（節(jié)選）外文翻譯--分光光度法測(cè)定維生素c的含量(英文）

1、1 IntroductionVitamin C occurs in different concentrations in a vari- ety of natural samples. It is added to several pharma- ceutical products as an essential ingredient, a stabilizer for vitamin B complex, and as an

2、antioxidant. Consequent upon its desirable effects, it is widely used in the treatment of certain diseases such as scurvy, common cold, anemia, haemorrhagic disorders, wound healing, and even infertility, to mention some

3、 stark cases. It is considered essential for the development and regeneration of muscles, bones, teeth and skin. The increasing use of pharmaceuticals and other natural samples containing vitamin C has meant that the pr

4、acti- cising chemists should develop analytical procedures for its determination which are simple to operate, rapid, accurate, sensitive and selective. The desire to develop methods with ideal characteristics has result

5、ed a large number of procedures with varying applicability. Many instrument-based analyses including fluorometry1–4, HPLC5–10, polarography11–13 and enzymatic14,15 methods are reported in the literature. But due to the

6、ir inherent limitations, these techniques are not commonly used for routine analyses. However, photometric methods are particularly attractive because of their speed and sim- plicity. Consequently, a large number of su

7、ch proce- dures have been developed for the determination of ascorbic acid (AA). Though some short reviews16–18have been reported, a critical assessment of these meth- ods is desirable to examine their salient features

8、and utility. This review is an attempt to assess exclusively the existing spectrophotometric methods for the deter- mination of vitamin C as regards their simplicity, rapid- ity, Beer’s law range, sensitivity, selectivi

9、ty and applic- ability. It is primarily based on the information collect-ed through the Chemical Abstracts for the period 1970 to mid-1997.2 Results and DiscussionSeveral dyes such as 2,6-dichlorophenolindophenol (DCI

10、P), dimethoxydiquinone (DMDQ), ninhydrin, fast red AL salt and 2′,7′-dichlorofluorescein etc. have been used for the determination of vitamin C. Among these dyes, DCIP has been most extensively studied. It is included

11、in the official titrimetric methods as reported in different pharmacopoeias19–21 and it also forms the basis of many colorimetric methods. The blue dye DCIP is reduced to the colorless form on addition of ascorbic acid

12、as shown in Fig. 1, but it gives a pink color to the acidic solutions. Using the dye, ascorbic acid present in human urine22 and processed potatoes23has been determined. The excess dye can be extracted with xylene or b

13、utanol.24 Many substances which are capable of reducing the dye resulting from the prepara- tion and processing of food samples interfere. Flow dialysis proposed by Gary et al.25 and continuous flow systems have been us

14、ed to monitor the decrease in absorbance of DCIP. Such automated systems appear to be justified only when routine analysis of a largeANALYTICAL SCIENCES OCTOBER 1998, VOL. 14Photometric Methods for the Determination o

15、f Vitamin CSatya P. ARYA?, Meenakshi MAHAJAN and Preeti JAINDepartment of Chemistry, Kurukshetra University, Kurukshetra–136119, Haryana State, IndiaThe importance of vitamin C to the human body is widely acknowledged th

16、roughout the globe. The deficiency of this vitamin leads to various diseases. In view of its importance, numerous methods including spectrophotometric ones have been developed for its determination in pharmaceuticals,

17、foods and biological samples. A comprehensive review of the available spectophotometric methods for the determination of ascorbic acid is presented.Keywords Vitamin C determination, spectrophotometric method? To whom co

18、rrespondence should be addressed. Fig. 1 The reduction of DCIP with ascorbic acid.Ascorbic acid DCIP (Oxidized, Blue-Pink)Dehydroascorbic acidDCIP (Reduced, Colorless)and trinitrobenzene47 in tartrate buffer when studied

19、 for its determination over the concentration ranges 2 – 50 and 0 – 125 µg ml–1 of ascorbic acid respectively. Methanolic solution of resorcinol48 gives a pale yellow color (λ max=425 nm) with ascorbic acid in hydro

20、chloric acid medium, obeying Beer’s law for 80 – 400 µg ml–1. 4-Chloro-7-nitrobenzofurazane49 forms a bluish green colored species with ascorbic acid in presence of 0.2 M sodium hydroxide. The absorbance is measure

21、d at 582 nm after diluting the reaction contents with 50% (v/v) aqueous acetone solution. Beer’s law is obeyed in the concentration range 5 – 20 µg ml–1. The colored prod- uct is stable for 30 min only when kept a

22、way from direct sunlight or artificial day light. The method is reported free from the interference of all other vitamins and minerals present in multivitamin preparations and can be applied to the analysis of pharmaceu

23、ticals, fresh fruit juices and vegetables. Hashmi et al.50 proposed a method based on the reac- tion of 2,3,5-triphenyltetrazolium chloride with ascor- bic acid in alkaline medium. The pink solution is allowed to stand i

24、n the dark for 30 min at 25?C; it obeys Beer’s law over the range 5 – 25 µg ml–1. Sugars (>15 µg ml–1) except sucrose interfere by forming a similar color to that of the reagent. Riboflavin, cyanocobalamin

