ISO 16962:2017

INTERNATIONAL ISO
STANDARD 16962
Second edition
2017-02
Surface chemical analysis — Analysis
of zinc- and/or aluminium-based
metallic coatings by glow-discharge
optical-emission spectrometry
Analyse chimique des surfaces — Analyse des revêtements métalliques
à base de zinc et/ou d’aluminium par spectrométrie d’émission
optique à décharge luminescente
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus . 2
5.1 Glow-discharge optical-emission spectrometer . 2
5.1.1 General. 2
5.1.2 Selection of spectral lines . 2
5.1.3 Selection of glow-discharge source type . 2
6 Adjusting the glow-discharge spectrometer system settings . 3
6.1 General . 3
6.2 Setting the parameters of a DC source . 4
6.2.1 Constant applied current and voltage . 4
6.2.2 Constant applied current and pressure . 5
6.2.3 Constant voltage and pressure . 5
6.3 Setting the discharge parameters of an RF source . . 6
6.3.1 General. 6
6.3.2 Constant applied power and pressure . 6
6.3.3 Constant applied power and DC bias voltage . 6
6.3.4 Constant effective power and effective RF voltage . 7
6.4 Minimum performance requirements . 7
6.4.1 General. 7
6.4.2 Minimum repeatability . . 7
6.4.3 Detection limit . 8
7 Sampling . 9
8 Calibration . 9
8.1 General . 9
8.2 Calibration samples .10
8.2.1 General.10
8.2.2 Brass calibration samples .10
8.2.3 Zn-Al alloy samples .10
8.2.4 Low alloy iron or steel samples.10
8.2.5 Stainless steel samples .10
8.2.6 Nickel alloy samples . .10
8.2.7 Aluminium-silicon alloy samples .10
8.2.8 Aluminium-magnesium alloy samples.10
8.2.9 High-purity copper and zinc samples .11
8.3 Validation samples and optional RMs for calibration .11
8.3.1 General.11
8.3.2 Zinc-nickel electrolytically coated RM .11
8.3.3 Zinc-iron electrolytically coated RM .11
8.3.4 Zinc-aluminium hot dip coated RM .11
8.3.5 Zinc-iron hot dip coated and annealed RM .11
8.4 Determination of the sputtering rate of calibration and validation specimens .11
8.5 Emission intensity measurements of calibration specimens .13
8.6 Calculation of calibration equations .13
8.7 Validation using reference materials .13
8.7.1 General.13
8.7.2 Checking analytical accuracy using bulk reference materials .13
8.7.3 Checking analytical accuracy using surface layer reference materials .14
8.8 Verification and drift correction .14
9 Analysis of test specimens .15
9.1 Adjusting discharge parameters .15
9.2 Setting of measuring time and data acquisition rate .15
9.3 Quantifying depth profiles of test specimens .15
10 Expression of results .15
10.1 Expression of quantitative depth profile .15
10.2 Determination of total coating mass per unit area (coating aeric mass) .17
10.2.1 General method . .17
10.2.2 Method for special applications .17
10.3 Determination of average mass fractions .17
11 Precision .17
12 Test report .18
Annex A (normative) Calculation of calibration constants and quantitative evaluation of
depth profiles .19
Annex B (informative) Suggestions concerning suitable spectral lines .31
Annex C (informative) Determination of coating mass per unit area (coating areic mass) .32
Annex D (informative) Additional information on international cooperative tests .38
Bibliography .40
iv © ISO 2017 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 8, Glow discharge spectroscopy.
This second edition cancels and replaces the first edition (ISO 16962:2005), which has been technically
revised.
Introduction
This document is a revision of ISO 16962. Developments in both GD-OES instrumentation and the types
of zinc- and/or aluminium-based metallic coatings currently produced have rendered ISO 16962 partly
obsolete, and this revision is intended to bring it up to date.
vi © ISO 2017 – All rights reserved

INTERNATIONAL STANDARD ISO 16962:2017(E)
Surface chemical analysis — Analysis of zinc- and/or
aluminium-based metallic coatings by glow-discharge
optical-emission spectrometry
1 Scope
This document specifies a glow-discharge optical-emission spectrometric method for the determination
of the thickness, mass per unit area and chemical composition of metallic surface coatings consisting of
zinc- and/or aluminium-based materials. The alloying elements considered are nickel, iron, silicon, lead
and antimony.
