A Vacuum Ultraviolet Ion Source (VUV-IS) for Iodide–Chemical Ionization Mass Spectrometry: A Substitute for Radioactive Ion Sources
Iodide chemical ionization mass spectrometry has been used extensively in atmospheric research to study low-volatility and inorganic compounds. The technique works by ionizing methyl iodide source gas to form I– ions, which then ionize analytes via clustering. A significant barrier to widespread adoption of this technique has been the primary ionization source. Until now, only two approaches to ionization were available and widely used: a limited-sensitivity x-ray source, and a high-efficiency radioactive source, usually 210Po. However, due to obvious safety and regulatory concerns, polonium sources are difficult to acquire, store, transport, and deploy.
Ji et al. (2020) describes a new ionization technique using vacuum ultraviolet (VUV) light, which is much safer than a radioactive source but similarly efficient. A commercial version of this VUV source is now included with Vocus Aim instruments. The source uses a krypton lamp, which has two emission lines at 116.486nm and 123.582 nm. Methyl iodide (CH3I), delivered in a stream of high-purity N2, efficiently absorbs this light, forming low-energy photoelectrons which in turn interact with CH3I to form I– ions. Additives can be added to increase sensitivity and decrease the amount of precursor (CH3I) in the N2 source gas. The geometry of the source is such that VUV light does not enter the ion-molecule reaction chamber (IMR) and is optimized for low reagent ion source flow rates, conserving nitrogen for field measurements.
The performance of various configurations of the VUV source were tested by determining the instrument sensitivity to formic acid, gaseous chlorine (Cl2), and ClNO2. Sensitivities to each compound of around 700 cps/ppt were achieved, corresponding to 1-minute detection limits of 0.05 to 0.3 ppt (50 to 300 parts-per-quadrillion).
Additionally, the VUV ion source produced primary ion spectra that were cleaner than those observed with the radioactive source. Measured backgrounds for HNO3– and NO3– were also substantially reduced. This means that the LOD of VOC using the VUV source are lower than the polonium based systems because many species have reduced background.
Finally, the VUV source can be used to produce many other reagent ions, in addition to I–. If a small amount C6H6 or other absorber is also added to the source, it generates photoelectrons able to ionize other compounds that have smaller VUV absorption cross sections. As an example, the authors experimented with using SF6– as a reagent ion to measure sulfur dioxide, formic acid, and acetic acid.
TOFWERK has characterized a suite reagent ions possible with the VUV ion source, each of which can be used to selectively analyze different groups of compounds.
The new ion source was deployed during both a ground-site field campaign and on aircraft, demonstrating its robustness, mobile deployment, and reliability.