ASTM D7740-20

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of the standard as published by ASTM is to be considered the official document.
Designation: D7740 − 11 (Reapproved 2016) D7740 − 20
Standard Practice for
Optimization, Calibration, and Validation of Atomic
Absorption Spectrometry for Metal Analysis of Petroleum
Products and Lubricants
This standard is issued under the fixed designation D7740; 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 Scope*
1.1 This practice covers information on the calibration and operational guidance for elemental measurements using atomic
absorption spectrometry (AAS).
1.1.1 AAS Related Standards—Test Methods D1318, D3237, D3340, D3605, D3831, D4628, D5056, D5184, D5863, D6732;
Practices D7260 and D7455; and Test Methods D7622 and D7623.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 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:
D1318 Test Method for Sodium in Residual Fuel Oil (Flame Photometric Method)
D3237 Test Method for Lead in Gasoline by Atomic Absorption Spectroscopy
D3340 Test Method for Lithium and Sodium in Lubricating Greases by Flame Photometer (Withdrawn 2013)
D3348 Test Method for Rapid Field Test for Trace Lead in Unleaded Gasoline (Colorimetric Method)
D3605 Test Method for Trace Metals in Gas Turbine Fuels by Atomic Absorption and Flame Emission Spectroscopy
D3831 Test Method for Manganese in Gasoline By Atomic Absorption Spectroscopy
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D4307 Practice for Preparation of Liquid Blends for Use as Analytical Standards
D4628 Test Method for Analysis of Barium, Calcium, Magnesium, and Zinc in Unused Lubricating Oils by Atomic Absorption
Spectrometry
D5056 Test Method for Trace Metals in Petroleum Coke by Atomic Absorption
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
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
D6732 Test Method for Determination of Copper in Jet Fuels by Graphite Furnace Atomic Absorption Spectrometry
This practice 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 April 1, 2016July 1, 2020. Published May 2016August 2020. Originally approved in 2011. Last previous edition approved in 20112016 as
D7740 – 11.D7740 – 11 (2016). DOI: 10.1520/D7740-11R16.10.1520/D7740-20.
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.
The last approved version of this historical standard is referenced on www.astm.org.
*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
D7740 − 20
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
D7455 Practice for Sample Preparation of Petroleum and Lubricant Products for Elemental Analysis
D7622 Test Method for Total Mercury in Crude Oil Using Combustion and Direct Cold Vapor Atomic Absorption Method with
Zeeman Background Correction
D7623 Test Method for Total Mercury in Crude Oil Using Combustion-Gold Amalgamation and Cold Vapor Atomic Absorption
Method
D7740 − 20
3. Terminology
3.1 Definitions:
3.1.1 absorbance, n—logarithm to the base 10 of the ratio of the reciprocal of the transmittance.
3.1.2 atomic absorption spectrometry, n—analytical technique for measuring metal content of solutions, based on a combination
of flame source, hollow cathode lamp, photomultiplier, and a readout device.
3.1.3 atomizer, n—usually a flame source used to decompose the chemical constituents in a solution to its elemental
components.
3.1.4 blank, n—solution which is similar in composition and contents to the sample solution but does not contain the analyte
being measured.
3.1.5 burner, n—flame device used to atomize the analyte by burning in a high temperature flame mixed of a fuel and an oxidant.
3.1.6 calibration, n—process by which the relationship between signal intensity and elemental concentration is determined for
a specific element analysis.
3.1.7 calibration curve, n—plot of signal intensity versus elemental concentration using data obtained by making measurements
with standards.
3.1.8 calibration standard, n—material with a certified value for a relevant property, issued by or traceable to a national
organization such as NIST, and whose properties are known with sufficient accuracy to permit its use to evaluate the same property
of another sample.
3.1.9 certified reference material, n—reference material one or more of whose property values are certified by a technically valid
procedure, accompanied by a traceable certificate or other documentation which is issued by a certifying body.
3.1.10 check standard, n—material having an assigned (known) value (reference value) used to determine the accuracy of the
measurement system or instrument.
3.1.10.1 Discussion—
This practice is not used to calibrate the measurement instrument or system.
3.1.11 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.
3.1.12 dilution factor, n—ratio of sample weight of the aliquot taken to the final diluted volume of its solution.
3.1.12.1 Discussion—
The dilution factor is used to multiply the observed reading and obtain the actual concentration of the analyte in the original
sample.
3.1.13 graphite furnace, n—electrothermal device for atomizing the metal constituents.
3.1.14 hollow cathode lamp, n—device consisting of a quartz envelope containing a cathode of the metal to be determined and
a suitable anode.
3.1.15 hydride generation, n—device to atomize some metals which form gaseous hydrides.
3.1.16 monochromator, n—device that isolates a single atomic resonance line from the line spectrum emitted by the hollow
cathode lamp, excluding all other wavelengths.
