Gas Chromatography

Gas Chromatography Basics Gas Liquid Chromatography (GLC) Gas Solid Chromatography (GSC) Mobile phase does not interact with analyte Separation occurs by interaction of analyte differentially w/liquid stationary phase and temperature GC preferred method, only applicable to volatile substances derivitiazation

Carrier Gas Supply Must be at a constant flow rate so that retention times & retention volumes may be equated Choice of detector will often Dictate the gases that are used, need to be inert Ex. FID used H2, air, and He

Injection Port Heated so sample virtually instantly turns into a vapor as sample plug goes onto column Septum Split/ SplitlessSplitSplitless Want to reproducibly get in & get out Sample focusSample focus

Flow rate Measurement Why is a known flow rate critical?

GC Columns Capillary: - have i.d. <1mm - have wide unrestricted flow through center & inner surface is coated with liquid stationary phase Variety of functional groups have been blended into polysiloxane chain to provide different polarity & selectivity

Capillary columns AdvantagesDisadvantages

HO-CH 2 -CH 2 -(O-CH 2 -CH 2 ) n -OH polyethylene glycol

Temperature

Chromatography Catalog Exercise Determine the best type of column to achieve separation for drugs of abuse. Draw the chemical structures of the stationary phase as best you can. Indicate what chromatographic conditions you would do your separation under. Why do you think the catalog recommended the conditions they did? Compare with someone else who used a different catalog, explain any differences.

Detectors in GC The ideal detector should:

Flame Ionization Detector

Thermal conductivity detector

Thermionic Detector (NPD)

Electron Capture Detector (ECD)

Atomic Emission Detector

Mass Spectrometer as a GC Detector

Total Ion Chromatogram

Selected (single) ion chromatogram:

Connecting the GC to the MS This is difficult! Why?

How is a GC interfaced to an MS? 1. Direct Connection

How is a GC interfaced to an MS? (2) Open Split Interface

Advantages of Open Split Design

Jet Separator

Cryogenic Focussing Technique for Beer Headspace Analysis

Kovat’s Retention Index