Transcript Document
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 Polarity 1. 100% Dimethyl Nonpolar Polysiloxane (Gum) Applicatiom Phenol, hydroc. amines, sulfur cpds pesticides, PCBs Reynolds Constants OV-1 SE-30 Limits 60C to 325C 2. 100% DimethyL Polysiloxane (Fluid) Nonpolar Amino Acid Der. Essential oits OV-101, SP-2100 OC to 280C 3. 5% Diphenyl to 95% Dimethyl Nonpolar Fatty Acids, Me Est SE-52 -60C Alkaloids, Drugs, Halogenated Cpds OV-23, SE-54 325C 7% Cyanopropyl Intermediate 7% Phenyl Polysiloxane Drugs. steroids, Pesticides OV-1701 5. 50% Phenyl, Intermediate 50% Me.Polysiloxane Drugs, Steroids, Pesticides, Glycols OV-17 60C to 240C 6. 50% Cyanopro. Intermediate me 50% Phenyl ethyl Polysiloxane 50% Trifluoropro Intermediate Polysiloxane Fatty Acids, Me Est Alditol Acetates OV-225 60C to 240C Halogenated Cpds Aromatics OV-210 45 to 240C 8. Polyethylene Polar Glycol-TPA modified Acids, Alcohols, Acrytates, Nitrites, Aldehydes, Ketones OV-351, SP-1000 60 to 240C 9. Polyethylene Gly Free Acids, Alc Carbowax 20M Ethers,Essential Oils, Glycols, Solvents 4. 7. Polar -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 = tr’2 tr’1 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?