Impurity studies

ISX-B Impurity Studies Experiment B Oak Ridge National Laboratory (ORNL), Oak Ridge, Term. 

However, the ICH guidelines do not include information about conducting or interpreting impurity studies, except to state that stability studies, chemical development studies, and routine batch analysis should be used to predict the impurities likely to occur in commercial production. 

If data are unavailable to qualify the proposed acceptance criterion of an impurity, studies to obtain such data can be appropriate when the usual qualification thresholds given are exceeded. 

In summary it can be said that Mbssbauer spectroscopy has provided valuable insight into the structure and bonding of iodine compounds. Already some 50 compounds have been investigated and the nuclear syste-matics are well established in broad outline. The comparison with n.q.r. data has proved particularly fruitful and further refinements to the bonding models can be expected in the future. Less has been done on the lattice-dynamical aspects or on impurity studies, but here again the prognosis is favourable. 

Experimental studies confirm that the CPE is often related to the adsorption of impurities. Studies performed on a polycrystalline Au electrode in the double-layer zone [352] in 1 M H2SO4 display almost ideal capacitive behavior with 4> 0.99, while in the same cell on the same electrode in 1 M NaNOs a larger CPE deviation with (j) 0.93 was observed because sodium nitrate is not as pure as sulfuric acid. Moreover, at well-polished and annealed polycrystalline Pt (roughness factor / f = 1.4) the parameter (p is between 0.97 and 0.977 in the double-layer zone in 0.1 M H2SO4 [352]. However, after electrochemical roughening (/ f = 5.1) (p increases to >0.99. This can only be explained by the fact that the surface (ionic) impurities become distributed over a larger surface area and their coverage becomes smaller. 

Here C ° and E denote a zeroth-order approximation for the quasi-particle states. In our Si calculation this zeroth-order approximation was extracted from an empirically fitted pseudopotential band-structure (see ref.4 and 35). This bandstructure is fitted in terms of a fourth-nearest neighbor (in the fcc-lattice sites) overlap model of bonding and antibond ng orbitals as described n our earlier work on optical properties and impurity screening. Also the calculation of the two-particle Green s function is based on this bandstructure and follows closely the impurity studies (for details see in particular, ref.35). 

In general, according to ICH guidelines on impurities in new drug products, identification of impurities below the 0.1% level is not considered to be necessary unless the potential impurities are expected to be unusually potent or toxic. (In all cases, impurities should be qualified.) If data are not available to qualify the proposed specification level of an impurity, studies to obtain such data maybe needed. 

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