As standard most gases are sold against a sale specification. In many cases this will be not much more than the headline purity - for example 99.999% nitrogen.

The next level of certification is a certificate of conformity (or C o C). This declares that the product supplied conforms to a specific product specification. Typically this will be managed by the gas supplier by taking a sample from a batch of cylinders filled at the same time. It is important to understand how the process of quality assurance is being achieved.

The next level is a Certificate of Analysis (C o A). This is an actual analysis of the product being supplied. In addition to these forms of certification there are also other forms of certification for accredited mixtures. In UK accreditation is managed by UKAS but in fact there is a mutual recognition agreement on a European wide basis so Accredited mixtures from across Europe can be used.