Compression and separation device, comprising a dynamic compressor having first and second compression stages (C2, C3) for compressing a first gas having a first molecular weight less than 10 g/mol, or even less than 7.5 g/mol, or even less than 5 g/mol, comprising: i. An inlet for the first gas, ii. Means for mixing at least one second fluid having a second molecular weight greater than 50 g/mol with the first gas to form a third gas to be compressed having a molecular weight greater than 10 g/mol, iii. Means for sending the third gas to the first compression stage, a first heat exchanger for cooling the third gas downstream of the first compression stage in order to partially condense it, a first phase separator, means for sending the partially condensed third gas to the first phase separator to form a fourth gas having a lower molecular weight than the third gas and a first condensed liquid having a higher molecular weight than the third gas, iv. Means for sending the fourth gas from the first phase separator to the second compression stage, a second heat exchanger downstream of the second compression stage, means for sending the compressed fourth gas to the second compression stage and cooling it in the second heat exchanger in order to partially condense it, a second phase separator and means for sending the partially condensed fourth gas to the second phase separator to produce a fifth gas having a lower molecular weight than the fourth gas and a second condensed liquid having a higher molecular weight than the fourth gas, and v. A separation device and means for sending the fifth gas or a gas derived from the fifth gas to the separation device to be separated therein to produce a gaseous product having a molecular weight of less than 10 g/mol, or even less than 7.5 g/mol, or even less than 5 g/mol, and a composition of less than 10 ppm, or even less than 5 ppm, or even less than 1 ppm of any component having a molecular weight greater than 50 g/mol. i. An inlet for the first gas, ii. Means for mixing at least one second fluid having a second molecular weight greater than 50 g/mol with the first gas to form a third gas to be compressed having a molecular weight greater than 10 g/mol, iii. Means for sending the third gas to the first compression stage, a first heat exchanger for cooling the third gas downstream of the first compression stage in order to partially condense it, a first phase separator, means for sending the partially condensed third gas to the first phase separator to form a fourth gas having a lower molecular weight than the third gas and a first condensed liquid having a higher molecular weight than the third gas, iv. Means for sending the fourth gas from the first phase separator to the second compression stage, a second heat exchanger downstream of the second compression stage, means for sending the compressed fourth gas to the second compression stage and cooling it in the second heat exchanger in order to partially condense it, a second phase separator and means for sending the partially condensed fourth gas to the second phase separator to produce a fifth gas having a lower molecular weight than the fourth gas and a second condensed liquid having a higher molecular weight than the fourth gas, and v. A separation device and means for sending the fifth gas or a gas derived from the fifth gas to the separation device to be separated therein to produce a gaseous product having a molecular weight of less than 10 g/mol, or even less than 7.5 g/mol, or even less than 5 g/mol, and a composition of less than 10 ppm, or even less than 5 ppm, or even less than 1 ppm of any component having a molecular weight greater than 50 g/mol.