“The only source of knowledge is experience” — Albert Einstein
The Gas Chromatograph appears to be a black box to the novice but an understanding of the components and their role will help raise your comfort level and help you generate data of highest reliability. You will also appreciate the role of preventive maintenance for long time usage and high reliability of results.
Gas Chromatography is helpful for separation, identification and quantification of component compounds in a mixture. In contrast to the High Performance Liquid Chromatography technique Gas Chromatography uses a gas as carrier mobile phase instead of a liquid. It is specially suited for compounds that are volatile, thermally stable and have low molecular weights.
The carrier gas is led at a constant flow rate to the column packed with the stationary phase. Before entering the column the sample mixture (gas or liquid) is injected into the carrier gas stream. The liquid mixture is vaporized to the gaseous state by the high temperature inside the injector before being led to the column. On reaching the column the sample components are selectively retained on the basis of physico – chemical interactions between the analyte molecules and the stationary phase. The mobile gas stream moving at a steady rate elutes the mixture components in a sequence determined by the operating conditions. Different detectors are employed for detection and quantification of eluted compounds. You will now be introduced to the significant role of each part of the GC system.
Carrier gas serves to transport the injected sample to the system. The gas selected for the purpose is inert to the sample and column packing material. Care should be taken to remove residual moisture or other gaseous impurities.
Injectors are used to provide constant volume injection of sample into the carrier gas stream. Inertness and reproducibility of injection are essential to maintain high level of accuracy. The injectors should be able to handle small volumes of the order of microliters with highest precision.
Columns used in Gas Chromatography can be either short length (1.5 – 2m) stainless steel or glass of long length (30m or above) . Capillary columns are made of fused silica flexible tubing. Packed columns are packed with stationary phase whereas capillary columns are coated with a layer of absorbing phase on the inside wall of the capillary tubing. A column is a vital component and should be maintained properly as per supplier instructions for getting long time usage and reproducible separations run after run.
Column oven houses the column and maintains constant temperature (isothermal operation) or variable temperature as per the requirement of analysis (temperature programming). The set temperature or the temperature variation should be precise and repeatable to ensure reproducibility of separation as minor temperature changes lead to changes of retention time of eluted components.
A detector provides specific response for the separated components. Majority of the organic compound applications require flame ionization detector and for specific requirements detectors such as NPD, FPD, TCD, etc. can be used.
Data Acquisition & Control
Modern GC systems are computer based and software controls operational parameters such as carrier gas flow rate, temperature programming, injection volume and sequence as well as acquisition and treatment of data output.
These are the main parts of the basic GC system configuration. Specialized applications require a combination of hardware or dedicated analyzers which are covered in our upcoming online certificate programme on GC. The subsequent modules will introduce you to the components in greater detail.
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