ISO 16962:2005

INTERNATIONAL ISO
STANDARD 16962
First edition
2005-10-01
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 2005
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Contents Page
Foreword. iv
1 Scope . 1
2 Normative references . 1
3 Principle. 1
4 Apparatus . 2
4.1 Glow discharge optical emission spectrometer. 2
4.2 Minimum performance requirements . 2
5 Sampling. 4
6 Procedure . 4
6.1 Selection of spectral lines . 4
6.2 Optimizing the glow discharge spectrometer system settings . 5
6.3 Calibration . 8
6.4 Validation of the calibration. 11
6.5 Verification and drift correction . 12
6.6 Analysis of test samples. 13
7 Expression of results . 13
7.1 Quantitative depth profile . 13
7.2 Determination of total coating mass per unit area. 13
7.3 Determination of average mass fractions . 14
8 Precision. 14
9 Test report . 15
Annex A (normative) Calculation of calibration constants and quantitative evaluation of depth
profiles . 16
Annex B (informative) Suggested spectral lines for determination of given elements . 24
Annex C (informative) Determination of coating mass per unit area. 25
Annex D (informative) Additional information on interlaboratory tests . 29
Bibliography . 32

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 16962 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee
SC 8, Glow discharge spectroscopy.
It is based on the results from project SMT-CT96-2080, funded by the EC Industrial and Materials
Technologies Programme. The project was initiated by ECISS/TC 20.

iv © ISO 2005 – All rights reserved

INTERNATIONAL STANDARD ISO 16962:2005(E)

Surface chemical analysis — Analysis of zinc- and/or
aluminium-based metallic coatings by glow-discharge
optical-emission spectrometry
1 Scope
This International Standard 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 10 mass %;
lead contents between 0,005 mass % and 2 mass %;
antimony contents between 0,005 mass % and 2 mass %.
2 Normative references
The following referenced documents are indispensable for the application 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 14707, Surface chemical analysis — Glow discharge optical emission spectrometry (GD-OES) —
Introduction to use
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 Principle
The analytical method described here involves the following processes:
a) Cathodic sputtering of the surface coating in a direct current or radio frequency glow discharge device.
b) Excitation of the analyte atoms in the plasma formed in the glow discharge device.
c) Spectrometric measurement of the intensities of characteristic emission spectral lines of the analyte
atoms as a function of sputtering time (depth profile).
d) 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.
4 Apparatus
4.1 Glow discharge optical emission spectrometer
[1]
Required instrumentation includes an optical emission spectrometer system consisting of a Grimm type or
similar glow discharge source (direct current or radio frequency powered) and a simultaneous optical
spectrometer as described in ISO 14707, incorporating suitable spectral lines for the analyte elements (see
Annex B for suggested lines).
The inner diameter of the hollow anode of the glow discharge source shall be in the range 2 mm to 8 mm. A
cooling device for thin samples, such as a metal block with circulating cooling liquid, is also recommended, but
not strictly necessary for implementation of the method.
Since the determination is based on continuous sputtering of the surface coating, 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 500 measurements/second per spectral channel is
recommended, but for the applications within the scope of this International Standard a speed of
2 measurements/second per spectral channel may be acceptable.
4.2 Minimum performance requirements
4.2.1 General
It is desirable for the instrument to conform to the performance specifications given in 4.2.2 and 4.2.3, and
evaluated in 6.2.7.
NOTE Setting up for analysis commonly requires an iterative approach to the adjustment of the various experimental
parameters.
4.2.2 Minimum repeatability
The following test shall be performed in order to check that the instrument is functioning properly in terms of
repeatability.
Perform 10 emission intensity measurements on a homogeneous bulk sample with an analyte content
exceeding 1 mass %. The glow discharge conditions shall be those selected for the actual 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 sample. 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.
2 © ISO 2005 – All rights reserved

4.2.3 Detection limit
4.2.3.1 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 Zn/Al-based
alloys considered here. For the purposes of this International Standard, the detection limit for each analyte will
be acceptable if it is equal to or less than one-fifth of the lowest mass fraction expected in the coating or one-
fifth of the lower end of the range of mass fractions quoted in Clause 1 of this International Standard,
whichever is greater.
4.2.3.2 SNR method
The first method is often called the SNR (signal-to-noise ratio) method. In order to determine the detection
limit for a given analyte, the following steps shall be performed:
1) Select a bulk sample to be used as a blank. The composition of the sample shall be similar to that of the
coatings to be analysed in terms of the elemental composition of the matrix. Further, it shall be known to
contain less than 0,1 µg/g of the analyte.
