Gas Chromatography Gas chromatography is a technique used - PowerPoint PPT Presentation

1 / 80
About This Presentation

Gas Chromatography Gas chromatography is a technique used


Gas Chromatography Gas chromatography is a technique used for separation of volatile substances, or substances that can be made volatile, from one another in a ... – PowerPoint PPT presentation

Number of Views:1393
Avg rating:3.0/5.0
Slides: 81
Provided by: monzirpal


Transcript and Presenter's Notes

Title: Gas Chromatography Gas chromatography is a technique used

Gas Chromatography
  • Gas chromatography is a technique used for
    separation of volatile substances, or substances
    that can be made volatile, from one another in a
    gaseous mixture at high temperatures. A sample
    containing the materials to be separated is
    injected into the gas chromatograph. A mobile
    phase (carrier gas) moves through a column that
    contains a wall coated or granular solid coated
    stationary phase. As the carrier gas flows
    through the column, the components of the sample
    come in contact with the stationary phase. The
    different components of the sample have different
    affinities for the stationary phase, which
    results in differential migration of solutes,
    thus leading to separation

  • Martin and James introduced this separation
    technique in 1952, which is the latest of the
    major chromatograhpic techniques. However, by
    1965 over 18000 publications in gas
    chromatography (GC) were available in the
    literature. This is because optimized
    instrumentation was feasible. Gas chromatography
    is good only for volatile compounds or those,
    which can be made volatile by suitable
    derivatization methods or pyrolysis. Thus, about
    20 of chemicals available can be analyzed
    directly by GC.

  • Gas chromatography can be used for both
    qualitative and quantitative analysis.
    Comparison of retention times can be used to
    identify materials in the sample by comparing
    retention times of peaks in a sample to retention
    times for standards. The same limitations for
    qualitative analysis discussed in Chapter 26 also
    apply for separations in GC. Quantitative
    analysis is accomplished by measurement of either
    peak height or peak area

Gas - Solid Chromatography (GSC)
  • The stationary phase, in this case, is a solid
    like silica or alumina. It is the affinity of
    solutes towards adsorption onto the stationary
    phase which determines, in part, the retention
    time. The mobile phase is, of course, a suitable
    carrier gas. This gas chromatographic technique
    is most useful for the separation and analysis of
    gases like CH4, CO2, CO, ... etc. The use of GSC
    in practice is considered marginal when compared
    to gas liquid chromatography.

Gas - Liquid Chromatography (GLC)
  • The stationary phase is a liquid with very low
    volatility while the mobile phase is a suitable
    carrier gas. GLC is the most widely used
    technique for separation of volatile species.
    The presence of a wide variety of stationary
    phases with contrasting selectivities and easy
    column preparation add to the assets of GLC or
    simply GC.

  • It may be wise to introduce instrumental
    components before proceeding further in
    theoretical background. This will help clarify
    many points, which may, otherwise, seem vague. It
    should also be noted that a detector will require
    special gas cylinders depending on the detector
    type utilized. The column temperature controller
    is simply an oven, the temperature of which can
    be varied or programmed

(No Transcript)
  • Three temperature zones can be identified
  • Injector temperature, TI, where TI should allow
    flash vaporization of all sample components.
  • Column temperature, Tc, which is adjusted as the
    average boiling points of sample components.
  • Detector Temperature, TD, which should exclude
    any possible condensation inside the detector.
  • Generally, an intuitive equation can be used to
    adjust all three zones depending on the average
    boiling point of the sample components. This
    equation is formulated as
  • TI TD Tc 50 oC

  • The Carrier Gas
  • Unlike liquid chromatography where wide varieties
    of mobile phase compositions are possible, mobile
    phases in gas chromatography are very limited.
    Only slight changes between carrier gases can be
    identified which places real limitations to
    chromatographic enhancement by change or
    modification of carrier gases

  • A carrier gas should have the following
  • Highly pure (gt 99.9)
  • Inert so that no reaction with stationary phase
    or instrumental components can take place,
    especially at high temperatures.
  • A higher density (larger viscosity) carrier gas
    is preferred.
  • Compatible with the detector since some detectors
    require the use of a specific carrier gas.
  • A cheap and available carrier gas is an advantage.

