Basic Gas Chromatography

History Separation of dyes by Runge Separation of plant pigments by Tswett Theoretical gc (Martin & Synge) First gc TC detector

Process Sample is vaporized (if it is not already a vapor) Passes through a column where interaction occurs - does analyte move with gas phase or stay with stationary phase (column coating) Separation occurs Detection - many types of detectors

High purity! Source of mobile phase - He or H Detector gases - none or air/H (Flame ionization detector)

Gas flow regulators Pressure regulators - stainless steel parts - not welding quality! Flow regulators - Determine gas flow rates through system (sensitive precision instruments)

Injection port Introduce sample Vaporize sample Split sample (?)

Injection ports - many versions Split - only a portion of injection goes on column Splitless - “all” material injected goes on column On-Column - cold injection (sensitive materials) Programmed temperature - sensitive materials (more durable method than OC) Large volume - Can inject 1 ml - solvent removal

Columns Packed (hard to find) Capillary (generally open tubular but can be a wall coated PLOT type)

Columns Generally fused silica - strong and inert Inner diameters mm Length m Coatings - several - range in thickness from um

Common Stationary Phase Coatings

Phase selection PUBLISHED INFORMATION Kovats indices compilations Journal articles Internal work INTUITION like structures NO IDEA? Sample information Nonpolar column Change to polar if needed

Separation theory 1.Adsorption 2.Molecular exclusion 3. Partition 4. Vapor pressure

Adsorption chromatography Interaction with a granular support e.g. Tenax, charcoal, silica gel,

Molecular exclusion Used for the separation of permanent gases e.g. Zeolites, Linde molecular sieves

Partition chromatography Partitioning between mobile phase and carrier gas vapor pressure SEPARATION BASED ON THE BOILING PT

Column coatings (stationary phases) Polar to nonpolar Polar - Carbowax Non Polar - silicone based phases

Column ovens Usually heat ovens to help in separations Ovens can be controlled from about C

Detectors Many types varying in sensitivity and selectivity Discuss most common types

Thermal conductivity detector

Characteristics of TC detector Specificity - very little - will detect almost anything including H 2 O - called the universal detector. Sensitivity to grams/sec - this is poor - varies with thermal condition of the compound. Linear dynamic range; this is poor - response easily becomes nonlinear.

Flame ionization detector

Characteristics of a Flame Ionization Detector (FID) Specificity - most organics. Sensitivity g/sec for most organics -- this is quite good. Linear range this is good. A special type of FID is called an alkali flame (AFID). Rubidium sulfate is burned in the flame and the detector becomes specific for N and P. Organics are not detected. Used for amines and nitrosoamines. (more commonly called the NPD)

Electron Capture Detector

Characteristics of an ECD Specificity - sensitive to halogens, conjugated carbonyls, nitriles, and a few others - no response with ordinary organics or H 2 O. Sensitivity 5 x g/sec - excellent Linear range 10 4 The radioactive detectors have definite temperature limits.

Separation - terms

RESOLUTION

SELECTIVITY = relative interaction of column stationary phase with both compounds to be separated  = tr’2 tr’1 CAPACITY = retention “time” of compounds to be separated k = tr - tm = tr’ tm tm THEORETICAL PLATES = column EFFICIENCY n = (tr/Wh) 2

Optimizing Gas Chromatography

Key factors influencing efficiency in gas chromatography are column phase (nonpolar are most efficient) and column diameter.

Carrier gas type and velocity

Phase thickness: Capacity and Efficiency – influenced by column diameter and phase thickness Thick phase – capacity Thin Phase – less capacity

Column length Longer means better separations but longer analysis times Time proportional to length Separation proportional to sq root of length Poor means of getting separation – costs too much in time. Use diameter, phase thickness or phase type

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