Ionization, chemical hybrid

Furthermore, chemical ionization in hybrid mode allows the selection of a reactant ion in the trap, for example CH5+ for methane in the positive mode, rather than making the analyte react with each of the ions classically produced by methane (CH5+, C2H5+, C2H3+, and C3H5 ) when one submits it to electron ionization and allows it to react at a pressure of 1 torr. This is particularly valuable for obtaining a simple Cl mass spectrum or carrying out reactivity studies. 

Various analyzers have been used to analyze phenolic compounds. The choice of the MS analyzer is influenced by the main objective of the study. The triple quadrupole (QqQ) has been used to quantify, applying multiple reaction monitoring experiments, whereas the ion trap has been used for both identification and structure elucidation of phenolic compounds. Moreover, time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) are mainly recommended for studies focused on obtaining accurate mass measurements with errors below 5 ppm and sub-ppm errors, respectively (Werner and others 2008). Nowadays, hybrid equipment also exists, including different ionization sources with different analyzers, for instance electrospray or atmospheric pressure chemical ionization with triple quadrupole and time-of-flight (Waridel and others 2001). 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

As discussed previously, such species are more tangibly understood to be intermediates on passing from covalent species to carbenium ions, rather than chemical entities, because their lifetime should be comparable to a bond vibration (Eq. 79). Stretched bond isomerism does not exist unless it is associated with a change in spin number, as in some inorganic compounds [266,267]. Since ionization is accompanied by a change in hybridization from sp3 to sp2, it is very unlikely that species with stretched covalent bonds could be considered real individual chemical species. Moreover, the chemoselectivity of reactions with alleged stretched covalent bonds is not significantly different than that in con-... 

Buser and co-workers have used different GC/MS techniques in the investigation of PCDTs in fly ash samples and in aquatic organisms [12, 31]. GC/MS and MS/MS were used for detection and to differentiate PCDTs from PCDDs. Various ionization [electron ionization (El) and negative-ion chemical ionization (NCI)] techniques and MS/MS experiments monitoring different daughter ions on a hybrid instrument were used to distinguish PCDTs from PCDDs [31]. 

It is of interest to mention some detailed mass spectrometric results that were obtained by performing distinct experiments (a) ion-molecule reactions in mixtures of haloben-zenes and ammonia under chemical ionization conditions (b) ion-molecule reactions of mass-selected ionized halobenzenes toward ammonia in a quadrupole collision cell of a hybrid tandem mass spectrometer (c) ion-molecule reactions of phenyl diazonium cations with ammonia in the same quadrupole collision cell and, finally, (d) electrospray ionization of anilines in a hybrid quadrupole-time of flight mass spectrometer (QTof). Characterization of the product ions relies on collisional activation experiments in the low or high kinetic energy regime98. 

Fig. 12.21 GC-FTIR-MS hybridization, (a) Scheme of the configuration, (b) Information available from linked accurate mass measurement (AMM) electron ionization (El)-chemical ionization (Cl) and infrared spectra. (Reproduced from [53] with permission of the American Chemical Society).

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