ASTM D7260-20

Standard Practice for Optimization, Calibration, and Validation of Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) for Elemental Analysis of Petroleum Products and Lubricants

SIGNIFICANCE AND USE
4.1 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.  
4.2 Inductively coupled plasma-atomic emission spectrometry is one of the more widely used analytical techniques in the oil industry for multi-element analysis as evident from at least twelve standard test methods (for example, Test Methods C1111, D1976, D4951, D5184, D5185, D5600, D5708, D6130, D6349, D6357, D7040, D7111, D7303, and D7691) published for the analysis of fossil fuels and related materials. These have been briefly summarized by Nadkarni (1).5  
4.2.1 Determination of mercury and trace metals in crude oils using atomic spectroscopic methods is discussed in Guide D8056.  
4.3 The advantages of using an ICP-AES analysis include high sensitivity for many elements of interest in the oil industry, relative freedom from interferences, linear calibration over a wide dynamic concentration range, single or multi-element capability, and ability to calibrate the instrument based on elemental standards irrespective of their elemental chemical forms, within limits described below such as solubility and volatility assuming direct liquid aspiration. Thus, the technique has become a method of choice in most of the oil industry laboratories for metal analyses of petroleum products and lubricants.  
4.4 In addition to the ICP-AES standards listed in 2.2, a new ICP-MS standard, Test Method D8110, has been issued for analysis of distillate products for multi-element determination of Al, Ca, Cu, Fe, Pb, Mg, and K.
SCOPE
1.1 This practice covers information on the calibration and operational guidance for the multi-element measurements using inductively coupled plasma-atomic emission spectrometry (ICP-AES).  
1.2 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.3 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.

General Information

• Status: Published
• Publication Date: 30-Jun-2020
• ICS: 75.080 - Petroleum products in general
• ICS: 75.100 - Lubricants, industrial oils and related products
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.03 - Elemental Analysis

Relations

• Refers: ASTM C1109-23 - Standard Practice for Analysis of Aqueous Leachates from Nuclear Waste Materials Using Inductively Coupled Plasma-Atomic Emission Spectroscopy
• Effective Date: 01-Dec-2023
• Refers: ASTM D6299-23a - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 01-Dec-2023
• Refers: ASTM D6792-23c - Standard Practice for Quality Management Systems in Petroleum Products, Liquid Fuels, and Lubricants Testing Laboratories
• Effective Date: 01-Nov-2023
• Refers: ASTM D6792-23b - Standard Practice for Quality Management Systems in Petroleum Products, Liquid Fuels, and Lubricants Testing Laboratories
• Effective Date: 01-Oct-2023
• Refers: ASTM D1976-20 - Standard Test Method for Elements in Water by Inductively-Coupled Plasma Atomic Emission Spectroscopy
• Effective Date: 01-May-2020
• Refers: ASTM D7040-04(2020) - Standard Test Method for Determination of Low Levels of Phosphorus in ILSAC GF 4 and Similar Grade Engine Oils by Inductively Coupled Plasma Atomic Emission Spectrometry
• Effective Date: 01-May-2020
• Refers: ASTM D1976-18 - Standard Test Method for Elements in Water by Inductively-Coupled Plasma Atomic Emission Spectroscopy
• Effective Date: 01-Jul-2018
• Refers: ASTM D8056-18 - Standard Guide for Elemental Analysis of Crude Oil
• Effective Date: 01-Jun-2018
• Refers: ASTM D5185-18 - Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
• Effective Date: 01-Apr-2018
• Refers: ASTM D6130-11(2018) - Standard Test Method for Determination of Silicon and Other Elements in Engine Coolant by Inductively Coupled Plasma-Atomic Emission Spectroscopy
• Effective Date: 01-Mar-2018
• Refers: ASTM D6299-17b - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 15-Dec-2017
• Refers: ASTM D6299-17a - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 15-Nov-2017
• Refers: ASTM D8110-17 - Standard Test Method for Elemental Analysis of Distillate Products by Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
• Effective Date: 01-May-2017
• Refers: ASTM D6299-17 - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 01-Jan-2017
• Refers: ASTM E1479-16 - Standard Practice for Describing and Specifying Inductively Coupled Plasma Atomic Emission Spectrometers
• Effective Date: 01-Nov-2016

Overview

ASTM D7260-20 establishes best practices for the optimization, calibration, and validation of inductively coupled plasma-atomic emission spectrometry (ICP-AES) when used for elemental analysis of petroleum products and lubricants. Accurate measurement of elemental content is critical for compliance with chemical, commercial, and regulatory specifications in the petroleum industry. This standard is published by ASTM International and follows globally recognized principles of standardization.

ICP-AES is widely recognized in the analysis of petroleum products owing to its high sensitivity, versatility for multi-element analysis, and robust calibration methods. The standard provides detailed protocols to ensure reliable and reproducible results, contributing to the overall quality assurance process within oil industry laboratories.

Key Topics

• Optimization of ICP-AES Instruments

  • Guidance on instrument setup, including warm-up, wavelength profiling, and environmental control
  • Importance of proper peristaltic pump and sample introduction system maintenance
  • Selection criteria for wavelength and background correction specific to petroleum matrices

• Calibration Techniques

  • Matrix matching for calibration standards to enhance accuracy
  • Use of internal standards for compensation of matrix effects and transport variations
  • Verification of calibration linearity and analytical performance using check standards

• Validation and Quality Control

  • Procedures for validating instrument performance before sample analysis
  • Strategies for the use of control charts and quality management systems (referenced in ASTM D6299)
  • Best practices for ongoing instrument maintenance and inspection

• Interfering Factors

  • Identification and mitigation of spectral, physical, and chemical interferences inherent to ICP-AES
  • Addressing potential issues such as spectral overlap, viscosity differences, carbon buildup, and sample heterogeneity

• Safety Considerations

  • Recommendations for safe operation of ICP-AES equipment
  • Handling of hazardous organic solvents and protection from UV/visible light and fumes

Applications

ASTM D7260-20 is vital in laboratories conducting elemental analysis of a variety of petroleum-based products, supporting:

• Product Quality Control: Assures compliance with industry specifications by determining trace and major elements in lubricants, base oils, crude oils, and refining products.
• Regulatory Compliance: Facilitates the determination of regulated elements such as sulfur, phosphorus, or heavy metals in petroleum materials.
• Additive Analysis: Enables the quantification of metallic additives to optimize lubricant formulation and performance.
• Failure Investigation: Provides data on elemental impurities or contaminants contributing to equipment wear or product degradation.
• Research and Development: Supports method development for new product formulations and alternative fuels.

ICP-AES, when calibrated, optimized, and validated according to ASTM D7260-20, serves as a cornerstone technique in petroleum industry laboratories for reliable multi-element quantification.

