ANALYTICAL METHODS/Geochemical Analysis (Including X-ray) 69 saturated, aromatic, and resin groups, and will not be covered further here Gas Chromatography Gas chromatography (GC) – specifically gas–liquid chromatography – involves a sample being vaporized and injected onto the head of the chromatographic column (Table 4) The sample is transported through the column by the flow of an inert gaseous mobile phase The column itself contains a liquid stationary phase that is adsorbed onto the surface of an inert solid (Figure 17) The carrier gas is chemically inert (e.g helium) A sample of gas or petroleum is injected into the column quickly as a slug to prevent peak broadening and loss of resolution The temperature of the sample port is somewhat higher than the boiling point of the least-volatile component of the sample Sample sizes typically range from tenths of a microlitre to 20 ml The carrier gas enters a mixing chamber, where the sample vaporizes to form a mixture of carrier gas, vaporized solvent, and vaporized solutes A proportion of this mixture passes onto the chromatography column Chromatography columns have an internal diameter of a few tenths of a millimetre and walls coated with a liquid but stationary phase The optimum column temperature depends on the boiling point of the sample; typically a temperature slightly above the average boiling point of the sample results in an elution time of 2–30 As the carrier gas containing the chromatographically separated sample passes out of the end of the column, it is passed into one of a number of detectors such as a flame ionization detector, which has high sensitivity, a large linear response range, and low noise An example of a flame-ionization-detector signal from a wholepetroleum sample injected onto a GC column is given in Figure 18 One of the problems of GC analysis of geochemical samples is that different compounds can have similar elution times, rendering identification and quantification difficult However, the output stream from a gas chromatograph can be passed into other types of analytical instrument (e.g a mass spectrometer) for further analysis of the separated compounds over and above simple quantification of compounds with a common elution time GC is useful for analysing organic compounds and can be used to quantify mixtures if suitable standards have been employed Ion Chromatography Ion chromatography can be used for both cations and anions However, it is in the analysis of non-metal ions that the technique has proved most useful mainly because there are no real alternatives for the simultaneous quantitative analysis of these important species in waters or synthetic solutions Ion chromatography is used to analyse aqueous samples containing ppm quantities of common anions (such as fluoride, chloride, nitrite, nitrate, and sulphate) Ion chromatography is a form of liquid chromatography that uses ion-exchange resins to separate atomic or molecular ions based on their interaction with the resin (Figure 19) Its greatest utility is for the Figure 17 Essential components of a gas chromatograph The sample injection port is heated to volatilize liquid phase organics The GC column is held in an oven at a temperature above the boiling point of the compounds of interest The inert carrier gas drives the sample through the capillary with its stationary phase The stationary phase retards larger molecules more efficiently than small molecules Smaller molecules thus pass out of the column more rapidly than large molecules