Adduct, chemical ionization

Lawrence, P, and J.T. Brenna, Acetonitrile Covalent Adduct Chemical Ionization Mass... 

Brodbelt J, Liou C-C and Donovan T 1991 Selective adduct formation by dimethyl ether chemical ionization is a quadrupole ion trap mass spectrometer and a conventional ion source Ana/. Chem. 63 1205-9... 

Cl is not the only ionization technique where this aspect of interpretation must be considered carefully fast-atom bombardment, thermospray, electrospray and atmospheric-pressure chemical ionization, described below in Sections 3.2.3, 4.6, 4.7 and 4.8, respectively, all produce adducts in the molecular ion region of their spectra. 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

A detailed description of sources used in atmospheric pressure ionization by electrospray or chemical ionization has been compiled.2 Atmospheric pressure has been used in a wide array of applications with electron impact, chemical ionization, pressure spray ionization (ionization when the electrode is below the threshold for corona discharge), electrospray ionization, and sonic spray ionization.3 Interferences potentially include overlap of ions of about the same mass-charge ratio, mobile-phase components, formation of adducts such as alkali metal ions, and suppression of ionization by substances more easily ionized than the analyte.4 A number of applications of mass spectroscopy are given in subsequent chapters. However, this section will serve as a brief synopsis, focusing on key techniques. 

The use of ammonia for the protonation of nitroarenes leads frequently to formation of aduct ions, e.g. [M + NH4]+, but not to the protonated species (MH+)112,113. The ammonia chemical ionization spectrum of nitrobenzene shows, in addition to a series of adduct ions, a dominant signal corresponding to the anilinium ion (m/z 94)112114115. Evidence for the isomerization of the [M + NR ]"1" adduct followed by successive loss of NO and OH or NH3 to give ions corresponding to the substitution products, e.g. the anilinium ion, has been given115 see Scheme 41. 

Some combinations of IC with MS detection are available, including ICP-MS (element-specific detection), particle beam MS and MS with atmospheric pressure ionization (API) operated in either electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) modes that give information on molecular ions or adducts. 

In chemical ionization (Cl) mass spectra of 1-phenyl- and 1-heteroarylsilatranes289,210 (X = 2-furyl, 3-furyl, furfuryl, 2-thienyl and 3-thienyl) employing NH3 as the carrier gas, the main peak corresponds to the adduct ion [(M — X)NH3]+. 

Most often positive ESI and only to a small extent positive atmospheric pressure chemical ionization (APCI) were used as ion source (interface) to generate desol-vated free ions of TTA or QTA suitable for MS or MS/MS detection. TTA were detected as their proton adducts [M+H]+, whereas QTA were simply monitored as the original cations [M]+ thus not requiring adduct formation (Table 9). 

In the gas phase, addition of an ionic electrophile to a neutral (M) is usually accompanied by elimination of a neutral molecule from the reagent ion. This elimination process stabilizes the reaction products by removing excess energy from the initial ion-molecule adduct. Typically, protonation and alkylation reactions normally used in chemical ionization (Cl) and radiolytic experiments are of this type, as shown in equations 3 (HA+ = CH5+, C2H5+, NH4+, etc.) and 4 (R2X+ = an halonium ion, vide infra), respectively. 

In electron ionization, the response is normally linear with respect to the concentration over a wide range, often six orders of magnitude. This is not true for the other ionization techniques because of the influence which the sample quantity can have on the number of ions that is produced and on the fragmentation, and thus on the production yield of the various ionic species. For instance, in the chemical ionization mode the formation of adducts appearing at higher sample pressures changes the relative intensities in the spectrum. 

Evidence for the operation of cation radical mechanisms for cycloaddition has often been provided by means of a comparison of the results obtained for various methods of generating cation radicals. For example, in the Diels-Alder cycloaddition of phenyl vinyl sulfide to 1,3-cyclopentadiene (Scheme 36) the same adducts are formed whether the cation radicals are generated by chemical ionization (aminium salt), photochemical ionization (the PET method), or electrochemical ionization (anodic oxidation) [65]. 

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