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An analytical technique known as gas chromatography (GC) is used to identify, separate, and quantify the chemical components of a sample mixture. Often, these chemical components are gases or organic compounds. These components must be volatile, typically with a molecular weight below 1250 Da, and thermally stable to avoid degradation in the Gas Chromatography system in order for GC to be successful in their analysis.
By dividing the sample between two phases—a stationary phase and a mobile phase—in gas chromatography, the components of a sample are dissolved in a solvent and vaporised to separate the analytes. The analyte molecules are transported through the heated column by the mobile phase, which is a chemically inert gas. One of the only types of chromatography that does not use the mobile phase to interact with the analyte is gas chromatography. Gas-solid chromatography (GSC) uses a solid adsorbent as the stationary phase, while Gas-Liquid Chromatography uses a liquid on an inert support (GLC).
An injection port, a column, carrier gas flow control equipment, ovens and heaters to regulate the temperatures of the injection port and the column, an integrator chart recorder, and a detector make up a conventional Gas Chromatograph.
In Gas-Liquid Chromatography, a solution sample containing the desired organic compounds is injected into the sample port and allowed to evaporate in order to separate the components. The injected vaporised samples are then carried by an inert gas, which is frequently nitrogen or helium. This inert gas passes through a silica-filled glass column that is liquid-coated. Less soluble materials in the liquid will have a faster rate of rise than more soluble materials. This module’s goal is to provide you a better grasp of separation and measurement techniques as well as their use.
The x-axis of a Gas Chromatogram typically shows the time it takes for analytes to pass through the column and reach the mass spectrometer detector. The peaks represent the times when each of the components arrived at the detector.
The retention time is greatly influenced by the type of column used in the analysis as well as the GC parameters (e.g. flow rate, injection temperature, oven temperature, etc.). As a result, it’s critical to use the same parameters when comparing retention times from different analyses or labs to ensure accuracy.
The y-axis, or peak area, is typically a reflection of the amount of a specific analyte present. The area of a GC/MS chromatogram is determined by the number of counts taken by the mass spectrometer detector at the point of retention.
However, some compounds will have a higher affinity with the detector and the peaks will appear larger than the actual concentration in relation to the other peaks on the chromatogram, which is common in compounds that ionise easily. To address this issue, our experts use standards containing known concentrations of compounds to ensure accurate counts. Furthermore, unknown compounds are identified based on their retention times with other detectors of known standards. The mass spectrometer detector then identifies a compound based on the mass spectrum obtained during testing.
The following is a list of the factors that affect the resolution of the peaks produced by the GC.
Temperature of the column the unusually high temperature of the column is the result of both a low RT and poor analyte separation, because all of the components are primarily in the gas phase. On the other hand, in order for the analytes to separate, they need to interact with the stationary phase. The polarity of compounds is frequently connected with the boiling point of those compounds. If the compost has a low boiling point, then a higher vapour pressure will result in a shorter retention period. This is because the compost will spend a greater proportion of its total duration in the gas phase.
The tops almost always have an irregular or asymmetrical shape. When the sample solution’s concentration and volume are both excessively high, there will be tailing of the peaks, which will be the cause of poor separation. The detectors that are utilised in the GC are incredibly sensitive and do not call for a significant amount of additional hardware in order to put off a detectable signal. For instance, Flame Ionisation Detectors (also known as FIDs), Mass Spectrometers (also known as MSs), Electrolytic Conductivity Detectors (also known as ELCDs), Flame photometric Detectors (also known as FPDs), Photoionization Detectors (also known as PIDs), and so on.
In most cases, it has been found that compounds having polarities that are comparable to one another have larger affinities towards one another. Because of the powerful interactions that take place, molecules that share polarities with the material that makes up the stationary phase will be kept for a longer period of time, and vice versa.
At higher flow rates, the carrier gas moves the sample components through the column at a faster rate, which in turn lowers the retention times. However, despite the fact that the analysis time is shortened, the resolution between the peaks worsens because the components have less time to interact with the stationary phase.
The resolution between the several components that are being separated gets better as the column length gets longer. Nevertheless, this is owing to enhanced longitudinal sample vapour diffusion occurring within the column, which results in peak broadening.
The selection of column diameter is determined by the sample concentration. If sample loading is allowed to exceed column capacity, the results will be a loss of resolution, poor repeatability, and peak distortions. The typical amount of sample loaded onto a column can range anywhere from 10 ng for 0.1 mm id columns all the way up to 2000 ng for 0.53 mm id columns. Resolution is improved by using narrow columns, however this comes at the expense of decreased loading capacity.
In most cases, the thickness of the film formed by bound liquids on the stationary phase falls somewhere in the range of 0.1 to 5.0 micrometres. Increased sample loading and longer residence times for highly volatile components can be accomplished with the use of thicker films, which also help to reduce co-elution. Unfortunately, this leads to longer retention durations and higher elution temperatures as the film becomes thicker. A more effective separation of the components of the sample can be achieved by selecting the optimal film thickness.
