INTE’RNATIONAL IS0

STANDARD 13321
First edition
1996-07-o I

Particle size analysis - Photon correlation

spectroscopy

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Analyse granulom&rique - Spectroscopic par corrdation de photons
(standards.iteh.ai)
ISO 13321:1996
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5c94d603327e/iso-13321-1996

Reference number
IS0 13321 :I 996(E)
IS0 13321:1996(E)

Contents Page

Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

1 Scope ........................................................................................... 1

2 Definitions ........ ........................................................................... 1

3 Symbols ....................................................................................... 1

4 Principle ....................................................................................... 1

5 Apparatus .................................................................................... 2

6 Preliminary procedures.. .............................................................. 2

7 Measurement procedure ............................................................. 3

8 Calibration and validation.. .. ......................................................... 4

9
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Repeatability ................................................................................ 4

IO (standards.iteh.ai)
Test report ................................................................................... 4

Annexes ISO 13321:1996

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Computation of the average particle diameter and polydispersity
5c94d603327e/iso-13321-1996 6
Index .... ........................................................................................
Recommended test report form ................................................. 8
Theoretical background ............................................................... 10
Typical PCS instruments and recommended specifications ....... 13

Recommendations for sample preparation ................................. 15

Bibliography.. ............................................................................... 20

0 IS0 1996
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copying and mrcrofilm, without permission In wnting from the publisher.
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Printed in Switzerland

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 @ IS0 IS0 13321:1996(E)

Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.

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.
iTeh STANDARD
International StandardPREVIEW
IS0 13321 was prepared by Technical Committee
lSO/TC 24, Sieves, sieving and other sizing methods, Subcommittee
(standards.iteh.ai)
SC 4, Sizing by methods other than sieving.

Annex ISOA forms an integral part of this International Standard. Annexes B to

13321:1996
F are for information only.
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IS0 13321:1996(E) @ IS0

Introduction

Particle sizing in the submicrometer size range is nowadays performed on

a routine basis using photon correlation spectroscopy (PCS). The success
of the technique is mainly based on the facts that it provides estimates of
average particle size in measuring times of a few minutes and that user-
friendly commercial equipment is available. Nevertheless, proper use of
the instrument and interpretation of the results require certain precautions.
Therefore there is a need for an International Standard for the determina-
tion of particle size by photon correlation spectroscopy, in order to provide
a methodology that allows the users to obtain good interlaboratory agree-
ment on accuracy and reproducibility.

Although PCS allows the determination of particle size distribution, this

International Standard is limited to the description of size distribution by
means of only two parameters: an average size and a polydispersity index,
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as obtained by so-called cumulants analysis (see annex A). This does not
exclude more detailed information about particle size distributions being
obtained. However, the reproducibility (standards.iteh.ai)
and reliability of the method of
calculation for full distributions is, at the present state of the art, not good
enough to include in an International Standard. Again, this does not ex-
clude determination of acceptable size distributions ISOin13321:1996
particular appli-
cations. https://standards.iteh.ai/catalog/standards/sist/b8e80dc0-661f-46aa-a27c-
5c94d603327e/iso-13321-1996
This International Standard recommends measurements at a single scat-
tering angle of 90° using a He-Ne laser light source with a wavelength in
vacua of 632,8 nm. Since solid state laser sources operating at other
wavelengths have become available and may be used in future instru-
ments, this International Standard already includes recommendations for
such instruments. Although the procedure given is limited to a single angle
measurement, with some instruments additional measurements and
valuable additional information can be obtained at other scattering angles
or by simultaneous analysis of measurements performed at different
angles.

This International Standard uses isotropic spherically shaped particles

within a test procedure. Measurement of nonspherical and/or nonisotropic
particles can be made by this technique where the size of such particles is
reported by a spherical equivalence.

A list of suitable references for further reading is given in annex F.

