Sample microcoil

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). 

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. 

Although one can potentially attempt to cool the rf microcoil, which according to Eq. (2.5.2) would further improve the SNR by reducing the Rcoii term in the denominator, in the case of microcoils the sample is extremely close to the coil and if the sample has to be kept at room temperature, cooling the coil alone is extremely difficult. 

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... 

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. 

Putzbach, K., Krucker, M., Grynbaum, M. D., Hentschel, P., Webb, A. G., and Albert, K. 2005. Hyphenation of capillary high-performance liquid chromatography to microcoil magnetic resonance spectroscopy—Determination of various carotenoids in a small-sized spinach sample. J. Pharm. Biomed Anal. 38 910-917. 

One practical difficulty in many studies is the limited amounts of well-characterized samples available, leading to a sensitivity problem for 2D NMR experiments. The recent development of microcoils for MAS-NMR experiments, either static [320] or rotating with the MAS rotor [321], offers much promise for... 

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... 

In order to maximize sample handling efficiency, microcoil experiments can be performed on samples of limited volume with a setup similar to that shown in Figure 7.3.1.8. The liquid susceptibility-matching approach [7] described earlier allows observe factors of up to 70 % without broadening the linewidth. One recent example uses perfluorinated liquid plugs to restrict the total sample... 

As techniques for chemical analysis are used in continually smaller domains, experimental challenges for inherently insensitive methods such as NMR spectroscopy become increasingly severe. Among the various schemes to boost the intrinsic sensitivity of an NMR experiment, the development of small-volume RF probes has experienced a renaissance during the past decade. Commercial NMR probes now allow analyses of nanomole quantities in microliter volumes from natural product extracts and combinatorial chemical syntheses. Figure 7.3.1.9 illustrates the range of volumes that can be examined by NMR probes and accessories such as microsample tubes and inserts. With recently reported advances in sample preconcentration for microcoil NMR analysis [51], dilute microliter-volume samples can now be concentrated into nanoliter-volume... 

Figure 7.3.1.9 Relative sample volumes required for an assortment of NMR probes (a) conventional 5-mm tubes (b) 3-mm tubes with microcells (c) Varian Nano NMR (d) 1.7-mm tubes with minimal solvent (e) microcoil with a Fobs 30 nl...

Schematics of a micromachined NMR probe. The probe consists of a multi-turn electroplated planar microcoil integrated on a glass substrate with etched microfluidic channels for sample containment. The coil has typical dimensions of 2 mm or less, with sample-containment capability ranging from a few nanoliters to 1 [lL, depending on coil size. As a reference, the coil lies in the yz plane with the static magnetic field B0 along the z-axis [797]. Reprinted with permission from Elsevier Science.

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