Basic Gas Chromatography History 1850 - Separation of
Basic Gas Chromatography History 1850 - Separation of dyes by Runge 1906 - Separation of plant pigments by Tswett 1941 - Theoretical gc (Martin & Synge) 1952 - First gc 1954 - 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 - 0.10 - 0.53 mm Length - 1 - 60 m Coatings - several - range in thickness from 0.1 5 um Common Stationary Phase Coatings Similar Temperature Composition
-20 to 280C 60C to 220C 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 -60 400C
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 H2O - called the universal detector. Sensitivity to 10-7 grams/sec - this is poor varies with thermal condition of the compound. Linear dynamic range; 104 - this is poor response easily becomes nonlinear. Flame ionization detector
Characteristics of a Flame Ionization Detector (FID) Specificity - most organics. Sensitivity - 10-12 g/sec for most organics -this is quite good. Linear range 106 - 107 -- 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 H2O. Sensitivity 5 x 10-14 g/sec - excellent Linear range 104 The radioactive detectors have definite temperature limits. Separation - terms RESOLUTION
SELECTIVITY = relative interaction of column stationary phase with both compounds to be separated = tr2 tr1 CAPACITY = retention time of compounds to be separated k = tr - tm = tr tm
tm THEORETICAL PLATES = column EFFICIENCY n = 5.545 (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 What do you need?
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