This method is applicable to zinc contents between 0,01 mass % and 100 mass %; aluminium contents
between 0,01 mass % and 100 mass %; nickel contents between 0,01 mass % and 20 mass %; iron
contents between 0,01 mass % and 20 mass %; silicon contents between 0,01 mass % and 15 mass %;
magnesium contents between 0,01 mass% and 20 mass%; lead contents between 0,005 mass % and
2 mass %, antimony contents between 0,005 mass % and 2 mass %.
NOTE Due to environmental and health risks, lead and antimony are avoided nowadays, but this document
is also applicable to older products including these elements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 14284, Steel and iron — Sampling and preparation of samples for the determination of chemical
composition
ISO 17925, Zinc and/or aluminium based coatings on steel — Determination of coating mass per unit area
and chemical composition — Gravimetry, inductively coupled plasma atomic emission spectrometry and
flame atomic absorption spectrometry
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
4 Principle
The analytical method described here involves the following processes:
a) preparation of the sample to be analysed, generally in the form of a flat plate or disc of dimensions
appropriate to the instrument or analytical requirement (round or rectangular samples with a
width of more than 5 mm, generally 20 mm to 100 mm, are suitable);
b) cathodic sputtering of the surface coating in a direct current or radio frequency glow-discharge
device;
c) excitation of the analyte atoms in the plasma formed in the glow-discharge device;
d) spectrometric measurement of the intensities of characteristic emission spectral lines of the
analyte atoms and ions as a function of sputtering time (qualitative depth profile);
e) conversion of the depth profile in units of intensity versus time to mass fraction versus depth by means
of calibration functions (quantification). Calibration of the system is achieved by measurements on
calibration samples of known chemical composition and measured sputtering rate.
5 Apparatus
5.1 Glow-discharge optical-emission spectrometer
5.1.1 General
The required instrumentation includes an optical-emission spectrometer system consisting of a
[1]
Grimm type or similar glow-discharge source (direct current or radio frequency powered) and a
simultaneous optical spectrometer as described in ISO 14707, capable of providing suitable spectral
lines for the analyte elements. It is also common to combine this with a sequential spectrometer
(monochromator), allowing the addition of an extra spectral channel to a depth profile measurement.
An array-type detector, such as a charge coupled device (CCD) or a charge injection device (CID) can
also be used for simultaneous detection to cover a wide spectral range of the analytical lines.
The inner diameter of the hollow anode of the glow-discharge source should be in the range 2 mm to
8 mm. A cooling device for thin specimens, such as a metal block with circulating cooling liquid, is also
recommended, but not strictly necessary for implementation of the method.
Since the principle of determination is based on continuous sputtering of the surface
layer, the spectrometer shall be equipped with a digital readout system for time-resolved
measurement of the emission intensities. A system capable of a data acquisition speed of at least
300 measurements/second per spectral channel is recommended, but for a large number of applications
speeds of > 50 measurements/second per spectral channel are acceptable. In practice, it has been
established that 10 to 100 measurements/second per spectral channel are suitable.
5.1.2 Selection of spectral lines
For each analyte to be determined, there exist a number of spectral lines which can be used. Suitable
lines shall be selected on the basis of several factors, including the spectral range of the spectrometer
used, the analyte mass fraction range, the sensitivity of the spectral lines and any spectral interference
from other elements present in the test specimens. For applications where several of the analytes
of interest are major elements in the specimens, special attention shall be paid to the occurrence of
self-absorption of certain highly sensitive spectral lines (so-called “resonance lines”). Self-absorption
causes non-linear calibration curves at high analyte mass fraction levels, and strongly self-absorbed
lines should therefore be avoided for the determination of major elements. Suggestions concerning
suitable spectral lines are given in Annex B. Spectral lines other than those listed may be used, so long
as they have favourable characteristics.
