How does an IONICON PTR-MS work?
(using H3O+ as reagent ion)
For efficient chemical ionization via reaction (1) an abundant supply of H3O+ ions is necessary. In IONICON PTR-MS instruments these reagent ions are generated in a dedicated ion source that was developed and has been continuously improved to perfection over many years by our renowned experts.
In the ion source H2O vapor is ionized and fragmented in a hollow cathode discharge. In a second step the fragments recombine to protonated water ions (H3O+) with 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 reagent ions and eventually result in an inferior detection limit.
PTR drift tube
The fundamental process in a PTR-MS instrument is
H3O+ + R → RH+ - H2O (1)
This means that protonated water (hydronium; H3O+) interacts with the analyte (R). During this interaction a proton transfers from the hydronium to the analyte, which leads to a protonated and therefore ionized product ion (RH+) and a neutral water molecule (H2O). The proton transfer reaction is energetically possible for all VOCs with a proton affinity higher than that of water (166.5 kcal/mol). Many further compounds with proton affinities below that of H2O can be detected using our proprietary Selective Reagent Ionization - Mass Spectrometry (SRI-MS) technology.
In the PTR drift tube the actual chemical ionization process (1) of the analytes takes place. It can be easily derived that the PTR process (1) follows the equation
[RH+] = [H3O+]0 (1 - e-k|R|t) (2)
which can be simplified in good approximation to
[RH+] ~ [H3O+]0 |R|kt if
[RH+] << [H3O+] ~ [H3O+]0 = const. (3)
In (2) and (3) [RH+] is the density of protonated trace constituents, [H3O+]0 is the density of reagent ions (in absence of the analytes [R]), k is the reaction rate coefficient and t the average time the ions spend in the reaction region. The assumption (3) 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.) have 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 concentrations, get ionized. Compared to electron impact ionization, the energy transfer in the PTR process is very low. This effectively suppresses fragmentation and leads to mass spectra that are easy to interpret.
Subsequent to the PTR drift tube and prior to the mass spectrometer there is a differentially pumped ion transfer region. In basic PTR-MS instruments this region consists of a conventional lens system. In more advanced models there are two types of ion focusing devices, which suppress ion losses during the transfer and thus improve the instruments' sensitivity and lower the detection limit.
Ion funnels are RF devices which have been used for decades to focus ion currents into narrow beams. In some PTR-MS setups of competitors the focusing properties of the ion funnel improves the sensitivity of the setup for a few compounds by a factor of >200 (compared to operating in DC only mode, i.e. with the ion funnel turned off), whereas the sensitivities of other compounds are only improved by a factor of <10. That is, because of the highly compound dependent instrumental response one of the main advantages of PTR-MS, namely that concentration values can be directly calculated, is lost and a calibration measurement is needed for each analyte of interest. Furthermore, with this approach unusual fragmentation of analytes has been observed which complicates interpretation of measurement results and comparison between different types of instruments even more.
In IONICON PTR-MS instruments the ion funnel is not predominantly part of the reaction region but mainly for focusing the ions into the transfer region to the Time-Of-Flight (TOF) mass spectrometer. This enables a considerable increase in sensitivity and thus also an improvement of the detection limit, while keeping the ion chemistry well-defined and thus avoiding problems with quantification and interpretation of the results.
Quadrupole, hexapole and other multipole ion guides can be used to transfer ions between different parts of an instrument with high efficiency. In PTR-MS they are particularly suitable for being installed in the differentially pumped interface between the reaction region and the mass spectrometer. While quadrupole ion guides are known to have high focusing power, but also rather narrow m/z transmission bands, hexapole ion guides have excellent focusing capabilities over a much broader m/z band. Additionally, less energy is put into the transmitted ions, i.e. fragmentation and other adverse effects are less likely to occur. Consequently, IONICON's latest high-end PTR-MS instruments are equipped with hexapole ion guides for considerably improved performance or even with a sequential arrangement of an ion funnel followed by a hexapole ion guide for even higher sensitivity and lower detection limit.
TOF mass analyzer
IONICON's in-house built orthogonal acceleration TOF mass analyzers are equipped with ion mirrors for highest possible mass resolution. The ions injected from the PTR drift tube via the transfer region are pulsed in packages into the field free TOF region. The time ions need to travel along the flight path depends on their m/z, i.e. by precisely measuring the flight time of each ion in the package, high resolution mass spectra are obtained within split-seconds. A microchannel plate is used for detecting the ions with utmost sensitivity.
Determination of concentrations
The mass analyzing and detection system of the PTR-MS instrument delivers count rates 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 coefficient k can be found in literature for many substances (alternatively it can be calculated or experimentally determined). Knowing all necessary variables in (3) makes it possible to calculate the concentrations of VOCs in the measured volume of air without the need of gas standards via equation:
[Concentration]ppbv = C * [RH+] / [H3O+] (4)
The highly sophisticated PTR-MS software automatically acquires and calculates all necessary data for equation (4) (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 or pptv of all measurable VOCs in online and in real-time.
The combination of the IONICON ULTRA-PURE ion source, the efficient PTR ionization process and a state-of-the-art mass analyzer in an IONICON PTR-MS instrument offer the possibility to monitor and quantify VOCs down to the single-digit pptv range while being compact, low cost in maintenance and reliable for a wide field of customers.