Dynamic dilution

The first element, dynamic dilution, provides a reproducible sample for each panelist. The system must minimize the loss of the odorant to the walls of the delivery apparatus, provide clean dilution air of odor-free quality, maintain a constant dilution ratio for the duration of a given test, and have no memory effect when going from high to low concentrations or switching between odorants of different character. The type of mask or port and the delivery flow rate have been found to influence the response of panelists in determining odor threshold and intensity. 

Sample preparation, injection, calibration, and data collection, must be automated for process analysis. Methods used for flow injection analysis (FLA) are also useful for reliable sampling for process LC systems.1 Dynamic dilution is a technique that is used extensively in FIA.13 In this technique, sample from a loop or slot of a valve is diluted as it is transferred to a HPLC injection valve for analysis. As the diluted sample plug passes through the HPLC valve it is switched and the sample is injected onto the HPLC column for separation. The sample transfer time typically is determined with a refractive index detector and valve switching, which can be controlled by an integrator or computer. The transfer time is very reproducible. Calibration is typically done by external standardization using normalization by response factor. Internal standardization has also been used. To detect upsets or for process optimization, absolute numbers are not always needed. An alternative to... 

Figure 4 Dynamic dilution of sample for HPLC analysis of amidation/cyclization reaction. Transfer solvent methoxyethanol flow rate 1 ml/min. Detection refractive index.

The 1/16" x 0.02" i.d. transfer line also functioned as a sample dilution device in other applications, a stainless steel column packed with glass beads has been found to be useful for dilution. This simple dynamic dilution technique has been used extensively in flow injection analysis.3 A refractive index detector is typically used to measure the sample transfer time. As shown in Figure 4, approximately 5 minutes is required to transfer the sample plug to the Rheodyne valve. As the apex of the sample band passes though the Rheodyne valve, the valve is activated and 1 pi injected onto the liquid chromatographic column. The sample transfer time was checked periodically over 1 year of operation and found to be stable. 

To obtain a sample, the reactor was manually pressurized with nitrogen to 10 to 12 psi, and the four-port switching valve was activated to divert the flow of reactor solvent to recycle. The pressure forced about 4 ml of sample through the 5-pl slot of the tantalum valve. The valve was then switched to transfer the sample to the HPLC located about 15 feet away. The solvent for transfer and dynamic dilution was ethoxyethanol, with a boiling point of... 

The olfactometer is a device which dilutes odourous air with odour free air. This dilutions are offered to a panel in order to determine the odour threshold. There can be distinguished between static and dynamic dilution... 

In Germany and U.K. no choice is made between statically and dynamically diluting systems. 

BEDBOROUGH, D.R. and TROTT, P.E. (1979). The Sensory Measurement of Odours by Dynamic Dilution, Warren Spring Laboratoiy Report, LR 299 (AP). 

In The Netherlands dynamic dilution devices are, with one exception, always used by research institutes, provincial authorities and industry. However these devices differ greatly in design and in method of application. In table I the principal data of four olfactometers are summarised. These were built by the research institutes themselves. 

Measurements of detection threshold values should use dynamic dilution techniques in odour free surroundings. 

The desired dilution in case of a dynamic dilution system is calculated by the formula ... 

Measurement of the strength of odours by dynamic dilution olfactometry and observers is a complex task. The observers require adequate training and sound psychophysical procedures are needed to maximise the validity of the measurements. 

Fig. 7. The effective free-energy potentials for retraction of the free end of arms in a mon-odisperse star polymer melt. The upper curve assumes no constraint-release, the lower two curves take the dynamic dilution approximation with the assumptions (Ball-...

The mathematical treatment that arises from the dynamic dilution hypothesis is remarkably simple - and very effective in the cases of star polymers and of path length fluctuation contributions to constraint release in Hnear polymers. The physics is equally appealing all relaxed segments on a timescale rare treated in just the same way they do not contribute to the entanglement network as far as the unrelaxed material is concerned. If the volume fraction of unrelaxed chain material is 0, then on this timescale the entanglement molecular weight is renormalised to Mg/0 or, equivalently, the tube diameter to However, such a... 

A naive application of dynamic dilution would introduce a dependence of the reptation time on the unrelaxed volume fraction. Since T gp L (aM/MJ this would imply the choice of in the case a=l for the dflution exponent. But... 

Using the Rouse result, the left hand side of Eq. (30) is just l/2t. In the case of star polymers, using the approximate result for t(x) from Eq. (21) and the corresponding dynamic dilution result 0(x)=(l-x) the criterion becomes... 

So the criterion that the effective constraint-release must be fast enough to allow local pieces of umelaxed chain to explore any dilated tube fully confirms the assumption of dynamic dilution for nearly the whole range of relaxation timescales exhibited by star polymers. 

So dynamic dilution is vahd in linear polymers only for the case of extremely wide polydispersity in which the weight distribution is broader than that of a power-law with exponent -5/2. In particular we see that it is not valid for near-monodisperse linear polymers around their terminal time. 

The experiments on H-polymers confirm another aspect of the dynamic dilution theory for constraint release in branched polymers the range of relaxation times clearly attributable to the arms of the H-polymers is typically much... 

A feature of theories for tree-like polymers is the disentanglement transition , which occurs when the tube dilation becomes faster than the arm-retraction within it. In fact this will happen even for simple star polymers, but very close to the terminal time itself when very little orientation remains in the polymers. In tree-like polymers, it is possible that several levels of molecule near the core are not effectively entangled, and instead relax via renormalised Rouse dynamics (in other words the criterion for dynamic dilution of Sect. 3.2.5 occurs before the topology of the tree becomes trivial). In extreme cases the cores may relax by Zimm dynamics, when the surroundings fail to screen even the hydro-dynamic interactions between the slowest sections of the molecules. 

Of course from a molecular point of view this is no longer surprising - we know that dynamic dilution is a highly cooperative process. However the quantitative prediction of the dynamic moduli of Fig. 14 is clearly a very demanding task for a theory with essentially no free parameters We outHne here how the tube model calculation is done in this case for details see [56]. 

Fig. 14. Data (points) for G (co) and G (co) for a range of compositions of a blend of two polyisoprene stars of molecular weights 28 and 144 kg mol The fractions of the bigger star are in order 0.0,0.2,0.5,0.8 and 1.0. Curves are theoretical predictions of the tube model with co-operative constraint release treated by dynamic dilution [56]. The choice of 2.0 rather than 7/3 for the dilution exponent p is compensated for by taking M = 5500 kg mol" ...

The correction to the coefficient of in the dynamic dilution (cubic) term in the potential (compare Eq. 22 for the pure star case) arises from the way the difference in arm molecular weights affects the fraction of unrelaxed arm at the same timescale. 

Reasons have been advanced for both an increase and a decrease of the tube diameter with strain. A justification of the former view might be the retraction process itself [38]. If it acts in a similar way to the dynamic dilution and the effective concentration of entanglement network follows the retraction then Cgjy < E.u > so that a < E.u On the other hand one might guess that at large strains the tube deforms at constant tube volume La. The tube length must increase as < E.u >,so from this effect a < E.u > . Indeed, Marrucci has recently proposed that both these effects exist and remain unnoticed in step strain because they cancel [69] Of course this is far from idle speculation because there is another situation in which such effects would have important consequences. This is in conditions of continuous deformation, to which we now turn. 

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