Pellet disc preparation

Initial tests were conducted at room temperature with pellet discs composed solely of PCL or 75%-acetate hydrolyzed PVA as the model component materials, with the amount of MB incorporated being 2.5% (w/w). It was found that the PVA pellet disc became swollen quickly and released most of the MB in 2 hrs. In contrast, the PCL pellet disc released very little even though the test was continued for over 5 days (Table 1). Changing the compression pressure during the preparation of the pellet disc to 2 or 10 tons did not alter the release characteristics as already observed. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

The effect of KBr pellet preparation discussed above brings about a well known - and possibly overlooked - class of mechanochemical reactions those between alkaline halogenides, such as KBr and Csl, used for IR pellets, and species, such as molecules carrying -COOH groups, which may cause an alteration of the absorption frequencies with respect to those measured in solution or Nujol [70]. The mechanochemical reactions that take place when milhng and pressing analytes with KBr to form discs for IR experiments have been recently reviewed by Fernandez-Bertran and Reguera [71]. 

Finally release the press and remove the die body (A) and the upper steel pellet from the face of the disc and then remove the disc itself with tweezers (the disc should never be handled with the fingers), and mount it in the specially designed holder. These latter operations are best conducted under a radiant heater. The disc holder is located in position in the sample beam path of the spectrophotometer if required a blank potassium bromide disc, similarly prepared, is introduced into the reference beam path. 

Weigh out 7 mg. of infrared-quality powdered KBr into a 2-cm. micro-porcelain boat, and with a spatula lightly tamp into a small cake at one end. Add the concentrated eluate from the thin layer plate to the KBr in 2.0-/Jiter increments, allowing sufficient time (about 30 seconds) between additions for the solvent to evaporate. After all the sample has been evaporated onto the KBr, prepare the disc, and obtain the spectrum in the usual manner. It is advisable to record the spectra of a blank KBr pellet and a reference standard on the same paper for comparison. 

Transmission spectroscopy (2) is the simplest sampling technique in IR spectroscopy and is used for routine spectral measurements on diverse samples. Resin samples such as polystyrene or TentaGel (3) beads are usually prepared as a potassium bromide disc (pellet). A small amount, usually 1-3 mg, of finely ground solid sample is mixed with approximately 400 mg powdered potassium bromide and then pressed in an evacuated die under high pressure. The resulting discs are transparent and yield good spectra. 

In a related type of electrode the membrane is a pressed disc of Ag2S + AgX where X may be Cl , Br , I or SCN and the electrode responds to X . With such a membrane, it is a very low concentration of Ag" ions on both sides of the disc which determines the membrane potential and the selectivity is a function of the solubility product any anion which forms a silver salt with a solubility product lower than the ion being determined will be a serious interference. Hence the electrode responds strongly to Ag and S and for the determination of other ions their presence is catastrophic. Similarly I and Br is a serious interference to the Cl electrode and I to the Br electrode. A electrode (M = Cu, Pb, Cd) may be prepared by making a pellet from a mixture of Ag2S and MS. In these electrodes, the ion activity controls the S activity and thereby the Ag activity and hence the electrode response. The pellet is again positioned in an inert electrode body with epoxy resin. 

Multi-component analysis can be readily apphed to the infrared spectra of minerals. The latter contain non-interacting components and so the spectrum of a mineral can be analysed in terms of a linear combination of the spectra of the individual components. However, the spectra of such solids exhibit a marked particle-size dependency. The particle size should be reduced (to 325 mesh) prior to preparation of an alkali halide disc. The pellet preparation involves separate grinding and dispersion steps because minerals tend not to be effectively ground in the presence of an excess of KBr. Figure 5.8 illustrates the analysis of a mineral containing several components. The sample spectrum (a) is shown, as well as the calculated spectrum (b) based on the reference spectra of a variety of standard mineral components. The residual difference spectrum (c) shows that the error between the two spectra is small. 

Sepiolite is a fibrous silicate, Sii2MggOjo(OH)4(H20)4, made up of microporous channels parallel to the fiber axis. The chemical composition and stmcture of sepiolite are responsible for good adsorbent behavior towards polar molecules such as water, ammonia, amines and aldehydes in both gas and liquid phases because of its hydrophilic surfaces. Activated carbon is essentially microporous and hydrophobic, making it suitable for nonpolar molecules such as hydrocarbons. As these properties are complementary, a mixture of both could be useful in specific applications such as adsorption of mixtures of molecules. The preparation of discs or pellets is straightforward because in mixtures of carbon and sepiolite, the latter acts as a binder when adding small quantities of water. 

For investigating the effect of temperature on viscosity, dynamic shear measurements were performed using the Rheometric Scientific ARES-RDAIII, with a parallel-plate fixture. Disc-shaped samples were prepared by compression molding for the measurements using the compounded pellets, or the as-received base polymers. Dynamic temperature sweep tests were carried out with temperatures ranging from 250 °C to 110 °C at a cooling rate of 5 °C/min, while the frequency was set at 1 rad/s. The plate gap was actively controlled at 1 mm to compensate for the thermal expansion of the fixtures. 

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