GAS CHROMATOGRAPHY (GC). GAS CHROMATOGRAPHY Sample is injected (using a syringe) into the injection...

GAS CHROMATOGRAPHY(GC)

He

Carrier Gas

Injectionport

Detector

Column

Recorder-computer

Oven

GAS CHROMATOGRAPHY

Sample is injected (using a syringe) into the injection port.

Sample vaporizes and is forced into the column by the carrier gas ( = mobile phase which in GC is usually helium)

Components of the sample mixture interact with the stationary phase so that different substances take different amounts of time to elute from the column.

The separated components pass through a detector. Electronic signals, collected over time, are sent to the GC software, and a chromatogram is generated.

AA

A

AA

A

A

A

B

B

B

B

B

BB B

Liquid Stationary Phase

B A

A A

A

A

A

A

AA

B BB

B

B

B

BB

Capillary Tube

He Carrier gas

Time0

Immediately after injection

He Carrier gas

After several minutes Resulting chromatogram

Compounds A and B interact with the stationary phase through intermolecular forces: (van der Waals or dipole-dipole forces, including hydrogen bonding).

A interacts more strongly with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A.

Typically, components with similar polarity elute in order of volatility. Thus alkanes elute in order of increasing boiling points; lower boiling alkanes will have shorter retention times than higher boiling alkanes.

GAS CHROMATOGRAPHY

GC - Alkane StandardsPentane

Isooctane

Gas Chromatograph of alkane standard mixture containing equimolar amounts of:

n-hexane, 2,3-dimethylbutane, 3-methyl-pentane, n-heptane, 3-methylhexane, n-octane, 3-methylheptane, 2,2,4-trimethyl-pentane

(= isooctane),

all dissolved in pentane.

GC of Alkane Standards vs. distillation fraction #1

Standards

Distillation Fraction #1

GC – Peak Areas and Resolution

GC – Isothermal vs Temperature Programming

GC – Example Chromatograms

Alltech Chromatography catalog, 550

GC – Packed vs. Capillary Columns

GC – Packed vs. Capillary Columns

Experimental Organic Chemistry D. R. Palleros, Wiley, NY, 2000

GC – Stationary Phase

GC – Stationary Phase

Alltech Chromatography catalog, 550

GC – Elution order vs Stationary Phase

Alltech Chromatography catalog, 550

GC - Derivatization

Why is chemical derivatization needed?

GC is best for separation of volatile compounds which are thermally stable.

Not always applicable for compounds of high molecular weight or containing polar functional groups. These groups are difficult to analyze by GC either because they are not sufficiently volatile, tail badly, are too strongly attracted to the stationary phase, thermally unstable or even decomposed.

Chemical derivatization prior to analysis is generally done to:

• increase the volatility and decrease the polarity of compounds; • reduce thermal degradation of samples by increasing their thermal stability; • increase detector response • improve separation and reduce tailing

Derivatizing Reagents Common derivatization methods can be classified into 4 groups depending on the type of reaction applied:

• Silylation• Acylation• Alkylation• Esterification

GC - Derivatization

Derivatizing Reagents Common derivatization methods can be classified into 4 groups depending on the type of reaction applied:

• Silylation

• Acylation

• Alkylation

• Esterification

Alltech.com

Skoog and Leary: Principals of Instrumental Analysis, 4 th ed. Suanders, 1992

GC – Resolution and Efficiency

GC – Resolution vs Column Efficiency (N, H)GC – Resolution vs Column Efficiency (N, H)

van Deemter Equation H = A + B/u +(Cs + Cm)u

H = L / N

Skoog and Leary: Principals of Instrumental Analysis, 5th ed. Suanders, 1998

CHROMATOGRAPHYPreparative vs Resolution vs Speed vs Expense