ISO 21501-1:2009

INTERNATIONAL ISO
STANDARD 21501-1
First edition
2009-06-01


Determination of particle size
distribution — Single particle light
interaction methods —
Part 1:
Light scattering aerosol spectrometer
Détermination de la distribution granulométrique — Méthodes
d'interaction lumineuse de particules uniques —
Partie 1: Spectromètre d'aérosol en lumière dispersée





Reference number
ISO 21501-1:2009(E)
©
ISO 2009

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ISO 21501-1:2009(E)
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ISO 21501-1:2009(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Terms and definitions. 1
3 Requirements . 3
3.1 Size range . 3
3.2 Counting efficiency. 3
3.2.1 General. 3
3.2.2 Lower size limit . 4
3.2.3 Upper size limit . 4
3.3 Size resolution . 4
3.4 Sizing accuracy. 5
3.5 Sampling flow rate. 5
3.6 Effective detection flow rate . 5
3.7 Maximum particle number concentration . 5
4 Test method. 5
4.1 Size calibration. 5
4.2 Effective detection flow rate . 6
4.3 Maximum particle number concentration . 7
4.4 Size resolution . 8
4.5 Counting efficiency. 9
Annex A (informative) Principle of the instruments . 11
Annex B (informative) Particle size standards . 18
Annex C (informative) Effects of the LSAS parameters on the particle size and particle number
concentration determination . 21
Annex D (informative) Representative sampling . 22
Annex E (informative) Example of an LSAS calibration with DEMS-classified PSL particles. 24
Bibliography . 26

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ISO 21501-1:2009(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.
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 21501-1 was prepared by Technical Committee ISO/TC 24, Particle characterization including sieving,
Subcommittee SC 4, Particle characterization.
ISO 21501 consists of the following parts, under the general title Determination of particle size distribution —
Single particle light interaction methods:
⎯ Part 1: Light scattering aerosol spectrometer
⎯ Part 2: Light scattering liquid-borne particle counter
⎯ Part 3: Light extinction liquid-borne particle counter
⎯ Part 4: Light scattering airborne particle counter for clean spaces

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ISO 21501-1:2009(E)
Introduction
Monitoring particle size distributions and particle number concentrations is required in various fields, e.g. in
filter manufacturing, in the electronic industry, in the pharmaceutical industry, in the chemical industry, in the
manufacture of precision machines and in medical operations. The aerosol spectrometer is a useful
instrument for the determination of the size distribution and number concentration of particles suspended in a
gas. The purpose of this part of ISO 21501 is to provide the calibration procedure and the validation method
for aerosol spectrometers, so as to improve the accuracy of the measurement result by aerosol spectrometers
in general, and to minimize the difference in the results measured by different instruments.

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INTERNATIONAL STANDARD ISO 21501-1:2009(E)

