GAS CHROMATOGRAPHY

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GAS CHROMATOGRAPHY
CHAPTER 21
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Gas Chromatography Produces Chromatograms
GC is used to identify, (compare to standards)
separate, and quantify (use peak area) samples.
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Stable, but Volatile Analytes

• The structure of the analyte could be changed, in
  a known way, to increase volatilityDerivatization
  .

• Volatility is related to vapor pressure and
  boiling point. Larger molecules (gt 600g/mol) are
  not volatile enough for GC.
• Boiling points lt 500oC help, as does low
  polarity.
• Heated injection and separation may
• degrade some
• compounds.
• Thermal stability is
  required.

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Factors That Determine Retention in GC

• Retention is related to the relative time the
  analyte is in the mobile phase compared to the
  stationary phase.
• Retention is affected by volatility, column
  temperature, and the degree of interaction with
  the stationary phase.
• More volatile analyte elutes first (volatility
  increases make retention decrease). Note the
  homolog alkane series.

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Factors That Determine Retention in GC

• TEMPERTURE Decreasing column temperature (via
  oven) leads to longer retention times due to a
  drop in volatility.
• Note this effect in the chromatograms where
  temperature is
• changed on the
• same system

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Factors That Determine Retention in GC

• Kovats Retention Index (I) retention of an
  analyte on one column is compared to the
  retention seen at the same temperature and column
  for a series of alkanes.
• tRz (adjusted retention time for n-alkane that
  elutes just prior to analyte)
• tR(z1) (adjusted retention time for n-alkane
  that elutes just after the analyte)
• z refers to the number of carbon atoms in the
  homolog series.
• Exercise 21.3 allows practice for Kovats index.

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Factors That Determine Retention in GC

• Interactions with the stationary phase can affect
  retention time.Examine the polarity of analytes
  and the stationary phase for interactions. Which
  of these two is more polar? Which would tend to
  retain polar analytes?

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Use Kovats to Compare Differences in Retention
Time Related to Polarity
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Mobile Phases and Elution Methods

• In the mobile phase, gas is low density and moves
  quickly through the column, giving rise to narrow
  peaks and quick separations. Low viscosity
  permits the use of longer columns (without undue
  pressure build up).
• Move solutes through with no or minimal
  interaction with analytes. (He, N2, and Ar
  common)
• The carrier gas (mobile phase) must be high
  purity to avoid contaminating the sample and
  possibly breaking down column materials.

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Mobile Phases and Elution Methods

• For simple samples a constant temperature may be
  maintained during the analysis (isothermal
  method).
• Complex samples often contain analytes with a
  wide range of volatility, causing a wide range of
  retention times.
• The figure shows, using the isothermal method,
  that some analytes emerge quickly, others much
  longer. Also, resolution varies.

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Mobile Phases and Elution Methods

• Temperature programming offers a solution to the
  general elution problem (separating with
  reasonable resolution in reasonable time).
• Start chromatography at a lower temperature, then
  gradually ramp up during analysisnote the four
  regions.

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GC Supports and Stationary Phases

• Columns may be discussed/compared by noting if
  they are packed or open-tubular.
• Packed columns are filled with small, absorbent
  particles or coated particles. Use a glass or
  metal column 12 meters in length and a few mm
  wide.
• Silica (SiO2) is used to make diatomaceous earth,
  which can be coated for use in Gas-Liquid
  chromatography.
• Molecular sieves or porous polymers are used with
  gas-solid chromatography.
• Packed columns have typical applications in
  simplier systems. A large surface area allows
  larger sample injection but has lower
  efficiencies.

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GC Supports and Stationary Phases

• An open-tubular column (capillary column) has a
  stationary phase coated on its interior. These
  are much longer than packed columns (10100m)
  with small diameters (0.10.75 mm). They also
  tend to be faster, have better resolution, and
  better efficiency.

WCOT SCOT PLOT
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GC Supports and Stationary Phases

• Compare packed and open-tubular columns

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Take a Closer Look

• Diatomaceous earth produces high surface area
  packing material.

• Molecular sieve have (zeolite) well defined pore
  size/structure.

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Gas-Liquid Chromatography

• Gas-solid chromatography utilizes solid,
  absorbent material, and molecular sieves with
  good applications to small hydrocarbons and
  gases. The surface area, the size of pores, and
  functional groups on the supports surface
  determine separation. GSC is not as common as
  GLC, but can be packed or open.
• Gas-Liquid Chromatography utilizes high viscosity
  and low volatility. Its easy to place the liquid
  spread over the support.
• The liquid coating is often
  based on polysiloxane
  with varying
  molar masses
  ( 1000106 g/mol).

