Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Proton transfer reaction mass spectrometry

Proton Transfer Reaction - Mass Spectrometry (PTR-MS) is a very sensitive technique for online monitoring of volatile organic compounds (VOCs) in ambient air developed by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria. A PTR-MS instrument consists of an ion source that is directly connected to a drift tube (in contrast to SIFT-MS no mass filter is interconnected) and an analyzing system (Quadrupole mass analyzer or Time-of-flight mass spectrometer). Commercially available PTR-MS instruments have a response time of about 100ms and reach a detection limit in the single digit pptv region. Established fields of application are environmental research, food and flavour science, biological research, medicine, etc.^,.

Theory

With H₃O⁺ as the primary ion the proton transfer process is (with $R$ being the trace component)

$H_{3}O^{+}+R\longrightarrow RH^{+}+H_{2}O$ (1).

Reaction (1) is only possible if energetically allowed, i.e. if the proton affinity of $R$ is higher than the proton affinity of H₂O (691 kJ/mol ). As most components of ambient air possess a lower proton affinity than H₂O (e.g. N₂, O₂, Ar, CO₂, etc.) the H₃O⁺ ions only reacts with VOC trace components and the air itself acts as a buffer gas. Moreover due to the low number of trace components one can assume that the total number of H₃O⁺ ions remains nearly unchanged, which leads to the equation

$\lbrack RH^{+}\rbrack =\lbrack H_{3}O^{+}\rbrack _{0}\left(1-e^{-k\lbrack R\rbrack t}\right)\approx \lbrack H_{3}O^{+}\rbrack _{0}\lbrack R\rbrack kt$ (2).

In equation (2) $\lbrack RH^{+}\rbrack$ is the density of product ions, $\lbrack H_{3}O^{+}\rbrack _{0}$ is the density of primary ions in absence of reactant molecules in the buffer gas, $k$ is the reaction rate constant and $t$ is the average time the ions need to pass the reaction region. With a PTR-MS instrument the number of product and of primary ions can be measured, the reaction rate constant can be found in literature for most substances and the reaction time can be derived from the set instrument parameters. Therefore the absolute concentration of trace constituents $\lbrack R\rbrack$ can be easily calculated without the need of calibration or gas standards.

Technology

In commercial PTR-MS instruments water vapour is ionized in a hollow cathode discharge:

$e^{-}+H_{2}O\longrightarrow H_{2}O^{+}+2e^{-}$

$e^{-}+H_{2}O\longrightarrow H_{2}^{+}+O+2e^{-}$

$e^{-}+H_{2}O\longrightarrow H^{+}+OH+2e^{-}$

$e^{-}+H_{2}O\longrightarrow O^{+}+H_{2}+2e^{-}$.

After the discharge a short drift tube is used to form very pure (>99.5%) H₃O⁺ via ion-molecule reactions:

$H_{2}^{+}+H_{2}O\rightarrow H_{2}O^{+}+H_{2}$

$H^{+}+H_{2}O\rightarrow H_{2}O^{+}+H$

$O^{+}+H_{2}O\rightarrow H_{2}O^{+}+O$

$H_{2}O^{+}+H_{2}O\longrightarrow H_{3}O^{+}+OH$.

Due to the high purity of the primary ions a mass filter between the ion source and the reaction drift tube is not necessary and the H₃O⁺ ions can be injected directly. The absence of this mass filter in turn greatly reduces losses of primary ions and leads eventually to an outstandingly low detection limit of the whole instrument. In the reaction drift tube a vacuum pump is continuously drawing through air containing the VOCs one wants to analyze. At the end of the drift tube the protonated molecules are mass analyzed (Quadrupole mass analyzer or Time-of-flight mass spectrometer) and detected.

Advantages of PTR-MS

• Low fragmentation: Only a small amount of energy is transferred during the ionization process (compared to e.g. electron impact ionization), therefore fragmentation is suppressed and the obtained mass spectra are easily interpretable.
• No sample preparation is necessary: VOC containing air and fluids headspaces can be analyzed directly.
• Real-time measurements: With a typical response time of 100ms VOCs can be monitored on-line.
• Real-time quantification: Absolute concentrations are obtained directly without previous calibration measurements.
• Compact and robust setup: Due to the simple design and the low number of parts needed for a PTR-MS instrument, it can be built in into space saving and even mobile housings.
• Easy to operate: For the operation of a PTR-MS only electric power and a small amount of distilled water are needed. Unlike other techniques no gas cylinders are needed for buffer gas or calibration standards.

Disadvantages of PTR-MS

• Not all molecules detectable: Because only molecules with a proton affinity higher than water can be detected by PTR-MS, the technology is not suitable for all fields of application.
• Maximum measurable concentration limited: Equation (2) is based on the assumption that the decrease of primary ions is neglectable, therefore the total concentration of VOCs in air must not exceed 10ppmv.

Applications

The most common applications for the PTR-MS technique are:

• Environmental Research
• Waste Incineration
• Food and Flavour Science
• Biological Research
• Process Monitoring
• Indoor Air Quality
• Medicine and Biotechnology

In 2008 C. Lindinger et al. published an article in "Analytical Chemistry" that found great response even in non-scientific media. Lindinger et al. developed a method to convert "dry" data from a PTR-MS instrument that measured headspace air from different coffee samples into expressions of flavour (e.g. "woody", "winey", "flowery", etc.) and showed that the obtained flavour profiles matched nicely to the ones created by a panel of European coffee tasting experts.
An extensive review about PTR-MS and some of its applications was published in 2007 in "Mass Spectrometry Reviews" by Joost de Gouw et al..