Optimum Sensitivity

Instrumental Interfaces. The ideal Ic/ms interface should place no compromises on either the Ic or the ms. Decomposition or loss of analyte should be avoided and efficient transfer provides for optimum sensitivity. 

The detection of spectral sensitizing action often depends on amplification methods such as photographic or electrophotographic development or, alternatively, on chemical or biochemical detection of reaction products. Separation of the photosensitization reaction from the detection step or the chemical reaction allows selection of the most effective spectral sensitizers. Prime considerations for spectral sensitizing dyes include the range of wavelengths needed for sensitization and the absolute efficiency of the spectrally sensitized process. Because both sensitization wavelength and efficiency are important, optimum sensitizers vary considerably in their stmctures and properties. 

To protect the sample from stray electrons from the anode, from heating effects, and from possible contamination by the source enclosure, a thin (-2 pm) window of aluminum foil is interposed between the anode and the sample. For optimum X-ray photon flux on the surface (i. e. optimum sensitivity), the anode must be brought as close to the sample as possible, which means in practice a distance of -2 cm. The entire X-ray source is therefore retractable via a bellows and a screw mechanism. 

Again, we need to define a coupling constant J to set up this experiment. Here for optimum sensitivity we have used an average value for direct (one-bond) carbon-hydrogen coupling constants of 160 Hz. This choice works well for most CH bonds, but is rather low if an acetylenic CH bond is present. 

The most convincing argument for using Si photodiodes in UV detection is the availability of strong expertise in electronic Si devices. Processing and performance of opto-electronic Si devices have been optimized for decades, and the UV-enhanced photodiode is a high-performance niche product that can be produced at a reasonable price, thanks to these efforts. Its probably most serious drawback is the necessity of using filters for visible-blind applications, which considerably increases the cost of sensors and reduces their otherwise optimum sensitivity. 

Furthermore, in a pharmaceutical development environment (working to GMP), redeveloping a method is not a simple task since peak tracking (by LC-MS) and validation may be required and, in the case of registered methods, results obtained by modified methods may not be acceptable. In such cases, it may be more efficient to use a preparative approach followed by conventional tube NMR where optimum sensitivity can be obtained through the use of cryogenic probes (Figure 6.48) or low-volume 1 mm probes [90]. 

The task of generating a display of heteronuclear X/Y-connectivities with optimum sensitivity can in principle be performed by recording a three-dimensional proton detected shift correlation in which the chemical shifts of both heteronuclei X and Y are each sampled in a separate indirect dimension. Three-dimensional fourier transformation of the data then generates a cube which is defined by three orthogonal axes representing the chemical shifts of the three nuclei 1H, X, Y, and the desired two-dimensional X/Y-correlation is readily obtained as a two-dimensional projection parallel to the axis... 

The optimum sensitization efficiency from 30 to 50% was observed at concentrations of dyes 10 3 M. The experimental data for photoconductivity of PAC sensitized by methylene blue are shown in Fig. 46. It is significant that a... 

As already shown, the sensitized spectra follow the dye absorption. Most of the dyes have sufficiently narrow absorption bands. This does not permit us to obtain the panchromatic sensitivity in the sufficiently broad spectral range. It was proposed to use the polymers with conjugated bonds as sensitizers [21]. The broad diffuse absorption spectra are inherent to such compounds. One can expect higher thermal stability from such sensitizers. In addition the application of binder may be omitted from the preparation of the photosensitive layers, for example, in electrophotography. Polymers with triple bonds, polyphenylenes and polyoxiphenylenes were used as sensitizers [10, 14, 278-280]. The typical results are shown in Fig. 47. The main rules for photoconductivity sensitized by polymers were the same as for the dyes. Optimum sensitization was obtained at the concentration of the sensitizer of 10 1-10-2 g/cm3 relative to the polymeric photoconductor weight. 

According to Prodolliet and Bruelhart (33) optimum sensitivity of acesulfame-K is obtained at 230 nm. However, quantification during HPLC analysis has also been performed at 254 (27,52), 233 (65), 227-228 (62,63,74), 217 (66), and 200-210 nm (16,34,47). Acesulfame-K has also been determined by a conductivity detector (17). 

At optimum sensitization, many reduction centers are formed per grain, as detected by a method devised by Spencer (52,53). These centers will not in themselves promote development, but will do so after treatment with an aurous thiocyanate solution. The centers can then be enlarged by arrested development to a size that is visible in electron micrographs. 

Amount of Sulfide Needed for Optimum Sensitization, sturmer and Blackburn (128) used radioactive sulfur to measure the concentration of sulfide ions formed on a silver bromide grain surface by thiosulfate. The number at optimum sensitization depended on the exposure irradiance to be used. The number was about 10,000 ions/pm for a high-irradiance exposure and about 20,000 for a medium-irradiance exposure. 

Cash (129) obtained a lower number, about 4000, but he used a gelatin that induced optimum sensitization upon heating even without added sulfur sensitizer. His gelatin may have contained a sensitizer that contributed to the optimum sensitization. 

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