Chapter 32

GAS CHROMATOGRAPHY

  • one of most widely used techniques for qualitative and quantitative analysis.
  • This chapter considers gas chromatography in detail, including the columns and stationary phases that are most widely used.
  • Various detection systems, including mass spectrometry, are described.
  • Although the chapter is primarily concerned with gas-liquid chromatography, there is a brief discussion of gas-solid chromatography.

INTRODUCTION:

In gas chromatography, the components of a vaporized sample are separated by being distributed between a mobile gaseous phase and a liquid or a solid stationary phase held in a column.

GAS CHROMATOGRAPHIC SEPARATION

  • 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.

MOBILE PHASE

  • does not interact with molecules of the analyte.
  • The only function of this is to transport the analyte through the column.

TWO TYPES OF GAS CHROMATOGRAPHY

  1. gas-liquid chromatography (GLC)
  2. the mobile phase is a gas, and the stationary phase is a liquid that is retained on the surface of an inert solid by adsorption or chemical bonding.
  3. gas-solid chromatography (GSC)
  4. the mobile phase is a gas, and the stationary phase is a solid that retains the analytes by physical adsorption.

INSTRUMENTS FOR GAS-LIQUID CHROMATOGRAPHY

CARRIER GAS SYSTEM

  • The carrier gas is an inert gas used to carry samples. Helium (He), nitrogen (N2), hydrogen (H2), and argon (Ar) are often used. Carrier gas must be dry, free of oxygen and chemically inert mobile-phase employed in gas chromatography.
  • The carrier gas system contains a molecular sieve to remove water and other impurities.

*SOAP BUBBLE-FLOW METER

– formed in the path of the gas when a rubber bulb containing an aqueous solution of soap or detergent is squeezed; the time required for this film to move between two graduations on the buret is measured and converted to volumetric flow rate.

SAMPLY INJECTION SYSTEM

  • For high column efficiency, a suitably sized sample should be introduced as a “plug” of vapor.

*CALIBRATED MICROSYRINGES

– used to inject liquid samples through a rubber or silicone diaphragm, or septum, into a heated sample port located at the head of the column.

*AUTOINJECTORS AND AUTOSAMPLERS

– syringes are filled, and the sample injected into the chromatograph automatically. The autoinjector syringe picks up the sample through a septum on the vial and injects the sample through a septum on the chromatograph.

– In the autosampler, samples are contained in vials on a sample turn-table.

COLUMN CONFIGURATION AND COLUMN OVENS

  • The columns in gas chromatography are of two general types: packed columns or capillary columns.
  • In the past, the vast majority of gas chromatographic analyses used packed columns. For most current applications, packed columns have been replaced by more efficient and faster capillary columns.

*TEMPERATURE PROGRAMMING

– is achieved by increasing the column temperature continuously or in steps during elution.

CHROMATOGRAPHIC DETECTORS

  • sense the solute vapours in the mobile phase as they emerge from the column

FLAME IONIZATION DETECTORS

  • is the most widely used and generally applicable detector for gas chromatography.
  • With a FID, effluent from the column is directed into a small air/hydrogen flame.

THERMAL CONDUCTIVITY DETECTORS

  • which was one of the earliest detectors for gas chromatography, still finds wide application.
  • This device consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas.
  • The heated element may be a fine platinum, gold, or tungsten wire or, alternatively, a small thermistor.
  • The electrical resistance of this element depends on the thermal conductivity of the gas.

ELECTRON CAPTURE DETECTOR

  • has become one of the most widely used detectors
  • for environmental samples because this detector selectively responds to halogen-containing
  • organic compounds, such as pesticides and polychlorinated biphenyls.

MASS SPECTROMETRY DETECTORS

  • One of the most powerful detectors for GC
  • The combination of gas chromatography and mass spectrometry is known as GC/MS.
  • a mass spectrometer measures the mass-to-charge ratio (m/z) of ions that have been produced from the sample.

OTHER GC DETECTORS

  • Other important GC detectors include the thermionic detector, the electrolytic conductivity or Hall detector, and the photoionization detector.
  • HYPHENATED METHODS: couple the separation capabilities of chromatography with the qualitative and quantitative detection capabilities of spectral methods.

GAS CHROMATOGRAPHIC COLUMNS AND STATIONARY PHASES

CAPILLARY COLUMNS

  • the stationary phase was a film of liquid a few tenths of a micrometer thick that uniformly coated the interior of a capillary tubing

2BASIC TYPES:

  1. WALL-COATED OPEN TUBULAR (WCOT): capillary tubes coated with a thin layer of the liquid stationary phase.
  2. SUPPORT COATED  OPEN TUBULAR (SCOT): the inner surface of the capillary is lined with a thin film (,30 mm) of a solid support material, such as diatomaceous earth, on which the liquid stationary phase is adsorbed.

PACKED COLUMNS

  • Modern packed columns are fabricated from glass or metal tubing.
  • The tubes are usually formed as coils with diameters of roughly 15 cm so that they can be Desirable properties for the immobilized liquid phase conveniently placed in a temperature-controlled oven.

LIQUID STATIONARY PHASES

  • Desirable properties for the immobilized liquid phase:
    • Low volatility
    • Thermal stability
    • Chemical inertness
    • Solvent characteristics such as k and a values

*SOME WIDELY USED STATIONARY PHASES

BONDED AND CROSS-LINKED STATIONARY PHASES

  • 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.

FILM THICKNESS

  • primarily affects 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, thus providing a greater time for separation to take place.

APPLICATIONS OF GAS-LIQUID CHROMATOGRAPHY

QUALITATIVE ANALYSIS

  • The areas under these extraneous peaks provide rough estimates of the extent of contamination. The technique is also useful for evaluating the effectiveness of purification procedures.

QUANTITATIVE ANALYSIS

  • Quantitative GC is based on comparison of either the height or the area of an analyte peak with that of one or more standards. If conditions are properly controlled, both of these parameters vary linearly with concentration.

*CALIBRATION WITH STANDARDS

– A series of standard solutions that approximate the composition of the unknown is prepared.

*THE INTERNAL STANDARD METHOD

– a carefully measured quantity of an internal standard is introduced into each standard and sample

ADVANCES IN GC

High-Speed Gas Chromatography

  • The basic idea of high-speed GC is that, for many separations of interest, higher speed can be achieved, albeit at the expense of some selectivity and resolution.

where kn is the retention factor for the last component of interest in the chromatogram.

Miniaturized GC Systems

  • For many years, there has been a desire to miniaturize GC systems to the microchip level.
  • Miniature GC systems are useful in space exploration, in portable instrument for field use, and in environmental monitoring.

Multidimensional Gas Chromatography

  • In multidimensional GC, two or more capillary columns of differing selectivities are connected in series.
  • Therefore, with two columns, one might contain a nonpolar stationary phase, while the second might have a polar stationary phase.

GAS-SLID CHROMATOGRAPHY

  • is based on adsorption of gaseous substances on solid surfaces.
  • Gas chromatography is performed with both packed and open tubular columns.

*For the latter, a thin layer of the adsorbent is affixed to the inner walls of the capillary. Such columns are sometimes called porous layer open tubular columns or PLOT columns.