Mass spectra, spectroscopy

HRMS high resolution mass spectra/spectroscopy... 

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). 

In contrast to IR and NMR spectroscopy, the principle of mass spectrometry (MS) is based on decomposition and reactions of organic molecules on theii way from the ion source to the detector. Consequently, structure-MS correlation is basically a matter of relating reactions to the signals in a mass spectrum. The chemical structure information contained in mass spectra is difficult to extract because of the complicated relationships between MS data and chemical structures. The aim of spectra evaluation can be either the identification of a compound or the interpretation of spectral data in order to elucidate the chemical structure [78-80],... 

Figure 6.7 Ammonia DCI mass spectrum of an LDPE extract. After Chen and Her [23]. From S.W. Chen and G.R. Her, Applied Spectroscopy, 47, 844-851 (1993). Reproduced by permission of the Society of Applied Spectroscopy...

Figure 7.31 Negative-ion thermospray mass spectrum of the disulfonated azo dye Direct Red 81 (mobile phase contains 10mmolL ammonium acetate). After Niessen [568]. Reprinted from W. Niessen, in Encyclopedia of Spectroscopy and Spectrometry (J.C. Lindon, ed.), Academic Press, pp. 2353-2360, Copyright (2000), with permission from Elsevier...

Evidence for a competing disproportionation mechanism (see Figure 1) for the termination of chain ends is provided by the combined presence of the peaks from 4 and 5 in the MALDI-TOF mass spectrum of this PMMA polymer (see Figure 6) [10]. Confirmation of the presence of the unsaturated and saturated chain ends, arising from disproportionation, was obtained by means of and 13C NMR spectroscopy, respectively [11]. 

Electrospray ionization mass spectroscopy have been carried out on aqueous tungstate solutions and species such as [HWOJ, [HW207]3, [W4013r, [HW4013r, rw60192 ], [ WioOga]2-, or [HW5016r have been observed in the mass spectrum (155). 

Mass spectroscopy is useful for qualitative analysis because the signals due to the different mass-to-charge ratios, as seen in the mass spectrum, represent molecular fragments that can be pieced together such that the compound can be identified. 

Above mentioned examples clearly show that if multivariate data processing methods are applicable, analytical information can be derived with a minimal amount of pre-information and a foreseeing of a maximum of problems. When the sampled object is homogenous, multivariate methods are only applicable when the analytical method itself produces multivariate signals. This is the case when several signals (e.g. spectra) are obtained for the sample as a function of another variable (e.g. time, excitation wavelength). For e mple in GC-MS, a mass spectrum is m sured of the eluents every. 1 a 1 second. In excitation-emission spectroscopy, spectra are measured at several excitation-wavelengths. The potentials of the application of multivariate... 

Some aspects of the chemistry of helicenes require still more attention. Since the interpretation of the mass spectrum of hexahelicene by Dougherty 159) no further systematic work has been done on the mass spectroscopy of helicenes, to verify the concept of an intramolecular Diels-Alder reaction in the molecular ion. Though the optical rotation of a number of helicenes is known and the regular increase of the optical rotation with increasing number of benzene rings has been shown, the dependence of the rotation on the helicity is still unknown. The asymmetric induction in the synthesis of helicenes by chiral solvents, or in liquid crystals, though small, deserves still more attention because application to other organic compounds will be promoted when the explanation of observed effects is more improved. 

Mass Spectroscopy. Thermal decomposition of CH2N2 in a special reactor attached to a mass spectrometer has enabled Eltenton29-30 to detect CH2 by its mass spectrum. No evidence of CH2 in the thermal decomposition of CH4 was found by this method. 

Figure 1. Versions of photoionization spectroscopy wherein not only the dependence of the multiphoton ionization efficiency on the laser wavelength is subject to measurement, but also the mass spectrum of photons and energy spectrum of photoelectrons (a) energy-level diagram (b) collision of a neutral particle with laser photons.

---