NMR microcoils

Fig. 2.5.8 COSY spectra of 300 mM D-xylose plus 400 mM borate at pD = 10. The spectra were recorded at 300 MHz with a single NMR microcoil using the instrumentation shown in Figure 2.5.5. (a) Continuous-flow. Flow rate = 2 pL min-1, corresponding to a reaction time t 165 s. The on-flow COSY is highlighted by the presence of intense reactant peaks in the region of 3.0-3.5 ppm. (b) Stopped-flow. The spectrum shows very weak reactant cross...

Providing that hid remains constant, the RF coil sensitivity is inversely proportional to the coil diameter for solenoidal coils of diameter greater than about 100 p,m. It can be shown theoretically and has been demonstrated experimentally that the sensitivity of a solenoidal coil is about three times that of a saddle coil of the same size [3], In order to translate the coil sensitivity into measurement of SIN defined by Equation 7.6, we also have to consider the noise characteristics of NMR microcoils. For coils of diameter less than 3 mm, the major noise source is from the resistance of the coil itself, and the noise from even lossy biological samples can be neglected [10]. 

To date, most NMR microcoils have been wound directly on to a capillary tube which functions as both sample container and coil form [26,27], According to electromagnetic field theory, a sample enclosed by a perfectly uniform and infinitely long hollow cylinder (e.g. a fused silica capillary) experiences a uniform static magnetic field [28]. In reality, the susceptibility variation of the... 

Figure 10 (A) H NMR spectrum of the trace impurity sample (200 pM atenolol and 200 mM sucrose in 50% TE/D20) from 5-mm probe. The expanded and vertically increased area is shown. Microcoil H NMR spectra shown in (B)-(D) recorded and processed with identical parameters. (B) Static NMR spectrum obtained with direct injection of 25 mM atenolol to the NMR microcoil. S/N of atenolol methyl peak is 21. (C) On-flow cITP-NMR spectrum of atenolol sample band at peak maximum during analysis of the trace impurity sample (200 pM atenolol and 200 mM sucrose in 50% TE/D20). No sucrose peaks can be observed. S/N atenolol methyl peak is 34. (D) Stopped-flow cITP-NMR spectrum of sucrose at peak maximum from the same experiment as in (C). (Adopted with the permission from Ref. 41. Copyright 1998 American Chemical Society.)...

Figure 12 Comparison of detection volumes of various NMR probes. Volume is indicated within brackets (A) CE-NMR microcoil (0.030 pL) (B) MRM flow probe (1.5 pL) (C) 1-mm Bruker probe (2.5 pL) (D) 1.7-mm Nalorac probe (20 pL) (E) Nalorac flow probe (24 pL) (F) 2.5-mm Bruker flow probe (30 pL) (G) Varian nanoprobe (40 pL) (H) 3-mm Varian probe (60 pL) (I) 5-mm probe (220 pL).

With the development of microcoil NMR techniques, it is now possible to couple NMR with some microseparation techniques, such as microbore L( 86,93,io3 capillary electrophoresis (CE), and capillary electrochro-matogrphy (CEC), where the inner diameters of the separation channels are in the range of 20 to 75 pm. After coupling these small separation channels with NMR microcoils whose diameters are in the same range, it is possible to achieve maximum mass sensitivity for NMR detec-... 

The size of sample required has been reduced by a number of technical developments including micro inverse probes and micro cells (references in Martin et al. 1998), and has been reduced even further using a newly developed 1.7-mm submicro inverse-detection gradient probe (Martin et al. 1998). The combined use of inverse detection probes with solenoid microcoils has also been developed to reduce sample volumes for NMR (Subramanian and Webb 1998). 

Subramanian R, AG Webb (1998) Design of solenoidal microcoils for high-resolution C NMR spectroscopy. Anal Chem 70 2454-2458. ... 

The development of microcoil techniques has been reviewed by Minard and Wind [24, 25] and by Webb [26]. In a more recent publication Seeber et al. reported the design and testing of solenoidal microcoils with dimensions of tens to hundreds of microns [27]. For the smallest receiver coils these workers achieved a sensitivity that was sufficient to observe proton NMR with an SNR of unity in a single scan of 10 pm3 (10 fL) of water, containing 7 x 1011 proton spins. Reducing the diameter of the coil from millimeters to hundreds of microns thus increases its sensitivity greatly, allowing analysis of pL to pL sample volumes. 

D.L. Olson, T. L. Peck, A. G. Webb, R. L. Magin, J. V. Sweedler 1995, (High resolution microcoil H-NMR for mass limited nanoliter-volume samples), Science 270, 1967... 

FIGURE 4.5 Schematic design of a microcoil NMR probe. (From Rehbein, J. et al., Characterization of Bixin by LC-MS and LC-NMR, John Wiley Sons Ltd., 2387, 2007. With permission.)... 

In summary, NMR spectroscopy is an extremely versatile tool useful that enables researchers to understand the structure of natural products such as carotenoids. For a full structural assignment, the compound of interest has to be separated from coeluents. Thus, it is a prerequisite to employ tailored stationary phases with high shape selectivity for the separation in the closed-loop on-line LC-NMR system. For the NMR detection, microcoils prove to be advantageous for small quantities of sample. Overall, the closed-loop system of HPLC and NMR detection is very advantageous for the structural elucidation of air- and UV-sensitive carotenoids. 

Krucker, M., Lienau, A., Putzbach, K., Grynbaum, M. D., Schuler, P., and Albert, K. 2004. Hyphenation of capillary HPLC to microcoil 11 NMR spectroscopy for the determination of tocopherol homologues. Anal. Chem. 76 2623-2628. 

Webb, A. G. 1997. Radio frequency microcoils in magnetic resonance. Prog. NMR Spec. 31 1-42. 

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