ASTM D7757 Standard Test Method for Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
12. Procedure
12.1 When using drift correction, prior to analyzing samples on a given day, analyze the drift-monitor sample measured at the time ofcalibration. Divide the total counts measured on the drift-monitor sample by T to convert to RS; this RS corresponds to factor B in Eq 4.
12.2 Analyze each sample of interest as follows:
12.2.1 Prepare a test specimen of the sample of interest according to 10.2.
12.2.2 Place the sample cell containing the test specimen in the X-ray beam, as directed by the instrument manufacturer's instructions (see Analyzer Preparation, 10.1).
12.2.3 Measure the total counts of silicon fluorescence (N), and divide the total counts by T to calculate RS (see Eq 2).
12.3 If RS for a test specimen is greater than the highest count rate in the calibration curve, quantitatively dilute a fresh portion of the sample with the base material used to prepare the calibration standards. Dilute the sample so the resultant count rate is within the limits of the calibration curve. Repeat the procedures described in 12.2 on a test specimen of the diluted sample.
12.4 Calculate the concentration of silicon in the test specimen as instructed in Section 13.
13. Calculation
13.1 When using a drift monitor sample, calculate a drift correction factor (F) for changes in daily instrument sensitivity according to Eq 4. If a drift monitor is not used, F is set equal to 1.
F = A/B
where:
A = RS for the drift monitor sample determined at the time of calibration (11.4), and
B = RS for the drift monitor sample determined at the time of analysis (12.1).
13.2 Calculate the drift-corrected count rate (Rcor) for the test specimen as follows:
Rcor = F x RS
F = drift correction factor, calculated by Eq 4, and
RS = total count rate for test specimen.
13.3 Calculate the silicon content (Si) of the test specimen by using the drift-corrected count rate (Rcor) in place of Rs in Eq 3.
13.4 If the test specimen was prepared from a quantitatively diluted sample, correct the measured concentration for sample dilution. The silicon concentration (Sio) in the original, undiluted sample is calculated as follows:
Sio = [Sid x (Mo + Mb)/Mo] - [Sib x (Mb/Mo)]
Sid = concentration of silicon in test specimen of the diluted sample (from 13.3), mg/kg,
Mo = mass of original sample, g,
Mb = mass of base material used to dilute sample, g, and
Sib = concentration of silicon in diluent, mg/kg.
13.5 If a correction factor was used to account for differences in the sample matrix versus the matrix of the calibration standards (see Section 5), apply the factor by multiplying the silicon concentration, Sio, obtained in Eq 6 by the correction factor.
