Photoionization-Generated Dibromomethane Cation Chemical Ionization Source for Mass Spectrometric Analysis of Volatile Organic Compounds

Volatile organic compounds (VOCs) have a serious impact on the ecological environment and human health for its characteristics of volatile and toxic. Single photoionization mass spectrometer (SPI-MS) based on vacuum ultraviolet (VUV) lamp is an efficient technique for rapid analysis of VOCs. However, its performance is limited due to relatively low photon flux density and ionization cross section. In this work, a novel photoionization-generated dibromomethane cation chemical ionization (PDCI) source based on VUV Kr lamp was developed for time-of-flight mass spectrometry (TOF MS) to analyze VOCs. A sufficient and stable flux of CH₂Br₂⁺ was generated by dibromomethane as reagent gas, and the VOCs were further ionized by reaction with CH₂Br₂⁺ cations through charge transfering. The mass spectrometer used in the experiments was a home-built radio frequency (rf)-only quadruple orthogonal acceleration reflectron TOF MS with the mass resolution of 3 000 (fwhm) at m/z 78. First of all, three bromoalkanes with the ionization energy higher than 10.0 eV while less than 10.6 eV were tested as reaction reagents, and the dibromomethane was chosen as the reaction reagent for PDCI source due to its simple reactant ions and high ionization efficiency. And then, to achieve better sensitivity and less backgrounds, the volume fraction of the reaction reagent and the electric field of PDCI source were optimized at 1 000 μL/L and 0.5 V/cm, individually. After that, the linearity and stability of the PDCI source were evaluated by benzene, toluene, and p-xylene. The linear concentration ranges, 0.8-349 μg/m³ for benzene, 0.9-411 μg/m³ for toluene, and 2.4-474 μg/m³ for p-xylene were obtained with the correlation coefficients of 0.996 5, 0.996 6, and 0.996 7, respectively. The relative standard deviations (RSDs) of the intensities of the reactant ions and the three compounds were 1.45%, 3.57%, 4.15% and 4.64%, respectively, within 6 h continuous measurement. The results indicated that the PDCI source owned good linearity and stability which can meet the needs for online quantitative and long-term monitoring of VOCs. Finally, EPA TO-14 and TO-15/17 calibration gases were used to demonstrate the performance of the PDCI source for complex VOCs samples. As a result, compared to SPI based on the same VUV lamp, the PDCI source not only distinctly enhanced the sensitivities by more than 100-fold of compounds with ionization energy close to 10.0 eV (103, 118 and 126-fold enhancement for 2-propanol, ethyl acetate and 3-allyl chloride) but also keep the soft character as well as SPI. Moreover, 42 kinds of EPA TO-14 and TO-15/17 calibration gases could be effectively detected with the lowest limit of detections at 0.06 μg/m³ in 10 s. Because of its high sensitivity, rapid analysis, and good stability, the PDCI-TOF MS has a broad prospect of applications in real-time and on-line monitoring of VOCs.