Introduction and Advantages

The negative electron affinity (NEA) photocathode is the most recent and generally the highest-performance photoemissive surface discovered to date [5.4,11-21]. Its operation is an extension of the same physical principles which apply to all other photoemissive surfaces, differing only in the means of obtaining low electron affinity and in the specific factors which determine photo yield. The principles of operation are so similar to those of classical emitters that the first NEA surface was completely predicted by theoretical extension of classical principles prior to its first experimental fabrication [5.66]. 

The two major physical differences between NEA and classical emitters are, first, that emitted electrons in NEA surfaces are not hot , i.e they have thermalized to the bottom of the conduction band, and second, that the value of electron affinity, although genuinely negative, is the effective value in the bulk and not the true value at the surface [5.15], 

As the discussion of the semitransparent GaAs photocathode will show, there are a tremendous number of trade-offs involved in the selection of materials parameters for NEA emitters, especially in those for imaging applications. As a result, a large number of variants of a seemingly identical device may become available when more devices are offered commercially, and none of these standard variants may perform as well as a device tailored to a user s specific application. This is similar to the forced choice of high sensitivity, low dark current, or fully extended infrared response in the case of the S-1 phototube. For NEA devices, however, the choices may be of even greater complexity. 

A general-purpose TM NE A photoemissive device active to 1.1 pm has yet to be developed. Nevertheless, from a comparison of spectral and performance characteristics of opaque NEA and classical photoemitters, it will be seen that NEA devices may soon permit high-gain room-temperature near-infrared imaging with orders of magnitude increase in performance over previous S-1 and modified S-20 characteristics. 

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Electrohydrogenation at Raney metal electrodes is a mild method of hydrogenation, the advantages and disadvantages of which have been pointed out in the Introduction and have been illustrated in the paper with selected... 

This protective group is introduced by an acid-catalyzed addition of the alcohol to the vinyl ether moiety in dihydropyran. />-Toluenesulfonic acid or its pyridinium salt is used most frequently as the catalyst,3 although other catalysts are advantageous in special cases. The THP group can be removed by dilute aqueous acid. The chemistry involved in both the introduction and deprotection stages is the reversible acid-catalyzed formation and hydrolysis of an acetal (see Part A, Section 8.1). 

Unlike tubocurarine, pancuronium does not produce ganglionic block or histamine release. For this reason it became popular soon after its introduction and became the drug of choice for use in sick patients. However, it increases the heart rate, arterial pressure, and cardiac output in clinical doses. While this may be advantageous when using high-dose opiate anaesthesia in cardiac surgery, it can be associated with arrhythmias and myocardial ischaemia. The... 

Mass spectrometry (MS) is an analytical technique of great interest, one that provides structural information and quantitative data not easily obtained by other techniques. In view of these advantages, mass spectrometers have been widely used as detectors in gas chromatography however, adapting them for use with HPLC systems has been more difficult, because the sample is not in the gaseous phase and the solvent must be removed prior to ionization. These difficulties have been overcome by the development of a number of sample-introduction and ionization tech-... 

We report here the results obtained by the use of a screen-printed electrode as electrochemical probe to be coupled with a microdialysis fibre for continuous glucose monitoring. The most significant advance is represented by the introduction of a mediator (PB) as the principal factor for hydrogen peroxide measurement. The improved operational stability observed with the PB-modified screen-printed electrodes has demonstrated that these sensors could serve as tool to be applied for the continuous monitoring of many analytes. The application to diabetic care seems to be the most promising and advantageous area in which to test these sensors. 

In the literature, the thermodynamic advantages of cosolvent addition have been emphasized however, the effect of cosolvents on other aspects of the process, such as mass transfer, overall cost, and product/residue properties, has not been considered in depth. Benefits of cosolvent addition must be balanced against its disadvantages for a specific application. Cosolvent introduction and solvent recovery (separation of the cosolvent from the extract, SCF, and solids residue) increase the complexity of process design. As well, an increase in solvent loading may result... 

RNAi presents important advantages compared to the antisense approach, it is highly specific, remarkably potent since only a few dsRNA molecules per cell are required for effective interference, and dsRNA can cause interference in cells and tissues outside the site of introduction and its effects are maintained through the progeny. 

The advantage of these industry-level studies is that data on product introductions and research expenditures are verifiable and readily available at the industry level. The disadvantage is that the introduction of NCEs in any year must be related... 

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