How Gas Chromatography Systems Work & How Leasing Benefits Labs

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Gas chromatography (GC) is a technique used to separate and analyze volatile and semi-volatile compounds, which easily vaporize at room temperature.

Chromatography diagram

This process relies on the interaction of molecules with the mobile and stationary phases within the GC system. Less volatile compounds interact more with the stationary phase, resulting in slower movement, while more volatile ones interact more with the mobile phase, leading to faster movement. When the analyte is detected, the computer generates a peak indicating the retention time of the sample. The area under this peak provides concentration information, collectively forming a chromatogram.

Gas chromatography systems, also known as GC systems or gas chromatographs, leverage these principles to conduct precise compound analyses, and are essential instruments in various industries where the isolation and analysis of volatile compounds is necessary.

There are a number of components used in GC systems, including (we’ll cover these components in further detail below):

  • Carrier Gas: The mobile phase in a GC system is a carrier gas, usually an inert gas like helium. Helium is the preferred choice for its nonreactive nature.
  • Stationary Phase: This refers to a microscopic layer of polymer or liquid set on an inert support inside tubing, typically made of glass or metal.
  • Injector: The injector is responsible for introducing the sample into the system. It must ensure that the sample is vaporized and transported into the column efficiently.
  • Column: This is the heart of the system, where separation of compounds takes place. The stationary phase-coated column interacts with the compounds, causing them to elute at different times.
  • GC Detector: The detector quantifies the separated compounds. Various types of detectors, like Flame Ionization Detectors (FID) or Mass Spectrometers (MS), can be used based on specific analytical needs.

During gas chromatography, the separation occurs due to the interaction between the molecules and the stationary phase. Less volatile molecules interact more with the stationary phase, leading to slower movement, while more volatile molecules interact more with the mobile phase, resulting in faster movement. When the analyte is detected, the computer generates a peak corresponding to the retention time of the sample. The area under the peak provides information about the sample's concentration, forming a chromatogram.

Several renowned companies specialize in manufacturing high-quality GC systems. Some of the industry leaders include Agilent Technologies, Thermo Fisher Scientific, Shimadzu Corporation, and PerkinElmer, among others.

Gas chromatography systems find extensive applications in diverse fields, including life sciences, pharmaceuticals, and industrial manufacturing. They are crucial for tasks such as compound separation in mixtures and purity testing.

One limitation to be aware of is that GC systems can only analyze compounds up to a certain molecular weight. It's essential to consider this factor when selecting an appropriate system for a specific analysis.

Methods, Benefits, & Aerograph Machine Cost

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While there are many modes of GC, such as vapor-phase chromatography (VPC) and gas-liquid partition chromatography (GLPC), they all refer to the same separation of gases. Many methods exist to customize the analytical uses of gas chromatography, and are as follows:

Carrier Gas

Standard carrier gases include helium, hydrogen, argon, and air, but the gas that is chosen is generally determined by the specific detector being used. Other times, it is chosen to match the sample as the chosen gas will not show up on the readout, which can be helpful on occasion.

The mobile phase carrier gas is typically held within a cylinder and is passed through a molecular sieve as it enters the GC column and interacts with the stationary phase. This sieve removes unwanted impurities, such as hydrocarbons, water vapor, and oxygen, which can all cause base line noise and reduce sensitivity, possibly increasing detection limits.

Furthermore, the flow rate of the carrier gas is an important consideration. That’s because, while a high gas flow rate can potentially reduce retention times, it can also lead to poor separation. Making sure to perform GC using an ideal flow rate will result in higher resolution and better measurement.

Inlet Type

The inlet, or injector, is dependent upon whether the analyte is in a liquid, gaseous, solid, or adsorbed form and whether the sample has to be vaporized. Already dissolved samples can be added directly onto the column, while gaseous samples are often injected using a gas valve system. Adsorbed samples that need to be vaporized have their own injection methods.

