ASTM E2490-09(2015)
(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
5.1 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.
General Information
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Designation: E2490 − 09 (Reapproved 2015)
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 E2490; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 ISO Standards:
ISO 13320-1 Particle Size Analysis—Laser Diffraction
1.1 This guide deals with the measurement of particle size
Methods—Part 1: General Principles
distribution of suspended particles, which are solely or pre-
ISO 14488Particulate Materials—Sampling and Sample
dominantly sub-100 nm, using the photon correlation (PCS)
Splitting for the Determination of Particulate Properties
technique. It does not provide a complete measurement meth-
ISO 13321 Particle Size Analysis—Photon Correlation
odology for any specific nanomaterial, but provides a general
Spectroscopy
overview and guide as to the methodology that should be
followed for good practice, along with potential pitfalls.
3. Terminology
1.2 This standard does not purport to address all of the
3.1 Definitions of Terms Specific to This Standard:
safety concerns, if any, associated with its use. It is the
3.1.1 Some of the definitions in 3.1 will differ slightly from
responsibility of the user of this standard to establish appro-
those used within other (non-particle sizing) standards (for
priate safety, health, and environmental practices and deter-
example, repeatability, reproducibility). For the purposes of
mine the applicability of regulatory limitations prior to use.
thisGuideonly,weutilizethestateddefinitions,astheyenable
1.3 This international standard was developed in accor-
the isolation of possible errors or differences in the measure-
dance with internationally recognized principles on standard-
ment to be assigned to instrumental, dispersion or sampling
ization established in the Decision on Principles for the
variation.
Development of International Standards, Guides and Recom-
3.1.2 correlation coeffıcient, n—measure of the correlation
mendations issued by the World Trade Organization Technical
(or similarity/comparison) between 2 signals or a signal and
Barriers to Trade (TBT) Committee.
itself at another point in time.
3.1.2.1 Discussion—If there is perfect correlation (the sig-
2. Referenced Documents
nals are identical), then this takes the value 1.00; with no
2.1 ASTM Standards:
correlation then the value is zero.
E177Practice for Use of the Terms Precision and Bias in
3.1.3 correlogram or correlation function, n—graphicalrep-
ASTM Test Methods
resentation of the correlation coefficient over time.
E691Practice for Conducting an Interlaboratory Study to
3.1.3.1 Discussion—This is typically an exponential decay.
Determine the Precision of a Test Method
3.1.4 cumulants analysis, n—mathematical fitting of the
E1617Practice for Reporting Particle Size Characterization
correlation function as a polynomial expansion that produces
Data
some estimate of the width of the particle size distribution.
F1877Practice for Characterization of Particles
3.1.5 diffusion coeffıcient (self or collective), n—a measure
of the Brownian motion movement of a particle(s) in a
This guide is under the jurisdiction of ASTM Committee E56 on Nanotech-
medium.
nology and is the direct responsibility of Subcommittee E56.02 on Physical and
3.1.5.1 Discussion—After measurement, the value is be
Chemical Characterization.
inputted into in the Stokes-Einstein equation (Eq 1, see
Current edition approved April 1, 2015. Published April 2015. Originally
approved in 2008. Last previous edition in 2009 as E2490–09. DOI: 10.1520/
7.2.1.2(4)). Diffusion coefficient units in photon correlation
E2490-09R15.
spectroscopy (PCS) measurements are typically µm /s.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 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 − 09 (2015)
3.1.6 Mie region, n—in this region (typically where the size reportedresults.VariationinpHislikelytoaffectthedegreeof
of the particle is greater than half the wavelength of incident agglomeration and so forth.
light), the light scattering behavior is complex and can only be
3.1.13 rotational diffusion, n—a process by which the equi-
interpreted with a more rigorous and exact (and all-
librium statistical distribution of the overall orientation of
encompassing) theory.
molecules or particles is maintained or restored.
