Purity

The minimum purity of a gas refers to the specified impurities that characterize the pure gas. The specific limits for the impurities are not exceeded. In the case of gases without details on impurities, the specification of their purity is to be seen as a typical value. In the case of a number of high purity gases, every filled cylinder is accompanied by a certificate of analysis confirming that the maximum limits for impurities have not been exceeded. This is noted for the gas purities to which it applies. It does not apply to "Gases in Small Containers". 

Cylinder fillings frequently display purities that are higher than those which have been guaranteed. Even if this should occur repeatedly, it should nevertheless not be taken as any assurance of uniformly higher purities. The maybe simpliest and most accurate definition of purity is: 

“Purity is the absence of impurity in a substance” 

Impurity can be defined as a material or substance in the finished product that differs from the chemical composition of the requested material or compound. BUT: 100% purity cannot be achieved due to the manufacturing process. 

Purity classification is normally written in two ways:

1. As a quality code, e.g. 5.5 where the number before the dot represents the number of nines and the last number indicates the last decimal (5.5 = 99,9995% or 6.0 = 99,9999%)

2. As impurity in percent, e.g. >99,9995.

Examples:   

Ethylene 2.8 means 99.8 % purity 

Argon 6.0 means 99.9999 % purity

Purity	Level	Reported impurity level as %	Reported impurity level as ppm
99,0%	2.0	1%	10 000 ppm
99,5%	2.5	0,5%	5 000 ppm
99,9%	3.0	0,1%	1 000 ppm
99,99%	4.0	0,01%	100 ppm
99,999%	5.0	0,001%	10 ppm
99,9995%	5.5	0,0005%	5 ppm
99,9999%	6.0	0,0001%	1 ppm (1000ppb)

Instead of the short suffix, a few gases have an application-related suffix, e.g. Nitrogen CO-free. Irrespective of the minimum purity indicated in the product description, these gases have an especially low residual content of certain otherwise troublesome impurities.