Quadrupole mass, generally spectrometers

Generally FAB produces protonated, MH+, or depro-tonated, (M — H) , quasi-molecular ions with a little excess energy which will sometimes produce fragment ions of low intensity. FAB is therefore a mild to soft ionisation technique which produces primarily molecular weight information and some structural information. Positive and negative ionisation mass spectra are produced with equal facility. FAB was originally used with magnetic sector mass spectrometers, but lately mainly with quadrupole mass spectrometers (Table 6.10). 

Quadrupole mass spectrometers are generally bench-top instruments, of relatively low cost, easy to manage and widely used for routine analysis. 

Both of the above examples illustrate that the sensitivity of the MDF method, in general, is comparable to PIS analysis and better than NLS methods (Figs 6.9 and 6.10). The sensitivity of the MDF method is attributed at least in part to the full-scan capability of high-resolution mass spectrometers such as the Q-TOF and LTQ-FTICR, which could be several times more sensitive than full-scan analysis by a triple-quadrupole mass spectrometer (Kostiainen et al., 2003). 

Plasma MS is usually based on quadrupole mass analysers. The atmospheric ICP, optimised for ion formation, is placed on its side facing a sample cone (Fig. 4.3). The mass spectrometer operates at reduced pressure and therefore a two- or three-stage differentially pumped interface is needed to transfer the ions from the plasma to the mass analyser. The interface for GC-ICP-MS is generally the same as for ICP emission systems. In one of the earliest GC-MS speciation studies (Chong and Houk, 1987) a packed GC column was used to obtain mass spectra of organic compounds with detection limits in the range 0.001-500 ngs The effects of isotopic fractionation by natural physico-chemical processes were also studied. 

In general, triple stage quadrupole mass spectrometer with ESI in positive ion mode is perceived as the most appropriate MS technique available at present for small—mostly basic—molecules and was used for the assay of nilotinib [112], sorafenib [115,116], lapatinib [117], sunitinib [113,114], bosutinib [88], vatalanib [119] axitinib [120], vandetanib [118] neratinib [87], and crizotinib [125], The latter assay for crizotinib was developed for preclinical experiments and does not contain... 

All major mass spectral data collections consist of El mass spectra, mostly recorded under accepted standardized conditions such as an ionization voltage of 70 eV, an emission current of 100-200 xA, and an ion source temperature of 150-200°C. Several types of GC/MS systems may be applied, for instance, magnetic sector, quadrupole, or ion trap analyzers. Ion trap systems are considered less applicable, when data comparison is required with spectra from a reference library. In particular, basic compounds related to VX or the three nitrogen mustards tend to produce protonated molecular ions by self-protonation. Magnetic sector and quadrupole mass spectrometers suffer less from interference of self-protonation, and spectra produced with these types of instruments are generally reproducible. 

Figure 2.10 shows the general diagram of an instrument with three quadrupoles [8], Quadrupole mass spectrometers are symbolized by upper case Q, and RF-only quadrupoles with a lower case q. A collision gas can be introduced into the central quadrupole at a pressure such that an ion entering the quadrupole undergoes one or several collisions. 

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

Electrospray mass spectra were recorded on a Perkin-Elmer Sciex API-Ill triple quadrupole mass spectrometer (Thornhill, Canada) with general MS and LC setup as previously described (5-8). Modifications of the instrument which improve sensitivity for negative-ion mode detection of phosphorylation and sulfation have also been described before (7,8). Because phosphorylated or sulfated peptides produce predominantly negatively charged marker ions while glycosylation or acylation yield positively charged species, these two classes of modification cannot both be detected in a single LCMS experiment. 

The triple quadrupole mass spectrometer is generally considered as the most versatile of all tandem MS instruments and is useful in the initial evaluation of metabolites for a new compound. ° Several types of... 

LC/MS and LC/MS/MS spectra were acquired on a triple quadrupole mass spectrometer (TSQ-45, Finnigan MAT) using a conventional Finnigan thermospray ion source. No electron filament or discharge electrode was employed. In general, ion source conditions for the polar conjugate metabolites were block temperature 200°-250°C vaporizer... 

Why are detection limits in ICP-MS generally lower with double-focusing. spectrometers than with quadrupole mass spectrometers ... 

In a quadrupole mass analyzer, only a single mass-to-charge ratio m/z) value is transmitted to the detector for any given combination of radio frequency (RF) and direct current (DC) potentials. Typically, the RF/DC ratio is held constant and scanned to provide a mass spectrum. If, for example, a quadrupole is scanned from m/z 1 to 1000 in 1 second, then any particular m/z is transmitted to the detector for only 1 millisecond, representing a duty cycle of 0.1 %. Thus, a quadrupole mass analyzer has a low transmission duty cycle in the full-scan mode, which results in limited full-scan sensitivity. In contrast, ion-trap and TOF mass analyzers have the theoretical potential to transmit all ions that enter the mass analyzer and yield far better sensitivity across the entire mass spectrum. In reality, the pulse sequences associated with these analyzers devote significant time to functions such as ionization and detection. The actual duty cycles are generally between 10 and 25%, still far better than a scanning quadrupole mass spectrometer. 

Multiple Knudsen cells for the determination of chemical activities have been reported by Chatillon and coworkers [148-151], Fraser and Rammensee [134], Zimmermann [152], Paulaitis and Eckert [153], and Hackworth et al. [154]. Multiple cells have practically the same temperature. One cell contains a reference material, the other for instance samples of different compositions. A comparison of the vapor in the different cells is generally rendered possible by their displacement from outside so that the molecular beams originating in the different cells can be analyzed successively. The quadrupole mass spectrometer and not the multiple Knudsen cell is displaced in the Knudsen cell-mass spectrometer system described by Ref. 153. No displacement of the mass spectrometer or the cell is necessary if the triple cell by Hackworth et al. [154] is used. Backdiffusion through the orifice has, however, to be considered for the two cells containing sample and reference. 

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