Ultrafast Time-of-Flight Method of Gasoline Contamination Detection Down to ppm Levels by Means of Terahertz Time-Domain Spectroscopy

Kamil Stelmaszczyk,Wojciech Knap,Thomas Skotnicki,Ewelina Karpierz-Marczewska,Valeri Mikhnev,Grzegorz Cywinski

doi:10.3390/app12031629

Abstract

In this article, we present the possibility of using terahertz time-domain spectroscopy to detect trace, ppm-level (%wt.) concentrations of admixtures in 95-octane lead-free gasoline in straightforward time-of-flight (pulse-delay) measurements performed directly in the liquid. The method was tested on samples containing 75–0.0125% of isopropanol and 0.3–0.0250% of water. The detection limits for isopropanol and water content were determined to be 125 ppm and 250 ppm, respectively, approaching the limits of much more complicated and time-consuming methods (like spectroscopy of the vapor phase). The measured pulse delays were compared with theoretical calculations using the modified Gladstone-Dale mixing rule for the solutions. The comparison demonstrated good agreement for gasoline-alcohol mixtures and large discrepancies for gasoline-water mixtures, suggesting that gasoline-water mixtures cannot be considered idealized binary mixtures. Our results clearly show that the pulse time delay measurement by THz-TDS is a fast and sensitive method of gasoline contamination detection and, as such, can be easily integrated with industrial online real-time quality control applications.

Highlights

Our work clearly shows that the pulse time delay measurement by terahertz time-domain spectroscopy (THz-TDS) is a sensitive and fast method of gasoline contamination detection and, as such, can be integrated with industrial real-time quality control applications
The stability performance of the THz-TDS spectrometer was verified with the beam delivery part and photoconductive antennas tightly closed in a plexiglass container, from which ambient air was removed by purging the container with purified clean dry air (CDA)
We have shown that a conceptually simple THz-TDS time-of-flight method relying on a straightforward THz pulse delay measurement can be successfully used to detect very small, on an order of parts-per-million of mass fraction, concentrations of water (H2 O) or isopropanol (C3 H8 O) in quasi-binary gasoline solutions

Summary

Introduction

Crude oil and its refined products, generally known as petrochemicals, are among the most important sources of energy, fuel and raw materials; without crude oil and petrochemicals, modern life would not be possible. Petrochemicals consist predominantly of solid, liquid and gaseous hydrocarbons which dissolve one another. The main components of petrochemicals are alkanes, cycloalkanes, aromatic hydrocarbons and, to a small extent, alkenes. The main contaminants are water, organic compounds containing oxygen, sulfur or nitrogen, as well as some inorganic compounds.

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