There are two kinds of gas capture methods,
1. IsoTube
Use a tube to capture the gas from mud-gas seperator ( Gas detector). Gas component analysis is done in Lab. The gas analysized is mainly free gas.
2. IsoJar
Use a gar to be filled some cuttings from shaker on well site. Heat the jar in Lab and get gas. capture this gas and analysis the componets. This gas is mainly adsorbed gas.
Need to combine these two methods to get the gas components for shale.
Look up internet about IsoTube and IsoJar and you will get more knowledge about these two methods. Hope this brief description is helpful to you.