ASTM D6445-99
(Test Method)Standard Test Method for Sulfur in Gasoline by Energy-Dispersive X-ray Fluorescence Spectrometry
Standard Test Method for Sulfur in Gasoline by Energy-Dispersive X-ray Fluorescence Spectrometry
SCOPE
1.1 This test method covers the measurement of sulfur in nonleaded gasoline and gasoline-oxygenate blends. The applicable concentration range is 48 to 1000 mg/kg sulfur.
1.2 The values stated in SI units are to be regarded as the standard. The preferred concentration units are mg/kg sulfur.
1.3 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific precautionary statements, see Sections 5 and 7.
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An American National Standard
Designation: D 6445 – 99
Standard Test Method for
Sulfur in Gasoline by Energy-Dispersive X-ray Fluorescence
Spectrometry
This standard is issued under the fixed designation D 6445; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Significance and Use
1.1 This test method covers the measurement of sulfur in 4.1 This test method provides a means of quantifying sulfur
nonleaded gasoline and gasoline-oxygenate blends. The appli- content in gasoline. It can be referenced in specification
cable concentration range is 48 to 1000 mg/kg sulfur. documents as a means to determine if the material meets the
1.2 The values stated in SI units are to be regarded as the desired sulfur content. It is a rapid and precise measurement of
standard. The preferred concentration units are mg/kg sulfur. total sulfur in petroleum products with a minimum of sample
1.3 This standard may involve hazardous materials, opera- preparation.
tions, and equipment. This standard does not purport to 4.2 The quality of gasoline is related to the amount of sulfur
address all of the safety concerns, if any, associated with its present. Knowledge of sulfur concentration is necessary for
use. It is the responsibility of the user of this standard to processing purposes. There are also regulations promulgated in
establish appropriate safety and health practices and deter- federal, state, and local agencies that restrict the amount of
mine the applicability of regulatory limitations prior to use. sulfur present in gasoline as it affects performance character-
For specific precautionary statements, see Sections 5 and 7. istics and potential corrosion problems and emission levels.
During combustion, the sulfur content in fuel affects SO
x
2. Referenced Documents
emissions, which degrade air quality. Certain jurisdictions may
2.1 ASTM Standards: restrict the amount of sulfur in gasoline to prevent or limit
D 3120 Test Method for Trace Quantities of Sulfur in Light
pollution to the environment.
Liquid Petroleum Hydrocarbons by Oxidative Microcou-
5. Apparatus
lometry
D 4057 Practice for Manual Sampling of Petroleum and 5.1 Energy-dispersive X-ray Fluorescence Analyzer—The
Petroleum Products analyzer needs to have sufficient sensitivity to measure the
D 4177 Practice for Automatic Sampling of Petroleum and concentration of sulfur at 500 mg/kg with a one standard
Petroleum Products deviation value due to counting statistics no greater than 10
mg/kg under optimized conditions. Any energy dispersive
3. Summary of Test Method
X-ray fluorescence analyzer may be used if its design incor-
3.1 The sample is placed in the beam emitted from an X-ray porates, as a minimum, the following features:
source. The resultant excited characteristic X radiation is
5.1.1 Source of X-ray Excitation—X-ray tube with energy
measured, and the accumulated count is compared with counts above 2.5 keV.
from previously prepared calibration standards to obtain the
NOTE 1—Operation of analyzers using X-ray tubes is to be conducted
sulfur concentration in mg/kg. One group of calibration stan-
in accordance with the manufacturer’s safety instructions and federal state
dards is required to span the concentration 5 to 1000 mg/kg
and local regulations.
sulfur.
5.1.2 Sample Cell, providing a sample depth of at least 4
mm and equipped with replaceable X-ray transparent film
window.
5.1.3 X-ray Detector, with a resolution value not to exceed
This test method is under the jurisdiction of ASTM Committee D-2 on
Petroleum Products and Lubricants and is the direct responsibility of Subcommittee
800 eV at 2.3 keV. A gas filled proportional counter has been
D02.03 on Elemental Analysis.
found suitable to use.
Current edition approved July 10, 1999. Published September 1999.
5.1.4 Filters, or other means of discriminating between
Annual Book of ASTM Standards, Vol 05.01.
Annual Book of ASTM Standards, Vol 05.02. sulfur K radiation and other X rays.
a
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D6445–99
NOTE 3—Purity on the label for di-n-butyl sulfide, thiophene, and
5.1.5 Signal conditioning and data handling electronics that
2-methylthiophene is only a nominal value. It is essential to know the
include the functions of X-ray intensity counting, spectral
concentration of sulfur in the sulfur standard, not the purity, since
overlap corrections, and conversion of sulfur X-ray intensity
impurities may also be sulfur containing compounds.
into mg/kg sulfur concentration. It is also imperative that the
instrument has the capability to monitor counts for at least one 7.5 Isooctane (2,2,4–trimethylpentane), with a certified
energy region distinct from the sulfur region to allow compen- analysis for sulfur content or checked by Test Method D 3120
sation for variations in spectral background (that is, calculation or equivalent test method as containing less than 3 mg/kg
of net intensities). sulfur.
