Ascorbic acid, absolute requirements

While many aspects of nonlinear pharmacokinetic behavior may impact on the above equation, the more relevant pharmacokinetic processes for ASOs are absorption and distribution at or below therapeutic or nontoxic plasma concentrations. Nonlinear absorption or distribution processes can affect AUC terms in a nonproportional manner when different doses are compared, thereby resulting in an inaccurate determination of bioavailability. This has been shown to occur on numerous occasions for compounds such as ascorbic acid or naproxen [59-62]. Such cases require an understanding of the capacity-limiting cause of the nonlinearity and the pharmacokinetic processes upon which this impacts, in case of ASOs absorption and intercompartmental distribution processes from the central compartment into the peripheral tissues. With this understanding, various methods may then be applied to best approximate the rate of change of the plasma concentrations from one sampling time to the next allowing for an estimate of the absolute BAV. 

The use of the HPA system is fairly simple the 30 mL or the 70 mL standard quartz vessels allow intakes of sample equivalent to 100 or 230 mg carbon, respectively, with nitric acid (65 to 69% w/v) volumes of 2 mL per 100 mg C. The heating programme from ambient to maximum temperature is as follows ambient to 250 °C 1 min 250 to 300 °C 30 min. This maximum temperature is maintained for 90 min then turned off. After cooling (approx. 60 min) the samples are made up to volume and treated as required for subsequent determination. In one of the author s laboratory (M.S.) the analyte solutions are treated with a Kl-ascorbic acid mixture for reduction to As(lll) and subsequent FIA-AAS-determination with excellent precision and absolute detection limit (Guo et al., 1990). 

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