Gas Chromatography - ?‘›...  Gas Chromatography Introduction 1.) Gas...

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Transcript of Gas Chromatography - ?‘›...  Gas Chromatography Introduction 1.) Gas...

  • 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

  • A A

    A

    AA

    A

    A

    A

    B

    B

    B

    B

    B

    B B B

    Liquid Stationary Phase

    B A

    A A

    A

    A

    A

    A

    A A

    B B B

    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 column Open 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

    2 10641 wtpeak heigh. peak Gaussian of Area

    P e a k A

    re a

    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 o