3.1.17 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.
3.1.18 NIST, n—National Institute of Standards and Technology, Gaithersburg, MD. Formerly known as National Bureau of
Standards.
3.1.19 precision, n—closeness of agreement between test results obtained under prescribed conditions.
3.1.20 quality assurance, n—system of activities, the purpose of which is to provide to the producer and user of a product,
measurement, or service the assurance that it meets the defined standards of quality with a stated level of confidence.
3.1.21 quality control, n—planned system of activities whose purpose is to provide a level of quality that meets the needs of
users; also the uses of such a system.
3.1.22 quality control sample, n—for use in quality assurance program to determine and monitor the precision and stability of
a measurement system; a stable and homogenous material having physical or chemical properties, or both, similar to those of
typical samples tested by the analytical measurement system.
D7740 − 20
3.1.22.1 Discussion—
This material should be properly stored to ensure sample integrity, and is available in sufficient quantity for repeated long term
testing.
3.1.23 reference material, n—material with accepted reference value(s), accompanied by an uncertainty at a stated level of
confidence for desired properties, which may be used for calibration or quality control purposes in the laboratory.
3.1.24 refractory elements, n—elements forming difficult-to-dissociate oxides during combustion.
3.1.25 repeatability, n—difference between two test results, obtained by the same operator with the same apparatus under
constant operating conditions on identical test material would, in the long term and correct operation of the test method, exceed
the values given only in one case in twenty.
3.1.26 reproducibility, n—difference between two single and independent results, obtained by different operators working in
different laboratories on identical test materials, would in the long run, in the normal and correct operation of the test method,
exceed the values given only one case in twenty.
3.1.27 spectrometer, n—instrument used to measure the emission or absorption spectrum emitted by a species in the vaporized
sample.
3.1.28 spectrum, n—array of the components of an emission or absorption arranged in the order of some varying characteristics
such as wavelength, mass, or energy.
3.1.29 standard reference material, n—trademark for reference materials certified by NIST.
4. Summary of Practice
4.1 An Atomic Absorption Spectrometer (AAS) is used to determine the metal composition of various liquid matrices. Although
usually AAS is done using a flame to atomize the metals, graphite furnace (GF-AAS) or cold vapor (CV-AAS) may also be used
for metals at very low levels of concentration or some elements not amenable to flame atomization. This practice summarizes the
protocols to be followed during calibration and verification of the instrument performance.
5. Significance and Use
5.1 Accurate elemental analysis of petroleum products and lubricants is necessary for the determination of chemical properties,
which are used to establish compliance with commercial and regulatory specifications.
5.2 Atomic Absorption Spectrometry (AAS) is one of the most widely used analytical techniques in the oil industry for
elemental analysis. There are at least twelve Standard Test Methods published by ASTM D02 Committee on Petroleum Products
and Lubricants for such analysis. See Table 1.
5.3 The advantage of using an AAS analysis include good sensitivity for most metals, relative freedom from interferences, and
ability to calibrate the instrument based on elemental standards irrespective of their elemental chemical forms. Thus, the technique
has been a method of choice in most of the oil industry laboratories. In many laboratories, AAS has been superseded by a superior
ICP-AES technique (see Practice D7260).
5.4 Some of the ASTM AAS Standard Test Methods have also been issued by other standard writing bodies as technically
equivalent standards. See Table 2.
6. Interferences
6.1 Although over 70 elements can be determined by AAS usually with a precision of 1-3 % 1 % to 3 % and with detection
limits of the order of sub-mg/kg levels, and with little or no atomic spectral interference. However, there are several types of
TABLE 1 Applications of AAS for Metal Analysis of Petroleum Products and Lubricants
ASTM Test Method Matrix Elements Determined
D1318 Residual Fuel Oil Sodium
D3237 Gasoline Lead
D3340 Greases Lithium and Sodium
D3605 Gas Turbine Fuels Calcium, Lead, Sodium, and Vanadium
D3831 Gasoline Manganese
D4628 Automotive Lubricants Barium, Calcium, Magnesium, and Zinc
D5056 Petroleum Coke Aluminum, Calcium, Iron, Nickel, Silicon, Sodium, and Vanadium
D5184 Fuel Oils Aluminum and Silicon
D5863 Crude and Fuel Oils Iron, Nickel, Sodium, and Vanadium
D6732 Jet Fuels Copper
D7622 Crude Oils Mercury
D7623 Crude Oils Mercury
D7740 − 20
A
TABLE 2 Equivalent AAS Test Methods
Analysis ASTM Standard EI Standard ISO Standard DIN Standard
Lead in Gasoline D3237 IP 428 8691
Analysis of Gas Turbine Fuels D3605 IP 413 51-790T3
Additive Elements in Lube Oils D4628 IP 308 51-391T1
Al and Si in Fuel Oils D5184 IP 377 10478 51-416
A
Excerpted from ASTM MNL44, Guide to ASTM Test Methods for the Analysis of Petroleum Products and Lubricants, 2nd edition, Ed., Nadkarni, R. A. Kishore, ASTM
International, West Conshohocken, PA, 2007.
interferences possible: chemical, ionization, matrix, emission, spectral, and background absorption interferences. Since these
interferences are well-defined, it is easy to eliminate or compensate for them. See Table 3.
