Principles of Instrumental Analysis

Transcription

1 Principles of Instrumental Analysis Chapter 27 Gas Chromatography Gas Chromatography (GC): vaporized analytes (solutes) are partitioned between a mobile gaseous phase and a liquid or a solid stationary phase held in a column. The mobile phase does not interact with molecules of the analytes (solutes). Gas-Liquid Chromatography (GLC): partition between mobile phase and liquid phase immobilized ( 固定化 ) on the surface of an inert solid packing or on the walls of a capillary tubing. Gas-Solid Chromatography (GSC): physical adsorption 27A Principles of GLC Due to compressibility of gaseous mobile phases, the mathematical relationships obtained in Chapter 26 need minor modification. Retention volume (V g ) instead of retention time (t R ): V g = (K/ S ) x (273/T c ) T c : column temperature S : density of liquid stationary phase 1

2 V R = t R F, V M = t M F F: average volumetric flow rate F = F m x (T c /T) x (P - P H2O )/P T c : column temperature (T: ambient) P: gas pressure at column end (ambient) P H2O : vapor pressure of water Corrected retention volume: V R0 = jt R F, V M0 = jt M F j: compressibility factor.. (27-4) Specific retention volume: V g = (V R0 -V M0 )/m S x 273/T c = jf(t R t M )/m S x 273/T c m S : mass of the stationary phase V g = jft M k/m S x 273/T c retention factor k = (t R -t M )/t M = V M0 k/m S x 273/T c = KV S /m S x 273/T c k = KV S /V M (K: distribution constant) S = m S /V S V g =K/ S x 273/T c (from eq. 27-4) At a given temperature, V g depends only on the distribution constant (K) and density ( S ) of stationary phase (liquid). van Deemter plot FIGURE 26-8 Effect of mobile-phase flow rate on plate height for (a) LC and (b) GC. Note very different flow rate and plate height scales. 1) The minimum for LC usually occurs at flow rates well below those for GC. 2) Plates heights (H) for LC columns are an order of magnitude or more smaller than those encountered with GC columns. Ch26 An Introduction to Chromatographic Separations P.772 2

3 27B Instruments for GLC FIGURE 27-1 Block diagram of a typical gas chromatograph. FIGURE 27-2 A soap-bubble flow meter. P y%20tour/organic/organic%20gc.htm 3

4 FIGURE 27-3 A set of micro-syringes for sample injection. P.791 FIGURE 27-4 Cross-sectional view of a microflash vaporizer direct injector. P.791 4

5 Gas sampling valve for quantitative work. FIGURE 27-5 A rotary sample valve: valve position (a) is for filling the sample loop ACB; position (b) if for introduction of sample into column. P.792 5

6 Column configuration: constructed of fused silica or stainless steel; formed as coils (diameter: 10~30 cm); housed in a thermostatted oven. Packed column: 1~5 m. Open tubular (capillary) column: ~100 m. Column temperature is an important variable: equal or slightly above the average boiling point of a sample reasonable elution time (2-30 min). In general, optimal resolution is associated with minimal temperature; the cost of lowered temperature is an increase in elution time [Figure 27-7(a)-(b)]. FIGURE 27-6 Fused-silica capillary columns. P.792 FIGURE 27-7 Effect of temperature on gas chromatograms: (a) isothermal at 45 ; (b) isothermal at 145 ; (c) programmed at 30 to 180. P.793 6

7 27B-4 Detection Systems Characteristics of the ideal detectors: 1)Adequate sensitivity. (10-8 ~ g solute/s) 2)Good stability and reproducibility. 3)A linear responses to solutes (several orders of magnitude). 4)A temperature range from r.t. to at least 400 o C. 5)A short response time independent of flow rate. 6)High reliability and ease of use. 7)Similarity in response toward all solutes. 8)The detector should be non-destructive. TABLE 27-1 Typical Gas Chromatographic Detectors P.793 FID responds to number of carbon atom: A mass-sensitive rather than concentration-sensitive. Suitable for organic samples: Functional groups (carbonyl, alcohol, halogen, and amine) yield fewer ions or none at all in a flame. Insensitive to non-combustible gases (H 2 O, CO 2, SO 2, CO, noble gases, and NO x ) Sensitivity: ~10-13 g/s. Linear response range: ~10 7 Low noise. Disadvantages: destructive FIGURE 27-8 A typical flame ionization detector (FID). P.794 7

