1. Introduction to Gas Chromatography

2. A Quick Historical Perspective
Gas Chromatography
A Quick Historical Perspective
1903 Adsorption chromatography
1952 Gas chromatography (GLC)
1968 High performance liquid chromatography (HPLC)

3. HPLC GC Gas Chromatography Mobile phase changes
Constant temperature
Compounds partition from the mobile phase based on solubility.
Elution is generally time or volume dependent
Mobile phase is constant
Increasing temperature
Compounds partition from the mobile phase based on volatility.
Elution is generally temperature dependent

4. Gas Chromatography

5. Gas Chromatography
Sample Injected
Low Temperature
High Temperature

6. Gas Chromatography
Typical Volatile Profile of Strained Carrots (Common Terpenoids Marked in Blue)

7. Sample Preparation Gas Chromatography
How do we obtain a volatile sample from foods?

8. Sample Preparation Gas Chromatography 1) Headspace Methods
2) Distillation Methods
3) Solvent Extraction
4) Solid-Phase Microextraction

9. Gas Chromatography 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.

10. Gas Chromatography 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.

11. Gas Chromatography 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

12. Gas Chromatography Solvent Extraction Separate the solvent containing
the analytes of interest and
concentrate by evaporation
for injection into GC.

13. Gas Chromatography Distillation Methods Produces a dilute aqueous
sample which is not the
best for GC. Sample can
be re-extracted with solvent.
Food
Sample
Sample

14. Gas Chromatography 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.
Probably the easiest method of sample prep.

15. Gas Chromatography
Hardware and Columns

16. Gas Chromatography Mobile Phase: He, Ar, N2, H2
Injection Port: Rubber septum barrier (usually maintained at a higher temperature than the boiling point of the least volatile component in the sample mixture)…why?
Column: (fused silica with a thin coating of stationary phase on the inner surface)
Oven: Thermostat controlled forced air oven
Detector: Many types
Data System

17. Gas Chromatography 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.
Sample is injected through the
septum with a syringe.

18. Gas Chromatography
The oven
Inside here
Column

19. Gas Chromatography 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?

20. Gas Chromatography
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.

21. Gas Chromatography
Gas chromatography separates molecules mostly based on:
Volatility (Oven)
Polarity (Column)

22. Oven Temperature Program
Gas Chromatography
Oven Temperature Program
Increase Temp. Linearly
or Stepwise
Temperature
Time

23. Gas Chromatography COLUMNS GC column nomenclature can be confusing.
Carbowax, DB-1, DB-5, PDMS …..
Columns are often referred to by their polarity, like most things with chromatography. The most non-polar stationary phase is polydimethyl siloxane (PDMS).
Polarity of a column is increased by adding phenyl groups to PDMS (1% = DB-1; 5% = DB-5).
For more polar analytes, polyethylene glycol (carbowax) is used as the stationary phase

24. Gas Chromatography COLUMNS
About a dozen different types in common usage.

25. GC Applications with Food
Gas Chromatography
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