Dynamic splitting

An alternative form of split injection is the timed split technique, Figure 6.11 [130,131,133]. In this case the column is connected directly to the valve and the valve actuator is controlled electronically to turn the valve to the inject position and back very rapidly with only a portion of the sample in the loop displaced to the column. Timed split allows variable volumes to be injected by changing the valve actuator tine and provides more reproducible splitting than the dynamic split technique. However, it suffers from many of the same problems as dynamic split, namely, poor accuracy, split ratios that depend on pressure, and high detection limits. 

Substitution of equation (2.3.62) into a set of equations (4.1.13) to (4.1.16) for noncharged (neutral) particles (Uvil r) = 0) does not affect equations (4.1.18) and (4.1.19) whereas the linear equation (4.1.23) describing the correlation dynamics splits now into three integro-differential equations. Main stages of the passage from general equations (4.1.14)—(4.1.16) for the joint densities to those for the joint correlation functions have been demonstrated earlier, see (4.1.20) and (4.1.21). Therefore let us consider only those terms which are affected by the use of superposition approximation. Hereafter we use the relative coordinates f=f — f(, f = r 2 — r[ and... 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Figure 9. A) Dynamic splitting of 3 B) Tracing of ganglion subsinglet locations.

A heterogeneous pore structure with varying aspect ratio would increase the frequency of breakup and coalescence, which should increase the observed mobile ganglia size distribution. However, the basic flow mechanism should remain unchanged. Also the relative importance of snap-off as a breakup mechanism would be increased relative to dynamic splitting. Here too a detailed study seems desirable. 

We remark that if condition (273) is not satisfied, which is the case if one misses the conical intersection in an intermediate regime (f Ac), the Landau-Zener formula shows that the dynamics splits the population into the two surfaces near the intersection. This gives rise afterwards to two states that will have their own adiabatic evolution. 

The delayed split method uses the same configuration as the dynamic split except for the addition of an on/off valve in the vent line [171-173]. With the split vent valve in the off position, the sample is injected in the normal way at a selected pressure. After a prescribed delay time the injection valve is returned to the load position and the vent valve opened, which aifects an immediate purge of the injection area. The amount of sample transferred into the column depends on the valve actuation time and the delay time. The delayed split injection techniques can be modified for solvent backflushing [171]. Immediately after the vent valve is opened a rapid negative pressure ramp is initiated. This causes a reversal of flow in the first part of the column and precipitation of the sample on the column wall. The solvent, and probably some of the sample, is backflushed out of the column and through the open vent line. The injection repeatability is similar to timed split injection. 

Other Langevin dynamics splitting schemes in common use utilize a different additive decomposition of the SDE vector field. For example, the Stochastic Position Verlet (SPV) method of [265] relies on the splitting strategy... 

Grishanov S, Tausif M and Russell S J (2012b), Application of dynamic splitting to estimate fibre entanglement in simulated nonwoven fibrous assemblies . Comp Sci Tech, 72(12), 1318-1323. 

In the dynamic split mode, which is the most frequently applied technique in open tubular column SFC, the split ratio is determined by the ratio of the flow out of the column to the flow out of the split restrictor. Thus, if the flow out of the split is 500 times faster than the flow out of the column, the split ratio is 1 500. The disadvantage of the dynamic split technique is that obtaining reproducible results can be difficult with certain samples. 

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