ASTM D7691-23

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of the standard as published by ASTM is to be considered the official document.
Designation: D7691 − 16 D7691 − 23
Standard Test Method for
Multielement Analysis of Crude Oils Using Inductively
Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
This standard is issued under the fixed designation D7691; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers the determination of several elements (including iron, nickel, sulfur, and vanadium) occurring in crude
oils.
1.2 For analysis of any element using wavelengths below 190 nm, a vacuum or inert gas optical path is required.
1.3 Analysis for elements such as arsenic, selenium, or sulfur in whole crude oil may be difficult by this test method due to the
presence of their volatile compounds of these elements in crude oil; but this test method should work for resid samples.
1.4 Because of the particulates present in crude oil samples, if they do not dissolve in the organic solvents used or if they do not
get aspirated in the nebulizer, low elemental values may result, particularly for iron and sodium. This can also occur if the elements
are associated with water which can drop out of the solution when diluted with solvent.
1.4.1 An alternative in such cases is using Test Method D5708, Procedure B, which involves wet decomposition of the oil sample
and measurement by ICP-AES for nickel, vanadium, and iron, or Test Method D5863, Procedure A, which also uses wet acid
decomposition and determines vanadium, nickel, iron, and sodium using atomic absorption spectrometry.
1.4.2 From ASTM Interlaboratory Crosscheck Programs (ILCP) on crude oils data available so far, it is not clear that organic
solvent dilution techniques would necessarily give lower results than those obtained using acid decomposition techniques.
1.4.3 It is also possible that, particularly in the case of silicon, low results may be obtained irrespective of whether organic dilution
or acid decomposition is utilized. Silicones are present as oil field additives and can be lost in ashing. Silicates should be retained
but unless hydrofluoric acid or alkali fusion is used for sample dissolution, they may not be accounted for.
1.5 This test method uses oil-soluble metals for calibration and does not purport to quantitatively determine insoluble particulates.
Analytical results are particle size dependent and low results may be obtained for particles larger than a few micrometers.
1.6 The precision in Section 18 defines the concentration ranges covered in the interlaboratory study. However, lower and
particularly higher concentrations can be determined by this test method. The low concentration limits are dependent on the
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.03 on Elemental Analysis.
Current edition approved June 1, 2016May 1, 2023. Published June 2016June 2023. Originally approved in 2011. Last previous edition approved in 20112016 as
ɛ1
D7961 – 11D7961 – 16. . DOI: 10.1520/D7691-16.10.1520/D7691-23.
Nadkarni, R. A., Hwang, J. D., and Young, L., “Multielement Analysis of Crude Oils Using Inductively Coupled Plasma Atomic Emission Spectrometry,” J. ASTM
International, Vol 8, No. 10, 2011, pp. 103837.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7691 − 23
sensitivity of the ICP instrument and the dilution factor used. The high concentration limits are determined by the product of the
maximum concentration defined by the calibration curve and the sample dilution factor.
1.7 Elements present at concentrations above the upper limit of the calibration curves can be determined with additional
appropriate dilutions and with no degradation of precision.
1.8 As a generality based on this interlaboratory study (see 18.1), the trace elements identifiable in crude oils can be divided into
three categories:
1.8.1 Element levels that are too low for valid detection by ICP-AES and hence, cannot be determined: aluminum, barium, lead,
magnesium, manganese, and silicon.
1.8.2 Elements that are just at the detection levels of the ICP-AES method and hence, cannot be determined with a great deal of
confidence: boron, calcium, chromium, copper, molybdenum, phosphorus, potassium, sodium, and zinc. Perhaps the determination
of these elements can be considered as semi-quantitative.
1.8.3 Elements that are at higher levels of concentration and can be determined with good precision: iron, nickel, sulfur, and
vanadium.
1.9 The detection limits for elements not determined by this test method follow. This information should serve as an indication
as to what elements are not present above the detection limits typically obtainable by ICP-AES instruments.
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Element mg/kg
Aluminum 1
Barium 0.2
Boron 1
Calcium 0.1
Chromium 0.1
Copper 0.1
Lead 1.4
Magnesium 1
Manganese 0.1
Molybdenum 0.2
Phosphorous 1
Potassium 0.5
Silicon 4
Zinc 0.5
1.10 This test method determines all possible elements simultaneously and is a simpler alternative to Test Methods D5184, D5708,
or D5863.
1.11 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.12 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 healthsafety, health, and environmental practices and determine
the applicability of regulatory limitations prior to use.
