ASTM E2490-08
(Guide)Standard Guide for Measurement of Particle Size Distribution of Nanomaterials in Suspension by Photon Correlation Spectroscopy (PCS)
Standard Guide for Measurement of Particle Size Distribution of Nanomaterials in Suspension by Photon Correlation Spectroscopy (PCS)
SIGNIFICANCE AND USE
PCS is one of the very few techniques that are able to deal with the measurement of particle size distribution in the nano-size region. This Guide highlights this light scattering technique, generally applicable in the particle size range from the sub-nm region until the onset of sedimentation in the sample. The PCS technique is usually applied to slurries or suspensions of solid material in a liquid carrier. It is a first principles method (that is, calibration in the standard understanding of this word, is not involved). The measurement is hydrodynamically based and therefore provides size information in the suspending medium (typically water). Thus the hydrodynamic diameter will almost certainly differ from other size diameters isolated by other techniques and users of the PCS technique need to be aware of the distinction of the various descriptors of particle diameter before making comparisons between techniques. Notwithstanding the preceding sentence, the technique is widely applied in industry and academia as both a research and development tool and as a QC method for the characterization of submicron systems.
SCOPE
1.1 This guide deals with the measurement of particle size distribution of suspended particles, which are solely or predominantly sub-100 nm, using the photon correlation (PCS) technique. It does not provide a complete measurement methodology for any specific nanomaterial, but provides a general overview and guide as to the methodology that should be followed for good practice, along with potential pitfalls.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E2490–08
Standard Guide for
Measurement of Particle Size Distribution of Nanomaterials
in Suspension by Photon Correlation Spectroscopy (PCS)
This standard is issued under the fixed designation E 2490; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope other (non-particle sizing) standards (for example, repeatabil-
ity, reproducibility). For the purposes of this Guide only, we
1.1 This guide deals with the measurement of particle size
utilize the stated definitions, as they enable the isolation of
distribution of suspended particles, which are solely or pre-
possible errors or differences in the measurement to be as-
dominantly sub-100 nm, using the photon correlation (PCS)
signed to instrumental, dispersion or sampling variation.
technique. It does not provide a complete measurement meth-
3.1.1 correlation coeffıcient, n—measure of the correlation
odology for any specific nanomaterial, but provides a general
(or similarity/comparison) between 2 signals or a signal and
overview and guide as to the methodology that should be
itself at another point in time.
followed for good practice, along with potential pitfalls.
3.1.1.1 Discussion—If there is perfect correlation (the sig-
1.2 This standard does not purport to address all of the
nals are identical), then this takes the value 1.00; with no
safety concerns, if any, associated with its use. It is the
correlation then the value is zero.
responsibility of the user of this standard to establish appro-
3.1.2 correlogram or correlation function, n—graphical
priate safety and health practices and determine the applica-
representation of the correlation coefficient over time.
bility of regulatory limitations prior to use.
3.1.2.1 Discussion—This is typically an exponential decay.
2. Referenced Documents
3.1.3 cumulants analysis, n—mathematical fitting of the
correlation function as a polynomial expansion that produces
2.1 ASTM Standards:
some estimate of the width of the particle size distribution.
E 177 Practice for Use of the Terms Precision and Bias in
3.1.4 diffusion coeffıcient (self or collective), n—a measure
ASTM Test Methods
of the Brownian motion movement of a particle(s) in a
E 1617 Practice for Reporting Particle Size Characteriza-
medium.
tion Data
3.1.4.1 Discussion—After measurement, the value is be
F 1877 Practice for Characterization of Particles
inputted into in the Stokes-Einstein equation (Eq 1, see
2.2 ISO Standards:
7.2.1.2(4)). Diffusion coefficient units in photon correlation
ISO 13320-1 Particle Size Analysis—Laser Diffraction
spectroscopy (PCS) measurements are typically µm /s.
Methods—Part 1: General Principles
3.1.5 Mie region, n—in this region (typically where the size
ISO 14488 Particulate Materials—Sampling and Sample
of the particle is greater than half the wavelength of incident
Splitting for the Determination of Particulate Properties
light), the light scattering behavior is complex and can only be
ISO 13321 Particle Size Analysis—Photon Correlation
interpreted with a more rigorous and exact (and all-
Spectroscopy
encompassing) theory.
3. Terminology
3.1.5.1 Discussion—This more exact theory can be used
instead of the Rayleigh and Rayleigh-Gans-Debye approxima-
3.1 Definitions of Terms Specific to This Standard—Some of
tions described in 3.1.7 and 3.1.8. The differences between the
the definitions in 3.1 will differ slightly from those used within
approximations and exact theory are typically small in the size
range considered by this standard. Mie theory is needed in
This guide is under the jurisdiction of ASTM Committee E56 on Nanotech-
order to convert an intensity distribution to one based on
nology and is the direct responsibility of Subcommittee E56.02 on Characterization:
volume or mass.
