Silica gels hydrocarbon retention

However, consider the separation of solutes that are more polar than the aromatic hydrocarbons, for example mixtures of ethers or aliphatic esters. If it were attempted to separate a series of aliphatic esters on silica gel employing n-heptane as the mobile phase it would be found that the retention of the later eluting solutes was inordinately long. If a slightly stronger solvent, such as chloroform was used as an alternative to n-heptane, it would be found that the less polar esters... 

As a result of its highly polar character, silica gel is particularly useful in the separation of polarizable materials such as the aromatic hydrocarbons and polynuclear aromatics. It is also useful in the separation of weakly polar solute mixtures such as ethers, esters and in some cases, ketones. The mobile phases that are commonly employed with silica gel are the n-paraffins and mixtures of the n-paraffins with methylene dichloride or chloroform. It should be borne in mind that chloroform is opaque to UV light at 254 nm and thus, if a fixed wavelength UV detector is being used, methylene dichloride might be a better choice. Furthermore, chloroform is considered toxic and requires special methods of waste disposal. Silica gel is strongly deactivated with water and thus, to ensure stable retentive characteristics, the solvent used for the mobile phase should either be completely dry or have a controlled amount of water present. The level of water in the solvent that will have significant effect on solute retention is extremely small. The solubility of water in n-heptane is... 

Figure 3.13 n-Electron selectivity of different octadecyl-bonded silica gels towards the retention of polycyclic aromatic hydrocarbons. Columns. , LOC-ODS-E, O, LOC-ODS-NE O, HIC-ODS-E, A, HIC-ODS-NE, eluent, 80% aqueous acetonitrile at 30°C. Compounds polycyclic aromatic hydrocarbons 1, benzene, 2, naphthalene 3, pyrene 4, 3,4-benzopyrene. 

Figure 4.1 Retention factors related to log P values. Column, 5/im octadecyl-bonded silica gel (LiChrosorb LC7) 25 cm x 4.1 mm i.d. eluent, 50% aqueous acetonitrile flow rate, 1ml min-1. Compounds , alkanols O, benzoates, O, polycyclic aromatic hydrocarbons, and A, alkylbenzenes.

One component of the eluent should have properties similar to those of the analytes, and this solvent is diluted by another solvent to control the retention time. The basic idea can be understood from the chromatographic behaviour of phthalic acid esters and polycyclic aromatic hydrocarbons (PAH). This approach can be applied to the separation procedure for a variety of stationary phase materials, including silica gel, polystyrene gel, and ion-exchangers. 

Several improved stationary phase materials have been synthesized for reversed-phase liquid chromatography. One material is vinyl alcohol copolymer gel. This stationary phase is quite polar and chemically very stable however, it demonstrated a strong retention capacity for polycyclic aromatic hydrocarbons.45 9 Although stable octadecyl- and octyl-bonded silica gels have been synthesized from pure silica gel50,51 and are now commercially available, such an optimization system has not yet been built. Further experiments are required to elucidate the retention mechanism, and to systematize it within the context of instrumentation. 

G.l.c. papers of interest include the classification of 22 acyclic monoterpenoid alcohols according to retention indexes, resolution of cyclic ketones [e.g. ( )-menthone, ( )-isomenthone] as diethyl (+)-tartrate acetals, and the use of lanthanide shift reagents to resolve non-terpenoid racemic epoxides.The occurrence and prevention of monoterpenoid hydrocarbon isomerization during silica gel chromatography has been examined and the separation of monoterpenoids and sesquiterpenoids by gel permeation chromatography is reported. Monoterpenoid hydrocarbons have been selectively extracted from essential oils using dimethylsilicone. ... 

Adsorption TLC selection of the mobile phase is conditioned by sample and stationary-phase polarities. The following polarity scale is valid for various compound classes in NPTLC in decreasing order of K values carboxylic acids>amides>amines>alcohols>aldehydes > ketones > esthers > nitro compounds > ethers > hal-ogenated compounds > aromatics >olefins > saturated hydrocarbons > fluorocarbons. For example, retention on silica gel is controlled by the number and functional groups present in the sample and their spatial locations. Proton donor/acceptor functional groups show the greatest retention, followed by dipolar molecules, and, finally, nonpolar groups. 

The C-8 reversed-phase sorbents (Table 1.2) are often the most popular sorbents for drug analysis because of a shorter hydrocarbon chain than a C-18 sorbent. The shorter chain length makes it much easier for secondary interactions between the analyte and the silica gel that enhances retention of the analyte. This added interaction is useful in the purification of drugs and metabolites from blood and urine because they contain basic nitrogen atoms (hat may hydrogen bond to the silica gel. 

Analysis of Products. The three fractions collected from each sample were analyzed by gas chromatography. The noncondensable fraction, containing hydrogen and methane, was analyzed on silica gel at room temperature. The fraction containing C2-C4 hydrocarbons was analyzed at 75 °C. on silica gel treated with didecyl phthalate. Aliquots of the liquid fraction were analyzed on three columns of different selectivity Bentone-34-didecyl phthalate silicone SE-30 and m-polyphenyl ether (five-ring). Products were identified, and their yields were determined by comparison of retention volumes and peak areas with values for known amounts of authentic samples. 

Hydroxylation of tertiary carbon atoms on silica gel. Israeli chemists have devised an experimental method for hydroxylation of saturated hydrocarbons at tertiary positions by ozone adsorbed on silica gel in this way concentrations of ozone of 4.5% by weight at -78 can be attained. First the hydrocarbon is impregnated on the silica gel and then ozone is passed through at -78° until the silica gel is saturated. After the silica gel has warmed to 20, the product is eluted. Tertiary alcohols can be obtained in this way in high yield with almost complete retention of configuration. The secondary alcohol adamantane-2-ol was oxidized to the ketone by this method. 

Fractions 3 and 4 from the tar trap tar, which primarily contained polycyclic aromatic hydrocarbons and their alkylated derivatives, were combined and subfractionated on a silica gel column. In sequential elution, toluene eluted 46 percent of the mass (subfraction A), propanol eluted 48 percent (subfraction B), methanol eluted 3 percent (subfraction C) and 3 percent was recovered by washing the silica gel with tetrahydrofuran (subtraction D). The toluene subfraction was found by GC/MS analysis to consist of a series of PAH, both parent compounds and numerous methylated derivatives. In addition, the heterocyclic compound dibenzothiophene was present as a major peak. Tentative identifications are listed in Table IX. Compounds whose identifications were confirmed by GC retention time comparisons with standards are noted. Although the identities of many of the individual PAHs have not been confirmed, it is clear that most of the subtraction is PAH in nature. 

Retention of Polycyclic Aromatic Hydrocarbons on Silica Gels... 

Paraffins are isolated from the sample matrix (edible oils or food extracts often containing much fat or oil) and separated from aromatics and olefins (e.g., sterenes from the raffination of edible oils) by NPLC. As it is difficult to maintain a high activity of silica gel to separate paraffins from olefins when large amounts of matrix material are injected, two columns are used up to 30 mg edible oil or fat is injected onto a first column of 25 cm x 2 mm i.d. The hydrocarbons are transferred to a second column of the same dimension. The first column is backflushed with dichloromethane to remove the oil and other materials of the food extract. The second column only comes in contact with pentane as the mobile phase and nonpolar sample components, i.e. it maintains its high retention power for unsaturated components. Transfer of the fraction of about 400 pi volume to GC mostly involves the retention gap... 

---