Beagle II had a miniature Mass Spectrometer
But you generally need to concentrate the input to one. That would need a small atmospheric processing plant, maybe based on pumps and membrane-sieve technology to concentrate the methane vs CO2, or do it by sequential liquefaction of the atmospheric components, the way liquid nitrogen and LOx are made on Earth, but smaller.
Yars' low temperature probably makes it quite easy to liquefy CO2 and CH4, under the right pressure.