_applied sciences_  
_Article_  
Ultrafast Time-of-Flight Method of Gasoline Contamination  
Detection Down to ppm Levels by Means of Terahertz  
Time-Domain Spectroscopy  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]  
CENTERA Laboratories, Institute of High Pressure Physics Polish Academy of Sciences, ul. Sokolowska 29/37, 01-142 Warsaw, Poland; [EMAIL_ADDRESS] (E.K.-M.); [EMAIL_ADDRESS] (V.M. ); [EMAIL_ADDRESS] (G.C. ); t.skotnicki@oezamateu. (T.S. ); [EMAIL_ADDRESS] (W.K.) CEZAMAT Centre for Advanced Materials and Technologies, Warsaw University of Technology, ul. Ploezeki 19, 02-82 Warsaw, Poland  
Institute of Microelectronics and Optoelectronics, Faculty of Electronics and Information Technology, Warsaw University of Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland  
Laboratoire Charles Coulomb, UMR, CNRS 5221, 34095 Montpellier, France  
Correspondence: [EMAIL_ADDRESS]; Tel. : +48-22-182-12-47  
Footnote 13: email: email: st and motor fuels are obtained by refining crude oils. Thus, even in the final fuel, there are always some traces of different contaminants, including water [1]. Water is a particularly undesirable component of fuels, especially in the aviation industry. Due to its polar structure, it is almost immiscible with the organic compounds of which fuel is composed. In fuels, water is present in three forms: dissolved water, free water (a separate liquid phase) and fuel-water emulsions [2; 3]. Dissolved water is not dangerous for the proper working of the engines, and it is regarded as a minor fuel constituent [3]. However, water is miscible in fuel only within a small concentration range, and its exact saturation concentration depends on the fuel's composition. For example, in the case of hydrocarbon mixtures containing higher contents of aromatics, more water can be dissolved. This occurs because water is more soluble in benzene than in other hydrocarbons. Above a certain concentration level and depending on external conditions (temperature, pressure), water begins to separate from the solution, forming free water [1; 4]. If surfactants are present in a mixture, the water and fuel form an emulsion where droplets of water less than 100 \(\upmu\)m in diameter are constantly dispersed in the fuel phase. Both free water and emulsions are dangerous forms of water in fuels. There are many factors that make water in fuel undesirable, of which the most important are corrosion, lubrication loss and fuel flow obstruction. Water can corrode fuel tanks, which creates a risk of contamination with solid particulates of rust or even fuel leakage. Water is less viscous than diesel and, therefore, provides a smaller \"cushion\" of lubrication between the opposing surfaces of the moving parts, leading to increased abrasive wear. Therefore, the presence of water deteriorates the lubricating properties of a fuel. Water will freeze at sub-zero Celsius temperatures, and the resulting ice crystals, like other solid particles, may clog fuel filters, cause mechanical damages and/or impede fuel flow. Water in the fuel may also facilitate the growth of microorganisms that can produce sludge and acids that contaminate the fuel and give rise to corrosion of the tank and fuel systems. The multiple problems caused by water in fuels (partially those enumerated above) mean that new, cheap and quick methods to determine its exact content are constantly being sought [2; 5]. One of the most intensively explored methods is terahertz time-domain spectroscopy (THz-TDS). THz-TDS is a rapidly developing spectroscopic technique often used for determining the properties of a material by probing it with short pulses of THz radiation [6]. THz-TDS is particularly attractive for analytical applications in a gas phase, where frequencies of molecular twisting or stretching or intramolecular or intermolecular hydrogen bonding often correspond to the THz range [7; 8].