2) Perform ten 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 shall be the same as those that will be used in the analysis of the coated samples. 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.
3) Compute the detection limit, expressed as a mass fraction, using the following equation:
3 ×S
DL = (1)
m
where
DL is the detection limit;
S is the standard deviation of the ten background intensity measurements performed in step 2;
m is the analytical sensitivity derived from the instrument calibration, expressed as the ratio of
intensity to mass fraction.
If the detection limit calculated is unacceptable, repeat the test. If the second value calculated is also
unacceptable, then the cause shall be investigated and corrected prior to analysing samples.
4.2.3.3 SBR-RSDB method
The second method, which does not require a blank, is often called the SBR-RSDB (signal-to-background
ratio — relative standard deviation of the background) method. The method is performed as follows:
1) Select a bulk sample that has a matrix composition that is similar to that of the coatings to be analysed,
and for which the mass fraction of the analyte is greater than 0,1 % and known. If an analytical transition
that is prone to self-absorption (see 6.1) is to be used, then the mass fraction of the analyte shall not
exceed 1 %.
2) Perform three replicate burns on the chosen sample. For each burn, integrate the emission intensity at
the analytical wavelength for 10 s. The glow discharge conditions used shall be similar to those that will
be used in the analysis of the coated samples. For each measurement, the sample 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 sample for each individual burn.
Average the three replicate emission intensities.
3) Select a peak-free region of the emission spectrum within 0,2 nm of the analytical peak. Perform ten
replicate burns on the chosen sample. For each burn, integrate the intensity at the peak-free region for
10 s. These are the measurements of the background intensity. The glow discharge conditions and
preburn shall be the same as those used in step 2. Once again, use an unsputtered area of the surface of
the sample for each individual burn. Compute the average and relative standard deviation of the
10 replicate measurements.
4) Calculate the detection limit using the following equation:
3××(MF RSDB/100)
DL = (2)
()SBB− /
where
DL is the detection limit;
MF is the mass fraction of the analyte in the sample;
RSDB is the relative standard deviation of the background from step 3, expressed as a
percentage;
B is the average background intensity from step 3;
S is the average peak intensity from step 2.
If the detection limit calculated is unacceptable, repeat the test. If the second value calculated is also
unacceptable, then the cause shall be investigated and corrected prior to analysing samples.
5 Sampling
As appropriate, carry out sampling in accordance with ISO 14284 and/or relevant national/international
standards. If such standards are unavailable, follow the instructions from the manufacturer of the coated
material or another appropriate procedure. Avoid the edges of coated strips. The size of the test samples shall
be suitable for the glow discharge source used. Typically, round or rectangular samples with sizes (diameter,
width and/or length) of 20 mm to 100 mm are suitable.
Rinse the surface of the sample with an appropriate solvent (high-purity acetone or ethanol) to remove oils.
Blow the surface dry with a stream of inert gas (argon or nitrogen) or clean, oil-free compressed air, being
careful not to touch the surface with the gas delivery tube. The wetted surface may be lightly wiped with a
wetted, soft, lint-free cloth or paper to facilitate the removal. After wiping, flush the surface with solvent and dry
as described above.
6 Procedure
6.1 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 spectral interference from other elements
present in the samples. In this type of application, where most of the analytes of interest are major elements in
the samples, 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 shall therefore be avoided for the determination of major
4 © ISO 2005 – All rights reserved

elements. In Annex B, some suggestions concerning suitable spectral lines are given. Spectral lines other
than those listed may be used, so long as they have favourable characteristics.
6.2 Optimizing the glow discharge spectrometer system settings
6.2.1 General
Follow the manufacturer’s instructions or locally documented procedures for preparing the instrument for use.
The source parameters shall be chosen to achieve three aims:
⎯ adequate sputtering of the sample, to reduce the analysis time without over-heating the coatings;
⎯ good crater shape, for good depth resolution;
⎯ constant excitation conditions in calibration and analysis, for optimum accuracy.
There are often tradeoffs among the three specified aims.
In particular, 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. For further information, see ISO 14707.
