10-04-2017, 09:00 PM
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Gas Chromatography
A Quick Historical Perspective
1903 Adsorption chromatography
1952 Gas chromatography (GLC)
1968 High performance liquid chromatography (HPLC)
Gas Chromatography
HPLC
Mobile phase changes
Constant temperature
Compounds partition from the mobile phase based on solubility.
Elution is generally time or volume dependent
GC
Mobile phase is constant
Increasing temperature
Compounds partition from the mobile phase based on volatility.
Elution is generally temperature dependent
Sample Preparation
1) Headspace Methods
2) Distillation Methods
3) Solvent Extraction
4) Solid-Phase Microextraction
Direct Headspace
In a closed container, take a sample of the air above the
sample and inject it.
While direct injection of gasses can be used, many volatile
analytes are often in too low of a concentration to
be quantified using most common detectors.
Dynamic Headspace
Typically the sample is placed in a closed container and a flow
of inert gas such as nitrogen is used to purge the headspace
onto an adsorbent or a cryogenic trap.
The trap can be eluted with solvent, the
solvent can be concentrated and injected.
Solvent Extraction
Blend sample and mix with non-miscible solvent. (a solvent that doesn t mix with water). Centrifuge to unmix solvent and food sample Separate the solvent containing the analytes of interest and concentrate by evaporation for injection into GC.
Distillation Methods
Produces a dilute aqueous
sample which is not the
best for GC. Sample can
be re-extracted with solvent.
Solid Phase Microextraction
Sampling devices (manual and autosampler)
consist of a coated silica fiber inside a
hollow needle. The coating on the fiber
adsorbs and concentrates volatiles from
the headspace inside the sample container.
The needle protects the fiber which can be
inserted through the septa of the GC injector
for direct analysis.
Split/Splitless Injector
Splitless Injection,
(where the split vent is closed)
attempts to transfer all of the
sample to the column and is
used for trace analysis.
Split Mode,
only a small portion
(maybe 1-10% of the sample
moves into the column, and
the rest is sent to waste. This
is used when the analytes are
in high concentration and would
overload the column.
Question:
The injection port is at a HIGH temperature
250oC is common.
What happens when the hot, volatized sample hits a cold (50oC) GC column?
The volatile compounds condense at the front end of the column. Raising the temperature of the column allows for the separation of the compounds as they boil away at different temperatures.
If two compounds have the same volatility, the compound with the least affinity for the stationary phase will volatilize first.
GC Applications with Food
Analysis of foods is concerned with the assay of lipids, proteins, carbohydrates, preservatives, flavors, colorants, vitamins, steroids, drugs, and pesticide residues.
Most of the components are non-volatile (thus the use of HPLC) but with modification, GC can be effectively used.
Derivatization of lipids and fatty acid to their methyl esters
Proteins are acid hydrolyed followed by esterification (N-propyl esters)
Carbohydrates derivatized by silylation to produce a volatile compound