5.1.3 Selection of glow-discharge source type
5.1.3.1 Anode size
Most GD-OES instruments on the market are delivered with options to use various anode diameters,
2 mm, 4 mm and 8 mm being the most common. Some older instruments have one anode only, usually
8 mm, while the most commonly used anode in modern instruments is 4 mm. A larger anode requires
larger specimens and higher power during analysis; therefore, the specimen is heated to a greater
extent. On the other hand, a larger anode gives rise to a plasma of larger volume that emits more light,
resulting in lower detection limits (i.e. higher analytical sensitivity). Furthermore, a larger anode helps
2 © ISO 2017 – All rights reserved

to mask inhomogeneity within a surface layer. This may or may not be an advantage, depending on
the application. In a large number of applications, the 4 mm anode is a good compromise. However,
in surface analysis applications, it is rather common to encounter problems of overheating of the
specimens due to surface layers of poor heat conductivity and/or very thin specimens, for example. In
such cases, a smaller anode (typically 2 or 2,5 mm) is preferable, even if there is some loss of analytical
sensitivity.
5.1.3.2 Type of power supply
The glow-discharge source can be either a type powered by a direct current (DC) power supply or a radio
frequency (RF) type. The most important difference is that the RF type can sputter both conductive
and non-conductive specimens; hence, this is the only type that can be used for polymer coatings and
insulating oxide layers, for example. On the other hand, it is technically simpler to measure and control
the electrical source parameters (voltage, current, power) of a DC type. Several commercially available
GD-OES systems can be delivered with the option to switch between DC and RF operation, but RF-only
systems are becoming increasingly common. In short, there are a very large number of applications
where DC or RF sources can be used and several where only an RF source can be used.
5.1.3.3 Mode of operation
Both DC and RF sources can be operated in several different modes with respect to the control of the
electrical parameters (current, voltage, power) and the pressure. There are several reasons for this:
— “historical” reasons (older instruments have simpler but functional power supplies, while the
technology has evolved, so newer models have more precise and easier-to-operate source control);
— different manufacturers have chosen different solutions for source control;
— there are some application-related issues where a particular mode of operation is to be preferred.
This document gives instructions for optimizing the source parameters based on several available
modes of operation. The most important reason for this is to make these instructions comprehensive
so as to include several types of instruments. In most applications, there is no major difference
between these modes in terms of analytical performance, but there are other differences in terms of
practicality and ease of operation. For instance, a system equipped with active pressure regulation will
automatically be adjusted to the same electrical source parameters every time a particular analytical
method is used. Without this technology, some manual adjustment of the pressure to achieve the
desired electrical source parameters is normally required.
[2][3][4][7]
NOTE In this context, what is known as the emission yield forms the basis for calibration and
quantification as described in this document. The emission yield has been found to vary with the current, the
[4][7]
voltage and, to a lesser extent, the pressure . It is impossible in practice to maintain all three parameters
constant for all test specimens, due to variations in the electrical characteristics of different materials. In several
instrument types, the electrical source parameters (the plasma impedance) can therefore be maintained constant
by means of automatic systems that vary the pressure during analysis. Alternatively, there exist methods to
[4][7]
correct for impedance variations by means of empirically derived functions , and this type of correction is
implemented in the software of commercially available GD-OES systems.
6 Adjusting the glow-discharge spectrometer system settings
6.1 General
Follow the manufacturer’s instructions or locally documented procedures for preparing the instrument
for use.
For the optical system, the most important preparation step is to check that the entrance slit to the
spectrometer is correctly adjusted, following the procedure given by the instrument manufacturer.
This ensures that the emission intensities are measured on the peaks of the spectral lines for optimum
signal-to-background ratio and good reproducibility. For further information, see ISO 14707.
The most important step in developing a method for a particular application is to optimize the
parameters of the glow-discharge source. The source parameters shall be chosen to achieve three aims:
a) adequate sputtering of the test specimen, to reduce the analysis time without overheating the
specimen;
b) good crater shape, for good depth resolution;
c) constant excitation conditions in calibration and analysis, for optimum accuracy.
Trade-offs are often necessary among the three specified aims. More detailed instructions on how to
adjust the source parameters are given in 6.2 and 6.3.
The settings of the high voltage for the detectors depend on the source parameters, but the procedure
is the same for all modes of operation of the source. This procedure is therefore only described for the
first mode of operation.
Similarly, the steps to adjust and optimize the source settings in terms of signal stability and sputter
crater shape are also similar in principle for all modes of operation. Therefore, these procedures are
only described in detail for the first mode of operation.
NOTE There is no difference between DC and RF concerning the possibilities to measure the pressure.
However, there are large pressures differentials in a Grimm type source, and pressure readings obtained depend
on the location of the pressure gauge. Some instrument models have a pressure gauge attached to measure the
actual pressure in the plasma, while others have a pressure gauge located on a “low pressure” side of the source
closer to the pump. Therefore, the pressure readings can, for several instruments, just be used to adjust the source
parameters of that particular instrument, not as a measure of the actual operating pressure in the plasma.
