Differential coating, definition

An earlier off-odor problem that surfaced was the presence of 4-phenyl-cyclohexene in styrene-butadiene coated paper (Koszinowski et al. 1980). This compound was created by a Diels-Alder condensation reaction involving a molecule of styrene and butadiene and is differentiated by its odor from the isomeric 3 and 1-phenyl-cyclohex-ene compounds which cannot be formed by such a condensation reaction. The recognition threshold of this compound in the headspace over an aqueous solution lies around a concentration of 10 pg/kg (10 ppb). The typical odor of this compound at concentrations of 4-phenyl-cyclohexene in paper over 4 mg/kg (4 ppm) is easily identified. AGC determination in this concentration range is also possible without difficulty and its identification with MS using the relative molecular mass of 158 and one of the retro Diels-Alder decomposition product fragments at m/e = 104 (styrene) and mJe = 54 (butadiene) is definitely possible. 

Not even this list, however, comprehensively describes a formulation. Many tablets carry printed identification markings or are color-coated dyestuffs are outside the definition of an excipient, and allergies to them have been documented. Differential efficacy exists among differently colored placebos, and this should therefore also be expected for active formulations. Furthermore, formulations have more subtle but nonetheless differential characteristics, such as whether the tablet was compressed at a higher or lower pressure. 

The patterning strategy (high molecular mass adsorbate and robust covalent attachment) can be extended to sub-lOOnm sized patterns by exploiting AFM-tip assisted transfer of PAMAM dendrimers. By scanning smfaces with an AFM tip, which has been previously coated with the dendrimers, molecules can be deposited onto, say, sihcon, mica or SAMs of NHS - CIO. In the case of mica and oxidized sihcon substrates, the originally deposited patterns were detectable, but the AFM friction images showed that dendrimer molecules may have diffused over the substrates. Based on the cmrently available data, spontaneous diffusion or tip-induced effects cannot be differentiated. It is, however, clear that the patterns produced do not possess sufficient stability and definition to be of any use. 

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