Ascorbic acid vitamin analytical methods

Figure 3.31 Analytical isotachophoresis. Ascorbic acid (vitamin C) is naturally present in many foods and is often added to others. Occasionally the cheaper isomer, is oascorbic acid, which has no vitamin action, is used and is distinguishable from the natural isomer by many analytical methods, (a) shows the analysis of a sample of commercial fruit juice while (b) shows the same fruit juice to which a known amount (4 nmol) of isoascorbic acid has been added. (Reproduced by permission of LKB, Stockholm, Sweden.)...

How analytical methods deal with interferences is one of the more ad hoc aspects of method validation. There is a variety of approaches to studying interference, from adding arbitrary amounts of a single interferent in the absence of the analyte to establish the response of the instrument to that species, to multivariate methods in which several interferents are added in a statistical protocol to reveal both main and interaction effects. The first question that needs to be answered is to what extent interferences are expected and how likely they are to affect the measurement. In testing blood for glucose by an enzyme electrode, other electroactive species that may be present are ascorbic acid (vitamin C), uric acid, and paracetamol (if this drug has been taken). However, electroactive metals (e.g., copper and silver) are unlikely to be present in blood in great quantities. Potentiometric membrane electrode sensors (ion selective electrodes), of which the pH electrode is the... 

Figure 5-5 shows data for an experiment in which ascorbic acid (vitamin C) was measured in orange juice by an electrochemical method. The current between a pair of electrodes immersed in the juice is proportional to the concentration of ascorbic acid. Eight standard additions increased the current from 1.78 to 5.82 p,A (column C), which is at the upper end of the desired range of 1.5- to 3-fold increase in analytical signal. 

Pharmaceuticals. In commerce, ascorbic acid is produced exclusively by synthesis (98). Because of its rather pure nature and high concentrations in vitamin-multivitamin tablets, analysis by conventional or sophisticated procedures can be performed easily. The USP provides a reference standard of L-ascorbic acid for assay purposes. The methods used can be chosen from the many discussed above. The method officially approved by the Association of Official Analytical Chemists is the micro-fluorometric procedure developed by Deutsch and Weeks (44). 

Research Needs. Over the years L-ascorbic acid has been shown to be an essential nutrient for many insects including species of Lepidoptera, Orthoptera, Coleoptera, and Diptera. Others such as cockroaches, houseflies, and mealworms are reared on simple diets without added ascorbic acid. Perhaps those insects require very low levels of vitamin C in their diets. A sensitive analytical method is needed to measure levels of L-ascorbic acid and dehydroascorbic acid in insect tissue and food. Such a method, which is likely to be developed using HPLC with electrochemical detection, could be used to monitor vitamin C levels in feed ingredients as well as in tissues during an insect s life cycle. This information is needed to determine whether ascorbic acid is used to... 

A number of research groups have reported concentrations of ascorbic acid in honey between 5.4 and 54 mg/kg (Abu-Tarboush et al. 1993 Bogdanov et al. 2008 Castro et al. 2001 Ciulu et al. 2011 Haydak et al. 1942 White 1975). Other studies (Alvarez-Suarez et al. 2010a, 2010b Casella and Gatta 2001 Gheldof et al. 2002) reported the substantial absence of vitamin C in honey using different analytical methods. 

The extraction of ascorbic acid with metaphosphoric acid and the reaction of a citrate-buffered solution of analyte with a known amount of 2,6-dichlor-ophenol indophenol is the base for the colorimetric method developed by Bessey (1938). This method, which was later applied by Haydak et al. (1942) to honey, takes advantage of the fact that the absorbance decrease at 520 nm is proportional to the concentration of ascorbic acid. More recently, Ferreira et al. (2009) measured the concentration of vitamin C in three Portuguese unifloral honeys using the same principle, but unfortunately no validation parameters were provided in this study. 

Parviainen and Nyyssonen (1992) reviewed the analytical techniques used to study vitamin C. They stated that although TLC was used in early analytical studies to increase selectivity in the analysis of ascorbic acid and related compounds, currently this method has little qualitative or quantitative use. They also... 

In 1997, Li et al. separated ascorbic acid, thiamine, and pyridoxine, three water-soluble vitamins by CZE and MECC with AD [18]. The detection electrode was made of carbon fiber. A constant detection potential of 510-540 mV (vs. SCE) was applied to the detection electrode for AD. The developed analytical methods have been successfully applied in the determination of the three vitamins in health-caring foods. 

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