Related Standards

ASTM D7260-20 references and supplements several other key standards that further enhance analytical accuracy and laboratory best practices:

• ASTM D5185: Multielement analysis of used and unused lubricating oils and base oils via ICP-AES
• ASTM D4951: Determination of additive elements in lubricating oils by ICP-AES
• ASTM D6299: Applying statistical quality assurance and control charting techniques
• ASTM D7031: Analytical procedures for determining trace elements in petroleum coke
• ASTM D8110: Elemental analysis of distillate products by ICP-MS
• ASTM E1479: Describing and specifying ICP-atomic emission spectrometers
• ASTM D7876: Practice for sample decomposition by microwave heating for atomic spectroscopic determination

These related standards help laboratories establish comprehensive protocols for sample collection, preparation, and analysis, thereby supporting the high-precision requirements of the petroleum sector.

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Keywords: ASTM D7260-20, ICP-AES, petroleum analysis, elemental analysis, lubricants, calibration, validation, standardization, oil industry, spectrometric methods, quality control, regulatory compliance

Standards Content (Sample)

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.
Designation: D7260 − 20
Standard Practice for
Optimization, Calibration, and Validation of Inductively
Coupled Plasma-Atomic Emission Spectrometry (ICP-AES)
for Elemental Analysis of Petroleum Products and
Lubricants
This standard is issued under the fixed designation D7260; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* 2.2 ICP-AES Related Standards:
C1111Test Method for Determining Elements in Waste
1.1 This practice covers information on the calibration and
StreamsbyInductivelyCoupledPlasma-AtomicEmission
operational guidance for the multi-element measurements us-
Spectroscopy
ing inductively coupled plasma-atomic emission spectrometry
C1109Practice for Analysis of Aqueous Leachates from
(ICP-AES).
Nuclear Waste Materials Using Inductively Coupled
1.2 This standard does not purport to address all of the
Plasma-Atomic Emission Spectroscopy
safety concerns, if any, associated with its use. It is the
D1976Test Method for Elements in Water by Inductively-
responsibility of the user of this standard to establish appro-
Coupled Plasma Atomic Emission Spectroscopy
priate safety, health, and environmental practices and deter-
D4951Test Method for Determination ofAdditive Elements
mine the applicability of regulatory limitations prior to use.
in Lubricating Oils by Inductively Coupled Plasma
1.3 This international standard was developed in accor-
Atomic Emission Spectrometry
dance with internationally recognized principles on standard-
D5184Test Methods for Determination of Aluminum and
ization established in the Decision on Principles for the
Silicon in Fuel Oils by Ashing, Fusion, Inductively
Development of International Standards, Guides and Recom-
Coupled Plasma Atomic Emission Spectrometry, and
mendations issued by the World Trade Organization Technical
Atomic Absorption Spectrometry
Barriers to Trade (TBT) Committee.
D5185Test Method for Multielement Determination of
Used and Unused Lubricating Oils and Base Oils by
2. Referenced Documents
Inductively Coupled Plasma Atomic Emission Spectrom-
2.1 ASTM Standards:
etry (ICP-AES)
D4057Practice for Manual Sampling of Petroleum and
D5600Test Method for Trace Metals in Petroleum Coke by
Petroleum Products
Inductively Coupled Plasma Atomic Emission Spectrom-
D4307Practice for Preparation of Liquid Blends for Use as
etry (ICP-AES)
Analytical Standards
D5708 Test Methods for Determination of Nickel,
D6299Practice for Applying Statistical Quality Assurance
Vanadium, and Iron in Crude Oils and Residual Fuels by
and Control Charting Techniques to Evaluate Analytical
Inductively Coupled Plasma (ICP) Atomic Emission
Measurement System Performance
Spectrometry
D6792Practice for Quality Management Systems in Petro-
D6130Test Method for Determination of Silicon and Other
leum Products, Liquid Fuels, and Lubricants Testing
Elements in Engine Coolant by Inductively Coupled
Laboratories
Plasma-Atomic Emission Spectroscopy
D6349Test Method for Determination of Major and Minor
Elements in Coal, Coke, and Solid Residues from Com-
This practice is under the jurisdiction ofASTM Committee D02 on Petroleum
bustion of Coal and Coke by Inductively Coupled
Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-
Plasma—Atomic Emission Spectrometry
mittee D02.03 on Elemental Analysis.
D6357TestMethodsforDeterminationofTraceElementsin
CurrenteditionapprovedJuly1,2020.PublishedJuly2020.Originallyapproved
Coal, Coke, and Combustion Residues from Coal Utiliza-
in 2006. Last previous edition approved in 2019 as D7260–19. DOI: 10.1520/
D7260-20.
tion Processes by Inductively Coupled Plasma Atomic
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Emission Spectrometry, Inductively Coupled Plasma
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Mass Spectrometry, and Graphite Furnace Atomic Ab-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. sorption Spectro
*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
D7260 − 20
D7040Test Method for Determination of Low Levels of fortheanalysisoffossilfuelsandrelatedmaterials.Thesehave
Phosphorus in ILSAC GF 4 and Similar Grade Engine been briefly summarized by Nadkarni (1).
Oils by Inductively Coupled Plasma Atomic Emission 4.2.1 Determination of mercury and trace metals in crude
Spectrometry oils using atomic spectroscopic methods is discussed in Guide
D7111Test Method for Determination of Trace Elements in D8056.
Middle Distillate Fuels by Inductively Coupled Plasma
4.3 The advantages of using an ICP-AES analysis include
Atomic Emission Spectrometry (ICP-AES)
high sensitivity for many elements of interest in the oil
D7303Test Method for Determination of Metals in Lubri-
industry,relativefreedomfrominterferences,linearcalibration
cating Greases by Inductively Coupled Plasma Atomic
over a wide dynamic concentration range, single or multi-
Emission Spectrometry
elementcapability,andabilitytocalibratetheinstrumentbased
D7691TestMethodforMultielementAnalysisofCrudeOils
onelementalstandardsirrespectiveoftheirelementalchemical
Using Inductively Coupled Plasma Atomic Emission
forms, within limits described below such as solubility and
Spectrometry (ICP-AES)
volatilityassumingdirectliquidaspiration.Thus,thetechnique
D7876Practice for Practice for Sample Decomposition Us-
has become a method of choice in most of the oil industry
ing Microwave Heating (With or Without Prior Ashing)
laboratories for metal analyses of petroleum products and
for Atomic Spectroscopic Elemental Determination in
lubricants.
Petroleum Products and Lubricants
4.4 In addition to the ICP-AES standards listed in 2.2,a
D8056Guide for Elemental Analysis of Crude Oil
newICP-MSstandard,TestMethodD8110,hasbeenissuedfor
D8110Test Method for Elemental Analysis of Distillate
analysis of distillate products for multi-element determination
Products by Inductively Coupled Plasma Mass Spectrom-
of Al, Ca, Cu, Fe, Pb, Mg, and K.
etry (ICP-MS)
E1479Practice for Describing and Specifying Inductively
5. Apparatus
Coupled Plasma Atomic Emission Spectrometers
5.1 Spectrometer—Aninductivelycoupledplasmaemission
2.3 Other Standards:
spectrometer with a spectral bandpass of 0.05nm or less is
IP 437Determination of Additive Elements in Unused Lu-
required. The spectrometer may be of the simultaneous multi-
bricating Oils and Additive Packages by Inductively
elemental or sequential scanning type. The spectrometer may
Coupled Plasma-Atomic Emission Spectrometry
be of the air path, inert gas path, or vacuum type, with spectral
ISO/TC 17/SC1N883Guidelines for the Preparation of
lines selected appropriately for use with specific instrument.
Standard Methods ofAnalysis Using Inductively Coupled
