Gas Chromatography

Introduction

1.) Gas Chromatography Mobile phase (carrier gas) is a gas

- Usually N2, He, Ar and maybe H2

- Mobile phase in liquid chromatography is a liquid

Requires analyte to be either naturally volatile or can be converted to a volatile

derivative- GC useful in the separation of small organic and inorganic compounds

Stationary phase:- Gas-liquid partition chromatography – nonvolatile liquid bonded to solid support

- Gas-solid chromatography – underivatized solid particles

- Bonded phase gas chromatography – chemical layer chemically bonded to solid

support

Magnified Pores in activated carbon Zeolite molecular sieveBonded phase

Gas Chromatography

Introduction

2.) Instrumentation Process:

- Volatile liquid or gas injected through septum into heated port

- Sample rapidly evaporates and is pulled through the column with carrier gas

- Column is heated to provide sufficient vapor pressure to elute analytes

- Separated analytes flow through a heated detector for observation

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

B

B

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

Sample Injection

System

• For quantitative work, more

reproducible sample sizes for

both liquids and gases are

obtained by means of a rotary

sample valve.

• Errors due to sample size can be

reduced to 0.5% to 2% relative.

• The sampling loop is filled by

injection of an excess of sample.

• Rotation of the valve by 45 deg

then introduces the reproducible

volume ACB into the mobile

phase.

Column Configurations

• Two general types of columns are encountered in gas chromatography, packed and

open tubular, or capillary.

• Chromatographic columns vary in length from less than 2 m to 50 m or more. They are

constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven

for thermostating, they are usually formed as coils having diameters of 10 to 30 cm.

• Column temperature is an important variable that must be controlled to a few tenths of

a degree for precise work. Thus, the column is ordinarily housed in a thermostated

oven. The optimum column temperature depends upon the boiling point of the sample

and the degree of separation required.

Gas Chromatography

Instrumentation

1.) Open Tubular Columns Commonly used in GC

Higher resolution, shorter analysis time, and greater sensitivity

Low sample capacity

Increasing Resolution- Narrow columns Increase resolution

- Resolution depends directly from column length

Easy to generate long (10s of meters)

lengths of narrow columns to maximize

resolution

Gas Chromatography

Instrumentation

1.) Open Tubular Columns Increasing Resolution

Decrease tube diameter

Increase resolution

Increase Column Length

Increase resolution

Gas Chromatography

Increase Stationary Phase Thickness

Increase resolution of early eluting compounds

Also, increase in

capacity factor and

reduce peak tailing

But also decreases

stability of stationary

phase

Instrumentation

1.) Open Tubular Columns Increasing Resolution

Gas Chromatography

Instrumentation

2.) Choice of liquid stationary

phase: Based on “like dissolves

like”

Nonpolar columns for

nonpolar solutes

Strongly polar columns for

strongly polar compounds

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

GC – Stationary Phase

Gas Chromatography

Instrumentation

3.) Packed Columns Greater sample capacity

Broader peaks, longer retention times and less resolution- Improve resolution by using small, uniform particle sizes

Packed columnOpen tubular column

Gas Chromatography

Retention Index

1.) Retention Time Order of elution is mainly determined by volatility

- Least volatile = most retained

- Polar compounds (ex: alcohols) are the least volatile and will be the most

retained on the GC system

Second factor is similarity in polarity between compound and stationary

phase

Gas Chromatography

Retention Index

2.) Describing Column Performance Can manipulate or adjust retention time by changing polarity of stationary

phase

Can use these retention time differences to classify or rate column

performance- Compare relative retention times between compounds and how they

change between columns

Can be used to identify unknowns

Gas Chromatography

Temperature and Pressure Programming

1.) Improving Column Efficiency Temperature programming:

- Temperature is raised

during the separation

(gradient)

- increases solute vapor

pressure and decrease

retention time

Temperature gradient improves

resolution while also decreasing

retention time

Gas Chromatography

Temperature and Pressure Programming

1.) Improving Column Efficiency Pressure Programming:

- Increase pressure increases flow of mobile phase (carrier gas)

- Increase flow decrease retention time

Pressure is rapidly reduced at the end of the run- Time is not wasted waiting for the column to cool

- Useful for analytes that decompose at high temperatures

Van Deemter curves indicate

that column efficiency is

related to flow rate

Flow rate increases N2 < He < H2

Detection Systems

Characteristics of the Ideal Detector:

The ideal detector for gas chromatography has the following characteristics:

1. Adequate sensitivity

2. Good stability and reproducibility.

3. A linear response to solutes that extends over several orders of magnitude.

4. A temperature range from room temperature to at least 400oC.

5. A short response time that is independent of flow rate.

6. High reliability and ease of use.

7. Similarity in response toward all solutes or a highly selective response toward one or

more classes of solutes.

8. Nondestructive of sample.

Flame Ionization Detectors (FID)

The flame ionization detector is the most widely used and generally applicable

detector for gas chromatography.

• The effluent from the column is mixed with hydrogen and air and then ignited

electrically.

• Most organic compounds, when pyrolyzed at the temperature of a hydrogen/air

flame, produce ions and electrons that can conduct electricity through the flame.

• A potential of a few hundred volts is applied.

• The resulting current (~10-12 A) is then measured.

• The flame ionization detector exhibits a high sensitivity (~10-13 g/s), large linear

response range (~107), and low noise.

• A disadvantage of the flame ionization detector is that it is destructive of sample.

GC-MS

GC-MS

Gas Chromatography

1.) Qualitative and Quantitative Analysis

Compare retention times between reference sample and unknown- Use multiple columns with different stationary phases

- Co-elute the known and unknown and measure changes in peak area

The area of a peak is proportional to the quantity of that compound

210641 wtpeak heigh. peak Gaussian of Area

Peak A

rea

Concentration of Standard

Peak area increases proportional

to concentration of standard if

unknown/standard have the

identical retention time same

compound

GC – Peak Areas and Resolution

Method Development

Basic parameteres

• Sample injection (split- splitless)

• Column selection (packed or capillary column)

• Selection of gradient or isothermal program

• Detector selection

Inlet temperature, detector temperature, column temperature and

temperature program, carrier gas and carrier gas flow rates, the

column's stationary phase, diameter and length, inlet type and flow

rates, sample size and injection technique

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

The Retention Index / Kovats index

The retention index I was first proposed by Kovats for identifying solutes from

chromatograms. The retention index for any given solute can be derived from a

chromatogram of a mixture of that solute with at least two normal alkanes having

retention times that bracket that of the solute. That is, normal alkanes are the standards

upon which the retention index scale is based. The retention index for a normal alkane

is equal to 100 times the number of carbons in the compound regardless of the column

packing, the temperature, or other chromatographic conditions. Within a homologous

series, a plot of the logarithm of adjusted retention time t`R (t`R = tR - t`M) versus the

number of carbon atoms is linear.

I = Kovats retention index,

n = the number of carbon atoms in the smaller

alkane,

N = the number of carbon atoms in the larger

alkane,

tr' = the adjusted retention time.

The Retention

Index / Kovats

index

CHROMATOGRAPHY

Preparative vs Resolution vs Speed vs Expense