Longitudinal Diffusion Term
  • This is an important factor contributing to band
    broadening which is a function of the diffusivity
    of the solute in the gaseous mobile phase as well
    as the molecular diffusion of the carrier gas
  • HL K DM /V
  • Where DM is the diffusion coefficient of solute
    in the carrier gas. This term can be minimized
    when mobile phases of low diffusion, i.e. high
    density, are used in conjunction with higher flow

  • The same van Deemter equation as in LC can be
    written for GC where
  • H A B/V CV
  • The optimum carrier gas velocity is given by the
    derivative of van Deemter equation
  • Vopt B/C 1/2
  • However, the obtained velocity is much greater
    than that obtained in LC.

  • The carrier gas pressure ranges from 10-50 psi.
    Higher pressures potentially increase compression
    possibility while very low pressures result in
    large band broadening due to diffusion. Depending
    on the column dimensions, flow rates from 1-150
    mL/min are reported. Conventional analytical
    columns (1/8) usually use flow rates in the
    range from 20-50 mL/min while capillary columns
    use flow rates from 1-5 mL/min depending on the
    dimensions and nature of column. In most cases, a
    selection between helium and nitrogen is made as
    these two gases are the most versatile and common
    carrier gases in GC.

  • Septum type injectors are the most common. These
    are composed of a glass tube where vaporization
    of the sample takes place. The sample is
    introduced into the injector through a
    self-sealing silicone rubber septum. The carrier
    gas flows through the injector carrying vaporized
    solutes. The temperature of the injector should
    be adjusted so that flash vaporization of all
    solutes occurs. If the temperature of the
    injector is not high enough (at least 50 degrees
    above highest boiling component), band broadening
    will take place.

(No Transcript)
Column Configurations and Ovens
  • The column in chromatography is undoubtedly the
    heart of the technique. A column can either be a
    packed or open tubular. Traditionally, packed
    columns were most common but fast developments in
    open tubular techniques and reported advantages
    in terms of efficiency and speed may make open
    tubular columns the best choice in the near
    future. Packed columns are relatively short
    (2meters) while open tubular columns may be as
    long as 30-100 meters

  • Packed columns are made of stainless steel or
    glass while open tubular columns are usually made
    of fused silica. The temperature of the column is
    adjusted so that it is close to the average
    boiling point of the sample mixture. However,
    temperature programming is used very often to
    achieve better separations. The temperature of
    the column is assumed to be the same as the oven
    which houses the column. The oven temperature
    should be stable and easily changed in order to
    obtain reproducible results.

Detection Systems
  • Several detectors are available for use in GC.
    Each detector has its own characteristics and
    features as well as drawbacks. Properties of an
    ideal detector include
  • High sensitivity
  • Minimum drift
  • Wide dynamic range
  • Operational temperatures up to 400 oC.
  • Fast response time
  • Same response factor for all solutes
  • Good reliability (no fooling)
  • Nondestructive
  • Responds to all solutes (universal)

a. Thermal Conductivity Detector (TCD)
  • This is a nondestructive detector which is used
    for the separation and collection of solutes to
    further perform some other experiments on each
    purely separated component. The heart of the
    detector is a heated filament which is cooled by
    helium carrier gas. Any solute passes across the
    filament will not cool it as much as helium does
    because helium has the highest thermal
    conductivity. This results in an increase in the
    temperature of the filament which is related to
    concentration. The detector is simple,
    nondestructive, and universal but is not very
    sensitive and is flow rate sensitive.