6.2 Setting the parameters of a DC source
6.2.1 Constant applied current and voltage
6.2.1.1 General
The two control parameters are the applied current and the applied voltage. Set the power supply for
the glow-discharge source to constant current/constant voltage operation (current set by the power
supply, voltage adjusted by pressure/gas flow regulation). Then, set the current and voltage to the
typical values recommended by the manufacturer. Alternatively, set the power supply to constant
voltage/constant current operation (voltage set by the power supply, current adjusted by pressure/gas
flow regulation). If no recommended values are available, set the voltage to 700 V and the current to
a value in the range 5 mA to 10 mA for a 2 mm or 2,5 mm anode, 15 mA to 30 mA for a 4 mm anode
or 40 mA to 100 mA for a 7 mm or 8 mm anode. If no previous knowledge of the optimum current is
available, it is recommended to start with a value somewhere in the middle of the recommended range.
NOTE For the purposes of this document, there is no difference between the two alternative modes of
operation described above. However, for applications to very thin films, there may be a small difference in the
very short start-up of the discharge, affecting the analytical results to some extent.
6.2.1.2 Setting the high voltage of the detectors
Select test specimens with surface layers of all types to be determined. For all test specimens, run the
source while observing the output signals from the detectors for the analyte atoms. Adjust the high
voltage of the photomultiplier (PMT) detectors in such a way that sufficient sensitivity is ensured at
the lowest analyte mass fraction without saturation of the detector system at the highest analyte mass
fraction. For array type detectors (CCD or CID), adjust the integration time in the same way as the high
voltage for PMT.
4 © ISO 2017 – All rights reserved

6.2.1.3 Adjusting the source parameters
For each type of test specimen, carry out a full-depth profile measurement, sputtering it in the glow
discharge for a sufficiently long time to remove the surface layers completely and continue well into the
base material. By observing the emission intensities as a function of sputtering time (often referred to
as the “qualitative depth profile”), verify that the selected source settings give stable emission signals
throughout the depth profile and into the substrate. If this is found not to be the case, reduce one of the
control parameters by a small amount and sputter through the surface layers again. If the stability is still
unsatisfactory, reduce the other control parameter by a small amount and repeat the measurements. If
found necessary, repeat this procedure for a number of control parameter combinations until stable
emission conditions are obtained.
NOTE Unstable emission signals could indicate thermal instability in the specimen surface layers; specimen
cooling is beneficial in this regard.
6.2.1.4 Optimizing the crater shape
If a suitable profilometer device is available, adopt the following procedure. Sputter a specimen with a
surface layer typical of the test specimens to be analysed to a depth of about 10 µm to 20 µm, but still
inside the surface layer. This is only possible for application where surface layers of such thickness are
available. If no such specimen is available, use a steel or brass specimen. Measure the crater shape by
means of the profilometer device. Repeat this procedure a few times using slightly different values of
one of the control parameters. Select the conditions that give an optimally flat-bottomed crater. These
conditions are then used during calibration and analysis, provided that the stability of the emission
conditions obtained in step 6.2.1.3 is not compromised. In some cases, there is a certain trade-off
between these two requirements.
6.2.2 Constant applied current and pressure
The two control parameters are the applied current and the pressure. Set the power supply for the glow-
discharge source to constant current operation. Then, set the current to a typical value recommended
by the manufacturer. If no recommended values are available, set the current to a value in the range
5 mA to 10 mA for a 2 mm or 2,5 mm anode, 15 mA to 30 mA for a 4 mm anode or 40 mA to 100 mA for a
7 mm or 8 mm anode. If no previous knowledge of the optimum current is available, it is recommended
to start with a value somewhere in the middle of the recommended range. Sputter a typical coated test
specimen, and adjust the pressure until a voltage of approximately 700 V is attained in the surface layer.
Set the high voltage of the detectors as described in 6.2.1.2.
Adjust the source parameters as described in 6.2.1.3, adjusting first the current and, if necessary, the
pressure.
Optimize the crater shape as described in 6.2.1.4 by adjusting the pressure. These conditions are then
used during calibration and analysis.
