tailieunhanh - Introduction to Modern Liquid Chromatography, Third Edition part 82

Introduction to Modern Liquid Chromatography, Third Edition part 82. High-performance liquid chromatography (HPLC) is today the leading technique for chemical analysis and related applications, with an ability to separate, analyze, and/or purify virtually any sample. Snyder and Kirkland's Introduction to Modern Liquid Chromatography has long represented the premier reference to HPLC. This Third Edition, with John Dolan as added coauthor, addresses important improvements in columns and equipment, as well as major advances in our understanding of HPLC separation, our ability to solve problems that were troublesome in the past, and the application of HPLC for new kinds of samples. . | 766 SAMPLE PREPARATION Theory The Nernst distribution law states that any species will distribute between two immiscible solvents so that the ratio of the concentrations remains constant. aq where Kd isdicdistr ibutioiiconsraiiWf is the concentrationof the analyte in the organic phase ant C isthe toiiscmesiiocof theanolyso inrheaqueoucphase. A more useful expression is the fraction E of andyte extracted given by E C0V0 K g 2 CtVe CaeVac 1 dTsV where Vo hthe oolucte ofarganic phese ahe oolumedfequnous phdsc edtltf is the phase ratio Vo V j. Many LLE procedures are carried out ig yeyaratory funnels and typically use tens or hundreds of milliliters of each phase For one-step extrantions KD or 1 KD must be large . 10 for the quantitative recovery of analyte in one of the two phases sincuhOe ehase-satio vhstbemaintaihcd withinupsoctiaai eangeof values such as hiA 10 aee Eq. . In most separatory-funnel LLE procedures quantitative recoveeies 9O ireqitiretwo os more exircctionn. Uoe uuccessive multiple ea i odctk u e w ia e oVe uei oheanclyse phdiestromhachevtraction 1 ci. --------c . 1 M where e Ae nnmbae oo extrweiuns. - examplc h KD V 5 foa eu 3 anti the volumes of the two phases are equal 1 three extractions n 3 would be required for 99 recovery of the analyte. Several approaches can be used to increase the value of organic solvent can be changed to increase Kp Kauam be deema if e ana c le op tmcpresom0 tat lotazaaionso asn make rtsness krblemshu osgan1cphate theooatdiehanbeaxtannted into the organic phase by ion pairing provided that the analyte is ionized and an ion-pair reagent is added to the organic phase salting onr can pe used ha elcassatean anal t aqueous pleose bood5ition o nineee auerel sah . eodium euhate to the aqueousphore. Practice Table provldreepempletof ttnicaSexeraetion soivanoaasweh essomeunsmt-able water-miscibk. cxtraceionsolvents Apart from miscibility considerations

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