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Catalogue of Optical Spectra of Oils Hengstermann T & R Reuter Published first in January 1999 by: Carl von Ossietzky University of Oldenburg, Institute of Physics, Marine Physics Group, 26129 Oldenburg, Germany |
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| Absorbance Analyses | ||||
The absorption characteristic of oils is an important auxiliary parameter in many fluorometric investigations. While the absorption of many chemicals can be easily determined with conventional spectrophotometers, other substances, like e.g. crude oils, have very high absorption coefficients of 1 μm-1, typically. Sophisticated methods have been developed to directly measure the absorption coefficients of these highly absorbing liquids directly (Hoge, 1982; Barbaro et al., 1991).
Alternatively, the sample can be diluted in an organic solvent down to a concentration level which ensures transmission of light over distances of a few millimetres. Experiments have shown that cyclohexane is most suitable for this purpose: other types of solvents are not suitable to bring all compounds of crude oil samples into solution. In this way the use of optical setups for analysis of highly absorbing liquids, that are not generally available, is avoided and absorption measurements are performed with standard cuvettes and laboratory photometers.
To cover the entire wavelength range of spectra, absorption measurements are done at wavelengths between 250 and 700 nm with 1 nm resolution. The absorption of the solvent utilized for diluting the sample is discriminated by a double beam measurement, with the solvent in the reference cuvette
Absorption data were obtained by using quartz cuvettes with a 1 cm optical path length. A deuterium lamp was used between 250 and 340 nm, a tungsten halogene lamp at higher wavelengths. Data output is given in Absorption Units (A) which corresponds to the negative decadic logarithm of I/I', with I the transmitted and I' the incident intensity. The dynamic range of the spectrophotometer used in these measurements is from -0.3 to 4 A, with an accuracy specified to 0.002 A at 1.0 A.
Oil samples were diluted with cyclohexane down to a concentration that yields data within the dynamical range of the instrument. To avoid possible errors due to interactions with the solvent a series of dilutions is made with each sample. From these data an average over the entire linear range is calculated, and absorption coefficients are derived from this average.
References
Hoge F E, 1982. Laser measurement of the spectral extinction coefficients of fluorescent, highly absorbing liquids. Applied Optics, 21: 1725-1729
Barbaro A et al., 1991. Oil UV extinction coefficient measurement using a standard photometer. Applied Optics, 30: 852-857