Figure 1. Spectrum of gasoline vapor.QASoft Application Examples
Measuring Aromatics (BTEX) in Gasoline
A sample of Exxon Gasoline was analyzed in the year 1992 for its content of aromatic hydrocarbons (BTEX–benzene, toluene, ethyl benzene and the three xylenes). A drop of the gasoline was fully vaporized into an evacuated 10 cm. cell, pressurized with nitrogen to one atmosphere, and its spectrum was run at half wavenumber resolution. This is shown in absorbance form in Figure 1. The bands of paraffinic hydrocarbons dominate the spectrum, but the distinctive region of absorption by aromatic compounds appears near 750 cm-1. This region is shown in detail in the upper spectrum of Figure 2. Analysis for BTEX was done with the method of QASoft, called RIAS, for Region Integration and subtraction. One at a time, each of the six components was measured by integrating over a distinctive feature of its spectrum.
Figure 1. Spectrum of gasoline vapor.
Figure 2. Aromatics spectral region before and after measurement.After each measurement, the whole spectrum of each compound was subtracted from the sample spectrum. The measurement sequence was run three times, with the measured values being added together. For details on this, read the book PROCEDURES IN INFRARED ANALYSIS OF GASES, which is available on the website InfraredAnalysisInc.com. The residual spectrum after the measurements and subtractions is shown as the lower trace in Figure 2.
The distinctive reference spectra of the individual compounds are reproduced in Figures 3 and 4.
Figure 4. Spectra of the three xylenes.
Figure 3. Spectra of benzene, ethyl benzene and toluene
The measured numbers are tabulated in Figure 5. Since the sample was equivalent to 100 Parts-per-million, the PPM values are the same as percent values on a partial pressure (molar) basis.
Figure 5. Measurement results for aromatics in gasoline.
For comparison, a similar analysis was done on a spectrum of 100 PPM-Meters of pure isooctane vapor. These PPM results are shown in Figure 6. Note that some of the small numbers there are negative, This indicates that the small numbers are the result of noise in the spectrum, and it indicates the degree of uncertainty in the BTEX numbers of Figure 5.
Figure 6. Results of BTEX measurement in pure isooctane.
CROSS REFERENCES
See the following relevant articles in the book: PROCEDURES IN INFRARED ANALYSIS OF GASES. These may be read by clicking the icon BOOK in the website InfraredAnalysisInc.com.
CALIBRATION COMES FROM THE QUANTITATIVE REFERENCE SPECTRA
COMPUTER OPERATIONS IN THE SEQUENCE ANALYSIS OF 13 TRACE GASES IN AIR
DATABASE OF GAS PHASE REFERENCE SPECTRA
HARDWARE REQUIREMENTS FOR QASOFT
HYDROCARBONS, AROMATIC
IMPURITIES IN INFRARED ACTIVE GASES
INTEGRATION REGION AND ZERO LEVEL SELECTION
QASOFT (QUANTITATIVE ANALYSIS SOFTWARE)
REGION INTEGRATION AND SUBTRACTION (RIAS)
SEQUENCE ANALYSIS
WRONG ANSWERS ARE CREATED BY HIGH ABSORBANCE VALUES