Either an analog or digital readout system may be used.
Plasma-Atomic Emission Spectrometry and for Use of
5.2 An ICP-AES instrument system is typically comprised
ICP Spectrometry for the Determination of Chemical
Composition (1991) of several assemblies including a radio-frequency (RF)
generator, an impedance matching network (where required),
3. Summary of Practice an induction coil, a plasma torch, a plasma igniter system, a
sampleintroductionsystem,alightgatheringoptic,anentrance
3.1 An inductively coupled plasma-atomic emission spec-
slit and dispersing element to separate and measure the
trometry(ICP-AES)instrumentisonethatisusedtodetermine
intensity of the wavelengths of light emitted from the plasma,
elemental composition of various liquid matrices. Details of
one or more devices for converting the emitted light into an
the instrument components are given in Practice E1479. This
electrical current or voltage, one or more analog preamplifiers,
practice summarizes the protocols to be followed during
one or more analog-to-digital converter(s), and a dedicated
calibration and verification of the instrument performance.
computer with printer. Solid state CCD or CID detectors if
used may not require extra analog-to-digital components.
4. Significance and Use
Recentlymoderncamera-typeinstrumentshavebeensupplant-
4.1 Accurate elemental analysis of petroleum products and
ing the photomultiplier tube type detectors. Cameras may not
lubricants is necessary for the determination of chemical
have high resolution, but they offer greater wavelength choice.
properties, which are used to establish compliance with com-
5.2.1 Plasmacanbemonitoredeitheraxiallyversusradially.
mercial and regulatory specifications.
Potential for improved sensitivity as much as tenfold is often
realized with axial monitoring. However, the increased inter-
4.2 Inductively coupled plasma-atomic emission spectrom-
etryisoneofthemorewidelyusedanalyticaltechniquesinthe ference from molecular background may compromise these
gains depending on the wavelength monitored and matrix used
oil industry for multi-element analysis as evident from at least
twelve standard test methods (for example, Test Methods (especially for organics versus aqueous).
5.2.2 Echelle Spectrometers—More recently echelle grat-
C1111,D1976,D4951,D5184,D5185,D5600,D5708,D6130,
D6349, D6357, D7040, D7111, D7303, and D7691) published ings are being increasingly used in several commercial plasma
spectrometers. A prism is used as an order-sorter to improve
sensitivity. To measure widely separated lines with useful
Available from Energy Institute, 61 New Cavendish St., London, W1G 7AR,
U.K., http://www.energyinst.org.
4 5
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St., The boldface numbers in parentheses refer to a list of references at the end of
4th Floor, New York, NY 10036, http://www.ansi.org. this standard.
D7260 − 20
efficiency, echelle instruments have to be operated in many yieldsignalsof100×to1000×thedetectionlimitsought.Also,
different orders. This involves complex wavelength scanning ensure that the chosen wavelength will not be interfered with
programs for computer controlled echelle monochromators. from unexpected elements. See Section 6.
While the resolution of a grating monochromator is relatively 5.5.3 Often ion lines may be chosen for use over atom lines
constant across its working range, practical resolution of an to avoid interelement interference and sensitivity of detection.
echelle monochromator can vary considerably with wave- Thischoicewillbedependentontheanalyteofinterestandthe
length. Inherently higher theoretical resolving power of the sample matrix being analyzed.
echelle when used in high order, relative to the diffraction
5.6 Peristaltic Pump—Differences in the viscosities of the
grating used in the first order, allows a relatively compact
test specimen solutions and standard solutions can cause
echelle instrument to achieve high resolving power. The
differences in the uptake rates adversely affecting the accuracy
detectionlimitsobtainedwithechelleplasmaspectrometersare
of the analysis. These effects can be minimized by using a
comparable to those achieved by grating spectrometers.
peristaltic pump (or an internal standard). If a peristaltic pump
is used, inspect the pump tubing and replace it, if necessary,
5.3 Spectrometer Environment:
before starting each day. Verify the solution uptake rate daily
5.3.1 Temperature fluctuations affect the instrument stabil-
and adjust to the desired rate. Compatibility of the solution
ity. Some manufacturers provide systems for maintaining a
with the peristaltic pump tubing must also be confirmed to
constant internal temperature within the optical compartment
prevent premature pump failure.Avariety of polymeric mate-
and sample introduction area that assumes changes in the
rial options are available for pump tubing to address this
outside temperature are not being controlled within the neces-
concern by simple empirical testing with the given solvent/
saryspecifiedrangeandrateofchangetoinsurestability.Other
sample matrix used. Generally speaking, the selected tubing
manufacturers design their spectrometers to be stable over a
soaked overnight in the solvent/sample matrix should not
specified temperature range without attempting to control the
soften, crack, or embrittle the tubing.
spectrometer’s internal temperature.
5.3.2 Since temperature and humidity changes may also 5.7 Depending on the nebulizer design, starving solvent
flow (that is, flow below natural free aspiration rates) can have
affectthesampleintroductionsystem,detectors,andelectronic
readout as well as the spectrometer alignment, some manufac- an effect on aerosol generation. Consistent flow can help
achieve enhanced analyte sensitivity.
turers specify that care be used in selecting a location for the
spectrometerthatexperiencesminimalvariationintemperature
5.8 Torch—Inspect the torches before use for cracks and
and relative humidity. The user needs to provide a controlled
discardedorrepairedasappropriate.Cleantorchesthatarefree
environment as specified by the manufacturer. This is a very
of carbon buildup should be used. The load coil should be
important factor in optimum performance of an ICP-AES
replaced or cleaned if oxidation or leaking of coolant is
system.
observed. The glass also can devitrify especially at the aerosol
5.3.3 The generator output power and the plasma gas flow
tip with oxygen injection.
determine the plasma temperature and thus significantly influ-
5.9 Safety—The ICP-AES instrument is not normally con-
ence the emission signal and the background. Thus, the power
sidered as a hazardous instrument. However, appropriate pre-
applied and gas flow adjustments may be used to control the
cautions should be taken regarding the fumes, heat, and
signal to background ratio and, matrix, and some spectral
UV/visible light radiation as well as appropriate RF shielding.
interferences.
The equipment should always be used according to the
5.4 Optical Path: manufacturer’s operating instructions. No attempt should be
madetobypasstheinterlocks.Adequatecoolingtimesmustbe
5.4.1 Since oxygen exhibits increasing absorbance with
allowed before handling any hot components. Any safety
decreasing wavelengths below 200nm, the performance of an
covers must be in position.
air path instrument degrades below that wavelength and is
5.9.1 Fumes from the plasma and any ozone generated by
generally not useful below approximately 190nm.
the UVradiation must be removed by means of a suitable heat
5.4.2 Purging the optical path with nitrogen or argon, or
and acid resistant (acids can be formed from halogens or
another gas with low absorption in this ultraviolet region may
sulfate and nitrates in the solution) chimney fitted with an
extend the spectral region to wavelengths below 167nm. Use
exhaust fan of sufficient capacity.
of these purge gases is in general less expensive to maintain
