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Gas Chromatography 24 Gas Chromatography WHAT DID THEY EAT IN THE YEAR 1000? Cholesterol Internal standard n • C34 H70 Gas chromatogram of cholesterol and other lipids extracted from bones and derivatized Detector response ß with trimethylsilyl ((CH 3)3Si ) groups to increase volatility for chromatography. Bone contains 2 to 50 ␮g cholesterol/gram of dry bone. [From A. W. Stott and R. P.Evershed, “ d13 C Analysis of Cholesterol Preserved in Archaeological Bones and 20 30 Teeth,” Anal. Chem. 1996, 68, 4402.] Time (min) δ13 C for marine diet 15 10 δ13 C for 13 C content of cholesterol from bones of terrestrial 50 people who lived on British coast in the Number of people 5 diet years A.D. 500–1800. d13 C is the deviation of the atomic ratio 13 C/ 12 C from that of a standard material, measured in parts per thousand. [Data from A. W. Stott and R. P.Evershed,“ d13 C Analysis 0 of Cholesterol Preserved in Archaeological Bones −28 −27 −26 −25 −24 −23 −22 −21 −20 and Teeth,” Anal. Chem. 1996, 68, 4402.] Cholesterol δ13 C value The 13 C content of cholesterol preserved in ancient bones provides information on the diets of people who lived long ago. Approximately 1.1% of natural carbon is 13 C and 98.9% is 12 C. Different types of plants and animals have consistent, slightly different ratios of 13 C/ 12 C, which reflect their biosynthetic pathways. To learn whether people from the ancient British coastal town of Barton•on•Humber ate mainly plants or fish, cholesterol from the bones of 50 people was extracted with organic 13 12 solvent, isolated by gas chromatography, combusted to convert it into CO 2, and its C/ C ratio measured by mass spectrometry. Observed 13 C/ 12 C ratios differ from that of a standard material by about Ϫ21 to Ϫ24 parts per thousand. A diet of local plants yields a d13 C value (defined in Box 22•3) in cholesterol of Ϫ28 parts per thousand. Values more positive than Ϫ28 parts per thousand are indicative of a marine diet. It appears that the population got much of its food from the sea. Chapter 23 gave a foundation for understanding chromatographic separations. Chapters 24 through 26 discuss specific methods and instrumentation. The goal is for you to understand how chromatographic methods work and what parameters you can control for best results. 24­1 The Separation Process Gas chromatography: in Gas Chromatography mobile phase: gas 1,2 stationary phase: usually a nonvolatile liquid, In gas chromatography, gaseous analyte is transported through the column by a gaseous but sometimes a solid mobile phase, called the carrier gas. In gas•liquid partition chromatography, the stationary analyte: gas or volatile liquid phase is a nonvolatile liquid bonded to the inside of the column or to a fine solid support 528 CHAPTER 24 Gas Chromatography Carrier Figure 24­1 Schematic diagram of a gas gas in chromatograph. Silicone rubber septum Exit Injector port Computer Injector Detector oven Detector oven Column Column oven (Figure 23•6, upper right). In gas•solid adsorption chromatography, analyte is adsorbed directly on solid particles of stationary phase (Figure 23•6, upper left). In the schematic gas chromatograph in Figure 24•1, volatile liquid or gaseous sample is injected through a septum (a rubber disk) into a heated port, in which it rapidly evaporates. Vapor is swept through the column by He, N2, or H2 carrier gas, and separated analytes flow The choice of carrier gas depends on the through a detector, whose response is displayed on a computer. The column must be hot enough detector and the desired separation efficiency to provide sufficient vapor pressure for analytes to be eluted in a reasonable time. The detector and speed. is maintained at a higher temperature than the column so that all analytes will be gaseous. Open Tubular Columns Compared with packed columns, open tubular columns offer The vast majority of analyses use long, narrow open tubular columns (Figure 24•2) made of 1. higher resolution fused silica (SiO ) and coated with polyimide (a plastic capable of withstanding 350°C) for 2 2. shorter analysis time 3 support and protection from atmospheric moisture. As discussed in Section 23•5, open tubu• 3. greater sensitivity lar columns offer higher resolution, shorter analysis time, and greater sensitivity than packed 4. lower sample capacity columns, but have less sample capacity. The wall­coated column in Figure 24•2c features a 0.1• to 5• ␮m•thick film of stationary Wall•coated open tubular column (WCOT): liquid phase on the inner wall of the column. A support­coated