Gas Chromatography Seminar 2005

In gas chromatography this equilibrium is ... electrons cause ionisation of the carrier gas nitrogen. This creates a current between two electrode pla...

0 downloads 181 Views 2MB Size
Gas Chromatography Chromatography Laboratory Course

Dr. Christian Jungnickel

Chromatography Course GC September 2005

The laboratory course experiments  General Aim:

Gain general experience using a GC  Constant Injection technique  Temperature variations  Qualitative and quantitative analysis of a ‘real’ sample. 

Dr. Christian Jungnickel

Chromatography Course GC September 2005

The laboratory course experiments  Test a number of compounds isothermally  Effect of temperature ramp on separation of a

sequence of alkanes  Create a calibration curve for the sequence of alkanes  Determine the concentration of the unknowns in a number of samples

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Principles of gas chromatography  Chromatography - separation method – one

phase is held, and the other moves past it

 Method of separation of substances in gas

chromatography is according to what?

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Retention times are due to?  Different retention times on column

Can be a function of the boiling points of the substances to be separated. Why is this not always true?  Varying sorption coefficients onto the stationary phase inside the column 

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Partitioning  Partitioning is the equilibrium of sorption and

desorption of molecules onto another phase.

cS K= cM  In gas chromatography this equilibrium is

between the gas and liquid phase

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Typical Chromatogram ts tms

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Band spreading

Detector response

45oC

145oC

Time (min) Dr. Christian Jungnickel

Chromatography Course GC September 2005

Plate height and Efficiency  Plate height is a measure of column

efficiency. The smaller the plate height, the narrower the peaks, the better the separation.

H=

σ

2

L

Where σ is the standard deviation of Gaussian band x is the length of the column

 Number of theoretical plates:

L N= H Dr. Christian Jungnickel

The higher the number of plates in a column The better the separation

Chromatography Course GC September 2005

Van Deemeter equation  Van Deemeter equation:

B H = A + + Cu u  It is a summary of effects

that can influence efficiency of a column  In open tubular columns the multiple paths term is zero Dr. Christian Jungnickel

Chromatography Course GC September 2005

Van Deemeter equation  A – known as ‘eddy diffusion’ is a constant

band spreading effect due to the tortuosity of the column (only in packed columns)  B/u – longitudinal diffusion, where B is ~ two times the diffusion coefficient of the solute in the mobile phase  Cu – Equilibrium time of solute with stationary phase

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Van Deemeter equation

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Temperature Programming  Increasing the temperature of the column

increases solute vapour pressure  This decreases the retention time of the substance  It also reduces the flow rate in the column

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Detector response

Temperature Program

Dr. Christian Jungnickel

Chromatography Course GC September 2005

What is the optimal temperature  Temperature influences  Flow rate of carrier gas  Rate of sorption and desorption  Therefore by changing temperature  Peak resolution  And column efficiency are varied  How to determine the optimal temperature? FlowCalc Dr. Christian Jungnickel

Chromatography Course GC September 2005

Basic Gas Chromatograph

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Carrier Gas supply  The carrier gas must be

chemically inert  



Examples include He, Ar, N2, and H2 Choice of gas is dictated by the detector used Efficiency dictated by equilibrium time (Cu)

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Sample Injection System  Responsible for introducing the sample into the

column in a ‘plug’ 

Common method is flash vaporisation

 Sample volumes injected are 0.01 µL to 20 µL.

Sample volume in capillary is much smaller (~10-3 µL)  Oversized samples causes ‘band spreading’ and ‘poor resolution’ 



How is the volume reduced?

 What temperature should the injector be? Dr. Christian Jungnickel

Chromatography Course GC September 2005

Sample Injector including a sample splitter

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Column

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Column Configuration  Types of columns  Packed  Open tubular (capillary)

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Properties of typical columns

Dr. Christian Jungnickel

Chromatography Course GC September 2005

The stationary phase – different phases

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Detectors  Characteristics of the ideal detector

Adequate sensitivity  Good stability and reproducibility  Linear response  Workable temperature range for optimal separation  Short response time  High reliability  Non destructive 

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Flame ionisation detector (FID)  Organics when pyrolysed form ions  The ions when inside a flame can conduct

electricity  A current of several hundred volts is applied across the burner tip, and a collector electrode above the flame.  The measured current is amplified  Selective towards carbon compounds. Ignores contaminants such as nitrogen and sulphur  Requires make up gas. Why? Dr. Christian Jungnickel

Chromatography Course GC September 2005

Flame Ionisation detector

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Thermal conductivity detector  The sensing element is a heated element

whose temperature depends on the thermal conductivity of the surrounding gas.  Helium, and hydrogen have a thermal conductivity roughly six times greater than most organic compounds.  The presence of organic compound therefore causes a marked increase of element temperature Dr. Christian Jungnickel

Chromatography Course GC September 2005

Thermal conductivity detector

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Thermal conductivity detector

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Electron Capture Detector  The sample passes over a β – emitter. The

electrons cause ionisation of the carrier gas nitrogen. This creates a current between two electrode plates. In the presence of organics the electrons are captured, and a decrease in signal is observed.  It is sensitive towards halides, peroxides, and nitro groups.  Highly sensitive, but not always linear. Dr. Christian Jungnickel

Chromatography Course GC September 2005

Atomic Emission Detector  Elemental analysis  Uses a microwave energised helium plasma

which atomises the complete sample, and excites all atoms, such that the emission spectra may be measured.  Coupled with a diode array optical emission spectrometer.

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Atomic Emission Detector

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Hyphenated Analysis Methods  GC-MS  A jet separator is used to split the sample from the carrier gas. The sample is then fed into a mass spectrometer

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Hyphenated Analysis Methods  GC-IR 

The sample is passed through a heated gold pipe through which the laser from the Fourier Transform Infrared spectrometer (FTIR) passes. The characteristic spectra are then compared to a stored library.

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Which detector?  The choice of detector depends on a number

of variables: Sample composition  Concentration of analytes in sample  Carrier gas 

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Kovats Index  Kovats retention index for linear alkanes is

100 times the number of carbon atoms.  The Kovats index for compounds other than alkanes can be calculated  log tr′ (unknown) − log tr′ (n)  I = 100  n + ( N − n)  ′ ′ log t ( N ) − log t ( n )  r r 

 The Kovats index can generally be used to

identify unknown compounds. But only with strict limitations. Dr. Christian Jungnickel

Chromatography Course GC September 2005

Kovats Index

Dr. Christian Jungnickel

Chromatography Course GC September 2005

Preparation for the experiments

 If you have any questions, please ask.

Dr. Christian Jungnickel

Chromatography Course GC September 2005