Chapter 27: Gas Chromatography
Gas Chromatography Mobile phase (carrier gas): gas (He, N 2, H 2 ) - do not interact with analytes - only transport the analyte through the column Analyte: volatile liquid or gas Stationary phase: - solid (GSC) or non-volatile liquid (GLC) GSC (gas-solid adsorption chromatography) - semi-permanent retention of active or polar molecules - severe tailing of elution peaks GLC (gas-liquid partition chromatography) - non-volatile liquid is coated on the inside of the column or on a fine solid support - In 1955, the first commercial apparatus for GLC appeared on the market
Instrument for gas chromatography
Instrument for gas chromatography Temp of a sample injector port: 50 o C above the b.p. of least volatile component of the sample rapidly evaporates (2-50 m) (thermostated) The column should be hot enough to provide sufficient vapor pressure for analyte to be eluted in a reasonable time.
Open Tubular Columns Outer-wall: fused silica coated with polyimide - Higher N, smaller H Higher resolution (H = A + B/U + CU, no A term in OTC) - Higher flow rate shorter analysis time Thin coating: small C-term (decreased H) : Compared with packed columns, OTC offers higher resolution, shorter analysis time, greater sensitivity, lower sample capacity
Length: 15-100 m Open Tubular Columns
Open Tubular Columns Because of increased surface area, support-coated columns can handle larger sample than can wall-coated column
GC Columns
Porous-Layer Design in Stat. Phase - Solid particles are active stationary phase Vapors from a beer can
Effect of Column Length The number of theoretical plates, N, on a column is proportional to the length Resolution is proportional to (N) 1/2 and therefore, the square root of column length
Effect of Stationary Phase Thickness
Liquid Sta. Phase Choice of liquid phase for a given problem: like dissolves like - Nonpolar columns: best for nonpolar solutes - Polar columns for polar solutes - As a column ages, stationary phases bakes off surface silanol groups (Si-OH) are exposed peak tailing (polar analyte) Therefore, stationary phase is covalently attached to silica surface
Polar Stat. Phase:-CN, -CO, -OH, -ester Non-polar Stat. Phase: hydrocarbon
Chiral Separation
Chiral Separation Separation of amino acid enatiomers
Chiral Separation: Cyclodextrin Stat. Phase
Packed Columns Packed columns contain - a fine solid support itself - a fine solid support coated with nonvolatile liquid stationary - solid support: silanized diatomite to reduce hydrogen bonding to a polar solute Packed columns - are made of stainless steel or glass - 3-6 mm diameter, 1-5 m in length Packed columns are good for preparative separations Uniform particle size decrease A term reduce H better resolution Small particle size decreased time for solute equilibration improved column efficiency (problem: higher pressure for carrier gas)
Prevention of Peak Tailing : Silanization - Sites that bind solute strongly cause peak tailing - Silica surface of columns and stationary phase particles have hydroxyl groups that form hydrogen bond with polar solutes, thereby leading to serious tailing - Silanization reduces tailing by blocking the hydroxyl groups with nonpolar trimethylsilyl groups
Chromatogram of alcohol mixture using packed column In comparison to the chromatogram using OTC, broader peaks, longer retention times, and less resolution
The Retention Index Non-polar column Polar column (retention time: hydrocarbon<ketone<alcohol) Dipole interaction H-bonding Column oven temp = 70 o C
The Retention Index Retention index relates the retention time of a solute to the retention times of linear alkanes Kovat retention index (I) for a linear alkane 100 times the number of carbon atoms (e.g.) octane, I = 800, nonane, I = 900
Temperature Programming Temp of column (oven) increases Solute vapor pressure increase decrease retention time Isothermal at 150 o C Temp programming: 50 250 o C at 8 o C/min Precaution: at too high temp. thermal decomposition of analyte
Temperature Programming
Carrier Gas in GC Carrier gas: chemically inert H = A + B/U + CU In GC, B term is important (D o of gas: 10 4 greater than liquid) Good resolution but slow analysis React with alkene at high temp & dangerous
Carrier Gas in GC
Sample Injection in GC Liquid samples are injected into GC by syringe through a rubber septum into a heated port Gaseous samples use gas-tight syringe <Sample size> Packed column: sub ml 20 ml, Capillary column: 10-3 ml (split injection) Spilt injection delivers only 0.2-2% of the sample to the column
Sample Injection in GC
