Elution continuous method

The GC broadening techniques for the measurement of gaseous diffusivities are the continuous elution method, introduced by Giddings, and its arrested elution modification invented by Knox and McLaren. 

The continuous elution method is conducted in an open mbe with circular cross section. The carrier gas flow rate is chosen such that the plate height, H, depends mainly on only one of the van Deemter terms, namely the longitudinal diffusion term. For correction reasons, the use of two different length columns is necessary, and the equation for H is ... 

Although the arrested elution method has two drawbacks (the need of several runs to get a Dab value with a precision of 2%, and of constant flow rates over long periods for runs at various arrested times), in comparison with the continuous elution method, the former has also the following advantages 1) Effects of zone broadening other than axial molecular diffusion and non-uniform flow profile do not affect the measurement 2) no assumptions are made about the precise form of the flow profile, the smoothness of the column wall, or the accuracy in the knowledge of the column diameter. 

The time needed for the determination of Dab values by the continuous elution method is very short ( 5 min), while that by the arrested elution method is longer ( 3 hr), due to the requirement of repeating the experiments at a number of delay times. The corresponding time for the RF-GC method is about 30-60 min, depending on the binary gas mixture and the length of the diffusion column used. 

The length of the empty diffusion column in RF-GC is relatively short (30-80 cm), compared to that used in the continuous elution method, where much longer (—15 m) columns are used. 

Stalcup aiid co-workers [14] adapted this method to a continuous elution mini-prep electrophoresis apparatus shown in Fig. 11-3. In this apparatus, the end of the electrophoretic gel is continuously washed with elution buffer. The eluent can then be monitored using an HPLC detector (Fig. 11-4) and sent to a fraction collector where the purified enantiomers, as well as the chiral additive, may be recovered. In this system, the gel configuration was approximately 100 mm x 7 mm, and was aircooled. The number of theoretical plates obtained for 0.5 mg of piperoxan with this gel was approximately 200. A larger, water-cooled gel was able to handle 15 mg of... 

Elution Methods. For every generator, the separation of the daughter nuclide from the parent can be performed by the continuous (steady state) method or the discontinuous (bolus) method. The continuous method involves the elution of the daughter nuclide from the generator as it is formed and direct administration as a gas or liquid phase. The bolus method involves the elution at one time of the available daughter... 

In frontal analysis the sample solution is continuously added to the stationary phase at the top of the column. During such addition a stage reaches when components of the given sample start emerging from the column, the beginning being made by the component which has the smallest ratio of distribution between the stationary phase and the mobile phase. Other components follow and exit from the column in order of increasing distribution ratio but not separated as well as in the elution method. 

The apparatus used for continuous-flow paper electrophoresis can be used for separation with an elution rather than a continuous-flow method. The sample is spotted at the top of the paper and only the buffer solution is continuously added. If the electrophoresis is stopped before the sample components drip from the bottom of the paper, the technique combines the separating ability of descending paper chromatography (separation occurring vertically) with that of electrophoresis (separation occurring horizontally) to yield spots simultaneously developed in two dimensions. 

The TREF elution curve resembles a chromatogram with a small peak at the beginning (typically obtained at isothermal elution), which corresponds to the fraction that has not crystallized at the lowest crystallization temperature chosen in the analysis method this is followed by the continuous elution of the fractions of increasing crystallinity as temperature rises (as shown in Fig. 15). Cooling down... 

Diffusion coefficients, for various binary gas mixtures, at various temperatures and pressures with their accuracy and precision, measured by gas chromatographic broadening techniques (GC-BTs continuous, as well as arrested elution methods), are given in Table 3 of Ref. and in Table 1 of Ref. Representative data are collected here in Table 1. 

The precision of the RF-GC method was calculated to be 0.9%. The precision of the continuous elution technique was about 1%, while that of the arrested elution method was about 2%P ... 

After separation, the enantiomers were eluted on the above-layer portion by using the continuous development method and were determined by fluorescence quenching at 254 nm, allowing to perform in situ quantitative measurements by using a Shimadzu CS9000 scanning densitometer. 

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... 

Elution development is by far the most common method of processing a chromatographic separation and is used in all types of chromatography. Elution development is best described as a series of absorption-extraction processes which are continuous from the time the sample is injected into the distribution system until the time the solutes exit from it. The elution process is depicted in Figure 1. 

Chromatography. A number of HPLC and TLC methods have been developed for separation and isolation of the brevetoxins. HPLC methods use both C18 reversed-phase and normal-phase silica gel columns (8, 14, 15). Gradient or iso-cratic elutions are employed and detection usually relies upon ultraviolet (UV) absorption in the 208-215-nm range. Both brevetoxin backbone structures possess a UV absorption maximum at 208 nm, corresponding to the enal moeity (16,17). In addition, the PbTx-1 backbone has an absorption shoulder at 215 nm corresponding to the 7-lactone structure. While UV detection is generally sufficient for isolation and purification, it is not sensitive (>1 ppm) enough to detect trace levels of toxins or metabolites. Excellent separations are achieved by silica gel TLC (14, 15, 18-20). Sensitivity (>1 ppm) remains a problem, but flexibility and ease of use continue to make TLC a popular technique. 

The combined use of a continuous flow system and a spectrophotometer for sample screening to discriminate between synthetic and natural colorants is also available. With a very simple flow system on a column packed with natural materials, one can discriminate natural and synthetic colorants. The natural (not retained) ones can be determined in the first step and the synthetic (retained) ones in the second step after their elution. For yellow, red, green, blue, and brown, natural or synthetic colorants were chosen as models. The specific maximum wavelength for each color (400,530, and 610 mn, respectively) was selected by a diode array system. A complete discrimination of natural and synthetic colorants was obtained for concentrations of natural colorants (in the absence of synthetic ones) up to 2000 (yellow), 2000 (red), and 10,000 (brown) times that of the detection limits (DLs) of synthetic additives. This method was applied to screen fruit drinks and candies. ... 

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