Gas Chromatography & Gas-Liquid Chromatography

Both gas chromatography and gas-liquid chromatography work in a very similar way

Components Carrier gas: moves through the column, carrying the sample along (mobile phase) Sample Injection: A small amount of sample is injected into the column: Oven: Sample is heated to a sufficient temperature to instantly vaporise the sample, which is then swept into the column by the gas. Column: series of loops of inert solid, in gas liquid chromatography the column is coated with porous solid coated with liquid hydrocarbon with a high boiling point, the liquid acts as the stationary phase. Detector: Flame ionisation

How does it work? Gas chromatography separates chemicals based on the ease with which they evaporate into a gas. The chemical mixture being analysed is injected and carried through the instrument by a carrier gas (commonly nitrogen). There is a column in the oven that makes it possible to separate the chemicals based on how quickly they travel through it.

Injection of the sample A very small quantity of the sample being analysed is injected into the machine via the injection port using a syringe. The injector is contained in an oven in which the temperature can be controlled. It is hot enough so that all the sample boils and is carried into the column with the carrier gas.

The column in the oven The column is normally made of stainless steel and is between 1 and 4 meters long. It has an internal diameter of just 4mm. The column is coiled up so that it will fit inside the temperature controlled oven. Inside the column, it is packed with a very porous rock (solid which contains many minute channels or open spaces). Which is coated the a high boiling liquid, normally a waxy polymer. The temperature of the oven gets up to 250°C,  it is cooler than the injector oven, so that some components of the mixture may condense at the beginning of the column.

The separation. A few things may happen to a particular molecule once injected into the column: A compound with a boiling point higher than the temperature of the column will condense at the start of the column. Some molecules may dissolve in the liquid stationary phase. Some compounds will be more soluble in the liquid than others. The more soluble ones will spend more of their time absorbed into the stationary phase; the less soluble ones will spend more of their time in the gas.

Retention Time Retention time is the time taken for a substance to emerge from the column (when a peak is seen on the recorder) This time is measured from the time at which the sample is injected to the point at which the display shows a maximum peak height for that compound. Different compounds have different retention times

Factors that effect retention time: Boling Point – a compound that boils at a higher temperature than that of the column temperature is going to spend most of its time condensed as a liquid at the beginning. High boiling point means long retention time. Solubility in the liquid phase – The more soluble, the less time it will spend being carried along by the gas. High solubility means high retention time Temperature of the column – A very high temperature will excite molecules into the gas phase, because they evaporate more readily or because they are so energetic that the attractions of the liquid no longer hold them. A high column temperature shortens retention times for everything in the column. Low temperatures in the column means better separation, but it could take a very long time. High temperatures in the column means everything will pass through very quickly although less will be separated out.

Detector

Detector There are many types of detectors that can be used, although flame ionisation detectors are often used. The whole detector is enclosed in its own oven which is hotter than the column temperature. That stops anything condensing in the detector. As the sample emerges from the column, the sample mixture is burnt in a hydrogen-oxygen flame. This causes the sample to become ionised and a small electric current is produced. This current is amplified and results as a peak on the chart. During the process of burning organic compounds, small amounts of ions and electrons are produced in the flame. The presence of these can be detected. The more of the organic compound there is in the flame, the more ions will be produced, and so the higher the current will be. Disadvantage: It destroys everything coming out of the column as it detects it.

Interpreting the data. The output will be recorded as a series of peaks - each one representing a compound in the mixture passing through the detector. The areas under the peaks are proportional to the amount of each compound which has passed the detector.

Calibration A gas chromatography test can be calibrated by running at least 3 standards of different know concentrations. Record the retention time and area of each peak. Plot area vs. concentration. Run an unknown if it has the same retention time as your standards use it's area to calculate concentration.

Advantages & Disadvantages of Gas chromatography/ Gas-liquid chromatography It is the most sensitive of the chromatographic techniques Capable of being able to detect 10^-12g of a compound Suitable for volatile organics such as petrol. Disadvantages: Limited to compounds that can be readily vaporised without decomposing. The sample is destroyed when it goes through the detector, and so cannot be analysed any other way.

What gas chromatography can analyse. Drug analysis  toxicology organic compounds analysis of body fluids for the presence of illegal substances  testing of fibber and blood from a crime scene detect residue from explosives Urine samples that are routinely taken from athletes competing in major events to ensure they are not benefitting from the use of illegal, performance-enhancing drugs.

Qualitative analysis: Gas chromatography can be used for qualitative analysis by using the retention time. Molecules of a substance should always take the same time to move through the column at the same conditions. Quantitative analysis: Gas chromatography can be used for quantitative analysis because the higher the concentration, the more the flame will flare up. The area under the peak reflects the concentration of the substance. It can be compared to a standard solution to determine concentration.

How does it compare to other techniques? Gas chromatography is the most sensitive of all the chromatographic techniques. Its limited to such compounds that have a relative molecular masses less than 300. High performance liquid chromatography, can separate compounds with a relative molecular masses of over 1000.

Links http://www.chem.agilent.com/cag/7890A_5975C_Videos/AgilentPlayer.html http://www.youtube.com/watch?v=08YWhLTjlfo