Chromatogram, gas

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. 

Schematic diagram showing the injection of a mixture of four substances (A, B, C, D) onto a GC column, foliowed by their separation into individuai components, their detection, and the dispiay (gas chromatogram) of the separated materiais emerging at different times from the coiumn.

A single peak from an ordinary gas chromatogram (a) is revealed as two closely separated peaks by resolution enhancement (b). 

Figure 2.7 (11) shows a gas chromatogram obtained by co-solvent trapping and concurrent eluent evaporation after injecting 500 p.1 of diluted gasoline. The main solvent was -pentane with 5% of -heptane as co-solvent. It is noteworthy that without the co-solvent, higher-boiling compounds could be lost. 

Figure 2.21 shows the on-line extraction gas chromatogram of 2.25 ml of water spiked at 5 ppb levels with 14 different organic pollutants (40). In this case, the authors concluded that wall-coated open tubular traps (thick-film polysiloxane phases) can be used for the on-line extraction of organic compounds from water. However, when using swelling agents such as pentane, non-polar analytes can be trapped quantitatively, while for more polar compounds chloroform is the most suitable solvent. 

Figure 3.6 Two-dimensional gas chromatogram of an oi ange oil extract, in which a 2 s heait-cut has been made in the region A where /3-miycene has eluted on a non-polar column. Secondary analysis on a polar Carbowax 20 M column indicated two compounds (marked B and C), both identified as odoi ous by organoleptic assessment. Reproduced from R A. Rodriguez and C. L. Eddy, ]. Chromatogr Sci. 1986, 24, 18 (32).

Comprehensive two-dimensional GC has also been employed for the analysis of pesticides from serum, which, although not strictly a forensic analytical problem , provides an example of the promise of this technique to forensic applications, such as the analysis of drugs of abuse (40). Two-dimensional gas chromatograms of a 17-pesticide standard and an extract from human serum are shown in Figure 15.13. The total analysis time of about 5 min, high peak capacity and the separation of all... 

Element-selective detectors. Many samples, e.g. those originating from environmental studies, contain so many constituent compounds that the gas chromatogram obtained is a complex array of peaks. For the analytical chemist, who may be interested in only a few of the compounds present, the replacement of the essentially non-selective type of detector (i.e. thermal conductivity, flame ionisation, etc.) by a system which responds selectively to some property of certain of the eluted species may overcome this problem. 

Gas chromatograms are usually obtained with the column kept at a constant temperature. Two important disadvantages result from this isothermal mode of operation. 

FIGURE 3 A gas chromatogram of a mixture of pesticides from farmland. The relative heights of the peaks indicate the relative abundances of the compounds. 

FIGURE 5 Gas chromatogram of degradation products of a linear polymer prepared from 3,9-bis-(ethylidene-2,4,8,10-tetraoxaspiro[5,5]undecane) and l,6-hexanediol. (From Ref. 16.)... 

Figure 15 Gas chromatogram of a gas condensate (NGL = natural gas liquid) sample [74]. The peak with retention time of 5.70 eluted between nCis and nCis is indicative of the probable existence of diamantane in the sample.

Figure 16. Gas chromatogram of a crude oil sample showing the possible existence of adamantane and diamantane in the sample.

Figure 17. Gas chromatogram of a diamondoid-rich gas condensate (NGL) sample showing clusters of peaks representing adamantanes, diamantanes, triamantanes, and tetramantanes. Taken from Ref. [11] with permission.

In Smoke. We compared the gas chromatograms of nitrosamines in matching aliquots of mainstream smoke derived from Burley type cigarettes that were identical except for the degree of nitrate fertilization during cultivation (Figure 2). This comparison supports the concept that the nitrate concentration in tobacco is a determining factor for the nitrosamine yields in the smoke. The data in Table II confirm this concept. These studies have... 

Figure 3. Gas chromatogram of initial and final gas composition for the maximum Cg yield point of Fig. 2a. (Reprinted with permission from the AAAS, ref. 12).