1.13 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C1109 Practice for Analysis of Aqueous Leachates from Nuclear Waste Materials Using Inductively Coupled Plasma-Atomic
Emission Spectroscopy
D1552 Test Method for Sulfur in Petroleum Products by High Temperature Combustion and Infrared (IR) Detection or Thermal
Conductivity Detection (TCD)
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D4307 Practice for Preparation of Liquid Blends for Use as Analytical Standards
D5184 Test Methods for Determination of Aluminum and Silicon in Fuel Oils by Ashing, Fusion, Inductively Coupled Plasma
Atomic Emission Spectrometry, and Atomic Absorption Spectrometry
D5185 Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively
Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
D5708 Test Methods for Determination of Nickel, Vanadium, and Iron in Crude Oils and Residual Fuels by Inductively Coupled
Plasma (ICP) Atomic Emission Spectrometry
D5854 Practice for Mixing and Handling of Liquid Samples of Petroleum and Petroleum Products
D5863 Test Methods for Determination of Nickel, Vanadium, Iron, and Sodium in Crude Oils and Residual Fuels by Flame
Atomic Absorption Spectrometry
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measure-
ment System Performance
D6792 Practice for Quality Management Systems in Petroleum Products, Liquid Fuels, and Lubricants Testing Laboratories
D7260 Practice for Optimization, Calibration, and Validation of Inductively Coupled Plasma-Atomic Emission Spectrometry
(ICP-AES) for Elemental Analysis of Petroleum Products and Lubricants
E135 Terminology Relating to Analytical Chemistry for Metals, Ores, and Related Materials
3. Terminology
3.1 For the definition of emission spectroscopy, refer to Terminology E135.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D7691 − 23
3.1 Definitions:
3.1.1 For definitions of terms used in this test method, refer to Terminology D4175.
3.1.2 For the definition of emission spectroscopy, refer to Terminology E135.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 analyte, n—element whose concentration is being determined.
3.2.2 Babington-type nebulizer, n—device that generates an aerosol by flowing a liquid over a surface that contains an orifice from
which gas flows at a high velocity.
3.2.3 calibration, n—process by which the relationship between signal intensity and elemental concentration is determined for a
specific element analysis.
3.2.4 calibration curve, n—plot of signal intensity versus elemental concentration using data obtained by making measurements
with standards.
3.2.5 detection limit, n—concentration of an analyte that results in a signal intensity that is some multiple (typically two) times
the standard deviation of the background intensity at the measurement wavelength.
3.2.6 inductively-coupled plasma (ICP), n—high-temperature discharge generated by flowing an ionizable gas through a magnetic
field induced by a load coil that surrounds the tubes carrying the gas.
3.2.7 linear response range, n—elemental concentration range over which the calibration curve is a straight line, within the
precision of the test method.
3.2.8 profiling, n—technique that determines the wavelength for which the signal intensity measured for a particular analyte is a
maximum.
3.2.9 radio frequency (RF), n—range of frequencies between the audio and infrared ranges (3 GHz to 300 GHz).
4. Summary of Test Method
4.1 This test method usually requires several minutes per sample. A weighed portion of a thoroughly homogenized crude oil is
diluted tenfold by weight with mixed xylenes, kerosene, or other suitable solvent. Standards are prepared in the same manner. A
mandatory internal standard is added to the solutions to compensate for variations in test specimen introduction efficiency. The
solutions are introduced to the ICP instrument by a peristaltic pump. By comparing emission intensities of elements in the test
specimen with emission intensities measured with the standards, the concentrations of elements in the test specimen are calculable.
5. Significance and Use
5.1 Most often determined trace elements in crude oils are nickel and vanadium, which are usually the most abundant; however,
as many as 45 elements in crude oils have been reported. Knowledge of trace elements in crude oil is important because they can
have an adverse effect on petroleum refining and product quality. These effects can include catalyst poisoning in the refinery and
excessive atmospheric emission in combustion of fuels. Trace element concentrations are also useful in correlating production from
different wells and horizons in a field. Elements such as iron, arsenic, and lead are catalyst poisons. Vanadium compounds can
cause refractory damage in furnaces, and sodium compounds have been found to cause superficial fusion on fire brick. Some
organometallic compounds are volatile which can lead to the contamination of distillate fractions, and a reduction in their stability
or malfunctions of equipment when they are combusted.
5.2 The value of crude oil can be determined, in part, by the concentrations of nickel, vanadium, and iron.
5.3 Inductively coupled plasma-atomic emission spectrometry (ICP-AES) is a widely used technique in the oil industry. Its
advantages over traditional atomic absorption spectrometry (AAS) include greater sensitivity, freedom from molecular
interferences, wide dynamic range, and multi-element capability. See Practice D7260.
