Vitamin detection

Dichomeris marginalis no vitamin detected in whole body 56... 

Locusta migratoria vitamin detected in tissues by histochemical staining 7... 

In food analysis using ME-ED technology, CNTs modification has been studied using both GCE and CSPEs for vitamin detection [51]. CSPE-CNTs modified have become the best approach since their improvement of sensitivity and lowered LCDs with the advantages of easy surface modification, inherent miniaturization, and disposability. The performance of these CNT detectors for vitamins determination integrating calibration has also been evaluated with an excellent results. Vitamins calibration and analysis were carried out in less than 400 s with recoveries higher than 95%. 

FIGURE 18.3 Extracted ion current profiles of the water-soluble vitamins detected in a green kiwi extract (see reference [138] for the details). The LC—SRM (standard reference material) chromatogram was acquired by a high-flow ESI source (TurboIonSpray source). Each analyte was identified on the basis of the retention time, the two selected SRM transitions and their relative abundance. Only the most intense SRM ion current is reported in the figure. 

FIGURE 18.4 Extracted ion current profiles of the fat-soluble vitamins detected in a green kiwi extract. This is an example of NARP chromatography (see Table 18.1, reference [137], for the details) a nonaqueos mobile phase was chosen as the best compromise between chromatographic resolution and support to positive APCl ionization of analytes. 

Determination of structural features. The ultraviolet spectrum has been of value in the determination of the structure of several vitamins. Thus the presence of an a-naphthoquinone system in vitamin K was first detected by this means. Also the 4-methylthiazole and the 2 5-dimethyl-6-aminopyridine system was first identified in vitamin Bj (thiamine), a- and /3-Ionones can be distinguished since the former contains two conjugated chromophores and the latter three conjugated chromophores. 

Description of Method. The water-soluble vitamins Bi (thiamine hydrochloride), B2 (riboflavin), B3 (niacinamide), and Be (pyridoxine hydrochloride) may be determined by CZE using a pH 9 sodium tetraborate/sodlum dIhydrogen phosphate buffer or by MEKC using the same buffer with the addition of sodium dodecyl-sulfate. Detection Is by UV absorption at 200 nm. An Internal standard of o-ethoxybenzamide Is used to standardize the method. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

As with many of the vitamins, biological assays have an important historical role and are widely used. For example, microbiological assays use l ctobacillusplantarum ATCC No. 8014 (57,59) or l ctobacillus arabinosus (60). These methods are appropriate for both nicotinamide and nicotinic acid. Selective detection of nictonic acid is possible if l euconostoc mesenteroides ATCC No. 9135 is used as the test organism (61). The use of microbiological assays have been reviewed (62). 

The development of rehable uv analysis permitted the dependable detection and assay of the provitamins and vitamins. Prior to this, the Lieberman-Bouchard chemical test was used, but the color reaction gave many false positives and was relatively inaccurate. 

Chromatographic methods including thin-layer, hplc, and gc methods have been developed. In addition to developments ia the types of columns and eluents for hplc appHcations, a significant amount of work has been done ia the kiads of detectioa methods for the vitamin. These detectioa methods iaclude direct detectioa by uv, fluoresceace after post-column reduction of the quiaone to the hydroquinone, and electrochemical detection. Quantitative gc methods have been developed for the vitamin but have found limited appHcations. However, gc methods coupled with highly sensitive detection methods such as gc/ms do represent a powerful analytical tool (20). 

Two techniques for sorption-spectroscopic determination of ascorbic acid have been proposed. The first one is the recovery by silica modified with tetradecyl ammonium nitrate of blue form of molibdophosphoric HPA in the presence of vitamin C. And the second one is the interaction between the ascorbic acid in solution and immobilized on silica ion associate of molibdophosphoric acid with lucigenine. The detection limits of vitamin C are 0.07 and 2.6 mg respectively. The techniques were successfully applied to the determination of ascorbic acid in fmit juices. 

These methods are employed for the detection and determination of antibiotics and substances with similar effects, like alkaloids, insecticides, fungicides, mycotoxins, vitamins, bitter principles and saponins [14]. 

Under long-wavelength UV light (2 = 365 nm) thiamine (/i/fr 40 — 45) appeared as a bluish fluorescent zone which could be employed for quantitative analysis (Fig. 1). The detection limit was 500 pg vitamin Bi per chromatogram zone. 

In the case of carbohydrates blue chromatogram zones are produced on a yellow background that slowly fades [2]. Steroids, vitamins, antioxidants, phenols and aromatic amines yield, sometimes even at room temperature, variously colored chromatogram zones [5]. -Blockers and laxatives also acquire various colors [7, 10]. The detection hmits are in the nanogram to microgram range [5]. 

This illness is mainly characterized by an age-related bone loss. The detection of osteocalcin in bone was the starting point for a series of studies on the role of vitamin K-dependent proteins in bone development and maintenance and on possible supplemantation therapies. Epidemiologic studies found differences in the risk of hip fractures depending on the dietary vitamin K... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Metagenomics has been applied to the search for novel genes encoding the synthesis of vitamins such as biotin and vitamin C [47,76]. Seven cosmids were detected in metagenomic libraries obtained after avidin enrichment of environmental samples. The highest levels of biotin production in this study were detected in a cosmid... 

By contrast, the cytoplasmic decarboxylation of dopa to dopamine by the enzyme dopa decarboxylase is about 100 times more rapid (Am 4x 10 " M) than its synthesis and indeed it is difficult to detect endogenous dopa in the CNS. This enzyme, which requires pyridoxal phosphate (vitamin B6) as co-factor, can decarboxylate other amino acids (e.g. tryptophan and tyrosine) and in view of its low substrate specificity is known as a general L-aromatic amino-acid decarboxylase. 

Rentel, C. et al.. Silver-plated vitamins a method of detecting tocopherols and carotenoids in LC/ESI-MS coupling. Ana/. Chem., 70, 4394, 1998. 

Lequea et al. used the activity of tyrosine apodecarboxylase to determine the concentration of the enzyme cofactor pyridoxal 5 -phosphate (vitamin B6). The inactive apoenzyme is converted to the active enzyme by pyridoxal 5 -phosphate. By keeping the cofactor the limiting reagent in the reaction by adding excess apoenzyme and substrate, the enzyme activity is a direct measure of cofactor concentration. The enzymatic reaction was followed by detecting tyramine formation by LCEC. The authors used this method to determine vitamin B6 concentrations in plasma samples. 

---