INTERNATIONAL STANDARD @ IS0 IS0 13321:1996(E)

Particle size analysis - hoton correlation spectroscopy

1 scope B max maximum value of the intercept B for a given

setting of the detection optics;
This International Standard describes the application
of photon correlation spectroscopy (PCS) to the c concentration of particulate material, in moles
measurement of an average particle size and a per litre;
measure of the broadness of the size distribution of
particles dispersed in liquids. It is applicable to particle G2@) intensity autocorrelation function;
sizes ranging from a few nanometres to about 1 Frn,
or to the onset of sedimentation. In the data analysis f-2 refractive index of the dispersion medium;
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procedure (see annexes A and C) it is assumed that
the particles are isotropic and spherically shaped. number of particles in scattering volume V;
Nv

NOTE I -The technique

(standards.iteh.ai)
is also known or referred to viscosity of the dispersion medium;
77
under other names, e.g. quasi-elastic light scattering (QELS)
and dynamic light scattering (DLS). ISO 13321:1996r decay rate;
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;lo laser wavelength in vacua (632,8 nm for He-Ne
2 Definitions laser);

For the purposes of this International Standard, the e particle volume fraction;
following definitions apply.
P particle density;
2.1 average particle diameter, xpcs: Harmonic
intensity-averaged particle diameter, as determined by 8 scattering angle;
equation (C.10) of annex C.
P2 second cumulant.
It is expressed in nanometres (1 O-9 m).

2.2 polydispersity index, PI: Dimensionless

measure of the broadness of the size distribution, as
determined by equation (C.9) of annex C. 4 Principle

2.3 scattering volume, V: Section of the incident A monochromatic and coherent laser light beam
laser beam viewed by the collecting or detector op- illuminates a representative sample for particle size
tics. analysis, dispersed at a suitable concentration in a
liquid. The light scattered by the particles at an angle
Typical order of magnitude is 1 O-6 cm? (typically 90°) is recorded by a detector whose output
is fed to a correlator. The decay of the autocorrelation
function of the scattered intensity is interpreted in
3 Symbols terms of average particle size and polydispersity index
by the so-called cumulants method.
B value of the intercept of the intensity autocor-
relation function [see equation (C.6) of an- Annex C provides some theoreticai background for
nex Cl; particle sizing by PCS.
IS0 13321:1996(E) @ is0

5 Apparatus beam is on. Observe local regulations for laser

radiation safety.
The main components of a typical photon correlation
spectrometer are listed below. 6.2 Sample preparation and inspection
NOTE 2 - Commercial or “home-made” instruments that 6.2.1 Samples shall consist of well-dispersed par-
meet the requirements of this International Standard may ticles in a liquid medium. The dispersion liquid shall
be used. There are several significant differences, both in
fulfil following requirements:
hardware and software, not only between instruments from
different manufacturers but also between different types
from one manufacturer. The instrument specifications do a) it shall be transparent (non-absorbing) at the laser
not always give adequate information for proper assess- wavelength;
ment of its specific features. Therefore, annex D has been
provided to recommend specifications for PCS instruments. b) it shall be compatible with the materials used in
the instrument;
5.1 Laser, monochromatic, emitting light polarized
cl it shall not dissolve, swell or coagulate the particu-
with its electric field component perpendicular to the late material;
plane formed by the incident and detected rays
(vertical polarization), e.g. a He-Ne laser capable of d) it shall have a refractive index different from that
2 mW to 5 mW power output. of the particulate material;

5.2 Sample holder, allowing control and measure- d its refractive index and viscosity shall be known
ment of the temperature to within + 0,3 “C. with an accuracy better than 0,5 %;

f 1 it shall be well filtered.

53. Primary beam stop.
Water is often used as a dispersion medium. The use
5.4 Optics and detector, to collect and digitize the of freshly distilled water (the still shall be built from
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radiation scattered by the sample at an angle of, e.g., quartz glassware) or of deionized and filtered (pore
size 0,2 pm) water is recommended. Since long-range
90”. If a polarization analyzer is included, it shall be
(standards.iteh.ai)
positioned in the vertical position, i.e. with a maximum particle interaction may affect the results for strongly
charge-stabilized dispersions, a trace of salt [c(NaCI)
transmission for light polarized with its electrical field
about IO-3 mol/l] may be added to such samples to
perpendicular to the plane of incident and scatteredISO 13321:1996
beams. reduce the range of particle interaction.
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Large fluctuations in recorded
scattered time-
5.5 Correlator. averaged signals (count rate) on short time scales
(e.g. 0,l s intervals) with bursts of high count rates
5.6 Computation unit.
indicate the presence of contaminating dust. The
appearance of sparkling centres in the beam also
usually indicates a dusty sample. Such liquids shall be
further cleaned (by filtration and/or distillation) before
6 Preliminary procedures use.