5.1.6 Display or Printer, that reads or prints out in mg/kg or 7.6 Toluene, with a certified analysis for sulfur content or
masspercent sulfur. checked by Test Method D 3120 or equivalent test method as
containing less than 3 mg/kg sulfur.
6. Matrix Effects
7.7 X-ray Transparent Film—Any film that resists attack by
the sample, is free of sulfur, and is sufficiently X-ray transpar-
6.1 Matrix effects refer to changes in measured intensity of
ent may be used. Films found to be suitable are polyester,
sulfur caused by concentration variations of the elements in a
polypropylene, polycarbonate, and polyimide films. Typical
sample. These variations directly influence X-ray absorption
film thicknesses range from 1.5 to 8 μm. Film thickness will
and change the measured intensity of each element. For
affect the transmission of X rays and the films resistance to
example, performance enhancing additives, such as oxygenates
chemical attack.
in gasoline, can affect the apparent sulfur reading. These types
7.7.1 Samples of high aromatic content may dissolve poly-
of interferences are always present in X-ray fluorescence
ester and polycarbonate films. In these cases, other materials
analysis and are completely unrelated to spectral interferences.
besides these films may be used for X-ray windows, provided
6.2 Many modern instruments have the capability to correct
that they do not contain any elemental impurities. An optional
for matrix effects by ratioing measured sulfur intensities to that
window material is polyimide film. While polyimide film
of X-ray radiation scattered from the sample (for example,
absorbs sulfur X rays more than other films, it may be a
scattered X-ray tube lines). This can be an effective method for
preferred window material as it is much more resistant to
compensating for matrix differences between samples and
chemical attack by aromatics and exhibits higher mechanical
standards, although it can result in some degradation of the
measurement precision. It is the user’s responsibility, however, strength.
7.8 Sample Cells, resistant to sample attack and meeting
to ensure that the matrix corrections applied are accurate. It is
recommended that these are checked by analyzing standard geometry requirements of spectrometer. Disposable cells are
preferred.
reference materials and that the software corrections offered by
the manufacturer not be accepted at face value. In addition,
corrections should be verified for new formulations. 8. Sampling and Specimen Preparation
8.1 Take samples in accordance with the instructions in
7. Reagents and Materials
Practice D 4057 or D 4177 where appropriate. Thoroughly mix
7.1 Purity of Reagents—Reagent grade chemicals shall be
and analyze samples immediately after pouring into a sample
used in all tests. Unless otherwise indicated, it is intended that
cell. Inspect the sample for any air bubbles or sediment. Allow
all reagents conform to the specifications of the Committee on
air bubbles to escape or resample if necessary.
Analytical Reagents of the American Chemical Society where
NOTE 4—The measured sulfur concentration may vary with the time
such specifications are available. Other grades may be used,
that the sample/standard contacts the film covering the sample cell. By
provided it is first ascertained that the reagent is of sufficiently
consistently minimizing the length of time the film comes into contact
high purity to permit its use without lessening the accuracy of
with the sample or standards, possible variations can be reduced.
the determination. The concentration should be known to at
8.2 If using reusable sample cells, clean and dry cells before
least three significant figures or nearest 1 mg/kg, whichever is
use. Do not reuse disposable sample cells. Replacement of the
higher.
X-ray film of a reused sample cell is essential for the
7.2 Di-n-Butyl Sulfide (DBS), a high purity standard, mini-
measurement of each sample. Avoid touching the inside of the
mum 96 % purity, with a certified analysis for sulfur content.
sample cell or portion of the window film in the cell or in the
Use the certified sulfur content when calculating the exact
instrument window that is exposed to X rays. Oil from
concentrations of the calibration standards (see 10.1).
fingerprints can affect the reading when analyzing for low
NOTE 2—Warning: Di-n-butyl sulfide is flammable and toxic.
levels of sulfur. Wrinkles in the film will affect the intensity of
sulfur X rays transmitted. Therefore, it is essential that the film
7.3 Thiophene, sulfur content 37.72 mass %, 99 % purity.
7.4 2-Methylthiophene, 32.00 % sulfur, 98 % purity. be taut and clean to ensure reliable results. The analyzer will
need recalibration if the type or thickness of the window
material is changed.
