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-Ám 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 (NaI) 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.
This article describes a breakthrough using a microfluidic interface to conduct sensitive time-of-flight secondary ion mass
spectrometry (TOF-SIMS) analysis and study liquid surfaces in situ under vacuum conditions. Applications in TOF-SIMS so far
have been limited to solid surfaces or low-vapor-pressure liquid surfaces.
TOF-SIMS has been applied in applications such as polymers (1), pharmaceuticals (2), semiconductors (3), environmental chemistry
(4), inorganic surfaces (5), corrosion and monitoring of organic coating processes, catalysis (6), investigation of surface
contamination (7), geology (8), and archaeology (9). However, a lot of chemistry takes place at the interface of liquid phases
with gases in environmental, industrial, and biological systems. The surfaces of these aqueous phases and films (when <1-nm
thick) have unique kinetics and thermodynamics that are distinct from those of the bulk materials (10–12). Although TOF-SIMS
has already been used to investigate ionic liquids (13), lubricants (14,15), and liquid crystals (16,17), which all have very
low vapor pressure, no one has been able to analyze high-vapor-pressure aqueous solutions (to the best of our knowledge).
Because TOF-SIMS is vacuum-based, one cannot easily probe these high-vapor-pressure interfaces. Our goal is to permit studies
of high-vapor-pressure liquid surfaces in TOF-SIMS.
TOF-SIMS offers great sensitivity and attractive spatial resolution as an advanced vacuum-based surface analytical technique.
It uses a focused ion beam to raster over the surface, ejecting molecules, atoms, and molecular fragments (neutral and ionic)
from the top layer, and a TOF mass spectrum is obtained for each pixel (see Figure 1). TOF-SIMS gives high mass resolution
(m/Δm > 10,000) and high lateral resolution (<100 nm), a great advantage in exploring complex samples. Automated charge compensation
permits the study of insulators. Simultaneous auxiliary ion sputtering permits depth profiling and 3D chemical maps. Negative
or positive emitted ions can be monitored. Sensitivity is typically in a range of 0.1–10 ppm.
Figure 1: A schematic of the TOF-SIMS instrument.
In this article, we will describe the first aqueous surface detections by TOF-SIMS enabled by a microfluidic interface designed
for operation in vacuum conditions. We demonstrate new TOF-SIMS applications using different liquid samples prepared in the
laboratory. Directions for using this unique technique in future developments are also presented.