Detailed recommendations for sample preparation are

61. Instrument location given in annex E.
The instrument shall be placed in a clean environment, The dispersion liquid alone shall give no (or very low)
free from excessive electrical noise and mechanical scattered signal when checked in the instrument for
vibration and out of direct sunlight. If organic liquids dust or contaminants.
are used (e.g. as an index-matched liquid and/or as the
suspension medium), there shall be due regard to
local health and safety requirements, and the area 6.2.2 The concentration of particulate material shall
shall be well ventilated. The instrument shall be placed be above some minimum level and shall not exceed a
on a rigid table or bench to avoid necessity for fre- maximum level. The minimum level is determined by
quent realignment of the optical system. the following two requirements:

NOTE 3 - Alternatively, it may incorporate a rigid optical 1) The scattered intensity (count rate) of the sample
bench internally. containing the dispersed particles shall be at least
10 times higher than the signal scattered by the
WARNING - PCS instruments are equipped with dispersion medium alone.
low or medium power lasers whose radiation can
cause permanent eye damage, Never look into the 2) The number NV of particles in the scattering vol-
direct path of the laser beam or its reflections. Do ume shall be at least about 1 000 (any number in
not use highly reflecting surfaces when the laser the range 500-I 000 is acceptable).

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NOTE 4 -This number can be estimated from the av- requirements 2), 3) and 4) can only be fulfilled in
erage PCS diameter xpcs, from the particle volume frac- exceptional cases.
tion @and from the value Vof the measuring volume by

NV = 6qiWlm,$,

A typical order of magnitude of V is 1 O-6 cm3 (its value 7 Measurement procedure

can be found in the specifications provided by the in-
strument manufacturer). This equation relates to The measurement procedure assumes a properly
monosized materials only; for polydisperse samples the installed and aligned instrument and an operator
actual number of particles in the scattering volume may familiar with the instrument manual.
be much larger than predicted by this equation. If this
larger number density compromises the single-
scattering criteria 3) to 5), it may be that further dilution 71 Switch the instrument on and allow it to warm
is necessary, leading to a requirement that either the U;.
coherence aperture in the receiver be increased or the
incident laser beam made larger to increase the Typically about 0,5 h is required to stabilize the laser
measurement volume. A subsequent reduction in intensity and to bring the sample holder to the desired
measured intercept will then be incurred. This com- temperature.
promise is not permitted for the purposes of calibration
and verification.
7.2 Check the dispersion medium and record the
The maximum level is mainly determined by the average count rate scattered by a blank sample of it.
condition that only single scattering shall be ob-
served, i.e. no multiple scattering. The absence of
7.3 Place a sample of the dispersion (dispersion
significant contributions from multiple scattering
medium + particulate sample) in the instrument and
shall be determined by the three following checks.
allow temperature equilibrium to be established
between sample and sample holder. The temperature
3) The samples shall not look opaque but shall look
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clear, or only slightly cloudy or turbid. This shall al-
shall be controlled and measured with an accuracy of
0,3 “C.
ways be verified before putting the sample into the
instrument. (standards.iteh.ai)
NOTE 6 - It can take about 10 min for the liquid in the
measurement volume of a l-cm measuring cell to equilib-
4) The ratio of the measured intercept (see annex A
ISO 13321:1996 rate by thermal diffusion for a temperature change of only
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to its maximal value (see the 3 “C. Errors in particle size determined from aqueous
specifications of the manufacturer or 5c94d603327e/iso-13321-1996
clause 8 for dispersions will be recorded at the rate of 2 % per “C if the
its determination) shall be at least 0,8. temperature of the sample does not reach equilibrium.

