High-resolution scans full scanning

Fig. 17. The He(I) UPS of ferrocene (a) full spectrum, (b) high-resolution scan of the first two band systems. (From Ref. 123.)...

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

MS11 capabilities. However, ions may then subsequently be detected at unit resolution using an electron multiplier or, alternatively, focused in a C-Trap (Figure 5.2) and then transferred and detected at high resolution using the Orbitrap. In our experience with the LTQ-Orbitrap, ions may be measured with a resolution of approximately 60,000 with online LC/MS in the full scan mode. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

MS, chemical ionization-MS, and sometimes GC/infrared spectroscopy (IR) have been used with GC/MS to obtain structural information. Examples of the use of GC/ MS for identifying new DBFs include the recent identification of iodo-acids. The iodo-acids were discovered in drinking water treated with chloramination through the use of full-scan GC/MS on the methylated extracts. Empirical formula information for both the molecular ions and the fragment ions was obtained by high-resolution electron ionization (EI)-MS, and the spectra were interpreted to yield tentative identifications of five new iodo-acids (iodoacetic acid, bromoiodoacetic acid, ( )-3-bromo-3-iodopropenoic acid, (Z)-3-bromo-3-iodopropenoic acid, and )-2-iodo-3-methylbutenedioic acid). Structural assignments were then confirmed by the match of mass spectra and GC retention times to authentic chemical standards, several of which had to be synthesized. 

The combination of enantio-MDGC with high-resolution MS or mass-selective detectors, both used in full scan or (at least) in the multiple ion monitoring (MIM) mode is currently the most potent analytical tool in enantioselective analysis of chiral compounds from complex mixtures. 

The emergence of high-resolution LC-MS technologies has greatly facilitated metabolite identification tasks with enhanced resolution, high mass accuracy, and high sensitivity in a full-scan mode (Meyer et al., 1996 Yoshitsugu et al., 1999 ... 

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). 

In conclusion, the sensitivity and specificity for screening for oxidative metabolites by the three LC-MS methods are variable, compound-dependent, and complementary to each other. The MDF method requires a simple, generic full-scan analysis using a mass spectrometer equipped with high-resolution capability, and the accurate mass information obtained can provide empirical formula information for metabolites and their fragments. Since MDF employs a different detection... 

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