1. Carrier gas supply. - Flow rate measurement

Transcription

1 Chapter 27 Gas chromatography Gas Chromatography - The components of a vaporized sample are separated as a consequence of being partitioned between a mobile gaseous phase and a liquid or a solid stationary phase held by a column. How to perform the separation in GC? - The sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase. Note : the mobile phase does not interact with the analyte; its only function is to transport the analyte through the column.

2 Type of GC 1. Gas-liquid chromatography (GLC) or Gas chromatography (GC) -liquid stationary phase -partition between gas and liquid -wild application 2. Gas-solid chromatography (GSC) -solid stationary phase -retention of the analyte is depend on the physical adsorption -limited application

3 27B Instruments for GLC

4

5 1. Carrier gas supply Mobile phase: He, Ar, N 2, H 2 Flow rate: ml /min for packed column 1 25 ml /min for capillary column - Flow rate measurement a. soap-bubble flow meter b. computer controlled electronic flow meter

6 Figure 27-3 A set of microsyrigings for sample injection

7 2. Sample injection system

8 The use of a microsyringe to inject a liquid or gaseous sample through septum into a flash vaporizer port located at the head of the column. The sample port is about 50 above the boiling point of the least volatile component of the sample. Sample size: 0.1 ~ 20 µl for packed column 10-3 µl for capillary column. Sample splitter (1:50 to 1:500) Reproducibility: rotary sample valve (~0.5% relative error) Fig.27-5

9 27B-3. Column Types: packed and capillary column Column temperature: slightly above the average boiling point of sample. FIGURE 27-7 Effect of temperature on gas chromatograms: (a) isothermal at 45 ; (b) isothermal at 145 ; (c) programmed at 30 to 180.

10 27B-4 Detection System Table 27-1 Typical Gas Chromatographic Detectors

11 Characteristics of the Ideal Detector 1. Adequate sensitivity. (10-8 ~ g solute /s) 2. Good stability and reproducibility 3. A linear response to solute that extends over several orders of magnitude. 4. Temperature range from room temperature to at least C 5. A short response time independent of flow rate 6. High reliability and ease of use. 7. Similarity in response toward all solutes or alternatively a highly predictable and selective response toward one or more classes of solutes 8. The detector should be nondestructive.

12 Flame ionization Detector (FID) Figure 27-8 A typical flame ionization detector

13 a) The effluent from the column is mixed with H 2 and air and then ignited electrically. Organic compounds produce ions and e - that can conduct electricity through the flame. A potential of a few hundred volts is applied across the burner tip and a collector electrode located above the flame. The resulting current (~10-12 A) is then directed into a high-impedance operational amplifier for measurement. b) The number of ions produced is roughly proportional to the number of reduced carbon atoms in the flame. c) The detector is insensitive toward noncombustible gases such as H 2 O, CO 2, SO 2, NO x. d) advantage: high sensitivity (~ g/s) large linear response range (~10 7 ) low noise e) disadvantage: destructive detection for organic compounds only

14 Thermal conductivity detector (TCD) FIGURE 27-9 Schematic of (a) a TCD cell, and (b) an arrangement of two sample detector cells and two reference detector cells. (Wheatstone bridge circuit)

15 a) the sensing element is an electrically heated element (Pt, Au, W) whose temperature at constant electrical power depends upon the thermal conductivity of the surrounding gas. b) mobile phase: H 2, He (high thermal conductivity) c) in the presence of organic materials, a relatively large decrease in the thermal conductivity of the column effluent take place; consequently, the detector undergoes a marked rise in temperature. d) advantage: simplicity large linear dynamic range (~10 5 ) universal detector non-destructive detection e) disadvantage: low sensitivity, can t used for capillary column. (~10-8 g solute/ml carrier gas)

16 Electron capture detector (ECD) Figure Schematic diagram of ECD a) The effluent from column is passed own a β emitter ( 63 Ni). An electron form the emitter causes ionization of the carrier gas and the production of a burst of e -. In the absence of organic species, a constant standing current between a pair of electrodes results from this ionization process. The current decreases markedly in the presence of those organic molecules that tend to capture e -.