NOTE Before sputtering a new specimen type, make a test run in order to ensure that the voltage has not
changed by more than 5 % from the previously selected value. If this is the case, readjust the pressure until the
correct value is attained.
6.2.3 Constant voltage and pressure
The two control parameters are applied voltage and pressure. Set the power supply for the glow-
discharge source to constant voltage operation. First, set the voltage to a typical value recommended
by the manufacturer. If no recommended values are available, set the voltage to 700 V. Sputter a typical
coated test sample, and adjust the pressure until a current is attained in the range 5 mA to 10 mA for
a 2 mm or 2,5 mm anode, 15 mA to 30 mA for a 4 mm anode, 40 mA to 100 mA for a 7 mm or 8 mm
anode in the surface layers. If no previous knowledge about the optimum current is available, it is
recommended to start with a value somewhere in the middle of the recommended range.
Set the high voltage of the detectors as described in 6.2.1.2.
Adjust the source parameters as described in 6.2.1.3, adjusting first the voltage and, if necessary, the
pressure.
Optimize the crater shape as described in 6.2.1.4, by adjusting the pressure. These conditions are then
used during calibration and analysis.
NOTE Before sputtering a new sample type, make a test run in order to ensure that the current is not altered
more than 5 % from the previously selected value. If this is the case, readjust the pressure until the correct value
is attained.
6.3 Setting the discharge parameters of an RF source
6.3.1 General
The most common operating modes of RF sources are the following: constant applied power and
constant pressure; constant applied voltage and constant pressure; or constant effective power and
applied voltage. All RF operational modes are allowed in this document, provided they meet the three
aims listed in 6.1. In the following, separate instructions are provided on how to set the parameters for
the different operational modes.
NOTE RF sources differ from DC sources in the respect that for several instrument models, only the applied
(forward) RF power can be measured, not the actual power developed in the glow-discharge plasma. The applied RF
power is normally in the range 10 W to 100 W, but it must be noted that the RF power losses in connectors, cables,
etc. vary considerably between different instrument models and the point of contact of the RF power to the sample.
Typical power losses are in the range 10 % to 50 % of the applied power. Furthermore, the possibilities to measure
the additional electrical parameters voltage and current in the plasma are more or less restricted due to technical
difficulties with RF systems, and several existing instrument models can only measure the applied RF power.
6.3.2 Constant applied power and pressure
The two control parameters are the applied power and the pressure. First, set the applied power and
adjust the source pressure to the values suggested by the manufacturer. If recommended values are not
available, set the applied power and pressure to somewhere in the middle of the ranges available for the
instrument used. Measure the penetration rate (i.e. depth per unit time) on an iron or steel specimen,
adjusting the power to give a penetration rate of about 2 µm/min to 3 µm/min.
Set the high voltage of the detectors as described in 6.2.1.2.
Adjust the source parameters as described in 6.2.1.3, adjusting first the applied power and, if necessary,
the pressure.
Optimize the crater shape as described in 6.2.1.4 by adjusting the pressure.
Remeasure the penetration rate on the iron or steel specimen and adjust the applied power, if necessary,
to return to about 2 µm/min to 3 µm/min. Repeat the cycle of power and pressure adjustment until no
significant change is noted in the penetration rate or crater shape. Note the power and pressure used in
units provided for the instrument type. These conditions are then used during calibration and analysis.
6.3.3 Constant applied power and DC bias voltage
The two control parameters are the applied power and the DC bias voltage. First, set the applied power
and adjust the source pressure to attain a DC bias typical of the values suggested by the manufacturer.
If recommended values are not available, set the applied power and DC bias voltage to somewhere in the
middle of the range commonly used for depth profiling of metal specimens. On instruments equipped
with active pressure control, this can be achieved automatically. Measure the penetration rate (i.e.
depth per unit time) on an iron or steel specimen, adjusting the power to give a penetration rate of
about 2 µm/min to 3 µm/min.
Set the high voltage of the detectors as described in 6.2.1.2.
6 © ISO 2017 – All rights reserved

Adjust the discharge parameters as described in 6.2.1.3, adjusting first the applied power and, if
necessary, the DC bias voltage.
Optimize the crater shape as described in 6.2.1.4 by adjusting the DC bias voltage.