6.2.2 Setting the discharge parameters of a dc source
6.2.2.1 General
Modern dc glow discharge spectrometers usually have provisions for complete control/measurement of the
electrical parameters (current, voltage, power), allowing any two of these parameters to be locked to constant
values by varying the pressure (active pressure regulation). Older spectrometers often lack an active pressure
regulation system, but the pressure can still be adjusted manually to achieve the same result. The user shall
adopt one of the following procedures.
6.2.2.2 Constant applied current and voltage
The two control parameters are applied current and voltage. Set the power supply for the glow discharge
source to constant current/constant voltage operation. First set the current and voltage to typical values
recommended by the manufacturer. 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 about the optimum current
is available, it is recommended that, to start with, a value somewhere in the middle of the recommended range
is used.
Set the high voltage of the detectors as described in 6.2.4.
Adjust the discharge parameters as described in 6.2.5, adjusting first the current and if necessary the voltage.
Optimize the crater shape as described in 6.2.6, by adjusting the voltage. These conditions are then used
during calibration and analysis.
6.2.2.3 Constant applied current and pressure
The two control parameters are applied current and pressure. Set the power supply for the glow discharge
source to constant current operation. First 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 to10 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 about the optimum current is available, it is recommended that, to start with, a value
somewhere in the middle of the recommended range is used. Sputter a typical coated test sample, and adjust
the pressure until a voltage of approximately 600 V is attained in the coating.
Set the high voltage of the detectors as described in 6.2.4.
Adjust the discharge parameters as described in 6.2.5, adjusting first the current and if necessary the
pressure.
Optimize the crater shape as described in 6.2.6, by adjusting the pressure. Before sputtering a new sample
type, make a test run in order to ensure that the voltage 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. These conditions
are then used during calibration and analysis.
Emission yields vary with the current, voltage and possibly the pressure (see Reference [4] in the
Bibliography). It is therefore essential that these parameters be kept as closely as possible at the same levels
during measurement of coated samples as during calibration. Since it is impossible in practice to maintain all
three parameters constant for all samples, priority is given to maintaining current and voltage constant,
utilizing the pressure as a variable parameter. There exists a method to correct for voltage and current
variations by means of an empirically derived function (see Reference [4]) and this type of correction is often
implemented in software based on the intensity normalization method in accordance with Equation (A.2) in
Annex A. However, such corrections for voltage and current are not included in this standard method. If
available in the spectrometer software, the user shall therefore ensure that the voltage-to-current corrections
are disabled in order to implement the method correctly.
6.2.3 Setting the discharge parameters of an rf source
6.2.3.1 General
Currently, most rf sources are operated with constant applied power and constant pressure. Other modes also
exist, such as constant applied voltage and pressure, and constant effective power and applied voltage.
These modes are likely to become more common in the future. All rf operational modes are allowed in this
International Standard provided they meet the three aims listed in 6.2.1. In the following, separate instructions
are provided on how to set the parameters for the operational modes that are currently used regularly.
6.2.3.2 Constant applied power and pressure
The two control parameters are applied power and pressure. First set the applied power and adjust the source
pressure to those suggested by the manufacturer. If recommended values are not available, set the applied
power and pressure to somewhere in the middle of the ranges commonly used for depth profiling of metal
samples. Measure the penetration rate (i.e. depth per unit time) on an iron or steel sample. Adjust 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 6.2.4.
Adjust the discharge parameters as described in 6.2.5, adjusting first the applied power and if necessary the
pressure.
Optimize the crater shape as described in 6.2.6, by adjusting the pressure.
Re-measure the penetration rate on the iron or steel sample and adjust the applied power if necessary to
return to about 2 µm/min to 3 µm/min. Repeat the cycles 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
by the instrument. These conditions are then used during calibration and analysis.
6.2.3.3 Constant applied power and dc bias voltage
The two control parameters are applied power and dc bias voltage. First set the applied power and adjust the
source pressure to attain a dc bias typical of those suggested by the manufacturer. If recommended values
6 © ISO 2005 – All rights reserved

are not available, set the applied power and dc bias voltage to somewhere in the middle of the ranges
commonly used for depth profiling of metal samples. 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
sample. Adjust 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 6.2.4.
Adjust the discharge parameters as described in 6.2.5, adjusting first the applied power and if necessary the
dc bias voltage.
Optimize the crater shape as described in 6.2.6, by adjusting the dc bias voltage.
Re-measure the penetration rate on the iron or steel sample and adjust the applied power if necessary to
return to about 2 µm/min to 3 µm/min. Repeat the cycles of power and dc bias voltage adjustment until no
significant change is noted in the penetration rate or 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 by the
instrument. These conditions are then used during calibration and analysis.
