Evaluating the Performance of Five Different Chemical Ionization Techniques for Detecting Gaseous Oxygenated Organic Species
In this work, the authors conclude that each CIMS technique is particularly appropriate for measuring a different chemical space in the atmosphere. The strengths and limitations of each technique should be a careful consideration when designing an experiment and selecting an instrument suite.
Chemical ionization mass spectrometry (CIMS) has emerged as a powerful tool to support fundamental research in climate and air quality and to address challenging aspects of atmospheric organic chemistry. CIMS techniques are particularly well-suited to study the formation of organic aerosol (fine particulates). The reaction chemistry that forms aerosol rapidly generates a wide spectrum of product compounds with diverse chemical and physical properties. Different CI reagent ions can be used to target different groups of compounds, and the use of a TOF mass analyzer allows the simultaneous measurement of hundreds to thousands of product ions. Additionally, the high sensitivity of many CIMS techniques has enabled the detection and quantification of increasingly more compounds, even at very dilute mixing ratios.
No single CIMS technique can measure all organic compounds in the atmosphere. It is important to understand the particular strengths and weaknesses of each technique, so that the appropriate instrument is selected for a study. This work presents a comprehensive study undertaken at the University of Helsinki to characterize and evaluate five different CIMS techniques. The team conducted a series of experiments oxidizing α-pinene, an important biogenic compound, in an environmental chamber. The experimental conditions were intentionally diverse, including various levels of nitrogen oxides, intensity of UV radiation, and different oxidants.
Approximately 1000 ion masses were measured by the instrumentation suite, which included iodide CIMS, two atmospheric pressure interface CIMS instruments (nitrate and amine CI-Api-TOF), and two PTR-MS instruments: a conventional PTR-ToF and a Vocus 2R PTR-TOF. This study is to-date the most comprehensive comparison of the Vocus PTR-TOF to other CIMS techniques in a controlled experimental environment.
The iodide CIMS proved to sensitively measure many semivolatile compounds, while the amine and nitrate CI-Api-TOFs were more sensitive to highly oxygenated species with five or more oxygen atoms. The two PTR-TOFs were the only instruments able to measure the α-pinene precursor. The Vocus PTR-TOF detected a surprisingly large range of oxygenated compounds, including species with up to 10 oxygen atoms and organonitrates. The sensitivity of Vocus to these species was markedly better than that of the conventional PTR-TOF instrument, and for some groups of compounds was comparable to the iodide CIMS. The authors suggest that the improved inlet design of the Vocus is responsible for its ability to measure highly oxygenated compounds. Good correlation was seen overall for the set of compounds measured by the iodide CIMS and the Vocus, which independently each measured a large fraction of reacted carbon in the gas phase.