Correlator hardware limitation

Correlator Hardware Limitation. The equivalent, average number of photopulses per sample time in this experiment was 16. The typical 4-bit circuitry found in many correlators would suffer overflow... 

Correlator hardware and linearly-spaced sample time limitations are discussed. Data transformation from an intensity-weighted to a mass-weighted size distribution is demonstrated. The artificial width of broad distributions is discussed. The use of multiangle measurements is considered, and the effect of baseline error is shown using computer simulated data. 

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. 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

Chee and Wan (1993) believe that CZE offers some advantages over MEKC for drug screening, particularly, simple background electrolyte preparation and shorter analysis times. They see the main limitation as the inability to analyze acidic, neutral, and basic drugs together. An additional advantage offered by CZE is that a peculiar separation mechanism, poorly correlated with MEKC, allows the use of this technique in parallel with MEKC for confirmation purposes, with the same instrumental hardware. 

Even if "X, "Y correlation spectroscopy imposes no principle differences with respect to H, Y correlations, its widespread use was until recently limited by hardware restrictions recording of such spectra under additional H... 

The progress in NMR hardware to support the structure elucidation of limited quantity samples allowed to exploit the ADEQUATE experiment to a greater extent [2,93,94]. The advantage of the ADEQUATE pulse sequence originates from C-C magnetization step transfers that allow to specifically differentiate between Jch and Jch bonds which is not possible from HMBC-based experiments. It also includes connectivity information to non-protonated carbon atoms. Thus, multiplicity-edited H-C HSQC and 1,1-ADEQUATE experiments were co-processed to yield a C—C correlation plot [28]. The map was diagonally symmetric in case of adjacent Jcc-coupled protonated carbons and asymmetric in case of Jcc between protonated and quaternary carbons. It was emphasized that the latter responses were observed at the C shift of the protonated carbon in the FI direction and the correlation at the C shift of the quaternary carbon in the F2 dimension, cf. exemplarily for the methylene Cl 1 and the carbonyl CIO in Fig. 5.12. The well-known compound strychnine served as proof-of-principle for the HSQC-1,1-ADEQUATE. 

Process qualification tests can be the same set of accelerated wearout tests used in design qualification. As in design qualification, overstress tests maybe used to qualify a product for anticipated field overstress loads. Overstress tests may also be exploited to ensure that manufacturing processes do not degrade the intrinsic material strength of hardware beyond a specified limit. However, such tests should supplement, not replace, the accelerated wearout test program, unless explicit physics-based correlations are available between overstress test results and wearout field-failure data. 

This article provides a survey of quantum chemical studies on the structure and properties of isolated DNA bases and their hydrogen-bonded and stacked pairs. Recent developments of computer hardware and software have allowed electron correlation to be considered even for very large systems, leading to a qualitative improvement in the accuracy of calculated properties. Experimental characterization of isolated bases and base pairs is very difficult, and only a limited number of reliable studies are available. There is still only one published gas phase experiment on the energetics of H-bonded nucleic acid bases, from the end of the 1970s, and experimental data on base stacking are absent the same is true for the structure of bases and base pairs. High-level quantum chemical calculations thus represent the only tool to obtain reference data on the structure, properties, and interactions of nucleic acid bases. These data help to properly understand the function and properties of nucleic acids and are exceptionally important for verification and/or parametrization of empirical potentials... 

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