gas chromatography

• http://www.studyhplc.com/chromatographyanimation.php

http://www.wooster.edu/chemistry/analytical/gc/default.html

Major Components

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/ + C

Remember, = flow rate,A = multiple paths, B/ = longitudinal diffusion effects, C = MT effectsWe 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 non-polar 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, byincreasing the sample “plug”.

Injection port temperature is usually held 50oC higher than the BP of the least volatile compounds.

GC injection and band broadeningand anomalies.

Extremely slow injections will cause band-broadening, 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 aconfirmation.