Electrospray ionization negative ions produced

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

As with GC/MS, LC/MS offers the possibility of unequivocal confirmation of analyte identity and accurate quantiation. Similarly, both quadrupole and ion-trap instruments are commercially available. However, the responses of different analytes are extremely dependent on the type of interface used to remove the mobile phase and to introduce the target analytes into the mass spectrometer. For pesticide residue analyses, the most popular interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Both negative and positive ionization can be used as applicable to produce characteristically abundant ions. 

Also, a brief note has appeared concerning electrospray ionization mass spectrometry of mixtures of -carotene with ft- and with y-cyclodexlrin in aqueous methanol solutions. Whereas negative ion ESI produced 1 1 adduct ions of -carotene with both of the cyclodextrin isomers, positive ESI gave these adducts only in the case of ft-cyclodextrin302. 

Figure 5 Examples of Data Generated on an Electrospray Ionization Mass Spectrometer, (a) Proteins Typically Produce Positive, Multiply Charged Ions and (b) Oligonucleotides Generate Negative, Multiply Charged Ions. Inset are the Computer-Generated Molecular Weight Spectra...

Rodrigues Filho et al. studied the fragmentation of dipyridamole and several of its derivatives by electrospray ionization combined with collisional activated decomposition mass spectrometry in both positive and negative modes [61]. These compounds produce abundant mono-charged ions ([M + H]+) under electrospray ionization. Interpretation of the collisional activated decomposition spectra showed that fragmentation occurs preferentially in the ethanolamine groups attached at C-2, C-4, C-6, and C-8. 2-Methoxyethanol is eliminated when ethanolamines are in positions C-2/C-6, and 2-aziridinethanol is eliminated from... 

The term "molecular ion" by definition refers to a radical cation or anion of an intact molecule. Molecular ions are odd-electron ions, which may thus be generated by El. Unfortimately, the term molecular ion is also frequently used to indicate the even-electron ionic species produced by electrospray and APCl. This obviously is not correct. In the soft ionization techniques, predominantly even-electron protonated molecules are generated in positive-ion mode, and deprotonated molecules in negative ions. In addition, various adduct ions may be generated (Table 2.2). These all are even-electron ions, and should therefore not be referred to as molecular ions. Alternatively, the term protonated molecular ions is used, which again is incorrect one cannot protonate a radical cation ... 

Improvements in the instrumentation, ionization sources, high-resolution mass analyzers, and detectors [67-69], in recent years have taken mass spectrometry to a different level of HPLC-MS for natural product analysis. Mass spectrometry detection offers excellent sensitivity and selectivity, combined with the ability to elucidate or confirm chemical structures of flavonoids [70-72]. Both atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) are most commonly used as ionization sources for flavonoid detection [73-76]. Both negative and positive ionization sources are applied. These sources do not produce many fragments, and the subsequent collision-induced dissociation energy can be applied to detect more fragments. Tandem mass spectrometry (MS , n> 2) provides information about the relationship of parent and daughter ions, which enables the confirmation of proposed reaction pathways for firagment ions and is key to identify types of flavonoids (e.g., flavones, flavonols, flavanones, or chalcones) [77-80]. 

The ionization mode most effective in ergot alkaloid analysis is positive electrospray ionization, which produces more intense protonated molecular ions than the deprotonated molecular ions that result from negative ionization, and this has become the preferred ionization method. When ergot alkaloids are ionized in positive electrospray mode the ions formed are predominantly the protonated molecular ions [M -i- H] -i- ions, with little evidence of adduct formation with ions of sodium, potassium, or ammonia, despite the ubiquitous presence of sodium in aqueous mobile phase solvents and the addition of comparatively high levels of ammonium salts. 

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