A hollow cathode proton transfer reaction time of flight mass spectrometer
Introduction
Proton transfer reaction mass spectrometry (PTR-MS) has been used for almost a decade as a fast online technique for detection and analysis of trace hydrocarbons in gaseous samples [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Major applications of the technique are in breath analysis and atmospheric chemistry, and detection limits as low as a few pptv are routine [10], [11]. The technique relies on chemical ionisation by gas phase proton transfer to produce protonated molecules with little or no fragmentation, facilitating the analysis of complex gas mixtures without the need for a separation step in the analytical method.
In PTR-MS, proton transfer is usually achieved using gas-phase hydronium ion (H3O+) as the protonating agent:H3O+ + R → RH+ + H2Owhere R is a gas phase analyte. The proton affinity of R, PA(R) is defined asPA(R) = −ΔHfor the above reaction, and is simply a measure of the exothermicity of the gas phase proton transfer. Molecules with a PA greater than that of gas phase water are readily protonated by the hydronium ion, with rate constants approaching the collisional limit. For small molecules, differences in PA are small compared to dissociation energies of the protonated ions, so soft chemical ionisation occurs with little or no fragmentation. Thus, PTR-MS produces a protonated molecule for each gas phase component that is capable of undergoing protonation. This includes most oxygenated organic molecules and a large number of hydrocarbons that possess more than four carbon atoms. A specific advantage of the technique is that only the trace components of the air are protonated, the proton affinity of molecular oxygen, carbon dioxide, molecular nitrogen and other main components of air being lower than that of water. Thus, the technique represents a fast, sensitive and selective method for analysing components of air that are of considerable interest to atmospheric scientists.
Quantification in PTR is possible via knowledge of the protonation rate constants [1], which are known for many species [12], [13]. However, a failing of the method as both a qualitative and a quantitative analytical tool lies in the mass spectrometric detection technology. Typically, quadrupole mass spectrometers (QMS) are used in PTR-MS; consequently, the mass resolution is limited. This results in a lack of discriminating power between nominally isobaric materials, that is, between non-isomeric materials with the same nominal molecular mass. Strategies to overcome this problem include the use of different protonating agents (e.g., NH4+) and the alteration of proton transfer collision energetics [5], though these techniques are not ideal for online-analysis. Recent work to develop an ion trap PTR instrument has made some progress towards addressing this drawback [14]. In the ion trap, ion-neutral collisions can be induced to promote fragmentation, and multistage tandem MS enables differentiation between isobaric molecules for example methacrolein and methyl vinyl ketone [14].
An alternative strategy is to use a time of flight (ToF) mass spectrometer to analyse the protonated molecules produced in the proton transfer reaction flow drift tube. ToF mass spectrometry has the potential to benefit over QMS in this application for two reasons. Firstly, the generally higher mass resolving capability of the technique can provide elemental composition leading directly to discrimination between nominally isobaric analytes. The second advantage is an increased spectral acquisition rate that is expected to lead to higher sensitivity.
Recently, the capabilities of PTR-ToF using a radioactive ionisation source (241Am, 1.2 mCi) have been demonstrated [15] with linearity of detection in the range 3–53 ppmv giving inferred detection limits in the ppbv range over integration times of around 1 min. Here we demonstrate PTR-ToF capability using a hollow cathode ionisation source with limits of detection below 10 ppbv for integration times of 10–60 s.
Section snippets
Experimental
The PTR-ToF developed at York comprises an aluminium flow drift tube interfaced to a custom built R500 time of flight mass spectrometer (KORE Technologies, Ely, UK) and is shown schematically in Fig. 1. The instrument requires a 1 m × 2 m footprint, weighs ∼300 kg, and has an average power requirement of 3.4 kW. Primary hydronium ions are produced in a tungsten hollow cathode interfaced to a short source-drift (SD) tube in which hydronium ion production is maximised by ion-neutral reactions with
Distribution of primary ions in the FDT
Our first generation PTR-ToF operates with a drift tube pressure of 0.6–1.0 mbar and a 240 V dc potential difference between injection and extraction orifices. The simple orifice design in this drift tube leads to relatively high humidity as a result of forward transport of water from the primary ion source into the drift tube. Because of this high humidity, we operate the FDT at relatively high values of reduced-field (E/N, where E is the electric field across the FDT and N is the gas number
Conclusions and future modifications
We have demonstrated the operation of our first generation hollow cathode PTR-ToF mass spectrometer. The instrument is capable of detection of a few ppbv of gaseous analyte on the order of a few seconds, with a sensitivity at this volume mixing ratio approaching 30 ncps ppbv−1 for acetone. The instrument response is linear over concentration ranges of important atmospheric trace gases. Use of a time-of-flight mass spectrometer, rather than quadrupole-MS, has offered the expected advantages
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