Detection of Ketones by a Novel Technology: Dipolar Proton Transfer Reaction Mass Spectrometry (DP-PTR-MS)
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Proton transfer reaction mass spectrometry (PTR-MS) has played an important role in the field of real-time monitoring of trace volatile organic compounds (VOCs) due to its advantages such as low limit of detection (LOD) and fast time response. Recently, a new technology of proton extraction reaction mass spectrometry (PER-MS) with negative ions OH– as the reagent ions has also been presented, which can be applied to the detection of VOCs and even inorganic compounds. In this work, we combined the functions of PTR-MS and PER-MS in one instrument, thereby developing a novel technology called dipolar proton transfer reaction mass spectrometry (DP-PTR-MS). The selection of PTR-MS mode and PER-MS mode was achieved in DP-PTR-MS using only water vapor in the ion source and switching the polarity. In this experiment, ketones (denoted by M) were selected as analytes. The ketone (molecular weight denoted by m) was ionized as protonated ketone [M + H]+ [mass-to-charge ratio (m/z) m + 1] in PTR-MS mode and deprotonated ketone [M – H]– (m/z m – 1) in PER-MS mode. By comparing the m/z value of the product ions in the two modes, the molecular weight of the ketone can be positively identified as m. Results showed that whether it is a single ketone sample or a mixed sample of eight kinds of ketones, the molecular weights can be detected with DP-PTR-MS. The newly developed DP-PTR-MS not only maintains the original advantages of PTR-MS and PER-MS in sensitive and rapid detection of ketones, but also can estimate molecular weight of ketones.
KeywordsDP-PTR-MS PTR-MS mode PER-MS mode VOCs Ketone
Proton transfer reaction mass spectrometry (PTR-MS) is a chemical ionization (CI) mass spectrometry that can be used for on-line detection of trace volatile organic compounds (VOCs). Because it has the advantages of short response time, low limit of detection (LOD), direct sampling, etc. [1, 2, 3, 4, 5], PTR-MS is widely used in many fields, such as environmental protection, food, medicine, and public safety [6, 7, 8, 9, 10, 11, 12, 13].
In traditional PTR-MS, H3O+ ions generated from water vapor with a discharge source are used as proton donor ions. The VOCs are ionized by undergoing proton transfer reaction with the H3O+. Although this is a soft ionization technique, it is inevitable that a few cluster ions and fragmental ions are produced, which causes difficulty in qualitatively identifying the VOCs. Therefore, successors exploited O2 +, NO+, and NH4 + as reagent ions, and the identification of VOCs was improved, owing to the production of different product ions resulting from changes in the reagent ion-molecule chemistry [14, 15, 16, 17, 18, 19]. However, these reagent ions must be prepared with specific reagent gasses (oxygen, dry air, ammonia).
Based on previous studies [20, 21], our research team recently developed a new proton extraction reaction mass spectrometry (PER-MS) that uses OH– ions as reagent ions, prepared with only water vapor in the discharge source . The detection of VOCs with PER-MS is based on the proton extraction reaction. With the development of this technology, it is possible to prepare different reagent ions using only water vapor.
Based on our previous research on PTR-MS and PER-MS [14, 15, 22], a new technology called dipolar proton transfer reaction mass spectrometry (DP-PTR-MS) is presented in this work. DP-PTR-MS prepares two kinds of reagent ions (H3O+ and OH-) with only water vapor discharged in a hollow cathode (HC) ion source, and the selection of PTR-MS mode and PER-MS mode was achieved by a circuit switch in the single instrument. The water vapor flow and reduced field in the drift tube were optimized. Ketone was tested as an example, and a single kind sample and mixed samples of eight kinds of ketones (acetone, 2-butanone, 2-pentanone, 2-hexanone, 2-heptanone, 2-octanone, cyclopentanone, and cyclohexanone) were analyzed, respectively, in the two modes of DP-PTR-MS. With the comparison of the characteristic ions for one ketone in two modes, the molecular weight of this ketone can be estimated with DP-PTR-MS.
Instrumentation and Principle
In this experiment, the pressure in the drift tube was 1.75 mbar in PTR-MS mode and 1.95 mbar in PER-MS mode. The temperature of drift tube was kept at 338 K. The pressure in the ion detection system was about 6.3 × 10–7 mbar. The voltage of the drift tube could be adjusted from 0 to 1000 V.