3.1.6.1 Discussion—This more exact theory can be used
3.1.14 translational diffusion, n—a process by which the
instead of the Rayleigh and Rayleigh-Gans-Debye approxima-
equilibrium statistical distribution of molecules or particles in
tions described in 3.1.8 and 3.1.9. The differences between the
space is maintained or restored.
approximations and exact theory are typically small in the size
3.1.15 z-average, n—harmonic intensity weighted average
range considered by this standard. Mie theory is needed in
particlediameter(thetypeofdiameterthatisisolatedinaPCS
order to convert an intensity distribution to one based on
experiment; a harmonic-type average is usual in frequency
volume or mass.
analyses) (see 8.9).
3.1.7 polydispersityindex(PI),n—descriptorofthewidthof
3.2 Acronyms:
theparticlesizedistributionobtainedfromthesecondandthird
3.2.1 APD—avalanche photodiode detector
cumulants (see 8.3).
3.2.2 CONTIN—mathematical program for the solution of
3.1.8 Rayleigh-Gans-Debye region, n—inthisregion(stated
non-linear equations created by Stephen Provencher and ex-
to be where the diameter of the particle is up to half the
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
describe the behavior of the particle with respect to incident
3.2.4 DLS—dynamic light scattering
light.
3.2.5 NNLS—non-negative least squares
3.1.9 Rayleigh region, n—size limit below which the scat-
3.2.6 PCS—photon correlation spectroscopy
tering intensity is isotropic—that is, there is no angular
3.2.7 PMT—photomultiplier tube
dependence for unpolarized light.
3.2.8 QELS—quasi-elastic light scattering
3.1.9.1 Discussion—Typically, this region is stated to be
3.2.9 RGB—Rayleigh-Gans Debye
where the diameter of the particle is less than a tenth of the
wavelength of the incident light. In this region a mathematical
4. Summary of Guide
approximation can be used to predict the light-scattering
behavior.
4.1 This Guide addresses the technique of photon correla-
tion spectroscopy (PCS) alternatively known as dynamic light
3.1.10 repeatability, n—in PCS and other particle sizing
scattering (DLS) or quasi-elastic light scattering (QELS) used
techniques, this usually refers to the precision of repeated
for the measurement of particle size within liquid systems. To
consecutive measurements on the same group of particles and
avoidconfusion,everyusageofthetermPCSimpliesthatDLS
isnormallyexpressedasarelativestandarddeviation(RSD)or
or QELS can be used in its place.
coefficient of variation (C.V.).
3.1.10.1 Discussion—The repeatability value reflects the
5. Significance and Use
stability (instrumental, but mainly the sample) of the system
over time. Changes in the sample could include dispersion 5.1 PCS is one of the very few techniques that are able to
(desired?) and settling. deal with the measurement of particle size distribution in the
nano-size region. This Guide highlights this light scattering
3.1.11 reproducibility, n—in PCS and particle sizing this
technique, generally applicable in the particle size range from
usually refers to second and further aliquots of the same bulk
the sub-nm region until the onset of sedimentation in the
sample (and therefore is subject to the homogeneity or other-
sample. The PCS technique is usually applied to slurries or
wise of the starting material and the sampling method em-
suspensions of solid material in a liquid carrier. It is a first
ployed).
principles method (that is, calibration in the standard under-
3.1.11.1 Discussion—In a slurry system, it is often the
standing of this word, is not involved). The measurement is
largest error when repeated samples are taken. Other defini-
hydrodynamically based and therefore provides size informa-
tions of reproducibility also address the variability among
tion in the suspending medium (typically water). Thus the
single test results gathered from different laboratories when
hydrodynamic diameter will almost certainly differ from other
inter-laboratory testing is undertaken. It is to be noted that the
size diameters isolated by other techniques and users of the
same group of particles can never be measured in such a
PCS technique need to be aware of the distinction of the
system of tests and therefore reproducibility values are typi-
various descriptors of particle diameter before making com-
cally be considerably in excess of repeatability values.
parisons between techniques. Notwithstanding the preceding
3.1.12 robustness, n—a measure of the change of the
sentence, the technique is widely applied in industry and
requiredparameterwithdeliberateandsystematicvariationsin
any or all of the key parameters that influence it.