8 Thermal Conductivity Detector (TCD): Temperature at constant electrical power depends on the thermal conductivity of the surrounding gas. Suitable to both organic and inorganic species. Simplicity & non-destructive. Low sensitivity: ~10-8 g/s. Linear response range: ~10 5 FIGURE 27-9 Schematic of (a) a TCD cell, and (b) an arrangement of two sample detector cells and two reference detector cells. P.794 Nickel-63 Ionization of N 2 FIGURE Schematic diagram of an Electron-Capture Detector (ECD). Electronegative functional groups tend to capture electrons decrease current. Selectively responds to halogen-containing organic compounds: for detection of environmental samples: pesticides, polychlorinated biphenyls. Selective in response: high sensitivity to halogens, peroxides, quinones, and nitro groups; insensitive to amines, alcohols, and hydrocarbons. Highly sensitive and non-destructive. But only two orders in linear response. P.795 8

9 FIGURE Diagram of a Hall electrolytic conductivity detector. P.796 AED is an element-selective detector. FIGURE An Atomic Emission Detector (AED) for GC. The plasma is sufficiently energetic to atomize all the elements in a sample and to excite their characteristic atomic emission spectra. P.797 9

10 C: 198 nm FIGURE 27-13(a). Chromatogram for a gasoline sample containing a small amount of MTBE and several aliphatic alcohols: monitoring a carbon emission line. P.798 O: 777 nm FIGURE 27-13(b) Chromatogram for a gasoline sample containing a small amount of MTBE and several aliphatic alcohols: monitoring an oxygen emission line. P

11 Mass Spectrometry Detectors FIGURE Schematic of a typical capillary GC/MS system. The effluent from the GC is passed into the inlet of the mass spectrometer, where the molecules in the gas are fragmented, ionized, analyzed, and detected. P.799 Time, min FIGURE 27-15(a). Typical outputs for a GC/MS system. In (a), the total ion (current) chromatogram were 1, N-nitrosodimethylamine, 2, bis(2- chloroethyl)ether, 3, bis(2-chloroisopropyl)ether, 4, N-nitrosodi-n-propylamine, and 5, bis(2-chloroethoxy)methane MW= MW= MW= MW= MW= P

12 m/z 74 FIGURE 27-15(b) Typical outputs for a GC/MS system. In (b), the mass chromatogram at m/z = 74 is shown. The peak is due to the parent ion of n-nitrosodimenthylamine (C 2 H 6 N 2 O). MW= P.799 m/z 93 FIGURE 27-15(c). Typical outputs for a GC/MS system. A selected-ion chromatogram m/z = 93 is shown in (c). Peaks 2 and 5 give a response at this m/z value due to fragmentation products. MW= MW= P

13 27C GC Columns and Stationary Phases TABLE 27-2 Properties and Characteristics of Typical GC Columns P.802 FIGURE A photomicrograph of a diatomaceous earth ( 矽藻土 ). Magnification 5000X. P

14 27C-3 Adsorption on Column Packings or Capillary Walls: Polar silanol silanization Acid washing before silanization removes metal oxide impurities. P C-4 Stationary phase (immobilized liquid) requires: (1) low volatility, (2) thermal stability, (3) chemical inertness, and (4) solvent characteristics (k and ). TABLE Some Common Liquid Stationary Phases for GLC P