Determination of particle size distribution — Single particle
light interaction methods —
Part 1:
Light scattering aerosol spectrometer
1 Scope
This part of ISO 21501 specifies characteristics of a light scattering aerosol spectrometer (LSAS) which is
used for measuring the size, number concentration and number/size distribution of particles suspended in a
gas. The light scattering technique described in this part of ISO 21501 is based upon single particle
measurements. The size range of particles measured by this method is between approximately 0,06 µm to
45 µm in diameter.
Instruments that conform to this part pf ISO 21501 are used for the determination of the particle size
11 3
distribution and particle number concentration at relatively high concentrations of up to 10 particles/m .
Application fields include:
⎯ characterization of metered dose inhalers (MDI), dry powder inhalers (DPI) and nebulizers in pharmacy;
⎯ production control of active agents;
⎯ cut-off determination: impactors, cyclones and impingers;
⎯ atmospheric aerosols: bio-aerosols, stables/composting facilities, nebulized droplets, measurements in
street tunnels;
⎯ fractional separation efficiency determination of filters.
For the above-mentioned applications, aerosol spectrometers should determine the particle size distribution,
particle number concentration, size resolution and sizing accuracy as accurately as possible. These aerosol
spectrometers are not suitable for the classification of clean rooms.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
particle
discrete element of the material regardless of size
[ISO 2395:1990]
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ISO 21501-1:2009(E)
2.2
aerosol
suspension in a gaseous medium of solid particles, liquid particles or solid and liquid particles having a
negligible falling velocity
[ISO 4225:1994]
NOTE In general, one divides the atmospheric aerosol into three size categories: the superfine range x < 0,1 µm, the
fine range 0,1 µm < x < 1 µm and the coarse range x > 1 µm, where x is the particle diameter.
2.3
particle size
size of a sphere having the same physical properties in the method of analysis as the particle being described
NOTE 1 See also equivalent particle diameter (2.4).
NOTE 2 There is no single definition of particle size. Different methods of analysis are based on the measurement of
different physical properties. The physical property to which the equivalent diameter refers shall be indicated using a
suitable subscript or reference to the documentary measurement standard according to which the particle size was
measured.
In ISO 9276 the symbol x is used to denote the particle size or the diameter of a sphere. However, it is
recognized there that the symbol d is also widely used to designate these values. Therefore the symbol x may
be replaced by d where it appears.
2.4
equivalent particle diameter
diameter of the sphere with defined characteristics which behaves under defined conditions in exactly the
same way as the particle being described
2.5
light scattering equivalent particle diameter
x
sca
equivalent diameter of a homogeneous sphere of a reference substance (e.g. latex) which scatters defined
incident light with the same radiation efficiency into a defined solid angle element
2.6
number concentration density distribution
density (frequency) distribution of the particle number concentration represented as a function of the particle
size
2.7
particle concentration
indication of, e.g. particle numbers, particle mass, particle surface related to the unit volume of the carrier gas
NOTE For the exact concentration indication, information on the gaseous condition (temperature and pressure) or the
reference to a standard volume indication is necessary.
2.8
coincidence error
probability of the presence of more than one particle inside the sensing zone simultaneously
NOTE Coincidence error is related to particle number concentration and size of sensing zone.
2.9
counting efficiency
relation of the concentration determined from the counting rate of the measuring instrument and the real
concentration of the aerosol at the inlet of the instrument
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ISO 21501-1:2009(E)
2.10
border zone error
particle sizing error that occurs when particles pass through the optical border of the sensing zone
3 Requirements
3.1 Size range
The measuring size range is defined by the lower and upper size limit of quantification.
3.2 Counting efficiency
3.2.1 General
The counting efficiency is the relation of the particle number concentration C — determined from
N,measured
the counting rate of a device and corrected for possible coincidence errors — to the real particle number
concentration C of the aerosol at the inlet of the device. The counting efficiency [η x ] is a function of
( )
N,actual
the particle size and is expressed as the ratio:
Cx
( )
N,measured
η x = (1)
()
Cx
()
N, actual
The counting efficiency is also a function of signal processing, the homogeneous illumination of the measuring
volume and the extent to which the particles enter the measuring volume and flow rate.
Figure 1 shows a graphical representation of counting efficiency. In an ideal case, the counting efficiency in
the middle of the measuring range — as represented here — has the value one. If an experimental
examination results in a value deviating from one, then this is to be accounted for as a correction to the
measurement result. Usually, one defines the lower and/or upper size limit of the measuring range for the
particle size with the particle diameters for which the counting efficiency shows the value 0,5.
For a proper evaluation of a measuring instrument, it is useful to determine the complete counting efficiency
curve, or to indicate the particle diameters corresponding to values of the counting efficiency (e.g. at 0,1 and
0,9) besides those corresponding to a counting efficiency value of 0,5. The counting efficiency can be
determined according to 4.5.

