Polystyrene Micro-spheres are spherical in design, highly accurate is size diameter with NIST traceabilty, provided with a narrow size distribution, and provided with a Size Certificate that includes the particle specification. Particle size standards are extremely accurate for calibration of your laser particle counters, water inspection systems and other metrology instruments used in your science studies. Here is a list of particles packaged with 1% or less micro-sphere concentration in 15 ml De-ionized water. Each bottle ships with a NIST-Traceable, Particle Size Certificate showing date of manufacture and manufacturing specs. If you handle the bottles with contamination control in mind, store in a cool, dry, dark environment, these particle size standards can last more than 2 years.