3.1.12.1 Discussion—For example, dispersion time (ultra-
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
sound time and duration) almost certainly will affect the this standard.
E2490 − 09 (2015)
academiaasbotharesearchanddevelopmenttoolandasaQC one looks for stable count level without jumps or leaps in the
method for the characterization of submicron systems. level of the scattering counts that could be produced by
particles (of dust or contamination) falling through the mea-
6. Reagents
surement zone (‘number fluctuations’). Ideally the form of the
6.1 In general, no reagents specific to the technique are
correlogramisanexponentialdecaytoaflatbaseline(approxi-
necessary.However,dispersingandstabilizingagentsoftenare
mating to the photon counts in the system without sample) and
requiredforaspecifictestsampleinordertopreservecolloidal
not rise again (again indicating number fluctuations in the
stability during the measurement.Asuitable diluent is used to
data). Manufacturers also provide other means of assuring the
achieve a particle concentration appropriate for the measure-
reliability of the data and is recommended that these protocols
ment. Particle size is likely to undergo change on dilution, as
are consulted, as appropriate.
theionicenvironment,withinwhichtheparticlesaredispersed,
7.1.2 Giventhenatureoftheproducedintensitydistribution
changes in nature or concentration. This is particularly notice-
and the likelihood that the size standard has been certified by
able when diluting a monodisperse latex. A latex that is
electron microscopy (number distribution) care needs to be
-3
measured as 60 nm in1×10 M NaCl can have a hydrody-
exercised in direct comparison of the results. For a completely
-6
namic diameter of over 70 nm in1×10 M NaCl (close to
monodisperse sample, (every particle identical) then the num-
deionized water). In order to minimize any changes in the
ber and intensity distributions are essentially identical. For the
system on dilution, it is common to use what is commonly
real-worldsituationwherethereissomepolydispersity(width)
called the “mother liquor”. This is the liquid in which the
to the distribution, then the number distribution is expected to
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
7. Procedure
interaction of light with matter and the random or Brownian
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
-6
ments can be made by the PCS or any other light scattering
the sub-micron (<1×10 m) region, then these standards (for
technique.
example, 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,
single mode distribution, PI < 0.1) and, while confirming the x se
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2490 − 09 E2490 − 09 (Reapproved 2015)
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 E2490; 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
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.
2. Referenced Documents
2.1 ASTM Standards:
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E1617 Practice for Reporting Particle Size Characterization Data
F1877 Practice for Characterization of Particles
2.2 ISO Standards:
ISO 13320-1 Particle Size Analysis—Laser Diffraction Methods—Part 1: General Principles
ISO 14488 Particulate Materials—Sampling and Sample Splitting for the Determination of Particulate Properties
ISO 13321 Particle Size Analysis—Photon Correlation Spectroscopy
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 Some of the definitions in 3.1 will differ slightly from those used within other (non-particle sizing) standards (for example,
repeatability, reproducibility). For the purposes of this Guide only, we utilize the stated definitions, as they enable the isolation of
possible errors or differences in the measurement to be assigned to instrumental, dispersion or sampling variation.
3.1.2 correlation coeffıcient, n—measure of the correlation (or similarity/comparison) between 2 signals or a signal and itself
at another point in time.
This guide is under the jurisdiction of ASTM Committee E56 on Nanotechnology and is the direct responsibility of Subcommittee E56.02 on Physical and Chemical
Characterization.
Current edition approved April 1, 2009April 1, 2015. Published June 2009April 2015. Originally approved in 2008. Last previous edition in 20082009 as
E2490 – 08.E2490 – 09. DOI: 10.1520/E2490-09.10.1520/E2490-09R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
3.1.2.1 Discussion—
If there is perfect correlation (the signals are identical), then this takes the value 1.00; with no correlation then the value is zero.
3.1.3 correlogram or correlation function, n—graphical representation of the correlation coefficient over time.
3.1.3.1 Discussion—
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2490 − 09 (2015)
This is typically an exponential decay.
3.1.4 cumulants analysis, n—mathematical fitting of the correlation function as a polynomial expansion that produces some
estimate of the width of the particle size distribution.