column has solid particles liquid stationary phase on inside wall of coated with stationary liquid phase and attached to the inner wall. In the porous­layer column column Support•coated open tubular column (SCOT): liquid stationary phase coated on solid support attached to inside wall of column Porous•layer open tubular column (PLOT): solid stationary phase on inside wall of column Outer wall of column Flow 0.1 −0.53 mm inner diameter Stationary phase (a) (0.1− 5 µm thick) (b) Solid support Stationary coated with Stationary liquid stationary solid­phase phase liquid phase particles Column wall Figure 24­2 (a) Typical dimensions of open tubular gas chromatography column. Wall­coated Support­coated Porous­layer (b) Fused silica column with a cage diameter open tubular open tubular open tubular of 0.2 m and column length of 15–100 m. column column column (c) Cross­sectional view of wall­coated, (c) (WCOT) (SCOT) (PLOT) support­coated, and porous­layer columns. 24­1 The Separation Process in Gas Chromatography 529 3 1 Acetaldehyde 2 Methanol 8 3 Ethanol 4 Ethyl acetate 5 2•Methyl­2­propanol 1 6 1•Butanol 7 2•Methyl­1­butanol 8 3•Methyl­1­butanol Carbon layer 9 Isoamyl acetate 4 7 10 Ethyl caproate 11 2•Phenylethanol Detector response 6 9 5 10 Fused silica 2 11 0 3 6 9 12 (a) (b) Time (min) Figure 24­3 (a) Porous carbon stationary phase (2 ␮m thick) on inside wall of fused silica open tubular column. (b) Chromatogram of vapors from the headspace of a beer can, obtained with 0.25­ mm­diameter ϫ 30­m­long porous carbon column operated at 30 ЊC for 2 min and then ramped up to 160 ЊC at 20 Њ/min. [Courtesy Alltech Associates, State College, PA. ] in Figure 24•3, solid particles are the active stationary phase. With their high surface area, support•coated columns can handle larger samples than can wall•coated columns. The per• formance of support•coated columns is intermediate between those of wall•coated columns and packed columns. Column inner diameters are typically 0.10 to 0.53 mm and lengths are 15 to 100 m, with 30 m being common. Narrow columns provide higher resolution than wider columns (Figure 24•4) but require higher operating pressure and have less sample capacity. Diame• ters of 0.32 mm or greater tend to overload the gas•handling system of a mass spectrome• ter, so the gas stream must be split and only a fraction is sent to the mass spectrometer. The number of theoretical plates, N, on a column is proportional to the length. In Equation 23•30, resolution is proportional to 2N and, therefore, to the square root of column length (Figure 24•5). 3 3 1 4 1 1. 1,3•Dichlorobenzene 4 2. 1,4•Dichlorobenzene 2 3. sec­Butylbenzene 4. 1,2•Dichlorobenzene 2 0.32­mm 0.25­mm inner inner diameter diameter Detector response 1.5 2.0 2.5 1.5 2.0 2.5 Time (min) Figure 24­4 Effect of open tubular column inner diameter on resolution. Narrower columns provide higher resolution. Notice the increased resolution of peaks 1 and 2 in the narrow column. Conditions: DB­1 stationary phase (0.25 ␮m thick) in 15­m wall­coated column operated at 95 ЊC with He linear velocity of 34 cm/s. [Courtesy J&W Scientific, Folsom, CA. ] 530 CHAPTER 24 Gas Chromatography 3 3 1 1 4 3 4 1 4 2 2 2 15 m 30 m 60 m Detector response 1.5 2.0 3.0 3.5 4.0 6.5 7.0 7.5 Time (min) Time (min) Time (min) Figure 24­5 Resolution increases in proportion to the square root of column length. Notice the increased resolution of peaks 1 and 2 as length is increased. Conditions: DB­1 stationary phase (0.25 ␮m thick) in 0.32­mm­diameter wall­coated column operated at 95 ЊC with He linear velocity of 34 cm/s. Compounds 1–4 are the same as in Figure 24­4. [Courtesy J&W Scientific, Folsom, CA. ] At the constant linear velocity in Figure 24•6, increasing the thickness of the stationary phase increases retention time and sample capacity and increases the resolution of early•eluting peaks with a capacity factor of k¿ Շ 5. (Capacity factor was defined in Equation 23•16). Thick films of stationary phase can shield analytes from the silica surface and reduce tailing (Figure 23•20) but can also increase bleed (decomposition and evaporation) of the stationary phase at elevated temperature. A thickness of 0.25 ␮m is standard, but thicker films are used for volatile analytes. The choice of liquid stationary phase (Table 24•1) is based on the rule “like dissolves like.” Nonpolar columns are best for nonpolar solutes (Table 24•2). Columns of intermediate polarity are best for intermediate polarity solutes, and strongly polar columns are best for strongly polar solutes.
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