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ISO 21501-1:2009(E)

Key
X particle size
Y counting efficiency η (x)
1 lower size limit
2 upper size limit
3 size range
[5]
Figure 1 — Graphical representation of the counting efficiency
3.2.2 Lower size limit
The lower size limit for the particle size is defined by convention to be the smallest diameter with which the
counting efficiency shall be 0,5 ± 0,15 (50 % ± 15 %; lower size limit of the measuring range).
3.2.3 Upper size limit
The upper size limit for the particle size is defined by convention to be the largest diameter with which the
counting efficiency shall be 0,5 ± 0,15 (50 % ± 15 %; upper size limit of measuring range). This is of particular
interest if the aerosol inlet is situated horizontally in the LSAS, as particle losses can occur in the LSAS
through impact and sedimentation.
3.3 Size resolution
The size resolution indicates which neighbouring particle sizes a particle measuring instrument can still
differentiate between and record separately. Aerosol spectrometers should determine the particle size
distribution and the particle number concentration as accurately as possible with high size resolution and good
sizing accuracy. The size resolution depends on particle size.
Almost all measuring instruments determine the particle number concentration in a limited number of size
classes which are firmly specified by the instrument design (e.g. instrument geometry, evaluation electronics,
evaluation software). In practical operation, the size resolution of an LSAS cannot be better than the width of
its size classes.
The size resolution can be determined according to 4.4.
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ISO 21501-1:2009(E)
3.4 Sizing accuracy
The sizing accuracy depends on the particle size. Therefore, the sizing accuracy can be evaluated for any
particle size as follows
xx−
sr
ε(x)=×100 (2)
x
r
where
ε()x is the ratio of particle size difference, in %;
x is the particle size of the certified reference material, in µm;
r
x is the particle size indicated by the LSAS, in µm.
s
The sizing accuracy of an LSAS describes the difference between the actual calibration particle size and the
particle size indicated by the instrument. The correlation between the particle size and size class stated by the
manufacturer (channel number) is normally based upon a calibration of the instrument with a known test
aerosol [mostly polymer latex (PL) particles]. Refer to Annex E.
3.5 Sampling flow rate
The sampling flow rate is the volumetric flow rate through the LSAS. Any error in the volume flow will affect
the reported particle number concentration with a proportional relationship. The error in the sampling flow rate
shall be within 5 %. Typical sampling flow rate is about 0,5 l/min to 5 l/min.
3.6 Effective detection flow rate
The effective detection flow rate is the volumetric flow rate through the optically and/or aerodynamically limited
sensing zone. For particle number concentration measurements, the effective detection flow rate shall be
evaluated. If the geometrical dimensions of the measuring volume are exactly known, the effective detection
flow rate can be determined by measuring the transit time of the particles through the measuring volume.
Otherwise, the effective detection flow rate has to be determined by a calibration experiment according to 4.2.
3.7 Maximum particle number concentration
11 3
The LSAS must be able to measure in high particle number concentrations up to 10 particles/m and the
maximum particle number concentration shall be specified by the manufacturer. According to 4.3, the
coincidence loss at the maximum particle number concentration of an LSAS shall be equal to or less than
10 %.
4 Test method
4.1 Size calibration
The light scattered by the individual particles is detected and transformed into a voltage pulse. These pulses
are classified according to their height in a multi-channel analyser (MCA). The result is a count rate histogram.
This histogram is transferred into a particle size distribution by applying the calibration curve. This calibration
curve, which is instrument specific, shall be determined either experimentally or theoretically. Normally,
aerosol spectrometers are calibrated with monodisperse test aerosols of known (traceable) size and refractive
index.
For the production of monodisperse test aerosols, several generation principles can be used. The size
distribution of test aerosols with small variances are obtained if aqueous suspensions of PL particles are
nebulized, dried and drawn through the instrument. Dry powder monodisperse PL particles are also useful for
calibration.
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ISO 21501-1:2009(E)

Figure 2 — Comparison of a theoretical calibration curve (line) and experimental data with
[14]
monodisperse aerosol particles (vertical bars)
Other techniques of providing narrow distributed aerosols are the vibrating-orifice generator or passing a
polydisperse aerosol through a differential mobility analyser, which extracts particles within a narrow size
range according to their electrical mobility.
4.2 Effective detection flow rate
The effective detection flow rate is obtained by relating the instrument particle count rate to a reference
particle number concentration measurement using an instrument with a higher particle number concentration
range than the instrument to be calibrated and that is traceable to internationally accepted standards.

