How Gas Chromatographs Work & How We Save You Time & Money

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.

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).

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.

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.

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.

These instruments are extremely useful but can seem somewhat confusing mechanically when inspecting them from the outside. Understanding the components of the 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.
• Inlets: The choice of inlet allows the user to introduce a continuous flow of carrier gas to the sample. There are many different injector options, with many being more effective depending on the type of carrier gas that is going to be used.
• 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.