25、 and folic acid interfere due to their own color. Beutler et al.51,52 investigated the use of methylthiazolyltetrazolium salt in presence of ascorbate oxidase enzyme and 3-(4,5-dimethylthiazolyl-2-yl)-2,5- diphenyltetra

26、zolium chloride or bromide in the pres- ence of 5-methylphenazinium methyl sulfate (electron carrier) at pH 3.5 for the determination of ascorbic acid in foods, fruit juices and vegetables juices. These reac- tions invo

27、lve the formation of formazon (λ max=578 nm). The interference of sulfur dioxide requires treat- ment with formaldehyde, and color interference from dark juices is removed by decolorization with 1% polyvinylpolypyrrolid

28、one before filtration. Sorbitol, alcohol and oxalate interfere with the ascorbic acid oxi- dase. However, the effect of oxalate can be checked by adding a slight excess of Ca(II) ions. Other derivatives such as 2,5-di

29、phenyl-3-thiazolyl tetrazolium chloride53at pH 12.2, 2-(p-iodophenyl)-3-(p-nitrophenyl)-5- phenyltetrazolium chloride at pH 10.5 (λ max=540 nm) and 2,2′,5,5′-tetra-(4-nitrophenyl)-3,3′-(3,3′-dimethoxy- 4,4′-biphenyl)dite

30、trazolium chloride54 have also been employed for the assay of ascorbic acid. The coupling of 2,4-dinitrophenylhydrazine (DNPH) with ketonic groups of DHAA and diketogulonic acid (DKGA) has been the basis of many methods

31、for the determination of total vitamin C contents. Proteins present in the samples are precipitated by adding trichloroacetic acid (TCA) and aliquots of filtrate are shaken with acid–washed charcoal (norit) or activated

32、 charcoal55 to clarify the solutions and to oxidize AA to DHAA. A reducing medium is produced by adding thiourea prior to DNPH addition, otherwise unspecific coloration is given by oxidants. The osazones (λ max=545 nm)

33、 thus formed during the 3 h incubation at37?C by the reaction of DNPH and DHAA are dis- solved by adding 85% H2SO4. Vitamin C can be extracted with metaphosphoric acid–stannous chloride solution without charcoal treatme

34、nt for differential determination of DKGA, DHAA and AA in the same tissue extracts. The interference of sugars can be mini- mized by carrying out incubation at 15?C and measur- ing the absorbance only after adding sulfu

35、ric acid for 75 min.56 The use of several acid mixtures has been proposed for replacing the tedious dropwise addition of sulfuric acid. Lack of specificity is found with many of these methods; interfering osazones can b

36、e separated by chromatographic methods such as TLC57 and HPLC58, but at the cost of making these procedures tedious and cumbersome. The nature of DNPH meth- ods for total vitamin C also makes it amenable to auto- matic

37、flow through analyses.59–61Phenylhydrazinium chloride62 produces a yellow color (λ max=395 nm) when treated with ascorbic acid in 0.1 M HCl medium. The reaction contents are kept for 1 h in an incubator or water bath at

38、 50±2?C, thus mak- ing the method time-consuming. Beer’s law is obeyed in the range 25 – 100 µg of ascorbic acid. No interfer- ence is observed from other vitamins, minerals, glucose, sucrose, excipients and

39、reducing agents. However, the presence of excessive amounts of riboflavin requires the addition of 0.5 g talc, which imparts a yellow color to the solution. 3-Methyl-2- benzothiazolone hydrazone63 reacts in the presence

40、 of sodium metaperiodate to form a blue colored solution (λ max=630 nm) which helps in the determination of ascorbic acid over the range 6 – 14 meq ml–1. Wang64 suggested the use of potassium iodate for the determination

41、 of vitamin C in pharmaceuticals. The absorbance is measured either in the UV region (288 nm) or in the visible region (445 nm). Besides aqueous phase measurements, the yellow precipitate can be extracted into chlorofo

42、rm65 (λ max=514 nm). The ICl2–generated in the oxidation of AA by iodate66 in acid medium in the presence of Cl– ions has been used to iodinate 2′,7′-dichlorofluorescein dye. The iodinated dye (λ max=525 nm) obeys Beer

43、’s law up to 300 µg (ε=8.81×103). Soft drinks67 have been analyzed using the reaction of iodine in an acetic acid medium (λ max= 350 nm). Sirividya and Balasubramanian68 reported an indirect procedure based o

44、n the oxidation of ascorbic acid by a known excess of iodate in the presence of acid for the analysis of pharmaceuticals and fresh fruit juices. The unreacted iodate is used for hydroxylamine oxidation to generate nitri

45、te, which is then diazotized with sulfanilic acid. The resulting diazonium salt is coupled with N-(1-naphthyl)ethylenediamine dihy- drochloride to form an azo dye (λ max=540 nm). The procedure is a complicated one as i

46、t involves many steps. The reaction of hexacyanoferrate(III)69 (5) was used for the determination of micro quantities of vitamin C by measuring the decrease in color intensity of the reagent (5) (λ max=420 nm) in McIlvai