Glass wool with a diameter of ~8 µm is commercially available and inexpensive. We tested their resistance to liquid flow, and found that it is virtually resistance-free, which is required for solid phase synthesis. Assuming a length of 1 cm, density of 2.2 g/ml, and 3.2 molecules/nm 2 (Table ), the loading is 1,208 nmol/g (see supporting information for calculations). For glass beads, to allow for close to resistance-free liquid flow, ideally their diameter is ~50 µm or larger. Assuming a diameter of 58 µm, a density of 2.2 g/ml, and 3.2 molecules/nm 2 , the loading is calculated to be 249 nmol/g. Therefore, the loading of glass wool is ~4.8 times of that of glass beads (Table ).