ISO/TS 25138:2019

TECHNICAL ISO/TS
SPECIFICATION 25138
Second edition
2019-08
Surface chemical analysis — Analysis
of metal oxide films by glow-discharge
optical-emission spectrometry
Analyse chimique des surfaces — Analyse de films d'oxyde de métal
par spectrométrie d'émission optique à décharge luminescente
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
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 . 3
6 Adjusting the glow-discharge spectrometer system settings . 4
6.1 General . 4
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 . 6
6.3 Setting the discharge parameters of an RF source . . 6
6.3.1 General. 6
6.3.2 Constant applied voltage and pressure . 6
6.3.3 Constant applied power and DC bias voltage . 7
6.3.4 Constant effective power and RF voltage . 7
6.4 Minimum performance requirements . 7
6.4.1 General. 7
6.4.2 Minimum repeatability . . 8
6.4.3 Detection limit . 8
7 Sampling . 9
8 Calibration .10
8.1 General .10
8.2 Calibration samples .10
8.2.1 General.10
8.2.2 Low alloy iron or steel samples.10
8.2.3 Stainless-steel samples . .11
8.2.4 Nickel alloy samples . .11
8.2.5 Copper alloy samples .11
8.2.6 Titanium alloy samples .11
8.2.7 Silicon samples .11
8.2.8 Aluminium alloy samples .11
8.2.9 High-oxygen samples .11
8.2.10 High-carbon samples .11
8.2.11 High-nitrogen samples .11
8.2.12 High-hydrogen samples .11
8.2.13 High-purity copper samples .11
8.3 Validation samples .12
8.3.1 General.12
8.3.2 Hot-rolled low-alloy steel .12
8.3.3 Oxidized silicon wafers . .12
8.3.4 TiN-coated samples .12
8.3.5 Anodized Al O samples .12
2 3
8.3.6 TiO -coated samples . .12
8.4 Determination of the sputtering rate of calibration and validation samples .12
8.5 Emission intensity measurements of calibration samples .14
8.6 Calculation of calibration formulae .14
8.7 Validation of the calibration .14
8.7.1 General.14
8.7.2 Checking analytical accuracy using bulk reference materials .15
8.7.3 Checking analytical accuracy using metal oxide reference materials .15
8.8 Verification and drift correction .15
9 Analysis of test samples .16
9.1 Adjusting discharge parameters .16
9.2 Setting of measuring time and data acquisition rate .16
9.3 Quantifying depth profiles of test samples .16
10 Expression of results .17
10.1 Expression of quantitative depth profile .17
10.2 Determination of metal oxide mass per unit area .17
10.3 Determination of the average mass fractions of the elements in the oxide .18
11 Precision .18
12 Test report .19
Annex A (informative) Calculation of calibration constants and quantitative evaluation
ofdepth profiles .20
Annex B (informative) Suggested spectral lines for determination of given elements .32
Annex C (informative) Examples of oxide density and the corresponding quantity ρ .34
O
Annex D (informative) Report on interlaboratory testing of metal oxide films .35
Bibliography .40
iv © ISO 2019 – 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
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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
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.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/TS 25138:2010), which has been
technically revised. The main changes compared to the previous edition are as follows:
— the element Zr has been added to the Scope;
— the description of the apparatus has been modified to include spectrometers with solid-state array
detectors of the types CCD and CID;
— Clause 6 has been modified for more clarity, and to include the spectrometer types;
— Clause 8 has been modified for more clarity, particularly in the recommendations for calibration
samples.
A list of all parts in the ISO TS 25138 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
TECHNICAL SPECIFICATION ISO/TS 25138:2019(E)
Surface chemical analysis — Analysis of metal oxide films
by glow-discharge optical-emission spectrometry
1 Scope
This document describes a glow-discharge optical-emission spectrometric method for the
determination of the thickness, mass per unit area and chemical composition of metal oxide films.
This method is applicable to oxide films 1 nm to 10 000 nm thick on metals. The metallic elements of
the oxide can include one or more from Fe, Cr, Ni, Cu, Ti, Si, Mo, Zn, Mg, Mn, Zr and Al. Other elements
that can be determined by the method are O, C, N, H, P and S.
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 14707, Surface chemical analysis — Glow discharge optical emission spectrometry (GD-OES) —
Introduction to use
ISO 16962:2017, Surface chemical analysis — Analysis of zinc- and/or aluminium-based metallic coatings
by glow-discharge optical-emission spectrometry
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
verification
confirmation, through the provision of objective evidence, that specified requirements have been
fulfilled
[SOURCE: ISO 9000:2015, 3.8.12]
3.2
validation
confirmation, through the provision of objective evidence, that the particular requirements for a
specific intended use or application have been fulfilled
[SOURCE: ISO 9000:2015, 3.8.13]
4 Principle
The analytical method described here involves the following processes.
a) Cathodic sputtering of the surface metal oxide 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 spectral-emission lines of the
analyte atoms as a function of sputtering time (qualitative depth profiling).
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.
e) 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).
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 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 principle of determination is based on continuous sputtering of the surface metal oxide,
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.
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 samples. For applications where several 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
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.
2 © ISO 2019 – All rights reserved