17 b) the detector is highly sensitive to molecules containing electronegative functional groups such as halogens, peroxides, quinines, and nitro groups. c) application: determination of chlorinated pesticides. d) advantage: high sensitivity not altering the sample significantly (FID consumes the sample) e) disadvantage: narrow linear range (10 2 )

18 Thermoionic detector (TID) a) The detector is similar in structure to the FID detector. The effluent is mixed with H 2, passed through the flame tip and is ignited. The hot gas then flows around an electrically heated rubidium silicate head. The heated bead forms a plasma having a temperature of 600 to 800. Exactly what occurs of ions from phosphorus or nitrogen-containing molecules is not understood; but large ion currents result, which are useful for determining compounds containing these two elements. b) compared with FID detector phorphorus-containing cpd: 500 times more sensitive nitrogen-containing cpd: 50 times more sensitive

19 Electrolytic Conductivity Detector Compounds containing halgens, sulfur or nitrogen are mixed with a reaction gas in a small reactor tube, usually made of nickel (850~1000 o C). The products are then dissolved in a liquid, which produce a conductive solution. The change in conductivity as a result of the ionic species in the conductance cell is then measured. Figure Diagram of a Hall electrolytic conductivity detector

20 Detection mode Reaction gas Products Conductivity solvent Linear range Detection limit Halogen mode Hydrogen (H 2 ) HX, H 2 S, NH 3 n-propyl alcohol 10 6 ~0.5 pg Cl/s 2- Sulfur mode Air SO 2 (SO 3 2-,SO4 in solvent), N2, NOx, HX(removed by scrubber) Nitrogen mode Hydrogen (H 2 ) NH 3 (NH 4 + in solvent), HX, H 2 X(removed by scrubber) (NPA) Methyl alcohol (methanol) Water + a little organic solvent pg S/s 10 3 ~4 pg N/s Dried mode H 2 HCl or HBr None N/A N/A

21 Photoionization Detector Molecules eluting from column are photoionized by UV (from H 2 lamp-10.2 ev or Ar lamp-11.7 ev) to produce ions and electrons, then they are collected at a pair of biased electrodes. Compounds with a higher potential do not absorbed the energy and thus not detected. Target compounds: easily photoionized compounds aromatic hydrocarbons, organsulfur oragnophosphorus compounds Learn range

22 Atomic Emission Detector (AED) Figure In the AED, the effluent from the GC column is introduced into a microwave induced plasma (MIP), in inductively coupled plasma (ICP) or a direct current plasms (DCP). The MIP (most widely used) is used in conjunction with a

23 diode array or charge coupled-device (CCD). AED is an element-selective detector. Figure Chromatogram for a gasoline sample containing a small amount of MTBE and several aliphatic alcohols (a) monitoring a carbon emission line; (b) monitoring an oxygen emission line.

24 Flame Photometric Detector (FPD) P and S -the eluent is passed into a low-temperature hydrogen-air flame, which converts part of the phosphorus to an HPO species that emits bands of radiation centered about 510 and 526 nm. Sulfur in the sample is converted to S 2 (394 nm) -wildly applied to the analysis of air and water pollutants, pesticides, coal hydrogenation products. -Suitable filters are required to isolate the appropriate band. Mass Spectrometry Detectors - One of the most powerful detectors for GC GC/MS, see Figure The flow rate from capillary columns is low enough that the column output can be directly into the ionization chamber of the mass spectrometry. - Electron impact ionization (EI) and Chemical ionization (CI) - Mass spectrometer scans the masses repetitively during the chromatographic experiment. - Total ion chromatogram- similar to a conventional chromatogram, see Figure 27-15(a) - Selective ion chromatogram (a single mass-to-charge, m/z, is selected), see Figure 27-15(b) and (c) - GC/MS/MS or GC/MS n

25 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.

26 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.

27 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 (C2H6N2O).

28 m/z 93 FIGURE 27-15(c) Typical outputs for a GC/MS system. A selected-ion chromatogram t m/z = 93 is shown in (c). Peaks 2 and 5 give a response at this m/z value due to fragmentation products.

29 GC coupled with Spectroscopic Detection-hyphenated methods -GC/FTIR, GC/NMR, GC/electrodes (electrochemistry) Other types of Detectors - Sulfur chemiluminescence detector (certain sulfur comp. react with O 3 pollutants like mercaptants) Linear range:10 5 LOD: 0.5 pg/s for sulfur. - Nitrogen-specific chemiluminescence detector - Linear range: LOD: 5 pg/s for nitrogen compound

30 GC columns and stationary phase Columns 1. Open tubular column (capillary column) 1. WCOT: wall-coated open tubular. Capillary tubes are coated with a thin layer of stationary phase. 2. SCOT: support-coated open tubular. The inner surface of the capillary is lined with a thin film (~30μm) of a support material, such as diatomaceous earth, then coated with stationary phase. (SCOT has a greater sample capacity, lower efficiency than WCOT) FSWC: fused-silica wall-coated open tubular. The capillary is drawn form fused-silica and given added strength by an outside protective polyimide coating. ID Resolution

31 TABLE 27-2 Properties and Characteristics of Typical GC Columns

32 2. Packed column material: glass, metal, Teflon length: 2~3m diameter: 2~4 mm packing material : diatomaceous earth + stationary phase (0.05~1μm thickness ) particle size of packing material : mesh ( μm) mesh ( μm) Solid supported Materials Ideal support: 1. Small; 2.Uniform; 3. Spherical particle; 4.Good mechanical; 5. Surface area 1 m 2 /g; 6. inert to temperature; 7. uniformly wetted by the liquid phase.

