Gas$Chromatography$ - Faculty...
Transcript of Gas$Chromatography$ - Faculty...
ì Gas Chromatography Se$ng Parameters
Defini0on of Parameter:
set of physical proper0es whose values determine the characteris0cs or behavior of something. Example: Parameters of the atmosphere can be temperature, pressure, and density.
-‐ Merriam-‐Webster Dic=onary
Example?
Comparing Gas & Liquid Chromatography
Type of Chromatography
Liquid: TLC or Column
Gas
Sta0onary Phase powdered silica Stainless steel column coated w/ PDMS
Mobile Phase liquids -‐ eluents (organic solvents)
gas (iden0ty depends on separa0on, air for GChem)
Force crea0ng flow! capillary ac0on (TLC) or gravity (column)
gas pressure & column temperature
1. Injector port for a µL syringe. 2. Injector port heater to evaporate
analyte chemicals. 3. Fan to create pressure for air carrier
gas. (In other GCs, port to connect carrier gas.)
4. 11 m long stainless steel column w/ an internal diameter of 0.53 mm.
5. Column Oven: hea0ng element & circula0ng fan
6. Detector 7. Recorder (Computer)
What is a Gas Chromatograph?
Stationary Phase
ì 11 m column: stainless steel coiled capillary tubing w/ an interior coa0ng of polydimethylsiloxane (PDMS)
ì Analytes adsorb onto PDMS. ì What is the polarity of PDMS? ì What kind of IM forces might exist b/w
PDMS & analyte molecules? ì How would measuring contact angles
help us understand this?
PDMS
Analyte ì Organic liquids or low MP solids. (V = 0.01 to 1.0 µL) Only
certain chemicals can be put on certain columns. Chemicals for the column used in lab:
ì Order of elu0on determined by vapor pressure – chemical w/ highest VP is eluted 1st. Unless IM Forces cause strong adsorp0on w/ sta0onary phase.
Cpd Family # of Cs BP (°C) Cpd Family # of Cs BP (°C)
Alcohols C1-‐C8 65-‐195 Esters C2-‐C10 30-‐120
Aldehydes C2-‐C8 20-‐170 Ethers C4-‐C8 35-‐145
Aroma0cs C6-‐C10 80-‐165 Ketones C3-‐C8 55-‐175
Carboxylic Acids C1-‐C4 100-‐165 Nitriles C2-‐C5 80-‐140
Mobile Phase
ì Carrier Gas… Air. Usually an inert gas like Ar or He.
ì What parameters control a gas? ì Carrier Gas Pressure (Flow Rate): 1-‐21kPa ì Column Temperature: 30-‐120 °C
ì Reten0on Time (tR): amount of 0me analyte stays on column
ì Coelu0on: when 2+ analytes come out together
ì Rela=onship to parameters: é P or T, ê tR but é coelu0on
Pressure… aka Flow Rate
ì Carrier Gas Pressure determines how quickly carrier gas flows thru column.
ì For our column: Flow Rate = 0.54 x P
ì High flow rate = low reten0on 0me = greater coelu0on = bad separa0on
Pressure (kPa) Flow Rate(mL/min) at 45 °C
2.5 1.4
5.0 2.7
7.5 4.1
10.0 5.4
12.5 6.8
15.0 8.1
17.5 9.5
20.0 10.8
GC Programming: P & T
ì Carrier Gas Pressure (Flow Rate)
ì Injector Temp
ì Start Temp
ì Hold Time: length of 0me temp is held constant
ì Ramp Rate: how quickly oven changes temperature
ì Final Temp
If P or T is…
ì too low: tR is long ì Analyte will not come off the column for a long 0me.
ì too high: coelu0on of peaks ì 2+ analytes have same short tR, no separa0on.
100 50
Parameters of a good Chromatogram
Bad Good
Trial & Error are used to find the right P & Ts to make a good chromatogram.
Injecting Analyte into GC ì Wait un0l green LED
light is lit & “Ready for Injec0on” appears
ì Clean syringe by flushing w/ acetone 3xs. ì Do not bend
needle. ì Never fill syringe to
more than 50% of its total volume.
ì Do not bend plunger as you push it down.
ì Load syringe w/ analyte soln ì Prerinse 3xs ì Fill syringe w/ 0.5 µL extra, holding
the syringe at eye level, depress the plunger to the correct volume, wipe excess on outside of needle w/ Kimwipe.
ì Hold syringe w/ one hand, pinch the needle to steady it w/ other.
ì Insert needle into injec0on port, rota0ng slightly if needle s0cks. Never force the plunger.
ì W/ a partner, simultaneously, depress syringe & select Collect. Remove needle from injec0on port & clean w/ ethanol.
Detector
ì ChemicapaciMve sensor: microcapacitor coated w/ a chemoselec0ve polymer.
ì Analyte vapor is absorbed as it exits the column changing the permi$vity of the polymer.
ì This change leads to more or less current flowing thru the capacitor.
e-‐ e-‐ e-‐ e-‐
polymer
- +
Electrically conduc0ve metal plates
Current in
Current out