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Fast Polarity Switching Meets Soft Ionization with the Vocus B

Fast Polarity Switching Vocus B

Product Release
November 2023
TOFWERK, Switzerland

The Vocus B is a compact, real-time chemical ionization mass spectrometer that provides soft ionization for simplified mass spectra and uses both positive and negative ion chemistries to target a broad range of volatile organic and inorganic compounds.

With model options that include varying levels of mass resolving power and sensitivity, the Vocus B Series provides unprecedented flexibility.

Historically, chemical ionization mass spectrometry has faced a fundamental compromise: use proton transfer reaction (PTR) ionization and correct for ionization-induced fragmentation or use soft ionization technologies – such as the TOFWERK Vocus Aim Reactor – and be limited in compound coverage through the use of more selective ionization chemistry. TOFWERK Vocus CI-TOF instruments have always offered use of a PTR or an Aim Reactor, allowing users to choose between broad coverage with ionization-induced fragmentation or more selective, softer ionization.

Vocus B Series: Unprecedented Flexibility Within One Compact Instrument

With the release of the Vocus B Series, instrument users can now harness the benefits of the Vocus Aim Reactor while achieving compound coverage that can exceed that of PTR ionization. The TOFWERK Vocus B series combines cutting-edge, millisecond polarity switching with the ultra-soft ionization technology of the Vocus Aim Reactor. Simplified mass spectra, negligible fragmentation and real-time sampling using both positive and negative ion chemistry makes the Vocus B mass spectrometers an attractive solution for air quality monitoring, atmospheric chemistry research, and other challenging applications where speed and sensitivity are essential. 

The Chemical Ionization Compromise

Proton transfer reaction (PTR) ionization is a popular method in chemical ionization time-of-flight (TOF) mass spectrometry. The Vocus PTR Reactor offers a powerful analysis platform for a broad range of volatile organic compounds (VOCs). However, the ionization of acids, alcohols, and peroxides among other functional groups often results in ionization-induced fragmentation during the PTR ionization process, complicating the resulting mass spectra and making it more difficult to interpret.

As a solution, the Vocus Aim Reactor, relies on compact VUV sources to generate a variety of more selective reagent ions which do not lead to significant ionization induced fragmentation – primarily via the formation of ion-molecule adducts. This, combined with a higher-pressure flow tube reactor, allows for “soft” ionization, suppressing ionization-induced fragmentation to negligible levels and simplifying the mass spectra. Figure 1 shows stick spectra of a hypothetical mixture of different VOC with a variety of functional groups measured by Vocus PTR and Aim Reactors. Some of the sample species are susceptible to ionization induced fragmentation. The hypothetical sample mixture contained lactic acid, pinonic acid, levoglucosan, a-pinene and xylene.

The top spectrum, produced using a database of fragmentation patterns from a PTR reactor operating at 100 Td, shows multiple peaks for each compound as a result of fragmentation during the PTR ionization process. For example, 13 different peaks associated with fragments of levoglucosan alone can be seen in blue in the top spectrum. Other peaks like lactic acid appear as predominantly a single peak, but this peak is a fragment, not the parent molecule. Other compounds, such as xylene are well measured by the PTR technique.

In the bottom spectrum, produced by the instrument using the Vocus Aim Reactor, fragmentation was eliminated from the ionization process and each molecule resulted in a single peak (plus isotopes) in the spectrum. This resulted in a much simpler mass spectrum that is easier to read and interpret.

fast polarity switching
Figure 1. Compounds susceptible to fragmentation are measured using both a Vocus PTR (top) and Aim Reactor (bottom) reactor coupled with a TOFWERK Vocus TOF mass analyzer. The bottom panel, generated by the instrument using Aim, shows a much simpler and easier-to-interpret mass spectrum due to soft ionization technology.

The main disadvantage of using only the Vocus Aim Reactor is that it is a more selective ionization chemistry, meaning that fewer compounds can be measured in real time compared to the PTR – albeit with negligible fragmentation. As a result, instrument operators and researchers have had to choose which reagent ions best suit their purposes based on the chemical make up and volatility of the compounds that they want to measure.

The Vocus B Fast Polarity and Reagent Ion Switching Solution

The Vocus B series eliminates the need to make this choice. For the first time in a chemical ionization time-of-flight mass spectrometer, millisecond polarity and reagent ion switching combined with the power of the Vocus Aim Reactor means that instrument users can use up to six different positive or negative reagent ions based on three radially mounted VUV ion sources switched in real time. Thus, the Vocus B provides real-time, comprehensive measurements of a wide range of compounds by combining reagent ions with very different selectivity while benefiting from fragmentation-free ionization.

Figure 2 below demonstrates the rapid reagent ion switching made possible by the Vocus B. Every half second the reagent ion polarity is switched, enabling new ion chemistry in the reaction chamber. The switching duty cycle is greater than 80%, demonstrating that little data is discarded due to the switching transients.

fast polarity switching
Figure 2. Fast polarity switching in the Vocus CI-TOF B mass spectrometer. Every half second the reactor switches between different positive and negative reagent ions, making measurement of many different compounds possible on extremely rapid time scales. A typical configuration involves bromide, iodide, acetone and benzene – though other combinations are also possible for different applications.

Figure 3 below shows a timeseries produced by the Vocus B measuring ambient air at the TOFWERK headquarters in Thun, Switzerland. It shows nitric acid, ammonia, other trace organic acids as well as lower volatility oxidation products from the photo-oxidation of isoprene and monoterpenes – all measured simultaneously and in concentrations ranging from single digit pptv to a few ppbv. This demonstrates the extreme versatility of the Vocus B; it can measure a vast array of compounds, spanning a large dynamic range all in a very short period of time. The Vocus B series effectively eliminates the issue of fragmentation and allows real-time measurement of a range of compounds, harnessing the power of soft ionization and ultra-fast polarity switching. 

Figure 3. Timeseries of ambient air measured by the Vocus B in Thun, Switzerland spanning three days in March 2023. This demonstrates that the Vocus B series can simultaneously measure a wide array of organic compounds, acids and bases as well as trace oxidation products from biogenic VOC oxidation over a vast dynamic range thanks to its fast polarity switching and soft ionization.

Why Choose the Vocus B Mass Spectrometer?

By combining the highly sensitive and soft ionization of the Vocus Aim Reactor with cutting-edge, 50 ms polarity and reagent ion switching, the Vocus B mass spectrometer solves a longstanding problem in chemical ionization mass spectrometry. Finally, instrument users in fields such as atmospheric chemistry and semiconductor cleanroom monitoring do not have to choose between the analytic power of PTR ionization and the soft ion chemistry of the Vocus Aim Reactor. Vocus B is one compact, robust instrument that can measure more, produce clearer-than-ever results and go anywhere. One small instrument, extraordinary flexibility.

Learn More

Vocus CI-TOF Product Page

Vocus B Product Sheet

App Note: Highly Reproducible and Reliable Results with the Vocus B Mass Spectrometer