5.9.2 A UV/visible light absorbing viewing window (with
thanthevacuumpathsystems.Sealedopticsfilledwithaninert
RF shielding) must always be in place to protect the eyes and
gas is also available for such work.
skin of the operator from radiation.
5.5 Wavelength Selection:
5.9.3 Often the organic samples and solvents used in or-
5.5.1 When selecting the fixed position wavelengths to be
ganic ICP-AES analysis are toxic and hazardous.All appropri-
utilized in a Paschen-Runge polychromator for particular
ate precautions must be taken in handling such materials to
applications, close collaboration between user and instrument
protect the operators. Consult MSDS and other safety infor-
manufacturer is critical. Camera instruments do not have this
mation before handling these chemicals.
problem.
6. Interferences
5.5.2 If possible, use the peak and background wavelengths
suggested in the methods. When there is a choice such as with 6.1 Several types of interference effects may contribute to
the sequential instruments, choose the wavelength that will inaccuracies in the elemental determination using ICP-AES.
D7260 − 20
Principally these interferences can be classifies as spectral, made using the computer software supplied by the instrument
physical, and chemical. manufacturer or the empirical method described in Test
Method D5185. Further details of the empirical method are
6.2 Spectral Interferences:
given in the Test Method C1109 and by Boumans (6). This
empirical method cannot be used with scanning spectrometer
NOTE 1—An empirical method for correcting spectral interferences is
detailed in Test Method D5185.
systems when both the analytical and interfering lines cannot
belocatedpreciselyandreproducibly.Withanyinstrument,the
6.2.1 Spectral interferences can be categorized as (1) unre-
analyst must always be alert to the possible presence of
solved overlap of a spectral line from another element, (2)
unexpected elements producing interfering spectral lines.
unresolved overlap of molecular band spectra, (3) background
6.2.3.1 The empirical method of spectral interference cor-
contribution from continuous or recombination phenomena,
rection uses interference correction factors that are determined
and (4) background contribution from stray light from line
by analyzing single-element high-purity solutions under con-
emission of high concentration of elements. With echelle
ditions matching as closely as possible those used for test
spectrometers it may be possible to look at two or more lines
specimen solution analysis. Unless plasma conditions can be
to identify interference.
accurately reproduced from day to day, and sample matrix to
6.2.2 Interelement Interferences—This interference can be
sample matrix, or for longer periods, interference correction
compensatedforbycomputercorrectionoftherawdata,which
factors found to significantly affect the results must be deter-
requires measurement of the interfering element at the wave-
mined each time specimens are analyzed. One way to accu-
length of interest. Various analytical systems may exhibit
rately do this is to daily monitor the ratio of Mg I to Mg II
somewhat different levels of interferences. Therefore, the
lines, or copper to magnesium ratio.
interference effects must be evaluated for each individual
6.2.4 Molecular Band Interferences—These arise from
system. Perhaps a 2% maximum correction may be accept-
overlapofmolecularbandspectraatthewavelengthofinterest,
able.
and can be eliminated by careful selection of wavelength.
6.2.2.1 Potential spectral overlaps from concomitant ele-
6.2.5 High Background Interferences—These can be com-
ments may be estimated by measuring the signal arising from
pensated for by background correction adjacent to the analyte
ahigh-puritysingle-elementreferencesolutionoftheconcomi-
line. Signal to background ratios should be of the order of at
tant element. It is useful to consult tables of spectral lines to
least 3 to 5 factor.
become aware of possible overlaps, especially when analyzing
6.2.5.1 Wavelength should be selected with best intensity,
samples of unknown composition (Refs 2-7). The overlaps
peak shape, and lack of interferences. When analysis is being
may appear at the measured wavelength peak or at one of the
done for elements at trace levels, background correction is
two background points selected, thus prompting correction on
required. Thus, for all elements possible, the baseline for the
onlyonesideofthepeakiftheinterferantissuspectedpresent.
emission peak should be set with points as close to both sides
If the overlap is severe (for example, 50%), alternate line
of the peak without measuring the element wavelength inten-
selection may be indicated to minimize spectral interferences.
sity(seeFigure1ofTestMethod D7111).Afterthesebaselines
Judicious selection of background correction points may also
are set, a check standard should be used to test the system
prevent potential interferences affecting analyte quantification
response and calibration.
from interfering concomitant elements. It is recommended that
6.2.5.2 Appropriate selection of wavelengths for back-
multiple high purity solutions be used to confirm consistency
ground corrections is extremely critical for the determination
of the suspected spectral source of a potential interference.
of some elements. For example, since the predominate sodium
Some analyte elements may be difficult to remove in manu-
emission line (588.995nm) resides near a significant argon
facturing a high purity solution for the suspected interfering
emission interference, it is recommended, if possible, compare
element, especially for organometallic standard solutions.
the spectra of the sample and the standards at the sodium
6.2.2.2 Potential interferences should be considered in the
emission wavelength to ensure that the signal integration
line selection process for polychromators. With sequential
occurs accurately.
instruments, it may be desirable to select an alternate line to
6.2.5.3 When emission wavelengths occur on a highly
avoid spectral overlaps even though the sensitivity of the
structured background (as in the example of sodium line
alternate line may be lower. This issue should be carefully
above), a single off-peak background measurement may not
considered for selection of fixed wavelength options for
provide accurate results. For such emission wavelengths on a
applicable polychromator when purchased.
structuredbackground,backgroundcorrectionisrecommended
6.2.2.3 There is also the possibility of spectral overlap from
at both lower and higher wavelengths from the emission
an element that is not being determined. With simultaneous
wavelengths. Additionally, some low resolution, photo-
instruments, it may be necessary to install additional hardware
multiplier tube-based instruments may require a comprised
to correct for concomitant elements or to allow determination
selection of background points, and this could also provide
of a given element at two or more wavelengths. Sequential
inaccurate results.
instruments permit measurement of other lines of interfering
elements to allow correction of their contributions at the
6.3 Physical Interferences—These are generally considered
analytical wavelength.
effects associated with the sample nebulization and transport
6.2.3 Interelement Interferences—When spectral interfer- processes. Such properties as change in viscosity and surface
ences cannot be avoided, the necessary corrections should be tension can cause significant inaccuracies, especially in
D7260 − 20
samples that may contain high dissolved solids or acid con- (see 8.8.4). This can be monitored by several means beyond
centrations (such as in aqueous ICP-AES analysis), or both. analysis of the sample beforehand to ascertain the un-doped
Use of a high-solid Babington type nebulizer (although op- internal standard concentration.This may include the monitor-
