Chromatographic separation open column chromatography

In order to reduce the time-consuming open-column chromatographic processes, conventional methods of hydrocarbon-group-type separation have been replaced by MPLC and HPLC. Flash column chromatography is a technique less commonly applied than open-column version, but several applications have been described [2,24—27]. The common technique version is to use a silica-gel-filled column for example, 230 to 400 mesh 20 X 1 cm column size with a back pressure of 1.5 X 10 Pa of an ambient gas such as nitrogen. Solvents are similar to the ones apphed in the case of open-column chromatography fractionations. 

Purification can be accomplished with different chromatographic protocols, but in most cases, open-column chromatography is used. For example, during the separation process of cucurbitacins from Cucurbita andreana, Jayaprakasam et al. [12] employed a medium pressure liquid chromatography (MPLC) system, using silica gel and mixtures of chloroform-acetone as eluent. A similar protocol was used by Peters et al. [27] during the isolation of the cucurbitacins from Wilbrandia ebracteata, but in this case the elution was performed first with mixtures of petroleum ether / ethyl acetate and then with ethyl acetate / i-propanol. 

The analytical procedure for extraction and class separation was adapted from Venkatesan et al. (1987) and consisted of repeated ultrasonic solvent extractions using hexane/acetone followed by saponification of the free fatty acids in KOH/methanol. The organic extract was then divided into three fractions containing aliphatics (w-alkanes), PAH and n-alcohols, and sterols using open column chromatography on 10% water-deactivated silica. Free fatty acids and n-alcohols were derivatized by BSTFA prior to analysis on GC-MS. All determinations were performed using a Hewlett Packard 5890 series 11 gas chromatograph equipped with a split/splitless injector and an electronic pressure control. The column was a 30 m X 0.25 mm ID coated with a 0.25 /rm, 5% phenyl-methylsilicone phase (J W DB-5MS) at a head pressure... 

Indeed, great emphasis was placed on the presentation of compounds in crystalline form for many years, early chromatographic procedures for the separation of natural substances were criticized because the products were not crystalline. None the less, the invention by Tswett (3) of chromatographic separation by continuous adsorption/desorption on open columns as applied to plant extracts was taken up by a number of natural product researchers in the 1930s, notably by Karrer (4) and by Swab and lockers (5). An early example (6) of hyphenation was the use of fluorescence spectroscopy to identify benzo[a]pyrene separated from shale oil by adsorption chromatography on alumina. 

The timely adoption of the cross-linked dextran gels (i.e., Sephadex) in late-fifties as a packing material for column chromatography opened an altogether new horizon of chromatographic separation whereby substances, in general, undergo separation more or less as per their molecular size. 

Gas chromatography and gas chromatography/mass spectrometry were employed to identify and quantitate individual molecular components. Both 25 and 50 meter glass support coated open tubular (SCOT) and fused silica wall coated open tubular (WCOT) capillary columns (SE.30, BP.l and BP.5 phases) were used with H2 as a carrier gas and F.I.D. detection. Acidic components were derivitized (BF3/methanol) to their methyl esters and hydroxyl groups to their silyl ethers (N,0-bis-(trimethylsilyl)trifluoroacetamide) in order to improve chromatographic separation. Carbon Preference Indices (CPI) were calculated using the equation -... 

Over the last two decades, HPLC has to a large extent superseded the classical modes of open column, thin-layer or paper chromatography previously used for natural product separation and has become an integral part of natural product analysis and preparative isolation. This can be attributed to various factors, including (1) availability of numerous chromatographic modes, robust high-resolution chromatographic materials and... 

Adsorption chromatographic separation can be carried out either in closed columns or open films the latter usually consists of a rectangular film of adsorbent supported on a glass plate. Chromatography on open films is referred to as thin-layer chromatography when the adsorbent particles adhere to each other and to the plate, and as loose-layer chromatography when the adsorbent particles are nonadherent. The combination... 

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