GAS CHROMATOGRAPHY

CONTENTS

HISTORY PRINCIPLE INSRTUMENTATION GASES INJECTORS COLUMNS DETECTORS HEADSPACE TECHNIQUE ADDITION METHOD

HISTORY

Chromatography dates to 1903 in the work of the Russian scientist, Mikhail SemenovichTswett. German graduate student Fritz Priordeveloped solid state gas chromatography in 1947. Archer John Porter Martin, who was awarded the Nobel Prize for his work in developing liquid-liquid (1941) and paper (1944) chromatography, laid the foundation for the development of gas chromatography and later produced liquid-gas chromatography (1950).

PRINCIPLEGas chromatography - specifically gas-

liquid chromatography - involves a sample being vapourised and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.

PRINCIPLE

The gaseous compounds being analyzed interact with the walls of the column, which is coated with different stationary phases. This causes each compound to elute at a different time, known as the retention timeof the compound. The comparison of retention times is what gives GC its analytical usefulness.

PRINCIPLE

Gas chromatography is in principle similar to column chromatography (as well as other forms of chromatography, such as HPLC, TLC), but has several notable differences. Firstly, the process of separating the compounds in a mixture is carried out between a liquid stationary phase and a gas moving phase, whereas in column chromatography the stationary phase is a solid and the moving phase is a liquid.

PRINCIPLE (Hence the full name of the procedure is "Gas-

liquid chromatography", referring to the mobile and stationary phases, respectively.) Secondly, the column through which the gas phase passes is located in an oven where the temperature of the gas can be controlled, whereas column chromatography (typically) has no such temperature control. Thirdly, the concentration of a compound in the gas phase is solely a functionof the vapor pressure of the gas.[1]

PRINCIPLE Gas chromatography is also similar to fractional

distillation, since both processes separate the components of a mixture primarily based on boiling point(or vapor pressure) differences. However, fractional distillation is typically used to separate components of a mixture on a large scale, whereas GC can be used on a much smaller scale (i.e. microscale).[1]

Gas chromatography is also sometimes known as vapor-phase chromatography (VPC), or gas-liquid partition chromatography (GLPC). These alternative names, as well as their respective abbreviations, are frequently found in scientific literature. Strictly speaking, GLPC is the most correct terminology, and is thus preferred by many authors

The chromatographic process

Two different substances are partitioned between two phases. Depending on their affinity (toward the stationary phase) will spent different times adsorbed by the stationary phase.

Sample Injected

Low Temperature

High Temperature

Factors affecting separation

Boiling points of components present in the sample

Length of the column Flow rate of carrier gas Intermolecular interactions

Orientation interactions• The interactions between two permanent dipoles

Debye interactions• The interactions between a permanent dipole in one

molecule and the induced dipole in a neighboring molecule Dispersion interactions

• The forces arising out of synchronized variations in the instantaneous dipole of the two interaction species

Determination of Retention Time

Since Velocity = DistanceTime

Retention Time = Dist(cm)Vel(cm/min)

Dist = Distance chart moves in cmVel = Velocity of chart in cm/min

Starting PointOn Chart

Distance

Schematic diagram of a gas chromatograph:

GASES

The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.

CARRIER GAS

Common gases--- Helium, Hydrogen, Nitrogen, argon

Requirements-----1) Suitable for particular detector in use. 2) Inert3) Dry and pure4) Free from organic impurities5) Regulated

TYPES OF INJECTORS

1) Packed column injectors

2) Split / splitless injectors

3) On column Capillary injectors

4) Programmable split/splitless injectors

Split / splitless injectors

For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapour - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a microsyringe is used to inject sample through a rubber septum into a flash vapouriser port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/splitless injection is used. Have a look at this diagram of a split/splitless injector;

Split / splitless injectorsThe injector can be used in one of two modes; split or splitless. The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). The sample vapourises to form a mixture of carrier gas, vapourised solvent and vapourisedsolutes. A proportion of this mixture passes onto the column, but most exits through the split outlet. The septum purge outlet prevents septum bleed components from entering the column

Schematic diagram

Split vs splitless!

Split injection!

