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ZETASIZER NANO SERIES AND HPPS TRAINING COURSE
Dr Mike Kaszuba (Product Technical Specialist)E-mail:[email protected]
PART ONETHEORY OF SIZE BY DLS 1
DLS and BROWNIAN MOTION
Dynamic Light Scattering or
Photon Correlation Spectroscopy or
Quasi-Elastic Light Scattering
measures Brownian motion
and relates it to size
2
BROWNIAN MOTION
Random movement of particles due to the bombardment
by the solvent molecules that surround them
2
BROWNIAN MOTION
Temperature must be accurately known because we need to know the viscosity
The temperature needs to be stable otherwise convection currents in the sample will cause non-random movements which will ruin correct size interpretation
The larger the particle the more slowly the Brownian motion will be
Higher the temperature the more rapid the Brownian motion will be
Velocity of the Brownian motion is defined by the translational diffusion coefficient (D)
2
STOKES-EINSTEIN EQUATION
d(H) =3 D
kT
D = diffusion coefficient
d(H) = hydrodynamic diameterk = Boltzmann’s constantT = absolute temperature= viscosity
where
2
HYDRODYNAMIC DIAMETER
The diameter which is measured in DLS is a value
that refers to how a particle moves within a liquid
It is called the HYDRODYNAMIC DIAMETER
The diameter that is obtained is the diameter of a
sphere that has the same translational diffusion
coefficient as the particle
2
HYDRODYNAMIC DIAMETER:Effect of Ionic Strength
1/K
1/K (Debye Length) is the thickness of the electrical double layer
It is dependent upon the ionic strength of the medium
In low ionic strength media (eg DI water) the double layer is extended
A latex standard diluted in DI water will give the wrong result (too high)
Particle Diameter
Hydrodynamic Diameter
3
HYDRODYNAMIC DIAMETER:Effect of Ionic Strength
1/K
1/K (Debye Length) is the thickness of the electrical double layer
It is dependent upon the ionic strength of the medium
In high ionic strength media the double layer is suppressed
Latex standards should be diluted in 10mM NaCl to suppress the double layer and hence give the correct result (ISO13321)
Particle Diameter
Hydrodynamic Diameter
3
HYDRODYNAMIC DIAMETER:Effect of Surface Coatings
A layer of molecules on the particle surface will slow the diffusion speed down
The hydrodynamic diameter will therefore be influenced
Reported hydrodynamic diameter
3
NON SPHERICAL PARTICLES
RAPID
SLOW
EQUIVALENT SPHERE
The hydrodynamic diameter reported will be based upon the sphere which has the same average diffusion coefficient as that of the particles
3
INTERACTION OF LIGHT WITH MATTER: Rayleigh Approximation
For small particles (d /10 or around 60nm for He-Ne laser) scattering is isotropic i.e. equal in all directions
Rayleigh approximation tells us that I d6 and I 1/4
where I = intensity of scattered light, d = particle diameter and = laser wavelength
4
INTERACTION OF LIGHT WITH MATTER: Rayleigh-Gans-Debye (RGD) Theory
A modification of Rayleigh theory
Used for particles somewhat larger than those described
as Rayleigh scatterers but with a small relative refractive
index
Used in HPPS software version 1 to convert the
measured Intensity distribution data into Volume
4
INTERACTION OF LIGHT WITH MATTER: Mie Theory
When the size of the particles become equivalent to the wavelength of the laser, the scattering becomes a complex function with maxima and minima with respect to angle
Mie theory is the only theory that explains correctly the maxima and minima in the plot of intensity with angle
This theory is used in version 3 of the HPPS and Nano Series software
4
INTERACTION OF LIGHT WITH MATTER: Mie Theory
Polystyrene latex spheres at 174.65o: RIParticle= 1.59, AbsParticle= 0.001, RIMedium= 1.33, = 633nm
5
OPTICAL CONFIGURATION OF THE NANO ZS
6
UNIQUE FEATURES OF THE HPPS, NANO S AND NANO ZS (1): NIBS
The HPPS, Nano S and Nano ZS measures the
scattered light at an angle of 173o
This is known as backscatter detection
The optics are not in contact with the sample
The detection optics are therefore said to be non-
invasive
7
Several reasons for using NIBS detection: The laser does not have to travel through the entire
sample This reduces multiple scattering effects This means that higher concentrations of sample can
be measured Contaminants such as dust particles within the
dispersant scatter light mainly in the forward direction Therefore backscatter detection reduces the effects
of dust
7
UNIQUE FEATURES OF THE HPPS, NANO S AND NANO ZS (1): NIBS
The measurement position within the cuvette of the HPPS, Nano S and Nano ZS can be changed
This allows a much larger range of sample concentrations to be measured
In the HPPS, the measurement position is changed by moving the cuvette
In the Nano S and Nano ZS, the measurement position is changed by moving the focussing lens.
