Lec 10 Dr.Thukaa.pdf

Gas – liquid chromatography (GLC) 2 Dr. Thukaa Z. Abdul-Jalil Lec 10 Detectors : Detectors detects the emerging sample in the gas form usually the solute or sample emerge from the column in the form of a gas or steam. Type of detectors 1- Destructive usually integrated for quantitative analysis and...

Gas – liquid chromatography (GLC) 2 Dr. Thukaa Z. Abdul-Jalil Lec 10 Detectors : Detectors detects the emerging sample in the gas form usually the solute or sample emerge from the column in the form of a gas or steam. Type of detectors 1- Destructive usually integrated for quantitative analysis and include: Mass spectral Flame ionization Nitrogen-phosphorus (NPD) Flame –photometric (FPD) Electrolytic conductivity 2- Non destructive usually integrated for qualitative analysis and include: Thermal conductivity (TCD) Electron capture (ECD) Photo ionization (PID) Qualitative analysis in GLC: One of the important parameters used in qualitative analysis in GLC is the retention time Rt The time taken for a particular compound to travel through the column to the detector is known as its RETENTION TIME. 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. For a particular compound retention time will vary depending on: Length of the column ex : when the column is short , then the retention time is reduced because it will take shorter time for solute to emerge from 3ft column than 5ft column. The boiling point of the compound. A compound which boils at a temperature higher than the column temperature is going to spend nearly all of its time condensed as a liquid at the beginning of the column, so high boiling means a long retention time. The solubility in the liquid phase. The more soluble a compound is in the liquid phase, the less time it will spend being carried along by the gas. High solubility in the liquid phase means a high retention time. The temperature of the column. A higher temperature will tend to excite molecules into the gas phase-either 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. The gas flow has also a big role in controlling the retention time , if the flow of the gas increase the retention time decrease because the gas will flow faster carrying the vaporized solute & so the solute will emerge faster & hence the Rt is decreased. Quantitative analysis : Quantitative estimation in GLC is done by measuring the area under the peak. This area is measured by several techniques : Estimate the area by triangulation i.e. draw tangents to the points of inflection on the peak sides & compute the area of the triangle formed with the base line. Use a mechanical or electronic accessory to the recorder which will automatically integrate the area ( some devices operate directly from the detector ). If the peak is too sharp (narrow) then the area can not be measured accurately , in this case we use the peak height as a measure of the amount provided a proper calibration factor is determined. Advantages of gas chromatography Very good separation Time (analysis is short) Small sample is needed-ml Good detection system Quantitative analysis Disadvantages of gas chromatography or limitations of its use Require specific workers with very high training and experience