Godin:Protocols/NIST traceable particle size standard
These microspheres are used to calibrate instrumentation. At 0.01x concentration of 1.51µm beads, we have just over 5 billion particles per ml:
This is pretty straight-forward. Simply dilute the beads to the desired concentration. The NIST particles should be kept in the refrigerator to prevent the growth of contaminants. Also, be sure to use the vortex mixer to fully dissolve all the microspheres; they tend to clump together when left alone for awhile.