Physical, Chemical, and Toxicological Properties.
3.1.6 polydispersity index (PI), n—descriptor of the width
Current edition approved Oct. 1, 2008. Published November 2008.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
of the particle size distribution obtained from the second and
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
third cumulants (see 8.3).
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2490–08
3.1.7 Rayleigh-Gans-Debyeregion,n—inthisregion(stated 3.2.2 CONTIN—mathematical program for the solution of
to be where the diameter of the particle is up to half the non-linear equations created by Stephen Provencher and ex-
tensively used in PCS (1)
wavelength of incident light), the scattering tends to the
3.2.3 CV—coefficient of variation
forward direction, and again, an approximation can be used to
3.2.4 DLS—dynamic light scattering
describe the behavior of the particle with respect to incident
3.2.5 NNLS—non-negative least squares
light.
3.2.6 PCS—photon correlation spectroscopy
3.1.8 Rayleigh region, n—size limit below which the scat-
3.2.7 PMT—photomultiplier tube
tering intensity is isotropic—that is, there is no angular
3.2.8 QELS—quasi-elastic light scattering
dependence for unpolarized light.
3.2.9 RGB—Rayleigh-Gans Debye
3.1.8.1 Discussion—Typically, this region is stated to be
where the diameter of the particle is less than a tenth of the
4. Summary of Guide
wavelength of the incident light. In this region a mathematical
4.1 This Guide addresses the technique of photon correla-
approximation can be used to predict the light-scattering
tion spectroscopy (PCS) alternatively known as dynamic light
behavior.
scattering (DLS) or quasi-elastic light scattering (QELS) used
3.1.9 repeatability, n—in PCS and other particle sizing
for the measurement of particle size within liquid systems. To
techniques, this usually refers to the precision of repeated
avoidconfusion,everyusageofthetermPCSimpliesthatDLS
consecutive measurements on the same group of particles and or QELS can be used in its place.
is normally expressed as a relative standard deviation (RSD) or
5. Significance and Use
coefficient of variation (C.V.).
5.1 PCS is one of the very few techniques that are able to
3.1.9.1 Discussion—The repeatability value reflects the sta-
deal with the measurement of particle size distribution in the
bility (instrumental, but mainly the sample) of the system over
nano-size region. This Guide highlights this light scattering
time. Changes in the sample could include dispersion (de-
technique, generally applicable in the particle size range from
sired?) and settling.
the sub-nm region until the onset of sedimentation in the
3.1.10 reproducibility, n—in PCS and particle sizing this
sample. The PCS technique is usually applied to slurries or
usually refers to second and further aliquots of the same bulk
suspensions of solid material in a liquid carrier. It is a first
sample (and therefore is subject to the homogeneity or other-
principles method (that is, calibration in the standard under-
wise of the starting material and the sampling method em-
standing of this word, is not involved). The measurement is
ployed).
hydrodynamically based and therefore provides size informa-
3.1.10.1 Discussion—In a slurry system, it is often the
tion in the suspending medium (typically water). Thus the
largest error when repeated samples are taken. Other defini-
hydrodynamic diameter will almost certainly differ from other
tions of reproducibility also address the variability among
size diameters isolated by other techniques and users of the
single test results gathered from different laboratories when
PCS technique need to be aware of the distinction of the
inter-laboratory testing is undertaken. It is to be noted that the
various descriptors of particle diameter before making com-
same group of particles can never be measured in such a parisons between techniques. Notwithstanding the preceding
system of tests and therefore reproducibility values are typi- sentence, the technique is widely applied in industry and
academia as both a research and development tool and as a QC
cally be considerably in excess of repeatability values.
method for the characterization of submicron systems.
3.1.11 robustness, n—a measure of the change of the
required parameter with deliberate and systematic variations in
6. Reagents
any or all of the key parameters that influence it.