5) if the instrument allows the laser beam passing A possible alternative method is to measure the room
through the sample to be viewed indirectly, a sharp temperature and then set the instrument to control the
parallel beam shall be observed; a spreading halo sample holder temperature to within 0,3 “C of the room
around the beam or any apparent absorbance shall temperature. Samples can then equilibrate at room tem-
not be present. perature and be measured immediately after insertion in
the sample holder. Alternatively, samples can be equilib-
rated in a thermostatted bath whose temperature is within
NOTE 5 - If possible, the following additional check is 0,3 “C of that of the instrument sample holder. In this case,
recommended. If a spectrophotometer is available, the remove the water from the cell before it contaminates the
optical density (O.D.) of the sample at the laser wavelength index-matching fluid.
used for a l-cm path length can be used as an indicator for
multiple scattering: for O.D. values larger than 0,04, mul-
tiple scattering effects are to be expected. 7.4 The following data shall be recorded: sample
identification, laser wavelength and scattering angle if
In many applications a volume fraction ((I) of dispersed necessary, measuring temperature, refractive index
particulate material in the range IO-5 to IO-4 fulfils the and viscosity of the dispersion medium, particle
requirements for particle sizes below about 500 nm. concentration and any other relevant details.
[In order to obtain the concentration range (c) in mass
of dispersed material per unit volume, multiply the
volume fraction by the particle density p (c = p&l For 7.5 Perform a preliminary measurement on the
poiydisperse and/or larger particles, it may not be dispersion to check that the particle concentration is
possible to find a concentration that satisfies require- not too low [less than about 1 000 particles in the
ments Z), 3) and 4) without either increasing the scattering volume, see 6.22 check Z)] or too high [see
coherence aperture of the receiver or increasing the 6.22, check 4)]. Check that the average scattered
diameter of the incident laser beam in order to in- intensity (count rate) is in the range 5 kcounts/s to
crease the measurement volume. If this is so, then 1 000 kcounts/s (5 000 counts/s to 1 000 000
the intercept values obtained may not meet the crite- counts/s). if these criteria are not met, repeat a
rion set out in 4). For particle sizes above 1 pm, the measurement on a sample with a different concen-

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IS0 13321:1996(E) @ IS0

tration of particulate material, or use another laser installation of the instrument and at time intervals
power level if possible. thereafter or in case of doubt.

Dispersions of polystyrene latex with narrow size

7.6 For each sample, at least six duplicate distribution with average particle diameter as
measurements of at least 60 s duration shall be measured by PCS of about 100 nm shall be used. For
performed and results stored. For sample count rates such dispersions the measured average particle size
below 20 000 counts/s (20 kcounts/s), the duration in shall be within 2 % of the stated size and the repeat-
seconds shall be equivalent to at least 1 200 000 ability, defined as IOOSN~I/(X), shall be better than
counts divided by the count rate in counts per second, 2 %; the polydispersity index shall be smaller than 0,l.
or 1 200 kilocounts divided by the count rate in kilo-
counts per second. NOTE 8 - Several suppliers provide traceable latices. A
polystyrene latex (catalogue No. 1963) with a certified
7.7 Record the average particle diameter xpcs and diameter of 100,7 nm can be obtained from the National
polydispersity index PI for each of the measurements. Institute of Standards and Technology (NIST), Gaithersburg,
MD, USA. Annex E provides guidelines for the preparation
Calculate the mean:
of a suitable sample.

(x)=$ Xi
The maximal value of the intercept of the autocorre-
lation function can be determined as follows:
i=l
a) select a collecting aperture for which a value of
and the estimated standard deviation: B max is available;

b) determine the intercept B (see annex A) with

dispersions of polystyrene latex (diameter about
SN-l =$T, f&i- b))’ 100 nm) with at least two different concentrations
i=l meeting requirements I), 2), 3) and 5) of 6.22;
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where N is the number
(i = 1 . . . IV) of xpcs and of PI.
(standards.iteh.ai)
of measurements xi c) if B depends in a systematic
concentration, extrapolate the
way on particle
results to infinite
dilution.
ISO 13321:1996
7.8 If a systematic concentration dependence of
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average particle size is observed, the results5c94d603327e/iso-13321-1996
of an
extrapolation to infinite dilution (or the results obtained
at the lowest acceptable concentration) shall be 9 Repeatability
reported.
The repeatability of the determination of average
particle size, defined as 1 00~~~I/(X), shall be better
NOTE 7 -Although checks 3) to 5) of 6.2.2 will exclude
than 5 %.
biasing effects due to multiple scattering, particle interac-
tions may, in particular for smaller particles (XPCSs 100 nm)
at volume fractions above O,Ol, bias the estimation of the
average PCS diameter via equation (C.5) of annex C. There-
fore for unknown dispersed systems it is recommended 10 Test report
that steps in 7.5 to 7.7 are repeated for several concen-
trations in a range of a factor of at least 2.
The average particle size xpcs and polydispersity index
PI shall be reported. These data shall be the mean and
7.9 Check at the end of the measurement that no standard deviation of at least six repeated measure-
significant sedimentation has occurred in the sample. ments on a sample and shall be calculated according
If sediment is found then either the sample may have to the method given in annex A. If the mean values of
aggregated and precipitated or the material may be xpcs and PI are concentration-dependent, their values
fundamentally unsuitable for measurement by PCS. extrapolated to infinite dilution, or the value obtained
at the lowest concentration, shall be reported.

