3º Congresso BrMass – 12 a 15 de Dezembro de 2009

trace gas atmospheric measurement. Once per months for four consecutive long-distance flights from Frankfurt (Germany) to North/South America, Southern India, South Africa or the Philippines the PTRMS measures chemically important volatile organic compounds (VOCs) such as acetone, acetonitrile, methanol, and acetaldehyde at 9-12 km altitude, which in the meantime led to the largest available airborne dataset of these compounds in this altitude range. The PTR-MS uses water vapor injected into a hollow cathode ion source to create Hydronium ions (H3O+), hereafter referred as primary ions, with 99 % purity or better. The primary ions are extracted into a drift tube and ionize many trace gases (but not the most abundant gases, such as N2, O2, Ar,…) by proton transfer reaction. Due to the selectivity of the reaction, air can be used as buffer gas without any prior treatment, allowing rapid response time (1-10 sec). For most small- and medium-sized organic molecules the reactions are slightly exothermic which reduces the possibility of fragmentation and simplifies the spectrum analysis. The ions are filtered by a quadrupole system and counted with an electron multiplier