Why a Mass Flow Controller is Supercritical for SFC​​​

​A mass flow controller precisely measures and regulates the mass flow rate of liquids and gases. In the Teledyne LABS ACCQPrep SFC (Supercritical Fluid Chromatography) system, the mass flow controller works in tandem with pumps and a back pressure regulator to maintain a stable flow rate of carbon dioxide (CO₂) within a preset range. This control is essential for achieving reproducible SFC results.

Why Mass Flow Control Ma​​​tters in SFC

Unlike liquid chromatography, where liquids are largely incompressible and flow can be reliably managed by pump stroke volume, supercritical CO₂ is compressible and highly sensitive to temperature and pressure fluctuations. This variability can impact flow rate and chromatographic performance. The MFC compensates for these changes, ensuring consistent delivery of CO₂ and co-solvent mixtures.

Supercritical CO₂ an​​d Mass Flow Control

In SFC, the mobile phase commonly used is supercritical carbon dioxide. CO₂​ is normally a gas (people and animals exhale it). When it is under a certain pressure and cold enough, it forms dry ice. Under specific conditions—high pressure and temperature above its critical point—it becomes a supercritical fluid. In this state, CO₂ exhibits both gas-like diffusivity and liquid-like solvating power, making it ideal for chromatography:

• Fast separations
  
• Environmentally friendly
• Non-toxic and non-flammable
  
• Easy to remove from samples 
  

However, supercritical CO₂ is highly sensitive to changes in pressure and temperature, which affect its density and compressibility. This variability makes mass flow control essential for maintaining consistent flow rates and achieving reproducible results in SFC. 

Because many compounds cannot be separated using supercritical CO₂ alone, it usually is used in combination with another solvent, such as methanol, ethanol, and others. You can learn more about supercritical carbon dioxide here.​

T​he Mass Flow Controller: How it works

The ACCQPrep SFC's MFC measures the actual mass of carbon dioxide flowing through it—regardless of variations in density due to incoming CO₂ temperature and pressure fluctuations.  The stability is maintained by a PID (Proportional-Integral-Derivative) control algorithm, a time-tested feedback mechanism originally developed for ship autopilots in the 1920s.​​ 

In SFC, where supercritical CO₂ behaves unpredictably due to its compressibility, the PID algorithm continuously adjusts flow to maintain consistent performance. Unlike liquid chromatography, which requires minimal correction, SFC demands dynamic control. The ACCQPrep SFC delivers it with precision and reliability.

Without mass flow controller.

With mass flow controller.

​The Proportional ​Component

Adjusts CO₂ flow based on the difference between the setpoint and the actual flow rate. The greater the difference, the larger the correction.​

The Integral Compo​​nent

Accounts for accumulated errors over time, such as reduced density when the pressure drops from CO₂ tank depletion. It dynamically adjusts the mass flow to maintain the programmed rate and solvent composition.

The Derivative ​​Parameter

Predicts future errors by analyzing the rate of change in flow difference. Helps prevent overshooting and stabilizes flow during rapid system changes, such as gradient transitions.

The Back Pressure​​ Regulator

The BPR maintains system pressure, which directly affects the density of the supercritical fluid inside the column. Higher backpressure increases fluid density, allowing:

• Higher sample concentration
• Earlier elution
• Potential changes in selectivity
  

The BPR adjusts dynamically during gradients, working in tandem with the MFC to ensure chromatographic stability and accurate solvent ratios.

The Mass Flow Controlle​​​r in Action

We've got everything under control!

With flow rate, co-solvent concentration, and backpressure tightly controlled, the ACCQPrep SFC achieves reproducibility comparable to preparative HPLC. This precision enables:

• Accurate method development
• Unattended operation
• Reliable stacked injections​
  

Frequently Aske​​d Questions 

What does a mass flo​w controller do?

A mass flow controller precisely measures and regulates the flow rate of liquids and gases. In the ACCQPrep SFC instrument, it is used to ensure stable CO₂ delivery within a preset range for consistent chromatography. 

What is the difference between a ​mass flow meter and a mass flow controller?

A mass flow meter only measures flow rate. A mass flow controller both measures and regulates flow, automatically adjusting to maintain desired conditions.

What are the two types of​​ flow control?

• Manual: Requires user intervention to adjust valves—less precise and inefficient for dynamic systems like supercritical fluid chromatography.
• Automatic: Uses electronic signals to continuously adjust flow—ideal for SFC and other high-precision applications.