ASTM D4053 Test Method for Benzene in Motor and Aviation Gasoline by Infrared Spectroscopy
9. Sampling
9.1 Follow the procedures and precautions contained in Practice D4057. (Warning - Gasolines are extremely flammable, harmful if inhaled.)
9.2 Cool the sample container and contents to 0 to 4°C before opening the container and transferring material to other containers.
10. Calibration and Standardization
10.1 Reference Standards - Prepare standard blends of benzene, using fresh, full-range gasoline of low benzene content (less than 1 volume %) as the solvent. Measure and dilute all components at ambient temperature. Accurately pipet the required volume of benzene into 100-mL volumetric flasks partially filled with the gasoline. Dilute to volume with additional gasoline. Prepare the blends in 1 volume % increments.
10.2 Toluene Standard - Prepare a blend of toluene in either isooctane or n-heptane as the solvent. Measure and dilute all components at ambient temperature. Accurately pipet 2 mL of toluene into a 10-mL volumetric flask partially filled with either isooctane or n-heptane. Dilute to volume with the chosen solvent.
10.3 Calibration:
10.3.1 Following the steps of Section 11, Procedure, for each of the standard blends and the gasoline base stock, determine three absorbance values: (1) at the point of maximum absorbance near 673 cm(-1) (14.86 µm), designated the benzene band; (2) at the point of maximum absorbance near 460 cm(-1) (21.74 µm), designated the toluene band; and (3) at 500 cm(-1) (20.00 µm), designated the baseline position.
10.3.2 Following the steps of Section 11, Procedure, for the toluene standard, determine the absorbances at the locations described in 10.3.1 for the benzene band, the toluene band, and the baseline position. Subtract the baseline position value at about 500 cm(-1) from those found for benzene at about 673 cm(-1) and toluene at about 460 cm(-1) in order to obtain the net absorbance for each. Take the ratio of the benzene band net absorbance to the toluene band net absorbance to obtain the toluene correction factor.
10.3.3 For the gasoline base stock and each blend examined in 10.3.1, obtain the net absorbances at the benzene and the toluene bands by subtracting the baseline position value from the absorbances found for the band maxima. Continuing, for each liquid, multiply the toluene band net absorbance by the toluene correction factor found in 10.3.2 and subtract this value from the benzene band net absorbance in order to obtain the corrected net absorbance for the benzene band.
10.3.4 Construct a curve by plotting the benzene band corrected net absorbance for each calibration liquid, as found in 10.3.3, divided by the cell path length in millimetres, versus the volume % of added benzene for each.
10.3.5 Extrapolate the curve to zero absorbance. The absolute value of the intercept is the concentration of benzene in the gasoline used as the solvent.
10.3.6 Construct a standard reference curve by replotting the baseline absorbances per millimetre thickness, corrected in 10.3.5, against total concentration of benzene in percent by volume so that the curve passes through the origin.
NOTE 2 - A linear equation can be used instead of the plot.