6.2.3.4 Constant effective power and rf voltage
The two control parameters are effective power and rf voltage. Constant effective power is defined here as the
applied power minus reflected power and the “blind power” measured with the sample in place but without
plasma (vacuum conditions). The rf voltage is defined here as the RMS voltage at the coupling electrode.
Set the power supply for the glow discharge source to constant effective power/constant rf voltage operation.
First set the power to typical values 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,
for example. If no previous knowledge about the optimum power is available, it is recommended that, to start
with, a value somewhere in the middle of the recommended range is used.
Set the high voltage of the detectors as described in 6.2.4.
Adjust the discharge parameters as described in 6.2.5, adjusting first the power and if necessary the rf voltage.
Optimize the crater shape as described in 6.2.6, by adjusting the rf voltage.
Re-measure the penetration rate on the iron or steel sample and adjust the applied power if necessary to
return to about 2 µm/min to 3 µm/min. Repeat the cycles of power and dc bias voltage adjustment until no
significant change is noted in the penetration rate or 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 by the
instrument. These conditions are then used during calibration and analysis.
6.2.4 Setting the high voltage of the detectors
Select test samples with coatings of all types to be determined. Using these samples, run the source while
observing the output signals from the detectors for the analyte atoms. Adjust the high voltage of the detectors
in such a way that sufficient sensitivity at the lowest analyte mass fraction is ensured, without saturation of the
detector system at the highest analyte mass fraction.
6.2.5 Adjusting the discharge parameters
For each type of test sample, carry out a full depth profile measurement, sputtering it in the glow discharge for
a sufficiently long time to remove the coating completely and continuing 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. Unstable emission signals may indicate thermal instability on the sample surface; sample
cooling is beneficial in this regard. If the emission signals are not found to be stable, reduce one of the control
parameters by a small amount and sputter through the coatings 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.
6.2.6 Optimizing crater shape
Sputter one of the brass samples (see 6.3.2), or a Zn- and/or Al-based coating typical of the unknown
samples, to a depth of about 10 µm to 20 µm (but still inside the coating if a coated sample is used) and
measure the crater shape by means of a suitable profilometer device. Repeat this procedure on the brass or
coating sample a few times using slightly different values of one of the control parameters. Select the
conditions that gives an optimally flat-bottomed crater.
6.2.7 Preliminary performance test
Ensure that the selected operating parameters are adequate to meet the minimum performance requirements
described in 4.2. If these minimum performance requirements are not satisfied, then additional adjustments to
the operating parameters will be necessary.
6.3 Calibration
6.3.1 General
Calibration of the system consists of determining, for each analyte and spectral line, the analytical response
curve as described in either Clause A.2 or Clause A.3 in Annex A. In order to carry out the calibration, it is
necessary to know both the chemical composition and the sputtering rates (mass loss rates) of the calibration
samples.
6.3.2 Calibration samples
6.3.2.1 General
Whenever possible, spectrometric calibration samples issued as CRMs (certified reference materials) shall be
used. Due to the quantification being based on emission yields, the calibration samples need not be very
similar to the coating materials in composition, but shall have sputtering rates which are well determined and
reproducible. In particular, pure or nearly pure zinc samples are not recommended, due to difficulties in
obtaining reproducible and stable sputtering rates in zinc. Furthermore, high-purity metals are not necessary
in order to calibrate correctly for high mass fractions, but they are valuable for the determination of the
spectral backgrounds. The following considerations are the most important in the selection of the calibration
samples:
a) there shall be at least five calibration samples for each analyte, covering a range from zero to the highest
mass fraction to be determined;
b) the samples shall be homogeneous.
Based on these general considerations, the following types of calibration sample are suggested. Additional
calibration samples of other alloy types containing the analytes may be used.
6.3.2.2 Brass calibration samples
Select at least two brass samples with zinc mass fractions of 25 % to 50 %, aluminium mass fractions of 1 %
to 4 % and lead mass fractions of 1 % to 4 %.
6.3.2.3 Zn-Al alloy samples
Select at least two Zn-Al alloy samples with zinc mass fractions of 40 % to 90 %.
8 © ISO 2005 – All rights reserved

6.3.2.4 Iron or low-alloy steel samples
Select at least two iron or low-alloy steel samples with iron mass fractions greater than 98 %. The iron mass
fraction may be determined by subtracting the sum of the mass fractions for all other known elements from
100 %.