6.1 In general, no reagents specific to the technique are
3.1.11.1 Discussion—For example, dispersion time (ultra-
necessary. However, dispersing and stabilizing agents often are
sound time and duration) almost certainly will affect the
required for a specific test sample in order to preserve colloidal
reported results.Variation in pH is likely to affect the degree of
stability during the measurement. A suitable diluent is used to
agglomeration and so forth.
achieve a particle concentration appropriate for the measure-
3.1.12 rotational diffusion, n—a process by which the
ment. Particle size is likely to undergo change on dilution, as
equilibrium statistical distribution of the overall orientation of
theionicenvironment,withinwhichtheparticlesaredispersed,
molecules or particles is maintained or restored.
changes in nature or concentration. This is particularly notice-
3.1.13 translational diffusion, n—a process by which the
able when diluting a monodisperse latex. A latex that is
-3
equilibrium statistical distribution of molecules or particles in measured as 60 nm in 1 3 10 M NaCl can have a hydrody-
-6
space is maintained or restored. namic diameter of over 70 nm in 1 3 10 M NaCl (close to
deionized water). In order to minimize any changes in the
3.1.14 z-average, n—harmonic intensity weighted average
system on dilution, it is common to use what is commonly
particle diameter (the type of diameter that is isolated in a PCS
called the “mother liquor”. This is the liquid in which the
experiment; a harmonic-type average is usual in frequency
analyses) (see 8.9).
3.2 Acronyms:
The boldface numbers in parentheses refer to the list of references at the end of
3.2.1 APD—avalanche photodiode detector this standard.
E2490–08
particles exist in stable form and is usually obtained by be smaller than the produced intensity distribution; the greater
centrifuging of the suspension or making up the same ionic the polydispersity, then the larger the differences between
nature of the dispersant liquid if knowledge of this material is intensity, volume and number distributions. Note that verifica-
available. Many biological materials are measured in a buffer tion of a system only demonstrates that the instrument is
(often phosphate), which confers the correct (range of) condi- performing adequately with the prescribed standard materials.
tions of pH and ionic strength to assure stability of the system. Practical considerations for real-world materials (especially
Instability (usually through inadequate zeta potential (2) can ‘dispersion’ if utilized or if the distribution is relatively
promote agglomeration leading to settling or sedimentation in polydisperse) mean that the method used to measure that
a solid-liquid system or creaming in a liquid-liquid system real-world material needs to be carefully evaluated for preci-
(emulsion). Such fundamental changes interfere with the sta- sion (repeatability).
bilityofthesuspensionandneedtobeminimizedastheyaffect
7.2 Measurement
the quality (accuracy and repeatability) of the reported mea-
7.2.1 Introduction:
surements.Thesearelikelytobeinvestigatedinanyrobustness
7.2.1.1 The measurement of particle size distribution in the
experiment.
nano- (sub 100 nm) region by light scattering depends on the
interaction of light with matter and the random or Brownian
7. Procedure
motion that particle exhibits in liquid medium in free suspen-
7.1 Verification:
sion. There must be an inhomogeneity in the refractive indices
7.1.1 The instrument to be used in the determination should
of particle and the medium within which it exists in order for
be verified for correct performance, within pre-defined quality
light scattering to occur. Without such an inhomogeneity (for
control limits, by following protocols issued by the instrument
example, in so-called index-matched systems) there is no
manufacturer. These confirmation tests normally involve the
scattering and the particle is invisible to light and no measure-
use of one or more NIST-traceable particle size standards. In
ments can be made by the PCS or any other light scattering
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the sub-micron (< 1 3 10 m) region, then these standards
technique.
(e.g., NIST, Duke Scientific- now part of Thermo Fisher
7.2.1.2 For particles < 100 nm, as considered in this guide,
Scientific) tend to be nearly monodisperse (that is, narrow,
several facts hold true:
single mode distribution, PI < 0.1) and, while confirming the x
(1) The amount of scattering is weak in relative terms and
(size) axis, do not verify the y (or quantity axis). Further, there
depends highly on the size of the particle. In the Rayleigh
is a lack of available standards for the sub-20 nm region and
approximation region (typically d < l/10 in which d is the
therefore biological materials (e.g., bovine serum albumin-
diameter of particle and l is the wavelength of light em-
–BSA, cholesterol, haem, size controlled dendrimers,Au sols)
ployed), then this intensity of scattering is proportional to
of known size (often by molecular modeling) can be utilized.
6 2 2
r – or (volume) or (relative molecular mass) . With a com-
Note that PCS is a first principles measurement and thus
monlyutilizedhelium-neon(He-Ne)laser(632.8nm),thenthis
calibration in the formal sense (adjustment of the instrument to
limit is approximately 60 nm.This means, in practice, that a 60
read a true and known value) cannot be undertaken. In the
nm particle scatters 1 million times as much light as a 6nm
event of a “failure” at the verification stage, then the issues to
particle of the same composition. Thus, it is imperative that
check involve quality of the dilution water, state of dispersion
solutions are kept free of any contaminating particles, for
and stability of the standard under dilution plus instrumental
example dust, that are often present in the local environment
issues such as thermal stability, cleanliness and alignment of
and is
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