Modified packings

Porous silica packings do, however, sometimes suffer from adsorption between the sample and silanol groups on the silica surface. This interaction can interfere with the size exclusion experiment and yield erroneous information. In many cases, this problem is easily overcome by selecting mobile phases that eliminate these interactions. In addition, the surface of porous silica packings is routinely modified in order to reduce these undesirable interactions. Trimeth-ylsilane modified packing is typically used with synthetic polymers. Diol modified packing is typically used with proteins and peptides. 

Diagram A shows a modified packing material which utilises bulky silane groups, end capping and a polar embedded functional group... 

Packed columns. In this technique separation is by virtue of selectivity. A number of different types of modified silica particles developed for HPLC are applicable. In addition, there are a number of in situ modified packings that are of interest in connection with lipid analysis (Demirbiiker and Blomberg, 1991, 1992 Demirbiiker, Hagglund and Blomberg, 1990 Hag-glund, Demirbiiker and Blomberg, 1994)... 

Small quantities of acids and bases may also be added to the stationary phase to cover or neufralize active sites on support. They usually have the same acid-base properties of the species being analyzed and are referred to as tail reducers. Phosphoric acid-modified packings are effective for analyzing fatty acids and phenols potassium hydroxide has been used with success for amines and other basic compounds. 

By employing a shghtly modified funnel, the glass nail can be utilised, in conjunction with a drying p. tube packed with desiccant, for drying the product... 

One report (13) describes the procedure for spinning dry asymmetric ceUulose acetate fiber with a bore skin. Such fibers are spun in a modified dry-spinning process in which a volatile Uquid (methyl formate) is used as the ceUulose acetate solvent. The bore coagulating Uquid is isopropyl alcohol, which is subsequentiy removed. The advantages of these dry fibers over most ceUulose acetate membranes are that they can be stored dry, they are wet-dry reversible, they can be sterilized and packed dry, and they are ready for use without removal of preservatives. 

The deviation from the Einstein equation at higher concentrations is represented in Figure 13, which is typical of many systems (88,89). The relative viscosity tends to infinity as the concentration approaches the limiting volume fraction of close packing ( ) (0 = - 0.7). Equation 10 has been modified (90,91) to take this into account, and the expression for becomes (eq. 11) ... 

The analysis of mefloquine in blood, using packed-column sfc, a mobile phase consisting of / -pentane modified with 1% methanol and 0.15% -butylamine, and electron capture detection has been reported (92). The method compares favorably to a previously pubflshed hplc-based procedure having a detection limit of 7.5 ng/mLin 0.1 mL blood sample. 

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