Peaks trapping

There are four main modes of operation of LC-NMR instruments, in terms of how the compound/fraction of interest is dealt with post chromatographic separation. These methods are continuous-flow, stop-flow, peak parking and peak trapping. There are several different variations within each of these modes (e.g. peak-slicing). Each one of these modes will be described in turn and advantages and disadvantages will be discussed. See Fig. 19.20. 

Fig. 19.22. Schematic showing an LC-NMR system incorporating peak trapping with SPE. The dotted arrows represent electronic controls and the solid lines represent the flow path of the eluent.

While the early days of LC-NMR and LC-NMR-MS were plagued by the poor sensitivity of the NMR spectrometer, the recent probe design advances have provided a means to potentially overcome this hurdle. As reported in the literature, it is possible to get both ID and 2D homo-nuclear and heteronuclear correlation data on sub micrograms of materials in quite complex mixtures utilizing cryogenic flow-probes in tandem with SPE peak trappings [98]. While these technologies are still in their infancy, they have the potential to revolutionize LC-NMR as a structure elucidation technique. 

Figure 6.40 Multiple peak trapping on a SPE cartridge (a) single trap and (b) four times trapping. 1 pg of hydrocortisone per injection, 2 mm Cis HD SPE cartridge, 600 MHz, 3-mm z-gradient LC-SEl probe with an active volume of 60 pi, 1024 transients.

Fig. 12.4 Instrumental set-up for the HPLC-NMR hyphenated approaches, (a) Peak sampling unit using storage loops, (b) Peak trapping onto SPE cartridges with parallel MS and C70genically cooled NMR detection [113], Source Reproduced with permission of Wiley.

Various tools have been developed to enhance the potential of the techniqne, snch as mass-directed peak selection [55], on-line SPE for on-line preconcentration [56] or post-column peak trapping and stop-flow NMR [57]. 

Table 21.1 classifies popular LC detectors according to several criteria for purposes of comparison. At the present time, LC detectors are generally less sensitive than GC detectors, which can detect picograms of material under good conditions. Most LC detectors provide only limited structural information. However, spectrophotometers fitted with micro flow-cells can be used to obtain a stop-flow ultraviolet-or visible-absorption spectrum of an LC peak trapped in the flow cell. On-line coupling of liquid chromatographs with mass or infrared spectrometers offers sophisticated, but indeed expensive, detection/identification methods. Such systems have been described in the literature, but are quite limited by the solvents that can be used in the chromatography step. 

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