Particle beam positive chemical ionization

The particle beam-liquid chromatography-mass spectrometry (PB-L0-M8) of phenols (phenol, 4-nitrophenol, and 1—naphthol) and their glucuronide and sulfate conjugates in electron impact (El) and positive chemical ionization (PCI) is described. 

Based on a new technology, particle beam enhanced liquid chromatography-mass spectrometry expands a chemist s ability to analyse a vast variety of substances. Electron impact spectra from the system are reproducible and can be searched against standard or custom libraries for positive compound identification. Chemical ionization spectra can also be produced. Simplicity is a key feature. A simple adjustment to the particle beam interface is all it takes. 

Particle beam is especially useful in identifying unknown compounds, despite its limitations with respect to the quantification and its lack of sensitivity at trace levels (51). Particle beam-MS is generally carried out in the positive El, because EI-MS provides very reproducible mass spectra, and a wide variety of mass spectra libraries, containing up to 130,000 entries, are available for fast identification. Chemical ionization with positive (PCI) or negative (NCI) ion de-... 

A mass spectrometer has three basic components something to volatilize and ionize the molecule into a beam of charged particles something to focus the beam so that particles of the same mEtssxharge ratio are separated from all others and something to detect the particles. All spectrometers in common use operate in a high vacuum and usually use positive ions. Two methods are used to convert neutral molecules into cations election impact and chemical ionization. 

This approach was pioneered by Hayward et al. [2] for the automated analysis of potential agricultural chemicals by means of eoluttm-bypass thermospray (TSP) MS-MS. The method was found to provide the required information in approximately 70% of the MS stmetural confirmations performed at the Agricultural Research Division of American Cyanamid. Subsequently, an automated routine MS eharaeterization of potential drug eompounds was reported by Tiller and Lane [3] at Glaxo. Both particle-beam (PBl) in positive-ion chemical ionization (Cl) mode and TSP interfaeing were investigated. The TSP system was found to be more robust, to require less maintenance, and to be easier to use. 

Table I. Practical quantitation limits (PQL, ng injected), correlation coefficients (R2 and quadratic regression parameters (a and b) of 21 compounds with electron impact, and positive and negative chemical ionization (methane) Particle Beam mass spectrometry, direct flow injection with full scan mode...

Mass spectrometric data to identify phenolic acids is getting more prevalent. Carini et al. [96] and Kammerer et al. [98] published valuable data about the determination of several phenolic acids and derivatives by HPLC/MS, in mate and black carrot, respectively. The data about the determination of phenolic compounds in wine was reviewed by Monagas et al. [72]. Positive ion thermospray LC/MS using a discharge electrode to effect solvent-mediated chemical ionization yields [M + H]+ ions and some fragmentation to principal losses of H2O, CO, and CO2, when applied to the analysis of simple phenolic acids [99]. Electrospray LC/MS is a more promising method and used successfully to determine phenolic acids and conjugates. Mass spectral data of several phenolic acids in flow injection analysis particle beam electron impact (FIA-PB-EI) MS is shown in Table 10.10 [100]. 

Figure 3 SPE-LC-APCI-MS chromatogram of 200 ml tap water spiked with 0.04 ng ml of pesticides and 0.05 ng ml IS tert-butylazine in positive-ion mode and dinoterb in negative-ion mode). Peak identification 1, bentazone 2, Vamidothion 3,4-nitrophenol 4, MCPA 5, mecoprop 6, dinoseb 7, atrazine 8, isoproturon 9, ametryn 10, malathion 11, fenotrothion 12, molinate 13, prometryn 14, terbutryn and 15, parathion-ethyl. (Reprinted with permission from Aguilar C, Ferrer I, Bormll F, Marce RM, and Barcelo D (1998) Comparison of automated on-line solid-phase extraction followed by liquid chromatography-mass spectrometry with atmospheric-pressure chemical ionization and particle-beam mass spectrometry for the determination of a priority group of pesticides in environmental waters. Journal of Chromatography A 794 147-163 Elsevier.)...

The most important characteristic of a particle-beam interface is the possibihty to obtain El spectra, which can be evaluated according to known rules and compared to those of spectra libraries. For obtaining a meaningful spectrum of a substance, an absolute amount of 10 to 100 ng is required. In 90% of all applications El ionization is applied in 20% of the applications El ionization is combined with chemical ionization in the positive ion mode. AH other Cl applications are carried out in the negative ion mode. 

The thermo-chemical or the initial (neutral, un-ionized specimen with n elec-trons)-final (radiation beam ionized specimen with n — 1 electrons) states relaxation dominates the CLS [6, 7]. The energy required for removing a core electron from a surface atom is different from the energy required for a bulk atom. The surface atom is assumed as a Z -F 1 impurity sitting on the substrate metal of Z atomic number. The energy states of atoms at a flat surface or at a curved surface are expected to increase/decrease while the initial states of atoms in the bulk decreases/increase when the particle size is reduced. This mostly adopted mechanism creates the positive, negative, or mixed surface shift in theoretical calculations. 

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