ASTM D5059 Standard Test Methods for Lead in Gasoline by X-Ray Spectroscopy
TEST METHOD A (BISMUTH INTERNAL STANDARD)
5. Apparatus
5.1 X-ray Spectrometer, capable of measuring radiations mentioned in 3.1.1 and of being operated under the following instrumental conditions or other giving equivalent results:
Tube Voltage: 50 kV
Tube Current: 20 to 45 mA
Analyzing Crystal: Lithium Fluoride (LiF)
Optical Path: Air, Helium (Warning - Compressed gas under pressure)
Detector: Proportional or Scintillation
NOTE 1 - The X-ray spectrometer and manner of use should comply with the regulations governing the use of ionizing radiation or recommendations of the International Commission of Radiological Protection, or both.
6. Reagents and Materials
6.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determinations.
6.2 Hydrocarbon-Soluble Bismuth.
NOTE 2 - Bismuth 2-Ethylhexoate has been found suitable to use. Other bismuth containing materials that are hydrocarbon-soluble may also be used when they are certified to conform to 6.1.
6.3 Bismuth Internal Standard Solution - Dilute the hydrocarbon-soluble bismuth with a suitable solvent. If bismuth 2-ethylhexoate is used, add 2-ethylhexanoic acid as a stabilizer (see Note 3) to obtain a solution containing the following:
3.00 g Bi/US gal at 15.5°C (60°F) or
3.60 g Bi/UK gal at 15.5°C (60°F) or
0.793 g Bi/L at 15°C
NOTE 3 - Some stability difficulties have been experienced with bismuth 2-ethylhexoate internal standard solution. If the standard is blended to contain 5 % 2-ethylhexanoic acid, the standard has been found to last almost indefinitely. The 2-ethylhexanoic acid stabilizes iso-octane, toluene, and benzene solutions of the bismuth 2-ethylhexoate which are otherwise stable for only a day or two. Normal octanoic acid does not stabilize solution.
6.4 Iso-octane. (Warning - Extremely flammable.)
6.5 Solvent, capable of dissolving the bismuth internal standard. Mixed xylenes and dodecane have been found suitable to use.
6.6 Hydrocarbon-Soluble Lead - Either tetraethyllead (TEL) or a lead-containing compound (for example, lead naphthenate) with a certifiable lead concentration.
6.7 Lead (Pb) Standard Solution - Dissolve tetraethyllead (TEL) (Warning - TEL is toxic by ingestion), lead naphthenate (see Note 4), or other suitable lead containing compound in iso-octane (Warning - Extremely flammable), toluene, or a mixture of these two solvents. This standard solution shall contain an accurately known lead concentration of approximately the following magnitude:
5 g Pb/US gal at 15.5°C (60°F) or
6 g Pb/UK gal at 15.5°C (60°F) or
1.3 g Pb/L at 15.5°C
6.7.1 Keep the standard solution refrigerated when not in use.
NOTE 4 - A lead naphthenate solution of same lead concentration has also proven satisfactory as a calibration material. Concentrated TEL is not used to make up standard solutions. The concentrated solution is too acutely toxic to be handled safely under normal laboratory conditions.
NOTE 5 - When this lead standard solution is prepared with TEL, the lead concentration can be determined with Test Method D3341.
6.8 Toluene. (Warning - Flammable. Vapor harmful.)
7. Calibration
7.1 Make dilutions of the lead (Pb) standard solution to give 0.10, 1.00, 2.00, 3.00, 4.00 and 5.00 g Pb/US gal at 15.5°C (60°F) or 0.10, 1.00, 2.50, 3.50, 5.00, and 6.00 g Pb/UK gal at 15.5°C (60°F) or 0.025, 0.264, 0.529, 0.793, 1.057, 1.322 g Pb/L at 15°C in toluene, iso-octane, or a mixture of these solvents.
7.2 Allow the lead standards and bismuth internal standard solutions to come to room temperature.
7.3 Pipet accurately 10 mL of each standard into separate glass-stoppered bottles or flasks and add an equal, accurately measured volume of the bismuth internal standard solution to each one. Mix thoroughly.
7.4 Place one of these solutions in the sample cell using techniques consistent with good operating practice for the spectrometer employed. Place the cell in the instrument, allow the spectrometer atmosphere to reach equilibrium (if appropriate), and determine the counting rate at the lead L-a1 line (1.175 Å) and at the bismuth L-a1 line (1.144 Å).
NOTE 6 - When possible, collect at least 100 000 counts at each line. When sensitivity or concentration, or both, makes it impractical to collect this many counts, the technique that allows the greatest statistical precision in the time allotted for each analysis should be used. Sample stability should also be considered in determining counting rate. Variation in counting rates should be observed and if the counting rate tends to go in one direction only, the sample is probably decomposing. If this occurs, shorter counting times should be used consistent with acceptable statistical precision.
7.5 Determine the ratio, R, for each standard as follows:
R = A/B
where:
A = counting rate at 1.175 Å, and
B = counting rate at 1.144 Å.
7.6 Plot a calibration curve relating R to the grams of lead per gallon.
NOTE 7 - Many modern X-ray spectrometer instruments will plot and store the calibration curve, slope, and related information in the instrument computer system, as an alternative to hand-plotting this information.
8. Quality Control Checks
8.1 Confirm the calibration of the instrument each day it is in use by analyzing a quality control (QC) sample containing a quantifiable concentration of lead, that is, independent of the calibration curve. It is advisable to analyze additional QC samples as appropriate, such as at the beginning and end of a batch of samples or after a fixed number of samples, to ensure the quality of the results. Analysis of result(s) from these QC samples can be carried out using control chart techniques. When the QC sample result causes the laboratory to be in an out-of-control situation, such as exceeding the laboratory's control limits, instrument re-calibration may be required. An ample supply of QC sample material shall be available for the intended period of use, and shall be homogeneous and stable under the anticipated storage conditions. If possible, the QC sample shall be representative of samples typically analyzed and the average and control limits of the QC sample shall be determined prior to monitoring the measurement process. The QC sample precision shall be checked against the ASTM method precision to ensure data quality.
9. Procedure
9.1 Obtain sample in accordance with Practice D4057.
9.2 Prepare the samples to be analyzed as described in 7.3 and 7.4 for the standard lead solutions and determine the ratio, R, as described in 7.5.
9.3 Determine the lead content of the samples by relating the R values obtained to the previously determined calibration curve.
10. Report
10.1 Report the lead content obtained as g Pb/US gal at 15.5°C (60°F) or g Pb/UK gal at 15.5°C (60°F) to the nearest 0.01 g, or g Pb/L at 15.5°C to the nearest 0.003 g, as appropriate.
NOTE 8 - To convert grams per US gallon at 15.5°C (60°F) to: (a) grams per UK gallon at 15.5°C (60°F) multiply by 1.200 and (b) grams per litre at 15.5°C, multiply by 0.2201.
