ISO/TR 18231:2016

TECHNICAL ISO/TR
REPORT 18231
First edition
Iron Ores — Wavelength dispersive
X-ray fluorescence spectrometers —
Determination of precision
Minerais de fer — Spectromètres à fluorescence à rayons X à longueur
d’onde dispersive — Détermination de la précision
PROOF/ÉPREUVE
Reference number
ISO/TR 18231:2016(E)
©
ISO 2016

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ISO/TR 18231:2016(E)

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ISO/TR 18231:2016(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Frequency of testing . 1
3 Counter tests . 2
3.1 Counter resolution . 2
3.1.1 General. 2
3.1.2 Procedure . 4
3.1.3 Assessment of results . 5
3.2 Conductivity of the gas flow proportional counter window . 6
3.2.1 General. 6
3.2.2 Procedure . 6
3.2.3 Assessment of results . 6
3.3 Pulse shift corrector . 7
3.3.1 General. 7
3.3.2 Procedure . 7
4 Spectrometer tests . 8
4.1 General . 8
4.2 Precision . 9
4.2.1 General. 9
4.2.2 Calculation of counting statistical error . 9
4.3 Test specimen .10
4.3.1 General.10
4.3.2 Sequential spectrometers .11
4.3.3 Simultaneous spectrometers .11
4.4 Instrumental conditions .11
4.4.1 General.11
4.4.2 Sequential spectrometers .11
4.4.3 Simultaneous spectrometers .11
4.5 Stability test .11
4.6 Specimen rotation test .12
4.7 Carousel reproducibility test .12
4.8 Mounting and loading reproducibility test .12
4.9 Comparison of sample holders .13
4.10 Comparison of carousel positions .13
4.11 Angular reproducibility .13
4.12 Collimator reproducibility (for sequential spectrometers fitted with an
interchangeable collimator) .14
4.13 Detector changing reproducibility (for sequential spectrometers fitted with more
than one detector).14
4.14 Crystal changing reproducibility .14
4.15 Other tests .14
4.16 Note on glass bead curvature .14
5 Determination of the dead time and the maximum usable count rate of the equipment .15
5.1 General .15
5.2 Methods of determination of dead time .16
5.2.1 General.16
5.2.2 Recommended method for determining dead time .16
Annex A (informative) Calculation of the coefficient of variation of duplicates .24
Bibliography .26
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ISO/TR 18231:2016(E)

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 meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 102, Iron ore and direct reduced iron,
Subcommittee SC 2, Chemical analysis.
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ISO/TR 18231:2016(E)

Introduction
If an X-ray fluorescence spectrometer is to be used for precise analyses, it needs to be functioning
correctly to specification, that is, the errors associated with the various functions of the instrument
have to be very small. It is important therefore that the spectrometer be tested to ensure that it is
indeed functioning to deliver the required precision. The objective of this Technical Report is to set
out tests that can be used to ascertain the extent of the errors and to suggest procedures for their
rectification. These tests are not used to ascertain whether the instrument is operating optimally but
to determine whether the instrument is capable of giving a preselected precision.
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TECHNICAL REPORT ISO/TR 18231:2016(E)
Iron Ores — Wavelength dispersive X-ray fluorescence
spectrometers — Determination of precision
1 Scope
This Technical Report describes methods of test that can be applied to wavelength dispersive X-ray
fluorescence (WD-XRF) spectrometers to ensure that the spectrometers are functioning in a manner
that allows precise analyses to be made.
The tests outlined are designed to measure the errors associated with the operation of certain parts of
the spectrometer. They are not designed to check every part of the spectrometer but only those parts
that may be the common sources of error.
It is assumed that the performance of the instrument has been optimized according to the
manufacturer’s instructions. For all tests, the two-theta angle should be carefully set for the line being
measured. The pulse height window should be set according to the manufacturer’s instructions and
should have a broad setting which may also include the escape peak for gas proportional counters. The
instrument and detector gas environment should be as specified by the manufacturer, as should the
power supply to the instrument.
NOTE Where no distinction has been made, it is assumed that a test is applicable to both sequential and
simultaneous spectrometers.
