Selective Reagent Ions for the Direct Vapor Detection of Organophosphorus Compounds Below Parts-per-Trillion Levels

Real-time low to sub parts-per-trillion (ppt_v) vapor detection of some organophosphorous compounds (OPCs) is demonstrated with an atmospheric flow tube–mass spectrometer. The chemical species investigated included dimethyl methylphosphonate, triethyl phosphate, and tributylphosphate. The atmospheric flow tube provides ambient chemical ionization with up to several seconds of ionization time. With sensitivities in the parts-per-quadrillion (ppq_v) range, there are many background contaminants competing for charge with the target analytes. Initially, the OPCs were not observable in direct room air analysis, presumably due to other trace components possessing higher proton affinities. However, the addition of a trialkylamine as a dopant chemical served to provide a single reagent ion that also formed a proton-bound heterodimer with the OPCs. These asymmetric proton-bound dimers had sufficiently high hydrogen bond energy to allow the cluster to remain intact during the analysis time of several seconds. Changes in stability were observed for some of these asymmetric proton-bound dimers with a shorter half-life for adducts with a larger proton affinity differences between the amine and the OPC. Detection levels approaching low ppt_v to high ppq_v were correlated by three different methods, including use of a permeation tube, direct injection of a fixed mass into the sample air flow, and calculations based upon signal intensity ratios, reaction time, and an estimated reaction rate constant. A practical demonstration showed real-time monitoring of a laboratory environment initially with low ppt_v levels of vapor observed to decay exponentially over about an hour while returning to baseline levels

Structures for Lossless Ion Manipulations Device as an Ion Mobility Filter (ASMS 2017)

Structures for Lossless Ion Manipulations (SLIM) allow confining and manipulating ions utilizing a combination of radio frequency (RF) and direct current (DC) fields or traveling waves (TW). TW can be employed in SLIM devices to separate ions based on their mobility. We have been exploring concepts for the continuous filtering of ions for the selection of specific and narrow mobility ranges. Such a device would be an IM analog of a e.g. quadruple mass filter. In this presentation we show the supporting simulations and the experiments to demonstrating the filtering capability of the SLIM device.The SLIM filter module (30.5 cm) was designed having two parallel arrays of electrodes, namely the rung and guard electrodes. Ions are confined laterally by the applied DC voltage to the guard electrodes, while confined between the surfaces by effective potentials created by applying alternating 1800 out of phase RF voltages. In the current design, ions are guided by a combination of TW and opposing DC drift fields. The SLIM was segmented into two mirror-image sections where the TW and opposing DC are applied. By choosing the suitable combination of DC gradient and TW parameters for the two sections, it is possible to transmit ions of certain mobility while filtering out other ions.In this presentation, we demonstrate a SLIM ion mobility filter allowing ions of specific mobility to be efficiently transmitted. Ion trajectories simulations showed the SLIM devices can filter ions according to their mobilities when opposing TW and DC drift fields are combined. By choosing the suitable combination of DC gradient and TW parameters for the two sections, we found it is possible to transmit ions of certain mobility while filtering out other ions. The SLIM filter is operated by combining a positive DC gradient in the first half and a negative DC gradient in the second half of the SLIM. Two TW were used, one moving forward in the first section, while with the other is moving in the reverse direction in the second section of the SLIM module. The filtering is determined by DC gradient and the TW parameters, such as frequency, amplitude and the sequence (or in other words, the duty cycle of the travelling waveTW). Experiments show that filtering with minor loss of ions could can be achieved by adjusting proper selection of TW frequencies. The difference in frequency, frequency window, determines the range of mobilities transmitted through the filter, which can be explained by the relative ion velocity obtained from the applied DC and TW potentials. The sequence of the TW was found to affect the sensitivity of the device. The velocity of the ions due to TW and that due to the DC field were extracted from the simulations. The filtering is due to the opposing effects of the TW and the DC gradient. Those ions whose mobility due to TW is higher than that due to the DC gradient will successfully pass the first section. While in the second section ions having a higher mobility due to DC gradient will be transmitted