PES (polyethersulfone) syringe filters, 0.22 μm. Two common sizes: - 4 mm: Less dead volume (less peptide lost in the filter), but can clog on larger volumes. Good for up to 1–2 mL. - 13 mm: Better for larger volumes (up to 3–6 mL capacity), slightly more dead volume but can be air-purged to recover residual liquid. Cobetter brand is well-regarded. Double-layer filters (0.65 μm / 0.22 μm) are ideal—the larger pore acts as a pre-filter.