Peak selectivity

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

In contrast to the peak selectivity just discussed, there is evidence that the larger, more flexible, ligands tend to exhibit plateau selectivity - a reflection that a number of the larger metal ions are accommodated by the cryptand without major variation in binding energy. 

Differentiation of vapor responses of the colloidal crystal film was accomplished with spectral measurements of the shape changes of the diffraction peak. Selectivity of response was obtained by applying multivariate data analysis to correlate these spectral changes to the effects of species of different chemical nature and to establish the identity and concentration of species. 

As a consequence of the development of extraction methods for STA based on mixed-mode SPE columns, as well as of the recent introduction of instruments for the automated sample preparation allowing efficient evaporation and derivatization of the extracts, full automation of STA methods based on GC-MS analysis is also available. It needs GC-MS instalments equipped with an HP PrepStation System. The samples directly injected by the PrepStation are analyzed by full scan GC-MS. Using macrocommands, peak identification and reporting of the results are also automated. Each ion of interest is automatically selected, retention time is calculated, and the peak area is determined. All data are checked for interference, peak selection, and baseline determination. 

Both the capillary LC and NMR are controlled by the interface software, which enables the operator to use the UV-detector output for peak selection. Only peaks of interest can be subjected to NMR analysis, while minor or unimportant compounds can be directed to waste. NMR acquisition can take place in either on-flow or stop-flow mode. The combination of capillary LC and NMR is suitable for sample-limited applications (e.g., proteomics) and allows for low nanograms detection. 

Peak Select Occ Peak Select Occ Peak Select Occ... 

Fig. 9. Selectivity behaviour of a typical rigid ligand (e. g. 30 peak selectivity) (left), and of a typical flexible ligand (e.g. 33 plateau selectivity) (right)...

Aishima used hierarchical cluster analysis on gas chromatographic profiles [10 peaks out of 93 measured peaks, 48 samples seleeted out of 200 samples of eight brands (categories) of soy sauce]. The obtained dendrograms were mainly discussed in connection with the results of linear discriminant analysis and the ten peaks selected for clustering. 

A more precise cavity control is found with the three-dimensional cryptands and Figure 26 shows the variation of Ks with alkali and alkaline earth metal cations for a range of cryptands.29 Two types of selectivity are displayed. Peak selectivity occurs when there is an excellent correlation between cavity and cation radius (Table 8) and so a sharp discrimination can occur (Figure 26). Plateau selectivity occurs when there is little discrimination between cations of higher radius but discrimination against cations of small radius which are not well accommodated (Figure 26). 

Finally, use of the loop-storage/loop-transfer technique provides an escape from this dilemma. The chromatogram is acquired without any interruption, the mass spectrometer acquires MS and MS2 spectra online, and provides spectroscopic information which allows reliable peak selection for the storage procedure. The MS unit is used above all as a very selective and sensitive detector. After the chromatographic separation, it is available for other purposes and the collected samples are then independently analysed in the NMR spectrometer. 

For efficient use of the mass spectrometer, data exchange between the MS software and the LC-NMR software must be possible. Preferably, peak selections based on the MS chromatograms is carried out by the LC-NMR software. 

As preferably one user should be able to operate the whole system, it becomes clear that a highly integrated system is needed. Even if the peak selection is carried out by the user, the positioning of the sample, followed by the start of the NMR and MS experiments, must take place automatically. An integrated LC-NMR/MS system therefore has to contain the following components ... 

In contrast to the crown ethers, the three-dimensional cryptands display peak selectivity. The macrobicyclic cavities are more rigid and restricted so that they are unable to constrict sufficiently... 

The capillary HPLC separation from a selected protein spot provides a base-peak profile shown in Figure 6.2A. The base-peak profile is similar to a total ion current (TIC) profile, but it contains only the most abundant mass spectral peak in each scan. The chromatogram is simplified and the contributions from background ion abundances are eliminated, resulting in an enhanced signal-to-ion ratio for an improved visualization of data. The molecular mass for each component is labeled along with corresponding amino acid residues. This format provides a comprehensive approach for peak selection and peptide identification. 

Dear GJ, Plumb RS, Sweatman BC et al. (2000) Mass directed peak selection, an efficient method of drug metabolite identification using directly coupled liquid chromatography-mass spectrometry-nuclear magnetic resonance spectroscopy. J Chromatogr B Biomed Sci Appl 748 281-293... 

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