Activity isotopic ratios of the CTBT relevant radionuclides detected at radionuclide stations of the international monitoring system (IMS) are used for characterization of a release event under assumed scenarios. Activities released from a nuclear event, activity concentrations in a plume passing over an IMS station, and activities collected in a sample at the IMS station can be related based on an analysis procedure with respect to the activity evolution in an assumed underground nuclear explosion, atmospheric transport modelling (ATM) simulations, and sample measurements. In a forward analysis, the concentration profile at an IMS station is estimated by using the assumed release scenario and ATM simulations. The activities collected in samples are determined by spectrum analysis of sample measurements first, then the activity concentrations are estimated based on an assumption on the concentration profile. Generally, activity concentrations are assumed constant during sampling. This is true for radionuclides with relative longer half-life such as 7 times compared with the sampling duration. It might be a challenge for the isotopes with short half-life, such as the 135Xe half-life of 9.14 hours with respect to the collection duration of 12 hours for SAUNA noble gas systems. In this study, the decay correction during sampling is investigated by two approaches, 1) Interval constant concentration: the collection duration is divided into multiple intervals in which a constant concentration is assumed in each one respectively; 2) Analytical solution: the activity collected in the sample is derived based on an analytical solution regarding the ordinary differential equations of the activity decay and accumulation. The impacts on isotopic ratios are demonstrated in three cases with short half-life, 140Ba/140La, and 133mXe/133Xe, and 135Xe/133Xe.