Gas Chromatography Chap 27 Types: Gas-solid chromatography (GSC) Limited application Gas-liquid chromatography (GLC) Widely used and called “GC”

Schematic of a Gas Chromatograph Fig. 27-1 He, N2, or H2 T = room + 10°  400°C

Effect of mobile-phase flow rate on plate height A van Deemter Plot: Effect of mobile-phase flow rate on plate height Analogous to Fig. 26-10

Injection Port for Split Injection into an Open Tubular Column Analogous to Fig. 27-3 ~ 50 °C > column oven ∼0.001 μL ∼0.1 μL

Injector System For samples that decompose above BP and for Quant Routine for small sample volume into OT capillary column Best for trace levels of analytes < 0.01%

Comparison of 1% methyl isobutyl ketone (BP 118 °C) and 1% p-xylene (BP 138 °C) in CH2Cl2 (BP 40 °C) (splitless)

GC Columns Two types: Packed (for large samples sizes) Al, Cu, SS, or glass packed with diatomaceous earth or Chromosorb® Open Tubular (capillary) commonly of fused silica (FSOT) coated with polyimides  flexible, non-wetting greater speed and higher N than packed columns

Open Tubular Columns Capillary Columns

Typical Fused Silica Open Tubular Column (FSOT) L = 20 – 100 m id = 0.1 – 0.53 mm N = 2 x 104 – 4 x 105

Stationary Phase Characteristics of immobilized liquid phase: low volatility (BP > oven + 100 °C) thermal stability chemical stability solvent characteristics (polar or nonpolar) Most are poly hydrocarbon siloxanes:

Table 27-3

Table 27-2

Chromatogram of Vapors From the Headspace of a Can of Beer FSOT column 0.25 mm x 30 m 20% Carbowax 30 °C for 2 min Ramped to 160 ° at 20 °/min FID detector

Common Detectors for GC Flame ionization detector (FID) Thermal conductivity detector (TCD) Electron capture detector (ECD) Photoionization detector (PID)

Flame Ionization Detector (FID) Fig 27-8 Most widely used Mass-sensitive High sensitivity (∼10-13 g) Large linear range Low noise Destructive of sample

Thermal Conductivity Detector (TCD) Fig. 27-9(a) Simple, classic design Concentration sensitive Measures change in conductivity of He when analyte is present Large linear range Relatively poor sensitivity ∴ can’t be used with capillary columns Nondestructive of sample

Thermal Conductivity Detector (TCD) Fig. 27-9(b)

Electron Capture Detector (ECD) Fig. 27-10 From column N2 To exhaust Radioactive β-emitter: Ni-63 + − Concentration sensitive Highly selective to molecules containing electronegative groups: halogens, (especially insecticides) peroxides, nitro groups, etc.

Photoionization Detector (PID) 10 – 12 eV Highly sensitive to aromatics & organo-sulfur & -phosphorous

Purge and Trap Method Extraction of a volatile analyte from a liquid matrix Uses He to purge or “sparge” analyte from matrix e.g., drinking water treatment includes chlorination but Cl2 reacts with organics to produce nasty THMs trap purge