Gas Chromatography. Chromatography Laboratory Course. Dr. Christian Jungnickel Chromatography Course GC September 2005

Transcription

1 Gas Chromatography Chromatography Laboratory Course

2 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.

3 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

4 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?

5 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

6 Partitioning Partitioning is the equilibrium of sorption and desorption of molecules onto another phase. K = In gas chromatography this equilibrium is between the gas and liquid phase c c S M

7 Typical Chromatogram t ms t s

8 Band spreading 45 o C Detector response Time (min) 145 o C

9 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 L 2 Where σ is the standard deviation of Gaussian band x is the length of the column Number of theoretical plates: N = L H The higher the number of plates in a column The better the separation

10 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

11 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

12 Van Deemeter equation

13 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

14 Temperature Program Detector response

15 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

16 Basic Gas Chromatograph

17 Carrier Gas supply The carrier gas must be chemically inert Examples include He, Ar, N 2, and H 2 Choice of gas is dictated by the detector used Efficiency dictated by equilibrium time (Cu)

18 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?

19 Sample Injector including a sample splitter

20 Column

21 Column Configuration Types of columns Packed Open tubular (capillary)

22 Properties of typical columns

23 The stationary phase different phases

24 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

25 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?

26 Flame Ionisation detector

27 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

28 Thermal conductivity detector

29 Thermal conductivity detector

30 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.

31 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.

32 Atomic Emission Detector

33 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

34 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.

35 Which detector? The choice of detector depends on a number of variables: Sample composition Concentration of analytes in sample Carrier gas

36 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 t r ( unknown) log t r ( n) I = 100 n + ( N n) log t r ( N) log t r ( n) The Kovats index can generally be used to identify unknown compounds. But only with strict limitations.

37 Kovats Index

38 Preparation for the experiments If you have any questions, please ask.