A method and device for purifying heptafluoroisobutyronitrile and a dilution gas from a used gas mixture comprising heptafluoroisobutyronitrile, a dilution gas and arcing by-products. The method comprising the steps of (a) contacting the used gas mixture with at least one adsorbent material to generate a gas stream depleted in arcing by-products; (b) contacting the gas stream depleted in by-products with a first membrane to obtain a first permeate stream rich in the dilution gas, and a first retentate stream rich in heptafluoroisobutyronitrile; (c) contacting the first permeate stream rich in the dilution gas with a second membrane to obtain a second permeate stream rich in the dilution gas and a second retentate stream rich in heptafluoroisobutyronitrile; and (d) combining the first and second retentate streams rich in heptafluoroisobutyronitrile.