Sample Injection in Capillary GC Split injection: routine means Splitless injection: best for trace analysis On-column injection: best for thermally unstable solutes
Quantitative and Qualitative Analysis by GC Qualitative analysis: - retention time (GC-FID, TCD, ECD ): comparison with authentic sample - mass (GC-MS) Quantitative analysis: - peak area or peak height
Thermal Conductivity Detector (TCD) - TCD responds to the changes in thermal conductivity of carrier gas stream (high conductivity) by the presence of analyte molecules (low conductivity) - The carrier gas of choice: H 2 or He (highest thermal conductivity) Pt, W, or thermister Temp of the filament depends on the thermal conductivity of surroundings
Thermal Conductivity Detector (TCD) <Advantages of TCD> - simple system - wide linear dynamic range (~ 10 4 ) - general response to organic and inorganic species - non-destructive <Limitation of TCD> - relatively low sensitivity
Flame Ionization Detector (FID) - Most widely used and generally applicable detector - Column eluate is mixed with H 2 /air and then burned in flame. CH + O CHO + + e - - Most organic compounds, when pyrolyzed at the temp of H 2 /air flame, produce ions (~1/10 5 ) and electrons that can conduct electricity through the flame - FID responds to the # carbons entering the detector per unit time : mass sensitive rather than concentration-sensitive - Functional groups (carbonyl, carboxyl, halogen) yield fewer ions - FID is insensitive to noncombustible gases (H 2 O, CO 2, O 2, N 2, SO 2, and NOx)
Sulfur Chemiluminescence Detector Sulfur compounds H 2 -O 2 flame SO + products SO + O 3 SO 2 * + O 2 SO 2 * SO 2 + light (blue) <FI Detector> * very small amount of sulfur compounds <SC Detector>
GC-Mass Spectrometer Mass spectrometer is a powerful detector for both qualitative and quantitative analysis of analyte in gas or liquid chromatography Analyte: ionized Ionized analyte: separated according to mass Ions: detected
GC-Mass Spectrometer
GC-Mass Spectrometer Ionization: electron impact 70 ev = 6.7 x 10 3 kj/mol Mass analyzer: quadrupole mass analyzer Typical chemical bond: 200-600 kj/mol compact, rugged, less expensive than magnetic sector, high scan rate ( <10 ms)
GC-Mass Spectrometer <Electron impact ionization> hard source much fragmentation <Chemical ionization> soft source, less fragmentation> MW=226 Electron Impact: good for structural information Chemical ionization: good for M.W. information CH 4 + e - CH 4+ + 2e - CH 4+ + CH 4 CH 5+ + CH 3 CH 5+ + M CH 4 + MH +
GC-Mass Spectrometer
Calibration A process that relates the measured signal to the concentration of analytes - Simple (external) calibration method (no matrix effect or pre-separation step) The plot between series of standards and signal - Standard addition method - Internal standard method Add standard solutions to sample (several aliquot of the same size) A substance is added in a constant amount to all samples, blank and calibration standards
Standard Addition Method - Useful for analyzing complex samples in which matrix effect is substantial. - Known quantities of analyte are added to the unknown: from the increase in signal, concentration of analyte in original unknown can be deduced.
Standard Addition Method
Internal Standard An internal standard (IS) is a known amount of compound, different from analyte, that is added to the unknown sample. IS: useful in analyses in which the quantity of sample analyzed or the instrument response varies slightly from run to run for reasons that are difficult to control Gas and liquid chromatography: - flow rate change response change - small quantity of solution is injected: not reproducible Relative response of the detector to the analyte and standard is constant: (e.g) flow rate change S(IS) 5% increase S(analyte) 5% increase The concentration of IS is known correct concentration of analyte can be derived. IS is also desirable when sample loss can occur during sample preparation before analysis.
Internal Standard If [X] = [S] = 1.0 mm, area Ax = 2.5 area S, then response factor = 2.5 Ax/[X] = R (As/[S]) ; R = response factor Example: (1) Preliminary experiment to determine R: [X] = 0.0837M, [S] = 0.0666M Ax =423, As = 347 423/0.0837 = R (347/0.666) R = 0.970 (2) To analyze the unknown ([X]=?), 10.0 ml of 0.146 M standard was added to the 10 ml unknown and diluted to 25 ml Ax = 553, As = 582 [S] = 0.146 M x dilution factor (10.0/25.0) = 0.0584 M 553/[X] = 0.970 (582/0.0584) [X] = 0.0572 M Thus, original concentration of X in unknown is 0.0572 x (25.0 ml/10.0ml) = 0.143M