The PTR-MS Technology
Theoretical background information for advanced readers
The fundamental process in a PTR-MS instrument can be written as
(1)
This means that protonated water (H3O+) interacts with the trace gas (R). During this interaction a proton switches from the hydronium to the trace gas molecule, which leads to a protonated and therefore ionized molecule (RH+) and a neutral water molecule (H2O). However, process (1) is only energetically possible if the proton affinity of the trace gas component is higher than the proton affinity of water (166.5 kcal/mol) and therefore only these components can be measured with the PTR-MS technology.
Click here to see an animation of how PTR-Quadropole-MS works!
Ion source
It is obvious that for an effective ionization via reaction (1) a powerful supply of H3O+ ions is necessary. In the IONICON PTR-MS instruments these primary ions are generated in a dedicated ion source that has been developed and was continuously improved to perfection over many years by our renowned experts.
The ion source is divided into two parts: In the first part water vapor (from standard distilled water) is drawn into a hollow cathode discharge chamber leading to the following reactions
(2)
The fragments produced by reactions (2) subsequently enter together with the water vapor the second part of the ion source consisting of a short drift chamber. In this region well defined ion-molecule reactions take place
(3)
and finally the H2O+ ions from (2) and (3) form hydronium via
(4)
The so produced protonated water is of very high purity (up to 99.5%) and can therefore be injected directly into the PTR drift tube without the need of an interconnected mass filter, which would lead to an inevitable loss of primary ions and eventually result in a worse detection limit.
PTR drift tube
In the PTR drift tube the actual ionization process of the trace gas molecules takes place. It can be easily derived that the PTR process (1) follows the equation
(5)
which can be simplified in good approximation to
(6)
In (5) and (6) [RH+] is the density of protonated trace constituents, [H3O+](0) is the density of primary ions (in absence of reactant neutrals [R]), k is the reaction rate constant and t the average time the ions spend in the reaction region. The assumption (6) is justified because only molecules with a proton affinity (PA) higher than the PA of water (166.5 kcal/mol) undergo a PTR reaction. As all common constituents of ambient air ( N2, O2, Ar, CO2 etc.) possess a lower PA than water, the air itself acts as a buffer gas and only volatile organic compounds (VOCs), which are usually present in very small densities, get ionized. Compared to electron impact ionization, where electrons are shot at neutrals at about 70eV, the energy transfer in the PTR process is very low. This effectively suppresses fragmentation and leads to mass spectra that are easily interpretable.
Calculation of concentrations
The mass analyzing and detection system (Quadrupole mass filter or Time-of-Flight mass spectrometer) of the PTR-MS instrument delivers count rates (or currents) which are proportional to [RH+] and to [H3O+]. The average time t can be calculated from system parameters (drift voltage, pressure, temperature, etc.) and the reaction rate constant k can be found in literature for many substances (alternatively it can be calculated or experimentally determined). Knowing all necessary variables in (6) makes it possible to calculate the concentrations of VOCs in the measured volume of air without the need of gas standards.
(7)
The highly sophisticated PTR-MS software automatically acquires and calculates all necessary data for equation (7) (constant C which includes k, t and a conversion factor as well as the ratio of the signal intensities) so that the user can monitor the absolute concentrations in ppbv of all measurable VOCs in real-time without the need of being an expert in mathematics, physics or chemistry.
Conclusion
The combination of a highly sophisticated ion source, the efficient PTR ionization process and a state-of-the-art mass analyzing system in an IONICON PTR-MS instrument offers the possibility to monitor and quantify VOCs down to the single-digit pptv range. As the measured air itself acts as a buffer gas and the primary ions are generated from distilled water, no additional gas cylinders are needed. This makes the IONICON PTR-MS instruments very compact, low in maintenance and reliable for a wide field of customers.
Related document:
Download a PTR-MS technology factsheet (0.2 MB .pdf)
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