8.3 Impurities or thickness variations, which may affect the
Reagent Chemicals, American Chemical Society Specifications, American
Chemical Society, Washington, D.C. For suggestions on the testing of reagents not
measurement of low levels of sulfur, have been found in
listed by the American Chemical Society, see Analar Standards for Laboratory
window materials films and may vary from lot to lot. There-
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
fore, check the calibration after starting each new package of
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
MD. film.
D6445–99
TABLE 2 Composition of Primary Standards When Using
9. Preparation of Apparatus
Thiophene/2-Methylthiophene Blend (TM)
9.1 Set up the apparatus in accordance with the manufac-
Sulfur Mass of Diluent Mass of TM
turer’s instructions. Whenever possible the instrument should
(mg/kg) (g) (g)
remain energized to maintain optimum stability. 100 540 0.15
2000 200.8 1.15
10. Calibration and Standardization
10.1 Preparation of Calibration Standards:
10.1.1 Prepare diluent by blending 20 % toluene and 80 % 10,000~DBS3 S 1 Diluent 3 S !
DBS DILUENT
S 5 (1)
isooctane by volume. DBS 1 Diluent
10.1.2 Use either di-n-butyl sulfide or a blend of thiophene/
2-methylthiophene as a source of sulfur in the primary stan- 10,000~TM 3 S 1 Diluent 3 S !
TM DILUENT
S 5 (2)
dards. If using di-n-butyl sulfide as the source of sulfur, TM 1 Diluent
proceed to 10.1.3.
where:
10.1.2.1 To prepare the thiophene/2-methylthiophene (TM)
S = mg/kg sulfur of the primary standards,
blend, mix 9.90 g thiophene with 9.55 g 2-methylthiophene.
DBS = if using the di-n-butyl sulfide, this is the actual
Weigh the materials into a tared volumetric flask and record the
mass in grams of di-n-butyl sulfide used,
mass to four significant digits. Calculate the exact sulfur
TM = if using the thiophene/2-methylthiophene blend,
content of the stock sulfur solution to the nearest mg/kg. Mix
this is the actual mass of the sulfur blend in
thoroughly (a polytetrafluoroethylene (PTFE)-coated magnetic
grams,
stirrer is suitable) at room temperature.
S = if using di-n-butyl sulfide, this is the mass %
DBS
10.1.3 Make primary standards independently at 100 and
sulfur of the di-n-butyl sulfide, typically
2000 mg/kg sulfur and not by serial dilutions from a single
21.91 %. For example, 21.91 % would be ex-
concentrate. Refer to 10.1.3.1 if using di-n-butyl sulfide and
pressed as 21.91 in the formula (see Note 3),
10.1.3.2 if using the thiophene/2-methylthiophene blend for the
S = if using the thiophene/2-methylthiophene blend,
TM
source of sulfur.
this is the mass % sulfur in the this blend,
10.1.3.1 Weigh the diluent and the di-n-butyl sulfide (DBS)
typically 34.9 mass %. For example, 34.9 %
into a tared volumetric flask, using the indicated mass in Table
would be expressed as 34.9 in the formula (see
1 (but record the mass to four significant digits). Mix thor-
Note 3),
oughly (a PTFE-coated magnetic stirrer is suitable) at room
Diluent = actual mass of isooctane/toluene diluent (g),
temperature.
and
10.1.3.2 Weigh the diluent and the thiophene/2-
S = mass % sulfur in the isooctane/toluene blend.
Diluent
methylthiophene (TM) blend into a tared volumetric flask
For example, 0.0001 % would be expressed as
using the indicated mass in Table 2 (but record the mass to four
0.0001 in the formula.
significant digits). Mix thoroughly (a PTFE-coated magnetic
10.1.4 Prepare calibration standards with the nominal con-
stirrer is advisable) at room temperature.
centration ranges identified in Table 3 for the two ranges by
10.1.3.3 If the isooctane/toluene diluent being used for the
diluting the appropriate primary standard with diluent. Adjust
preparation of standards contains sulfur, incorporate this value masses as needed if preparing more or less than 100 g of
into the calculated sulfur content of the prepared standards
standard solutions.
(consult your supplier for a certified sulfur concentration or test 10.1.4.1 Calculate the exact sulfur content in each of the
the isooctane/toluene using Test Method D 3120 or any other
calibration standards to the nearest mg/kg. Calculate the
equivalent low level sulfur analyzing method). concentration of sulfur using the following equation:
10.1.3.4 It is important that the actual mass is known and
thus the actual concentration of the prepared standards is
TABLE 3 Calibration Standards
calculated and entered into the instrument for calibration
Standard # Nominal Mass of Primary Mass of Diluent
purposes. Calculate the exact sulfur content in each of the
Concentration Standard (g)
prepared standards to the nearest 1 mg/kg. Calculate the
(mg/kg)
...
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