Time, separation

It is of value to the cliromatographer to observe the influence of changing various parameters on the separation time, so that an optimum separation can be achieved in minimum time (in addition, this approach will minimise solvent consumption). It can be shown that the time taken to elute the last peak is given by  

When the critical pair is separated, so are all other peaks in the chromatogram. If the critical pair is not the last two peaks in the chromatogram the separation time will be tR(l + nk) where n is the ratio of the retention factor of the last peak to elute in the chromatogram to the retention factor of the second peak of the critical pair. The fastest separation is obtained when tR is minimized. This will be the case if the following criteria are met. The minimum useful value for the resolution of the critical pair is accepted. The separation system is optimized to maximize the separation factor (a) for the critical pair. The retention factor for the critical pair is minimized (k 1 -5). The column is operated at the minimum value of the plate height, Hmin, corresponding to Uopt. 

Liquid chromatographic separations will never be as fast as gas chromatographic separations because mass transfer properties in liquids are inferior to those in gases [138,154,235,236,251]. Most fast separations in liquid chromatography are accomplished at the maximum available inlet pressure. Adopting reduced parameters (section 1.5.3) the separation time is given by 

The fastest possible separation with a given column pressure drop (AP) and Nreq is obtained at the minimum reduced plate height (hmin) corresponding to the optimum reduced mobile phase velocity (Vopt)- This will be achieved at a specified optimum particle size (dpopt) according to 

Since sorbents are available in a limited number of particle sizes, a compromise is necessary. An available particle size is selected that is larger but closest to dpopt. The column is made proportionately longer and operated at a slightly higher velocity than Vopt. The retention time, however, will be slightly longer than the theoretical minimum. Elevated temperatures are useful for fast separations because of decreased mobile phase viscosity (q) and increased solute diffusion (Dm). 

An alternative strategy for fast liquid chromatography uses short columns packed with small particles operated at high flow rates and often elevated temperatures to separate simple mixtures under conditions were resolution is compromised but still adequate for identification purposes [252-258]. Small diameter particles provide larger plate numbers by virtue of their relatively small interparticle mass transfer resistance combined with a shallow increase in the reduced plate height as the reduced mobile 

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Gas bubble separation time of petroleum oils NFT 60-149 ASTM D 3427 Time for air liberation after supersaturation (measurement of density)... 

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

More generally, the relaxation follows generalized first-order kinetics with several relaxation times i., as depicted schematically in figure B2.5.2 for the case of tliree well-separated time scales. The various relaxation times detemime the tiimmg points of the product concentration on a logaritlnnic time scale. These relaxation times are obtained from the eigenvalues of the appropriate rate coefficient matrix (chapter A3.41. The time resolution of J-jump relaxation teclmiques is often limited by the rate at which the system can be heated. With typical J-jumps of several Kelvin, the time resolution lies in the microsecond range. 

The stationary phase is selected to provide the maximum selectivity. Where possible, the retention factor is adjusted (by varying the mobile phase composition, temperature, or pressure) to an optimum value that generally falls between 2 and 10. Resolution is adversely affected when k 2, while product dilution and separation time... 

Classical gels had a low degree of cross-linkage and were of a large particle size. This resulted in that modest flow rates could only be applied and the separation time was typically 10 hr, which at that time was perfectly acceptable, keeping in mind that preparation of the column could take up to 2 days or more. After the introduction of Sephadex, new materials have been introduced continuously on the market, and still, 30 years after the introduction of the first commercial material, new media are still introduced, also from the originators of Sephadex. What are the driving forces behind this development and what are the features of these new media ... 

Elow rate determines the separation time and can significantly affect resolution and efficiency. The effect of flow rate on HETP for TSK-GEL SW and TSK-GEL SWxi analytical columns is shown in Fig. 4.6. Resolution is typically higher at slower flow rates, although results shown in Fig. 5B indicate that, with increasing sample load, the faster flow rates can give higher resolution. 

Previously, Bruno and Santoro had found complete exchange occurred between the ions Ag(dipy)2 or Ag(phen)2 and Ag, in nitric acid solution, within the separation time. 

Tewes et al., using both precipitation and extraction separation techniques, observed essentially complete exchange within the separation time (1 min) during an isotopic study ( V) of this exchange. The media ranged from 0.3 M perchloric acid to 7 M hydrochloric acid reactant concentrations were 10 M. 

Separation of phases is sometimes a problem, especially in the case of mixtures of alkaline aqueous solutions and organic liquids. Addition of materials that decrease the surface tension, filtration of the emulsion to remove interfacial contamination, and the use of sedimentation centrifuges should be considered if the separation time is too long. 

The highest column efficiency will be obtained at In practice, higher values are frequently used to minimize the separation time. For gas chromatography a velocity of about 2 u, called the practical operating gas velocity, is frequently recommended [67]. Provided that the ascending portion of the van Deemter curve is... 

For coluzms with large plate numbers or operated with a vacuum outlet (as might be the case in GC/MS) such that P 1 the optimum separation time is given by equation (1.59)... 

The temperature program mode is the most widely used separation technique in gas chromatography [155]. As well as reducing separation times for samples with a wide tailing point... 

The most common types of preparative-scale gas chromatographic instruments are based on pacXed column technology [489-491]. The primary objective in preparative-scale gas chromatography is to obtain a high sample throughput. An inevitable result of this goal is that either resolution or separation time, or both, must be compromised. The primary method... 

Another approach to defining the separation capacity of a column is by its peak capacity (the number of peaks than can be resolved at any specific resolution, usually R, i, in a given separation time). For SEC the pe2dc capacity, PCgc, is given approximately by... 

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