Useful lifetime

An important problem with all liquid stationary phases is their tendency to bleed from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a column s useful lifetime is significantly shortened. Capillary columns with bonded or... 

Fig. 7. Useful lifetime of Parylenes N, C, and D as a function of temperature in air. Failure = 50% loss in tensile strength.

Design for degradation. Products should be designed to be biodegradable at the end of their useful lifetimes. 

Promotion We use the term promotion, or classical promotion, to denote the action of one or more substances, the promoter or promoters, which when added in relatively small quantities to a catalyst, improves the activity, selectivity or useful lifetime of the catalyst. In general a promoter may either augment a desired reaction or suppress an undesired one. For example, K or K2O is a promoter of Fe for the synthesis of ammonia. A promoter is not, in general, consumed during a catalytic reaction. If it does get consumed, however, as is often the case in electrochemical promotion utilizing O2 conducting solid electrolytes, then we will refer to this substance as a sacrificial promoter. 

Performance of polymers during use is a key feature of any composite material, which decides the real fate of products in outdoor applications. Whatever the application, there is concern regarding the durability of polymers, partly because of their useful lifetime, maintenance and replacement. The deterioration of these materials depends on the duration and extent of interaction with the environment (Homminga et ah, 2005). 

Determination of the useful lifetime of a resin occupies much effort in any process development program. In fact, because of validation considerations, it is perhaps the major cost factor. A very useful model has been presented for determining the number of cycles for which a given resin can be used.59 This program involves activities in six main areas ... 

The useful lifetime of hplc columns is shortened by the appearance in the column packing of cracks or voids, especially at the top of the... 

Posokhov, Y. O., Merzlyakov, M., Hristova, K. and Ladokhin, A. S. (2008). A simple proximity correction for Forster resonance energy transfer efficiency determination in membranes using lifetime measurements. Anal. Biochem. 380, 134—6. 

The selection of a fixed bed Co-LTFT process supported the objective to apply the SMDS process for beneficiation of remote gas fields. The Co-LTFT catalyst has a useful lifetime of 5 years and the robustness of fixed bed reactor technology has been proven. For example, the fixed bed Arge Fe-LTFT process has now been in operation for more than 50 years at Sasol 1. 

Half-life is defined as the time required for a radioisotope to reduce its initial radioactivity (disintegration rate) to one-half (or 50%). The half-life is represented by the symbol, t a, and it is unique for a given radioisotope. The useful lifetimes of radiopharmaceuticals are usually determined by radioactive decay, which constantly decreases the amount of radioactivity present. The half-life is related to decay constant, X of a radioisotope (discussed in the subsequent section), as follows ... 

Weighing. The filters are not desiccated prior to weighing, and are always handled with nonserrated tweezers. Since it is easy to build up static charges on the filter, a static eliminator is necessary. These should be replaced at about six month Intervals, since there is some question about their useful lifetime. 

However, it has been mentioned before, and must be bourne in mind, that it is not the pressure capability of the solvent pump that normally determines the maximum pressure that can be employed, but the maximum pressure the whole chromatographic system can tolerate. Although valves have been designed to operate at 10,000 p.s.i., or even higher, their useful lifetime at that pressure is often relatively short, usually as a result of sample contamination scoring the valve seats. For long-term continuous operation the maximum inlet pressure a valve can tolerate is often only about 3000 p.s.i.. 

There are a few hundred stable nuclei but several thousand nuclei that are radioactive and have experimentally useful lifetimes. Since 1990, one of the fastest growing areas of research in nuclear science has been the study of nuclear reactions induced by radioactive projectiles. Using either ISOL (/sotope Separator On-Line) or PF (Projectile Fragmentation) techniques, several hundred new radioactive nuclear beams have become available (see Chapter 14). 

Most literature on enzyme kinetics is devoted to initial rate data and the analysis of reversible effects on enzyme activity. In many applications and process settings, however, the rate at which the enzyme activity declines is of critical importance. This is especially true when considering its long-term use in continuous reactors. In such situations the economic feasibility of the process may hinge on the useful lifetime of the enzyme biocatalyst. The focus of this section is on the mechanisms and kinetics of loss of enzyme activity. It should also be recognised that the alteration of protein structure is central to the practical manipulation of proteins (e.g. precipitation, affinity and other forms of protein chromatography, and purification in general). 

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