Figure 3 — Example for effective detection flow rate and counting efficiency test set-up
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ISO 21501-1:2009(E)
The effective detection flow rate is as follows.
N
q = (3)
E
C
N,ref
where
q is the effective detection flow rate, in volume per time;
E
N is the measured particle count rate by LSAS, in number per time;
C is the measured particle number concentration by reference instruments, in number per volume.
N,ref
Effective detection flow rate calibration is necessary for counting efficiency evaluation. If the effective flow rate
is unknown, the effective detection flow rate calibration shall be performed with particle sizes much smaller
than the dimensions of the optical sensing zone, because larger particles effectively increase the resulting
measuring volume. However, the particle size must be such that both the LSAS under test and the reference
device have a counting efficiency close to 100 %.
4.3 Maximum particle number concentration
The maximum particle number concentration of an LSAS is determined by the coincidence loss, which is
normally limited to 10 %. The coincidence loss is determined by the sampling flow rate, the time required for
particles to pass through the sensing zone and the electrical signal processing time. These values are
determined by the design of the LSAS. Calculation of coincidence loss is as follows:
⎡⎤
Lq=−1 exp− ×t×C ×100 (4)
()
E,Nmax
⎣⎦
where
L is the coincidence loss, in %;
3
q is the effective detection flow rate, in m /s;
E
t is the time of passing through the effective sensing region plus electrical processing time, in s;
3
C is the maximum particle number concentration, in particles per m .
N,max
There are several practical ways to ensure sufficiently low coincidence loss.
⎯ A defined and reproducible concentration change of the same aerosol can be performed. If the number
concentration measured by the LSAS shows an equivalent change according to the defined conditions,
both concentrations are below the maximum particle number concentration.
⎯ A reference instrument may be used to measure the reference concentration C . In this case, the
N, ref
particle size must be chosen such that the counting efficiency of both the LSAS and the reference
instrument is close to 100 %. The number concentration range of the reference instrument must be
greater than the maximum particle number concentration of the LSAS. As long as C /C is
N, LSAS N, ref
greater than 0,9, the number concentration is below the maximum particle number concentration of the
LSAS.
In both cases, increasing the particle number concentration until a coincidence loss of 10 % is found will
determine the maximum particle number concentration of the LSAS.
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ISO 21501-1:2009(E)

Key
X particle number concentration from defined concentration changes or C
N, ref
Y LSAS particle number concentration
Figure 4 — Example for a concentration comparison measurement to determine the coincidence loss
and the maximum particle number concentration
4.4 Size resolution
The size resolution depends on the particle size. It can be determined by using monodisperse test aerosols
with known geometric standard deviation (σ ). Determine the median MCA channel (Ch2 in Figure 5). The
PSL
lower channel (Ch1) and upper channel (Ch3) are selected so that the relative counts are 61 % ± 10 % of the
median channel. Using the calibration curve, determine the particle sizes corresponding to Ch1 and Ch3.
Calculate the absolute value of the differences in particle sizes between the PL particle size and the particle
sizes corresponding to Ch1 and Ch3. The larger difference in particle size is the observed standard deviation
σ. Calculate the percentage of size resolution of the LSAS using Equation (5). (For further information, refer to
Annex E.)
22
σσ−
P
R=×100 (5)
x
where
R is the size resolution, in %;
σ is the observed standard deviation of the LSAS, in µm;
σ is the standard deviation of the particle size of the calibration particles, as reported by the reference
P
material producer, in µm;
x is the particle size of the calibration particle, in µm.

