CHAPTER 32: GAS CHROMATOGRAPHY

by Jorene Kyla V. De Villa

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. The vaporized sample is injected onto the head of a chromatographic column.

TWO TYPES

GAS-LIQUID CHROMATOGRAPHY (GLC)/ GAS CHROMATOGRAPHY (GC)

  • The mobile phase is gas and the stationary phase is a liquid that is retained on the surface of an inert solid by adsorption or chemical bonding.

GAS-SOLID CHROMATOGRAPHY (GSC)

  • The mobile phase is a gas and the stationary phase is a solid that retains the analytes by physical adsorption.
  • Permits separation of and determination of low-molecular-mass gases
  • Has limited application because of semi-permanent retention of active or polar molecules and severe tailing of elution peaks. The tailing is due to the nonlinear character of adsorption process.

COMPONENTS

CARRIER GAS SYSTEM

  • Mobile phase in gas chromatography
  • Chemically inert
  • Helium is commonly used. Others are argon, nitrogen, and hydrogen. The gases are available in pressurized tanks.
  • Flow rates are regulated by:
    • Two-stage pressure regulator
    • Pressure regulator/ flow regulator mounted in the chromatograph
  • Inlet pressure: 10-50 psi (lb/in2)
  • Flow rate: 25-150 mL/min (packed columns), 1-25 mL/min (open tubular capillary columns)

SAMPLE INJECTION SYSTEM

  • Where sample is introduced as a plug of vapor
  • Slow injection/ oversized samples cause band spreading and poor resolution
    • 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.
  • Sample port is kept at 50oC > boiling point of least volatile component of the sample
  • Ordinary packed analytical column sample size: few tenths of a microliter to 20 μL
  • Capillary column sample size: smaller by a factor of 100 or more
  • SAMPLE VALVE– used for introducing gases instead of syringe. Sample sizes can be reproduced to better than 0.5% relative.
  • Solid samples are introduced as solutions or alternatively are sealed into thin-walled vials that can be inserted at the head of the column and punctured or crushed from the outside.

COLUMN

  • Two types: Packed and Capillary columns (currently used)
  • Length: less than 2m – 60m
  • Material: stainless steel, glass, fused silica, Teflon
  • Formed as coils having diameters of 10-130 cm
  • Housed in thermostated oven for temperature control. Temperature equal to or slightly above the average boiling point of a sample results in a reasonable elution time (2 to 30 min)
  • Samples with broad boiling range use temperature programming where temp. is increased continuously/ in steps
  • Optimal resolution is achieved with minimal temp., however; lowered temp. increases elution time. Proper control range of temp. must be used.

CHROMATOGRAPHIC INDICATORS

Characteristics:

  1. Adequate sensitivity.
  2. Good stability and reproducibility.
  3. A linear response to solutes that extends over several orders of magnitude.
  4. Temperature range from room temperature to at least 4008oC.
  5. A short response time that is independent of flow rate.
  6. High reliability and ease of use.
  7. Similarity in response toward all solutes or, alternatively, a highly predictable and selective response toward one or more classes of solutes.
  8. Nondestructive of sample.

Different detectors give different types of selectivity:

  • Non-selective detector responds to all compounds except the carrier gas.
  • Selective detector responds to a range of compounds with a common physical or chemical property.
  • Specific detector responds to a single chemical compound.

Four most widely used detectors:

FLAME IONIZATION DETECTOR

  • Mass-sensitive because it responds to the rate at which solute molecules enter the detector; although it destroys the sample during the combustion step.

MECHANISM:

  1. Effluent from the column is directed into a small air/hydrogen flame
  2. Organic compounds produce ions and electrons when pyrolyzed
  3. Ions and electrons are collected by a collector electrode located above the flame
  4. The resulting current (~10-12A) is then measured with a sensitive picoammeter
  • Functional groups (carbonyl, alcohol, halogen, amine) yield fewer ions or none at all in a flame
  • Insensitive toward noncombustible gases, such as H2O, CO2, SO2, and NOx.

THERMAL CONDUCTIVITY DETECTOR

  • Consists of an electrically heated source (fine platinum, gold, tungsten, small thermistor) whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas.
  • Thermally sensitive resistive elements used: reference pair located ahead of the sample injection chamber; sample pair immediately beyond the column.
  • The detectors are incorporated in two arms of a simple bridge circuit, cancelling the thermal conductivity of the carrier gas.

ELECTRON CAPTURE DETECTOR

  • Most widely used for environmental samples because it selectively responds to halogen-containing organic compounds, such as pesticides and polychlorinated biphenyls

MECHANISM:

  1. Sample eluate from a column is passed over a radioactive β emitter, usually nickel-63.
  2. Electron from the emitter causes ionization of the carrier gas (often nitrogen) and the production of a burst of electrons
  • Halogens, peroxides, quinones, and nitro groups, are detected with high sensitivity. The detector is insensitive to functional groups such as amines, alcohols, and hydrocarbons.

MASS SPECTROMETRY DETECTORS

  • Combination of gas chromatography and mass spectrometry (GC/MS3)
  • Measures the mass-to-charge ratio (m/z) of ions that have been produced from the sample.

MECHANISM:

  1. Mass spectrometer scans the masses repetitively during a chromatographic experiment. If a chromatographic run is 10 minutes, for example, and a scan is taken each second, 600 mass spectra are recorded.
  2. A computer data system is needed to process the large amount of data obtained.

WAYS DATA CAN BE ANALYZED:

  • Ion abundance in each spectrum can be summed and plotted as a function of time to give a total-ion chromatogram.
  • Mass spectrum can be displayed at a particular time during the chromatogram to identify the species eluting at that time.
  • Single mass-to-charge (m/z) value can be selected and monitored throughout the chromatographic experiment, a technique known as selected-ion monitoring

Mass chromatograms are mass spectra of selected ions during a chromatographic experiment.

USE IN QUANTITATIVE ANALYSIS

Based on comparison of either the height or the area of an analyte peak with that of one or more standards.

CALIBRATION WITH STANDARDS

  • Series of standard solutions that approximate the composition of the unknown is prepared
  • Chromatograms for the standards are obtained, and peak heights or areas are plotted as a function of concentration to obtain a working curve.
  • Plot of the data should yield a straight line passing through the origin; this would be the basis of quantitative analyses.
  • Frequent standardization is needed for the highest accuracy

INTERNAL STANDARD METHOD

  • Exhibits highest precision because the uncertainties introduced by sample injection, flow rate, and variations in column conditions are minimized.
  • A carefully measured quantity of an internal standard is introduced into each standard and sample
  • The ratio of analyte peak area (or height) to internal standard peak area (or height) is used as the analytical parameter.
  • The internal standard peak is separated from the peaks of all other components in the sample. However, it must appear close to the analyte peak.