Gas Chromatography

  The main components of the gas chromatographers are: carrier gas supply, injection ports or inlets, column (inside an oven), flow controllers, detector and a data system. The column is the essential part of the technique because it is where the separation occurs. The first columns were the packed columns, metal tubes of 1-2 meters length and 0.2-0.4 cm of inner diameter, packed with inert supports which were coated with the stationary liquid phases. Today, the fused silica columns are of widespread use and are open tubes called capillary columns, due to their small inner diameter (0.1-0.53 mm). The stationary liquid phase is coated on the inner surface of the tube, forming a coating with a thickness of around 0.1-5 µ m.

  Only volatile samples can be analyzed Difficult to implement for thermally labile samples Not appropriate for preparative scale or large samples Usually requires additional techniques, like mass spectrometry, for confirmation of peak identities.

  Efficiency is a measure of how well the separations can be performed and is expressed in plate numbers. Capillary columns are known to possess several hundred thousand plate numbers. The efficiency of a column increases with column length. In theory, having very long columns can make analysis very efficient, however and unlimited increase in length is not practical and nowadays complex separations are performed by using comprehensive two-dimensional GC.

  GC provides fast separations, and current commercial instrumentation permit analysis to be performed in seconds. A technique called comprehensive two-dimensional GC (GCxGC) has been developed recently, which implies the injection of the eluent from a traditional separation into a very short column, a second column. The second analysis requires just a few seconds to be completed. The result obtained is a GCxGC contour plot where we have the second dimension, represented in a y-axis.

  Fast analysis, in the order of minutes High resolution and efficient High sensitivity, can detect ppm and ppb. Is a nondestructive technique, can be coupled to a mass spectrometer Quantitative analysis is very accurate with RSDs in the order of 1-5%. Only small samples are needed (microliters) Reliable and relatively simple technique Inexpensive

  In a typical chromatogram for a single solute A, we can see a small peak at the beginning, short time after the injection (which is performed at time zero). The solutes are characterized by their retention times or retention volumes (tr, or Vr). These quantities are depicted as the distance between the injection point and the peak maximum. The formula to calculate the retention volume Vr is: Retention volume equals (“retention time” multiplied by “gas flow rate”) Or we can express it in terms of the retention time as: retention time equals (“Retention volume” divided by “gas flow rate”) Or in formula Vr = tr x Fc   or   tr = Vr / Fc; where Fc is the gas flow rate The small initial peak corresponds to a component that does not is sorb into the stationary phase, thus it is an un-retained component. The IUPAC defines Vm, the holdup volume as: “the volume of the mobile phase (MP) required to elute the un‐retained compound from the chromatographic column and report...

  Distribution constant Kc: defined as the concentration of a solute in the stationary phase, divided by its concentration in the mobile phase. Is a thermodynamic value, dependent on temperature. Different migration rates through the column among compounds are a consequence of the differences in distribution constant values.

 Symbols recommended by the IUPAC are indicated with (IUPAC) after their definition. The terms without this clarification are other symbols and names in use. Kc: distribution constant (for GLC) (IUPAC) Kp: partition coefficient KD: distribution coefficient k: retention factor (IUPAC) k′: capacity factor; capacity ratio; partition ratio N: plate number (IUPAC) n: theoretical plate number; no. of theoretical plates H: plate height (IUPAC) HETP: height equivalent to one theoretical plate R: retardation factor (in columns) (IUPAC) RR: retention ratio Rs: peak resolution R (IUPAC) alpha: separation factor (IUPAC) - Selectivity; solvent efficiency tR: retention time (IUPAC) VR: retention volume (IUPAC) VM: holdup volume (IUPAC) Volume of the mobile phase; VG: volume of the gas phase VO void volume; dead volume

  The distribution constant or partition coefficient Kc, is a measure of the tendency of a component to be attracted to the stationary phase. In chromatography, the greater this value, the greater the attraction of that component to the stationary phase. The differences in distribution constants, which are controlled thermodynamically, are the responsible for chromatographic separations. There are two types of sorption processes: Absorption: sorption into the bulk of stationary phase Adsorption: sorption on the surface of the stationary phase One of these processes is usually dominant, but both can be present.

  The output signal of the detector produces what is called a chromatogram. Its representation is a series of peaks at different times (independent variable) and the relative sizes of the peaks are indicative of the relative masses of each component. The retention factor “k” is provided by the ratio of the mass in the stationary phase to the mass in the mobile phase and is a very important chromatographic variable.

  At the beginning, a sample containing two components A and B, is introduced to the column in a narrow zone and then, it is carried through the column by the mobile phase. The components partition between the liquid and gas phase. The component A has greater affinity to the mobile phase, and is carried down the column faster than the component B, who has more affinity for the liquid phase. As a consequence of this process, component A leaves the column first and is detected. After that, B leaves the column and is detected.

  The use of a gas phase requires contained and leak-free systems. Such systems are metal or glass tubes, called columns. The columns contain the stationary phase. The columns are named using the name of the stationary phase. For instance, if the stationary liquid phase is polydimethylsiloxane (PDMS), then one can refer to it as a PDMS column.

  All techniques within GC can be classified according to the state of the stationary phase as follows Gas-solid chromatography (GSC): the stationary phase is a solid. Gas-liquid chromatography (GLC): liquid stationary phase. OT or capillary: are Open tubular or capillary columns and can be of GSC or GLC type. GLC is, by far, the most used technique.

  Elution: The process in which a mobile phase is continuously passed through or along the chromatographic bed and the sample is fed to the system as a finite slug. Chromatographic processes names are linked to the state of the mobile phase. Hence, the mobile phase is a gas in GC and a liquid in LC. In GC, the sample is vaporized and carried through the column by the carrier gas. The components of the sample equilibrate into and out the stationary liquid phase in the column. These equilibriums are dependent on temperature and the components separate from one another based on their affinities for the stationary phase and relative vapor pressures.

 Gas chromatography (GC) is a technique used for separation and analysis of volatile compounds. It can be used to analyze gases, liquids or solids, which are usually dissolved in volatile solvents. The molecular weight of the compounds analyze can vary in the molecular weight range from 2 to over 1000 Da. For example, it can be used to separate and analyze more than 450 components in coffee aroma.