Volatility and boiling points determine how quickly the components of the mixture move through the column.
The components of the mixture leave the column after definite intervals of time, known as the retention time. Each component has its own retention time, which can be compared with tables of known retention times to identify unknown compounds. Retention times are always the same, so long as the conditions remain the same. A detector detects changes in the carrier gas as it leaves the column, and a recorder records a peak for each component. Each peak identifies a different component in the mixture being separated.
Gas/liquid chromatography is used by the police and forensic scientists, to detect small quantities of organic compounds, and can be very useful in day to day life.
Instrumental components
Carrier gas
The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.
Sample injection port
For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapour - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro syringe is used to inject sample through a rubber septum into a flash vapouriser port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/splitless injection is used. Have a look at this diagram of a split/splitless injector;
The injector can be used in one of two modes; split or splitless. The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). The sample vapourises to form a mixture of carrier gas, vapourised solvent and vapourised solutes. A proportion of this mixture passes onto the column, but most exits through the split outlet. The septum purge outlet prevents septum bleed components from entering the column.
The difference between Medicines and Drugs
People often get confused between the definition of a drug, and that of a medicine. There is a distinct difference- a drug is something which the doctor prescribes, whereas a medicine is what you as a patient can purchase from the chemist. It is an overall name given to the mixture, which consists of the active drug which the doctor prescribed, and any other substances such as sugar and gelatine. Contrary to what many people believe, medicine does not just come liquidised in a bottle- it can come in all shapes and sizes, in the form of tablets, capsules and even injections.