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A
TABLE 1 Elements Determined and Suggested Wavelengths
Element Wavelength, nm
Aluminum 308.215, 396.153, 309.271, 237.01
Barium 233.53, 455.403, 493.410
Boron 249.773, 182.59, 249.68
Calcium 315.887, 317.933, 364.44, 422.67
Chromium 205.552, 267.716, 298.92, 283.563
Copper 324.754, 219.226
Iron 259.94, 238.204, 271.44, 259.837
Lead 220.353, 224.688, 283.306
Magnesium 279.079, 279.553, 285.21, 293.65
Manganese 257.61, 293.31, 293.93, 294.92
Molybdenum 202.03, 281.616, 204.598, 203.844
Nickel 231.604, 227.02, 221.648, 341.476
Phosphorus 177.51, 178.289, 214.914, 253.40
Potassium 766.491, 769.896
Sodium 588.995, 330.29, 589.3, 589.592
Silicon 288.159, 251.611, 212.412, 282.851
Sulfur 180.731, 182.04, 182.62
Vanadium 292.403, 309.31, 310.23, 311.07
Zinc 202.551, 206.209, 213.856, 334.58, 481.05, 202.48
A
These wavelengths are only suggested and do not represent all possible
choices. Not all of these elements were determined in this interlaboratory study.
6. Interferences
6.1 Spectral—There are no known spectral interferences between elements covered by this test method when using the spectral
lines listed in Table 1. However, if spectral interferences exist because of other interfering elements or selection of other spectral
lines, correct for the interference using the technique described in Test Method D5185.
6.2 Check all spectral interferences expected from the elements listed in Table 1. Follow the manufacturer’s operating guide to
develop and apply correction factors to compensate for the interferences. To apply interference corrections, all concentrations shall
be within the previously established linear response range of each element listed in Table 1. (Warning—Correct profiling is
important to reveal spectral interferences from high concentrations of some elements on the spectral lines used for determining
trace metals.)
6.2.1 Spectral interferences can usually be avoided by judicious choice of analytical wavelengths. When spectral interferences
cannot be avoided, the necessary corrections should be made using the computer software supplied by the instrument manufacturer
or the empirical method described below. Details of the empirical method are given in Test Method C1109 and by Boumans. This
empirical correction method cannot be used with scanning spectrometer systems when both the analytical and interfering lines
cannot be located precisely and reproducibly. With any instrument, the analyst shall always be alert to the possible presence of
unexpected elements producing interfering spectral lines.
6.2.2 The empirical method of spectral interference correction uses interference correction factors. These factors are determined
by analyzing the single-element, high purity solutions under conditions matching as closely as possible those used for test specimen
analysis. Unless plasma conditions can be accurately reproduced from day to day, or for longer periods, interference correction
factors found to affect the results significantly shall be redetermined each time specimens are analyzed.
6.2.3 Interference correction factors can be negative if off-peak background correction is employed for element, i. A negative Kia
correction factor can result when an interfering line is encountered at the background correction wavelength rather than at the peak
wavelength.
6.3 Viscosity Effects—Differences in the viscosities of test specimen solutions and standard solutions can cause differences in the
uptake rates. These differences can adversely affect the accuracy of the analysis. The effects can be reduced by using a peristaltic
pump to deliver solutions to the nebulizer or by the use of internal standardization, or both. When severe viscosity effects are
encountered, dilute the test specimen and standard twentyfold rather than tenfold while maintaining the same concentration of the
internal standard. See Table 2.
Boumans, P. W. J. M., “Corrections for Spectral Interferences in Optical Emission Spectrometry with Special Reference to the RF Inductively Coupled Plasma,”
Spectrochimica Acta, Vol 31B, 1976, pp. 147-152.
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TABLE 2 Suggested Internal Standards
A
Element Concentration, mg/kg Wavelength, Nm
Cadmium 10 226.502; 228.802; 214.438
Cobalt 10 228.616; 238.892; 237.662
Lanthanum 10 379.48; 379.08
Scandium 10 255.237; 361.384; 357.253
Yttrium 10 371.030; 324.228; 360.073
A
These wavelengths are only suggested and do not represent all possible
choices.
6.4 Particulates—Particulates can plug the nebulizer thereby causing low results. Use of a Babington type high-solids nebulizer
helps to minimize this effect. Also, the specimen introduction system can limit the transport of particulates, and the plasma can
incompletely atomize particulates, thereby causing low results.
7. Apparatus
7.1 Balance—Top loading or analytical, with automatic tare, capable of weighing to 0.001 or 0.0001 g, 0.001 g or 0.0001 g, with
sufficient capacity to weigh prepared solutions.
7.2 Inductively-Coupled Plasma Atomic Emission Spectrometer—Either a sequential or simultaneous spectrometer is suitable, if
equipped with a quartz ICP torch and RF generator to form and sustain the plasma. Suggested wavelengths for the determination
of the elements in crude oils are given in Table 1. For the analysis of sulfur, the spectrometer shall be capable of operating in the
wavelength region of 180 nm.
5,6
7.3 Nebulizer—A Babington-type high-solids nebulizer is strongly recommended. This type of nebulizer reduces the possibility
of clogging and minimizes aerosol particle effects.