3.1.5 diffusion coeffıcient (self or collective), n—a measure of the Brownian motion movement of a particle(s) in a medium.
3.1.5.1 Discussion—
After measurement, the value is be inputted into in the Stokes-Einstein equation (Eq 1, see 7.2.1.2(4)). Diffusion coefficient units
in photon correlation spectroscopy (PCS) measurements are typically μm /s.
3.1.6 Mie region, n—in this region (typically where the size of the particle is greater than half the wavelength of incident light),
the light scattering behavior is complex and can only be interpreted with a more rigorous and exact (and all-encompassing) theory.
3.1.6.1 Discussion—
This more exact theory can be used instead of the Rayleigh and Rayleigh-Gans-Debye approximations described in 3.1.8 and 3.1.9.
The differences between the approximations and exact theory are typically small in the size range considered by this standard. Mie
theory is needed in order to convert an intensity distribution to one based on volume or mass.
3.1.7 polydispersity index (PI), n—descriptor of the width of the particle size distribution obtained from the second and third
cumulants (see 8.3).
3.1.8 Rayleigh-Gans-Debye region, n—in this region (stated to be where the diameter of the particle is up to half the wavelength
of incident light), the scattering tends to the forward direction, and again, an approximation can be used to describe the behavior
of the particle with respect to incident light.
3.1.9 Rayleigh region, n—size limit below which the scattering intensity is isotropic—that is, there is no angular dependence
for unpolarized light.
3.1.9.1 Discussion—
Typically, this region is stated to be where the diameter of the particle is less than a tenth of the wavelength of the incident light.
In this region a mathematical approximation can be used to predict the light-scattering behavior.
3.1.10 repeatability, n—in PCS and other particle sizing techniques, this usually refers to the precision of repeated consecutive
measurements on the same group of particles and is normally expressed as a relative standard deviation (RSD) or coefficient of
variation (C.V.).
3.1.10.1 Discussion—
The repeatability value reflects the stability (instrumental, but mainly the sample) of the system over time. Changes in the sample
could include dispersion (desired?) and settling.
3.1.11 reproducibility, n—in PCS and particle sizing this usually refers to second and further aliquots of the same bulk sample
(and therefore is subject to the homogeneity or otherwise of the starting material and the sampling method employed).
3.1.11.1 Discussion—
In a slurry system, it is often the largest error when repeated samples are taken. Other definitions of reproducibility also address
the variability among single test results gathered from different laboratories when inter-laboratory testing is undertaken. It is to be
noted that the same group of particles can never be measured in such a system of tests and therefore reproducibility values are
typically be considerably in excess of repeatability values.
3.1.12 robustness, n—a measure of the change of the required parameter with deliberate and systematic variations in any or all
of the key parameters that influence it.
3.1.12.1 Discussion—
For example, dispersion time (ultrasound time and duration) almost certainly will affect the reported results. Variation in pH is
likely to affect the degree of agglomeration and so forth.
E2490 − 09 (2015)
3.1.13 rotational diffusion, n—a process by which the equilibrium statistical distribution of the overall orientation of molecules
or particles is maintained or restored.
3.1.14 translational diffusion, n—a process by which the equilibrium statistical distribution of molecules or particles in space
is maintained or restored.
3.1.15 z-average, n—harmonic intensity weighted average particle diameter (the type of diameter that is isolated in a PCS
experiment; a harmonic-type average is usual in frequency analyses) (see 8.9).
3.2 Acronyms:
3.2.1 APD—avalanche photodiode detector
3.2.2 CONTIN—mathematical program for the solution of non-linear equations created by Stephen Provencher and extensively
used in PCS (1).