The cuvette position is automatically determined by the software
7
UNIQUE FEATURES OF THE HPPS, NANO S AND NANO ZS (2): VARIABLE MEASUREMENT POSITION
Small particles or dilute samples Measure close to cell centre to maximise measurement volume and minimise flare
7
UNIQUE FEATURES OF THE HPPS, NANO S AND NANO ZS (2): VARIABLE MEASUREMENT POSITION
Concentrated samplesMeasure close to the cell wall to reduce the light path through
the sample and hence minimise multiple scattering
7
UNIQUE FEATURES OF THE HPPS, NANO S AND NANO ZS (2): VARIABLE MEASUREMENT POSITION
HOW DOES A DLS EXPERIMENT WORK?
We need a way of measuring the Brownian motion
of particles and relating this to size
In DLS, the intensity fluctuations of scattered light
arising from Brownian motion are measured.
How do these fluctuations in the intensity of
scattered light arise?
8
THE SPECKLE PATTERN
LaserIncident Beam
Axis
SampleCell
ScreenSpecklePattern
8
BROWNIAN MOTION AND SCATTERED LIGHT
Screen
Two beams interfere and‘cancel each other out’
resulting in a decreased intensity in the scattered light
Consider 2 stationary particles
9
BROWNIAN MOTION AND SCATTERED LIGHT
Screen
Two beams interfere and‘enhance each other’
resulting in an increased intensity in the scattered light
9
BROWNIAN MOTION AND SCATTERED LIGHT
Many scattered beams interfere with one anotherresulting in a very complex
intensity pattern of ‘speckles’
Screen
Consider many
particles
9
INTENSITY FLUCTUATIONS
For a system of particles undergoing Brownian motion, a
speckle pattern is observed where the position of each
speckle is seen to be in constant motion
This is because the phase addition from the moving
particles is constantly evolving and forming new patterns
The rate at which these intensity fluctuations occur will
depend on the size of the particles
9
INTENSITY FLUCTUATIONS
Large Particles
Small Particles
Time
Time
Inte
nsity
Inte
nsity
10
HOW A CORRELATOR WORKS
Inte
nsity
Time
0 t tt t t
11
HOW A CORRELATOR WORKS
If the particles are large, the signal will be changing
slowly and the correlation will persist for a long time
If the particles are small and moving rapidly then the
correlation will disappear more rapidly
11
TYPICAL CORRELOGRAM FROM SAMPLE CONTAINING LARGE PARTICLES
12
0
0.1000
0.2000
0.3000
0.4000
0.5
0.6000
0.7000
0.8000
0.9000
0.1000 10. 1000. 1.e+5 1.e+7 1.e+9
Cor
rela
tion
Coe
ffici
ent
Time (us)
Raw Correlation Data
TYPICAL CORRELOGRAM FROM SAMPLE CONTAINING SMALL PARTICLES
12
0
0.1000
0.2000
0.3000
0.4000
0.5
0.6000
0.7000
0.1000 10. 1000. 1.e+5 1.e+7 1.e+9
Cor
rela
tion
Coe
ffici
ent
Time (us)
Raw Correlation Data
THE CORRELATION FUNCTION
The correlation function G()
= <I(t).I(t+)>
where is the time difference (the sample time) of the correlator
13
THE CORRELATION FUNCTION
G() = A[1 + B exp(-2)]where:- A = the baseline of the correlation functionB = intercept of the correlation function = Dq2where:-D = transational diffusion coefficientq = (4n / o) sin (/2)where:-n = refractive index of solutiono = wavelength of the laser=scattering angle
For monodisperse particles the correlation function is
13
THE CORRELATION FUNCTION
G() = A[1 + B g1()2]For polydisperse particles the correlation function is
where:- g1() is the sum of all exponential decayscontained in the correlation function
13
OBTAINING SIZE FROM THE CORRELATION FUNCTION
Size is obtained from the correlation function by using various algorithms
In the DTS software, the mean diameter (z-average) and the width of the distribution (polydispersity) are obtained by using the cumulants analysis as described in ISO13321
13
OBTAINING SIZE FROM THE CORRELATION FUNCTION
The size distribution displayed is obtained from a non-negative least squares (NNLS) analysis
The size distribution is a plot of the relative intensity of light scattered by particles in various size classes
