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Introduction

Magnetic Suspension Balances (MSBs) have become indispensable tools for gravimetric analysis under extreme conditions—including high pressures, elevated temperatures, and corrosive atmospheres—where traditional microbalances fail. A critical innovation in AMI’s next-generation MSB is the Quadrupole Magnetic Control System (QMS), which advances the precision and stability of mass measurements even further.

This application note explains the unique advantages of the QMS, how it addresses the challenges of high-pressure adsorption experiments, and how it improves the reliability and flexibility of the AMI MSB platform.

Technical Challenge: Precision Under Dynamic Forces

In high-pressure sorption studies, maintaining stable suspension of the sample while loading or when subjected to external disturbances is a key challenge. Traditional MSB designs can experience:

• Transient forces during sample loadingcausing reading instabilities.
• Influence from external magnetic fieldsaffecting the electromagnetic coupling system, leading to signal drift or noise over long experiments.

Such disturbances can compromise the accuracy of both static sorption isotherms and kinetic adsorption curves—particularly in dynamic or long-duration experiments.

QMS Solution: Locking Stability and Shielding Precision

The Quadrupole Magnetic Control System (QMS) resolves these challenges through two core innovations:

1. Suspension Locking Mechanism
   During sample loading, the QMS activates a quadrupole magnetic fieldthat locks the suspension mechanism in place. This eliminates transient forces that could otherwise displace the sample or distort early data points.
2. External Magnetic Field Shielding
   The QMS generates a magnetic field precisely tuned to counteract external magnetic interferences. By stabilizing the electromagnetic coupling system, the QMS ensures that even in laboratories with variable magnetic fields, the measurement remains accurate and repeatable.

Key Benefits for Sorption and Gravimetric Applications

• Measurement Stability Over Time:
  QMS minimizes drift during long-term weighing experiments—critical for adsorption kinetics and endurance testing.
• Improved Accuracy in Dynamic Loading:
  Locking the suspension during sample introduction reduces error and improves reproducibility, especially for delicate samples or small mass changes.
• Broader Experimental Versatility:
  By reducing sensitivity to external magnetic fluctuations, QMS-equipped MSBs can operate in a wider range of laboratory environments without performance degradation.

Typical Use Cases

• High-pressure gas sorption (up to 700 bar)
• Multi-component competitive adsorption
• Gravimetric density measurements at extreme conditions
• Long-duration kinetic adsorption studies
• Corrosive environment testing (e.g., H₂S, SO₂)

Conclusion

The QMS exemplifies AMI’s commitment to pushing the boundaries of gravimetric measurement under real-world and extreme conditions. By enhancing suspension control and eliminating external magnetic influences, the QMS not only safeguards precision but also expands the experimental capabilities of the AMI MSB platform.

For laboratories demanding the highest standards in sorption science, material research, and gas storage studies, QMS delivers measurable improvements in accuracy, repeatability, and operational confidence.

QMS-MSB Schematic

QMS Performance: Measurement Deviation vs. External Magnetic Field Distance

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1. Research Background

Carbon dioxide (CO₂) is a major greenhouse gas, and technologies for its capture and storage are central to combating climate change. Among the available approaches, adsorption using porous materials has gained significant traction due to its low energy demands and high efficiency. However, conventional CO₂ adsorption at ambient conditions suffers from low capacity and poor selectivity.

Supercritical CO₂ (scCO₂)—formed when CO₂ surpasses its critical point of 31.1°C and 7.39 MPa—exhibits a hybrid of gas and liquid properties: high diffusivity, tunable density, and low viscosity. These attributes make it uniquely useful in materials science, separation, and enhanced oil recovery (EOR).

FIGURE 1 – Phase Diagram of Supercritical CO₂

Geological Sequestration

scCO₂ geological sequestration is a widely recognized method for long-term CO₂ storage. Injecting scCO₂ into deep geological formations—such as depleted reservoirs and saline aquifers—allows CO₂ to be trapped through structural, solubility, or residual mechanisms. Mineral carbonation reactions further convert CO₂ into stable carbonate solids, significantly reducing the risk of leakage [1].

FIGURE 2– Investigation of CO₂ Adsorption under Normal vs. High Pressure

Enhanced Oil Recovery (CO₂-EOR)

EOR using scCO₂ serves both economic and environmental goals. When scCO₂ reaches miscibility pressure with crude oil (typically >10 MPa), it significantly reduces oil viscosity and improves flow characteristics.

EOR injection methods include:

• Continuous injection
• Water-Alternating-Gas (WAG)

scCO₂-EOR can increase recovery by 10–25% and extend oil field lifetimes by decades. In the U.S., such projects already sequester 30 million tons of CO₂ annually, or 1.5% of industrial emissions.

2. Gravimetric Method for scCO₂ Adsorption

2.1 Why Magnetic Levitation?

Volumetric methods—though simple—require larger sample quantities and cannot effectively resolve kinetic behavior or low-pressure responses [2].

