System gradient

In case of fast gradient (below 15 min), S could be considered constant for all the investigated molecules and wiU only have a small influence on the retention time of the compounds. Thus, the gradient retention times, of a calibration set of compounds are linearly related to the ( )o values [39]. Moreover, Valko et al. also demonstrated that the faster the gradient was, the better the correlation between t, and < )o [40]. Once the regression model was established for the calibration standards, Eq. 8 allowed the conversion of gradient retention times to CHI values for any compound in the same gradient system. Results are then suitable for interlaboratory comparison and database construction. The CH I scale (between 0 and 100) can be used as an independent measure of lipophilicity or also easily converted to a log P scale. 

NMR microscopy systems are built as add-ons to NMR spectrometers. The general technology of an NMR spectrometer is not the subject of this chapter, only the parts that are of particular importance in NMR microscopy applications, such as the magnets, the shim systems, the gradient systems, the NMR microscopy probes and the gradient amplifiers, will be described in some detail. 

The whole NMR imaging sensor system usually consists of a magnet, a shim system mounted inside the room-temperature bore of the magnet, a gradient system mounted inside the shim system and the rf coil mounted inside the gradient system. In the case of a saddle coil or a birdcage resonator, open access can be realized from the bottom to the top of the entire system with the coil diameter. 

Some applications are shown in the following sections, where the standard hardware, software and methods have been partially modified or connected to special experimental constructions. In the past, other applications have been performed by specific groups, who built their own dedicated rf probes and/or gradient systems. These originally specialized products have now found their way into the commercially available NMR microscopy products of today [18, 19]. 

A single cell layer of an excised piece of onion epidermis was selected as a test object for this micro-coil, (Figure 2.1.14), and an imaging experiment was carried out, similar to the one by Mansfield et al., who used a laboratory-made micro-coil probe and gradient system [31]. Other micro-coil types, e.g., volume coils or coils that are immersed into the objects, can be adapted to specific applications and mounted on commercial imaging probes. 

Fig. 2.1.17 Conventional NMR microscopy gradient system (Micro2.5, 2.5 C cm-1 A-1, 40-mm inner diameter) (right) and a conventional MAS probe (left). The probe is mounted into the separate gradient system instead of the original imaging probe.

Box, K., Bevan, C., Comer, J., Hill, A., Allen, R., Reynolds, D., High throughput measurement of pKa values in a mixed-buffer linear pH gradient system. Anal. Chem. In press. 

If screening for general intestinal absorption = pH gradient system, only report to projects a —> b direction... 

If screening for carrier-mediated transport in any direction = both a nongradient and a pH gradient system should be used and bidirectional transport data should be reported... 

Most chromatographers limit themselves to binary or tertiary gradient systems however, it should be noted that pumps capable of quaternary gradient are available (see reference 3, Chapter 3) and should be considered in the equipment selection process to allow greater versatility in method development. 

Advanced gradient system For optimum functionality in automated systems designed primarily for reversed-phase chromatography and other gradient techniques, the LKB advanced-gradient system is recommended (Fig 1.2 (c)). Key features include ... 

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