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GAS CHROMATOGRAPHY

GAS CHROMATOGRAPHY. COLUMNS. COLUMNS. Open Tubular Columns Increasing Resolution. Decrease tube diameter. Increase resolution. Increase Column Length. Increase resolution. COLUMNS. Open Tubular Columns Increasing Resolution. Increase Stationary Phase Thickness.

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GAS CHROMATOGRAPHY

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  1. GAS CHROMATOGRAPHY

  2. COLUMNS

  3. COLUMNS • Open Tubular Columns • Increasing Resolution Decrease tube diameter Increase resolution Increase Column Length Increase resolution

  4. COLUMNS Open Tubular Columns • Increasing Resolution 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

  5. COLUMNS Choice of liquid stationary phase: • Based on “like dissolves like” • Nonpolar columns for nonpolar solutes • Strongly polar columns for strongly polar compounds • To reduce “bleeding” of stationary phase: • bond (covalently attached) to silica • Covalently cross-link to itself

  6. COLUMNS Packed Columns • Greater sample capacity • Broader peaks, longer retention times and less resolution • Improve resolution by using small, uniform particle sizes Open tubular column Packed column

  7. COLUMNS 500 L chromatography column • Packed Columns • The major advantage and use is for large-scale or preparative purification • Industrial scale purification maybe in the kilogram or greater range Oil refinery – separates fractions of oil for petroleum products

  8. COLUMN TEMREATURE(OVEN)

  9. 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.)

  10. 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.

  11. 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."

  12. 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.

  13. ISOTHERMAL TEMPERATURE: best only if the boiling point range of the sample is narrow • FOR CASE OF WIDE BOILING POINT RANGE: • Low isothermal column temperature • Low boiling fractions are well resolved but the high boiling fractions are slow to elute with extensive band broadening • High isothermal column temperature • higher boiling components elute as sharp peaks but the lower boiling components elute so quickly there is no separation.  The effect of column temperature on the shape of the peaks. 

  14. OVEN Multi-ramp temperature program Single-ramp temperature program

  15. Van Deemter H = A + B/û + C*û where: 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

  16. 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

  17. EDDY DIFFUSION • Smaller particles of uniform spherical shape result in low values of term A • For open tubular columns, this term is zero

  18. 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.

  19. ( 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

  20. 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.

  21. Van Deemter • H is minimum for a specific value of u • H vs. u is the “Van Deemter Plot”

  22. Van Deemter Plot

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