Remeasure the penetration rate on the iron or steel specimen and adjust the applied power, if necessary,
to return to about 2 µm/min to 3 µm/min. Repeat the cycle of power and DC bias voltage adjustment
until no significant change is noted in the penetration rate or in the crater shape. If this is not the case,
readjust the DC bias voltage until the correct value is attained. Note the power and DC bias voltage used
in units provided for the instrument. These conditions are then used during calibration and analysis.
6.3.4 Constant effective power and effective RF voltage
The two control parameters are the effective power and the RF voltage. Constant effective power is
defined here as the applied power minus the reflected power and the “blind power” measured with the
specimen in place but without plasma (vacuum conditions). The RF voltage is defined here as the RMS
[8][9]
voltage at the coupling electrode without DC bias .
Set the power supply for the glow-discharge source to constant effective power/constant RF voltage
operation. First, set the power to a typical value recommended by the manufacturer. If no recommended
values are available, set the RF voltage to 700 V and the power to a value in the range 10 W to 15 W
for a 4 mm anode, 5 to 10 W for a 2 mm anode to give an example. If no previous knowledge of the
optimum power is available, it is recommended to start with a value somewhere in the middle of the
recommended range.
Set the high voltage of the detectors as described in 6.2.1.2.
Adjust the discharge parameters as described in 6.2.1.3, adjusting first the effective power and, if
necessary, the RF voltage.
Optimize the crater shape as described in 6.2.1.4 by adjusting the RF voltage. Select the conditions that
give an optimally flat-bottomed crater. These conditions are then used during calibration and analysis.
6.4 Minimum performance requirements
6.4.1 General
It is desirable for the instrument to conform to the performance specifications given in 6.3.2 and
6.3.3 below.
NOTE Setting up for analysis commonly requires an iterative approach to the adjustment of the various
instrumental parameters described in this document.
6.4.2 Minimum repeatability
The following test shall be performed in order to check that the instrument is functioning properly in
terms of repeatability.
Perform 10 measurements of the emission intensity on a homogeneous bulk specimen with a content of
the analyte exceeding a mass fraction of 1 %. The glow-discharge conditions shall be those selected for
analysis. These measurements shall be performed using a discharge stabilization time (often referred
to as “preburn”) of at least 60 s and a data acquisition time in the range 5 s to 20 s. Each measurement
shall be located on a newly prepared surface of the specimen. Calculate the relative standard deviation
of the 10 measurements. The relative standard deviation shall conform to any requirements and/or
specifications relevant to the intended use.
NOTE Typical relative standard deviations determined in this way are 2 % or less.
6.4.3 Detection limit
6.4.3.1 General
Detection limits are instrument-dependent and matrix-dependent. Consequently, the detection limit for
a given analyte cannot be uniquely determined for every available instrument or for the full range of
surface layers considered here. For the purposes of this document, the detection limit for each analyte
will be acceptable if it is equal to or less than one-fifth of the lowest expected mass fraction in the
surface layer to be analysed.
6.4.3.2 SNR method
The first method is often called the “signal-to-noise ratio (SNR) method”. In order to evaluate the
detection limit for a given analyte, the following steps are performed.
Select a bulk specimen to be used as a blank. The composition of the specimen should preferably be
similar, in terms of the elemental composition of the matrix, to that of the surface layers to be analysed.
Further, the specimen shall be known to contain less than 1 µg/g of the analyte.
Perform 10 replicate burns on the blank. For each burn, acquire the emission intensity at the analytical
wavelength for 10 s. These are the background emission intensity measurements. The glow-discharge
conditions used should preferably be the same as those that will be used in the analysis of the coated
specimens. For each measurement, the blank shall be preburned at these conditions for a sufficient
length of time to achieve stable signals prior to the quantification of the emission intensity. Use an
unsputtered area of the surface of the blank for each individual burn.
Compute the detection limit, expressed as a mass fraction, using Formula (1):
3×σ
DL = (1)
S
where
DL is the detection limit;
σ is the standard deviation of the background intensity measurements performed in step 2;
S is the analytical sensitivity derived from the instrument calibration, expressed in the appro-
priate units (the ratio of intensity to mass fraction).
If the detection limit calculated is unacceptable, the test shall be repeated. If the second value calculated
is also unacceptable, then the cause shall be investigated and corrected prior to analysing specimens.
6.4.3.3 SBR-RSDB method
The second method, which does not require a blank, is often called the “signal-to-background ratio —
relative standard deviation of the background (SBR-RSDB) method”. The method is perfor
...