6.1.1 Chemical Interferences—If the sample for analysis contains a thermally stable compound with the analyte that is not
totally decomposed by the energy of the flame, a chemical interference exists. They can normally be overcome or controlled by
using a higher temperature flame or addition of a releasing agent to the sample and standard solutions.
6.1.2 Ionization Interferences—When the flame has enough energy to cause the removal of an electron from the atom, creating
an ion, ionization interference can occur. They can be controlled by addition of an excess of an easily ionized element to both
samples and standards. Normally alkali metals which have very low ionization potentials are used.
6.1.3 Matrix Interferences—These can cause either a suppression or enhancement of the analyte signal. Matrix interferences
occur when the physical characteristics – viscosity, burning characteristics, surface tension – of the sample and standard differ
considerably. To compensate for the matrix interferences, the matrix components in the sample and standard should be matched
as closely as possible. Matrix interferences can also be controlled by diluting the sample solution until the effect of dissolved salts
or acids is negligible. Sometimes, the method of standard addition is used to overcome this interference. See 6.2.
6.1.4 Emission Interferences—At high analyte concentrations, the atomic absorption analysis for highly emissive elements
sometimes exhibits poor analytical precision, if the emission signal falls within the spectral bandpass being used. This interference
can be compensated for by decreasing the slit width, increasing the lamp current, diluting the sample, and / or using a cooler flame.
6.1.5 Spectral Interferences—When an absorbing wavelength of an element present in the sample but not being determined falls
within the bandwidth of the absorption line of the element of interest a spectral interference can occur. An interference by other
atoms can occur when there is a sufficient overlapping between radiation and emitted by the excited atoms and other absorbing
atoms. Usually the bandwidth is much wider than the width of the emission and absorption lines. Thus, interferences by other
atoms are fortunately quite limited in AAS. The interference can result in erroneously high results. This can be overcome by using
a smaller slit or selecting an alternate wavelength.
6.1.6 Background Absorption Interferences—There are two causes of background absorption: light scattering by particles in the
flame and molecular absorption of light from the lamp by molecules in the flame. This interference cannot be corrected with
standard addition method. The most common way to compensate for background absorption is to use a background corrector which
utilizes a continuum source.
6.2 Standard Addition Method—One way of dealing with some of the interferences in the AAS methods is to use a technique
called standard addition. IUPAC rule defines this technique as “Analyte Addition Method,” however, the phrase “standard addition
method” is well known and is widely used by the practitioners of AAS; hence, there is no need to adopt the IUPAC rule. This
technique takes longer time than the direct analysis, but when only a few samples need to be analyzed, or when the samples differ
from each other in the matrix, or when the samples suffer from unidentified matrix interferences this method can be used. The
TABLE 3 Elemental Analysis of Petroleum Products by AAS
Element Wavelength, Flame Typical Detection Matrix ASTM Test Method
nm Limits, mg/L
Aluminum 309.3 N O + C H 0.03 Petroleum Coke; Fuel Oils D5056; D5184B
2 2 2
Barium N O + C H 0.008 Lubricants D4628
2 2 2
Calcium 422.7 N O + C H 0.001 Gas Turbine Fuels; Lubricants; D3605; D4628; D5056
2 2 2
Petroleum Coke
Copper 324.8 GF-AAS 0.001 Jet Fuel D6732
Iron 248.3 Air + C H 0.003 Crude Oils; Fuel Oils D5184
2 2
Lead 283.3 Air + C H 0.01 Gasoline; Gas Turbine Fuels D3237; D3340
2 2
Magnesium 285.2 N O + C H 0.00001 Lubricants D4628
2 2 2
Manganese 279.5 Air + C H 0.001 Gasoline D3831
2 2
Mercury 253.65 CV-AAS 0.000008 Crude Oil D7622; D7623
Nickel 232.0 Air + C H 0.004 Crude Oils; Fuel Oils D5863
2 2
Silicon 251.6 N O + C H 0.06 Fuel Oils D5184B
2 2 2
Sodium 589.6 Air + C H 0.0002 Residual Fuel Oil; Gas Turbine Fuels; D1318; D3605; D5056; D5863
2 2
Petroleum Coke; Crude Oils; Fuel
Oils
Vanadium 318.34 N O + C H 0.04 Gas Turbine Fuels; Petroleum Coke; D3605; D5056; D5863
2 2 2
Crude Oils; Fuel Oils
Zinc 213.9 N O + C H 0.0008 Lubricants D4628
2 2 2
D7740 − 20
method of standard addition is carried out by: (1) dividing the sample into several (at least four) aliquots, (2) adding to all but the
first aliquot increasing amount of analyte, (3) diluting all to the same final volume, and (4) measuring the absorbance, and (5)
plotting the absorbance against the amount of analyte added. The amount of the analyte present in the sample is obtained by
extrapolation beyond the zero addition. The method of standard addition may be less accurate than direct comparison; but when
matrix interferences are encountered, it is necessary to use standard addition.