There are 70 size classes logarithmically spaced It is an INTENSITY distribution
13
EVALUATING THE CORRELATION FUNCTION
If the INTENSITY distribution is a fairly smooth peak, there is little point in conversion to a VOLUME distribution using Mie theory
However, if the INTENSITY plot shows a substantial tail, or more than one peak, then a VOLUME distribution will give a more realistic view of the importance of the tail or second peak
NUMBER distributions are of little use because small errors in data acquisition can lead to huge errors in the distribution by number and are not displayed
13
INTENSITY, VOLUME AND NUMBER DISTRIBUTIONS
NUMBER43
VOLUME= r3
INTENSITY= d 6
Diameter (nm)
Rel
ativ
e %
in c
lass
5 10 10050
1 1
Diameter (nm)
Rel
ativ
e %
in c
lass
5 10 10050
1
1000
Diameter (nm)R
elat
ive
% in
cla
ss5 10 10050
1
1,000,000
Mixture containing equal numbers of 5 and 50nm spherical particles
THE CUMULANTS ANALYSIS
The decay in the correlation function [G] is exponential
The simplest way of obtaining a size from this curve is to use the cumulants analysis as described in ISO13321
A 3rd order fit of a polynomial to a semi-log plot of the correlation function is performed
If the distribution is polydisperse, the semi-log plot will be curved
14
THE CUMULANTS ANALYSIS
ISO13321 states that a 3rd order fit of a polynomial should be used
Ln[G1] = a + b + c2
The value of b = z-average diffusion coefficient 2c/b2 = polydispersity index (the width of the
distribution) This method only gives a mean and a width and is
only a good description for reasonably narrow monomodal samples
It is an INTENSITY mean size15
THE CUMULANTS ANALYSIS
0.2
0.3
0.5
0.4
0.7
0.6
0.8
0 1 20.5 1.5
Ln G1
Time (ms) (x 103)
b = z-average diffusion coefficient
2c/b2 = polydispersity
index
15
POLYDISPERSITY INDEX
0 to 0.05 - Only normally encountered with latex standards or particles made to be monodisperse
0.05 to 0.08 - Nearly monodisperse sample. PCS cannot normally extract a distribution within this range
0.08 to 0.7 - This is a mid-range polydispersity, it is the range over which the distribution algorithm based on NNLS best operates over
Greater than 0.7 - Very polydisperse. Care should be taken in interpreting results as the sample may not be suitable for the technique, e.g. a sedimenting high size tail may be present
16
16
DETERMINATION OF THE INTENSITY SIZE DISTRIBUTION
The intensity distribution shown in the DTS software is obtained by analysis of the correlation function by a non-negative least squares (NNLS) analysis
The analysis of light scattering data can produce a number of possible solutions for the same set of data (smooth distributions/multiple peaks)
In order to cater for different applications, two analysis models are available in the software
Both of these algorithms are NNLS with the only difference being the level of smoothing applied to the data prior to analysis
DETERMINATION OF THE INTENSITY SIZE DISTRIBUTION
17
DETERMINATION OF THE INTENSITY SIZE DISTRIBUTION
A general purpose algorithm is available that is suitable for most samples
However this will not maximize the resolution available by the technique where some information is known about the sample
A second analysis model is provided so that knowledge about the sample can be used to improve the measurement resolution
17
DETERMINATION OF THE INTENSITY SIZE DISTRIBUTION
For a sample where no information is available, then
the general purpose model should be used
For a sample known to be multimodal, such as a
synthetic mixture of materials, or a single molecular
species containing aggregates, then multiple narrow
modes should be selected
17
PART TWODLS EXPERIMENTAL 18
UPPER SIZE LIMIT OF DLS
DLS will have an upper limit with respect to size When particle motion is not random (e.g.
