The chemistry lab is full of tools and equipment that are completely unknown by most engineers, but offer significant capabilities for analysis. One of the fundamental pieces of equipment in a well-equipped lab is the Gas Chromatograph, or “GC” as we typically call it.
Gas Chromatography Overview
The technique that separates gases by the selective adsorption of molecules based on chemical properties is known as gas chromatography. This technique separates known or unknown chemical species (known as analytes). Analyte is a term used to describe the sample characteristics of a particular analysis. To achieve this separation, a thin film is coated inside a column, commonly called the stationary phase. A column is a path length designed to force analytes to interact with the coated film. Separation of analytes occurs because of the interaction with the coated column. When the sample is injected into the GC, it is vaporized into the gas phase, then carried into the column by a carrier gas (typically an inert gas such as nitrogen or helium).
The carrier gas is called the mobile phase. The components will separate based on the column chemistry, and those individual analytes will enter a detector. In GC, flame ionization (FID) or thermal conductivity (TCD) are common methods to probe the separated chemicals by measuring energized components, through which a unique signal is sent to a computer. The computer software then builds a spectrum called a chromatograph that is analyzed, and the sample’s composition is determined. A typical spectrum is the amount of measured signal plotted versus the time through the column. A very basic diagram of a GC setup is as follows:
The peak separation on the gas chromatograph can be tweaked based on type and length of column used, flow rate of the carrier gas, and temperature of the oven. Most GC columns are capillary columns where the inner lining is thinly coated with a stationary phase, such as silica. By playing with these different parameters, distinct peaks can typically be obtained and used for further analyses of samples.
Types of Detectors
Two common types of detectors are flame ionized detector (FID) and thermal conductivity detector (TCD). FIDs use a hydrogen flame to combust various organic compounds to form ions. These ions are then captured by an electrode and the current is measured. The more ions generated, the more of an organic species is present. The largest downside of a FID is that it cannot detect water, CO2, or other inorganic compounds. TCDs measure the thermal conductivity of a gas stream compared to a reference gas stream. Thus, it detects most chemical compounds and is typically a safer detector because it does not require hydrogen gas to operate. TCDs are also non-destructive samplers thus putting a TCD in series with an FID is also a common strategy.
Inverse GC
Inverse Gas Chromatography is useful for determining the relationship between a solute and a desired stationary phase, and is shown above. The columns used for this method are packed columns. If the stationary phase is a solid such as some polymers or specialized particles, the columns can be easily packed with the solid. If the coating is a liquid, the liquid needs to be loaded on a stable support, such as inert silica, and packed into a tube. The solute, either a gas or a liquid, is injected into the column. The residence time, how long the solute takes to go through the column, is recorded and can be used to calculate infinite dilution activity coefficients or Henry’s Law Constants (for liquids or gases respectively). These values can then be used comparatively to determine what stationary phase to use in a desired separation process such as liquid-liquid absorption.
Both methods use the same instrument, just modified to fit the different types of columns used in an experiment, making a GC a versatile and quick tool for use in a separations-oriented laboratory.
Our chemists and chemical engineers have a deep understanding of these techniques and apply them to a wide range of projects here at Sparx Engineering. If you need assistance in the development of chemical-related technologies, we are here to help.
One Response
This was a great and informative article. If you can do one one spectrometry and how light waves can identify different elements, I would be very interested. This is something I have never fully understood.