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 liquid 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, packed 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 using a vapor rather than a fluid as a mobile phase and liquid samples as opposed to a solid for the stationary phase.

How Does Gas Chromatography Work?

An inert gas such as helium (He) or hydrogen (H2) are used as a carrier for vaporized molecules of interest. This gaseous mixture flows 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 affinity with the stationary fluid. The flow-through rates of the components of a sample can be used for compound detection, identification, quantitation, and purification.

Elution is the central process of gas chromatography. It refers to the process of separating and detecting different compounds in a sample. It involves the use of a permeable column, which is packed with a stationary phase material. As the sample is injected into the column, it interacts with the stationary phase, causing different compounds to be retained for different lengths of time.

To monitor the elution process, a detector is placed at the outlet stream of the column. This detector is capable of electronically measuring the retention time (tR) of each compound. The retention time is the amount of time it takes for a compound to travel through the column and reach the detector.

By measuring the retention time, the detector can provide qualitative information about the adsorption characteristics of the compounds. The packing media used in the column have distinct chemical compositions, which interact differently with the compounds in the sample. This interaction determines how strongly the compounds are retained and how long it takes for them to elute from the column.

The detector’s electronic monitoring allows for accurate and precise measurement of the retention time, enabling the identification and characterization of different compounds in the sample. This information can be used to determine the composition of a mixture, identify unknown compounds, or assess the purity of a sample. Overall, the electronic monitoring of elution in gas chromatography plays a crucial role in the analysis and separation of compounds in various fields such as pharmaceuticals, environmental analysis, and forensic science.

Challenges in Gas Chromatography

One of the ongoing challenges in GC analysis remains the calibration of detectors. Numerous detector types are integrated with gas chromatographs, including mass spectrometers, flame ionization detectors, and thermal conductivity detectors. Each system 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 depends on 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 calibration challenges can affect the performance of the detector by reducing its accuracy and selectivity in detecting minute gas concentrations in real time. Calibration requires specialized gas mixers to generate a test mobile phase with desirable concentrations, which must match the desired thermal mass flow rate and pressure ranges of test conditions. Gas chromatography calibration is a crucial aspect of the analytical process, as it ensures accurate and reliable results. The effectiveness of this calibration technique is highly dependent on the specific application area in which it is being used. Different industries and scientific fields may have varying requirements and standards for calibrating GC systems.

Succesful GC calibration hinges on the requisite mass accuracy. Often, the desired accuracy level is on the order of parts per million (ppm) or even parts per billion (ppb). This means the calibration process must must deal with small quantities. Achieving such high levels of mass accuracy is essential for various reasons.

  • Environmental analysis: GC calibration enables researchers to quantify trace amounts of pollutant in air, water, and soil samples. These can harm their surrounding ecosystem. Left untreated, they may impinge on human health. It is thus vital to ensure analytical accuracy in detection and quantification.
  • Pharmaceutical industry: Calibration ensures the reliability of QA/QC, linking to drug purity and potency. Tiny impurities or concentration variations can affect the efficacy and safety of medications.

To achieve the required mass accuracy levels, gas chromatography calibration involves several steps. First, a known standard sample with a known concentration of the target substance is analyzed using the chromatographic system. The system’s response to this standard sample is then used as a reference to calibrate the instrument.

During calibration, the instrument’s response to the standard sample is compared to the expected response based on the known concentration. Any discrepancies are used to adjust the instrument’s settings and parameters, ensuring that it accurately measures the target substance in unknown samples.

The calibration process may also involve the use of internal standards, which are substances added to the sample in a known concentration. These internal standards help correct for any variations in sample preparation or instrument performance, further enhancing the accuracy of the calibration.

In conclusion, gas chromatography calibration is a critical step in the analytical process, particularly in applications that require high levels of mass accuracy. By ensuring precise measurements in the parts per million (ppm) and parts per billion (ppb) range, gas chromatography calibration enables accurate and reliable analysis in various industries and scientific fields.

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.