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Chapter 26 & 27. Introduction to Chromatographic Separations and Gas Chromatography. Introduction. In gas chromatography (GC), the sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase.

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Chapter 26 27

Chapter 26 & 27

Introduction to Chromatographic Separations and Gas Chromatography


Introduction
Introduction...

  • In gas chromatography (GC), the sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase.


Chapter 26 27


Retention volume
Retention Volume... analyte between a gaseous mobile phase and a liquid phase immobilized on the surface of an inert solid.

The specific retention volume, Vg is defined as follows:

Vg = [JF(tr-tm)]/W(273/Tc)

where, J = the pressure drop factor, F = average flow rate, tr and tm = retention times W = mass of stationary phase and Tc = column temperature in Kelvin.


Instruments for gc
Instruments for GC... analyte between a gaseous mobile phase and a liquid phase immobilized on the surface of an inert solid.

The schematic of a gas chromatograph is shown below:


Carrier gas supply
Carrier Gas Supply... analyte between a gaseous mobile phase and a liquid phase immobilized on the surface of an inert solid.

Carrier gases, which must be chemically inert, include helium, argon, nitrogen, carbon dioxide, and hydrogen. The choice of gases is often dictated by the detector used. Associated with the gas supply are pressure regulators, gauges, and flow meters.


Chapter 26 27


Sample injector system
Sample Injector System... molecular sieve to remove water or other impurities.

The most common method of injection involves the use of a micro-syringe to inject a liquid or gaseous sample through a silicone rubber diaphragm or septum into a flash vaporizer port located at the head of the column.


Column configurations and column ovens
Column Configurations and Column Ovens... molecular sieve to remove water or other impurities.

Two general types of columns are encountered in gas chromatography:

(1) packed

(2) open tubular or capillary


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Characteristics of the ideal detector
Characteristics Of The Ideal Detector... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

1. Adequate sensitivity

2. Good stability and reproducibility.

3. A linear response to analyses that extends over several orders of magnitude.

4. A temperature range from room temperature to at least 400 deg C.

5. A short response time that is independent of flow rate.

6. High reliability and ease of use.

7. Similarity in response toward all analyses.

8. Nondestructive of sample.


Flame ionization detector
Flame Ionization Detector... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • The flame ionization detector (FID) is one of the most widely used and generally applicable detectors for gas chromatography. The effluent from the column is mixed with hydrogen and air and then ignited electrically.


A diagram of a typical fid is shown in below
A diagram of a typical FID is shown in below. 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.


Thermal conductivity detector
Thermal Conductivity Detector... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • A very early detector for gas chromatography is based upon changes in the thermal conductivity of the gas stream brought about by the presence of analyte molecules. This device is sometimes called a katharometer.


Thermoionic detector
Thermoionic Detector... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • The thermoionic detector (TID) is selective toward organic compounds containing phosphorous and nitrogen. It is similar in structure to the flame detector.


Electron capture detector ecd
Electron Capture Detector (ECD)... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

Electron-capture detector (ECD) operates in much the same way as a proportional counter for measurement of X-radiation. Here the effluent from the column passes over a beta-emitter, such as nickel-63 or tritium (adsorbed on platinum or titanium foil). An electron from the emitter causes ionization of the carrier gas (often nitrogen) and the production of a burst of electrons.


Atomic emission detector aed
Atomic Emission Detector (AED)... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • The newest commercially available gas-chromatographic detector is based upon atomic emission. In this device, the eluent is introduced into a microwave-energized helium plasma that is coupled to a diode-array optical emission spectrometer.


Packed columns
Packed Columns... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • Present day packed columns are fabricated from glass, metal (stainless steel, copper, aluminum), or Teflon tubes that typically have lengths of 2 to 3 m and inside diameters of 2 to 4mm. These tubes are densely packed with a uniform, finely divided packing material, or solid support, that is coated with a thin layer of stationary liquid phase. The tubes are formed as coils having diameters of roughly 15cm.


Solid support materials
Solid Support Materials... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • The solid support in a packed column serves to hold the stationary phase in place so that as large a surface area as possible is exposed to the mobile phase.

  • The ideal support consists of small, uniform spherical particles with good mechanical strength and a specific surface area of at least 1m/g.


Particle size supports
Particle Size Supports... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • The efficiency of a gas-chromatographic column increases rapidly with decreasing particle diameter of the packing. The pressure difference required to maintain a given flow rate of carrier gas, however, varies inversely as the square of the particle diameter


Open tubular columns
Open Tubular Columns... 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

  • Open tubular, or capillary, columns are of two basic types, namely, wall-coated open tubular (WCOT) and support-coated open tubular(SCOT). WCOT columns are simply capillary tubes coated with a thin layer of the stationary phase. In SCOT columns the inner surface of the capillary is lined with a thin film of a support material, such as diatomaceous earth.


Chapter 26 27
Desirable properties for the immobilized liquid phase in a gas liquid chromatographic column include...

  • low volatility

  • thermal stability

  • chemical inertness

  • solvent characteristics such that k’ and alpha values for the solutes to be resolved fall within a suitable range.


Bonded and cross linked stationary phases
Bonded and Cross-linked Stationary Phases... gas liquid chromatographic column include...

  • The purpose of bonding and cross-linking is to provide a longer-lasting stationary phase that can be rinsed with a solvent when the film becomes contaminated.


Chiral stationary phases
Chiral Stationary Phases... gas liquid chromatographic column include...

  • There has been recent developments of stationary phases which can separate chiral compounds. One method is to form a derivative of the analyte with an optically active reagent that forms a pair of diasteromers that can be separated on an achiral column. The alternative method is to use a chiral liquid as the stationary phase.


Film thickness
Film Thickness... gas liquid chromatographic column include...

  • Film thickness primarily affect the retentive character and the 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.


The retention index
The Retention Index... gas liquid chromatographic column include...

  • The retention index, I, 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 having retention times that bracket that of the solute


Below is a diagram of gc ms a instrument
Below is a diagram of GC/MS a instrument gas liquid chromatographic column include...


References
References... gas liquid chromatographic column include...

  • http://www.acs.org

  • http://www.cas.org

  • http://www.chemcenter/org

  • http://www.sciencemag.org

  • http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html