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ISO 21501-1:2009(E)

Key
X MCA channel
Y relative count
1 count rate distribution
2 lower side resolution
3 upper side resolution
Figure 5 — Size resolution
4.5 Counting efficiency
Counting efficiency can be experimentally determined by using the measuring set-up shown in 4.2.
For the test of the counting efficiency according to 3.2, it is assumed that C = C .
N, actual N, ref
It must be verified that
a) the LSAS under test and the reference counting device are below the 10 % coincidence limit,
b) the effective detection flow rate of the LSAS is correct,
c) the LSAS under test and the reference counter are calibrated correctly concerning particle size, the optics
is not polluted and the electronics is adjusted correctly.
If Y > 1 in Figure 1, then C ≠ C or a), b) and c) are not adhered to.
N, ref N, actual
If Y = 1 in Figure 1, then the LSAS under test has 100 % counting efficiency.
If Y < 1 in Figure 1, then this is due to the sensitivity of the sensor (point 1 in Figure 1; see also point B in
Figure 6) or to transport losses (point 2 of Figure 1).
If Y decreases with increasing C , then this is due to coincidence losses (see point A in Figure 6).
N, ref
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ISO 21501-1:2009(E)

Key
3
X reference particle number concentration C , in particles/cm
N, ref
Y counting efficiency C /C
N, LSAS N, ref
0,156 µm PL particles
0,234 µm PL particles
0,312 µm PL particles
Figure 6 — Counting efficiency is affected by coincidence A, incorrect flow rate (4.2)
and decreasing sensitivity B of the instrument
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ISO 21501-1:2009(E)
Annex A
(informative)

Principle of the instruments
A.1 Light scattering
A.1.1 General
The measuring method of an LSAS is based on the Lorentz-Mie theory. The particle characteristic, the
diameter, is determined for individual particles which are assumed to be spherical, and the number of
measured particles is registered at the same time.
If light with the wavelength λ meets a spherical particle with a diameter d and a refractive index n, then the
light is scattered in different directions (Figure A.1). The scattering of light at the particle is caused by
diffraction, refraction and reflection. The polarization plane of the incident light wave is also affected.
The intensity Ι of the light scattered from the single particles depends on the incident light intensity Ι , the
0
polarization angle Φ, the detection angle of the scattered light Θ, the refractive index n, the light wavelength λ
and the particle diameter d.
I=×IfΦΘ,,n,λ,d (A.1)
( )
0
By means of the scattering parameter α, introduced by Mie
π× d
α= (A.2)
λ
the relation between the sphere circumference π × d to the wavelength λ is used in Equation (A.1):
II=×fΦΘ,,n,α (A.3)
( )
0
With regard to the particle-size-depending scattering power, three ranges can be distinguished in terms of the
scattering parameter α (see Figure A.2).
a) Rayleigh range: α << 1; here the scattering power rises with the sixth power of the particle diameter, see
6 4
References [6] and [17]. The scattered light will be proportional to d /λ . This means if in the Rayleigh
range one wants to be able to measure a particle half as large as before (lower size limit), then doubling
the supplied quantity of light is not enough. The required quantity of light must be possibly 64-times
stronger than for a particle twice as large.
b) Mie range: 0,1 u α u 10; here the relation between the scattered light intensity and the particle size is
not monotonic for certain optical configurations (Figure A.2).
c) Fraunhofer range: α >> 1; here a quadratic relation between the scattering power and particle diameter
is valid.
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ISO 21501-1:2009(E)

Key
1 incident light
2 scattering area
α scattering parameter
λ wavelength of light
Θ scattering angle
n refractive index
r radius
Φ polarization angle
Figure A.1 — Principle of the scattering incident light by particle
A.1.2 Theoretical response function
Although it is advisable to calibrate an optical instrument experimentally by means of test aer
...