Gas Chromatography (GC) is an analytical technique used across various scientific fields to separate and analyze complex mixtures of compounds. It works by vaporizing a sample and separating its components based on their individual interactions with two phases: a stationary phase fixed inside a column and a mobile phase that flows through it. The mobile phase, often called the carrier gas, is an indispensable component, serving as the transport system for the sample.
Defining the Mobile Phase
The mobile phase in Gas Chromatography is always a gas, known as the carrier gas. Its primary requirement is chemical inertness, meaning it must not react with the sample components or the stationary phase inside the column. To maintain column integrity and analysis accuracy, the carrier gas must also be of extremely high purity, often 99.995% or higher. Trace impurities like oxygen or water vapor can damage the stationary phase, especially at the elevated temperatures used during separation.
Helium (He), Nitrogen (N₂), and Hydrogen (H₂) are the most common gases employed. Helium has historically been the preferred choice due to its inert nature, stability, and compatibility with many detectors. Hydrogen is frequently used because its low molecular weight offers advantages in speed and efficiency. Nitrogen, while abundant and inexpensive, has a higher molecular weight and lower optimal performance compared to the other two.
The Role in Separation
The carrier gas’s main functional purpose is to physically transport the vaporized sample from the injection port, through the separating column, and onward to the detector. Unlike in liquid chromatography, the mobile phase in GC does not actively participate in the separation mechanism by chemically interacting with the analytes. The separation itself is driven by the distribution, or partitioning, of the sample components between the stationary liquid phase and the moving gaseous mobile phase.
For a chromatographic analysis to be reproducible, the mobile phase must maintain a constant and controlled linear velocity, or flow rate, through the column. This velocity directly affects how long each compound stays in the column, which is known as its retention time. Precise control over the flow rate is necessary to ensure that retention times remain consistent across different runs, allowing for reliable identification of the separated compounds.
The linear velocity of the carrier gas directly influences the efficiency of the separation. If the flow is too slow, the components spend too much time diffusing longitudinally, causing the separated peaks to broaden. Conversely, if the flow is too fast, the sample components do not have enough time to properly partition between the stationary and mobile phases, leading to reduced separation quality.
Selecting the Carrier Gas
The choice of carrier gas is a practical decision that weighs performance against cost, availability, and detector requirements. The most significant factor is compatibility with the detector being used in the gas chromatograph. For example, the Thermal Conductivity Detector (TCD) often requires Helium because its low thermal conductivity compared to most organic compounds ensures a high signal difference.
In contrast, the highly sensitive Flame Ionization Detector (FID) is largely compatible with all three primary gases—Helium, Hydrogen, and Nitrogen—since the detector uses a separate supply of hydrogen as its fuel. For Mass Spectrometry (MS) detectors, Helium has traditionally been the standard due to its inertness and compatibility with the vacuum systems used in the MS interface. However, due to recent helium shortages and rising costs, laboratories are increasingly exploring alternatives.
Hydrogen offers the best chromatographic efficiency and provides the fastest analysis times because its optimal linear velocity is significantly higher than that of Helium or Nitrogen. The optimal velocity for hydrogen is typically around 40 to 80 cm/s, while Helium’s is lower, often 25 to 40 cm/s. However, the use of hydrogen introduces a safety consideration because it is flammable and requires additional safety measures. Nitrogen, while the safest and cheapest option, requires much slower flow rates to achieve good separation efficiency, often leading to longer analysis times.