ASTM D6277 Method for Determination of Benzene in Spark-Ignition Engine Fuels
ASTM D6277 Standard Test Method for Determination of Benzene in Spark-Ignition Engine Fuels Using Mid Infrared Spectroscopy
12. Procedure
12.1 Equilibrate the samples to between 15 and 38°C before analysis.
12.2 Clean the sample cell. If a separate baseline using the empty cell is required, and if residual fuel is in the sample cell, remove the fuel by flushing the cell and inlet-outlet lines with enough pentane to ensure complete washing. Evaporate the residual pentane with either dry air or nitrogen.
12.3 If needed, obtain a baseline spectrum in the manner established by the manufacturer of the equipment.
12.4 Prior to the analysis of unknown test samples, establish that the equipment is running properly by collecting the spectrum of the quality control standard(s), by analyzing the spectrum with the calibration model, and by comparing the estimated benzene concentration to the known value for the QC standard(s). Introduce enough standard to the cell to ensure that the cell is washed a minimum of three times with the standard solution.
12.5 Introduce the unknown fuel sample in the manner established by the manufacturer. Introduce enough of the fuel sample to the cell to ensure the cell is washed a minimum of three times with the fuel.
12.6 Obtain the spectral response of the fuel sample.
12.6.1 If a filter based mid IR instrument is used, acquire the absorbance for the fuel sample at the wavelengths corresponding to the specified filters.
12.6.2 If an FTIR is used, acquire the digitized spectral data for the fuel sample over the frequency region from 4000 cm(-1) to 600 cm(-1).
12.7 Determine the benzene concentration (volume %) according to the appropriate calibration equation developed in Annex A1.
12.7.1 For filter based mid IR instruments, apply the calibration equation determined in A1.2.4 to convert the absorbances at each of the wavelengths to the benzene concentration expressed in volume %.
12.7.2 For FTIR instruments using a PLS calibration, determine the benzene concentration using the calibration models developed in A1.2.5 by following the steps outlined as follows.
12.7.2.1 Baseline correct the spectrum using a linear baseline fit to absorbances measured between 712 and 658 cm(-1).
12.7.2.2 Estimate the benzene concentration in the fuel sample by applying the low calibration (see A1.2.5.1) to the baseline corrected spectrum in the region of 712 to 664 cm(-1).
12.7.2.3 If the estimated benzene concentration (determined in 12.7.2.2) is equal to or less than 1.30 volume %, determine the benzene concentration by applying the low calibration (see A1.2.5.2) to the baseline corrected spectrum in the region of 712 to 664 cm(-1).
12.7.2.4 If the estimated benzene concentration (determined in 12.7.2.2) is greater than 1.30 volume %, estimate the benzene concentration by applying the high calibration (see A1.2.5.3) to the baseline corrected spectrum in the region of 712 to 664 cm(-1).
12.7.2.5 If the value estimated by application of the high calibration (determined in 12.7.2.4) is less than or equal to 1.30 volume %, report the value determined by the low calibration (even if the value is greater than 1.30 volume %). For estimated values greater than 1.30 volume % (determined in 12.7.2.4), report the value obtained.
12.7.3 For FTIR instruments using a classical least squares peak fitting calibration, fit the absorption spectrum in the region of 710 through 660 cm(-1) using a classical least squares fit (k-matrix method). The fit matrix must include the derived spectra of toluene, 1,3-dimethylbenzene, 3-ethyltoluene, 1,3,5–trimethylbenzene, ethylbenzene, and propylbenzene (as determined in A1.2.6.1).
12.7.3.1 To eliminate spectral overlaps, subtract the derived spectra of toluene, 1,3-dimethylbenzene, 3-ethyltoluene, 1,3,5-trimethylbenzene, ethylbenzene and propylbenzene, multiplied by the coefficients that resulted from the classical least squares fit to the absorption spectrum. In this way, a residual benzene peak is obtained.
12.7.3.2 Fit the residual benzene peak with a Lorentzian line shape function (as defined in A1.2.6.4) with a linear background in the region of 691 through 660 cm(-1) and determine the peak height of the residual benzene peak.
12.7.3.3 Determine the benzene concentration expressed in mass % in the fuel sample by applying the calibration (see A1.2.6) using the peak height of the residual benzene peak determined in 12.7.3.2.
12.7.3.4 Determine the density of the fuel sample by Test Method D1298 or Test Method D4052.
12.7.3.5 Convert the determined mass % to volume % for the sample using the equation in Section 13.
13. Calculation
13.1 Conversion to Volume % of Benzene - To convert the calibration and qualification standards to volume % use Eq 1.
Vb = Mb(Df/0.8844)
where:
Vb = benzene volume %,
Mb = benzene mass %, and
Df = relative density at 15.56°C of the calibration or qualification standard being tested as determined by Practice D1298 or Test Method D4052.