(No Transcript)
(No Transcript)
  • Note that gases should always be flowing through
    the detector including just before, and few
    minutes after, the operation of the detector.
    Otherwise, the filament will melt. Also, keep
    away any oxygen since oxygen will oxidize the
    filament and results in its destruction.
  • Remember that TCD characteristics include
  • Rugged
  • Wide dynamic range (105)
  • Nondestructive
  • Insensitive (10-8 g/s)
  • Flow rate sensitive

b. Flame Ionization Detector (FID)
  • This is one of the most sensitive and reliable
    destructive detectors. Separate two gas
    cylinders, one for fuel and the other for O2 or
    air are used in the ignition of the flame of the
    FID. The fuel is usually hydrogen gas. The flow
    rate of air and hydrogen should be carefully
    adjusted in order to successfully ignite the

(No Transcript)
(No Transcript)
  • The FID detector is a mass sensitive detector
    where solutes are ionized in the flame and
    electrons emitted are attracted by a positive
    electrode, where a current is obtained.
  • The FID detector is not responsive to air, water,
    carbon disulfide. This is an extremely important
    advantage where volatile solutes present in water
    matrix can be easily analyzed without any

  • Remember that FID characteristics include
  • Rugged
  • Sensitive (10-13 g/s)
  • Wide dynamic range (107)
  • Signal depends on number of carbon atoms in
    organic analytes which is referred to as mass
    sensitive rather than concentration sensitive
  • Weakly sensitive to carbonyl, amine, alcohol,
    amine groups
  • Not sensitive to non-combustibles H2O, CO2,
    SO2, NOx
  • Destructive

Electron Capture Detector (ECD)
  • This detector exhibits high intensity for halogen
    containing compounds and thus has found wide
    applications in the detection of pesticides and
    polychlorinated biphenyls. The mechanism of
    sensing relies on the fact that electronegative
    atoms, like halogens, will capture electrons from
    a b emitter (usually 63Ni). In absence of
    halogenated compounds, a high current signal will
    be recorded due to high ionization of the carrier
    gas, which is N2, while in presence of
    halogenated compounds the signal will decrease
    due to lower ionization.

(No Transcript)
  • Remember the following facts about ECD
  • 1. Electrons from a b-source ionize the carrier
    gas (nitrogen)
  • 2. Organic molecules containing electronegative
    atoms capture electrons and decrease current
  • 3. Simple and reliable
  • 4. Sensitive (10-15 g/s) to electronegative
    groups (halogens)
  • 5. Largely non-destructive
  • 6. Insensitive to amines, alcohols and
  • 7. Limited dynamic range (102)
  • 8. Mass sensitive detector

Gas Chromatographic Columns and Stationary Phases
  • Packed Columns
  • These columns are fabricated from glass,
    stainless steel, copper, or other suitable tubes.
    Stainless steel is the most common tubing used
    with internal diameters from 1-4 mm. The column
    is packed with finely divided particles (lt100-300
    mm diameter), which is coated with stationary
    phase. However, glass tubes are also used for
    large-scale separations.

  • Several types of tubing were used ranging from
    copper, stainless steel, aluminum and glass.
    Stainless steel is the most widely used because
    it is most inert and easy to work with. The
    column diameters currently in use are ordinarily
    1/16" to 1/4" 0.D. Columns exceeding 1/8" are
    usually used for preparative work while the 1/8"
    or narrower columns have excellent working
    properties and yield excellent results in the
    analytical range. These find excellent and wide
    use because of easy packing and good routine
    separation characteristics. Column length can be
    from few feet for packed columns to more than 100
    ft for capillary columns.

Capillary/Open Tubular
  • Open tubular or capillary columns are finding
    broad applications. These are mainly of two
  • Wall-coated open tubular (WCOT) lt1 mm thick
    liquid coating on inside of silica tube
  • Support-coated open tubular (SCOT) 30 mm thick
    coating of liquid coated support on inside of
    silica tube
  • These are used for fast and efficient separations
    but are good only for small samples. The most
    frequently used capillary column, nowadays, is
    the fused silica open tubular column (FSOT),
    which is a WCOT column.

