Glycerol cluster ions

Ion bombardment of glycerol with sample dissolved can produce molecular ions and glycerol cluster ions. Field (34) studied the effects of bombarding a glycerol sample to study the products. Better molecular ion spectra were found to occur when acid or alkali salts were added (2). 

A typical FAB mass spectrum of glycerol alone, showing a protonated molecular ion at m/z 93 accompanied by decreasing numbers of protonated cluster ions (m/z, 1 + nx92 n = 2, 3, 4,. ..). 

Mass spectral analysis of quaternary ammonium compounds can be achieved by fast-atom bombardment (fab) ms (189,190). This technique rehes on bombarding a solution of the molecule, usually in glycerol [56-81-5] or y -nitroben2yl alcohol [619-25-0], with argon and detecting the parent cation plus a proton (MH ). A more recent technique has been reported (191), in which information on the stmcture of the quaternary compounds is obtained indirectly through cluster-ion formation detected via Hquid secondary ion mass spectrometry (Isims) experiments. 

Fig. 7.7. Comparison of (a) 70 eV El and (b) isobutane-d spectrum of glycerol. Instead of a molecular ion, an [M+H] quasimolecular ion is observed in El mode, too. In addition to [M+H], the Cl spectrum shows few fragment ions and a weak [2M+H] cluster ion signal.

Note Csl and Au both bear the advantage of being monoisotopic. This insures the peak top to exactly represent the theoretical isotopic mass of the respective cluster ion, independent of its m/z ratio or actual resolution (Chap. 3.3.5, 3.4). Csl, KI, and other alkali salts providing more narrow-spaced cluster ion series can alternatively be employed as saturated solutions in glycerol. [45-47]... 

The amount of energy released by the incident beam is sufficiently high in the vicinity of the point of impact that sample molecules are ionized by proton transfer from a protonated matrix. Typically, the spectrum obtained shows cluster ions formed from the matrix. In particular, the positive ion spectrum of glycerol (mw 92 u) shows a series of ions 92 +l where n can be 1 up to 12 or more. This spectrum can be used to calibrate the mass spectrometer and also provide internal lock masses for accurate mass measurement, described in applications below. 

For completeness, it should be noted that other cluster ion beams, including C24H12+ and Cgo [370], giant glycerol [371], Ir4(CO)7+ [372], Au4oo [373], and Aruoo [374] also have potential for molecular depth profiling and some of them have already proved their ability in... 

Barber et al. introduced FAB in 1981. In this technique, bombardment of a liquid target surface by a beam of fast atoms such as xenon or argon, causes the continuous desorption of ions that are characteristic of the liquid. In a typical FAB spectrum, the analyte ion is usually formed as protonated or cationized ions in positive FAB, and deprotonated ions in negative FAB mode. A few fragmented ions may also be formed. The spectrum usually contains peaks from the matrix, such as protonated matrix clusters of glycerol if it is used as the matrix solvent. FAB utilizes a liquid matrix such as glycerol. The matrix is used to enhance sensitivity and ion current stability. 

The fast atom bombardment ionization (FAB) technique is a soft ionization method, typically requiring the use of a direct insertion probe for sample introduction in which a high energy beam of Xe atoms, Cs+ ions, or massive glycerol-NH4+ clusters sputter the sample and matrix from the probe surface (Figure 8). 

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