Primary Standard - Highly accurate mixtures prepared gravimetrically on high-precision electronic balances. These standards are analyzed against Praxair Primary Laboratory Standards (PPLS) and named to a gravimetrically generated concentration. Certified Standard - These routine calibration mixtures are prepared by either gravimetric, volumetric or partial pressure methods. These standards are analyzed against Praxair Primary Laboratory Standards (PPLS). Non-Certified - Mixtures are prepared by the same methods and the same care used for Certified Standard Grades. Analyses are not reported. Custom - Mixtures prepared to the exact blend tolerance and analytical uncertainty requested