  • The external surface of the fused silica columns
    is coated with a polyimide film to increase their
    strength. The most frequently used internal
    diameters occur in the range from 260-320
    micrometer. However, other larger diameters are
    known where a 530 micrometer fused silica open
    tubular column was recently made and is called a
    megapore column, to distinguish it from other
    capillary columns. Megapore columns tolerate a
    larger sample size.

(No Transcript)
(No Transcript)
(No Transcript)
(No Transcript)
  • It should be noted that since capillary columns
    are not packed with any solid support, but rather
    a very thin film of stationary phase which
    adheres to the internal surface of the tubing,
    the A term in the van Deemter equation which
    stands for multiple path effects is zero and the
    equation for capillary columns becomes
  • H B/V CV

  • Capillary columns advantages compared to packed
  • higher resolution
  • shorter analysis times
  • greater sensitivity
  • Capillary columns disadvantage compared to packed
  • smaller sample capacity

Solid Support Materials
  • The solid support should ideally have the
    following properties
  • Large surface area (at least 1 m2/g)
  • Has a good mechanical stability
  • Thermally stable
  • Inert surface in order to simplify retention
    behavior and prevent solute adsorption
  • Has a particle size in the range from 100-400 mm

Selection of Stationary Phases
  • General properties of a good liquid stationary
    phase are easy to guess where inertness towards
    solutes is essential. Very low volatility
    liquids that have good absolute and differential
    solubilities for analytes are required for
    successful separations. An additional factor
    that influences the performance of a stationary
    phase is its thermal stability where a stationary
    phase should be thermally stable in order to
    obtain reproducible results. Nonvolatile liquids
    assure minimum bleeding of the stationary phase

Weight of liquid stationary phase 100
  • Loading
  • Increasing percent loading would allow for
    increased sample capacity and cover any active
    sites on the solid support. These two advantages
    are very important, however increasing the
    thickness of stationary phase will affect the C
    term in the van Deemter equation by increasing
    HS, and therefore Ht.

Weight of stationary phase plus solid support
  • Generally, the film thickness primarily affects
    the retention character and the sample capacity
    of a column. Thick films are used with highly
    volatile analytes, because such films retain
    solutes for a longer time and thus provide a
    greater time for separation to take place. Thin
    films are useful for separating species of low
    volatility in a reasonable time. On the other
    hand, a thicker film can tolerate a larger sample
    size. Film thicknesses in the range from 0.1 5
    mm are common.

Liquid Stationary Phases
  • In general, the polarity of the stationary phase
    should match that of the sample constituents
    ("like" dissolves "like"). Most stationary phases
    are based on polydimethylsiloxane or polyethylene
    glycol (PEG) backbones

  • The polarity of the stationary phase can be
    changed by derivatization with different
    functional groups such as a phenyl group.
    Bleeding of the column is cured by bonding the
    stationary phase to the column or crosslinking
    the stationary phase.
  • Liquid Stationary Phases should have the
    following characteristics
  • Low volatility
  • High decomposition temperature (thermally
  • Chemically inert (reversible interactions with
  • Chemically attached to support (to prevent
  • Appropriate k' and a for good resolution

Bonded and Crosslinked Stationary Phases
  • The purpose of bonding and cross-linking is to
    prevent bleeding and provide a stable stationary
    phase. With use at high temperatures, stationary
    phases that are not chemically bonded or
    crosslinked slowly lose their stationary phase
    due to bleeding in which a small amount of the
    physically immobilized liquid is carried out of
    the column during the elution process.
    Crosslinking is carried out in situ after a
    column is coated with one of the polymers

(No Transcript)
  • In summary, stationary phases are usually bonded
    and/or crosslinked and the following remarks are
    usually helpful
  • 1. Bonding occurs through covalent linking of
    stationary phase to support
  • 2. Crosslinking occurs through polymerization
    reactions to join individual stationary phase
  • 3. Nonpolar stationary phases are best for
    nonpolar analytes where nonpolar analytes are
    retained preferentially
  • 4. Polar stationary phases are best for polar
    analytes where polar analytes are retained

