Sampling, discrete

Digital signal. A signal that represents information in a computer-compatible form as a sequence of (binary) numbers that can describe discrete samples of an analog signal. 

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... 

In tlie case of a discrete sample space (i.e., a sample space consisting of a finite number or countable infinitude of elements), tliese postulates require tliat tlie numbers assigned as probabilities to tlie elements of S be noimegative and have a sum equal to 1. These requirements do not result in complete specification of tlie numbers assigned as probabilities. The desired interpretation of probability must also be considered, as indicated in Section 19.2. The matliematical properties of the probability of any event are tlie same regardless of how tliis probability is interpreted. These properties are formulated in tlieorems logically deduced from tlie postulates above without tlie need for appeal to interpretation. Tliree basic tlieorems are ... 

Fig. 2) and b) discrete sample processing using small test tubes, in order to ensure the physical separation of each enzyme assay (i.e., the LKB-8600) (Table I). 

Other analyzers such as the Gilford Automated Enzyme Analyzer and the LKB-8600 Reaction Rate Analyzer analyze discrete samples one at a time. These instruments provide kinetic analyses, digital data reduction at the time each sample is analyzed, and excellent electronic and optical characteristics. Recently, Atwood has developed kinetic enzyme analyzers which require only 9 seconds for measuring an enzyme activity, using highly stable and sensitive electronic circuits (12). This short read out time allows a large number of samples to be processed by one instrument in an automated mode. 

When considering how much urine to collect, one must decide whether to collect individual voids as discrete samples or to collect larger samples where the test subject voids several times in one collection vessel. If the researcher is interested in examining the analyte in the urine in each void, smaller 500-mL wide-mouthed jars can be used to collect each void over a 24-h period. These are generally referred to as spot void samples. Taking void spot samples in this manner allows the researcher to examine each void for the test analyte and also to composite a portion of each void into one 24-h sample in order to look at the overall concentration of the test analyte in the 24-h urine sample. If the spot samples are to be composited, aliquots of each spot sample should be removed based on each void s percentage of the total weight of the 24-h sample. The spot sample aliquots can be composited to form one 24-h sample. The leftover spot samples can be used to obtain individual void measurements of the analyte in question. 

D. Beauchemin, D.C. Gregoire, D. Gunther, V. Karanassios, J.-M. Mermet and T.J. Woods (eds), Discrete Sample Introduction Techniques for Inductively Coupled Plasma Mass Spectrometry, Elsevier, Amsterdam (2000). 

Automated titrations can be divided into discontinuous and continuous, the former representing a discrete sample analysis, as a batch titration is the usual laboratory technique and the latter a flow technique, which is used less frequently in the laboratory, e.g., in kinetic studies, but is of greater importance in plant and environment control. 

Another interesting development, in which continuous flow was combined with discrete sample titration, is continuous flow titration by means of flow injection analysis (FIA) according to Ruzicka and co-workers70. Fig. 5.16 shows a schematic diagram of flow injection titration, where P is a peristaltic pump, S the sample injected into the carrier stream of diluent (flow-rate fA), G a gradient chamber of volume V, R the coil into which the titrant is pumped (flow-rate fB), D the detector and W waste. 

Elemental Speciation - New Approaches for Trace Element Analysis Discrete Sample Introduction Techniques for Inductively Coupled Plasma Mass Spectrometry... 

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. 

Lan and Mottola [14] have presented two continuous-flow-sensing strategies for the determination of C02 in gas mixtures using a special reaction cell. Both approaches are based on the effect of the complex of Co(II) with phthalocyanine as a rate modifier of the CL emission generated by luminol in the absence of an added oxidant agent, which is enhanced by the presence of C02 in the system. This enhancement allows the fast and simple determination of carbon dioxide at ppm levels (v/v) in atmospheric air and in human breath. In the first case, a continuous monitoring system was applied however, because the flow of expired gas is not constant, a discrete sample introduction approach was used in the analysis of C02 in breath. 

Fig. 6.13. Comparison of rates of sampling for a bioprocess (fermentation) the ability of NIR to measure in real time is compared with discrete sampling techniques.

Tailing refers to a separation pattern in which the desired discrete sample zone is spoiled by a trait of analyte behind the initial zone. 

In practice, the concentration gradient is estimated from obserrational data by computing the concentration difference (C - Q ) between two samples collected from two different depths (z. - z ). Real concentration gradients are not usually linear because of the effects of other processes that concurrently affect solute distributions, such as advection and biological activity. Thus, estimating fluxes from the concentration differences of discrete samples is best done over small depth intervals. 

Titration techniques can be appUed both to continuous monitoring and to analyses of discrete samples in flow-through systems. Generally it holds that the equivalence point for the reaction... 

Fig. 2.14 Standard response crrrve for continuous flow analysis of discrete samples.

In the continuous processing of discrete samples in the AutoAnalyzer system, the reaction-time is held constant by the manifold design, and because the rise-curve is exponential the degree of attainment of steady-state conditions is independent of concentration. Consequently it is unnecessary for the analytical reaction to proceed to completion for Beer s Law to be obeyed. This confers a considerable advantage upon the AutoAnalyzer approach and one which is frequently emphasized. The relationship between degree of attainment of steady state and IT,/, can be generaHzed in the semi-logarithmic plot of Fig. 2.16 [10], where time is expressed in units of IT,/,. 

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. 

A number of applications of flow-injection techniques have been made to flame atomic absorption spectrometry [22]. Although manifolds can be connected directly to the nebuhzer, the response of the spectrometer is dependent on the flow rate of the sample into the nebuhzer [23], and some adjustment to the manifold may be required. The optimum flow rate for maximum response when the sample enters the nebuhzer as a discrete sample plug can be different from that found for analysis of a continuous sample stream. 

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