Success of a future Mars Sample Return mission will depend on the correct choice of samples. Pyrolysis-FTIR can be employed as a triage instrument for Mars Sample Return. The technique can thermally dissociate minerals and organic matter for detection. Identification of certain mineral types can determine the habitability of the depositional environment, past or present, while detection of organic matter may suggest past or present habitation. In Mars history, the Theiikian Era represents an attractive target for life search missions and the acquisition of samples. The acidic and increasingly dry Theiikian may have been habitable and followed a lengthy neutral and wet period in Mars history during which life could have originated and proliferated to achieve relatively abundant levels of biomass with a wide distribution. Moreover, the sulfate minerals produced in the Theiikian are also known to be good preservers of organic matter. We have used pyrolysis-FTIR and samples from a Mars analogue ferrous acid stream with a thriving ecosystem to test the triage concept. Pyrolysis-FTIR identifies those samples with the greatest probability of habitability and habitation. A three tier scoring system was developed based on the detection of i) organic signals, ii) carbon dioxide and water and iii) sulfur dioxide. The presence of each component is given a score of A, B or C depending on whether the substance has been detected, tentatively detected or not detected respectively. Single-step (for greatest possible sensitivity) or multi-step (for more diagnostic data) pyrolysis-FTIR methods can inform the assignments. The system allows the highest priority samples to be categorised as AAA (or A*AA if the organic signal is complex) while the lowest priority samples can be categorised as CCC. Our methods provide a mechanism to rank samples and identify those that should take the highest priority for return to Earth during a Mars Sample Return mission.