INTEGRATED
COMPRESSOR
ZA2A
Zero air, nitrogen and hydrogen for gas chromatography
What is gas chromatography?
Gas chromatography (GC) is a widely used and very sensitive chemical method for separating and analyzing the components of a gas mixture. It is particularly suitable for volatile samples that are easily converted into gases and are stable when heated. Examples include residual solvent analysis, blood alcohols, metabolic fatty acids and the analysis of drug abuse.
How does the process work?
Gas chromatography is based on a separation principle. The sample is injected into the injector of the gas chromatograph and vaporized. A carrier gas transports the vaporized sample through a column. This is a long, hollow, coated glass tube with a narrow inner diameter. The inside of the column is coated with a substrate (the stationary phase) through which the gas (the mobile phase) containing the sample mixture is passed. As the mobile phase moves through the column, the components separate due to their different interactions with the stationary phase based on physical and chemical properties. As a result, different compounds move through the column at different velocities and allow the complete or partial separation of mixtures into the individual components.
The separated components then leave the column and pass through a detector which records the quantity of each component. This is done using suitable detection methods such as flame ionization detectors (FID), thermal conductivity detectors (TCD) or mass spectrometry (GC-MS).
Which gases are used in gas chromatography?
High-purity operating gases and an appropriate gas supply system are important prerequisites for the trouble-free and reliable operation of the gas chromatograph. Different gases are used for this purpose:
As detector gas
The most common use of zero air in GC is to provide oxidizing gas for detection. The most common flame ionization detectors (FID) measure the electrical conductivity of a very clean hydrogen/zero air flame to measure the presence of hydrocarbons in the sample. As a hydrocarbon detector, good performance depends on the absence of residual hydrocarbon from sources other than the sample, such as the burner air supply. For this reason, zero air is essential for sensitive and reproducible GC-FID analysis.
As carrier gas
Nitrogen is often used as a carrier gas as it does not react with the sample components. Carrier gases transport the sample through the GC column. As helium has become significantly more expensive and difficult to obtain as a carrier gas in recent years, nitrogen is gaining in importance. Nitrogen is chemically inert, readily available, inexpensive and an ideal choice for general applications in gas chromatography.
As detector gas
In the thermal conductivity detector (TCD), nitrogen is used as a pure carrier gas for comparative measurement with the gas from the separation column. The gas to be analyzed flows through one cell, while pure gas flows continuously through the other measuring cell and is used for comparative measurement. If pure carrier gas such as nitrogen flows through the measuring cell, the thermal conductivities in the measuring and reference cells are the same. However, if a sample component is added to the carrier gas, the thermal conductivity of the gas mixture changes in comparison to the pure carrier gas in the reference cell. This change generates a signal that is recorded.
Hydrogen is another frequently used carrier gas
Hydrogen offers the advantage that it enables faster separations and therefore shorter analysis times due to its lower viscosity and higher diffusion coefficient compared to helium.
As a detector gas
Flame ionization detectors (FID detectors) require hydrogen as a fuel gas for the flame. The sample from the GC column is fed into a hydrogen/air flame. This ionizes the organic compounds in the sample. The ions generate an electric current, which is measured and converted into a signal indicating the amount of hydrocarbon-containing substances present in the sample. Hydrogen is also used in the Thermal Conductivity Detector (TCD) as a pure carrier gas for comparative measurement with the gas from the separation column. TCD is used to detect permanent gases and noble gases, among other things, but nitrogen, hydrogen, carbon and sulphur oxides can also be detected.
Installation example
Suitable products
ZERO-AIR GENERATORS
ULTRA-HIGH PURITY NITROGEN GENERATORS - PURITY UP TO 99.9995%
HYDROGEN GENERATORS
The advantages of gas generators in analytical chemistry
GC detectors require carrier and fuel gases. These can be supplied from a cylinder or alternatively from a gas generator.
The main advantages of gas generators over cylinders are:
- Long-term cost reduction due to the relatively high demand for gases.
- Reduced risk due to the delivery, handling and storage of high-pressure cylinders.
- Elimination of batch-to-batch reproducibility between different cylinders.
- No risk of the cylinder running out during long or night-time analysis runs.
- Low workload for maintenance and manual handling.