Dilution quenching

For general purpose tracer work, however, and particularly in polymer chemistry, the liquid scintillation counter surpasses all other instruments in its sensitivity and adaptability. There is no question on the author s mind that at the present time such an instrument would be the first choice, particularly where tritium, carbon-14 or sulphur-35 were involved. Samples for assay are dissolved in a phosphor whose major solvent usually consists of toluene, toluene-alcohol, or dioxan. Many polymers and low molecular weight compounds are readily soluble in these solvents. Prospective users should not be deterred by alleged complications due to "variable quench effects" as these effects are readily corrected for via internal or external standards or the channels ratio method (7, 46, 91). Dilution quench corrections, though valid, are tedious and unnecessary. Where samples are insoluble in phosphor they may be suspended (e.g. as gels or as paper cut from chromatograms, etc.) or they can be burnt and the combustion products absorbed in a suitable phosphor solution. A modification of the Schoniger flask combustion technique is particularly suitable for this purpose (43—45). 

Dilution quenching results when a large volume of liquid radioactive sample is added to the scintillation solution. In most cases, this type of quenching cannot be eliminated, but it can be corrected by one of the techniques discussed below. 

FIGURE 7SI Quenching (a) Heat transfer quenching by double pipe heat exchanger in an aerosol reactor, (b) Dilution quenching by the addition of a cold gas. 

Dilution quenching results when a large volume of liquid radioactive sample is added to the scintillation solution. In most cases, this type of quenching cannot be eliminated, but it can be corrected. Modem scintillation counters are equipped with a standard radiation source inside the instrument but outside the scintillation solution. The radiation source, usually a gamma emitter, is mechanically moved into a position next to the vial containing the sample, and the combined system of standard and sample is counted. Gamma rays from the standard will excite solvent molecules in the sample, and the scintillation process occurs as previously described. However, the instrument is adjusted to register only scintillations due to y particle collisions with solvent molecules. 

Reversal of fluorescence intensity or self-quenching at high concentrations is a problem in quantitative analysis but can be eliminated by successive dilutions. Quenching by impurities can also occur and can cause signiflcant problems in analysis. Changes in pH can frequently change structure, as we saw with phenolphthalein in Fig. 5.33, and thereby change fluorescence intensity pH must therefore be controlled. Temperature and viscosity need to be controlled as well for reproducible results. 

As pointed out before, quenching means a reduction in the efficiency of transferring energy from the P-particles to the photomultiplier. In other words, the number of photons reaching the photomultiplier per P-particle is reduced this results in production of a pulse of reduced voltage. There are basically three types of quenching. They are (i) chemical, (ti) color, and (iiO dilution quenching. The three are discussed below briefly. 

Dilution Quenching. This is just what the title says. This is caused by the dilution of the solvent and fluor by the sample. High dilution will reduce the probability of scintillation event occurring. Naturally, this type of quenching normally takes place when working with Uquid samples. It can be corrected by correcting the data obtained for the dilution factor. 

In order for a soHd to bum it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the soHd phase and may continue in the Hquid (melt) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and, therefore, further combustion. Thus the burning of a soHd is like a chain reaction. For a compound to function as a flame retardant it must intermpt this cycle in some way. There are several mechanistic descriptions by which flame retardants modify flammabiUty. Each flame retardant actually functions by a combination of mechanisms. For example, metal hydroxides such as Al(OH)2 decompose endothermically (thermal quenching) to give water (inert gas dilution). In addition, in cases where up to 60 wt % of Al(OH)2 may be used, such as in polyolefins, the physical dilution effect cannot be ignored. 

Quenching. After solution treatment, the product is generally cooled to room temperature at such a rate to retain essentially all of the solute in solution. The central portions of thicker products caimot be cooled at a sufficient rate to prevent extensive precipitation in some alloys. Moreover, some forgings and castings are dehberately cooled slowly to minimize distortion and residual stress produced by differential cooling in different portions of the products. Cold water, either by immersion or by sprays, is the most commonly used cooling medium. Hot water or a solution of a polymer in cold water is used when the highest rates are not desired. Dilute Al—Mg—Si and Al—Mg—Zn extmsions can be effectively solution heat treated by the extmsion process therefore, they may be quenched at the extmsion press by either air or water. 

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

Thermal isomerization of a-pinene, usually at about 450°C, gives a mixture of equal amounts of dipentene (15) and aHoocimene (16) (49,50). Ocimene (17) is produced initially but is unstable and rearranges to aHoocimene, which is subject to cyclization at higher temperatures to produce a- and P-pyronenes (18 and 19). The pyrolysis conditions are usually optimized to give the maximum amount of aHoocimene. Ocimenes can be produced by a technique using shorter contact time and rapid quenching or steam dilution (51). 

The Quench and Drain control is good for removing heat without contaminating or diluting the pumped product. 

Our final focus in this review is on charged quenched-annealed fluid systems. Very recently Bratko, Chakraborty and Chandler have addressed this problem [34-36]. A set of grand canonical computer simulation results for infinitely diluted electrolyte adsorbed in an electroneutral matrix of ions has been presented and an attempt to describe them at the level of... 

A mixture of 10 g of the above piperazine carboxylate ester, 8 g of phosphorus pentoxide and 20 ml of phosphorus oxychloride is heated under reflux for about 1 day, diluted with 100 ml each of chloroform and benzene and quenched with 200 g of ice. The mixture is made basic with 10% sodium hydroxide. Theorganic layer is Isolated and extracted with 150 ml of dilute hydrochloric acid. The product is precipitated from the aqueous layer by addition of 10% sodium hydroxide, extracted with benzene and dried over potassium carbonate. Recrystallization from benzene-petroleum ether gives 2[Pg.77]

The reaction mixture is diluted with dry tetrahydrofuran (25 ml) and allowed to stand at room temperature for 20 hours. Excess Grignard reagent is quenched by adding a saturated solution of ammonium chloride. The organic layer is separated and the aqueous layer is extracted with ethyl acetate. 

Under a nitrogen atmosphere 0.23 g (1 mmol) of tm-butyl (S)-4-formyl-2,2-dimethyl-3-oxazolidinecar-boxylate(3) is dissolved in 5 mL ofabs. THF. The clear solution is stirred, cooled to —78 °C, then a solution of 1.3 mmol of the Grignard or lithium reagent is added dropwise. The mixture is stirred at — 78 °C for a further 1 h, then quenched by the addition of 20 mL of sat. aq NH4C1. After dilution with II20 the aqueous... 

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