Negative-ion mode

Mostly, positive-ion FAB yields protonated quasi-molecular ions [M -i- H]+, and the negative-ion mode yields [M - H]. In the presence of metal salts (e.g., KCl) that are sometimes added to improve efficiency in the LC column, ions of the type [M -i- X]+are common, where X is the metal. Another type of ion that is observed is the so-called cluster, a complex of several molecules with one proton, [M -i- H]+ with n = 1, 2, 3,. .., etc. Few fragment ions are produced. 

S. Lacorte and D. Barcelo, Determination of parts per trillion levels of organophospho-rus pesticides in groundwater by automated on-line liquid- solid extraction followed by liquid chr omatography/atmospheric pressure chemical ionization mass spectrometry using positive and negative ion modes of operation . Anal. Chem. 68 2464- 2470 (1996). 

Negative Cl can give excellent results for certain types of compounds. Compounds with electronegative substituents and unsaturation can be expected to have a large electron capture cross-section and thus work well in the negative ion mode. Frequently, much higher sensitivity is obtained for these compound types in the negative ion mode than under positive ion conditions. In addition, the molecular ion is usually very abundant. The... 

To check the identity and purity of the products obtained in the above reactions it is not sufficient to analyze for the sulfur content since a mixture may incidentally have the same S content. Either X-ray diffraction on single crystals or Raman spectra of powder-like or crystalline samples will help to identify the anion(s) present in the product. However, the most convincing information comes from laser desorption Fourier transform ion cyclotron resonance (FTICR) mass spectra in the negative ion mode (LD mass spectra). It has been demonstrated that pure samples of K2S3 and K2S5 show peaks originating from S radical anions which are of the same size as the dianions in the particular sample no fragment ions of this type were observed [28]. 

Electrospray is the softest mass spectrometry ionization technique and electrospray spectra therefore usually consist solely of molecular ions. Electrospray is unique, however, in that if the analyte contains more than one site at which protonation (in the positive-ion mode) or deprotonation (in the negative-ion mode) may occur, a number of molecular ions with a range of charge states is usually observed. For low-molecular-weight materials (< 1000 Da), the number of sites... 

The reagent species in the positive-ion mode may be considered to be proto-nated solvent ions, and in the negative ion mode 02, its hydrates and clusters. 

As with conventional Cl, this is a very mild form of ionization leading to molecular species with little or no fragmentation, i.e. (M + H)+ and (M — H) . This is not, however, always the case. The use of chromatographic modifiers may change the composition of the Cl plasma to such a state that, as in Cl and thermospray, other ions may be formed, e.g. the presence of ammonium acetate may lead to (M - - NH4)+ and (M - - CHsCOO)" ions in the positive- and negative-ion modes, respectively. The chemistry of the analyte may also have an effect, as has been discussed for ESI, with, for example, the spectra of fullerenes extracted from soot particles yielding an M+ molecular species [16],... 

Arita and coworkers introduced a mixed matrix composed of triethanolamine, 1,1,3,3-tetramethylurea (8) and triethylenetetramine for analyzing glycosphingolipids in the negative-ion mode this gives results similar to those obtained with 1-thioglycerol. ... 

Description glycosidic cleavage with a hydrogen transfer charge retained on reducing end positive- and negative-ion modes often referred to as )S-cleavage. 

Fig. 15.—Sequence of One of the Oligosaccharides Obtained from the Trehalose-containing Glycolipids of Mycobacteria kansasii. [Cleavage positions in the positive-ion mode (Pathway C, Section IV,2) are shown above the sequence. Cleavage positions in the negative-ion mode (Pathway B, Section IV,2) are shown below the sequence.

Electrospray (Turbo lonSpray), negative ion mode MS/MS with multiple reaction monitoring (MRM) -4500 V... 

Analysis of potential endocrine disrupters, such as alkylphenols (degradation products of non-ionic surfactants), diphenolalkanes (cumylphenol, bisphenol-A, bisphenol-F) and benzophenones (from the paper industry) with RPLC-SSI-QITMS in the negative ion mode has been described [631]. 

ESI operating in the negative ion mode has been the interface most widely used for the analysis of anionic perfluorinated surfactants. In addition, ESI has also been optimized for the determination of neutral compounds such as the sulfonamides perfluorooctanesulfonamide (PFOSA), perfluorooctanesulfonami-doacetate (A-EtFOSAA), and t-Bu-PFOS. The use of APPI has been explored in few works [126-128], Takino et al. [126] found as the main advantage of this technology, the absence of matrix effects, but the LODs were considerably higher than those obtained by LC-ESI-MS-MS. 

One of the specificities of ToF-SIMS is the possibility to switch easily from positive to negative ion mode by reversing the extraction potential. The mass calibration of spectra is internal, using low mass ions (H+, H2+, H3+, C+, CH+, CH2+ and CH3+ ion peaks in... 

Adduct ions are quite frequent in the mass spectra. In positive ion mode, sodium or potassium cluster ions are commonly found. Mineral compounds often lead to multiple cluster ions. For example, spectra of FeCl3 in negative ion mode lead to several peaks from m/z 35 (Cl ) to m/z 487 ([(FeCl3)2FeCl3] ) [Van Ham et al. 2004],... 

Different categories of Zonyl polymers are studied by ToF-SIMS both in the positive and negative ion mode. Studies have shown that, for each polymer, a specific fingerprint is obtained and the peaks corresponding to the specific chemical moieties of each polymer are detected (Figure 15.4). To represent this good selectivity, Principal Component Analysis is performed on the obtained spectra. The result clearly discriminates the different polymers. ToF-SIMS is then suited to the characterization of these materials. 

Carr et al. [2004] studied the incidence of different cleaning procedures on wool. ToF-SIMS analyses performed on the commercially scoured wool (negative ion mode) showed the presence of 18-methyleicosanoic acid thioester species (m/z 341), attributed to the presence of 18-methyleicosanoic acid (18-MEA) which is normally the predominant compound of the surface layer of wool. Other lipids are also detected. After artificial sunlight exposure, analyses show that 18-MEA disappears from the surface. 

Spectra of the greasy wool are more complicated than in the previous study. In negative ion mode, different fatty acids, fatty alcohols and alkanes are detected, whereas the positive ion mode shows mainly the presence of cholesterol and the cholesterol oxidation product (Figure 15.6). These ions are attributed to the presence of wool wax on the surface of raw wool. 

The aim of the study was to reveal whether or not egg tempera was mixed with oil as binding media. The cross-section was studied in both positive and negative ion modes. Lead white with linseed oil, tripalmitin, stearic acid and lead white with egg tempera were studied as reference samples. 

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