2 Frequency of testing
Testing is not required to be carried out with each batch of analyses. The frequency of testing varies
depending on the test involved. Table 1 lists the suggested frequency with which each test should be
carried out. Where specific problems are encountered, more frequent testing may be required and
remediation work performed.
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Table 1 — Suggested frequency of precision tests
Frequency Test
Monthly Resolution of the gas-flow proportional counter
Resolution of the scintillation and sealed gas counters
a
Operation of the pulse height shift corrector
Half yearly Conductivity of gas-flow proportional counter window
General stability
Collimator reproducibility
Detector changing reproducibility
Crystal changing reproducibility
Angular reproducibility
Yearly Carousel reproducibility
Comparison of carousel positions
Comparison of sample holders
Sample loading and unloading
a
The position of the pulse height peak should also be checked after changing
a bottle of detector gas since a variation in the methane content of the gas will
change the position of the peak.
The frequencies with which the tests listed in Table 1 are carried out are suggested on the basis that
there have been no changes to the spectrometer. If mechanical or electronic maintenance of a major
nature is carried out, the appropriate tests should be made before the spectrometer is taken back into
routine service.
3 Counter tests
3.1 Counter resolution
3.1.1 General
3.1.1.1 Theoretical resolution
Impurities in the flow gas and contamination of the anode wire may cause gas flow proportional
counters to gradually deteriorate, which will result in both a shift and a broadening of the energy
distribution (pulse height) curve. Similarly, scintillation counters and sealed gas counters may, for
various reasons, exhibit the same gradual deterioration. This can, ultimately, adversely affect the
measurements. Impurities in detector gas can be minimized by the use of gas filters.
The resolution (RES) of a counter is related to its energy distribution curve, and is given by the measured
peak width at half height (W) expressed as a percentage of the maximum of the pulse amplitude
distribution (V), using Formula (1) where the values of W and V are in terms of arbitrary units (which
vary between instrument manufacturers) obtained from the X-axis (see Figure 1):
W
RES =×100 (1)
V
The theoretical resolution (RES ), using the full width at half height of a Gaussian distribution, can be
th
calculated using the following formulae:
RES = 23, 6σ (2)
th
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σ = n (3)
Expressed as a percentage relative to n, Formula (3) becomes:
100
σ = in% (4)
()
n
where
n is the number of primary electrons per incident photon (gas counters) or number of photo-
electrons collected by the first dynode of the photomultiplier tube (scintillation counters),
calculated using Formula (5):
E
x
n= (5)
V
i
E is the energy of the incident radiation, in kilo electron volts (keV);
x
V is the effective ionization potential of Argon for a flow counter, in kilo electron volts (keV)
i
= 0,026 4.
Substituting Formula (5) into Formula (4), and Formula (4) into Formula (2) gives:
236× 0,026 4
38,4
RES = = (6)
th
EE
x x
Hence, for Cu Kα (E = 8,04 keV), the theoretical resolution of an Ar gas counter is 13,5 %.
3.1.1.2 Scintillation counter
For a scintillation counter:
128
RES = (7)
th
E
x
[1]
and for Cu Kα, the resolution should be approximately 45 %.
3.1.1.3 Practical resolution
In practice, however, the measured resolution achieved (RES ) is given in Formula (8):
m
RESk= R (8)
m
where
k is a factor that varies with the design of the counter, phosphor efficiency (scintillation coun-
ters), diameter, cleanliness and composition of the anode wire (gas counters).
For a well-designed and clean gas-flow proportional counter, k should be less than 1,15. Hence, for such
a counter, RES should be less than 15,6 % for Cu Kα radiation. For the scintillation counter, this value
m
should be less than 52 %.
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3.1.2 Procedure
This test should be carried out on all counters used in the spectrometer. Most modern instruments
provide the facility to measure pulse height distributions and to print out the counter resolution and
this facility should be used if available.
For sequential spectrometers, it is recommended that the test be carried out using either Cu Kα or
Fe Kα radiation for both detectors. However, if these lines are measured using only scintillation counter
in actual analysis, measure an X-ray line of a major element analysed with the gas proportional counter
for testing.
