Direct Vacuum Inlet System Enabling Ultra-Sensitive in-Situ Analysis of Chemical Reaction Products
Authors: Trimarco Daniel Bøndergaard , Scott Søren Bertelsen , Pedersen Thomas , Hansen Ole , Chorkendorff Ib , Vesborg Peter C. K. 
Published: February 2020

We present a first-of-its-kind vacuum inlet system enabling operation of vacuum analysis instruments, like mass spectrometers, directly in liquid or gaseous environments. Our inlet system overcomes the limitations of differential pumping while maintaining an ultra-fast time-response and delivering up to 1000 times higher sensitivity than conventional technologies [1]. We are thus able to perform quantitative analysis of chemical reaction products with unprecedented sensitivity on millisecond time-scales.

Across many scientific and industrial fields, it is a challenge to analyse chemical reaction products with high sensitivity and fast time-response. This is especially true in systems related to electrochemical energy conversion and storage, such as batteries and fuel cells, where accurate monitoring of fast and intermediate reaction phenomena often reveals key insights. At present, in-situ detection of electrochemical reaction products in liquid (most often aqueous) electrolytes is achieved by using a porous membrane inlet system together with differential pumping stages coupled with a mass spectrometer, a technique known in literature as Differential Electrochemical Mass Spectrometry (DEMS) [2]. However, by having an inlet system operating with differential pumping, vast amounts of both analyte and solvent are pumped away before entering the mass spectrometer. This translates into a significant drop in sensitivity and a large background-signal in the mass spectrometer due to rapid solvent evaporation. Furthermore, since DEMS systems are not commercially available, they presently only exist as custom-made and expensive instruments in a few research laboratories.

Here we present a novel vacuum inlet system, allowing mass spectrometers to operate directly in a liquid electrolyte without the use of differential pumping, thus delivering a 1000-fold sensitivity increase over conventional techniques, while maintaining an ultra-fast time-response. The inlet system consists of a disposable micro-fabricated membrane chip made of silicon, which forms a stable permeable interface between the vacuum of a mass spectrometer and the liquid of an electrochemical test environment. Due to the direct injection nature of this system no analyte is lost to differential pumping, which increases sensitivity of the system tremendously and makes calibration for quantitative measurements easier. In addition, the full analysis system is built more compactly and with fewer components than conventional DEMS systems, making it a significantly cheaper instrument. Finally, this inlet system operates equally well in liquid and gaseous test environments, making it highly versatile and easy to use.

The capabilities of this analysis system are showcased by various examples, including electrochemical water-splitting at low overpotentatial and low surface area CO-stripping. It is shown how the evolution of hydrogen and oxygen can be quantified in-situ at Faradaic currents on the order of 10 nA. In comparison, conventional techniques are capable of achieving detections down to about 10 µA. In addition it is shown, that it is possible to make a quantitative in-situ detection of < 1 % of a monolayer of surface adsorbed CO being electrochemically stripped off an electrode at standard potential scan-rates of 50 mV/s.

References:

[1] D. B. Trimarco et al., Rev. Sci. Instrum. 86, 075006 (2015)

[2] H. Baltruschat, J. Am. Soc. Mass Spectrom. 15 (2004), 1693–1706

Figure 1