Discrete sampling methods

This type of chromatographic development will only be briefly described as it is rarely used and probably is of academic interest only. This method of development can only be effectively employed in a column distribution system. The sample is fed continuously onto the column, usually as a dilute solution in the mobile phase. This is in contrast to displacement development and elution development, where discrete samples are placed on the system and the separation is subsequently processed. Frontal analysis only separates part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the... 

Let us leave, for a moment, the domain of discrete torsion angles and consider torsion-angle moves in a torsion continuum. Much effort has been directed to the development of efficient phase-space sampling methods for MC [17,23,37,128-138]. The simplest off-lattice move - being a pivotal move [128,131] of a single torsion angle at a time - hardly samples configuration... 

If we were to choose the ideal method for the analysis of any component of seawater, it would naturally be an in situ method. Where such a method is possible, the problems of sampling and sample handling are eliminated and in many cases we can obtain continuous profiles rather than limited number of discrete samples. In the absence of an in situ method, the next most acceptable alternative is analysis on board ship. A real-time analysis not only permits us to choose our next sampling station on the basis of the results of the last station, it also avoids the problem of the storage of samples until the return to a shore laboratory. 

The dry combustion-direct injection technique provides many advantages over other methods, such as quick response and complete oxidation for determining the carbon content of water. Its primary shortcoming is the need for rapid discrete sample injection into a high-temperature combustion tube. When an aqueous sample is injected into the furnace, it is instantaneously vapourised at 900 °C and a 5000-fold volume increase can be expected. Such a sudden change in volume causes so-called system blank and limits the maximum volume of injectable water sample, which in turn limits the sensitivity [106,107]. 

Depending on the configuration of the device and the method for sample and reagents introduction, it is possible to classify the systems into static (batch or discrete sampling instrument) or flowing stream, both using continuous-flow or stopped-flow systems. 

The philosophies for automation have been described in the foregoing sections. However, to solve an analytical problem there may well be more than one approach that offers potential. The Hterature abounds with methods that have been automated by flow-injection and by continuous-flow methodologies. Also, very often a procedure which involves several stages prior to the actual measurement can be configured by combining two of the approaches. An example of this is the automated Quinizarium system described by Tucker et al. [46]. This was a continuous extraction followed by a hatch extraction which is finally completed by a batch measurement on a discrete sample for quantification and measurement. Whereas sample preparation is almost always required, there is no doubt in my mind that the best approach to this area of activity is to avoid it totally. The application of near infra-red spectroscopy is an example of this strategy. 

Azad et al. [ 186] used a similar technique for the determination of selenium in soil extracts using a nondispersive spectrometer, with which it was possible to observe fluorescence from the 196.1, 214.3 and 204.0 lines simultaneously, thus enabling a detection limit of 10 ng/ml to be observed using discrete sample introduction via the hydride generation technique. In this method, soil... 

Discrete Variational Method (DVM) ( ) numerical sampling of Slater or numerical basis quite rapid good energy requires fine grid... 

Creatinine in urine. The assay of creatinine in body fluids is one of the core assays in clinical chemistry since its level in blood and urine reflects the functional status of the kidney. There are many methods for its assay ranging from the simple colorimetric Jafle reaction to dedicated creatinine analysers using discrete sampling technologies. For many metabolic assays, the so-called creatinine correction can be applied since creatinine excretion is considered to be constant throughout the day. The clinic therefore only needs to collect a random specimen of urine rather than a full 24 h specimen. 

Elemental Spedation - New Approaches for Trace Element Analysis Discrete Sample Introduction Tbchniques for Inductively Coupled Plasma Mass Spectrometry Modem Fourier Transform Infrared Spectroscopy Chemical Test Methods of Anal3rsis... 

The compound is eluted from the chromatographic plate, collected, and introduced into the mass spectrometer as a discrete sample. In this method, samples collected from a TLC spot, identified with an independent method of visualization, must be sufficiently volatile to evaporate into the source of the mass spectrometer. 

The use of an enzyme thermistor as a specific detector for monitoring different enzymes [17,63,45] in the eluents from chromatographic procedures had the advantage of being applicable in optically dense solutions, where spectro-photometric methods fail, and of being able to operate on-line for discrete samples. 

A related issue is that while microdialysis is a continuous process, it is coupled to an analytical separation step that requires discrete sample volumes. Individual samples can be collected off-line with a fraction collector and assayed later (Fig. 3). The temporal resolution is defined by the time interval at which the microdialysis samples are collected. Without the need for further sample cleanup, the temporal resolution for off-line analysis will ultimately be dependent on the perfusion rate and the volume of sample needed for quantitation. If the analytical method does not have sufficient limits of detection, larger sample volumes must be collected, decreasing the temporal resolution of the method. 

For polymodal or wide distributions the histogram method 03-9u (or the exponential sampling method) is more representative. In this method, the particle size distribution is presented by a finite number of discrete sizes, each of them an adjustable fraction of the total concentration. Then, the... 

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