If the spectrometer does not provide automatic functions to determine RES then the following
m
procedure should be used.
a) Select a sample containing the appropriate analyte and, using a lower level setting and the pulse
height analyser (PHA) window set to “threshold” (no upper level), adjust the X-ray tube power to
4
give a count rate of about 2 × 10 c/s (counts per second).
b) Select a narrow pulse height window (2 % to 4 % of the peak voltage V of Figure 1) and decrease
the lower level setting until the count rate drops to essentially zero.
c) Increase the lower level stepwise, noting the count rate at each step, until the peak has been passed
and the count rate drops again to a very low value. Each step should be of the same width as the
pulse height window width, i.e. if the pulse height window width corresponds to 0,2 units, then
each step of the lower level should be 0,2 units.
d) Plot the count rate obtained at each step against the lower level values. An example is shown in
Figure 1.
1
2
Key
1 half peak height
2 peak width at half peak height
Figure 1 — Intensity as a function of lower level setting (arbitrary energy units) displayed for
Fe Kα radiation measured on a flow counter
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The measured counter resolution RES (in %) is obtained from the plot as follows:
m
BA−
RES = ×100 (9)
m
V
where
B and A are the lower level settings at the half height positions on either side of the peak;
V is the lower level setting at the maximum of the pulse height distribution.
The pulse height distribution should be determined at a count rate indicated by the instrument
4
manufacturer, or just below 2 c/s × 10 c/s (see Figure 1).
When determining the pulse height distribution automatically, A, B, V and W are not usually displayed,
but the graph, peak position and detector resolution are shown together with instrument specific data.
3.1.3 Assessment of results
3.1.3.1 Gas flow proportional counter
Since the resolution of a proportional counter depends on design, an absolute resolution value cannot be
given but the value should be near to that given in Formula (6). The optimum resolution of a particular
counter may be specified by the manufacturer or it may be found with use. When RES increases by a
m
factor of 1,2 times that value, the counter should be serviced.
3.1.3.2 Scintillation and sealed gas counters
These counters usually do not exhibit significant changes in resolution unless the counter is beginning
to fail, which can be quite early for some sealed gas counters for light elements and where they have
been used at very high flux for extended periods. Their resolution can be calculated according to
Formula (9) but the k factor is instrument specific. Hence, it is sufficient to measure their resolution
experimentally on a regular basis. If a significant increase in resolution is noted or the pulse height
distribution extends outside the energy window when it did not on installation, then the counter should
be replaced.
Scintillation counters can fail due to leaks in the beryllium window, thereby admitting moisture to the
very hygroscopic NaI crystal. The effect of such moisture reduces count rates for the longer wavelengths,
e.g. ~2Å but may have relatively little effect on short wavelengths, ~0.6Å. As more moisture reacts with
the crystal, its sensitivity will be reduced for all wavelengths.
Similarly, the sensitivity of sealed counters will reduce due to leaks in the detector window. A leaking
sealed gas counter window will result in a loss of gas density and thus gas amplification.
Although a reduction in sensitivity of the scintillation and sealed gas counters does not affect counting
precision, the signal-to-noise ratio will be reduced and longer counting times will be required to achieve
a given precision. Furthermore, if the reduction in sensitivity is due to failure of the window, then this
reduction will be continuous and total detector failure will be imminent.
The best method of detecting deterioration of such counters is to set aside a stable specimen (such as an
instrument monitor) and to periodically measure the intensity of a reasonably long wavelength line (for
example, Fe Kα for the scintillation counter). Any pulse height shift or decrease of intensity over time
should be investigated to ascertain whether the scintillation (or sealed gas) counter requires replacing.
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3.2 Conductivity of the gas flow proportional counter window
3.2.1 General
The gas-flow proportional counter window consists of a thin layer of plastic material (such as Mylar or
polypropylene) coated with a very thin electrically conductive layer (typically aluminium). For practical
considerations, the window material should be able to support atmospheric pressure without breakage.
Where air or helium paths are frequently used over a period of time, this material may stretch and cause
the aluminium coating to suffer from micro-cracks, resulting in the window becoming non-conducting.