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Bonded Phase Support

• Some column bleed occurs during heating when
  the liquid stationary phase breaks down or
  vaporizes. This affects retention and even
  detectors.
• If the liquid can be covalently bonded to the
  solid support, a bonded phase support is
  formed, which is more stable (allows higher temp
  to be used.)
• This is formed by reacting a polysiloxane liquid
  phase with a silanol group on a solid silica
  support.
• A cross-linked stationary phase can also be used.

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Stationary Phases and Polarity
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Detectors Thermal Conductivity

• TCD can be used for organic and inorganic
  analytes.
• The key aspect is the ability of the carrier gas
  and the analytes to change the conductivity of a
  wire filament, which will vary with different
  analytes.
• The carrier gas should have different thermal
  conductivity of analytes.
• TCD is a non-destructive type of detection that
  uses a Wheatstone bridge style. Downsides are
  the response to impurities, leakage in air, and
  poor response to LOD.

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Wheatstone Bridge TCD
Most common carrier
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Detectors Flame Ionization

• FID uses fuel mixed with carrier and organic
  analyte. Analyte forms ions in the flame.
• Cations from the flame are gathered by the
  negative electrodeproduces a current.
• Advantageinorganics do not respond (He carrier
  gas), so the low background signal allows for LOD
  100- to 1000-fold lower then TCD.
• Disadvantage destructive

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Detectors Selective Detectors

• Specific for specific types of chemicals (i.e.,
  nitrogen-phosphorus detector, NPD)
• Measures ions produced from eluting N or P
  compounds, but generates e from a heated surface
  (not flame) that combine with electronegative
  elements to form negative ions
• Good selectivity, good LOD, but must periodically
  change heated material

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Detectors Electron Capture

• Detects electronegative atoms or groups (Cl,
  -NO2) and also polynuclear aromatics and
  conjugated carbonyl compounds
• These groups capture e that are produced from
  nuclear radiation from 3H
• Carrier gas hit by these e can release secondary
  e, which will be absorbed by analyte
• Has good LOD, but a narrow linear range and
  requires radioactive source

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Detectors Mass Spectrometry

• Detects and measures by converting eluting
  analytes into gas phase ions (forming a molecular
  ion or fragmenting analyte and ionizing.
  Compare patterns of ions and fragments to known
  values. Intensity relates to amounts.
• A portion of analytes is converted to anions via
  electron impact or chemical impact (softer, less
  fragments).
• Gas phase ions are separated by mass/charge ratio
  using a quadrupole mass analyzer. (Uses four
  parallel rods with well-defined potentials so
  that only certain mass/charge species may pass
  sorted)
• See Exercise 21.4.

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Detectors Mass Spectrometry

• Mass chromatogram

• A Mass spectrum plots intensity of mass/charge
  vs. timecould be set a certain retention time in
  a GC.

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Sample Injection and Pretreatment

• Gas Sample if present at moderate to high
  concentration, may be directly put into column
  via gas-tight syringe.
• Another technique is to use a gas-tight valve

For trace levels like volatile organic compounds
(VOC), the sample may need to be preconcentrated
by passing it through a solid-absorbing cartridge
or using a cryogenic trap.
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Sample Injection and Pretreatment

• Liquid Samples direct injection of a relatively
  large sample (6-10 mL). A calibrated microsyringe
  may be injected through a gas-tight septum into a
  heated chamber that volatilizes the sample.
• Note that there are direct, split, and splitless
  column styles.
• Sometimes pretreatment requires dervitatization
  or moving the sample into a different solvent
  (remove from H2O).
• Static or Dynamic Headspace analysis of vapor
  phase above a sample avoids water in the sample.

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Sample Injection and Pretreatment

• Solid Samples First extract compounds of
  interest from a solid matrix by liquid-liquid
  extraction or supercritical fluid. Samples are
  placed in solvent, then treated as liquid
  samples.
• Thermal desorption may be used on some solids as
  analytes may be collected during heating of a
  solid.
• Pyrolysis GC is employed for substances that are
  not volatile and cannot be easily derivatized to
  volatile forms. The solid is heated in a
  controlled way to break it into smaller, more
  volatile pieces separated by the column to form a
  pyrogram, which can be matched to known standards.