According to an example aspect of the present invention, there is provided a method of detecting carbon dioxide in a gaseous sample, the method comprising: flowing the gaseous sample through an anion exchange resin that is capable of selectively adsorbing CO2 present in the gaseous sample; releasing the adsorbed CO2 from the resin by heating the resin to a temperature in the range 80 to 250° C. to obtain a concentrated gaseous sample; determining the amount of an isotopic form of CO2 in the concentrated gaseous sample by infrared absorption spectroscopy.