tional in most methods) is highly recommended for avoiding ingofmultipleinternalstandardelementsorcomparisonofthe
plugging, particularly when analyzing used oils (which would expected magnitude of correction imposed by the internal
contain particulates). The use of a peristaltic pump may also standard relative to a known or blank solution analyzed as a
lessen these interferences. If these types of interferences are sample.Also,theinternalstandardmustbeanalyte-freeaswell
operative, they may be reduced by dilution (for example, as free of concomitant species that may impose a spectral
tenfold) of these samples or utilization of standard addition interference on the analyte lines applied.
techniques,orboth.However,dilutionorstandardadditionwill
6.4 Chemical Interferences:
not compensate for volatilization of some species.
6.4.1 Chemical interferences are caused by molecular com-
6.3.1 The torch aperture constriction is as much a problem
pound formation, ionization effects, and thermochemical ef-
with carbon soot for organics as it is for salt deposits in
fects associated with sample vaporization and atomization in
aqueous solutions that generally have the higher dissolved (as
the plasma. Normally these effects are not pronounced and can
opposed to suspended) solids content. Also, for organics the
beminimizedbycarefulselectionofoperatingconditionssuch
matrix effects are exacerbated relative to aqueous matrix. The
as incident power, plasma observation position, and so forth,
inherent diversity in organic matrix solution characteristics
by matrix matching, and by standard addition procedures.
commonlyimpacttransportmoresignificantlyduetovolatility,
Thesetypesofinterferencescanbehighlydependentonmatrix
polarity, surface tension, and so forth, as well as viscosity.
type and the specific analyte.
Without a doubt, analyte volatility will most significantly
6.4.2 Selective volatilization can occur when the analyte is
impact transport effects and accuracy of the analysis.
present in the sample in a relatively volatile form and can
6.3.2 Suspendedanalyteandotherwearparticlesintheused
vaporize during the nebulization process. Whereas the trans-
oilalsocauseproblemingettingaccurateanalysis.Itiscritical portefficiencyofdropletsproducedfromastandardpneumatic
to keep the sample solution homogenized to get good results.
nebulizer is typically around 1% or 2%, for vapor it can be
Often the used oil samples are mixed using ultrasonic bath or much higher giving rise to enhanced results that can vary
a vortex mixer before actual ICP-AES measurements;
depending on the relative concentrations of various chemical
however, the time lag between such homogenization and species present in the sample (for example, volatile sulfur
ICP-AES measurement should be kept to a minimum so that speciessuchasH S,leadalkyls,andsomesiliconcompounds).
the particles do not settle out during the waiting period. Thus,
6.5 Salt buildup at the tip of the nebulizer can occur from
for such samples autosampler may not be useful.
high dissolved solids (for example, in solutions prepared by
6.3.3 Tominimizenebulizertransporteffectscausedbyhigh
alkali fusion with subsequent dilution with acids). This salt
viscosity oils or viscosity improvers and additives in the oil,
buildup affects aerosol flow rate that can cause instrumental
and to reduce potential spectral interferences, dilute the
drift. To control this problem, in cases of aqueous analysis
samples and standards as appropriate to minimize transport
argon should be wetted prior to nebulization, using a tip
effects (minimum tenfold dilution). Both calibration standards
washer, or by diluting the sample.
and sample solutions should not contain more than 10% by
6.6 Carbon Buildup—Inspect the torch for carbon build-up
mass oil. The calibration standards should be prepared with
during the warm up period. If it occurs, replace the torch
analyte-free oil added to the solution matrix such that all
immediately and consult the manufacturer’s operating guide to
solutions contain the same mass% oil. Also, when diluting
take proper steps to remedy the situation.
samples to greater dilution factors, consistent oil to solvent
6.6.1 Carbon that accumulates on the tip of the torch
ratioshouldbemaintained.See7.2aboutthedilutionsolvents.
injector tube can be removed by using nebulizer gas that
6.3.4 Use of internal standard(s) could also be used in place
consists of approximately 1% oxygen in argon. However,
of dilution or standard additions. With samples containing
oxygenabsorbsradiationandcoolstheplasmathataffectsmost
acids,thebiggestvariationcanactuallybeobservedatlowacid
of the elements. Hence, it may not be a useful practice, except
concentrations.Acommonsolutiontothisproblemistoensure
whenhighsensitivityforsodiumisdesired.Also,oxygeninthe
that all samples and standards contain the same acid concen-
nebulizer or auxiliary gas streams may defeat the purpose of
tration (for example, 2% nitric acid).
purged optical path or vacuum for lines below 190nm.
6.3.5 Viscosity Index Improver Effect—Viscosity index im-
6.6.2 The carbon buildup can also be cleaned off-line by
proversthatmaybepresentinmulti-gradelubricatingoils,can
bakingthetorchinafurnaceat500°Cfor1hto3hdepending
bias the measurements (8). However, these biases can be
on the amount of carbon buildup.
reduced to negligible proportion by using the above specified
6.6.3 Gen
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7260 − 19 D7260 − 20
Standard Practice for
Optimization, Calibration, and Validation of Inductively
Coupled Plasma-Atomic Emission Spectrometry (ICP-AES)
for Elemental Analysis of Petroleum Products and
Lubricants
This standard is issued under the fixed designation D7260; 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 practice covers information on the calibration and operational guidance for the multi-element measurements using
inductively coupled plasma-atomic emission spectrometry (ICP-AES).
1.2 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.3 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:
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4307 Practice for Preparation of Liquid Blends for Use as Analytical Standards
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
2.2 ICP-AES Related Standards:
C1111 Test Method for Determining Elements in Waste Streams by Inductively Coupled Plasma-Atomic Emission Spectroscopy
C1109 Practice for Analysis of Aqueous Leachates from Nuclear Waste Materials Using Inductively Coupled Plasma-Atomic
Emission Spectroscopy
D1976 Test Method for Elements in Water by Inductively-Coupled Plasma Atomic Emission Spectroscopy
D4951 Test Method for Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic
Emission Spectrometry
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)
D5600 Test Method for Trace Metals in Petroleum Coke 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
D6130 Test Method for Determination of Silicon and Other Elements in Engine Coolant by Inductively Coupled Plasma-Atomic
Emission Spectroscopy
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 May 1, 2019July 1, 2020. Published May 2019July 2020. Originally approved in 2006. Last previous edition approved in 20172019 as
D7260 – 17.D7260 – 19. DOI: 10.1520/D7260-19.10.1520/D7260-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*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
D7260 − 20
D6349 Test Method for Determination of Major and Minor Elements in Coal, Coke, and Solid Residues from Combustion of
Coal and Coke by Inductively Coupled Plasma—Atomic Emission Spectrometry
D6357 Test Methods for Determination of Trace Elements in Coal, Coke, and Combustion Residues from Coal Utilization
Processes by Inductively Coupled Plasma Atomic Emission Spectrometry, Inductively Coupled Plasma Mass Spectrometry,
and Graphite Furnace Atomic Absorption Spectro
