Gas chromatography preparative

Gas chromatography can be utilized for preparative-scale separations as well as for analysis. For the qualitative and quantitative analyses of a small sample (see Chapter 4), microliter or microgram sample sizes are used. Once having worked out the conditions for such an analytical separation, one may wish to isolate larger amounts of one or more components of a mixture. Use of GC for this purpose is referred to as preparative gas chromatography. 

Preparation of milligram quantities of substances can readily be performed with an analytical gas chromatographic column by repetitive injection and collection. Larger sample quantities require modifications to an analytical apparatus, but are more easily obtained with the use of a special preparative unit. It has been postulated that the sample size approximately increases with the fourth power of the column diameter (up to 1 g). 

Samples larger than 1 g necessitate different gas chromatographic equipment than for analysis. 

Rotating Columns and Moving Bed Equipment. Preparative GC can be carried out where the sample is continuously introduced into a set of columns which may move in a transverse direction to the mobile phase flow and sample injection point. Using such an arrangement, the elution point on the cylindrical base, x, is 

Maximum efficiency is realized when the nth component is eluted adjacent to the first component. The frequency of revolutions must be 

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K2C03 CaH2, CaO or sodium, then fractionally distd. Near-dry alcohol can be further dried by refluxing with magnesium activated with iodine, as described for ethanol. Further purification is possible using fractional crystn, zone refining or preparative gas chromatography. 

Cyclobutanone [1191-95-3] M 70.1, b 96-97 , d 0.931, n 1.4189. Treated with dilute aqueous KMn04, dried with molecular sieves and fractionally distd. Purified via the semicarbazone, then regenerated, dried with CaS04, and distd in a spinning-band column. Alternatively, purified by preparative gas chromatography using a Carbowax 20-M column at 80°. (This treatment removes acetone). 

Diphenyl carbonate [102-09-0] M 214.2, m 80 . Purified by sublimation, and by preparative gas chromatography with 20% Apiezon on Embacel, and crystn from EtOH. 

If s-l,2-divinylcyclobutane is desired, it can be isolated in 7—8% yield from the reaction mixture by preparative gas chromatography with the Beckman Megachrom instrument, using columns packed with Apiezon J. 

When 2,7-dimethyloxepin is treated with potassium in liquid ammonia at — 70 C, a mixture of oct-4-en-2-one (1) and octa-4,6-dien-2-one (2) in a ratio of 75 20 is obtained.203 The major product can be separated by preparative gas chromatography in 23% yield. The analogous reaction of 3-benzoxepin gives, in 30% yield, a mixture of (2-cthylphenyl)acetaldehyde (3) and (2-ethynylphenyl)acetaldehyde (4) that resists separation.203 The Latter product can be formed exclusively in 17% yield when 3-benzoxepin is treated with sodium amide in tetra-hydrofuran at 33 C for 210 minutes.203... 

A portion of the product was heated to reflux with methanolic sodium methoxide to convert it into the thermodynamic mixture of trans- (ca. 65%) and cis- (ca. 35%) isomers. Small amounts of the isomers were collected by preparative gas chromatography using an 8 mm. by 1.7 m. column containing 15% Carbowax 20M on Chromosorb W, and each isomer exhibited the expected spectral and analytical properties. The same thermodynamic mixture of isomers was prepared independently by lithium-ammonia reduction5 of 2-allyl-3-methyl-cyclohex-2-enone [2-Cyclohexen-l-one, 3-methyl-2-(2-propcnyl)-],6 followed by equilibration with methanolic sodium methoxide. 

The isomers were separated by preparative gas chromatography. " The product is a mixture of cis and trans isomers. 

Interaction of the yellow hexafluoride with silica to give xenon tetrafluoride oxide must be interrupted before completion (disappearance of colour) to avoid the possibility of formation and detonation of xenon trioxide [1]. An attempt to collect the hexafluoride in fused silica traps at — 20°C after separation by preparative gas chromatography failed because of reaction with the silica and subsequent explosion of the oxygen compounds of xenon so produced [2],... 

