Comparison of Washing Methods
Multichannel pipettes, versatile and ubiquitous in almost every laboratory, may be used for any aspirate and dispense step in the magnetic bead-based assay process when used along with a permanent magnetic support, although by design their use is ultimately inappropriate for these tasks.
Consistency is highly dependent on the operator and difficult to reproduce from plate-to-plate or even from one procedural step to another. Tip depth and force used are also operator-dependent. Incorrect tip depth can lead to residual liquid, unwanted aspiration of beads and damage to the microwell surface, while incorrect aspirate/dispense force can contribute to aspiration of beads or splashing and aerosol creation.
A vacuum manifold, considered the traditional standard for many bead-based assays, may be used if the assay is performed in a filter-bottom microplate. Per each wash step, the filter-bottom microplate is sealed in an airtight chamber, and a vacuum is applied, drawing out the liquid. Additional reagents are dispensed via an additional device or instrument.
Vacuum manifolds have been used for decades, and their simplicity and low relative cost are attractive. It is difficult, however, to regulate vacuum pressure from well-to-well and plate-to-plate—too low, and residual liquid remains in the well; too high, and beads may become trapped within the filter membrane. Once removed from the chamber, residual liquid on the underside of the microplates must be blotted—a messy step that is difficult to build into automated systems. Finally, vacuum manifolds are not adaptable to various fluid viscosities, and their inherent vibrations may also contribute to blockages in the filter membrane.
Microplate washers are readily integrated into automated systems, and those equipped with magnets such as the ELx405™ Magnetic Bead Washer from BioTek Instruments can easily be integrated into magnetic bead assay procedures (Table). These washers significantly reduce human error and produce consistent results through every well and microplate. Gentle yet thorough aspiration and dispensing are highly repeatable and minimize bead loss through numerous wash cycles and assay parameters.