1) Are mixed mode resins, such as hydrophobic IEX resins equally robust against alkaline cleaning compared to IEX or HIC resins?
Mixed mode ligands are often very complex. However, TOYOPEARL MX-Trp-650M is still very robust and can be cleaned in place with 0.5 M NaOH
2) Can I apply high salt concentrations (similar to HIC) for protein adsorption to MX-Trp-650M?
The ligand of MX-Trp-650M has a weak cation exchange and a hydrophobic moiety. Capacity of the resin for proteins is usually much higher when applying conditions dedicated to charge based adsorption. Hydrophobic interactions support salt tolerance and selectivity during protein elution.
3) Applying pH or salt gradients; which one does provide higher selectivity?
In general, both is possible and the preferred gradient mode may depend upon an individual application. Most often, robust sodium chloride gradients allow for successful separation. Occasionally, a combination of salt and pH gradients may increase selectivity of the resin.