Volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) provide critical information across many scientific fields including atmospheric chemistry and soil and biological processes. Chemical ionization (CI) mass spectrometry has become a powerful tool for tracking these chemically complex and temporally variable compounds in a variety of laboratory and field environments. It is particularly powerful with time-of-flight mass spectrometers, which can measure hundreds of compounds in a fraction of a second and have enabled entirely new branches of VOC and/or VIC research in atmospheric and biological chemistry. To accurately describe each step of these chemical, physical, and biological processes, measurements across the entire range of gaseous products is crucial. Recently, chemically comprehensive gas-phase measurements have been performed using many CI mass spectrometers deployed in parallel, each utilizing a different ionization method to cover a broad range of compounds. Here we introduce the recently developed Vocus AIM (adduct ionization mechanism) ion–molecule reactor (IMR), which samples trace vapors in air and ionizes them via chemical ionization at medium pressures. The Vocus AIM supports the use of many different reagent ions of positive and negative polarity and is largely independent of changes in the sample humidity. Within the present study, we present the performance and explore the capabilities of the Vocus AIM using various chemical ionization schemes, including chloride (Cl⁻), bromide (Br⁻), iodide (I⁻), nitrate (NO${}_{\mathrm{3}}^{-}$ ), benzene cations (${{\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{6}}}^{+}$ ), acetone dimers ((C₃H₆O)₂H⁺), and ammonium (NH${}_{\mathrm{4}}^{+}$ ) reagent ions, primarily in laboratory and flow tube experiments. We report the technical characteristics and operational principles, and compare its performance in terms of time response, humidity dependence, and sensitivity to that of previous chemical ionization approaches. This work demonstrates the benefits of the Vocus AIM reactor, which provides a versatile platform to characterize VOCs and VICs in real time at trace concentrations.

Chemical ionization mass spectrometers are widely used for the detection of trace gases, particularly in the field of atmospheric science. Depending on the analytes of interest, chemical ionization instruments are operated under varying reactor conditions, which can make it difficult to compare instrument performance, even for the same reagent ion chemistry. This variability leads to inconsistent sensitivity distributions, particularly for weakly bound or labile analytes. As a result, determining sensitivity – instrument response per unit analyte concentration – is challenging, even when comparing the same compound detected with the same reagent ion across different studies. To address this issue, we employed multiple Vocus AIM reactors (Tofwerk AG) to systematically identify the critical parameters affecting sensitivity in flow tube chemical ionization mass spectrometers. Controlling these parameters for a given reactor geometry can significantly reduce sensitivity variations across instruments and operators. We demonstrate that sensitivity normalized to reagent ion concentration serves as a fundamental metric for interpreting results from different datasets operating under uniform chemical ionization conditions, such as those within regional networks or other monitoring applications. Calibrating the sensitivity of benzene cations to a group of hydrocarbons, and comparing it to the sensitivity of iodide anions to levoglucosan, a molecule known to react near the collision limit, reveals that it is possible to map kinetic constraints on sensitivity from one ion mode polarity to another, as long as the critical parameters are held constant. Additionally, we show that collision-limited sensitivity relative to the reagent ion is nearly constant across different ionization mechanisms for a given reactor geometry and set of conditions. This consistency enables the determination of the upper limit of sensitivity, even for reagent ions where the specific molecules reacting at the collision limit are unknown. As a result, the use of the voltage-scanning approach can be extended to a broader range of reagent ion chemistries. This study highlights how collision-limited sensitivity can enhance our understanding of the relationships between different instruments and simplify calibration requirements across various reagent ion chemistries.

Volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) provide critical information across many scientific fields including atmospheric chemistry, soil, and biological processes. Chemical ionization (CI) mass spectrometry has become a powerful tool for tracking these chemically complex and temporally variable compounds in a variety of laboratory and field environments. It is particularly powerful with time-of-flight mass spectrometers, which can measure hundreds of compounds in a fraction of a second and have enabled entirely new branches of VOC/VIC research in atmospheric and biological chemistry. To accurately describe each step of these chemical, physical, and biological processes, measurements across the entire range of gaseous products is crucial. Recently, chemically comprehensive gas-phase measurements have been performed using many CI mass spectrometers deployed in parallel, each utilizing a different ionization method to cover a broad range of compounds. Here we introduce the recently developed Vocus AIM (Adduct Ionization Mechanism) ion-molecule reactor (IMR), which samples trace vapors in air and ionizes them via chemical ionization at medium pressures. The Vocus AIM supports the use of many different reagent ions of positive and negative polarity and is largely independent of changes in the sample humidity. Within the present study, we present the performance and explore the capabilities of the Vocus AIM using various chemical ionization schemes, including Chloride (Cl^–), Bromide (Br^–), Iodide (I^–), Nitrate (NO₃^–), Benzene cations (C₆H₆⁺), Acetone dimers ((C₃H₆O)₂H⁺), and Ammonium (NH₄⁺) reagent ions primarily in laboratory and flow tube experiments. We report the technical characteristics, operational principles, and compare its performance in terms of time response, humidity dependence, and sensitivity to that of previous chemical ionization approaches. This work demonstrates the benefits of the Vocus AIM reactor which provides a versatile platform to characterize VOCs and VICs in real time at trace concentrations.

Volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) provide critical information across many scientific fields including atmospheric chemistry, soil, and biological processes. Chemical ionization (CI) mass spectrometry has become a powerful tool for tracking these chemically complex and temporally variable compounds in a variety of laboratory and field environments. It is particularly powerful with time-of-flight mass spectrometers, which can measure hundreds of compounds in a fraction of a second and have enabled entirely new branches of VOC/VIC research in atmospheric and biological chemistry. To accurately describe each step of these chemical, physical, and biological processes, measurements across the entire range of gaseous products is crucial. Recently, chemically comprehensive gas-phase measurements have been performed using many CI mass spectrometers deployed in parallel, each utilizing a different ionization method to cover a broad range of compounds. Here we introduce the recently developed Vocus AIM (Adduct Ionization Mechanism) ion-molecule reactor (IMR), which samples trace vapors in air and ionizes them via chemical ionization at medium pressures. The Vocus AIM supports the use of many different reagent ions of positive and negative polarity and is largely independent of changes in the sample humidity. Within the present study, we present the performance and explore the capabilities of the Vocus AIM using various chemical ionization schemes, including Chloride (Cl^–), Bromide (Br^–), Iodide (I^–), Nitrate (NO₃^–), Benzene cations (C₆H₆⁺), Acetone dimers ((C₃H₆O)₂H⁺), and Ammonium (NH₄⁺) reagent ions primarily in laboratory and flow tube experiments. We report the technical characteristics, operational principles, and compare its performance in terms of time response, humidity dependence, and sensitivity to that of previous chemical ionization approaches. This work demonstrates the benefits of the Vocus AIM reactor which provides a versatile platform to characterize VOCs and VICs in real time at trace concentrations.

Chemical ionization mass spectrometers are widely used for the detection of trace gases, particularly in the field of atmospheric science. Depending on the analytes of interest, chemical ionization instruments are operated under varying reactor conditions, which can make it difficult to compare instrument performance, even for the same reagent ion chemistry. This variability leads to inconsistent sensitivity distributions, particularly for weakly bound or labile analytes. As a result, determining sensitivity – instrument response per unit analyte concentration – is challenging, even when comparing the same compound detected with the same reagent ion across different studies. To address this issue, we employed multiple Vocus AIM reactors (Tofwerk AG) to systematically identify the critical parameters affecting sensitivity in flow tube chemical ionization mass spectrometers. Controlling these parameters for a given reactor geometry can significantly reduce sensitivity variations across instruments and operators. We demonstrate that sensitivity normalized to reagent ion concentration serves as a fundamental metric for interpreting results from different datasets operating under uniform chemical ionization conditions, such as those within regional networks or other monitoring applications. Calibrating the sensitivity of benzene cations to a group of hydrocarbons, and comparing it to the sensitivity of iodide anions to levoglucosan, a molecule known to react near the collision limit, reveals that it is possible to map kinetic constraints on sensitivity from one ion mode polarity to another, as long as the critical parameters are held constant. Additionally, we show that collision–limited sensitivity relative to the reagent ion is nearly constant across different ionization mechanisms for a given reactor geometry and set of conditions. This consistency enables the determination of the upper limit of sensitivity, even for reagent ions where the specific molecules reacting at the collision limit are unknown. As a result, the use of the voltage scanning approach can be extended to a broader range of reagent ion chemistries. This study highlights how collision–limited sensitivity can enhance our understanding of the relationships between different instruments and simplify calibration requirements across various reagent ion chemistries.

Volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) provide critical information across many scientific fields including atmospheric chemistry, soil, and biological processes. Chemical ionization (CI) mass spectrometry has become a powerful tool for tracking these chemically complex and temporally variable compounds in a variety of laboratory and field environments. It is particularly powerful with time-of-flight mass spectrometers, which can measure hundreds of compounds in a fraction of a second and have enabled entirely new branches of VOC/VIC research in atmospheric and biological chemistry. To accurately describe each step of these chemical, physical, and biological processes, measurements across the entire range of gaseous products is crucial. Recently, chemically comprehensive gas-phase measurements have been performed using many CI mass spectrometers deployed in parallel, each utilizing a different ionization method to cover a broad range of compounds. Here we introduce the recently developed Vocus AIM (Adduct Ionization Mechanism) ion-molecule reactor (IMR), which samples trace vapors in air and ionizes them via chemical ionization at medium pressures. The Vocus AIM supports the use of many different reagent ions of positive and negative polarity and is largely independent of changes in the sample humidity. Within the present study, we present the performance and explore the capabilities of the Vocus AIM using various chemical ionization schemes, including Chloride (Cl^–), Bromide (Br^–), Iodide (I^–), Nitrate (NO₃^–), Benzene cations (C₆H₆⁺), Acetone dimers ((C₃H₆O)₂H⁺), and Ammonium (NH₄⁺) reagent ions primarily in laboratory and flow tube experiments. We report the technical characteristics, operational principles, and compare its performance in terms of time response, humidity dependence, and sensitivity to that of previous chemical ionization approaches. This work demonstrates the benefits of the Vocus AIM reactor which provides a versatile platform to characterize VOCs and VICs in real time at trace concentrations.