Natural Trona/Nahcolite and Natorn deposits have multiple other salts in the mixture collected near an alkaline brine. The measurement of d13/PDB and 18O/PDB on IRMA (Isotope Ratio Mass Spectrometer, with double inlet system for the sample and the reference gases) requires pure Phosphoric acid attack on the natural alkaline salt mixture that mainly also includes sodium chloride and sulfate. Normally, the released CO2 is not obtained alone but colored gas (violet/lilac) is also observed. The colored gas makes trouble to the IRMS measurement. The nature of the colored gas in not known to me. Do yo have an idea bout its nature/chemical composition? I made a simple pretreatment to get ride of halides and recrystallization of Barium carbonate before the acid attack and this works fine, however, this pretreatment is accompanied by some artificial isotopic fractionation that I was able to determine using individual AR lab salts.