6.3.2.5 Stainless-steel samples
Select at least two stainless steels with nickel mass fractions of 10 % to 40 %.
6.3.2.6 Nickel alloy samples
Select at least one nickel-based alloy sample with a nickel mass fraction higher than 70 % (an Ni mass
fraction higher than the 20 % defined in Clause 1 is necessary due to the high sputtering rate of Zn-Ni alloys,
the calibration points being defined by the product of sputtering rate and mass fraction).
6.3.2.7 Aluminium-silicon alloy samples
Select at least one aluminium-silicon alloy sample with a silicon mass fraction of 5 % to 10 %.
6.3.2.8 High-purity copper sample
Select a high-purity copper sample with analyte mass fractions of less than 0,001 %. This sample can be used
as the zero point for all analytes except copper.
6.3.3 Validation samples and optional RMs for calibration
6.3.3.1 General
Validation (see 6.4) samples shall be prepared in order to check the accuracy of the analytical results. The
following sample types are suggested, but other samples may be used where appropriate. These samples
may also be used as additional calibrants.
6.3.3.2 Electrolytically coated zinc-nickel RM
Prepare an electrolytically coated RM with a nickel mass fraction less than 20 %. Determine the coating mass
per unit area and chemical composition of the coated RM by specified reference methods such as ISO 17925.
6.3.3.3 Electrolytically coated zinc RM
Prepare an electrolytically RM with a zinc mass fraction higher than 30 % and an iron mass fraction higher
than 5 %. Determine the coating mass per unit area and chemical composition of the coated RM by specified
reference methods such as ISO 17925.
6.3.3.4 Zinc-aluminium coated RM
Prepare a zinc-aluminium coated RM with a zinc mass fraction higher than 10 % and an aluminium mass
fraction higher than 5 %. Determine the chemical composition of the coated RM by specified reference
methods such as ISO 17925.
NOTE 1 A reference material (RM) is a material or substance one or more of whose property values are sufficiently
homogeneous, stable and well established to be used for the calibration of an apparatus, for the assessment of a
measurement method or for assigning values to materials.
NOTE 2 A certified reference material (CRM) is a reference material, accompanied by a certificate, one or more of
whose property values are certified by a procedure which establishes its traceability to an accurate realization of the unit in
which the property values are expressed, each certified value being accompanied by an uncertainty at a stated level of
® ®
confidence. A Standard Reference Material (SRM ) is a CRM issued by the National Institute of Standards and
Technology, Gaithersburg, MD, USA.
6.3.4 Determination of the sputtering rate of calibration samples
The term “sputtering rate” is understood here to be equivalent to the mass loss rate during sputtering in the
glow discharge. The term “relative sputtering rate” is understood here to be the sputtering rate of the sample
divided by the sputtering rate of a reference material sputtered under the same conditions. If the sputtered
areas of the sample and the reference sample are the same, then the relative sputtering rate is equivalent to
the relative sputtering rate per unit area. Proceed with sputtering rate determinations as follows (sputtering
rates may also be available from the manufacturer):
a) If laboratory facilities are available, measure the density of each calibration sample. A suitable method for
homogeneous samples is sample mass divided by sample volume where the sample volume is measured
by immersion of the sample in water, using Archimedes' principle. Alternatively, the sample volume could
be estimated from the sample dimensions, or the density calculated from the sample composition as
described in Annex A [see Equation (A.29)]. The accuracy of the measured or calculated density shall be
better than 5 %.
b) Prepare the sample surface in accordance with the instrument manufacturer's recommendations or
another appropriate procedure.
c) Adjust the glow discharge parameters to those selected in 6.2.
d) Sputter the sample for a time estimated to result in a crater 20 µm to 40 µm deep, recording the total
sputtering time.
e) Repeat d) several times if the sample surface area is sufficiently large, recording the total sputtering time
for each crater.
f) Measure the average depth of each crater by means of an optical or mechanical profilometer device,
performing at least four profile traces in different directions across the centre of the crater.
g) For absolute sputtering rates:
1) measure the area of at least one crater;
2) calculate the sputtered volume of each crater by multiplying the sputtered area by the average
sputtered depth;
3) calculate the sputtered mass as the sputtered volume multiplied by the density of the sample;
4) calculate the sputtering rate for each crater as the mass loss divided by the total sputtering time;
5) calculate the average sputtering rate and the standard deviation from the measurements of each
crater.
