Chromatography is a widely used laboratory technique that allows scientists to separate complex mixtures into their individual chemical components. This method is employed across many fields, including health, environmental science, and forensics. All forms of chromatography rely on the differential distribution of a sample between two distinct components: the stationary phase, which remains fixed, and the mobile phase, which flows through the system. The mobile phase is the active fluid that drives the entire separation process.
What Defines the Mobile Phase
The mobile phase is the moving fluid that travels through the chromatography system and carries the sample mixture (analytes) along with it. It can be a liquid or a gas, depending on the specific technique used. Its primary function is to transport the analytes through the stationary phase, which is a fixed material typically packed inside a column or coated onto a surface.
The mobile phase must be able to dissolve the sample components so they can be transported smoothly through the system. One can visualize the mobile phase as a river providing the continuous force that moves everything forward. The movement of the mobile phase ensures that the sample components have repeated opportunities to interact with the fixed material. This constant motion sets up the dynamic equilibrium necessary for separation to occur.
Driving the Separation Process
The mobile phase facilitates separation by controlling the balance of interactions experienced by the sample components. Each component competes for time between being dissolved in the flowing mobile phase and being temporarily held by the stationary phase. Components that spend more time dissolved in the mobile phase are carried along faster and exit the system sooner. Conversely, components more attracted to the stationary material lag behind.
This differential migration is the core principle of chromatography and is directly influenced by the mobile phase’s properties. The speed at which a substance travels through the system is its elution rate. The mobile phase possesses “elution strength,” which describes its ability to pull the analytes away from the stationary phase. A mobile phase with high elution strength causes components to move quickly, resulting in shorter analysis times.
Scientists can precisely control the separation by adjusting the properties of the mobile phase. For instance, in reversed-phase liquid chromatography, the mobile phase is a mixture of water and an organic solvent, such as acetonitrile or methanol. Increasing the proportion of the organic solvent increases the mobile phase’s elution strength, causing the analytes to spend less time on the stationary phase. Adjusting this solvent ratio allows the analyst to fine-tune the interactions and achieve the desired separation. The selection of the mobile phase’s composition, polarity, and pH level determines the overall success and resolution of the analysis.
Physical States of the Mobile Phase
The physical state of the mobile phase dictates the type of chromatography being performed and the kinds of samples that can be analyzed.
Liquid Mobile Phase
The most common form is a liquid mobile phase, which is used in all types of Liquid Chromatography (LC). In this technique, the mobile phase is a single solvent or, more commonly, a controlled mixture of solvents, such as water combined with organic liquids like methanol or acetonitrile. The liquid mobile phase acts as a solvent to keep non-volatile and thermally unstable compounds dissolved as they traverse the separation column.
Gas Mobile Phase
A gas mobile phase is employed in Gas Chromatography (GC), a method reserved for volatile compounds that can be easily vaporized. The mobile phase here is not a solvent but an inert carrier gas, typically helium, nitrogen, or hydrogen. This gas does not interact chemically with the sample components; instead, it simply serves as an unreactive stream to push the vaporized analytes through the column. Since the gas mobile phase is unreactive, the separation in GC is driven almost entirely by the components’ interaction with the stationary phase.
Supercritical Fluid Mobile Phase
A third, less common state is the supercritical fluid mobile phase, used in Supercritical Fluid Chromatography (SFC). A supercritical fluid is a substance, often carbon dioxide, that is heated and pressurized beyond its thermodynamic limits, where it no longer exists as a distinct liquid or gas. In this hybrid state, the fluid gains the density and dissolving power of a liquid while retaining the low viscosity and high diffusivity of a gas. This combination allows it to penetrate the stationary phase quickly and dissolve samples effectively, offering faster analysis times for certain compounds compared to traditional liquid chromatography.