6.3 Chemical Suppressants—In some cases, ionization suppressors or other chemical reagents are added to the sample and
standard solutions to suppress such interferences. Examples include: Test Method D3237 (lead in gasoline) uses iodine solution
in toluene, Test Method D3831 (manganese in gasoline) uses bromine solution, and Test Method D4628 (additive elements in
lubricating oils) uses potassium salt as ionization suppressant.
7. Apparatus
7.1 A simple schematic representation of AAS is shown in Fig. 1.
7.2 The basic AAS instrument consists of a suitable light source emitting a light spectrum directed at the atomizer through single
or double beam optics. The light emitted by the source is obtained from the same excited atoms that are measured in the atomizer.
The light leaving the atomizer passes through a simple monochromator to a detector. The measured intensity is electronically
converted into analytical concentration of the element being measured. Quantitative measurements in AAS are based on Beer’s
Law. However, for most elements, particularly at high concentrations, the relationship between concentration and absorbance
deviates from Beer’s Law and is not linear. Usually two or more calibration standards spanning the sample concentration and a
blank are used for preparing the calibration curve. After initial calibration, a check standard at mid range of calibration should be
analyzed.
7.3 The ground state atom absorbs the light energy of a specific wavelength as it enters the excited state. As the number of atoms
in the light path increase, the amount of the light absorbed also increases. By measuring the light absorbed, a quantitative
determination of the amount of the analyte present can be calculated.
7.4 Two types of AAS instruments use either single beam or double beam. In the first type, the light source emits a spectrum
specific to the element of which it is made, which is focused through the sample cell into the monochromator. The light source
is electronically modulated to differentiate between the light from the source and the emission from the sample cell. In a double
beam AA spectrometer, the light from the source lamp is divided into a sample beam which is focused through the sample cell,
and a reference beam which is directed around the sample cell. In a double beam system, the readout represents the ratio of the
sample and the reference beams. Therefore, fluctuations in the source intensity do not become fluctuations in the instrument
readout, and the baseline is much more stable. Both types use the light sources that emit element specific spectra.
7.5 In AAS, the sample solution whether aqueous or non-aqueous, is vaporized into a flame, and the elements are atomized at
high temperatures. The elemental concentration is determined by absorption of the analyte atoms of a characteristic wavelength
emitted from a light source, typically a hollow cathode lamp which consists of a tungsten anode and a cylindrical cathode made
of the analyte metal, encased in a gas-tight chamber. Usually a separate lamp is needed for each element; however, multi-element
lamps are in quite common use. The detector is usually a photomultiplier tube. A monochromator separates the elemental lines and
the light source is modulated to discriminate against the continuum light emitted by the atomization source.
7.6 Burner System—A dual option burner system consists of both a flow spoiler and an impact bead for optimal operation under
different analytical conditions. Equivalent precision is obtained with the air–acetylene flame using the flow spoiler or the impact
bead. However, for nitrous oxide–acetylene flame, noticeably poorer precision is obtained when using impact bead.
7.7 Flame Sources:
7.7.1 Usually, AAS instruments use flame as the atomization source. An air-acetylene flame is used for most elements; the
nitrous oxide-acetylene flame reaches higher temperature (2300°C(2300 °C for air-C H versus 2955°C2955 °C for N O-C H ),
2 2 2 2 2
and is used for atomizing the more refractory oxide forming metals. Flame conditions used in AAS are summarized in Table 4.
7.7.2 Out of several possible combinations (Table 4), air-acetylene and nitrous oxide–acetylene are the most commonly used
flames as atomization sources in AAS. Over 30 elements can be determined with the air–acetylene flame. The nitrous
oxide–acetylene flame is the hottest of the flames used and produces a maximum temperature of 3000 °C. It can atomize refractory
elements such as aluminum, silicon, vanadium, and titanium, and others, all forming highly refractory oxide molecules in the
flame. Although nitrous oxide–acetylene flame can be used for the determination of over 65 elements, in practice it is used only
where air–acetylene flame is ineffective.
7.8 Hollow Cathode Lamps:
HOLLOW CATHODE LAMP → NEBULIZER → FLAME →
...