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 samples and higher power during analysis; therefore the sample 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 to mask
inhomogeneity within a surface metal oxide. 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 samples
due to e.g. surface layers of poor heat conductivity and/or very thin samples. In such cases, the smaller
2 mm anode 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 samples; hence this is the only type that can be used for e.g. polymer
coatings and insulating oxide layers. 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 instrument. 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]
NOTE It should be noted in this context that 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
[8]
the current, the voltage and, to a lesser extent, the pressure . It is impossible in practice to maintain all three
parameters constant for all test samples, 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
[8]
exist methods to 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 an optical system with photomultiplier detectors in fixed spectral positions, 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. For further
information, see ISO 14707. For an optical system with CCD detectors, the corresponding control is
to check that the wavelength calibration is correct, following the procedure given by the instrument
manufacturer.
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 sample, to reduce the analysis time without overheating the sample;
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 the following subclauses.
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
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.
4 © ISO 2019 – All rights reserved

6.2.1.1 Setting the high voltage of the detectors
Select test samples with surface layers of all types to be determined. For all test samples, 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 and CID), adjust the integration time in the same way as the high voltage
for PMT.
6.2.1.2 Adjusting the source 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 metal oxide 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 metal oxide 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 sample surface layers; sample
cooling is beneficial in this regard.
6.2.1.3 Optimizing the crater shape
If a suitable profilometer device is available, adopt the following procedure. Sputter a sample with a
metal oxide typical of the test samples to be analysed to a depth of about 10 µm to 20 µm, but still
inside the metal oxide. This is only possible for applications where surface metal oxide layers of such
thickness are available. If no such sample is available, use a steel or brass sample. 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
sample, and adjust the pressure until a voltage of approximately 700 V is attained in the metal oxide.
Set the high voltage of the detectors as described in 6.2.1.1.
Adjust the discharge parameters as described in 6.2.1.2, adjusting first the current and, if necessary,
the pressure.
Optimize the crater shape as described in 6.2.1.3 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 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 the 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 of approximately 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 6.2.1.1.
Adjust the source parameters as described in 6.2.1.2, adjusting first the voltage and if necessary the
pressure.
Optimise the crater shape as described in 6.2.1.3, 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 RF voltage (the RF voltage is defined here as the RMS voltage at the coupling electrode without
DC bias). In addition, the mode constant applied power and DC bias voltage is sometimes used, but less
common. 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 voltage 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 commonly used
for depth profiling of metal samples. Measure the penetration rate (i.e. depth per unit time) on an iron
or steel sample, 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 6.2.1.1.
Adjust the discharge parameters as described in 6.2.1.2, adjusting first the applied power and, if
necessary, the pressure.
Optimize the crater shape as described in 6.2.1.3 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 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 © ISO 2019 – All rights reserved

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 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, 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 6.2.1.1.
Adjust the discharge parameters as described in 6.2.1.2, adjusting first the applied power and, if
necessary, the DC bias voltage.
Optimize the crater shape as described in 6.2.1.3 by adjusting the DC bias voltage.
Remeasure 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 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.
NOTE This mode can only be can be used if conductive samples are measured, since the DC bias vanishes at
non-conductive samples.
6.3.4 Constant effective power and 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 sample in place but without plasma (vacuum conditions). The RF voltage is defined here as the RMS
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 W 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 6.2.1.1.
Adjust the discharge parameters as described in 6.2.1.2, adjusting first the effective power and, if
necessary, the RF voltage.
Optimize the crater shape as described in 6.2.1.3 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 6.4.2 and
6.4.3 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 sample 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 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.
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 metal
oxides 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 metal oxide.
6.4.3.2 SNR method
The first method is often called the SNR (signal-to-noise ratio) method. In order to evaluate the
detection limit for a given analyte, the following steps are performed.
1) Select a bulk sample to be used as a blank. The composition of the sample should preferably be
similar, in terms of the elemental composition of the matrix, to that of the metal oxides to be
analysed. Further, the sample shall be known to contain less than 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 should preferably 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 i
...