Gas Chromatography: Detectors

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Gas Chromatography Detectors
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The Ideal Detector

• Adequate sensitivity - range 10-18 to 10-15 g
  analyte/s
• good stability and reproducibility
• a linear response to analyte that extends over
  several orders of magnitude
• a temperature range from room temperature to at
  least 400 degrees C

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The Ideal Detector

• A short response time that is independent of flow
  rate
• high reliability and ease of use
• similarity in response toward all analytes of
  alternatively a higher predictable and selective
  response toward classes of ananlytes
• nondestructive of sample

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Types of Detectors

• Thermal Conductivity Detector
• Thermionic Detectors
• Atomic Emission Detector
• Flame Ionization Detector
• Electron Capture Detector

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Thermal Conductivity Detector (TCD)

• Q. How does this work?
• Answer
• Electronically measures the changes in the
  thermal conductivity of the gas.
• The thermal conductivity changes due to the
  presence of the analyte

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The Modulate Single Filament (TCD)

• A method to get rid of some noise !
• How?
• Both the analytical and reference gases are
  passed altrenately over a tiny filament
• This is done at a set frequency, that sets the
  frequency of the output of the electrical signal
• The amplitude fo the signal is proportional to
  the difference in the thermal conductivity of
  both gases

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TCD

• What is the best type of carrier gas for the TCD?
• He and H have high thermal conductivity therefore
  they show the presence of other components of the
  gas because of the large drop in thermal
  conductivity
• Other gases are not used in TCD

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Advantages

• simplicity
• large linear dynamic range ( about 105)
• general response to organic and inorganic samples
• non-destructive character

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Limitations

• Low sensitivity (10-8 solute/mL carrier gas)
• Others exceed this by factor of 104 to 107

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Thermionic Detectors (TID)

• How does this work?
• The effluent is mixed with H
• Passes through a flame tip assembly to ignite
• Hot gas flows around an electrically heated
  rubidium silicate bead
• Heated bead forms a plasma
• What occurs in the plasma to produce large
  amounts of ions for D and N containing molecules
  is not understood

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TID

• There is a large ion current that results

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Advantages

• Selective for organic compounds with P and N
• This is useful for detecting pesticides that
  contain P

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Atomic Emission Detection (AED)

• Eluent introduced into a microwave - energized
  helium plasma coupled to a diode-array optical
  spectrometer
• Plasma sufficiently energetic to atomize all of
  the elements in the sample and excite their
  characteristic atomic emission spectra

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AED

• Spectra observed using a spectrometer that
  employs a movable, flat diode array is capable of
  detecting omitted radiation between 170 to 1780
  nm
• The positional diode array is capable of
  measuring 2 to 4 elements at any given setting
• Present software allows measurement of the
  concentration of 15 elements

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Flame Ionization Detector

• Consists of a stainless steel burner assembly
  installed in the detector compartment and a
  electrometer system in a separate unit adjacent
  to the gas chromatograph
• Often it is installed in the tandem with the
  thermal conductivity cell
• Effluent form the column enters burner base
  through millipore filters which remove
  contaminates

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Flame Ionization Detector

• hydrogen mixed with gas stream at bottom of jet
  and air or oxygen is supplied axially around the
  jet
• hydrogen flame burns at the tip, which also
  functions as the cathode and is insulated form
  the body by a ceramic seal
• Collector electrode is above the burner tip and
  is made of platinum

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Flame Ionization Detector

• In series with flame gases is a selection of
  resistors 107 to 1010 ohms
• vibrating reed electrometer used to provide
  sensitivities up to 5 x 1013 Amps
• Carbon counting device that produces a current
  proportional to number of ions or electrons
  formed in the flamed gases

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Flame Ionization Detector

• Responds to all organic compounds except for
  formic acid
• Response greatest with hydrocarbons and decreases
  with substitution
• Except for vapor of elements in Groups I and II,
  does not respond to inorganic compounds
• Sensitivity high due to low noise level

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Flame Ionization Detector

• Insensitivity to water, the permanent gases, and
  inorganic compounds simplifies the resolution of
  components in analysis of aqueous extracts and in
  air pollution studies

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Electron Capture Detector

• operates similar to proportional counter for
  measurement of X-radiation
• effluent form the column passes over a
  beta-emitter - I.e.) nickel-63 or tritium

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ECD

• electron from emitter causes ionization of
  carrier gas (N)
• produces a burst of electrons
• standing current between electrodes decreases in
  presence of organic species that capture the
  electrons

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ECD

• selective in its response and highly sensitive
• Hewlett Packard makes one with a detection limit
  of less than 8 fg/sec for lindane
• sensitive toward molecules with electronegative
  functional groups (halogens, peroxides, quinones,
  nitro groups)
• insensitive towards amines, alcohols and
  hydrocarbons

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ECD

• important application detection and
  determination of chlorinated insecticides
• advantages
• does not alter the sample significantly (in
  contrast to flame detection)
• quick and easy
• relatively cheap

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ECD

• disadvantage linear response range is usually
  limited to around 2 orders of magnitude