To enforce a uniform flow of solvents and reagents across the width of the array surface (at the flow rates relevant to nucleic acid synthesizers), the depth of the flow chamber needs to be quite small, in the range of about 40 to 70 µm. This can be achieved with a 250 µm thin gasket recessed by 200 µm relative to the quartz block surface in the interior of the flow cell (Figure ). Once assembled, the gasket is compressed by about 5 to 10 µm, providing a reliably leak-free seal. For the gasket material, which is cut to size with a CO 2 laser, we use Chemraz FFKM 584 with a thickness of 100 mils or 254 µm. Because the distance between the glass slide surface and the DMD image depends on gasket compression, we use a defined torque of 0.45 Nm on each of the four screws holding the assembly together in order to ensure consistency. Mechanical drawings and specifications for the flow cell and its subcomponents can be found in the supplementary materials. An alternative approach , based on the use of two closely spaced glass slides separated by a 50 µm PTFE gasket, can also be implemented, but requires the use of apertures to reduce the numerical aperture of the system to 0.08 in order to sufficiently increase the depth of focus.