10/27/10. Chapter 27. Injector typically 50 C hotter than oven

Transcription

1 Sample and solvent are vaporized onto the head of a column Vaporized solvent and solute are carried through the column by an inert gas (mobile phase) The mobile phase does not interact with compounds of interest Chapter 27 Separation occurs by interaction of solute with a stationary phase Detection occurs by a variety of means Filters/Traps Introduce sample as plug at the head of the column Regulators Syringe/Sampler Injector typically 50 C hotter than oven Air Hydrogen Gas Carrier Inlets Column Detectors Data System - Chromatography Software Package on PC Sample is flash evaporated and expands into gas expansion chamber Injection volumes are small Capillary columns ~ 1 µl Packed columns 1-20 µl 1

2 Commonly Employed GC Detectors Characteristics of the ideal detector Sensitivity wide range (~ g solute/second) Stability and reproducibility Linear response to analyte concentration Operates in wide temperature range (ambient to 400 C) Short response time to all solutes Similar response to all solutes Non-destructive to sample Detection Principle: analyte gases have different thermal conductivities than carrier gases (e.g. He and H 2 ) A platinum, gold or tungsten wire (or a thermister) is placed in the exit gas stream from the column A constant voltage is applied to heat the wire Temperature/Resistance of the wire is proportional to the thermal conductivity of the surrounding gas + wide dynamic range (10 5 ) + non destructive - poor limits of detection (10-8 g/s) - non-uniform response Air/H 2 Flame Analytes are pyrolyzed in an air/h 2 flame Ions are produced in the plasma around the flame Positive voltage is applied to collector; negative to the flame body Ions migrate to collector producing a current + low lod (10-13 g/s) + wide dynamic range (10 7 ) +/- not sensitive to non-combustibles - H 2 O, CO 2, SO 2, NO x ) - destructive - response is highly flow-rate dependent 2

3 Column effluent is passed over a ß - emitter (e.g. 63-Ni) The carrier gas is ionized A burst of e - is produced with each radioactive decay Potential is applied between the collector (anode) and the detector body (cathode) Produces a constant background current The current flow decreases in the presence of analyte molecules that analyte capture the emitted electrons Detector Characteristics + selective for molecules containing electronegative functional groups (e.g. Cl - ) + lod g for certain analytes + wide linear dynamic range ( ) - non-linear response to analyte concentration General organic compounds Halocarbons Compound Relative Response Compound Relative response Benzene 0.06 CF 3 CF 2 CF Acetone 0.50 CF 3 Cl butanol 1.00 CF 2 =CCl Bromobenzene 450 CF 3 CHClBr 4 x 10 5 Chloroform 6 x 10 4 CF 3 CF 2 CF 2 I 6 x 10 5 CCl 4 4 x 10 5 CFCl x 10 6 N 2 or Ar/CH 4 (carrier gas) + ß > N +n (ionized carrier gas) + e - (burst) + lod g + wide dynamic range ( ) a UV source ionizes all the molecules in the column effluent Ions produced are collected resulting in a current flow Mass Spectrometer Sulfur Chemiluminescence Detector (SCD) Atomic Emission Detector Fourier-Transform Infrared (FTIR) + tunable sensitivity hexane ev Thermionic or Nitrogen/Phosphorus Detector cyclohexane ev methyl ethyl ketone ev benzene e V (lod 2 pg vs. 50 pg for FID 3

4 Packed Columns Typically made of glass, Teflon and aluminum Length typically 2-3 m; ID ~3 mm Solid supports have Large surface areas (>1 m 2 /g) Good strength characteristics Inert (w.r.t. the solutes) Uniformly wetted Open Tubular (or Capillary) Columns Mostly made of fused silica Have small IDs (0.1-1 mm) and long lengths (10-30 m) Uses same stationary phases as packed columns Stationary phase is coated on the inside of the column Common Stationary Phases R R R R Si O Si O Si R R R R n Polydimethyl siloxane (all R = CH 4 ) is a common backbone for creating different stationary phases Replacing methyl groups with other groups changes its polarity and separation capabilities Phenyl C 6 H 5 Cyanopropyl C 3 H 3 CN Trifluropropyl - C 3 H 6 CF 3 4

5 Retention indices (Kovats. Rohrshneider, McReynolds, etc.) Compare retention of model test compounds on squalane (2,6,10,15,19,23- hexamethyltetracosane) to retention on another support I for heptane, by definition, is 700; for octane 800, etc. 4.5 Log t r benzene I = x 100 2,2-dimethylbutane I = 5.37 x Number of paraffinic carbon atoms Characterization of GC columns cont d ΔI = X + Y + Z +.. e.g. X = ΔI = I phase benzene - Isqualane benzene Symbol test substance interactions measured characteristic substance group X benzene dispersion aromatics, olefins Y butanol orientation properties alcohols, acids with proton donor/acceptor Z pentanone orientation properties ketones, ethers, aldehydes, with proton acceptors esters, epoxides Chromatographic properties of some typical liquid stationary phases McReynolds s constants Name type X Y Z Squalane hydrocarbon OV-101 dimethylsilicone OV-7 phenylmethyl dimethylsilicone Typical Chromatograms using WCOT columns High-Performance, Static-Coated Silicon Microfabricated Columns for Gas Chromatography, Anal. Chem. 2006, 78, Design, Fabrication, and Evaluation of Microfabricated Columns for Gas Chromatography Anal. Chem. 2004, 76, a) polydimethyl siloxane b) 5% (phenyl methyldimethyl) siloxane c) 50% (phenyl methyldimethyl siloxane d) 50% poly(trifluoropropyl-dimethyl) siloxane e) polyethylene glycol f) 50% poly(cyanopropyl-dimethyl siloxane 5