Rate of data acquisition

Using MRI as a substitute for X ray tomography IS only the first of what are many medical applica tions More he on the horizon If for example the rate of data acquisition could be increased then it would become possible to make the leap from the equivalent of still photographs to motion pictures One could watch the inside of the body as it works— see the heart beat see the lungs expand and con tract—rather than merely examine the structure of an organ... 

Other properties that define instrument capahihty include sensitivity, hmits of particle size resolution, and rate of data acquisition. For example, detection limits for green fluorescence range from 600 molecules of equivalent soluble fluorochrome (MESF)/cell to <50 MESF/cell, and particle sizes of 0.5 pm to 50 pm may be resolved in some instruments. Acquisition rates in analytical instruments vary from <3,500 cells (events)/s in older instruments, to 10,000 events/s in the newest models. High acquisition rates and the availability of automatic samphng devices for microplate-based assays have enhanced the efficiency of analyzing large sample sets. 

Unfortunately, the size of the crystallographic problem presented by elastase coupled with the relatively short lifedme of the acyl-enzyme indicated that higher resolution X-ray data would be difficult to obtain without use of much lower temperatures or multidetector techniques to increase the rate of data acquisition. However, it was observed that the acyl-enzyme stability was a consequence of the known kinetic parameters for elastase action on ester substrates. Hydrolysis of esters by the enzyme involves both the formation and breakdown of the covalent intermediate, and even in alcohol-water mixtures at subzero temperatures the rate-limidng step is deacylation. It is this step which is most seriously affected by temperature, allowing the acyl-enzyme to accumulate relatively rapidly at — 55°C but to break down very slowly. Amide substrates display different kinetic behavior the slow step is acylation itself. It was predicted that use of a />-nitrophenyl amid substrate would give the structure of the pre-acyl-enzyme Michaelis complex or even the putadve tetrahedral intermediate (Alber et ai, 1976), but this experiment has not yet been carried out. Instead, over the following 7 years, attention shifted to the smaller enzyme bovine pancreatic ribonuclease A. 

As will become apparent in the later chapters, the statistical nature of the conclusions is such that the present amount of data in the crystallographic data base is inadequate. We find ourselves presenting results that show well-defined trends, but in doing so, have come to realize that we need a tenfold increase in the information from X-ray and neutron crystallography before the conclusions drawn can become really definitive. This tenfold increase in structural data will surely come within the next decade, and will be useful only if the publication, data storage, and retrieval mechanisms keep pace with the accelerating rate of data acquisition. 

We saw in Section 3.7 that in ordinary NMR spectroscopy the rate of data acquisition determines the maximum spectral range. For imaging it is clear from Eq. 14.2 that for a given value of Gx the spectral width determines the maximum range, Fx, that can be observed in the x direction. Fx is called the field of view (usually abbreviated as FOV) in the x direction. 

Basically, the flow control and sampling unit allows three alternative methods of operation. Firstly the eluent from the column can flow directly from the UV detector to the NMR sample tube and the spectra can be continuously monitored during the development of the separation. The success of this procedure will depend on the volume of the cell, the sample size, the column flow rate, the resolution of the NMR spectrometer and the rate of data acquisition by the computer. In general, unless the new micro-cell facilities mentioned above are exploited, this procedure will rarely be successful, particularly if microbore columns are used and multi-component mixtures are being examined. 

In contrast to XPS, a certain amount of depth profiling with a rastered Ar beam is performed in most AES investigations. The reason for this is that most Auger lines fall in a KE range that renders them more su,sceptible to attenuation from surface contamination than is the case for XPS. Also, the rapid rate of data acquisition in AES makes it easy to review the results of a particular surface treatment, such as ion bombardment. 

Optical sectioning can be obtained also with SDCM, but, as an advantage, imaging is in this case performed at very high speed, and, as the microscope repeatedly scans many points in parallel, the increased rate of data acquisition also decreases photobleaching and phototoxicity [78, 79]. 

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