The outbreak of coronavirus disease 2019 (COVID-19) has led to substantial infections and mortality around the world. Fast screening and diagnosis are thus crucial for quick isolation and clinical intervention. In this work, we showed that attenuated total reflection−Fourier transform infrared spectroscopy (ATR−FT-IR) can be a primary diagnostic tool for COVID-19 as a supplement to in-use techniques. It requires only a small volume (∼3 μL) of the serum sample and a shorter detection time (several minutes). The distinct spectral differences and the separability between normal control and COVID-19 were investigated using multivariate and statistical analysis. Results showed that ATR−FT-IR coupled with partial least squares discriminant analysis was effective to differentiate COVID-19 from normal controls and some common respiratory viral infections or inflammation, with the area under the receiver operating characteristic curve (AUROC) of 0.9561 (95% CI: 0.9071−0.9774). Several serum constituents including, but not just, antibodies and serum phospholipids could be reflected on the infrared spectra, serving as "chemical fingerprints" and accounting for good model performances.

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One kind of ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate, is used to investigate the interaction between electrospray and corona discharge under positive and negative polarity. A high-speed camera, a digital camera, and an oscilloscope are applied to observe the shape of the meniscus and the jet, photograph the discharge plume, and observe the current waveform of discharge, respectively. The electrospray and discharge modes are studied under different flow rates and voltage conditions, and the experimental results show that there is only corona discharge with no jet at a low flow rate, while the intermittent jet or droplet emission is accompanied by corona discharge at a high flow rate, and no steady cone jet mode is obtained under all experimental conditions. The inevitability of corona discharge in the electrospray process of 1-ethyl-3-methylimidazolium tetrafluoroborate is analyzed from the viewpoint of electric field. Pulse discharge at low voltage depends on the oscillation of the meniscus. The frequency of pulse discharge can be changed indirectly by changing the voltage to control the oscillation of the meniscus. The influence of corona discharge and resultant space charge is not only on the electric field strength, but also on the shape, which is the reason why the liquid surface morphology is significantly different under positive and negative polarities.

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