The pure, more highly substituted olefin, n2Sd 1.3840, could also be separated by preparative gas chromatography, and the unchanged ketone could be separated from the two triflate isomers by chromatography on a silica gel column with pentane as the eluant. The pure product has infrared absorption (CC14 solution) at 1700 (enol C=C), 1210 and 1140 cm.-1 (S02) with end absorption in the ultraviolet (heptane... 

Examples of the use of this procedure to prepare vinyl triflates from ketones are provided in Table II. Often mixtures of cis and trans isomers as well as the various double bond isomers of vinyl triflates are obtained by this procedure, and the amounts of these isomers produced may vary with the base and solvent used. Also, small amounts of unchanged ketone may contaminate the initial crude product. Consequently, separation procedures such as preparative gas chromatography or efficient fractional distillation may be required to obtain a single vinyl triflate isomer. 

In addition to the analytical columns (columns used mainly for analytical work), so-called preparative columns may also be encountered. Preparative columns are used when the purpose of the experiment is to prepare a pure sample of a particular substance (from a mixture containing the substance) by GC for use in other laboratory work. The procedure for this involves the individual condensation of the mixture components of interest in a cold trap as they pass from the detector and as their peak is being traced on the recorder. While analytical columns can be suitable for this, the amount of pure substance generated is typically very small, since what is being collected is only a fraction of the extremely small volume injected. Thus, columns with very large diameters (on the order of inches) and capable of very large injection volumes (on the order of milliliters) are manufactured for the preparative work. Also, the detector used must not destroy the sample, like the flame ionization detector (Section 12.6) does, for example. Thus, the thermal conductivity detector (Section 12.6) is used most often with preparative gas chromatography. 

In order to satisfy the spedflcation outlined above, a special interface was developed. TTie detailed design of this interface, the transport system, and over-pressure system have been the subject of a previous pubhcation and a patent application [24]. The system devised for the interface is based on modifications to an injection system presented by Heilbronner et al. [2S] for preparative gas chromatography. A commercial version of the preparative unit was developed by Boer [26]. In this appfication. samples are monitored into the injection vessel which is connected to the gas chromatographic column by a length of stainless steel capillary tubing. 

Cyclization by amidomercuration has been reported (391). Reaction of N-methoxycarbonyl-6-aminohept-l-ene (211) with mercuric acetate gave the organomercurial (212). Reductive coupling of 212 with l-decen-3-one in the usual way gave the cis and trans isomers (213). Successive treatment of 213 with ethanedithiol, Raney nickel, and ethanolic hydrogen chloride afforded ( )-sole-nopsin A (Id, 2 parts) and its isomer (Ic, 3 parts), which were separable by preparative gas chromatography (GC) (Scheme 5) (391). 

Nitropolyzonamine (49) (Table IV) can be isolated as colorless crystals from the secretion of the millipede, P. rosalbum, by preparative gas chromatography or as its crystalline perchlorate from an ethereal solution of the crude defensive secretion. The structure and stereochemistry have been determined by an X-ray analysis of the base perchlorate 153). Racemic nitropolyzonamine (49) can be synthesized from polyzonimine (19). Treatment of polyzonimine (19) with 3-nitropropyl iodide gives a crystalline salt (344) which is readily cyclized to racemic nitropolyzonamine (49) in boiling pyridine (Scheme 37) 153). 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

The mixture of the crude product hydrazone 138 and the excess of hydrazone 129 was used directly in an acidic acetalization reaction utilizing aqueous 3n HCl in a biphasic system and gave a diastereomeric mixture of sordidin (126) and 7-epi-sordidin (7-epi-126) in 84% yield over two steps in a 1.5 1 ratio. Gratifyingly, we succeeded in separating the desired sordidin epimers by preparative gas chromatography. As a result, both could be obtained in diasteromerically pure form (sordidin de > 99% 7-epi-sordidin de > 97%) and with a high enantiomeric excess for each epimer (ee > 98%). 

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