Direct Mass Spectrometric Detection of n-Alkanes via Water Radical Cation-Induced C-H Activation

The rapid, sensitive, selective, and real-time measurement of n-alkanes in mixtures is significant for safety, health, and the environment. However, detecting n-alkanes is challenging due to their optoelectronic and chemical inertness, particularly in ambient mass spectrometry (MS) measurements, which often necessitate hardware modifications or environmentally harmful derivatization reagents. Here, an ambient MS approach was proposed for direct n-alkane detection in complex mixtures, leveraging water dimer radical cations ((H₂O)₂^+•) generated by a low-energy ambient corona discharge to activate C-H bonds and form detectable dihydroxyalkane adducts (R-C⁺(OH)₂). Using hexane as a model, the mechanism involves the following: (a) (H₂O)₂^+• reacts with hexane to produce a stable complex, followed by (b) stepwise hydroxylation reaction forming R-C⁺(OH)₂. The method was successfully extended to other n-alkanes (e.g., pentane, heptane, octane, nonane, and decane) with their corresponding dihydroxyalkane adducts R-C⁺(OH)₂ detected as anticipated. The presented MS approach enables simple and real-time analysis of n-alkanes in complex matrices, with a detection limit down to 0.22 parts per trillion. By leveraging (H₂O)₂^+•-induced C-H activation, this work provides an environmentally friendly strategy for direct n-alkane determination in mixtures under ambient conditions without sample pretreatment, offering a novel pathway for direct n-alkane determination across various fields including environmental monitoring, petroleum analysis, etc.