1	Chapter 27: Gas Chromatography Principles Instrumentation Detectors Columns and Stationary Phases Applications
2	"Basic Principle of GC –..." Basic Principle of GC – sample vaporized by injection into a heated system, eluted through a column by inert gaseous mobile phase and detected Three types (or modes) gas – solid chromatography early gas – liquid “ important gas – bonded phase “ relatively new An estimated 200,000 GC in use worldwide
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4	"Carrier gases (mobile phase" Carrier gases (mobile phase) – must be chemically inert He, Ar, N₂, CO₂ even H₂ and mixtures 95/5 N₂/CH₄ Often detector dictates choice of carrier gas In GC sample doesn’t really interact with carrier gas (unlike HPLC), temp controls partitioning Often necessary to purify cylinder gas with a trap, scrubber or cartridge of molecular sieves (or buy high purity gas) O₂ ppm Hc The move today is away from gas cylinders toward gas generators (extract pure carrier gas from air)
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6	"Flow control – 10 to..." Flow control – 10 to 50 psi with regulator Regulators vary in quality, material & control, typically use a 2 stage regulator with the best material being stainless steel Ultimately flow rate is checked by a soap bubble meter for accurate flow
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10	"Injector – use micro syringe..." Injector – use micro syringe 99.9 % of the time injecting 1 to 20 µL, rapidly shoot in plug of sample Old GCs had separate injection area Today use on-column & microflash vaporizers – all have septum of synthetic rubber which is punctured by syringe Injector usually 50 ^oC hotter than boiling point of sample – also hotter than column Can use rotary injector valve (as for HPLC)
11	"Rotary Injection Valve" Rotary Injection Valve Common for HPLC, rare in GC
12	"Alternate view of injector valve" Alternate view of injector valve Position A = Load (i.e. fill loop) Position B = Inject (sample swept onto column)
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14	"Column housed in Column Oven..." Column housed in Column Oven to maintain temperature Types – packed, open tubular, capillary oldest ------------------ newest Capillary columns will take over completely Packed – tube (steel, glass, fused silica, Teflon) packed with material Open Tubular – coated on walls Capillary – coated on walls, long & narrow Length range – 2 to 50 m (typically 30 m)
15	"Column Concepts" Column Concepts In GC since mobile phase is under pressure & we operate at various temperatures given that P V is proportional to T Sometimes use retention volumes (V_R, V_M) V_R = t_R F for retained species t_R = retention time V_M = t_M F for unretained F = flow rate
16	"Problem - pressure drop across..." Problem - pressure drop across a column Pressure at head of column may be 5 atm & at end of column may be 1 atm Need a correction factor 3[(P_i/P)² -1] j = ------------------ 2[(P_i/P)³ – 1] Where P_i = inlet pressure & P = outlet pressure (atmospheric)
17	"Can define specific retention volume..." Can define specific retention volume (V_g) V_R^o – V_M^o 273 V_g = --------------- x ------- W T_c Where W = mass of stationary phase T_c = column temp. (^oK) V_R^o = j t_R F V_M^o = j t_M F Can relate V_g to K (partition ratio) K 273 W V_g = ---- x ------- r_s = ------ r_s T_c V_s
18	"Detectors – dozens of detectors..." Detectors – dozens of detectors available Characteristics of an ideal detector: Adequate sensitivity for desired analysis (typical 10^-8 to 10^-15 g analyte/sec) Stable – background constant with time Reproducible – good precision Linear response over several orders of magnitude Temperature range – room temp - 400 ^oC
19	"Characteristics of ideal detector:" Characteristics of ideal detector: (continued) 6) Rapid response time 7) Independent of flow rate 8) Reliable 9) Easy to Use – inexperienced operators 10) Either selective or universal response 11) Nondestructive No detector exhibits all these characteristics
20	"Flame Ionization Detector (FID" Flame Ionization Detector (FID) - one of most widely used GC detectors - good sensitivity to almost all organic compounds
21	"FID Basics" FID Basics - column effluent mixed with air and burned in H₂ flame producing ions & electrons that conduct electricity a few hundred volts applied between burner tip & a collector electrode above the flame producing currents on the order of 10^-12 amps amplify & measure signal approximately proportional to number of reduced carbon atoms in flame
22	"FID Basics (continued)" FID Basics (continued) mass sensitive rather than concentration insensitive to non combustible gases – H₂O, CO₂, SO₂, NO_x FID exhibits High sensitivity Large linear response range 10-13 g/s Easy to use Rugged DESTRUCTIVE
23	"Flame Ionization Detector (FID" Flame Ionization Detector (FID) - one of most widely used GC detectors - good sensitivity to almost all organic compounds
24	"Thermal Conductivity Detector (TCD" Thermal Conductivity Detector (TCD) One of earliest GC detectors Not popular today Low sensitivity Several designs Use heated wire or semiconductor - Resistance of wire changes with analyte vs carrier
25	"TCD uses bridge circuit with" TCD uses bridge circuit with Sample & Reference Cells
26	"TCD" TCD New TCDs use pulsed current to increase sensitivity & reduce drift Thermal conductivity of He & H₂ are about 6 to 10 times greater than most organic compounds (must use these carrier gases) Other carrier gases (N₂, Ar, etc) have thermal conductivities too close to organics
27	"Advantages of TCD" Advantages of TCD Simple à Reliable & Easy to use Universal response (organic & inorganic) Large linear dynamic range 10⁵ Nondestructive, can use in tandem Older instruments have built-in TCD Disadvantages Low sensitivity Often can’t use with capillary columns because amount of analyte is small