15 Compatibility (solubility) of an analyte with stationary phase leads to reasonable residence time (k). Like dissolves like. Like refers to the polarities of the analyte and the immobilized liquid (stationary phase). In GC the polarity of stationary phase should match with that of the sample components. The order of elution is determined by the boiling point of the analytes. E coh = H vap RT (Cohesive energy) CED = E coh /V (Cohesive Energy Density) = (CED) 1/2 (Solubility Parameter) 溶劑之溶解參數 ( ) (cal/ml) 1/2 n-hexane n-octane Cyclohexane Benzene Toluene Acetone Methylethylketone (MEK) Methyl acetate Ethyl acetate Butyl acetate Tetrahydrofuran (THF) Methanol Ethanol n-butanol Water Polarity Liquid Stationary Phase fro GLC Hydrocarbon Polysiloxane bonded or cross-linked R= methyl: Polydimethyl siloxane R= methyl, phenyl: x% Phenyl-polydimethyl siloxane R= methyl, trifluoropropyl: x% Trifluoropropyl-polydimethyl siloxane R= methyl, cyanopropyl: x% Cyanopropyl-polydimethyl siloxane Polyethylene glycol: H-(OCH 2 CH 2 ) n -OH Polyester: H-(O-RO-CO-R -CO) n -OH Analytes: Polar: alcohols, acids, amines. Medium polar: ethers, ketones, aldehydes. Non-polar: saturated hydrocarbons. The polarity of the stationary phase should match that of the analytes. When the match is good, the elution order is determined by the boiling point of the analytes P

16 FIGURE 27-17(a)-(c) Typical chromatograms from open tubular columns coated with (a) polydimethyl siloxane; (b) 5% (phenyl methyldimethyl) siloxane; (c) 50% (phenyl methyldimethyl) siloxane. P.805 FIGURE 27-17(d)-(f) (d) 50% poly (trifluoropropyl-dimethyl) siloxane; (e) polyethylene glycol; (f) 50% poly (cyanopropyl-dimethyl) siloxane. P

17 Chiral Stationary Phases (CSP): chiral liquid as stationary phase for separation of enantiomers. Thalidomide was sold in a number of countries across the world from 1957 until 1961 when it was withdrawn from the market after being found to be a cause of birth defects in what has been called "one of the biggest medical tragedies of modern times". [4 It is not known exactly how many worldwide victims of the drug there have been, although estimates range from 10,000 to 20,000. [5] R 1 ' R 1 R 1 R 4 ' R 2 ' R 2 R 4 R 4 R 2 (S)-thalidomide (R)-thalidomide R 3 ' CSP (R) (S) enantiomer Three-point interaction R 3 R 3 27D Applications of GC 1)Perform separations 2)Analysis: a) Qualitative analysis: retention times or volumes. * Selectivity factors ( ): * The Retention Index (I): The retention index scale is based on normal alkanes: 100 x no. carbon. b) Quantitative analysis: peak heights or areas. 17

18 FIGURE Graphical illustration of the method for determining retention indexes for three compounds. Stationary phase: squalane. Temperature: 60. Retention indexes for normal alkane standards nonane and hexane are indicated. P E Advances in GC 1)High-speed GC: t R = L/u x (1 + k n ); V g = (K/ S ) x (273/T c ) 2)Miniaturized GC Systems FIGURE High-speed chromatogram obtained with isothermal operation (30 ) for 27 s followed by a 35 /min temperature ramp to 90. P

19 FIGURE Microfabricated columns (a) and chromatogram (b). The columns in (a) were 0.9-m-long spiral and serpentine channels. P.809 FIGURE 27-20(b) The mixture (b) was 1, acetone; 2, 2-butanone; 3, benzene; 4, trichloroethylene; 5, 2,5-dimethyl-furan; and 6, toluene. Air was used as the carrier gas with an outlet pressure of 0.5 atm. P

20 27F Gas-Solid Chromatography (GSC): physical adsorption packed or (porous-layer) open tubular columns (PLOT) 1) Molecular Sieves: aluminum silicates 2) Porous polymers: cross-linked polystyrenes FIGURE Typical gas-solid chromatographic separations: (a) a 5 ft. 1/8 in. molecular sieve column; (b) a 30 m 0.53 mm PLOT column. C n = hydrocarbon with n carbons. P