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Proton-Transfer Reaction Mass Spectrometer

PTRMSProton Transfer Reaction Mass Spectrometry (PTR-MS) is a recently developed technique for sensitive, on-line measurements of trace amounts of volatile organic compounds (VOCs) in air. In short, in PTR-MS the organic trace gases are ionized by undergoing a proton-transfer reaction with H3O+ ions. This proton-transfer reaction is a soft ionization process, which hardly fragments the molecules. The product ions are mass analyzed and detected with a quadrupole mass spectrometer (1 amu resolution) and yield information about the neutral precursors. The reaction is exothermic and efficient for those compounds with a proton affinity (PA) higher than the proton affinity of water.

H3O+ + R  ->  RH+ + H2O

This includes most of the VOCs of interest to the presently proposed work. The table gives a number of important VOCs which can be measured with PTR-MS. This list is far from complete: PTR-MS is able to detect most of the higher aldehydes (e.g. formaldehyde, acetaldehyde, propanal), ketones, alcohols (e.g. methanol, ethanol, propanol, iso-propanol, n-propanol), acids (e.g. formic acid, acetic acid, propanoic acid) and esters as well as many nsaturated and aromatic hydrocarbons.

For many of the compounds the proton-transfer reaction leads to the formation of one product ion only. Determining the ion mass is therefore an excellent indication of the identity of the compound detected. The assignment of the product ions can be further supported based on natural isotope abundance ratios (e.g. 13C/12C ratio). For this, the measured concentrations for the supposed 12C compound are correlated with the measured concentration at its isotope containing one 13C atom. By comparing the thus found isotope ratio with the natural abundance of 13C (1.1%) the number of carbon atoms in the compound can be determined.

The proton affinity of water and some volatile organic compounds which can be detected with PTR-MS.
Proton Affinity
(kcal mole-1)
Detection mass
Water H2O 166.5 18 19
Formaldehyde CH2O 171.7 30 31
Methanol CH3OH 181.9 32 33
Formic acid HCOOH 178.8 46 47
Acetaldehyde CH3CHO 186.6 44 45
Ethanol C2H5OH 188.3 46 29/47
Acetic acid CH3COOH 190.2 60 43/61
Acetone CH3COCH3 196.7 58 59
Propanal C2H5CHO 189.6 58 59
Iso-propanol C3H7OH 191.2 60 43
n-propanol C3H7OH 190.8 60 43
Propanoic acid C2H5COOH 191.8 74 75
Isoprene C5H8 200.4 68 69
Hydrogen sulfide H2S 170.2 34 35
Di-methyl sulfide CH3SCH3 200.6 62 63