B.K. Lavine, H. Mayfield, P.R. Kromann and A. Faruque, Source identification of underground fuel spills by pattern recognition analysis of high-speed gas chromatograms. Anal. Chem., 67 (1995) 3846-3852. 

If interfering peaks hinder sample quantitation on the gas chromatogram, better resolution could be obtained by using an FFAP or DB-17 gas chromatographic column. 

In multi-residue analysis, an analyte is identified by its relative retention time, e.g., relative to aldrin when using ECD or relative to parathion or chlorpyrifos when using a flame photometric detection (FPD) and NPD. Such relative retention times are taken from corresponding lists for the columns used. Further evidence for the identity of an analyte is provided by the selectivity of the different detectors (Modules D1 to D3), by its elution behavior during column chromatography (Modules Cl and C2) and in some cases even by the peak form in a gas chromatogram. In a specific analysis for only some individual analytes, their retention times are compared directly with the corresponding retention times of the analytes from standard solutions. 

Figure 6. Gas chromatograms of silylated hydrolyzate of (a) control sample (b) heparinized PET film (c) heparin.

Fig. 3. (Top left) Chemical methods used to depolymerize the polyesters. (Top right) Thin-layer and gas-liquid chromatograms (as trimethylsilyl derivatives) of the monomer mixture obtained from the cutin of peach fruits by LiAlD4 treatment. In the thin-layer chromatogram the five major spots are, from the bottom, C18 tetraol, C16 triol, and C18 triol (unresolved), diols, and primary alcohol. Nx = C16 alcohol N2= C18 alcohol Mj = C16 diol M2 = C18 diol D = C16 triol D2 and D3 = unsaturated and saturated C18 triol, respectively, T4 and T2, unsaturated and saturated C18 tetraol, respectively. (Bottom) Mass spectrum of component D3 in the gas chromatogram. BSA = bis-N,O-trimethylsilyl acetamide...

FIGURE 9.2 Gas chromatograms of an FT product, original and precolumn hydrogenated. Ampoule samples taken from the gaseous product flow at ca. 200°C. GC conditions Capillary, 100 m dr 0.25 mm df, 0.5 pm methyl silicone cross-linked temperature programm -80 to 270°C carrier gas H2 introducing gas N2 FID. 

The gas chromatograms of most oil samples examined had similar basic features. All were dominated by the n-paraffins, with as many as 13 resolved but unidentified smaller peaks appearing between the n-paraffin peaks of adjacent carbon numbers. Each oil had the same basic peaks, but their relative size within bands of one carbon number varied significantly with crude source. 

Figure 2.3. Capillary gas chromatogram of Si(OCH3)4 (3M) (a) after hydrolysis/ condensation with H20 (1.8 M) and HC1 (0.05 M) showing assignments of molecular formulas and structures and (b) with H20 (1.5 M) and catalysis conditions shown. For (a), linear and cyclical structures are indicated along the x-axis. All plots illustrate relative concentrations of species (y-axis) as a function of GC column retention time (x-axis). Higher mass species (e.g., hexamers (Si6) and pentamers (Si5)) demonstrate longer retention times. [Reprinted from Ref. 72, with permission.]...

Figure 5.15. Gas chromatograms of liquid precursors for producing n-type Si films, namely, solution of CPS with white phosphorus dissolved at 1 wt% initial (nonirradiated) solution 20-min-, 25-min-, and 30-min-irradiated solutions and white phosphorus alone solution, for comparison, all of which were diluted with toluene before analysis. Trans-decalin was used as a standard for a quantitative estimation of an unreacted CPS monomer. [Reproduced with permission from Ref. 25. Copyright 2007 The Japan Society of Applied Physics.]...

Gas chromatogram of the products obtained from n-hexane when passed over a Si02/Pt catalyst at 450°C. Column 50 m x 0.32 mm PLOT, alumina modified with KC1. Temperature ... 

---