Polystyrene Microspheres are spherical in design, highly accurate is size diameter, and are provided with a narrow size distribution, making these particle size standards extremely accurate for size response calibration of your laser particle counters, water particle counters and other metrology instruments used in your aerosol science studies. These particle size standards are packaged with 1% or less concentration of micro-spheres in 15 ml De-ionized water. Each bottle ships with a NIST-Traceable, Particle Size Certificate showing date of manufacture and size specifications with NIST Traceability to SRM Standards. Store in a cool dry, dark environment to minimize contamination issues, and these particle size standards can last more than 2 years when handle and stored properly.