Film coating alternatives

Solid phase extraction (SPE) is a very simple, rapid and reproducible cleanup technique that is now widely accepted as an alternative to the time-consuming liquid-liquid extractions. Additionally, SPE uses relatively small volumes of solvents, and is easy to automate. It is available in a number of different formats, including cartridges, disks, loose material, well plates or SPME using film-coated capillaries. SPE can be considered as an extraction technique when used for isolation and concentration or a cleanup technique when used to remove co-extractives from solvent extracts. The use of SPE for cleanup is discussed later. 

The film-coating process can be carried out in conventional pans, although operation variables such as speed of pan rotation, angle of pan axis, and temperature and humidity control may be more critical. Newer pans with one-way air flow through the tablet bed offer an even better alternative, because the pan environment can be controlled within finer limits. 

Raman often is evaluated as an alternative to an existing high performance liquid chromatography (HPLC) method because of its potential to be noninvasive, fast, simple to perform, and solvent-free. Raman was compared to HPLC for the determination of ticlopidine-hydrochloride (TCL) [43], risperidone [44] in film-coated tablets, and medroxyprogesterone acetate (MPA) in 150-mg/mL suspensions (DepoProvera, Pfizer) [45] it was found to have numerous advantages and performance suitable to replace HPLC. In an off-line laboratory study, the relative standard deviation of the measurement of the composition of powder mixtures of two sulfonamides, sulfathiazole and sulfanilamide, was reduced from 10-20% to less than 4% by employing a reusable, easily prepared rotating sample cell [46]. 

As an alternative, stable high-coverage nonpolar RPC sorbents phases have been prepared by cross-linking hydrophobic polymers at the silica surface, either via free radical 143 or condensation 101 polymerization chemistry. In this case, the underlying silica becomes partly protected from hydrolytic degradation due to the presence of the hydrophobic polymer film coating that effectively shields the support material. Similar procedures have been employed to chemically modify the surface of other support materials, such as porous zirconia, titania, or alumina, to further impart resistance to degradation when alkaline mobile-phase conditions are employed. Porous polystyrene-divinylbenzene sorbents, be-... 

To date, use of such continuous processes has been restricted primarily to the manufacture of large-volume products, an application for which continuous processes potentially have a major advantage. Nonetheless, continuous coating processes provide a potentially viable alternative for the scale-up of any film-coating process, where many of the tasks potentially become much simpler, since they would always be the same irrespective of whether the production batch size is 250 kg or 5(K)() kg. Simplification arises from reducing or eliminating tasks that would... 

A number of different methods can be used to prepare polymer film-coated electrodes. The simplest is to dip the surface to be coated into a solution of the polymer, remove the electrode from the solution, and allow the solvent to evaporate. While this method is simple, it is difficult to control the amount of material that ends up on the electrode surface. Alternatively, a measured volume of solution can be applied to the surface to be coated. This allows for accurate control of the amount of polymer applied. The polymer film may also be spin-coated onto the electrode surface. Spin-coating is used extensively in the semiconductor industry and yields very uniform film thicknesses. 

In spray pyrolysis the precursor reacts thermally in particle form and the product species is deposited as particles on the surface. Such films must be annealed or sintered after the particles have been deposited to make a uniform coating. Alternately, the precursor may be deposited on a heated surface and react thermally upon contact. In this case, any remaining solvent is driven from the surface by evaporation to the gas phase. 

An alternative to solvent-phase synthesis of CEPs is vapor-phase polymerization (VPP). This route was first used for PAn s in 1998 for the deposition of conducting PAn films on cotton fibers, by impregnating the thread with (NH SjOg oxidant and then exposing the surface to aniline vapor.94 Polyacrylamide films coated with conducting PAn (conductivity ca. 10-5 S cm-1) have been similarly prepared.95... 

A refractive index change on the order of 10-2 could be induced in the polymer on irradiation with blue light. This technology can also be utilized in the fabrication of multilayer or thick films. By alternately coating urethane-urea copolymer with azobenzene and nonphotosensitive polyvinylalcohol multilayer films could be obtained. In this case, the bitmaps are recorded as images on individual layers. 

Soc., 1999, 121, 3435 T. Hoshi, H. Saiki, S. Kuwazawa, Y. Kobayama and A. Anzai, Polyelectrolyte multilayer film-coated electrodes for amperometric determination of hydrogen peroxide in the presence of ascorbic acid, uric acid and acetaminophen, Anal. Sci., 2000, 16, 1009 E.S. Forzani, VM. Solis and E.J. Calvo, Electrochemical behavior of polyphenol oxidase immobilized in self-assembled structures layer by layer with cationic polyallylamine, Anal. Chem., 2000, 72, 5300 Y.M. Lvov, Z. Lu, J.B. Schenkman, X. Zu and J.F. Rusling, Direct electrochemistry of myoglobin and cytochrome P450cam in alternate layer-by-layer film with DNA and other polyions, J. Am. Chem. Soc., 1998, 120, 4073. 

---