D7040 Test Method for Determination of Low Levels of Phosphorus in ILSAC GF 4 and Similar Grade Engine Oils by
Inductively Coupled Plasma Atomic Emission Spectrometry
D7111 Test Method for Determination of Trace Elements in Middle Distillate Fuels by Inductively Coupled Plasma Atomic
Emission Spectrometry (ICP-AES)
D7303 Test Method for Determination of Metals in Lubricating Greases by Inductively Coupled Plasma Atomic Emission
Spectrometry
D7691 Test Method for Multielement Analysis of Crude Oils Using Inductively Coupled Plasma Atomic Emission Spectrometry
(ICP-AES)
D7876 Practice for Practice for Sample Decomposition Using Microwave Heating (With or Without Prior Ashing) for Atomic
Spectroscopic Elemental Determination in Petroleum Products and Lubricants
D8056 Guide for Elemental Analysis of Crude Oil
D8110 Test Method for Elemental Analysis of Distillate Products by Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
E1479 Practice for Describing and Specifying Inductively Coupled Plasma Atomic Emission Spectrometers
2.3 Other Standards:
IP 437 Determination of Additive Elements in Unused Lubricating Oils and Additive Packages by Inductively Coupled
Plasma-Atomic Emission Spectrometry
ISO/TC 17/SC 1 N 883 Guidelines for the Preparation of Standard Methods of Analysis Using Inductively Coupled
Plasma-Atomic Emission Spectrometry and for Use of ICP Spectrometry for the Determination of Chemical Composition
(1991)
3. Summary of Practice
3.1 An inductively coupled plasma-atomic emission spectrometry (ICP-AES) instrument is one that is used to determine
elemental composition of various liquid matrices. Details of the instrument components are given in Practice E1479. This practice
summarizes the protocols to be followed during calibration and verification of the instrument performance.
4. Significance and Use
4.1 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.
4.2 Inductively coupled plasma-atomic emission spectrometry is one of the more widely used analytical techniques in the oil
industry for multi-element analysis as evident from at least twelve standard test methods (for example, Test Methods C1111,
D1976, D4951, D5184, D5185, D5600, D5708, D6130, D6349, D6357, D7040, D7111, D7303, and D7691) published for the
analysis of fossil fuels and related materials. These have been briefly summarized by Nadkarni (1).
4.2.1 Determination of mercury and trace metals in crude oils using atomic spectroscopic methods is discussed in Guide D8056.
4.3 The advantages of using an ICP-AES analysis include high sensitivity for many elements of interest in the oil industry,
relative freedom from interferences, linear calibration over a wide dynamic concentration range, single or multi-element capability,
and ability to calibrate the instrument based on elemental standards irrespective of their elemental chemical forms, within limits
described below such as solubility and volatility assuming direct liquid aspiration. Thus, the technique has become a method of
choice in most of the oil industry laboratories for metal analyses of petroleum products and lubricants.
4.4 In addition to the ICP-AES standards listed in 2.2, a new ICP-MS standard, Test Method D8110, has been issued for
analysis of distillate products for multi-element determination of Al, Ca, Cu, Fe, Pb, Mg, and K.
5. Apparatus
5.1 Spectrometer—An inductively coupled plasma emission spectrometer with a spectral bandpass of 0.05 nm or less is
required. The spectrometer may be of the simultaneous multi-elemental or sequential scanning type. The spectrometer may be of
the air path, inert gas path, or vacuum type, with spectral lines selected appropriately for use with specific instrument. Either an
analog or digital readout system may be used.
5.2 An ICP-AES instrument system is typically comprised of several assemblies including a radio-frequency (RF) generator,
an impedance matching network (where required), an induction coil, a plasma torch, a plasma igniter system, a sample introduction
Available from Energy Institute, 61 New Cavendish St., London, W1G 7AR, U.K., http://www.energyinst.org.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
The boldface numbers in parentheses refer to a list of references at the end of this standard.
D7260 − 20
system, a light gathering optic, an entrance slit and dispersing element to separate and measure the intensity of the wavelengths
of light emitted from the plasma, one or more devices for converting the emitted light into an electrical current or voltage, one
or more analog preamplifiers, one or more analog-to-digital converter(s), and a dedicated computer with printer. Solid state CCD
or CID detectors if used may not require extra analog-to-digital components. Recently modern camera-type instruments have been
supplanting the photomultiplier tube type detectors. Cameras may not have high resolution, but they offer greater wavelength
choice.
5.2.1 Plasma can be monitored either axially versus radially. Potential for improved sensitivity as much as tenfold is often
realized with axial monitoring. However, the increased interference from molecular background may compromise these gains
depending on the wavelength monitored and matrix used (especially for organics versus aqueous).
5.2.2 Echelle Spectrometers—More recently echelle gratings are being increasingly used in several commercial plasma
spectrometers. A prism is used as an order-sorter to improve sensitivity. To measure widely separated lines with useful efficiency,
echelle instruments have to be operated in many different orders. This involves complex wavelength scanning programs for
computer controlled echelle monochromators. While the resolution of a grating monochromator is relatively constant across its
working range, practical resolution of an echelle monochromator can vary considerably with wavelength. Inherently higher
theoretical resolving power of the echelle when used in high order, relative to the diffraction grating used in the first order, allows
a relatively compact echelle instrument to achieve high resolving power. The detection limits obtained with echelle plasma
spectrometers are comparable to those achieved by grating spectrometers.
5.3 Spectrometer Environment:
5.3.1 Temperature fluctuations affect the instrument stability. Some manufacturers provide systems for maintaining a constant
internal temperature within the optical compartment and sample introduction area that assumes changes in the outside temperature
are not being controlled within the necessary specified range and rate of change to insure stability. Other manufacturers design their
spectrometers to be stable over a specified temperature range without attempting to control the spectrometer’s internal temperature.
5.3.2 Since temperature and humidity changes may also affect the sample introduction system, detectors, and electronic readout
as well as the spectrometer alignment, some manufacturers specify that care be used in selecting a location for the spectrometer
that experiences minimal variation in temperature and relative humidity. The user needs to provide a controlled environment as
specified by the manufacturer. This is a very important factor in optimum performance of an ICP-AES system.
5.3.3 The generator output power and the plasma gas flow determine the plasma temperature and thus significantly influence
the emission signal and the background. Thus, the power applied and gas flow adjustments may be used to control the signal to
background ratio and, matrix, and some spectral interferences.
5.4 Optical Path:
5.4.1 Since oxygen exhibits increasing absorbance with decreasing wavelengths below 200 nm, the performance of an air path
instrument degrades below that wavelength and is generally not useful below approximately 190 nm.
5.4.2 Purging the optical path with nitrogen or argon, or another gas with low absorption in this ultraviolet region may extend