7.4 Peristaltic Pump—A peristaltic pump is strongly recommended to provide a constant flow of solution. The pumping speed
shall be in the range 0.5 mL ⁄min to 3 mL ⁄min. The pump tubing shall be able to withstand at least 6 h exposure to the dilution
solvent. Viton tubing is typically used with hydrocarbon solvents, and polyvinyl chloride tubing is typically used with methyl
isobutyl ketone.
7.5 Solvent Dispenser, (Optional)—A solvent dispenser calibrated to deliver the required weight of dilution solvent for a tenfold
dilution of test specimen is very useful.
7.6 Specimen Solution Containers—Of appropriate size, glass or plastic vials or bottles, with screw caps.
7.7 Ultrasonic Homogenizer, (Recommended)—A bath-type or probe-type ultrasonic homogenizer to homogenize the sample.
7.8 Vortexer, (Optional)—Vortexing the sample is an alternative to ultrasonic homogenization.
7.9 High Speed Homogenizer, (Optional).
8. Reagents and Materials
8.1 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
Babington, R. A., Popular Science, May 1973, pp. 102.
Fry, R. C., and Denton, M. B., Analytical Chemistry, Vol 49, 1977.
D7691 − 23
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 determination.
8.2 Internal Standard—Oil-soluble cadmium, cobalt, lanthanum, scandium, or yttrium (or other suitable metal) is required for
internal standardization.
8.3 Organometallic Standards—Multi-element standards, containing 0.0500 mass % 0.0500 % by mass of each element, can be
prepared from the individual concentrates. Refer to Practice D4307 for a procedure for preparation of multi-component liquid
blends. When preparing multi-element standards, be certain that proper mixing is achieved. An ultrasonic bath is recommended.
Standard multi-element concentrates, containing 0.0500 mass % 0.0500 % by mass of each element, are also satisfactory.
(Warning—Some commercially available organometallic standards are prepared from metal sulfonates and therefore contain
sulfur. For sulfur determinations, a separate sulfur standard would be required.)
8.3.1 More than one multi-element standard can be necessary to cover all elements, and the user of this test method can select the
combination of elements and their concentrations in the multi-element standards. It can be advantageous to select concentrations
that are typical of crude oils. However, it is imperative that the concentrations are selected such that the emission intensities
measured with the working standards can be measured precisely (that is, the emission intensities are significantly greater than
background) and that these standards represent the linear region of the calibration curve. Frequently, the instrument manufacturer
publishes guidelines for determining linear range.
8.4 Sulfur Standard—To use a metal sulfonate as a sulfur standard, analyze the sulfonate by Test Method D1552. Alternatively,
prepare a sulfur standard by diluting NIST SRM 1622c in white oil. If sulfur is to be determined, the internal standard compound
should not contain sulfur. Use metal napthenate or similar compounds rather than metal sulfonates. Non-sulfonate oil based sulfur
standards are available commercially and can be used.
8.5 Dilution Solvent—A solvent that is free of analytes and is capable of completely dissolving all standards and samples. Mixed
xylenes, kerosine, toluene, and ortho-xylene were successfully used as dilution solvents in the interlaboratory study on precision.
8.6 Base Oil or White Oil.
9. Sampling
9.1 It is critical that a representative sample be obtained for analysis from the bulk material. Maintaining compositional integrity
of these samples from the time of collection until their analysis requires care and effort. Sampling procedure also should not
introduce any contaminants into the sample or otherwise alter the sample composition so that the subsequent test results are
affected.
9.2 See Practices D4057 and D4177 for manual and automatic sampling of petroleum and petroleum products, respectively. In
sampling of crude oils, the material may contain a heavy component, such as free water, which tends to separate from the main
component. Guide D5854 provides a guide for selecting suitable containers for crude oil samples for various analyses.
10. Preparation of Apparatus
10.1 Instrument—Design differences between instruments, ICP excitation sources, and different selected analytical wavelengths
for individual spectrometers make it impractical to detail the operating conditions. Consult the manufacturer’s instructions for
operating the instrument with organic solvents. Set up the instrument for use with the particular dilution solvent chosen.
10.2 Peristaltic Pump—Before using the peristaltic pump, inspect the pump tubing and replace it, if necessary, before starting each
day. Verify the solution uptake rate and adjust it to the desired rate.
10.3 ICP Excitation Source—Initiate the plasma source at least 30 min before performing analysis. During this warm up period,
Reagent Chemicals, American Chemical Society Specifications,ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference
Materials, American Chemical Society, Washington, DC. For Suggestionssuggestions on the testing of reagents not listed by the American Chemical Society, see
AnnualAnalar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharmacopeial
Convention, Inc. (USPC), Rockvil
...