Acquisition time definition

In the previous chapter we presented an overview of protein fluorescence. We described the spectral properties of the aromatic amino acids and how these properties depend on protein structure. We now extend this discussion to include time-resolved measurements of intrinsic protein flu( scence. Prior to 1983, most measurements of time-resolved fluorescence were performed using TCSPC. The instruments employed for these measurements typically used a flashlamp etcitation source and a standard dynode-chain-type PMT. Such instruments provided instrument response functions with a half-width near 2 ns, which is comparable to thedecay time of most proteins. The limited repetition rate of the flashlamps, near 20 kHz. resulted in data of modest statistical accuracy, unless the acquisition times were excessively long. Given the complexity of protein intensity and anisotropy decays, and the inherent difficulty of resolving multiexponential processes. ii was difficult to obUun definitive information on the decay kinetics of proteins. 

Figure 6.24 Schematic illustration of the difference with respect to accurate definition of a chromatographic peak between fixed acquisition times ( scan times ) as used by a linear quadrupole, and variable acquisition times dictated by automated gain control (AGC) applied in order to control space charge effects in a Paul ion trap. Since the rate of ion arrival is lower at the extremities of the peak, the acquisition times for the trap are longer than near the peak maximum, so that the start and end points for peak area integration are less well defined that in the case of fixed acquisition intervals (can be scan times or SIM/MRM channel dwell times).

In certain organs the onset of transamination is definitely coincident with critical stages of functional differentiation, e.g., in muscle — with the time of appearance and accumulation of phosphocreatine and establishment of rapid contraction (Koshtoyants and Ryabinovskaya, 103), in heart — with acquisition of definitive heart rate, in kidney — with the beginning of active excretory function. In the early period of postnatal life transamination values gradually increase, and only at the age of one month do they approach the levels characteristic of adult rabbits. 

Several points have to be taken into account when setting up, such as contrast, acquisition time. X-ray energy and so forth. These points are based on our experience using SRpCT. The definition of an experiment using a commercial scanner is slightly more complicated and must take into account specificities of these apparatuses (polychromatic light, diverging beams, etc.). 

By definition, learning is the acquisition of new information, whereas memory is the retention of acquired information. Driven by knowledge obtained by scientists before them, various disciples of neuroscience over the course of the past 100 years have learned more about various components of learning and memory, whether at the molecular level of synapses or at the systems level of brain circuitry. The concept of memory of mind has existed since the time of Aristotle. It is only during the past 50 years or so that scientists have begun to unravel some of the anatomical and cellular bases underlying such a complex mental process. 

With 2D experiments the situation is a little more complicated as the size of the overall digitised matrix depends on the number of time increments in tl as well as parameters specific to the 2D acquisition mode. Nevertheless, a digitised matrix of TD(2) X TD(1) complex data points is acquired and stored. Similar to ID the effective number o measured data points used for calculation TD(used) and the total number of data points SI to be transformed in t2 and tl may be defined prior to Fourier transformation. These parameters may be inspected and defined in the General parameter setup dialog box accessible via the Process pull-down menu. With 2D WIN-NMR the definitions for TD(2) and TD(1) are the same as for TD with ID WIN-NMR. However, unlike ID WIN-NMR, with 2D WIN-NMR SI(2) and SI(1) define the number of pairs of complex data points, instead of the sum of the number of real and imaginary data points. Therefore the 2D FT command (see below) transforms the acquired data of the current data set into a spectrum consisting of SI data points in both the real and the imaginary part. 

Spectroscopic imaging denotes knaging with acquisition of an NMR spectrum for each voxel. This increases the dimensionality of the imaging experiment by one for ID spectroscopic imaging and is expensive in terms of measurement time. The benefit though is access to all spectroscopic parameters for definition of images with contrast from different spectroscopic parameters (cf. Section 7.3) [Brol, Decl]. 

Data acquisition and interpretation is comparatively fast, and processes from time scales of tens of milliseconds up to 100 seconds and more can be studied. The accuracy of the method depends directly on the pressure transducer. It amounts to 0.1 mN/m for example for the setup described by Nagarajan Wasan (1993). By applying definite volume changes, linear or step-type ones, this set-up can be used simultaneously for relaxation studies. Such experiments are described in the next chapter. 

Graphical analysis must always precede the statistical analysis. It is imperative to keep short the time elapsed between data acquisition and data analysis, and in most cases, it is advisable to perform the graphical analysis even while the experiment is still in progress. Wien the data clearly define the nature ofthe rate or binding equation, statistical analysis is not needed to do this. Nevertheless, for a definitive work, statistical methods are necessary for parameter estimation as well as for model discrimination (Senear Bolen, 1992). Computer programs are now available for even the most sophisticated problems in enzyme kinetics (see Section 18.2.4). 

All data accumulated in the monitoring program should be maintained in a permanent tile, preferably for each employee but definitely for each facility. These records should be dated, signed by the individual responsible for the data, and documented as to the measurement parameters, i.e., instrument used, duration of sample collection, characteristics of the detection device, location of the person wearing the device, and, if possible, supporting data on the operations being conducted at the time of data acquisition... 

The method has been criticized [Ar 70], and the authors themselves were aware of its limitations [No 70, Pa 68, Va 70], Although there can be no doubt that values obtained in this way have a fairly large error, the simplicity of the method (especially when compared with the considerable time- and labour demands of calorimetric measurements) definitely makes it suitable for the acquisition of informatory enthalpy data. Among limitations recognized more recently, it is worth mentioning. 

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