Gas Chromatographic Phases

Because volatility is such an important factor in GC, the chromatographic column is contained in an oven, the temperature of which can be closely and reproducibly controlled. For very volatile 

The McReynolds number gives an approximate indication of polarity on a scale of 0 (nonpolar) to about 1000 (extremely polar). 

Two gas chromatograms showing the effect of polarity of the stationary phase on the separation efficiency for three substances of increasing polarity toluene, pyridine, and benzaldehyde. (a) Separation on silicone SE-30, a nonpolar phase, and (b) separation on elastomer OV-351, a more polar phase. Note the greatly changed absolute and relative retention times the more polar pyridine and benzaldehyde are affected most by the move to a more polar stationary phase. 

In general, the longer a chromatographic column, the better will be the separation of mixture components. In modem gas chromatography, columns are usually made from quartz and tend to be very long (coiled), often 10-50 m, and narrow (0.1-1.0 mm, internal diameter) — hence their common name of capillary columns. The stationary phase is coated very thinly on the whole length of the inside wall of the capillary column. Typically, the mobile gas phase flows over the stationary phase in the column at a rate of about 1-2 ml/min. 

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Kreis P, Dietrich A, Mosandl A (1996) Elution order of the furanoid linalool oxides on common gas chromatographic phases and modified cyclodextrin phases. J Essent Oil Res 8 339 Weinert B, Wiist M, Mosandl A Hanssum H (1998) Stereoisomeric flavour compounds LXX-Vlff. Separation and structure elucidation of the pyranoid linalool oxide stereoisomers using common gas chromatographic phases, modified cyclodextrin phases and nuclear magnetic resonance spectroscopy. Phytochem Anal 9.T0... 

Figure 12. Cyclolinear methylsiloxane polymers pyrolysis products separated in OV-1 gas chromatographic phase [80],...

FIGURE 20. Pyrolysis products from cyclo-linear methylsiloxane polymers separated on OV-1 gas chromatographic phase. Reproduced from Reference 178 by permission of Elsevier... 

Solid-phase microextractions also have been developed. In one approach, a fused silica fiber is placed inside a syringe needle. The fiber, which is coated with a thin organic film, such as poly(dimethyl siloxane), is lowered into the sample by depressing a plunger and exposed to the sample for a predetermined time. The fiber is then withdrawn into the needle and transferred to a gas chromatograph for analysis. 

Two examples from the analysis of water samples illustrate how a separation and preconcentration can be accomplished simultaneously. In the gas chromatographic analysis for organophosphorous pesticides in environmental waters, the analytes in a 1000-mL sample may be separated from their aqueous matrix by a solid-phase extraction using 15 mb of ethyl acetate. After the extraction, the analytes are present in the ethyl acetate at a concentration that is 67 times greater than that in... 

Caffeine is extracted from beverages by a solid-phase microextraction using an uncoated fused silica fiber. The fiber is suspended in the sample for 5 min and the sample stirred to assist the mass transfer of analyte to the fiber. Immediately after removing the fiber from the sample it is transferred to the gas chromatograph s injection port where the analyte is thermally desorbed. Quantitation is accomplished by using a C3 caffeine solution as an internal standard. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

Another important characteristic of a gas chromatographic column is the thickness of the stationary phase. As shown in equation 12.25, separation efficiency improves with thinner films. The most common film thickness is 0.25 pm. Thicker films are used for highly volatile solutes, such as gases, because they have a greater capacity for retaining such solutes. Thinner films are used when separating solutes of low volatility, such as steroids. 

As described above, the mobile phase carrying mixture components along a gas chromatographic column is a gas, usually nitrogen or helium. This gas flows at or near atmospheric pressure at a rate generally about 0,5 to 3.0 ml/min and evenmally flows out of the end of the capillary column into the ion source of the mass spectrometer. The ion sources in GC/MS systems normally operate at about 10 mbar for electron ionization to about 10 mbar for chemical ionization. This large pressure... 

Static headspace GC/MS. The partitioning of volatile and semivolatile compounds between two phases in a sealed container. An aliquot of the headspace gas generated is injected onto a gas chromatographic column. This is followed by mass spectrometric analysis of compounds eluting from the gas chromatograph. 

SPME has been utilized for deterrnination of pollutants in aqueous solution by the adsorption of analyte onto stationary-phase coated fused-siUca fibers, followed by thermal desorption in the injection system of a capillary gas chromatograph (34). EuU automation can be achieved using an autosampler. Eiber coated with 7- and 100-p.m film thickness and a nitrogen—phosphoms flame thermionic detector were used to evaluate the adsorption and desorption of four j -triazines. The gc peaks resulting from desorption of fibers were shown to be comparable to those obtained using manual injection. 

An advantage of Hquid chromatography is that the composition of the mobile phase, and perhaps of the stationary phase, can be varied during the experiment to provide greater efficacy of the separation. There are many more combinations of mobile and stationary phases to effect a separation in Ic than one would have in a similar gas chromatographic experiment, where the gaseous mobile phase often serves as Httle more than a convenient carrier for the components of the sample. 

The specialities of chromatographic behaviour of cypermethrin, permethrin, X-cyhalothrin, deltamethrin and fenvalerate were investigated in this work. Gas chromatographic determination was cai ry out with use of packed column with stationai y phase of different polarity (OV-101, OV-210 OV-17) and capillary and polycapillary columns with non-polai ic stationary phase. Chromatographic peak identification was realized with attraction GC-MS method. 

METHOD OF BINARY PHASES OF VARIABLE CAPACITY FOR GAS CHROMATOGRAPHIC ANALYSIS OF HIGH PURITY VOLATILE SUBSTANCES... 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

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