FIGURE 3 – Principle of Volumetric scCO₂ Testing

Gravimetric methods are preferred for high-precision adsorption work, but traditional balances suffer from:

• Buoyancy effects (especially with large samples)
• Balance drift
• Inability to operate in extreme environments

AMI’s Magnetic Suspension Balance (MSB) overcomes these issues:

• Operates under extreme temperatures and pressures
• Transfers weight via non-contact magnetic coupling
• Eliminates drift through periodic position switching
• Corrects buoyancy using reference masses or known fluid densities.

FIGURE 4 – Magnetic Levitation Instrument Structure

3. Application Cases

Case 1: Supercritical CO₂ Adsorption in Coal

High-pressure, high-temperature coal seams are ideal for CO₂ sequestration. Li and Ni et al. investigated the CO₂ adsorption behavior of Tunliu and Sihe coal under supercritical conditions [3].

FIGURE 5 – Isothermal Adsorption/Desorption Curves of Two Coal Types

Key Observations:

• Adsorption capacity peaks then declines
• Higher temperature = lower capacity
• Sihe coal has greater uptake than Tunliu
• Behavior follows Type I (IUPAC) isotherms before saturation [4]

However, corrections for excess vs. absolute adsorption and phase density assumptions are required to obtain true capacity data. Post-correction, adsorption shows stable increase with pressure and fully reversible desorption.

FIGURE  6– Density-Corrected Isotherms with Binary Langmuir Fitting

Case 2: Kinetics of scCO₂ Adsorption

Monitoring real-time uptake at fixed pressure provides kinetic insight. Adsorption is faster at higher pressure, but at supercritical pressures, adsorption approaches saturation more rapidly, reducing the number of available sites over time [5].

Studies by Charrière and Song show that:

• Residual adsorption capacity declines with time
• Fluctuations at high pressure are caused by density shifts near critical conditions [6][7]

FIGURE 7– CO₂ Residual Adsorption vs. Time at Various Pressures

Case 3: CO₂-EOR in Shale Reservoirs

Due to the complexity of core testing in shale (tight pore networks), magnetic levitation offers a direct and non-invasive method to monitor oil extraction.

Li and Chen used MSB readings at fixed intervals and compared results to NMR, showing strong correlation [8].

Table 1 – Comparison of Extraction Efficiency by Magnetic Levitation vs. NMR

With increasing CO₂ pressure:

• Smaller pore throats are accessed
• Extraction efficiency improves sharply until minimum miscibility pressure is reached

FIGURE  8– Pore Size Distribution of Tested Samples

4. Experimental Validation

4.1 Conditions

• Instrument: AMI Magnetic Suspension Balance
• Calibration: Helium for volume and mass
• Adsorbate: CO₂ up to 8 MPa
• Temperature: 50°C
• Sample Pretreatment: 200°C for 4 hours

4.2 Results

• Adsorption peaked between 40–50 bar
• Maximum uptake: 34.4%
• Minimal deviation between repeated tests
• Long equilibration times observed due to larger sample size
• Recommended: narrow pressure steps for smooth isotherms

BET analysis showed (on AMI Instruments):

• Surface Area: 1789 m²/g
• Micropore Volume: 0.6 cm³/g

FIGURE 9 – Measured Supercritical CO₂ Adsorption Capacity

5. Summary

Magnetic suspension gravimetric systems are essential tools for modern supercritical CO₂ adsorption research. Their advantages include:

• High-pressure and temperature compatibility
• Automatic elimination of drift via dual-position calibration
• In-situ kinetic and isotherm measurements
• Real-time buoyancy correction

Looking forward, future research should address:

• Internal density gradients and convective turbulence
• Adsorbent expansion corrections under extreme conditions

References

[1] Lv, Y., Tang, D., Xu, H., et al. CO₂ sequestration technology for improving coalbed methane recovery. Environmental Science and Technology, 2011, 34(5): 95–99.
[2] Sudibandriyo M., Pan Z., Fitzgerald J.E., et al. Langmuir, 2003, 19: 5323–5331.
[3] Li, Q., Ni, X., Wang, Y., et al. Coalfield Geology and Exploration, 2014, 42(3): 36–40.
[4] Fu, X., Qin, Y., Wei, C. Coalbed Methane Geology, China University of Mining and Technology Press, 2007.
[5] Charrière D., Pokryszka Z., Behra P. International Journal of Coal Geology, 2010, 81(4): 373–380.
[6] Song Y., Xing W., Zhang Y., et al. Adsorption, 2015, 21(1/2): 53–65.
[7] Siemons N., Wolf K.A.A., Bruining J. International Journal of Coal Geology, 2007, 72(3): 315–324.
[8] Li, B., Hou, J., Lei, Z., et al. Petroleum Drilling Technology, 2024, 52(4): 94–103.