. 

In electrospray ionization, compounds with a mass in excess of 500 Da may be prone to the formation of multiple-charge ions, either in positive-ion or in negative-ion mode [22-23], The averaging algorithm [23-24] can be used for molecular-weight determination from an ion envelope of multiple-charge ions (Ch. 16.2.2). Various automated computer-based procedures for the deconvolution or transformation of the electrospray mass spectra of proteins have been introduced. 

Figure 16.3 Principle of molecular-mass determination from the ion envelope of multiple-charge protein ions.

A compact ion attachment mass spectrometer was designed that is simple and small and fulfills all the basic requirements for lAMS the system can be used to obtain only molecular ions by detecting any chemical species in real time [93]. This custom-made apparatus (Fig. 6.11) consists of a Li+ ion attachment ion source into which a stream of gas from a capillary leak inlet is directed, an electrostatic lens system (ELS), and a quadrupole mass spectrometer and detector, all of which are installed in a vacuum-separated envelope. The system employs a single tuibomo-lecular pirmp on the vacuum envelope instead of a differential pumping system. 

The ionisation provided by cosmic-rays and UV photons drives an ion-molecule chemistry in CSEs which leads to a rich variety of molecular species. In order to be important, chemical processes must occur on a time-scale faster than that of the expansion. Figure 1 from Nejad (1986) shows a number of such time-scales for the case of lRC+10216. One sees that a number of fast processes such as reaction with Hj and dissociative recombination can occur faster than the expansion time but that grain surface processes will be unimportant in the outer envelope as discussed in 2.2. An important point to note in this figure is that fast reaction of a molecular ion with H2 can dominate even dissociative recombination out to a radial distance of - 10 cm. 

Because of the low collision rate in the high vacuum environment of a Fourier transform mass spectrometer (FTMS), vibrationally excited molecular ions cool predominantly by IR fluorescence. For typical IR transition dipole moments and frequencies in the mid-IR, spontaneous emission is expected to occur at a rate in the range of 1-100 s To energize an ion efficiently using IR multiple-photon excitation (MPE), the rate of photon absorption - the product of absorption cross section and photon flux - should exceed the emission rate. From such a back-of-an-envelope estimate, one finds that radiation sources producing several Watts/cm are required to induce efficient dissociation [141], Note that the demands on laser power may further increase because of the limited residence time of the ions in the laser field, collisional deactivation in traps at higher pressures, limited spectral overlap between molecular absorption and laser emission profiles, etc. 

Progress is also made in software for mass spectral interpretation, especially for the so-called deconvolution of (mixed) ion envelopes of multiply charged protein ions generated by electrospray ionization, and for the peptide sequencing by MS-MS techniques. In that respect, the development of tools to search extensive protein and DNA databases using peptide maps, peptide molecular masses, LC retention time data, and sequence tags is an enormous... 

Figure 4.10 Deamidation products of Fc IgG for a sample stressed for 28 h at 37°C and pH 7.4. (a) Extracted ion chromatogram (EIC), (b) molecular ion isotope envelopes of peaks in the EIC, and (c) kinetic profile of deamidation. Adapted from Sinha, S., et al. Protein Sci. 2009, with permission.

MS allows the measurement of the mass of the isotopes of molecular ions. An isotope is any of the forms of an element that differ in the number of neutrons in the nucleus. The isotopic envelope for an ion of a known empirical formula is measured... 

A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

---