Detection of formaldehyde oxidation catalysis by MCR-ALS analysis of multiset ToF-SIMS data in positive and negative modes

Toxicity of formaldehyde is extremely high even at very low concentration in air. We have used ToF-SIMS (Time of Flight Secondary Ion Mass Spectroscopy) to explore formaldehyde oxidation catalyzed by birnessite (Kx (Mn4+,Mn3+)2O4) in the temperature range from 25 to 200 °C. ToF-SIMS is a powerful tool for the generation of chemical maps because of its high spatial resolution, its sensitivity and relatively low acquisition time. In this method the mass analysis of positive and negative ions sputtered out of the uppermost layers of the analyzed sample is performed. ToF-SIMS produces large raw data sets with rich chemical information but rather complex to be analyzed and interpreted. This is why the application of chemometric methods is proposed in this work for the exploration of ToF-SIMS complex data sets. In this work, MCR-ALS (Multivariate Curve Resolution-Alternating Least Squares) has been applied to resolve both the positive and negative ions present in ToF-SIMS data sets simultaneously analyzed using a data matrix augmentation strategy. Birnessite without catalyzed formaldehyde was first analyzed to resolve background contributions and use them to implement a selectivity constraint for MCR-ALS to remove them in the analysis of the formaldehyde oxidation data sets. Results show that, following the temperature increase, concentration of formaldehyde combined with manganese ions decreased whereas concentration of manganese oxide increased. Conformation changes of manganese formaldehyde metal complex were then inferred. It is concluded that the formation of the metal complex species formed between two manganese ions and only one formaldehyde molecule is very unlikely to exist.