Fast Data Acquisition

As we have seen, conventional spin echo imaging (Section II.E) typically takes approximately a few minutes because an independent r.f. excitation is required for acquisition of each row of k-space data. Hence, sampling of the complete raster is limited by the repetition/recycle time of the pulse sequence used, which in turn is governed by the inherent 7) relaxation time(s) of the system under study. In general, the acquisition speed of an MR image may be improved by two basic methods  

T2- and Ti -relaxation contrast in the RARE acquisition is exploited such that the images are made selective to lysozyme and urea, respectively. 

Three sampling strategies are now introduced echo planar imaging (EPI), rapid acquisition with relaxation enhancement (RARE), and low excitation angle imaging. The first two are based on the sampling of more than one line of k-space for each r.f. excitation, and the third uses rapid multiple r.f. excitations. 

Schematic representation of the GERVAIS pulse sequence, identifying all pulse and delay timings. The pulse sequence shown is that for acquisition of successive velocity images. The magnitude and orientation of the g-gradient are changed as determined by the velocity or acceleration vector that is to be measured. 

An example of the application of RARE for rapid image acquisition is shown in Fig. 19, in which a single frame is shown from a series of 2-D images acquired of an oscillatory chemical reaction occurring within a fixed bed. Relaxation contrast is used to discriminate between the reaction products Mn and Mn (49). In this example, MR offered the opportunity to map the detailed structure of the fixed bed and the product distribution within it. This pulse sequence has also recently been applied to obtain quantitative images of the evolution of a lysozyme urea separation within a chromatography column (50). 

Sederman et al. (51) recently implemented a variant of RARE, which yields multiple images from full k-space data at multiple echo times following a single r.f. 

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The advantage of the imaging mode is fast data acquisition. Because all pixels are projected and detected simultaneously the measurement time for one distribution is extremely low. 

Fast chromatography involves the use of narrow-bore columns (typically 0.1-mm i.d.) that will require higher inlet pressures compared with the conventional wide-bore capillary columns. These columns require detectors and computing systems capable of fast data acquisition. The main disadvantage is a much-reduced sample loading capacity. Advances in GC column technology, along with many of the GC-related techniques discussed below, were recently reviewed by Eiceman et... 

Careful setting of the scan range is recommended. The narrower the scan range, the higher the number of data points per peak. This is not necessarily a linear relation it can depend on the MS. If a large mass range and fast data acquisition rate are required, a time-of-flight (ToF) MS would be... 

The instrumentation for voltammetry is relatively simple. With the advent of analog operational amplifiers, personal computers, and inexpensive data acquisition-control system, many computer-controlled electrochemical systems are commercially available or custom made. Programming complex excitation waveforms and fast data acquisition have become a matter of software writing. 

Combinatorial chemistry and HT E are powerful tools in the hands of a scient ist, as they are a source for meaningful consistent records of data that would be hard to obtain via conventional methods within a decent timeframe. This blessing of fast data acquisition can turn into a curse if the experimentalist does not take precautions to carefully plan the experiments ahead and the means of handling the data and analyzing them afterwards. The two essential elements that ensure a successful execution of ambitious projects on a rational and efficient basis are, therefore, tools that enable the scientist to carefully plan experiments and get the most out of the minimum number of experiments in combination with the possibility of fast and reliable data retrieval from databases. Therefore, experimental planning and data management are complementary skillsets for the pre- and post experimental stages. 

In most of the research summarized here, a homebuilt UHV-compatible Aarhus STM instrument (Fig. lb) was used, which represents a successful solution to the problem of designing a stable high-resolution microscope (55). It features state-of-the-art atomic resolution, and the compact, rigid design with a high mechanical frequency also allows for high sampling frequencies (i.e., fast data acquisition that enables observation of dynamic processes on the surface) (57). 

Consideration of Surface Tool Concerns. Reasonably fast data acquisition, small probing beam size (allowing both faster sputter profiling and spatial resolution) and semiquantitative data analysis, give AES a primary role in each of these two experiments. 

Fiber optics can be integrated into a normal spectrometer, but complete solutions consisting of a combination of spectrometer and probe are commercially available (Oceanoptics, 2006 Avantes, 2006 Stellarnet, 2006). Some spectrometers are combined with CCD cameras for detectors, enabling fast data acquisition, one UV-vis spectrum in 6 ms (Weckhuysen, 2003). Besides the excellent time resolution, the main advantage of the fibers is that they can be integrated into typical reactors or added as a second or third method to a cell optimized for a different method. 

Over the last decade both the speed and accuracy of data acquisition and the sensitivity of the LEED experiment have been significantly improved. In most structure determination experiments the LEED patterns are displayed on a phosphor screen. LEED I-V curves are then measured by photometric or photographic means. Fast data acquisition systems have been developed that record LEED patterns with video cameras interfaced to computers and generate I-V curves during the experiment./3,4/ The sensitivity of LEED is also being greatly increased with the development of instruments that can detect single... 

Other recently developed methods have also become available for adsorption studies. The availability of synchrotrons as excellent x-ray sources allows x-ray studies at liquid interfaces (Mohwald et al. 1990, Meunier Lee 1991, Daillant et al. 1991). The same applies for small angle neutron scattering (SANS, based on the different scattering cross section of hydrogen and deuterium). The use of SANS for dynamic studies of structures in membranes and at interfaces has been shown by different authors (Grundy et al. 1988, Bayerl et al. 1990, Vaknin et al. 1991). The method is characterised by a fast data acquisition and should allow dynamic investigations at freshly formed surfaces, as discussed by several authors (Blake, Howe, Penfold, private communication). 

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