h) For relative sputtering rates:
1) calculate the sputtered mass per unit area for each crater as the sputtered depth multiplied by the
density of the sample;
2) calculate the sputtering rate per unit area for each crater as the sputtered mass per unit area divided
by the total sputtering time;
3) choose a reference sample (iron or low-alloy steel is recommended) and measure the average
sputtering rate per unit area for this reference sample as described above for the calibration samples;
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4) calculate the relative sputtering rate for each crater as the sputtering rate per unit area divided by the
average sputtering rate per unit area of the reference;
5) calculate the average relative sputtering rate and the standard deviation from the measurements of
each crater.
The profilometer used in the depth calibration shall have an accuracy of better than 5 %.
NOTE The sputtered mass can also be determined by weighing samples before and after sputtering. However, this
requires the use of balances of extremely high accuracy, and the uncertainty in such measurements is generally inferior to
the uncertainty in crater depth measurements.
6.3.5 Emission intensity measurements of calibration samples
The procedure for measuring the calibration samples is as follows:
a) Prepare the surfaces of the calibration samples in accordance with the instrument manufacturer's
instructions. If such instructions are not available, dry grinding with 220 grit abrasive paper is usually
sufficient for any bulk sample. However, wet grinding may be beneficial. Wet samples can be dried by
thoroughly rinsing them with alcohol and then blowing the solvent away with a stream of inert gas, such
as argon or nitrogen. Be careful not to touch the sample surface with the gas delivery tube.
b) Adjust the source parameter settings to those selected in 6.2. Choose a preburn time of 50 s to 200 s and
a signal integration time of 5 s to 30 s.
c) Measure the emission intensities of the analytes. The units in which the intensities are given is of no
importance. Commonly used units are counts per second (cps) or volts (V). Measure each sample at
least two times and calculate the average value.
6.3.6 Calculation of calibration constants
Perform the calibration computations in accordance with the methods specified in either Clause A.2 or
Clause A.3 in Annex A.
NOTE Depending on the type of source, the mode of operation and the calibration samples chosen, the calibration
graphs for some elements may show a large separation between samples from different matrices. The separation typically
occurs between two matrix groups: one group including low-alloy steel, stainless steel and brass, and a second group
including aluminium and alminium-zinc alloys. This separation has been noted in elemental calibration graphs where
samples from both matrix groups are included. This separation is proof of a difference in emission yield, and has been
shown to be well correlated to matrix-dependent variations in the glow discharge plasma. If some facility is provided by the
manufacturer to minimize this effect, it should be used. Otherwise the solution is to choose, from each calibration curve,
calibration samples which most resemble the samples to be analysed. This normally does not present any special difficulty.
For zinc, for example, when analysing aluminium-zinc coated steel, the brass samples are excluded from the calibration
graph.
6.4 Validation of the calibration
6.4.1 General
Carry out the following procedure immediately after calibration in order to confirm that the calibration
equations are accurate. This process is called validation of calibration (see the Note). It is not necessary to
validate the calibration every time a new sample is analysed. A related procedure (verification) shall be used
on a more routine basis to check for instrument drift over time, as described in 6.5.
Two validation procedures are included in the subclauses that follow. The first procedure (see 6.4.2) makes
use of bulk reference materials, and the second (see 6.4.3) employs coated reference materials. Such coated
reference materials are often difficult to obtain. As a result, the validation procedure described in 6.4.3 is
optional.
NOTE Validation is the confirmation, through the provision of objective evidence, that the particular requirements for
a specific intended use or application have been fulfilled (see ISO 9000:2005, Subclause 3.8.5). Validation of the method
is defined in ISO/IEC 17025:2005, Subclause 5.4.5. Validation of the calibration is analogous to it (cf. Note to 6.5).
6.4.2 Checking analytical accuracy using bulk reference materials
a) Select an appropriate number of bulk reference materials to be used for validation of the calibration, in
accordance with 6.3.2.
b) Measure the emission intensities for these validation samples under the same glow discharge conditions
and preburn and integration times as selected for calibration. At least three independent burns shall be
placed on each sample, using a freshly prepared surface for each burn.
c) Compute the average mass fractions of the analytes for each validation sample, based upon the
calibration equations.
d) Confirm that the average mass fractions of the analytes measured in this way agree with the known
values to within appropriate statistical bounds. If statistical disagreement
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