The following details shall also be provided:

8 Calibration and validation
a) complete sample identification, including available
Since particle size determination by PCS is an abso- information about particle shape and homogen-
lute method based upon first principles, calibration as eity;
such is unnecessary. However, verification of the
instrument’s performance should be performed with b) instrument type and number, or its detailed de-
dispersions of particles of certified size after the first scription if a “homemade” instrument;

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0 IS0 IS0 13321:1996(E)

NOTE 9 - The following laser source details should value of the intercept B of the autocorrelation
also be provided: type, wavelength, power and that it function and value of the ratio BIBmax,
was vertically polarized;
count rate observed for the dispersion me-
c) dispersion conditions: dium,

- dispersing liquid and its cleaning/filtering pro- viscosity and refractive index of the dispersion
cedure, liquid,
temperature of the sample,
- concentration of particulate material,
- dispersing agents and their concentration, average count rate observed for the sample,

- dispersing procedure, number of analyses and their duration,

the variance of the fit [see annex A, equation
- sonication conditions: frequency and applied
power (if necessary); Mm
a ‘r
e) IT available, any other useful results or comments
d measurement conditions:
obtained at other scattering angles or from multi-
- number of concentrations investigated, angle results. If available, results obtained by other
data analysis methods and results from other par-
- number of particles in the measuring volume, ticle sizing methods;
as estimated from particle concentration and
average particle size xpcs, f) analyst identification:
- maximum value of the intercept B,,, of the - name and location of laboratory,
autocorrelation function attainable for the
given optical arrangement of the instrument. - operator’s name or initials,
- date.
NOTE 10 -This value can be determined with a
known material (see clause 8 and annex A),
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model test report form is given in annex B.

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IS0 13321:1996(E)

Annex A
(normative)

Computation of the average particle diameter and polydispersity index

In this International Standard the two parameters The parameters ao, al and a2 are determined by least
describing particle size distribution, i.e. the average squares fitting of the experimental estimates of $5) to
PCS diameter xpcs and the polydispersity index Pl, are equation (C.12) whereby the following function is
determined by a variant of the so-called cumulants minimized:
methodW

The basis for this analysis is that the experimentally s(a0, al, a2)= f,wj(yj - a0 + alz - azz!)’ . . . (A.2)
recorded intensity autocorrelation function Gz(Z) is j=l
approached by equation (C.12) of annex C. In order to
obtain a linear regression, this equation is transformed
as follows: In equation (A.2) the normalized weighting factor

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(standards.iteh.ai) J=.I

y(zj)=a()-aJTj +azZj ’ (j=1,2,3...m) . . .(A.l) ISO 13321:1996

accounts for the nonlinear transformation of the raw
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data G2 into the values for y(7).
where j is the number of the delay channel 5c94d603327e/iso-13321-1996
of the
correlator.
The average PCS diameter xpcs is calculated from al
The baseline or far point A can be determined in two bY
ways: by the total number of photon counts in a total
time duration of the considered experiment, or from
an estimate of Gz(z) for delay times z>> 25/(r). It is . . . (A.3)
recommended that both estimates of the baseline A
are determined and that the largest of both is re-
tained. However, for relative differences between the
two estimates of the baseline larger than IO-3 times
the smallest value, measurement shall be discarded
for further analysis.
k is the Boltzmann constant;

The range of values to be retained for yj = y(Zj> T is the absolute temperature;

shall correspond to a range in [c2(zj) - A] Of v is the viscosity of the dispersion medium;

[G2(Z1)-A]> [G2(2i)-A]> ([G2(q) - A]/ 1 00} with at n is the refractive index of the dispersion
medium;
least one value smaller than {[G2(q)-A]I50}. All
values of [Gz(Tj) -A] in this range must be positive, 0 is the scattering angle;

otherwise the measurement shall be discarded for is the laser wavelength in vacua.
further analysis.
The polydispersity index PI is related to a2 and al by
Finally, the number m of values of yj in the acceptable
range shall be at least 20. PI = 2a21aj2 . . . (A.41

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The actual value of the intercept B is computed from Measurements for which the ratio B/Bmax < 0,8 shall
a0 and A by be discarded.

B = [exp(2ao)]lA . . . (A.51 The variance, equal to

slim - 4) . . . VW
and is to be compared to the maximum value obtain-
able in the given experimental conditions, Bmax. can be used as a criterion of goodness of the fit.

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(standards.iteh.ai)
ISO 13321:1996
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