Gas-liquid chromatography (GLC)
  • Packed columns are fabricated from glass, metal,
    or Teflon with 1 to 3 m length and 2 to 4 mm in
    internal diameter. The column is packed with a
    solid support (100-400 mm particle diameter made
    from diatomaceous earth) that has been coated
    with a thin layer (0.1-5 mm) of the stationary
    liquid phase. Efficiency increases with
    decreasing particle size as predicted from van
    Deemter equation. The retention is based on
    absorption of analyte (partition into the liquid
    stationary phase) where solutes must have
    differential solubility in the stationary phase

  • Open tubular capillary columns, either WCOT, SCOT
    are routinely used. In WCOT the capillary is
    coated with a thin film (0.1-0.25 mm) of the
    liquid stationary phase while in SCOT a thin film
    of solid support material is first affixed to the
    inner surface of the column then the support is
    coated with the stationary phase. WCOT columns
    are most widely used. Capillary columns are
    typically made from fused silica (FSOT) and are
    15 to 100 m long with 0.10 to 0.5 mm i.d.

  • The thickness of the stationary phase affects the
    performance of the column as follows
  • Increasing thickness of stationary phase allows
    the separation of larger sample sizes.
  • Increasing thickness of stationary phase reduces
    efficiency since HS increases.
  • Increasing thickness of stationary phase is
    better for separation of highly volatile
    compounds due to increased retention.

  • Much more efficient separations can be achieved
    with capillary columns, as compared to packed
    columns, due to the following reasons
  • Very long capillary columns can be used which
    increases efficiency
  • Thinner stationary phase films can be used with
    capillary columns
  • No eddy diffusion term (multiple paths effect) is
    observed in capillary columns

(No Transcript)
Temperature Programming
  • Gas chromatographs are usually capable of
    performing what is known as temperature
    programming gas chromatography (TPGC). The
    temperature of the column is changed according to
    a preset temperature isotherm. TPGC is a very
    important procedure, which is used for the
    attainment of excellent looking chromatograms in
    the least time possible. For example, assume a
    chromatogram obtained using isothermal GC at 80
    oC, as shown below

(No Transcript)
(No Transcript)
(No Transcript)
(No Transcript)
The General Elution Problem
  • Look at the chromatogram below in which six
    components are to be separated by an elution
    process using isothermal conditions at for
    example 120 oC

  • It is clear from the figure that the separation
    is optimized for the elution of the first two
    components. However, the last two components have
    very long retention and appear as broad peaks.
    Using isothermal conditions at high temperature
    (say for example 200oC) can optimize the elution
    of the last two compounds but, unfortunately,
    results in bad resolution of the earlier eluting
    compounds as shown in the figure below where the
    first two components are coeluted while the
    resolution of the second two components becomes
    too bad

(No Transcript)
One can also optimize the separation of the
middle too components by adjusting the isothermal
conditions (for example at say 160 oC). In this
case, a chromatogram like the one below can be
  • However, in chromatographic separations we are
    interested in fully separating all components in
    an acceptable resolution. Therefore, it is not
    acceptable to optimize the separation for a
    single component while disregarding the others.
    The solution of this problem can be achieved by
    consecutive optimization of individual components
    as the separation proceeds. In this case,
    temperature should be changed during the
    separation process. This is called temperature
    programming gas chromatography (TPGC)

  • First, a temperature suitable for the separation
    of the first eluting component is selected, and
    then the temperature is increased so that the
    second component is separated and so on. The
    change in temperature can be linear, parabolic,
    step, or any other formula. The chromatographic
    separation where the temperature is changed
    during the elution process is called TPGC. A
    separation like the one below can be obtained

(No Transcript)
Temperature Zones in GC
  • Three temperature zones should be adjusted before
    a GC separation can be done. The injector
    temperature should be such that fast evaporation
    of all sample components is achieved. The
    temperature of the injector is always more than
    that of the column, which depends on the
    operational mode of the separation. The detector
    temperature should be kept at some level so as to
    prevent any solute condensation in the vicinity
    of the detector body.

