Grain Size Analysis
- PDF / 3,713,187 Bytes
- 92 Pages / 595.276 x 790.866 pts Page_size
- 57 Downloads / 305 Views
GAS CHROMATOGRAPHY Ruth Ann Armitage Department of Chemistry, Eastern Michigan University, Ypsilanti, MI, USA
Synonyms Gas-liquid chromatography (GLC); Gas-solid chromatography (GSC) Definition Chromatography is a method of separating mixtures based on how the molecules interact with an immobilized stationary phase and a mobile phase. It was invented in 1906 by Russian botanist Mikhail Tswett for separating various pigments in plants (thus the derivation of the name: chroma ¼ color, graph ¼ writing). Separation occurs because each component of the mixture undergoes different intermolecular interactions with the stationary phase through physical adsorption or interactions between the uneven distributions of charge in some molecules, called dipoles. Components that interact more strongly with the stationary phase are slowed in relation to those that interact less or not at all. In gas chromatography (GC), the separation occurs in the gas phase. The mixture is propelled by an inert gas, usually helium or hydrogen, as the mobile phase. In gas-solid chromatography, the stationary phase is a solid like alumina, where separation occurs due to differential surface adsorption. In gas-liquid chromatography, an immobilized liquid serves as the stationary phase; in this case, separation is effected by dipole interactions. Background A schematic for a basic gas chromatograph is shown in Figure 1. The instrument consists of a heated injection
port, which converts a liquid sample mixture into vapor. Samples may be injected by hand using a syringe or using an autosampler. In either case, the liquid sample is contained within a syringe equipped with a sharp needle, which is used to pierce a polymer septum. When the liquid is injected into the heated port, it flashes to a vapor and is swept by the mobile phase carrier gas into the column containing the stationary phase. Packed columns consist of stainless steel tubing filled with finely divided silicabased material or alumina. A packed column for gas-liquid chromatography (GLC) has the liquid stationary phase chemically bonded to the packing material. Capillary columns are open tubes (and are sometimes called open tubular columns), the interior wall of which supports the stationary phase. Porous layer open tubular (PLOT) capillary columns are used in gas-solid chromatography, while wall-coated open tubular columns are used in GLC applications. One common liquid stationary phase is the highly nonpolar (meaning no uneven distribution of charge is present) polymer polydimethylsiloxane, made up of the repeating structure shown in Figure 2. In addition to the chemical bonding between the stationary phase and the column material, the stationary phase material is also usually cross-linked, wherein bonds are formed between the polymer chains for added mechanical stability. In a mixture of molecules with varying polarity, those that are most nonpolar (e.g., straight-chain hydrocarbons) will interact most strongly with this nonpolar stationary phase, while those that are less so (e.g., arom
Data Loading...