Some of the common sample injection methods used are categorized as either hot and cold injection. Hot injection methods include split/splitless (S/SL) and total volume injection, while cold injection methods include cold on-column injection (OCI) and programmed temperature vaporization (PTV).

Column Selection

GC column choice is similarly dependent on the sample, with the polarity of the mixture being paramount. The polarity of the column stationary phase needs to match closely with the sample to increase resolution and decrease run time.

The two most common columns used in gas chromatography include packed columns and capillary columns. Packed columns are short, thick columns made of glass or stainless steel tubes and have been used since gas chromatography was introduced. Despite having low separation performance, packed columns can handle large sample volumes and are typically not susceptible to contamination. Using packed columns, the vapors (carrier gas) come into the stationary phase, achieve equilibrium with the moving gas phase and come back again in a series of steps, adsorbing and desorbing. Each step is called a theoretical plate.

Capillary columns are more commonly used these days due to their ability to perform excellent separation. These columns are well-suited for analysis that requires high-sensitivity. They consist of a thin, fused silica glass tube that has an internal liquid phase coating.

Column Temperature

The column in a GC is located inside an electronically controlled oven, so when speaking about the column temperature it refers to the heat of the oven which governs the column’s temp. The most important factor in determining temperature is finding the right compromise between how long the analysis will go and how much separation needs to occur.

This is important in the same way gas flow rate is important. If you use a high temperature, the sample will move through the column faster, (in conjunction with flow rate of the mobile phase). However, the faster it moves, the less it interacts with the stationary compound, and the less it interacts with the stationary compound, the less the sample separates. This can lead to suboptimal analyses.

Stationary Compound

Polarity is once again paramount when choosing the stationary compound, as it should have similar polarity to the solute. Common choices include, but are not limited to, cyanopropylphenyl dimethyl polysiloxane, bis cyanopropyl cyanopropylphenyl polysiloxane, carbowax polyethylene glycol, and diphenyl dimethyl polysiloxane.

GC Instruments

The GC instrument itself is an important part of the entire system, and while extremely useful, can appear confusing mechanically when inspecting them from the outside. Understanding the components of the instrument, or chromatograph, can help make troubleshooting issues simpler. There are three main components:

  • GC Detectors: There are many possible detectors that can be used in a GC, but the two most common are the thermal conductivity detector (TCD) and the flame ionization detector (FID). These are commonly chosen as the TCD is universally applicable and FIDs are very sensitive toward hydrocarbons. In addition to TCD and FID, electron capture detectors (ECD) and nitrogen phosphorous detectors (NPD) are also used. ECDs are specifically used when halogenated compounds must be analyzed, and NPDs are used when nitrogen- or phosphorous-containing compounds must be analyzed.
  • Autosamplers: This component provides the inverse of what the inlet offers, offering a means of introducing the sample automatically into the inlets. There are several different types of autosamplers that can be interchanged based on the needs of your lab.

Furthermore, gas chromatography systems are often integrated with other chromatography techniques and systems. They are commonly paired alongside mass spectrometers as well. This is due to the fact that the combination of these powerful analytical methods results in increased functionality and throughput.

For example, GC/MS systems allow for the separation of complex mixtures, the quantification of analytes, and the identification of unknown peaks, as well as the determination of trace levels of contamination in various samples at increased rates.

GC/MS System Leases for R&D in Biotechnology & Pharmaceuticals

If you're looking to utilize gas chromatography systems for the precise separation and analysis of organic compounds, or if you're considering combining the analytical capabilities of mass spectrometry with gas chromatography, Excedr has you covered.

By leasing your next GC/MS system with Excedr, you'll enjoy benefits such as reduced upfront costs, safeguarding your instrumentation with expert repair and preventive maintenance, and managing your finances more effectively with affordable and predictable scheduled payments.

Additionally, if you're in need of standalone or tandem mass spectrometers, we have the expertise to assist you in that area as well.

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Popular GC Equipment Manufacturers

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