33 TABLE2.6 THE COMPOSITION OF CALCINED DIATOMACEOUS EARTH Component percent composition SiO Al 2 O Fe 2 O CaO 1.4 MgO 0.4 Volatile 0.3 Other 0.4

34 Figure A photomicrograph of a diatom. Magnification 5000X Diatoms: diatomaceous earth ( 矽藻土 )

35 27C-3 Adsorption on column packings or capillary walls a) adsorption of polar species results in distorted peaks. The SiOH groups on the support surface have to retain them by adsorption b) support can be deactivated by silanization with dimethylchlorosilane (DMCS)

36 Washing with methanol Si

37 27C-4 Stationary phase for GLC a) Desirable properties for GLC stationary phase : low volatility thermal stability chemical inertness solvent characteristics such that k and αvalues within a suitable range b) "Like dissolves like " The polarity of the stationary phase should match that of sample components. polar stationary phase:-cn, -CO, -OH nonpolar stationary phase: hydrocarbon, dialkyl siloxane

38 Classification of stationary phase -- Some widely used stationary phase TABLE 27-3 Some Common Liquid Stationary Phases for GLC

39 - Five of the liquids listed in Table 27-3 are polydimethyl siloxanes that have the general structure R = CH 3 relatively nonpolar. R = phenyl ( C 6 H 5 ), cyanopropyl ( C 3 H 6 CN), trfluoropropyl ( C 3 H 6 CF 3 ), will increase difference polarity. - The fifth entry in Table 27-3 is a polyethylene glycol (PEG) with the structure HO CH 2 CH 2 (O CH 2 CH 2 )n OH It finds widespread use for separation polar species

40 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.

41 FIGURE 27-17(d)-(f) (cyanopropyl-dimethyl) siloxane. (d) 50% poly (trifluoropropyl-dimethyl) siloxane; (e) polyethylene glycol; (f) 50% poly

42 -- Bonded and cross-linked stationary phase will provide a longer-lasting stationary phase that is not disrupted at elevated temperature or during temperature programming. It can be rinsed with solvent (backflushed) when contaminated. - Column bleeding: a small amount of immobilized liquid is carried out of the column during the elution process. (chemical bonding and cross-linking inhibit bleeding) - Chemical bonding - Cross-linking (1) incorporated a peroxide into the original liquid. The reaction occurs when heated. - (free radical mechanism) - (2)exposing column to gamma ( ) radiation -- Film thickness (0.1~ 5 m) retentive character and capacity of a column Thick film: for highly volatile analytes (retain solute for a longer time) Thin film: for low volatile analytes Most application : 0.25 m ~0.32 m

43 27D Application of GC 1. Qualitative analysis Retention time - confirming the presence or absence of a suspected compound. Retention index I retention index for normal alkane = 100 number of carbon. a plot of log(t R -t M ) vs number of carbon is linear: retention index for X I = log( t R ) x log( t R ) n 100[ ] 100 n log( t R ) n 1 log( t R ) n 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.

44 2. Quantitative Analysis- See 26F-2 27E Advances in GC 27E-1 High-speed GC

45 FIGURE High-speed chromatogram obtained with isothermal operation (30 ) for 27 s followed by a 27E-2 Miniaturized GC Systems 35 /min temperature ramp to 90.

46 FIGURE Microfabricated columns (a) and chromatogram (b). The columns in (a) were 0.9-m-long spiral and serpentine

47 FIGURE 27-20(b) The mixture (b) was 1, acetone; 2, 2-butanone; 3, benzene; 4, trichloroethylene; 5, 2,5-dimethylfuran; and 6, toluene. Air was used as the carrier gas with an outlet pressure of 0.5 atm. (using photoionization detector) 27F Gas Solid Chromatography - Separation is Based upon adsorption - - Air, H 2 S, CS 2, NO X CO, CO 2, rare gases can not be retained in GLC, but can be separated in GSC. - - Perform with both packed column and porous-layer open tubular column (PLOT) - Two absorbents (1) molecular sieves Al x Si y, 4, 5, 10, 13 A (10-10 m), Figure 27-21(a)He, O2, N2, CH3OH, CO.. - (2) porous polymers, Figure 27-21(b)

48 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. Cn = hydrocarbon with n carbons.