Sensitivity problem

The negative sign in equation (b 1.15.26) implies that, unlike the case for electron spins, states with larger magnetic quantum number have smaller energy for g O. In contrast to the g-value in EPR experiments, g is an inlierent property of the nucleus. NMR resonances are not easily detected in paramagnetic systems because of sensitivity problems and increased linewidths caused by the presence of unpaired electron spins. 

Note that the problem analysed in this section can be viewed as a shape sensitivity problem (see Sokolowski, Zolesio, 1992 Pironneau, 1984 Oht-suka, 1986, 1994 Mroz, 1963 Schaeffer, 1975 Kinderlehrer, 1982 Athana-sopoulos, 1981). 

In analyzing these data it is necessary to go beyond the curves and determine the expected behavior of the material with respect to notch sensitivity. Problems with notch sensitivity can often be corrected by modifying the processing steps and/or heat treatment. 

Internal surfactants, i.e., surfactants that are incorporated into the backbone of the polymer, are commonly used in PUD s. These surfactants can be augmented by external surfactants, especially anionic and nonionic surfactants, which are commonly used in emulsion polymerization. Great attention should be paid to the amount and type of surfactant used to stabilize urethane dispersions. Internal or external surfactants for one-component PUD s are usually added at the minimum levels needed to get good stability of the dispersion. Additional amounts beyond this minimum can cause problems with the end use of the PUD adhesive. At best, additional surfactant can cause moisture sensitivity problems with the PUD adhesive, due to the hydrophilic nature of the surfactant. Problems can be caused by excess (or the wrong type of) surfactants in the interphase region of the adhesive, affecting the ability to bond. 

Thin-layer chromatography (TLC) is used both for characterization of alcohol sulfates and alcohol ether sulfates and for their analysis in mixtures. This technique, combined with the use of scanning densitometers, is a quantitative analytical method. TLC is preferred to HPLC in this case as anionic surfactants do not contain strong chromophores and the refractive index detector is of low sensitivity and not suitable for gradient elution. A recent development in HPLC detector technology, the evaporative light-scattering detector, will probably overcome these sensitivity problems. 

We have the very important result that choices for the eigenvalues for the pole-placement design and the observer design can be made independently. Generally, we want the observer response to be two to five times faster than the system response. We should not have to worry about saturation since the entire observer is software-based, but we do have to consider noise and sensitivity problems. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

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... 

In most applications, the electrochemical compounds are usually oxidized, yielding one or more electrons per molecule reacted. The oxidized form is usually unstable and reacts further to form a stable compound that flows past the carbon electrode surface. Unfortunately, this is not always the case, with the stable oxidized form occasionally building up at the surfaces of the carbon electrode. This creates sensitivity problems and decreases the efficiency of the detector. However, the problem is usually overcome by regularly cleaning the carbon electrode surfaces, removing any oxidizable products. Eluents for EC detection must be electrochemically conductive, which is achieved by the addition of inert electrolytes (to maintain a baseline current) such as phosphate or acetate. All solvents and buffers used in preparation of an eluent must be relatively pure and selected so as to not undergo electrochemical changes at the applied electrode potentials. 

Inaccurate temperature Unstable temperature Incorrect slit Broken cartridge Inaccurate voltage Unstable voltage Inaccurate current Unstable current Inaccurate wavelength Sensitivity problem Noise... 

Sensitivity problem Inaccurate determination of the impurity profile at the lower concentration level... 

This method suffered from sensitivity problems initially as the bile-acid molecules lack a chromophore, but did offer the distinct advantage that conjugated bile acids could be determined without hydrolysis. The sensitivity issue was addressed by use of fluorescent derivatives such as dimethoxycoumarin esters with a C18 reverse phase column and were able to resolve endogenous mixtures of bile acids. The combination of hplc and mass-spectroscopy detection has further improved the sensitivity along with providing specific identification, important as the resolution of bile acids by hplc is not as good as capillary column glc. ... 

For an analytical LC injection, there is a shortfall of analyte in the active volume by between 50 and 100-fold, and therefore LC injections loadings must be increased by this order just to obtain a one-dimensional proton spectrum. The sensitivity problem is exacerbated by the fact that LC peaks are generally of larger volume than the active volume (Figure 6.34) and so not all the anal54e is detected [57]. 

The use of glass and quartz capillaries is, in principle, not restricted to what nowadays are medium frequencies (400-600 MHz). This type of high pressure NMR equipment will certainly be further improved, mainly to study biological samples by high-field H NMR. To overcome the serious sensitivity problem inher-... 

In addition, the detailed considerations of the above listed investigators are of great practical importance in understanding and coping with explosion hazards. It seems clear from their, studies that the sensitivity problem is contained wholly within the framework of the heat-balance equations ... 

We have noted the noise-sensitivity problem of the simple inverse filter and introduced modifications to alleviate these difficulties. Modifications yielded different functional forms for y(co). The convenient single-step property of the basic method was nevertheless retained. This property contrasts with the need for possibly arbitrary stopping criteria when we use iterative methods, which are computationally more expensive. The iterative methods do, however, allow the user to control the signal-to-noise versus resolution tradeoff by stopping the process when the growth of spurious... 

Typically, a positive -intercept indicates the existence of interference with the response or the saturation of responses at high concentrations. A negative -intercept indicates the possibility of method sensitivity problem (i.e., a low response cannot be detected) or analytes get retained in the glassware or HPLC system (i.e., a compatibility issue between sample solvent and mobile phase). 

With 2D spectra, depending on the kind of experiment, problems with resolution and/or sensitivity may arise. Resolution is an inherent problem with 2D experiments and is mainly caused by the limited amount of spectrometer time and disk storage capacity. Sensitivity may be a problem when measuring small sample amounts or weak effects (NOESY, ROESY,. ..). In heteronuclear 2D experiment.s the sensitivity problem is more pronounced with X- rather than with H detection. 

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