Small Volume-High Specialization

Numerous polymer compositions are applied as electrical insulation. Some of them are only used in small volumes for special applications. The materials used in relatively large volume in 1980 are polyethylene (high density + low density), 211,000 tons poly(vinyl chloride) (PVC), 177,000 tons and polyesters, 77,000 tons. 

Compounding as a separate industry was originally devoted mainly to the addition of colouring, especially for special colour matches. This gradually developed to skill in short-run production, with high efficiency in distribution, to meet the demand from plastics processors for small volumes of special grades. Many independent compounders now operate under franchise from polymer producers, while also marketing their own formulations. 

Despite its advantages, SFE is employed routinely in only a few pesticide laboratories, for the extraction of low-moisture samples such as grains, pulses, dried fruit and tea. The technique requires specialized equipment because the extractions are performed at high pressure (45 psi) and elevated temperatures (around 60 °C) to maintain aC02 density ofO.85 gmL The analyte(s) may be trapped on Cig solid-phase material and eluted with a small volume (around 5 mL) of a polar solvent, such as acetonitrile. 

As the name suggests, flat-sheet membranes are flat, like a sheet of paper, and can be made as thin as less than 1 pm. However, they need special holders to hold them in place. Hollow-fiber membranes are shaped like tubes (200 to 500 pm ID), allowing fluids to flow inside as well as on the outside. Hollow fibers are self-supported and offer the advantage of larger surface area per unit volume and high packing density. A large number of parallel fibers can be packed into a small volume. 

Let us consider some of the special problems encountered in the operation of a radioisotope detector and the compromises that must be considered. Like any chromatographic detector, a carbon-14 detector should have a small volume and a short hold-up time in order to minimize band spreading and loss of resolution. Unfortunately radioisotopes are measured with an inherent time factor - disintegrations per minute. Therefore, the smaller the cell and the shorter the hold-up, the lower will be the sensitivity, a circumstance which is totally at odds with the first requirement. In practice, we have found that a U-tube with a cross-section diameter of 2mm is generally satisfactory. This gives a cell with a void volume of 200-300 yl, which is high compared to the 2-10 yl volumes of many UV flow cells, and may introduce some band spreading when used with the best new HPLC columns. 

The analysis of brines perhaps deserves special mention as the high sodium chloride concentrations are extremely unfavourable for electrothermal atomisation and most troublesome in flame analysis. The preferred approach is probably solvent extraction with either oxine or APDC to remove the trace metals into a small volume of MIBK for flame atomisation or chloroform for electrothermal cells. Care must be taken to avoid interference from chloro-complexes in the extraction, and if this is suspected an ion-association extraction of these complexes might be preferable. 

The application of microanalytical techniques, such as flow injection in ICP-MS (FI-ICP-SFMS), is also of special interest in medical research where very small sample volumes have to be characterized, e.g. for Cr determination in DNA by sector-field ICP-MS.For the separation of isobaric interferences of Cr and Ar C the measurements were performed at a mass resolution of m/Am = 3000. Transient signals of Cr and Cr analyzed by FI-ICP-SFMS of a I0p,gl chromium solution (sample loop 20p,l) are illustrated in Figure 9.31a. For quantification by the isotope dilution technique, the small volume of DNA available (diluted 1 10) was injected into a continuous flow of 2 % HNO3, which is spiked with high-enriched Cr solution (the isotope abundance of Cr was 83.4%). The application of the isotope dilution technique in flow injection ICP-SFMS is shown in Fig. 9.31 b. ... 

Phosphazene elastomers were very successful throughout the 1980s, being used mainly in military and aerospace industry. However, because of their high cost and relatively small volume market, they are not available commercially other than on special orders. 

The preponderance of analytical (and preparative) separations that precede mass spectrometric analyses are carried out using HPLC. Despite its potential for extremely high efficiencies, capillary electrophoresis (CE) remains something of a specialized application, particularly with mass spectrometric detection. The technique is hampered by its reliance on small sample volumes (typically nL injections) in order to avoid loss of separation efficiency. The consequence of these small volumes is a concentration detection limit that is substantially higher than that of techniques such as HPLC. Despite the exquisite sensitivity of mass spectrometers, characterization of related substances present at low femtomole levels (assuming a hypothetical related substance compound with molecular weight 500 daltons present at 0.1% w/w in an equally hypothetical drug substance sample... 

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