Chemical Ionization by Protonation

Ionization in Cl is the result of one or several competing chemical reactions. Therefore, the sensitivity in Cl strongly depends on the conditions of the experiment. In addition to primary electron energy and electron current, the reagent gas, the reagent gas pressure, and the ion source temperature have to be stated with the sensitivity data to make a comparison. Modem magnetic sector instmments are specified to have a sensitivity of about 4 x 10 C pg for the [Mh-H] quasi-molecular ion of methylstearate, m/z 299, at / = 1000 in positive-ion Cl mode. This is approximately one order of magnitude less than for El. 

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Negative chemical ionization by proton removal (deprotonation) by a base to provide (M - H) ions is used rarely now except in research laboratories. It concerns molecules whose acidic hydrogens are easy to detach with a base. If one wants to use hydroxyl OH ions as basic reagents, one can for example form a hydroxyl by using a mixture of nitrous oxide and methane. Figure 9.62 illustrates the mechanisms of dissociative electron capture on N2O to form the O ion (reaction 3) and abstraction of a hydrogen atom from methane by O to form OH (reaction 4). The OH ion can then abstract a proton from the analyte to form the (M - H) ion (reaction 5). 

In positive ion chemical ionization (PICI), the neutral analyte is most commonly ionized by proton transfer (Equation (4)) or adduct formation (Equations (5) and (6)). 

Atmospheric pressure chemical ionization (APCI) [15] involves the primary formation of ions by corona discharge in a solvent spray under atmospheric pressure. A solution of the sample is nebulized pneumatically and the droplets formed are vaporized by heating (120 °C). The resulting sample gas is chemically ionized by the transfer of protons from the primary reactant ions.The APCI technique is widely used in the analysis of drugs or metabolic studies. Detection is limited to a molecular mass of about 1000 Da. 

Since the introduction of chemical ionization by Munson and Field in 1966 (10), ion-molecule reactions have been studied extensively for the structural analysis of unknown compounds. In brief, the reagent gas is allowed into the ion source at pressures significantly higher than that of the analyte. The reagent gas is ionized by electron impact and a variety of ion-molecule reactions can occur between charged and neutral gas molecules. These products, in turn, may ionize the analyte by one of five mechanisms. The most important by far is proton transfer, which is the only positive ion Cl reaction that has seen broad acceptance for analytical purposes. Three that apply to the case of acetonitrile are proton transfer, charge transfer, and assodation (also called addition or covalent adduct formation). Acetonitrile Cl is unique in providing very selective analytical information as a result of adduct formation. 

The PTR-MS technique has been extensively discussed in a series of review papers [4 6,8-13]. Briefly, it combines a soft, sensitive, and efficient mode of chemical ionization, adapted to the analysis of trace VOCs, with a quadrupole mass filter. The gas to be analyzed is continuously introduced into the chemical ionization cell (drift tube) and ionized by proton transfer from H3O+. The protonated VOCs are extracted by a small electrical field from the drift tube and mass analyzed by a quadrupole mass spectrometer. The specific aspect of the chemical ionization scheme in PTR-MS is that the generation of the primary H30 ions, and the chemical ionization process, VOC + H30 V0CH" " + H2O, are spatially and temporally separated and can therefore be individually optimized. 

Most of the ions produced by either thermospray or plasmaspray (with or without the repeller electrode) tend to be very similar to those formed by straightforward chemical ionization with lots of protonated or cationated positive ions or negative ions lacking a hydrogen (see Chapter l).This is because, in the first part of the inlet, the ions continually collide with neutral molecules in the early part of their transit. During these collisions, the ions lose excess internal energy. 

Chemical ionization produces quasi-molecular or protonated molecular ions that do not fragment as readily as the molecular ions formed by electron ionization. Therefore, Cl spectra are normally simpler than El spectra in that they contain abundant quasi-molecular ions and few fragment ions. It is advantageous to run both Cl and El spectra on the same compound to obtain complementary information. 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

Protonated molecule. An ion formed by interaction of a molecule with a proton abstracted from an ion, as often occurs in chemical ionization according to the reaction ... 

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

We have previously shown (8) that the chemical ionization spectra using methane as reactant are generated by the combination of dissociative proton transfer from CH5 + and hydride ion abstraction and alkyl ion... 

It appears that gas-phase basicity of nitro compounds has been studied only scarcely. Thus, only the values of the parent compounds, nitromethane (179.2 kcalmol-1) and nitrobenzene (193.4 kcalmoD1), are found in the comprehensive listing given in Reference 39. The rather high PA values for nitro compounds suggest protonation by common chemical ionization reagent systems, such as hydrogen (H3+) and methane (CH5+). 

The chemical ionization MS of a selection of substitued nitrobenzenes have been studied106. The H2 chemical ionization MS appeared significantly more useful for characterization of, e.g., isomeric compounds than the corresponding methane spectra, apparently due to the higher internal energy deposited in the protonated molecule by the reaction with H3+, and consequently in the more extensive fragmentation106. 

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