the spectral region to wavelengths below 167 nm. Use of these purge gases is in general less expensive to maintain than the vacuum
path systems. Sealed optics filled with an inert gas is also available for such work.
5.5 Wavelength Selection:
5.5.1 When selecting the fixed position wavelengths to be utilized in a Paschen-Runge polychromator for particular
applications, close collaboration between user and instrument manufacturer is critical. Camera instruments do not have this
problem.
5.5.2 If possible, use the peak and background wavelengths suggested in the methods. When there is a choice such as with the
sequential instruments, choose the wavelength that will yield signals of 100× to 1000× the detection limit sought. Also, ensure that
the chosen wavelength will not be interfered with from unexpected elements. See Section 6.
5.5.3 Often ion lines may be chosen for use over atom lines to avoid interelement interference and sensitivity of detection. This
choice will be dependent on the analyte of interest and the sample matrix being analyzed.
5.6 Peristaltic Pump—Differences in the viscosities of the test specimen solutions and standard solutions can cause differences
in the uptake rates adversely affecting the accuracy of the analysis. These effects can be minimized by using a peristaltic pump
(or an internal standard). If a peristaltic pump is used, inspect the pump tubing and replace it, if necessary, before starting each
day. Verify the solution uptake rate daily and adjust to the desired rate. Compatibility of the solution with the peristaltic pump
tubing must also be confirmed to prevent premature pump failure. A variety of polymeric material options are available for pump
tubing to address this concern by simple empirical testing with the given solvent/sample matrix used. Generally speaking, the
selected tubing soaked overnight in the solvent/sample matrix should not soften, crack, or embrittle the tubing.
5.7 Depending on the nebulizer design, starving solvent flow (that is, flow below natural free aspiration rates) can have an effect
on aerosol generation. Consistent flow can help achieve enhanced analyte sensitivity.
5.8 Torch—Inspect the torches before use for cracks and discarded or repaired as appropriate. Clean torches that are free of
carbon buildup should be used. The load coil should be replaced or cleaned if oxidation or leaking of coolant is observed. The glass
also can devitrify especially at the aerosol tip with oxygen injection.
D7260 − 20
5.9 Safety—The ICP-AES instrument is not normally considered as a hazardous instrument. However, appropriate precautions
should be taken regarding the fumes, heat, and UV/visible light radiation as well as appropriate RF shielding. The equipment
should always be used according to the manufacturer’s operating instructions. No attempt should be made to bypass the interlocks.
Adequate cooling times must be allowed before handling any hot components. Any safety covers must be in position.
5.9.1 Fumes from the plasma and any ozone generated by the UV radiation must be removed by means of a suitable heat and
acid resistant (acids can be formed from halogens or sulfate and nitrates in the solution) chimney fitted with an exhaust fan of
sufficient capacity.
5.9.2 A UV/visible light absorbing viewing window (with RF shielding) must always be in place to protect the eyes and skin
of the operator from radiation.
5.9.3 Often the organic samples and solvents used in organic ICP-AES analysis are toxic and hazardous. All appropriate
precautions must be taken in handling such materials to protect the operators. Consult MSDS and other safety information before
handling these chemicals.
6. Interferences
6.1 Several types of interference effects may contribute to inaccuracies in the elemental determination using ICP-AES.
Principally these interferences can be classifies as spectral, physical, and chemical.
6.2 Spectral Interferences:
NOTE 1—An empirical method for correcting spectral interferences is detailed in Test Method D5185.
6.2.1 Spectral interferences can be categorized as (1) unresolved overlap of a spectral line from another element, (2) unresolved
overlap of molecular band spectra, (3) background contribution from continuous or recombination phenomena, and (4) background
contribution from stray light from line emission of high concentration of elements. With echelle spectrometers it may be possible
to look at two or more lines to identify interference.
6.2.2 Interelement Interferences—This interference can be compensated for by computer correction of the raw data, which
requires measurement of the interfering element at the wavelength of interest. Various analytical systems may exhibit somewhat
different levels of interferences. Therefore, the interference effects must be evaluated for each individual system. Perhaps a 2 %
maximum correction may be acceptable.
6.2.2.1 Potential spectral overlaps from concomitant elements may be estimated by measuring the signal arising from a
high-purity single-element reference solution of the concomitant element. It is useful to consult tables of spectral lines to become
aware of possible overlaps, especially when analyzing samples of unknown composition (Refs 2-7). The overlaps may appear at
the measured wavelength peak or at one of the two background points selected, thus prompting correction on only one side of the
peak if the interferant is suspected present. If the overlap is severe (for example, 50 %), alternate line selection may be indicated
to minimize spectral interferences. Judicious selection of background correction points may also prevent potential interferences
affecting analyte quantification from interfering concomitant elements. It is recommended that multiple high purity solutions be
used to confirm consistency of the suspected spectral source of a potential interference. Some analyte elements may be difficult
to remove in manufacturing a high purity solution for the suspected interfering element, especially for organometallic standard
solutions.
6.2.2.2 Potential interferences should be considered in the line selection process for polychromators. With sequential
instruments, it may be desirable to select an alternate line to avoid spectral overlaps even though the sensitivity of the alternate
line may be lower. This issue should be carefully considered for selection of fixed wavelength options for applicable polychromator
when purchased.
6.2.2.3 There is also the possibility of spectral overlap from an element that is not being determined. With simultaneous
instruments, it may be necessary to install additional hardware to correct for concomitant elements or to allow determination of
a given element at two or more wavelengths. Sequential instruments permit measurement of other lines of interfering elements to
allow correction of their contributions at the analytical wavelength.
6.2.3 Interelement Interferences—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 in Test Method D5185.
Further details of the empirical method are given in the Test Method C1109 and by Boumans (6). This empirical 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 must always be alert to the possible presence of unexpected elements producing
interfering spectral lines.
6.2.3.1 The empirical method of spectral interference correction uses interference correction factors that are determined by
analyzing single-element high-purity solutions under conditions matching as closely as possible those used for test specimen
solution analysis. Unless plasma conditions can be accurately reproduced from day to day, and sample matrix to sample matrix,
or for longer periods, interference correction factors found to significantly affect the results must be determined each time
specimens are analyzed. One way to accurately do this is to daily monitor the ratio of Mg I to Mg II lines, or copper to magnesium
ratio.
D7260 − 20
