Small volume conventional probes

Smaller diameter probes reduce sample volumes from 500 to 600 pi typical with a 5 mm probe down to 120-160 pi with a 3 mm tube. By reducing the sample volume, the relative concentration of the sample can be correspondingly increased for non-solubility limited samples. This dramatically reduces data acquisition times when more abundant samples are available or sample quantity requirements when dealing with scarce samples. At present, the smallest commercially available NMR tubes have a diameter of 1.0 mm and allow the acquisition of heteronuclear shift correlation experiments on samples as small as 1 pg of material, for example in the case of the small drug molecule, ibu-profen [5]. In addition to conventional tube-based NMR probes, there are also a number of other types of small volume NMR probes and flow probes commercially available [6]. Here again, the primary application of these probes is the reduction of sample requirements to facilitate the structural characterization of mass limited samples. Overall, many probe options are available to optimize the NMR hardware configuration for the type and amount of sample, its solubility, the nucleus to be detected as well as the type and number of experiments to be run. 

For mass-limited samples, small-diameter (conventional) probes present an attractive alternative to cryogenic probes. In collaboration with Hoffmann-La Roche, Bruker has recently developed a 1 mm probe with a sample volume of 5 pL. This probe offers a mass sensitivity that typically is fourfold higher than a conventional 5 mm probe. An example displaying a [13C, 1H]-HSQC of 1 pg Ibuprofen at 13C natural abundance is shown in Fig. 3.6 below [25]. 

These small-volume systems are good choices for either high-throughput semiautomated analysis or in environments with multiple users, because the probes can be easily switched with other standard probes.10 S/N comparisons between conventional tube systems and flow solenoids are especially problematic. However, based on comparisons between a 1-mm cryogenic HTS probe93 and values in the literature from a 1-mm room temperature solenoid,94 about 30 pg of a 500 Da sample would give comparable results to a 1 mmol F 1 sample in a 5-mm warm copper probe. 

There are probes specifically designed for small volumes of sample (e.g., the Bruker microprobe, which handles 80 pL volumes and the Varian nanoprobe, which handles 40 pL volumes), but these probes suffer from problems with either low spectral resolution and/or low throughput, not to mention the order-of-magnitude-plus higher cost associated with sample vessels versus conventional NMR tubes. Nonetheless, if our application requires and benefits from the use of one of these small-sample-volume probes, we will be glad that these probes exist. 

Conventional LC-NMR employs separation columns which are connected to NMR detection probes via open tubular capillaries with internal diameters substantially smaller than the column to avoid extra-column band broadening. Since the band broadening introduced by the connections and transfer lines causes significant peak dispersion in capillary separations, on-column detection for small-volume LC is desirable. Because of the need for deuterated solvents for NMR detection, the relatively large volumes and flow rates used with conventional analytical HPLC columns make coupling of LC and NMR an expensive experiment. 

In surface studies, one is confronted with the difficulty of detecting a small number of surface atoms in the presence of a large number of bulk atoms a typical solid surface has 10 atoms/cm as compared with 10 atoms/cm in the bulk. In order to be able to probe the properties of solid surfaces using conventional methods, one needs the use of powders with very high surface-to-volume ratio so that surface effects become dominant. However, this technique suffers from the distinct disadvantage of an entirely uncontrolled surface structure and composition which are known to play an important role in surface chemical reactions. It is thus desirable to use specimens with well-defined surfaces which generally means small surface area, of the order of 1 cm, and examine them with tools that are surface sensitive. 

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