3.2.3 CV—coefficient of variation
3.2.4 DLS—dynamic light scattering
3.2.5 NNLS—non-negative least squares
3.2.6 PCS—photon correlation spectroscopy
3.2.7 PMT—photomultiplier tube
3.2.8 QELS—quasi-elastic light scattering
3.2.9 RGB—Rayleigh-Gans Debye
4. Summary of Guide
4.1 This Guide addresses the technique of photon correlation spectroscopy (PCS) alternatively known as dynamic light
scattering (DLS) or quasi-elastic light scattering (QELS) used for the measurement of particle size within liquid systems. To avoid
confusion, every usage of the term PCS implies that DLS or QELS can be used in its place.
5. Significance and Use
5.1 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.
6. Reagents
6.1 In general, no reagents specific to the technique are necessary. However, dispersing and stabilizing agents often are required
for a specific test sample in order to preserve colloidal stability during the measurement. A suitable diluent is used to achieve a
particle concentration appropriate for the measurement. Particle size is likely to undergo change on dilution, as the ionic
environment, within which the particles are dispersed, changes in nature or concentration. This is particularly noticeable when
-3
diluting a monodisperse latex. A latex that is measured as 60 nm in 1 × 10 M NaCl can have a hydrodynamic diameter of over
-6
70 nm in 1 × 10 M NaCl (close to deionized water). In order to minimize any changes in the system on dilution, it is common
to use what is commonly called the “mother liquor”. This is the liquid in which the particles exist in stable form and is usually
obtained by centrifuging of the suspension or making up the same ionic nature of the dispersant liquid if knowledge of this material
is available. Many biological materials are measured in a buffer (often phosphate), which confers the correct (range of) conditions
of pH and ionic strength to assure stability of the system. Instability (usually through inadequate zeta potential (2) can promote
agglomeration leading to settling or sedimentation in a solid-liquid system or creaming in a liquid-liquid system (emulsion). Such
fundamental changes interfere with the stability of the suspension and need to be minimized as they affect the quality (accuracy
and repeatability) of the reported measurements. These are likely to be investigated in any robustness experiment.
7. Procedure
7.1 Verification:
The boldface numbers in parentheses refer to the list of references at the end of this standard.
E2490 − 09 (2015)
7.1.1 The instrument to be used in the determination should be verified for correct performance, within pre-defined quality
control limits, by following protocols issued by the instrument manufacturer. These confirmation tests normally involve the use
-6
of one or more NIST-traceable particle size standards. In the sub-micron (< 1 × 10 m) region, then these standards (e.g., (for
example, NIST, Duke Scientific- now part of Thermo Fisher Scientific) tend to be nearly monodisperse (that is, narrow, single
mode distribution, PI < 0.1) and, while confirming the x (size) axis, do not verify the y (or quantity axis). Further, there is a lack
of available standards for the sub-20 nm region and therefore biological materials (e.g., (for example, bovine serum albumin–BSA,
cholesterol, haem, size controlled dendrimers, Au sols) of known size (often by molecular modeling) can be utilized. Note that PCS
is a first principles measurement and thus calibration in the formal sense (adjustment of the instrument to read a true and known
value) cannot be undertaken. In the event of a “failure” at the verification stage, then the issues to check involve quality of the
dilution water, state of dispersion and stability of the standard under dilution plus instrumental issues such as thermal stability,
cleanliness and alignment of optical components. The raw correlogram data can be examined during and after acquisition. Such
examination requires some experience and training. During data acquisition one looks for stable count level without jumps or leaps
in the level of the scattering counts that could be produced by particles (of dust or contamination) falling through the measurement
zone (‘number fluctuations’). Ideally the form of the correlogram is an exponential decay to a flat baseline (approximating to the
photon counts in the system without sample) and not rise again (again indicating number fluctuations in the data). Manufacturers
also provide other means of assuring the reliability of the data and is recommended that these protocols are consulted, as
appropriate.
7.1.2 Given the nature of the produced intensity distribution and the likelihood that the size standard has been certified by
electron microscopy (number distribution) care needs to be exercised in direct comparison of the results. For a completely
monodisperse sample, (every particle identical) then the number and intensity distributions are essentially identical. For the
real-world situation where there is some polydispersity (width) to the distribution, then the number distribution is expected to be
smaller than the produced intensity distribution; the greater the polydispersity, then the larger the differences between intensity,
volume
...
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