6.2.4 Molecular Band Interferences—These arise from overlap of molecular band spectra at the wavelength of interest, and can
be eliminated by careful selection of wavelength.
6.2.5 High Background Interferences—These can be compensated for by background correction adjacent to the analyte line.
Signal to background ratios should be of the order of at least 3 to 5 factor.
6.2.5.1 Wavelength should be selected with best intensity, peak shape, and lack of interferences. When analysis is being done
for elements at trace levels, background correction is required. Thus, for all elements possible, the baseline for the emission peak
should be set with points as close to both sides of the peak without measuring the element wavelength intensity (see Figure 1 of
Test Method D7111). After these baselines are set, a check standard should be used to test the system response and calibration.
6.2.5.2 Appropriate selection of wavelengths for background corrections is extremely critical for the determination of some
elements. For example, since the predominate sodium emission line (588.995 nm) resides near a significant argon emission
interference, it is recommended, if possible, compare the spectra of the sample and the standards at the sodium emission
wavelength to ensure that the signal integration occurs accurately.
6.2.5.3 When emission wavelengths occur on a highly structured background (as in the example of sodium line above), a single
off-peak background measurement may not provide accurate results. For such emission wavelengths on a structured background,
background correction is recommended at both lower and higher wavelengths from the emission wavelengths. Additionally, some
low resolution, photo-multiplier tube-based instruments may require a comprised selection of background points, and this could
also provide inaccurate results.
6.3 Physical Interferences—These are generally considered effects associated with the sample nebulization and transport
processes. Such properties as change in viscosity and surface tension can cause significant inaccuracies, especially in samples that
may contain high dissolved solids or acid concentrations (such as in aqueous ICP-AES analysis), or both. Use of a high-solid
Babington type nebulizer (although optional in most methods) is highly recommended for avoiding plugging, particularly when
analyzing used oils (which would contain particulates). The use of a peristaltic pump may also lessen these interferences. If these
types of interferences are operative, they may be reduced by dilution (for example, tenfold) of these samples or utilization of
standard addition techniques, or both. However, dilution or standard addition will not compensate for volatilization of some
species.
6.3.1 The torch aperture constriction is as much a problem with carbon soot for organics as it is for salt deposits in aqueous
solutions that generally have the higher dissolved (as opposed to suspended) solids content. Also, for organics the matrix effects
are exacerbated relative to aqueous matrix. The inherent diversity in organic matrix solution characteristics commonly impact
transport more significantly due to volatility, polarity, surface tension, and so forth, as well as viscosity. Without a doubt, analyte
volatility will most significantly impact transport effects and accuracy of the analysis.
6.3.2 Suspended analyte and other wear particles in the used oil also cause problem in getting accurate analysis. It is critical
to keep the sample solution homogenized to get good results. Often the used oil samples are mixed using ultrasonic bath or a vortex
mixer before actual ICP-AES measurements; however, the time lag between such homogenization and ICP-AES measurement
should be kept to a minimum so that the particles do not settle out during the waiting period. Thus, for such samples autosampler
may not be useful.
6.3.3 To minimize nebulizer transport effects caused by high viscosity oils or viscosity improvers and additives in the oil, and
to reduce potential spectral interferences, dilute the samples and standards as appropriate to minimize transport effects (minimum
tenfold dilution). Both calibration standards and sample solutions should not contain more than 10 mass % 10 % by mass oil. The
calibration standards should be prepared with analyte-free oil added to the solution matrix such that all solutions contain the same
mass % oil. Also, when diluting samples to greater dilution factors, consistent oil to solvent ratio should be maintained. See 7.2
about the dilution solvents.
6.3.4 Use of internal standard(s) could also be used in place of dilution or standard additions. With samples containing acids,
the biggest variation can actually be observed at low acid concentrations. A common solution to this problem is to ensure that all
samples and standards contain the same acid concentration (for example, 2 % nitric acid).
6.3.5 Viscosity Index Improver Effect—Viscosity index improvers that may be present in multi-grade lubricating oils, can bias
the measurements (8). However, these biases can be reduced to negligible proportion by using the above specified
solvent-to-sample dilution or an internal standard, or both. Following internal standards have been successfully used in the
laboratories: Ag, Be, Cd, Co (most common), La, Mn, Pb, Sc, and Y. Often a viscosity index improver is included in the diluting
solution to help reduce the effect on multi-grade oils.
6.3.6 The use of an internal standard assumes the sample to contain essentially no quantity of this internal standard element (see
8.8.4). This can be monitored by several means beyond analysis of the sample beforehand to ascertain the un-doped internal
standard concentration. This may include the monitoring of multiple internal standard elements or comparison of the expected
magnitude of correction imposed by the internal standard relative to a known or blank solution analyzed as a sample. Also, the
internal standard must be analyte-free as well as free of concomitant species that may impose a spectral interference on the analyte
lines applied.
6.4 Chemical Interferences:
6.4.1 Chemical interferences are caused by molecular compound formation, ionization effects, and thermochemical effects
associated with sample vaporization and atomization in the plasma. Normally these effects are not pronounced and can be
D7260 − 20
minimized by careful selection of operating conditions such as incident power, plasma observation position, and so forth, by matrix
matching, and by standard addition procedures. These types of interferences can be highly dependent on matrix type and the
specific analyte.
6.4.2 Selective volatilization can occur when the analyte is present in the sample in a relatively volatile form and can vaporize
during the nebulization process. Whereas the transport efficiency of droplets produced from a standard pneumatic nebulizer is
typically around 1 % or 2 %, for vapor it can be much higher giving rise to enhanced results that can vary depending on the relative
concentrations of various chemical species present in the sample (for example, volatile sulfur species such as H S, lead alkyls, and
some silicon compounds).
6.5 Salt buildup at the tip of the nebulizer can occur from high dissolved solids (for example, in solutions prepared by alkali
fusion with subsequent dilution with acids). This salt buildup affects aerosol flow rate that can cause instrumental drift. To control
this problem, in cases of aqueous analysis argon should be wetted prior to nebulization, using a tip washer, or by diluting the
sample.
6.6 Carbon Buildup—Inspect the torch for carbon build-up during the warm up period. If it occurs, replace the torch
immediately and consult the manufacturer’s operating guide to take proper steps to remedy the situation.
6.6.1 Carbon that accumulates on the tip of the torch injector tube can be removed by using nebulizer gas that consists of
approximately 1 % oxygen in argon. However,
...