Sensitive-point method

The sensitive-point method was introduced in 1974 by Hinshaw [Hinl, Hin2]. The z-component of the magnetic field is modulated in a sinusoidal fashion by application of time-dependent field gradients in x-, y-, and z-directions (Fig. 5.1.2),... 

Biological Aspects.— The application of n.m.r. spectroscopy to biological systems continues to grow. There have been reviews of its use in the study of living tissue and for n.m.r. imaging. In the latter field an alternative sensitive-point method gives improved full spectra but it has limited sensitivity. ... 

Criteria for solvent purity include careful measurement of physical constants such as melting point, refractive index, or conductance, but even these techniques may not detect some trace impurities. In many instances gas-liquid or other sensitive chromatographic methods may be used. 

To verify this point, a much more sensitive analytical method, fluo-rescencet was used to search for traces of quinine in the permeate solution. The magnitude of the absorbance in Fig. 18C indicates that the pyridine concentration in the permeate is 7 X 10 M. With fluorescence analysis it is possible to detect 5 X 10 M quinine in the presence of 7 X 10 M pyridine. However, no quinine fluorescence could be detected from the... 

By modulating the electric field and using phase-sensitive detection methods, Uehara et al. 8 ) were able to increase the sensitivity considerably and they could even detect Stark splittings of less than the doppler width of the components. Fig. 3 shows the Stark spectrum of HDCO for different electric field strengths. Because of the Stark modulation technique the absorption lines appear differentiated the zero points represent the center of each line. 

The first two points represent a general motivation for miniaturization in separation science independent of the actual fabrication technology. The benefit of a reduction of the consumption of sample, reagents, and mobile phase in chemical and biochemical analysis is self-evident and does not need to be discussed further (reduced consumption of precious samples and reagents, reduced amounts of waste, environmental aspects). This advantage is, however, sharply contrasted by its severe implications on the detection side, as discussed elsewhere in this volume in detail. The detection of the separated zones of very small sample volumes critically depends on the availability of highly sensitive detection methods. It is not surprising that extremely sensitive laser-induced-fluorescence (LIF) has been the mostly used detection principle for chip-based separation systems so far. 

Multiple-point fluorescent deteclion has been proposed to enhance detection sensitivity. This method is based on the use of a detector function, such as the Shah function. The time-domain signals were first detected, and they were converted into a frequency-domain plot by Fourier transformation. Therefore, this technique was dubbed Shah convolution Fourier transform detection (SCOFT). As a comparison, the single-detection point time-domain response is commonly known as the electropherogram [698,699,701]. 

Quantitative methods of sensitivity analysis and metrics for measuring sensitivity are widely available. The most commonly used sensitivity analysis methods are often relatively simple techniques that evaluate the local linearized sensitivity of model response at a particular point in the input domain. This type of approach is typically used if the model inputs are treated as point estimates, often representing the best guess as to the true but unknown value of each... 

Any self-developed method or discipline will consist of personal features and individual points of view that may or may not reflect the opinion of the majority which may not necessarily be the most valid position. One should expect, therefore, that the criteria of those readers who have already created their own systematic approach to problem solving in mechanistic organic chemistry may be in conflict with some sensitive points expressed here. In addition, there are those who feel that rules in general jeopardize their ability to create and thus to find original solutions to problems. 

We have used sensitivity equation methods (Leis and Kramer, 1985) for gradient evaluation as these are simple and efficient for problems with few parameters and constraints. In general, the balance in efficiency between sensitivity and adjoint methods depends on the type of problem being addressed. Adjoint methods are particularly advantageous for optimal control problems in which the inputs are represented as a large number of piecewise constant input values and few interior point constraints exist. Sensitivity methods are preferable for problems with few parameters and many constraints. 

The determination of catecholamines requires a highly sensitive and selective assay procedure capable of measuring very low levels of catecholamines that may be present. In past years, a number of methods have been reported for measurement of catecholamines in both plasma and body tissues. A few of these papers have reported simultaneous measurement of more than two catecholamine analytes. One of them utilized Used UV for endpoint detection and the samples were chromatographed on a reversed-phase phenyl analytical column. The procedure was slow and cumbersome because ofdue to the use of a complicated liquid-liquid extraction and each chromatographic run lasted more than 25 min with a detection Umit of 5-10 ng on-column. Other sensitive HPLC methods reported in the literature use electrochemical detection with detection limits 12, 6, 12, 18, and 12 pg for noradrenaline, dopamine, serotonin, 5-hydroxyindoleace-tic acid, and homovanillic acid, respectively. The method used very a complicated mobile phase in terms of its composition while whilst the low pH of 3.1 used might jeopardize the chemical stability of the column. Analysis time was approximately 30 min. Recently reported HPLC methods utilize amperometric end-point detection. 

With line methods, the spin density along one line of n voxels in the object is interrogated simultaneously (Fig. 5.0.1 (b)) [Manl ]. Thus the sensitivity improves by a factor of n compared to point methods. 

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