The Environics, Inc. Post

The Working Principles of Gas Chromatography

Posted by Dr. Rachel Stansel on Tue, Nov 20, 2018 @ 04:29 AM

Gas chromatography is a novel technique for separating and quantitating vaporized compounds using an inert carrier gas. It operates on similar principles to column permeation chromatography, where a sample is dissolved in a mobile phase and passed through a porous stationary structure.

Compounds are characterized and quantified by the time it takes for them to elute from the permeable column. This factor is determined by multiple characteristics, including molecular weight, hydrodynamic behavior, and concentration in the mobile phase.

The differentiating factor between gas chromatography and standard chromatographic methods is the use of a vapor rather than a fluid as a mobile phase, and a liquid as opposed to a solid for the stationary phase.

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How Does Gas Chromatography Work?

Non-reactive gases such as helium (He) or hydrogen (H) are used as a carrier for vaporized molecules of interest. This gaseous mixture is flowed through the column of a gas chromatograph, which comprises a microscopic fluidic membrane and an inert, solid substrate. This column partitions vapors based on their mechanical properties and their affinity with the stationary fluid. The flow-through rates of the sample’s constituent parts can be used for compound detection, identification, quantitation, and purification.

Elution in gas chromatography is electronically monitored using a detector at the outlet stream of the permeable column. This measures the retention time (tR) of compounds to qualitatively determine their adsorption characteristics with packing media of distinct chemical compositions.

Challenges in Gas Chromatography

One of the ongoing challenges in gas chromatography remains calibration of detectors. Numerous detector types are integrated with gas chromatographs including mass spectrometers, flame ionization detectors, and thermal conductivity detectors. Each of these systems operates on unique principles and must be calibrated to detect minute gas concentrations in real-time with high selectivity.

The performance of gas chromatography calibration requires specialized gas mixers to generate a test mobile phase with desirable concentrations. This is dependent upon the application area, but typical gas chromatography studies require mass accuracy levels to the parts per million (ppm) and parts per billion (ppb) range. Mixers must also be able to match the desired thermal mass flow rate and pressure ranges of test conditions.

Gas Chromatography Solutions from Environics

Environics has decades of experience producing gas mixers, diluters, and dividers for gas chromatography calibration. The Model 4000 systems are as much as ten times more accurate than standard thermal mass flow meters and can be customized for the properties of the gases of interest. This enables reliable detection of compounds of interest and preparatory observation of concentration levels, with the establishment of an accurate and repeatable calibration curve.

Our Zero Air Generators (ZAG) are designed for ultimate flexibility and low-cost calibration of gas chromatographs. They can provide a continuous stream of contaminant-free air at flow rates of 20 liters per minute (SLPM) and pressures of 30psi either as a stand-alone system or using an existing compressed air source.

Our range of systems for gas chromatography calibration includes:

  • Series 4000 Multi-Component Gas Mixing System;
  • Series 4020 Computerized Gas Mixing and Dilution System.
  • Series 4040 Gas Dilution System.
  • Series 7000 Stand-Alone ZAG;

If you would like any more information about our products or services for gas chromatography applications, please do not hesitate to contact us.