Abstract
Abstract. Knowledge about mass discrimination effects in a chemical ionization mass spectrometer (CIMS) is crucial for quantifying, e.g., the recently discovered extremely low volatile organic compounds (ELVOCs) and other compounds for which no calibration standard exists so far. Here, we present a simple way of estimating mass discrimination effects of a nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer. Characterization of the mass discrimination is achieved by adding different perfluorinated acids to the mass spectrometer in amounts sufficient to deplete the primary ions significantly. The relative transmission efficiency can then be determined by comparing the decrease of signals from the primary ions and the increase of signals from the perfluorinated acids at higher masses. This method is in use already for PTR-MS; however, its application to a CI-APi-TOF brings additional difficulties, namely clustering and fragmentation of the measured compounds, which can be treated with statistical analysis of the measured data, leading to self-consistent results. We also compare this method to a transmission estimation obtained with a setup using an electrospray ion source, a high-resolution differential mobility analyzer and an electrometer, which estimates the transmission of the instrument without the CI source. Both methods give different transmission curves, indicating non-negligible mass discrimination effects of the CI source. The absolute transmission of the instrument without the CI source was estimated with the HR-DMA method to plateau between the m∕z range of 127 and 568 Th at around 1.5 %; however, for the CI source included, the depletion method showed a steady increase in relative transmission efficiency from the m∕z range of the primary ion (mainly at 62 Th) to around 550 Th by a factor of around 5. The main advantages of the depletion method are that the instrument is used in the same operation mode as during standard measurements and no knowledge of the absolute amount of the measured substance is necessary, which results in a simple setup.
Highlights
The nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer has been established as an important tool for atmospheric sciences over the last years
It is capable of measuring sulfuric acid down to levels of 5 × 104 cm−3 (Kürten et al, 2011; Jokinen et al, 2012) and clusters of sulfuric acid with dimethylamine (Kürten et al, 2014), as well as the newly described class of extremely low volatile organic compounds (ELVOCs, Ehn et al, 2014), which play a role in the formation of secondary organic aerosol (SOA)
This method is in use already for characterizing the transmission efficiency of APi-TOF instruments (Junninen et al, 2010) and does not account for mass discrimination effects of the CI source
Summary
The nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer has been established as an important tool for atmospheric sciences over the last years. The UV system is characterized with a separate experiment, using photolytical conversion of N2O to NOx (Edwards et al, 2003), where both reactant and product can be measured With this system calibration factors for the CI-APi-TOF can be obtained for sulfuric acid with an overall uncertainty of around 33 %. Ehn et al (2014) have performed a gravimetrical calibration of the CI-APi-TOF using perfluorinated heptanoic acid, which appears mainly in the m/z range of 319 to 426 Th (monomers) and at 727 Th (dimers) They obtained a similar calibration factor for this compound as for sulfuric acid; the detailed transmission efficiency of the instrument over the entire mass range cannot be resolved with this method
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
