Hydrocarbons packed columns chromatography

Styragel columns are compatible with most solvents commonly used in size exclusion chromatography. Exceptions are found on both sides of the polarity scale the use of standard general-purpose Styragel columns with aliphatic hydrocarbons or with alcohols (except hexafluoroisopropanol) and water is generally not recommended. However, it is possible to pack columns in special solvents for special-purpose applications. The interested user should contact Waters for additional information. 

For hydrocarbon gases, the concentration measurement tool is exclusively gas chromatography with a packed column and an FID detector, capable of precisely determining lO levels of C1-C5 in less than 5 minutes. Heavier hydrocarbons are sometimes determined using a quadrupole mass spectrometer. As these instruments and the techniques for loading gas samples onto them are described elsewhere (e.g.. Chapter 5), only field methods and, where applicable, sample pre-treatment methods for releasing gases from soils are discussed here. 

Materials with carbon on the surface used as column packings for chromatography include carbosils, which are prepared mainly by pyrolysis of aliphatic alcohols [92], aromatic hydrocarbons [93], chloroalkanes [94], and other organic compounds on the surface of silica gel. The carbon deposit obtained under the standard pyrolysis conditions is amorphous and possesses different physicochemical properties than graphitized carbon blacks. The transformation of such amorphous carbon deposit to graphitized carbon black on a silica gel surface is not possible because of the high temperature of the graphitization process (ca.3000 K). 

One of the major problems with packed columns is the pressure drop across the column, CO2 is the only practical SF for these systems. In contrast, a wider range of SFs can be used with open tubular capillary columns, e.g. ammonia and hydrocarbons. SF chromatography requires equipment capable of generating mobile phase pressures of up to lOOOOpsi (69 MPa), together with suitable detectors. Ultraviolet detectors are used with carbon dioxide since it is transparent in this region of the spectrum. 

The C1-C4 hydrocarbon gases are easily separated by a variety of packed columns. Most modern packing materials for this are based on synthetic porous polymers, or on porous silica beads of controlled texture (Chromosorbs 102, 104 Carbosphere, Hey Sep polymers, porapacks polymers, etc.). Both the analytical literature and the commercial catalogues offer many alternative materials which can be applied for gas-liquid and gas-solid chromatography. 

The relative distribution of aromatic hydrocarbons in xylene products can be quantitatively determined by gas chromatography. A flame ionization or thermal conductivity detector is used with a capillary or packed column containing crosslinked polyethylene glycol as the stationary phase. The peak area of each component is measured and the weight percentage concentration is calculated by dividing the peak area of the component by the sum of the areas of all peaks. 

Traditional gaseous fuel analysis is based on packed columns. Multidimensional chromatography, i.e. the use of columns with different retention characteristics coupled in series in one GC system, improves resolution of complex samples. But packed-column systems are not normally able to separate all compounds. To obtain more precise information about the composition of a gas sample, capillary columns were introduced in fuel analysis. The first were wall-coated open tubular (WCOT) columns with a nonpolar methylsilicone as stationary phase for hydrocarbon separation. However, the permanent gases were still separated in packed columns containing cross-linked polystyrene carbosieve, and molecular sieve as separation phases. 

Fig. 5-14 [63] shows van Deemter plots obtained for columns packed with 5, 10 and 30 p,m oc-tadecylsilica. The smaller the particle the less was the dependence of linear velocity on the theoretical plate height. Chromatograms of aliphatic and aromatic hydrocarbons are shown in Fig. 5-15 [65]. Use of the octadecylsilica in micropacked columns allows improvement of the efficiency of capillary packed columns. No tailing was observed. Application of these sorbents to gas chromatography, especially in packed capillary columns, is, in our opinion, a very promising direction [55, 63-69]. We also think that this type of modified adsorbents can be successfully used in gas adsorption on open capillary columns.

In reverse-phase column chromatography, the packing material for the stationary phase consists of glass beads coated with a nonpolar hydrocarbon film. 

Fig. 4. Examples of gas chromatographic separations on capillary and packed columns, (a) Packed column separation of mixed hydrocarbons (b) capillary column separation of some flavor compounds. Reproduced from D.W. Grant, Capillary Qas Chromatography, 1996. John Wiley Sons Ltd. Reproduced with permission.

High-performance liquid chromatography, referred to as HPLC, uses partitioning to separate and identify the components in a mixture. The stationary phase is a nonvolatile liquid, such as a long-chain hydrocarbon liquid, bonded onto a solid support, e.g. small particles of silica. This is packed tightly into a column. The solvent chosen for the mobile phase is usually polar, e.g. a methanol/water solvent. This has to be forced under pressure through the densely packed column where separation occurs (Figure 29.8). 

Boylan and Tripp [76] determined hydrocarbons in seawater extracts of crude oil and crude oil fractions. Samples of polluted seawater and the aqueous phases of simulated samples (prepared by agitation of oil-kerosene mixtures and unpolluted seawater to various degrees) were extracted with pentane. Each extract was subjected to gas chromatography on a column (8 ft x 0.06 in) packed with 0.2% of Apiezon L on glass beads (80-100 mesh) and temperatures programmed from 60 °C to 220 °C at 4°C per minute. The components were identified by means of ultraviolet and mass spectra. Polar aromatic compounds in the samples were extracted with methanol-dichlorome-thane (1 3). 

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