A field-deployable, chemical ionization time-of-flight mass spectrometer

Abstract

We constructed a new chemical ionization time-of-flight mass spectrometer (CI-TOFMS) that measures atmospheric trace gases in real time with high sensitivity. We apply the technique to the measurement of formic acid via negative-ion proton transfer, using acetate as the reagent ion. A novel high pressure interface, incorporating two RF-only quadrupoles is used to efficiently focus ions through four stages of differential pumping before analysis with a compact TOFMS. The high ion-duty cycle (>20 %) of the TOFMS combined with the efficient production and transmission of ions in the high pressure interface results in a highly sensitive (>300 ions s<sup>−1</sup> pptv<sup>−1</sup> formic acid) instrument capable of measuring and saving complete mass spectra at rates faster than 10 Hz. We demonstrate the efficient transfer and detection of both bare ions and ion-molecule clusters, and characterize the instrument during field measurements aboard the <i>R/V Atlantis</i> as part of the CalNex campaign during the spring of 2010. The in-field short-term precision is better than 5 % at 1 pptv (pL/L), for 1-s averages. The detection limit (3 σ, 1-s averages) of the current version of the CI-TOFMS, as applied to the in situ detection of formic acid, is limited by the magnitude and variability in the background determination and was determined to be 4 pptv. Application of the CI-TOFMS to the detection of other inorganic and organic acids, as well as the use of different reagent ion molecules (e.g. I<sup>−</sup>, CF<sub>3</sub>O<sup>−</sup>, CO<sub>3</sub><sup>−</sup>) is promising, as we have demonstrated efficient transmission and detection of both bare ions and their associated ion-molecule clusters

An electrospray chemical ionization source for real-time measurement of atmospheric organic and inorganic vapors

“…The sensitivity of iodide-based CIMS to a given compound mainly depends on the polarity and hydrogen binding energy of a compound to the I − ion (Lee et al, 2014;Iyer et al, 2016). In the atmospheric pressure ESCIMS, the ion molecule reaction time (a few milliseconds) is set by the electric field, and is up to a factor of 30 or more less than those (30-120 ms) in low-pressure CIMS instruments (Bertram et al, 2011;Lee et al, 2014, Lopez-Hilfiker et al, 2016a. The shorter reaction time should linearly lower sensitivities.…”

“…The SS capillary projects 3.5 mm into the IMR and acts as the atmospheric pressure interface between the IMR and the vacuum chamber of a commercial HR-ToF-MS (Tofwerk AG, Thun, Switzerland), effectively dropping the atmospheric pressure to 1.5 Torr in the first quadrupole of the MS, and resulting in a sample flow of ∼ 270 sccm (standard cubic centimeters per minute) into the MS. The HR-ToF-MS and its data acquisition procedures have been described in detail previously (Junninen et al, 2010;Bertram et al, 2011;Lee et al, 2014). The evaporation tube, lens and IMR tube are electrically connected, while the mass spectrometer entrance capillary is electrically isolated from the IMR by a ∼ 1 mm thick jacket of Teflon.…”

“…The recent coupling of chemical ionization to high-resolution time-of-flight mass spectrometers (HR-ToF-MS) enables the simultaneous determination of the abundance and molecular composition of a wide array of atmospheric inorganic and organic compounds with fast time response and high sensitivity (Junninen et al, 2010;Bertram et al, 2011;Yatavelli et al, 2012;Aljawhary et al, 2013;Lee et al, 2014;Lopez-Hilfiker et al, 2014, 2016aFarmer, 2015, 2016;Yuan et al, 2016). The use of HR-ToF-CIMS has allowed for groundbreaking progress in atmospheric organic chemistry, such as the observation of 3610 Y.…”

Understanding the primary emissions and secondary formation of gaseous organic acids in the oil sands region of Alberta, Canada

“…A detailed description of the instrument and principles of operation has been given elsewhere (Bertram et al, 2011;Lee et al, 2014) and in the Supplement. Briefly, the HR-ToF-CIMS used in this study was a differentially pumped time-of-flight mass spectrometer configured to use acetate ion as the reagent ion in the ionization of molecules of interest (Veres et al, 2008;Brophy and Farmer, 2015) via the following reaction:…”

Gas-phase pesticide measurement using iodide ionization time-of-flight mass spectrometry

“…As pesticides are typically quite large with molecular weights of 200-500 Da, a separation step before analysis of ambient air is often required to eliminate other isobaric molecules that may act as interferences. CIMS is increasingly used to measure trace gas species in the atmosphere because of high sensitivities, resolution, and selectivities of the different reagent ions employed (Bertram et al, 2011;Crounse et al, 2006;Lopez-Hilfiker et al, 2014;Nowak et al, 2007;Brophy and Farmer, 2015;Wentzell et al, 2013). Chemical ionization can be achieved by positive or negative ions, including hydronium, acetate, iodide, and nitrate.…”