Ti₃C₂(OH)_x-assisted LDI-TOF-MS for the rapid analysis of natural small molecules

The inhomogeneous distribution of co-crystallized analytes and the traditional organic matrices as well as the intensive background interference in the low molecular weight range hinder the application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) in the analysis of small-molecular compounds. New two-dimensional material MXene (e.g., Ti₃C₂) exerts better hydrophilicity, homogeneity and repeatability, and higher laser desorption efficiency, as well as less background interference than traditional organic matrices and other nanomaterial matrices such as titanium oxide, graphene, and gold nanostructures. This study was aimed to design Ti₃C₂ matrix with abundant hydroxyls on its surface, enhance the stability of this hydroxyl-rich Ti₃C₂ (Ti₃C₂(OH)_x), and evaluate the analytical performances of Ti₃C₂(OH)_x-assisted laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) for small-molecular natural compounds in complex samples. The developed Ti₃C₂(OH)_x showed the distinct advantages such as minimum background interference, high peak intensity (~10⁵), high salt (0.6 M) and protein (0.5 mg/mL) tolerance, good repeatability (relative standard deviation<20%), and good stability after eight months of storage. Ti₃C₂(OH)_x-assisted LDI-TOF-MS analysis could be used to rapidly identify Artemisia annua (a world-famous traditional Chinese medicine) and quantify the contents of the main chemical ingredients (oxymatrine (OXY) and matrine) of Compound Kushen Injection (CKI). Interestingly, the content of OXY in CKI could be accurately quantified by Ti₃C₂(OH)_x-assisted LDI-TOF-MS, and there was a good linear relationship (R² -0.9929), a low limit of detection (400 pg), and a low limit of quantification (600 pg) of OXY. Taken together, the rapid and accurate analysis of small-molecular natural compounds in complicated samples could be achieved by the Ti₃C₂(OH)_x-assisted LDI-TOF-MS analysis.