Gas-solid chromatography (GSC)
  • Gas-solid chromatography is based upon adsorption
    of gaseous substances on solid surfaces.
    Distribution coefficients are generally much
    larger than those for gas-liquid chromatography.
    Consequently, gas-solid chromatography is useful
    for the separation of species that are not
    retained by gas-liquid columns, such as the
    components of air, hydrogen sulfide, carbon
    disulfide, nitrogen oxides, and rare gases.
    Gas-solid chromatography is performed with both
    packed and open tubular columns.

Molecular Sieves
  • Molecular sieves are metal aluminum silicate ion
    exchangers, whose pore size depends upon the kind
    of cation present, like sodium in sodium aluminum
    silicate molecular sieves. The sieves are
    classified according to the maximum diameter of
    molecules that can enter the pores. Commercial
    molecular sieves come in pore sizes of 4, 5, 10,
    and 13 angstroms. Molecular sieves can be used to
    separate small molecules from large ones.

Porous Polymers
  • Porous polymer beads of uniform size are
    manufactured from styrene crosslinked with
    divinylbenzene. The pore size of these beads is
    uniform and is controlled by the amount of
    crosslinking. Porous polymers have found
    considerable use in the separation of gaseous
    species such as hydrogen sulfide, oxides of
    nitrogen, water, carbon dioxide, methanol, etc.

Quantitative Analysis
  • GC is an excellent quantitative technique where
    peak height or area is proportional to analyte
    concentration. Thus the GC can be calibrated with
    several standards and a calibration curve is
    obtained, then the concentration of the unknown
    analyte can be determined using the peak area or
    height. The detector response factor for each
    analyte should be considered for accurate
    quantitative analysis.

  • Gas chromatographs are widely used as criteria
    for establishing the purity of organic compounds.
    Contaminants, if present, are revealed by the
    appearance of additional peaks. Qualitative
    Analysis is usually done by comparison with
    retention times of standards, which are very
    reproducible in GC, provided good injection
    practices are followed. Injection should be done
    with a suitable Hamilton type syringe through the
    heated septum injector till all needle
    disappears, then the needle is drawn back as
    steadily and fast as possible. This is important
    for reproducible attainment of retention times.

The Retention Index
  • The retention index, RI, was first proposed by
    Kovats in 1958 as a parameter for identifying
    solutes from chromatograms. The retention index
    for any given solute can be derived from a
    chromatogram of a mixture of that solute with at
    least two normal alkanes (chain length gtfour
    carbons) having retention times that bracket that
    of the solute. That is, normal alkanes are the
    standards upon which the retention index scale is

  • By definition, the retention index for a normal
    alkane is equal to 100 times the number of
    carbons in the compound regardless of the column
    packing, the temperature, or other
    chromatographic conditions. The retention index
    system has the advantage of being based upon
    readily available reference materials that cover
    a wide boiling range. The retention index of a
    compound is constant for a certain stationary
    phase but can be totally different for other
    stationary phases.

  • In finding the retention index, a plot of the
    number of carbons of standard alkanes against the
    logarithm of the adjusted retention time is first
    constructed. The value of the logarithm of the
    adjusted retention time of the unknown is then
    calculated and the retention index is obtained
    from the plot.
  • The adjusted retention time, tR, is defined as
  • tR tR - tM

(No Transcript)
Interfacing GC with other Methods
  • As mentioned previously, chromatographic methods
    (including GC) use retention times as markers for
    qualitative analysis. However, this
    characteristic does not absolutely confirm the
    existence of a specific analyte as many analytes
    may have very similar stationary phases. GC, as
    other chromatographic techniques, can confirm the
    absence of a solute rather than its existence.
    When GC is coupled with structural detection
    methods, it serves as a powerful tool for
    identifying the components of complex mixtures. A
    popular combination is GC/MS.

Mass Spectrometry
Mass Spectrometer
Typical sample isolated compound (1 nanogram)
Mass Spectrum
Mass (amu)
(No Transcript)
Write a Comment
User Comments (0)