Probing Aqueous Surfaces by TOF-SIMS

We report the first observations of aqueous surfaces by time-of-flight secondary ion mass spectrometry (TOF-SIMS) via a self-contained microfluidic module compatible with a vacuum. The interface uses a microfluidic channel with a 3-mu m diameter window into the flowing fluid beneath it. This window supports the liquid against the vacuum by the liquid's surface tension and limits the high-density vapor region traversed by the probe beams to only a few micrometers. We demonstrate detection of aqueous surfaces such as deuterium water and sodium iodide (Nal) solution through the small aperture by TOF-SIMS. A molecular signal (C5H8NO4-=[M-H](-)) of glutamic acid also was observed. TOF-SIMS coupled with the interface provides a molecular recognition capability, making it a great choice to detect short-lifetime reaction intermediates in aqueous solutions. This novel microfluidic interface makes multimodal vacuum-based analysis of liquid surfaces possible.