Atmospheric-pressure chemical ionization negative ions produced

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. 

Unlike with GC-MS, quality criteria for identification of drug residues by LC-MS have not been yet defined within the European Union, but this is currently under review. Criteria for GC-MS stipulate the measurement of preferably at least four diagnostic ions. However, this is not always possible with LC-MS because most compounds will only produce an M ion in positive mode or a M ion in negative mode, with little fragmentation when using thermospray (TSP), electrospray (ESP), or atmospheric pressure chemical ionization (APCI). Even where the ions and ratios are in agreement, there will be still possibility of misidentification. For this reason, mass spectra data are often interpreted with additional supporting data such as the LC retention times, as, for example, in the LC-MS analysis of sulfadimethoxine and sulfadoxine that present identical mass spectra (24). 

HPLC with ESI produces mostly molecular ions and few to no molecular fragments. This limits the use of LC-MS in the differentiation of isomers. Two atmospheric pressure ionization (API) interfaces allow for the formation of molecular ion, [PAH]" , yet in general, derivatization and additives are required to induce fragmentation. ESI is an interface that transfers ions from the mobile phase into the gaseous phase for introduction into the mass spectrometer so that atmospheric pressure chemical ionization (APCI) can cause ionization of chemical species in the gaseous phase. Both techniques can be operated in the positive- or negative-ion mode. Reports exist for the ionization of PAHs by both techniques, although there are few actual applications to the analysis of real environmental samples. 

Chemical ionization is an ionization mechanism that allows the formation of protonated or deprotonated molecules via a gas-phase ion—molecule reaction. It exists under two different forms one under vacuum (Cl) and the second one at atmospheric pressure referenced as atmospheric pressure chemical ionization (APCI). The principal difference between Cl and El mode is the presence of reagent gas, which is typically methane, isobutene, or ammonia (Mimson, 2000). The electrons ionize the gas to form the radical cations (in the case of methane, CH4 -I- e CH4 -I- 2e ). In positive chemical ionization (PCI), the radical cations undergo various ion—molecule reactions to form CHs and finally lead to the formation, after proton transfer (CHs + M [M + H] ), of protonated molecules. Negative chemical ionization (NCI) (Budzikiewicz, 1986), after proton abstraction, leads to the formation of deprotonated molecules [M — H] . Negative ions can be produced by different processes such as by capture of low-energy electrons present in the chemical ionization... 

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]. 

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