Chapter 32 Gas Chromatography

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. Two types of gas chromatography are encountered: In gas-liquid chromatography, 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. In gas-solid chromatography, the mobile phase is a gas, and the stationary phase is a solid that retains the analytes by physical adsorption. Gas-solid chromatography permits the separation and determination of low- molecular-mass gases, such as air components, hydrogen sulfide, carbon monoxide, and nitrogen oxides.

32 A Instruments for gas-liquid chromatography The basic components of a typical instrument for performing gas chromatography are shown here.

Carrier Gas System The mobile phase gas in gas chromatography is called the carrier gas and must be chemically inert. Helium is the most common mobile phase, although argon, nitrogen, and hydrogen are also used. Flow rates in gas chromatographs were regulated by controlling the gas inlet pressure. Newer chromatographs use electronic pressure controllers both for packed and for capillary columns.

In a classical soap-bubble meter, a soap film is 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.

Sample Injection System For high column efficiency, a suitably sized sample should be introduced as a “plug” of vapor. Slow injection or oversized samples cause band spreading and poor resolution. Calibrated microsyringes are 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.

Column Configurations and Column Ovens The columns in gas chromatography are of two general types: packed columns or capillary columns. Chromatographic columns vary in length from less than 2 m to 60 m or more. They are constructed of stainless steel, glass, fused silica, or Teflon.

Column temperature is an important variable that must be controlled to a few tenths of a degree for precise work. The optimum column temperature depends on the boiling point of the sample and the degree of separation required.

Chromatographic Detectors The ideal detector for gas chromatography has the following characteristics: 1.Adequate sensitivity. 2.Good stability and reproducibility. 3.A linear response to solutes that extends over several orders of magnitude. 4.A temperature range from room temperature to at least 400  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 solutes or, alternatively, a highly predictable and selective response toward one or more classes of solutes. 8. Nondestructive of sample.

The flame ionization detector responds to the number of carbon atoms entering the detector per unit of time. It is a mass-sensitive rather than a concentration-sensitive device. It is useful for the analysis of most organic samples including those that are contaminated with water and the oxides of nitrogen and sulfur.

The electron capture detector (ECD) 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. Electron capture detectors are highly sensitive and have the advantage of not altering the sample significantly. The linear response of the detector is limited to about two orders of magnitude. One of the most powerful detectors for GC is the mass spectrometer. The combination of gas chromatography and mass spectrometry is known as GC/MS.

A computer data system is needed to process the large amount of data obtained by GC/MS mass spectrometers. The data can be analyzed in several ways. 1. The ion abundance in each spectrum can be summed and plotted as a function of time to give a total-ion chromatogram. 2. One can also display the mass spectrum at a particular time during the chromatogram to identify the species eluting at that time. 3. A single mass-to-charge (m/z) value can be selected and monitored throughout the chromatographic experiment, a technique known as selected- ion monitoring. Mass spectra of selected ions during a chromatographic experiment are known as mass chromatograms.

Other important GC detectors include the 1.thermionic detector - widely used for organo-phosphorous pesticides and pharmaceutical compounds. 2.the electrolytic conductivity or Hall detector - compounds containing halogens, sulfur, or nitrogen are mixed with a reaction gas in a small reactor tube. The products are then dissolved in a liquid that produces a conductive solution. The change in conductivity as a result of the presence of the active compound is then measured. 3.the photoionization detector - often used for aromatic and other molecules that are easily photoionized.

32 B Gas chromatographic columns and stationary phases Capillary columns are also called open tubular columns because of the open flow path through them. They are of the following types: 1. Wall-coated open tubular (WCOT) are capillary tubes coated with a thin layer of the liquid stationary phase. 2. Support-coated open tubular columns (SCOT) have an inner surface lined with a thin film (<30  m) of a solid support material. 3. Fused-silica open tubular (FSOT) columns are currently the most widely used GC columns. 4. Capillary columns with 530  m inside diameters, sometimes called megabore columns, are also used.

Modern packed columns are fabricated from glass or metal tubing. They are typically 2 to 3 m long and have inside diameters of 2 to 4 mm. Solid Support Materials The packing, or solid support, in a packed column serves to hold the liquid stationary phase in place so that as large a surface area as possible is exposed to the mobile phase. Packings for gas chromatography were prepared from naturally occurring diatomaceous earth, which consists of the skeletons of thousands of species of single-celled plants.

Liquid Stationary Phases Desirable properties for the immobilized liquid phase in a gas-liquid chromatographic column include (1) low volatility (ideally, the boiling point of the liquid should be at least 100  C higher than the maximum operating temperature for the column), (2) thermal stability, (3) chemical inertness, and (4) solvent characteristics such that k and  values for the solutes to be resolved fall within a suitable range. To have a reasonable residence time on the column, an analyte must show some degree of compatibility (solubility) with the stationary phase especially with respect to the polarities of the analyte and the immobilized liquid.

32 C Applications of gas-liquid chromatography It is applicable to species that are appreciably volatile and thermally stable at temperatures up to a few hundred degrees Celsius.

Qualitative Analysis Gas chromatography is widely used to establish the purity of organic compounds. Contaminants, if present, are revealed by the appearance of additional peaks in the chromatogram. 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. Although a chromatogram may not lead to positive identification of the species in a sample, it often provides sure evidence of the absence of species.

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. Peak area is independent of the broadening effects.

Calibration with Standards A series of standard solutions that approximate the composition of the unknown is prepared. Chromatograms for the standards are then obtained, and peak heights or areas are plotted as a function of concentration to obtain a working curve. A plot of the data should yield a straight line passing through the origin; quantitative analyses are based on this plot.

The Internal Standard Method The highest precision for quantitative GC is obtained using internal standards 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 and the ratio of analyte peak area (or height) to internal standard peak area (or height) is used as the analytical parameter.

Advances in GC High-Speed Gas Chromatography The basic idea is that, for many separations of interest, higher speed can be achieved at the expense of some selectivity and resolution. The principles of high-speed separations can be demonstrated by: where k n is the retention factor for the last component of interest in the chromatogram. The advantage is faster separations by using short columns, higher-than-usual carrier gas velocities, and small retention factors. The disadvantage is reduced resolving power caused by increased band broadening and reduced peak capacity.

Multidimensional Gas Chromatography In multidimensional GC, two or more capillary columns of differing selectivities are connected in series. Multidimensional GC can take several forms. In one implementation, called heart cutting, a portion of the eluent from the first column containing the species of interest is switched to a second column for further separation. In another methodology, comprehensive two-dimensional GC or GC  GC, the effluent from the first column is continuously switched to a second short column. Although the resolving power of the second column is necessarily limited, the fact that a column precedes it produces high-resolution separations.

Gas-solid chromatography is based on adsorption of gaseous substances on solid surfaces. Distribution coefficients are generally much larger than those for gas-liquid Chromatography, hence it is useful for separating species that are not retained by gas-liquid columns. Gas-solid 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.