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Major Components
http://www.wooster.edu/chemistry/analytical/gc/default.html
Column Stationary Phases:
Packed
- liquid coated silica particles (<100-300 mm diameter) in glass tube
-best for large scale but slow and inefficient
Capillary/Open Tubular
- wall-coated (WCOT) <1 mm thick liquid coating on inside of silica tube
- support-coated (SCOT) 30 mm thick coating of liquid-coated support on
inside of silica
tube
- best for speed and efficiency but only small samples diameter 150-400 mm
Why are modern GC based on a capillary column?
Back to the van Deemter Eqn. H = A + B/n + Cn
Remember, n = flow rate,
A = multiple paths, B/n = longitudinal diffusion effects, Cn = MT effects
We want to minimize H as much as possible.
Many based on polysiloxanes or polyethylene glycol (PEG):
Non-polar stationary phases best for non-polar analytes nonpolar analytes retained preferentially
Polar stationary phases best for polar analytes polar analytes
retained preferentially
Temperature Programming
-As column temperature raised, vapor pressure analyte increases, eluted
faster
- Raise column temperature during separation – temperature programming
– separates species with wide range of polarities or vapor pressures
Sample Injection
GC Injection Syringe
It is important to rapidly vaporize the sample.
Slow vaporization increases band broadening, by
increasing the sample “plug”.
Injection port temperature is usually held 50oC
higher than the BP of the least volatile
compounds.
GC injection and band broadening
and anomalies.
Extremely slow injections will cause bandbroadening, wide sample “plug”.
Jerky injections may cause double peaks for
the same analyte species.
Split vs. Splitless Injection
Sample injection is done by a syringe – 1 to 5 μL or ng’s of analyte for the
average capillary column.
Capillary columns usually require split in injections, a sample reduction
method.
Depending on the spilt ratio (adjustable) only 0.2 to 2 % of the sample injection
makes its way to the column. The rest is discarded.
Split
Splitless
GC Detectors
-Ideal detector characteristics, for flowing systems (e.g. GC)
-large dynamic/linear range
-response independent of flow rate, i.e. fast response times
-Universal detection, responds to all species.
-Keep this in mind when we discuss HPLC and CE.
-Additional requirements for GC
-operation from RT to 400 oC
- detector response independent of detector oven T
Flame Ionization Detector (FID).
-Sensitive towards organics
-Analyte is burned in H2/air, which produces CH and CHO+,
radicals, remember our discussion regarding the blue cone
in AA.
-CHO+ radicals are reduced at a cathode which produces a
current proportional to the radical quantity. About 10-12
amps
-Specific for organic carbon, insensitive to inorganics,
CO2, SO2 etc.
- Response to specific organic depends on the number of
organic carbons.
Electron Capture Detector (ECD)
Sensitive to electron withdrawing groups especially
towards organics containing –F, -Cl, -Br, -I also, -CN, NO2
Nickel-63 source emits energetic electrons collides with
N2 (introduced as make-up gas or can be used as carrier
gas) producing more electrons:
Ni-63  e- + N2  2e- + N2 +
The result is a constant current that is detected by the
electron collector (anode).
GC-MS offers structural determinations
With other detectors identification is possible with retention times of
analyte and standard, however it’s best if another method is used as a
confirmation.