Information taken from the Airgas web site that might be of interest to readers:
“The name of the purge gas does not indicate its true purity level. Differing manufacturers and use similar names to one another but these names do not help in knowing the purity.

The true way to find a Purge Gases true purity is through the digital system which identifies a gas’s true purity in percentage terms.

The first digit indicates the number of 9’s and the second digit indicates the next number that is different to a 9.

EG Argon 4.5

So, this means that there are four nines and then a five

Giving a purity level of 99.995%

Or 6.0 gives a purity level of 99.9999%

The label on the bottle indicates the level of impurities by gas with a purity table as follows:

Percentage Purity    Grade    Total Impurities                       
99.99%                      4.0           100 ppm
99.995%                    4.5             50 ppm
99.999%                    5.0             10 ppm
99.9995%                  5.5               5 ppm
99.9999%                  6.0               1 ppm