The sample was introduced into a 20 cm capillary through the bypass of a tee fitting and then introduced into the drift tube. The sample flow in the capillary was 2.84 mLmin–1. The temperature of the capillary was kept at 373 K.
Reagents and Samples
Acetone used in this experiment was purchased from Shanghai Zhenqi Chemical Reagent Co., Ltd., China; 2-butanone, 2-pentanone, 2-hexanone, 2-heptanone, and 2-octanone were purchased from Shanghai Jingchun Reagent Co., Ltd. Cyclopentanone and cyclohexanone were purchased from National Pharmaceutical Group Shanghai Chemical Reagent Co., Ltd., China. All these reagents are analytically pure. The high-purity water used to generate water vapor was produced by combining two lab water purification systems (KNTR-I-10 and Micropure UF). High-purity nitrogen (99.999%) was purchased from Nanjing Special Gas Factory Co., Ltd., China. The syringe pump (model no. 601553) was purchased from KD Scientific, Holliston, USA.
The gas samples of ketones detected with DP-PTR-MS were obtained by diluting the saturated vapor of ketones with high-purity nitrogen gas. The flow of high-purity nitrogen was controlled by a flowmeter with a valve. The saturated vapor of ketone was pushed into the high-purity nitrogen by a syringe pump. The concentration of ketone could be prepared to an appropriate value by adjusting the push velocity of the syringe pump and the flow of the high-purity nitrogen. This method can ensure that the product ions’ intensity I PI was far less than the reagent ions’ intensity I RI (I PI /I RI was less than 10%).
Results and Discussion
Optimization of the Water Vapor Flow
Optimization of Reduced Field
The Background of Mass Spectrum in DP-PTR-MS
The Detection of Ketones
Although the peaks of the quasi-molecular ions (at m/z m + 1, m – 1) can be observed in both modes, obvious fragmental ions of these ketones (except for acetone) were also observed in PTR-MS mode. There were even eight kinds of fragmental ions for 2-heptanone. For 2-pentanone, 2-hexanone, 2-heptanone, 2-octanone, and cyclohexanone in PTR-MS mode, the highest intensity peak in the mass spectrum is not the quasi-molecular ion peak. However, in PER-MS mode, the relative abundance of quasi-molecular ions at m/z m – 1 is more than 90%, and the intensities of their fragmental ions are negligible. This shows that DP-PTR-MS can detect ketones in both modes and can help to identify their molecular weight. Additionally, the new PER-MS mode has more effective soft ionization ability for ketones.
This work introduces a novel DP-PTR-MS technology that combines the functions of PTR-MS and PER-MS. The switching between the two modes is simple, without any complex tasks such as preparing multiple reactive gasses. Since both reagent ions, H3O+ and OH–, were generated by water vapor discharge in a single ion source, there is no pollution or interference from different reagent gasses caused by switching between two modes. The same substance underwent different ion molecular reactions (proton transfer reaction and proton extraction reaction) in the two modes to produce different characteristic ions. Taking ketones as examples, the molecular weight of ketones, even in a mixture, can be estimated by comparing the m/z values of the two kinds of characteristic ions in the two modes. In this work, the conditions of water vapor flow and reduced field were optimized. In PTR-MS mode and PER-MS mode, the optimal water vapor flow was 0.7 mL min–1 and 2.8 mL min–1, respectively, and the optimal reduced field E/N was 78.3 Td. DP-PTR-MS could detect molecular weight in single-ketone samples and in mixed samples containing eight kinds of ketones. The PER-MS mode in DP-PTR-MS has more obvious advantages for the detection of mixed sample of ketones. The novel DP-PTR-MS may have important application value in the field of VOC detection.
This work was supported by grants from the National Natural Science Foundation of China (21577145, 21477132, 81401756), National Key R&D Program of China (2016YFC0200200, 2015BAI01B04), Science and Technology Service Network Initiative, Chinese Academy of Sciences (KFJ-SW-STS-161), Anhui Provincial Program for Science and Technology Development, China (1604d0802001), and Innovation Program of the Development Foundation of the Hefei Center for Physical Science and Technology, China (2014FXCX007).
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