Terahertz spectroscopy plus analysis distinguishes gasoline mixtures

It is important in the petroleum industry to be able to characterize different refined-oil mixtures, which include diesel/gasoline and gasoline/gasoline (different octane grades); one way to do this is with terahertz time-domain spectroscopy, which takes advantage of the fact that different kinds of oil have different terahertz-frequency characteristics. However, in some instances, this approach has not been precise enough: for example, when testing mixtures of 90# and 97# gasoline, the absolute error between the real and fitted value was too large (25%). Now, researchers at Tianjin University (Tianjin, China) and North Automatic Control Technology Institute (Taiyuan, China) have improved this approach by subjecting it to a multiparameter combined analysis, which reduced the absolute error in the example case to 6%.

Terahertz time-domain waveforms of the samples were acquired using a mode-locked Ti:sapphire ultrafast laser to generate electron-hole pairs that created single-cycle terahertz pulses, which were then focused by parabolic optics to a 3.5 mm spot. Changing the relative time delay between a gated pulse and the detected pulse allowed the pulse shape to be mapped out. Absorption-
coefficient spectra of the sample (which was placed in a quartz cell) were taken. The analysis was based on four parameters, two time-related and the other two absorption-related. Eleven gasoline-mixture samples were tested. The researchers believe most types of oil mixtures can be tested this way. Contact Jian Li at [email protected].