Chapter 27 – Gas Chromatography Sample is vaporized, then injected onto the GC column. It is carried through the column by the MP, which is an inert gas. The column contains the SP, which can be a solid (gas-solid adsorption) or more likely a liquid (gas-liquid partitioning).

Columns

Stationary phase is coated onto support: Directly onto the walls of the capillary – Onto a support that is lining the capillary, such as diatomaceous earth –

Diatomaceous Earth Fossilized remains of diatoms, a hard-shelled algae. Has extremely high porosity - it is very light-weight. Mostly silica (86%), some metals. WCOT Made of fused silica - high purity Lower reactivity with sample components Flexible, so it can be coiled Physically strong, so it is less likely to break. i.d. Packed Columns Packed with diatomaceous earth which is coated with Surface area ~ Particle size Stationary Phase must have 0.1 - 5 µm thickness

Molecular Sieve Column: Separation is based on Pores in the packing material (aluminum silicate) allow small molecules to enter, are retained for some amount of time. Column: 30M x 0.53mm I.D., 25.0 um film Temperature: 35 - 100 C Injector: 50C Detector: 150C, TCD Carrier Gas: He, 5 psi ELUTION ORDER

Detectors

• • • •

FID: Eluent mixes with Burned in the flame, repelled from going back by a cathode Resulting ions are repelled toward collector plates, connected to ammeter, then to exhaust. Analyte must have TCD: The detector, a has a constant resistance when only carrier gas is flowing through. The resistance changes when a component of the mixture passes by. This produces an electricalsignal. The sample can be collected, it is a A _________________________ detector, but not as __________________________ as FID.

ECD: e- from emitter is continuously ionizing the carrier gas When an organic species passes through, it prevents this process and is detected. Very sensitive to Thermionic Detector: Other Specialty Detectors:

Type

Applicable Samples

Flame Ionization

Hydrocarbons

Thermal Conductivity

Electron Capture

Halogenated Compounds

Mass Spectrometer (MS)

Thermionic

Nitrogen & Phosphorus compounds

Electrolytic Conductivity

Compounds containing halogens, sulfur, or nitrogen

Photoionization

Fourier Transform IR (FTIR)

Organic Compounds