The reliable online measurement of traces in air or process gases is of utmost importance for environmental and atmospheric studies, but also for industrial applications. Well-founded data are needed and used in simulations and to optimise purification. The elucidation of all possible artefacts is crucial in order to avoid misinterpretations in the assignment of compounds and misjudgements in quantitative analysis.
As part of the Carbon2Chem® research project, a team of scientists at MPI CEC has succeeded in distinguishing individual traces in the ambient air and identifying them as artefacts, gas contaminants or other compounds, although isomeric substances cannot be differentiated. Until now, information about the origin of many ions was missing. In a recent study published in the Journal of Mass Spectrometry, a comprehensive classification of all ions in the PTR mass spectra was performed under dry and wet conditions. It was found that some ions show positive moisture dependencies, while others show negative moisture dependencies.
Detailed information
In catalytic reactions at the lab or industrial level, the analytical methods play a paramount role in the characterization and monitoring of main compounds, as well as of traces like by-products or impurities, which are needed to determine process stability and product quality.
Due to the growing need of making processes more sustainable and to minimize CO2 emissions in order to fulfil the tougher environmental regulations of the European Union, the utilisation of until now “wasted” carbon sources in combination with hydrogen generated from renewable sources is becoming a promising alternative to reduce emissions but at the same time brings new challenges.
By using unconventional sources as feedstock such as metallurgical gases (coke oven gas, blast furnace gas and converter gas) for the production of chemicals like for instance methanol, as pursued in the Carbon2Chem® project, one of the main challenges is to identify and monitor trace compounds at the ppb (parts per billion) level before and after a purification process, which could deactivate through poisoning subsequent downstream catalysts and thus incurring in short catalyst lifetimes and high operational costs.
Therefore, a highly sensitive analytic method is needed in order to characterise potential catalysts poisons on real time (on-line) but at the same time to monitor their eventual concentration fluctuations. Because PTR-QiTOF-MS* (Proton Transfer Reaction Quadrupole interface Time-Of-Flight Mass Spectrometer) was chosen as the main characterisation technique for the online monitoring of trace compounds in the ppb and ppt concentration range in the raw and purified metallurgical gases used at the Carbon2Chem® technical Centre, it was necessary to understand first the peaks observed in the background mass spectra of the diluting gas, nitrogen, and their dynamic dependency with humidity before the identification of real traces in the metallurgical gases and the estimation of their concentrations are made.
In an article recently published in Journal of Mass Spectrometry, in the framework of the project Carbon2Chem® initiated by Prof. Robert Schlögl, Director at the MPI CEC, our PostDoc Dr. Jorge Iván Salazar Gómez reports in a meticulous study the elucidation of all ions in the mass range m/z 0-800 in the dilution gas nitrogen and their differentiation as instrumental artefact, memory effect or real gas impurity (VOC) with their respective humidity dependencies measured in the HüGaprop-Container, enabling the differentiation from potential catalysts poisons and thus, avoiding misinterpretations and misestimations when measuring real metallurgical gases.
The first step was already done in which the main instrumental artefacts (parasitic ions) originating from the ion source were discovered (Publication Nov 2019) and with the present work (Publication July 2021) several new ions were discovered, such as cyclic sulphur species [Sx]H+.
While conventional analytical techniques like gas chromatography are time consuming and do not possess the sufficient sensitivity to detect traces at the low ppb level, PTR-TOF-MS enables a real-time determination even at the ppt level without the need of pre-separation processes or long sampling preparation. The present study set the basis for the build-up of a mass spectral database, showing that some ions are measurable under humid conditions but unmeasurable when the gas matrix is dry. This will help the characterisation of the traces in the metallurgical gases in the Carbon2Chem® project, especially after a purification process, which depending on the operating conditions can produce a humid or dry purified gas. The gained knowledge will facilitate the quality assurance of the estimations made, so that the planned up-scaling can be realised based on substantiated data.
“The main goal of this study was to understand the dynamic of the instrumental background before measuring the real metallurgical gases” explains Dr. Jorge I. Salazar Gómez, who leads the so-called Gas Analysis Team within the Catalytic Technology group, led by Dr. Holger Ruland.
This study was financed in the frame of the project Carbon2Chem®, which strives the utilisation of metallurgical gases in combination with hydrogen from renewable sources to generate chemicals like methanol or ammonia and thus, reduce the CO2 emissions of the steel industry, improving efficiency and making this industry more competitive. The authors hope to have a reliable analytic tool to characterise all the different processes carried out at the Carbon2Chem® project.
Contact: Dr. Jorge Iván Salazar Gómez
Original publication: Salazar Gómez, J. I., Sojka, M., Klucken, C., Schlögl, R., Ruland, H. (2021). Determination of trace compounds and artifacts in nitrogen background measurements by proton transfer reaction time-of-flight mass spectrometry under dry and humid conditions. Journal of Mass Spectrometry. https://doi.org/10.1002/jms.4777
* PTR-TOF-MS stands for proton-transfer-reaction mass spectrometry and is a method to monitor volatile organic compounds and minute traces in ambient air online.