A gas-flow proportional counter having a poorly conducting or non-conducting window appears to
function normally in most respects. However, measurements made with such a counter will be in error,
as the counter will give reduced count rates for samples which would normally give a high count rate.
This is especially noticeable when low and high count rates are measured consecutively.
Where 6 μm windows are used, loss of conductivity is seldom encountered, as the window usually has
a life of some years, requiring replacement only when the counter is serviced for other reasons. The
useful life of thinner windows (≤1 μm polypropylene windows are commonly used in most modern
spectrometers) varies from several months to more than a year. Where the spectrometer chamber
is continuously under vacuum, the window life is longer than for those where the spectrometer is let
down to normal air or Helium pressure frequently.
NOTE If the gas-flow proportional counter has a beryllium window, this test may be necessary to occasionally
check for the formation of hairline cracks.
3.2.2 Procedure
3.2.2.1 Sequential spectrometers
The procedure for sequential spectrometers is as follows.
a) Carry out the test using K Kα radiation.
b) Using low X-ray tube power (low kV and mA), select a sample that gives a count rate of between
1 000 c/s and 5 000 c/s.
c) Set the 20 angle and pulse height analyser using this sample and then, using these settings, measure
the count rate for 10 s.
d) Replace the sample with one having a high potassium concentration (a briquette of potassium
chloride or potassium hydrogen phthalate is particularly suitable or a sample advised by the
instrument manufacturer); set the X-ray tube to such a power that the maximum count rate allowed
by the manufacturer is achieved. Maintain these conditions for 2 min.
e) Reduce the X-ray tube power to its starting value and measure the count rate for the original
sample again.
3.2.2.2 Simultaneous spectrometers
For simultaneous spectrometers, the test is made on those channels having gas-flow proportional
counters fitted with thin plastic windows. The test is carried out in a similar manner to the procedure
specified in 3.2.2.1, except that the radiation used is that specific to the selected channel.
NOTE Tests for simultaneous spectrometers are also applicable to fixed channels in a sequential
spectrometer.
3.2.3 Assessment of results
If the window is conducting properly, the initial count rate will be within 5 % of the final count rate
when the X-ray tube power is returned to the original settings. If the window is poorly conducting, a
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reduced count rate will be observed. In such cases, the count rate should slowly return to the original
rate but for non-conducting windows, the time involved may be in excess of 12 h. The window should be
replaced if there is any evidence of poor conduction.
3.3 Pulse shift corrector
3.3.1 General
At high count rates, high detector gains will result in a shift to lower pulse amplitudes. In modern
spectrometers, the effects of pulse shift have been minimized by designing counters with a lower
dependence on gas density and composition and by the use of automatic pulse shift correction
electronics. Nevertheless, the effect of pulse shift errors on analytical accuracy can be significant.
All modern spectrometers are fitted with automatic pulse shift correction but the operation of
detectors should also be checked when the automatic shift correction is not operating at low count
rates. The threshold count rate at which the automatic shift correction operates can vary between each
detector type, instrument model and spectrometer manufacturer. This may even be programmable
with automatic pulse shift disabled or enabled in software or hardware.
For gas-flow detectors, since the pulse shift is dependent on the gas composition (proportion of argon
to methane and the level of contaminants) and gas density (temperature or pressure of the gas within
the detector), pulse shift should be checked whenever the flow gas supply cylinder has been replaced or
whenever the gas flow detector or gas density stabilizer has been repaired.
NOTE Leaks in the gas-flow circuitry may result in the erroneous entry of air, or helium in the case of liquid
measurements, which will alter the flow gas composition.
As modern spectrometers are fitted with gas density stabilizers of significant volume, detector gas lines
should be allowed to purge prior to checking pulse heights after any of the above work has been carried
out. As the volume of flow gas circuits may be up to 3 l, the purging of lines may take up to 3 h unless
the possibility exists for a higher purge rate for a short period of time. Advice from the manufacturer
should be taken here.
3.3.2 Procedure
a) For all XRF instruments, use a sample whose radiation wavelength i
...