Light absorbing impurities

The light-absorbing impurities of oxytetracycline hydrochloride and oxytetracycline dihydrate [2,4,6] are detected using a UV-spectrophotometric method. In all the compendia, the absorbance of a solution of 2.0 mg/mL in a mixture of 1 volume of hydrochloric acid solution (0.1 mol/L) and 99 volumes of methanol at 430 nm not greater than 0.50 is required the absorbance of a solution of 10 mg/mL in the same solvent at 490 nm is not greater than 0.20. The measurements are carried out within 1 h of the preparation of the solutions. 

Photo-oxidation of PE and PP is mainly due to slight absorption of light above 285 nm by the commercial grades due to the presence of light-absorbing impurities, particularly oxygen-containing species, and trace levels of... 

Light—absorbing impurities the absorbance of a 5% solution of silver sulfadiazine in 35% ammonia, measured at 400 nm, may not exceed 0.20. This demand seems to be sharp because Indemans (14) reported for three batches an absorbance between 0.199 and 0.273, while the three batches meet the other requirements. 

Some secondary antioxidants, such as nickel dimethyl dithiophosphate and nickel dimethyldithiocarbamate, can act as photoantioxidants, countering the effect of light-absorbing impurities. 

Many polymers which are intrinsically transparent to the shorter wavelengths of the sun s spectrum (<290 nm), become sensitized by the presence of light absorbing impurities and trace level of metals and adventitious species produced during manufacture and fabrication. Hence the susceptibility of commercial polymers (e.g. PE and PP) to outdoor weathering. The outdoor performance of polymers can, however, be markedly improved by photoantioxidants and UV-stabilizers. 

Polymers (PH) containing intra-molecular chromophoric groups and/or light absorbing impurities (RH) (inter-molecular impurities) can produce radicals in the presence of air (oxygen) under UV/VIS (visible) irradiation ... 

Industrially produced polymers contain a number of light-absorbing impurities produced in side-reactions during polymerization, processing and storage. These impurities can be divided into two groups [1759,1766,1768] ... 

The UV method performs well in the absence of interferences of other additives or pigments but is liable to be in error owing to interference by highly absorbing impurities that may be present in the sample or in the solvent [56,57]. Commercial erucamides with variable light stability have been analysed by means of SEC-UV (254nm)/RI and UV absorption in MeOH [58]. 

The processes of photochemistry are the same for polymers and small molecules. The Grotthus-Draper law sfafes fhat no photochemical reactions can occur unless a photon of lighf is absorbed. This means, for example, thaf many commercial plastics transparent in the near UV can undergo photodegradation only as a result of the absorption of light by impurities. 

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