URL

COLUMNS

There are two general types of column, packedand capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumental_Analysis/Chromatography/Gas_Chromatography

COLUMNS Capillary columns have an internal diameter of a few

tenths of a millimeter. Two types

Wall-coated open tubular (WCOT) • consist of a capillary tube whose walls are coated with liquid stationary

phase. Support-coated open tubular (SCOT)

• inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed.

SCOT columns are generally less efficient than WCOT columns.

Both types of capillary column are more efficient than packed columns.

COLUMNS

COLUMNS In 1979, a new type of WCOT column was devised -

the Fused Silica Open Tubular (FSOT) column; aka fused-silica wall-coated (FSWC) column

the walls of the fused-silica columns are drawn from purified silica containing minimal metal oxides. much thinner than glass columns, with diameters as small

as 0.1 mm and lengths as long as 100 m. Polyimide coating gives strength.

flexible and can be wound into coils. commercially available and currently replacing older

columns due to increased chemical inertness, greater column efficiency and smaller sampling size requirements.

COLUMNS Cross-linking of phase

covalently attach stationary phase to the solid support material

Helps to avoid column bleeding in GLC Bonded phases are prepared by reacting the

desired phase with a silica-surface Advantages:

more stable than coated liquid phases can be placed on support with thinner and more

uniform thickness than liquid phases

COLUMNS

COLUMNS

Computer Generated Image of a FSWC column

Computer Generated Image of a FSWC column

(specialized to withstand extreme heat, also acidic

and basic samples)

COLUMNS

COLUMN TEMREATURE(OVEN)

A gas chromatography oven, open to show a capillary column

The column(s) in a GC are contained in an oven, the temperature of which is precisely controlled electronically. (When discussing the "temperature of the column," an analyst is technically referring to the temperature of the column oven. The distinction, however, is not important and will not subsequently be made in this article.)

OVEN

The rate at which a sample passes through the column is directly proportional to the temperature of the column. The higher the column temperature, the faster the sample moves through the column. However, the faster a sample moves through the column, the less it interacts with the stationary phase, and the less the analytes are separated.

OVEN

In general, the column temperature is selected to compromise between the length of the analysis and the level of separation.

A method which holds the column at the same temperature for the entire analysis is called "isothermal." Most methods, however, increase the column temperature during the analysis, the initial temperature, rate of temperature increase (the temperature "ramp") and final temperature is called the "temperature program."

OVEN

A temperature program allows analytes that elute early in the analysis to separate adequately, while shortening the time it takes for late-eluting analytes to pass through the column.

Van Deemter

H = A + B/û + C*ûwhere:

H = height of the theoretical plate

A = Eddy diffusion term (packed column only)

B = Longitudinal band broadening

C = Resistance to mass transfer

u = Average linear gas velocity

EDDY DIFFUSION-(A term) Analyte molecules follow different pathways around the particles of

the stationary phase, some shorter and some longer.

These variations cause residence time of gas molecules to vary giving rise to broadening

Broadening depends on the particle size and geometrical packing factor

EDDY DIFFUSION

Smaller particles of uniform spherical shape result in low values of term A

For open tubular columns, this term is zero

MOLECULAR DIFFUSIONTerm (B)

Molecules of any analyte dissolved in a fluid will diffuse in all the directions with time

If along the axis of the column ---Axial spreading

Extent of spreading is dependent on :- a)- coefficient of diffusion of analyte in MP b)- the total time the sample is in MP.

( Term RESISTANCE TO MASS TRANSFER C)

Transfer of molecules of the analyte can occur only at interface between the two phases, to mantain distribution ratio

Both phases have a finite thickness At the front edge of the peak the M.P. is rich in analyte

and the stationary phase is deficient

RESISTANCE TO MASS TRANSFER ( Term C)

The extent of broadening depends on diffusion rates of analyte in the two phases

Diffusion is time dependent and the broadening will be worsened if the flow rate of the M.P. increases

A compromise between the values for A,B and C is required to achieve optimum column efficiency.

Van Deemter

•H is minimum for a specific value of u

•H vs. u is the “Van Deemter Plot”

Van Deemter Plot