Biacore 2000 – uses an optical technique to measure changes in mass as an interaction occurs between biomolecular partners on a sensor surface
- Autosampling with an unattended run time of 48 hours
- Classic chip configuration with 4 flow cells
- Analysis temperature range of 4-40oC
- Maximum mass sensitivity in liquid = pg/mm2
- Affinity range = interactions at nanomolar concentrations
- Minimum sample volume = <100 µl
- Examples of chip matrices and model systems: Carboxymethylated dextran e.g. CM5 (immobilization of proteins, nucleic acids, carbohydrates or small molecules), streptavidin (immobilization of biotinylated molecules), gold (customized surface chemistries)
- Key applications:
- Screening for binding events
- Kinetic rates of interactions
Q-Sense E4 Analyzer (Quartz Crystal Microbalance with dissipation monitoring; QCM-D) – uses acoustic parameters, frequency (related to mass) and dissipation (related to viscoelasticity), to monitor molecular layers formed on a sensor surface.
- Manual handling of samples by user
- 4-sensor channel system with the same measurement sequence
- Analysis temperature range of 15-65oC
- Maximum mass sensitivity in liquid = 0.5 ng/cm2
- Minimum sample volume = 200 µl
- Examples of sensor crystals and model systems: Gold (universal surface), cellulose (fabric), platinum (electrodes) and titanium (medical implants)
- Key applications:
- Changes in mass and structural properties
- Interactions, reactions and other surface phenomena
What is the difference between SPR and QCM-D measurements?
Whilst both systems measure changes in mass from a technical point of view, there are major differences in sensitivity. QCM-D systems are more sensitive for water-rich and extended layers, whereas SPR systems are favoured for compact and dense layers. QCM-D being an acoustic technique, allows measurement of thicker films, while SPR being an optical technique, has limitations concerning film thickness.
This is related to the different physical principles by which the coupled mass is measured. The mass-uptake estimate from SPR data is based on the difference in refractive index between the adsorbed biomolecules and water displaced by the biomolecules upon adsorption. This means that water associated with the film is essentially not included in the mass determination. In contrast, changes in frequency acquired with QCM-D, measure water coupled as an inherent mass via direct hydration or entrapment in cavities in the adsorbed film. The SPR response is proportional to the coupled ‘molar mass’ while in QCM, the layer is sensed as a ‘hydrogel’ composed of the macromolecules and coupled water.
The additional information collected from energy dissipation by QCM-D is related to the shear viscous losses induced by the adsorbed layers, and thus can help in identifying structural differences between different adsorbed systems, or structural changes in the same type of molecule during adsorption processes.