sedimentation, creaming) DLS is not the correct technique to use
Upper limit is set by the onset of sedimentation Upper size limit is therefore sample dependent No advantage in suspending particles in a more
viscous medium to prevent sedimentation because Brownian motion will be slowed down by the same extent
19
UPPER SIZE LIMIT OF DLS
Another factor that needs to be considered is the number of particles that will be present in the detection volume
The amount of scattered light produced by large particles would be sufficient to make successful measurements
However, the number of particles in the scattering volume may be so few that severe fluctuations of the momentary number of particles in the scattering volume will occur
This results in a phenomenon called number fluctuations which leads to problems in the definition of the baseline of the correlation function
19
UPPER SIZE LIMIT OF DLS
In order to overcome number fluctuations, the concentration of the sample needs to be increased
This may lead to multiple scattering effects which can be minimised by using backscatter detection (such as the Nano S)
This is the reason why the upper size limit of a backscatter system is higher than an instrument which has 90o detection
19
DETECTION VOLUME: Nano S90/Nano S
19
DETECTION VOLUME: Nano S90/Nano S
Detector
Laser
Detectionvolume
19
DETECTION VOLUME: Nano S90/Nano S
Detector Laser
Detectionvolume
19
LOWER SIZE LIMIT OF DLS
Lower size limit depends on
the sample concentration
refractive index of sample compared to diluent
laser power and/or wavelength
detector sensitivity
optical configuration of the instrument
The lower size limit is typically 2nm
19
SAMPLE PREPARATION:Sample Concentration
Measurements can be made on any sample in which the particles are mobile
However, each type of sample material has its own ideal range of concentration where measurements should be made
If the concentration is too low, there may not be enough light scattered to make a measurement
This is unlikely in the Nano S/Nano ZS and HPPS except in extreme circumstances
19
SAMPLE PREPARATION:Sample Concentration
If the sample is too concentrated, an effect called multiple scattering may occur
This will reduce the apparent size and the intercept value (signal to noise ratio)
When multiple scattering is insignificant, the size and intercept will be independent upon concentration
Instruments which have backscatter detection extend the concentration over which samples can be measured before seeing the effect of multiple scattering
19
Sample Concentration For DLS Measurements
CORRECT CONCENTRATION
z-A
VE
RA
GE
DIA
ME
TER
(nm
)
SAMPLE CONCENTRATIONLOW HIGH
MULTIPLESCATTERING CONVENTIONAL
DLS INSTRUMENT
BACKSCATTERDLS INSTRUMENT
CORRECT CONCENTRATION
MULTIPLE SCATTERING
20
SAMPLE PREPARATION:Sample Concentration
The upper limit of concentration is also governed by the onset of particle/particle interactions
This phenomenon will influence the diffusion speed of particles and hence influence the apparent size
20
SAMPLE PREPARATION:Sample Concentration
The contribution of multiple scattering and
particle/particle interactions at different concentrations
for a particular sample on the apparent size is not
known
During method development, determining the correct
sample concentration may involve several size
measurements at different concentrations
20
SAMPLE PREPARATION:Sample Concentration
An important factor in determining the maximum concentration a sample can be measured at is the size of the particles
Particle Size Minimum Recommended Concentration
<10nm 0.5g/l Only limited by thesample materialinteraction, aggregation, gelation etc.
10nm to 100nm 0.1mg/l 5% mass (assuming a density of 1gcm-3)
Maximum Recommended Concentration
100nm to 1m 0.01mg/ml 1% mass (assuming a density of 1gcm-3)
>1m 0.1g/l1% mass (assuming a density of 1gcm-3)
21
SAMPLE CONCENTRATION:Minimum Concentration For Small Particles
For particle sizes smaller than 10nm, the major factor in determining the minimum concentration is the amount of scattered light the sample generates
The concentration should generate a minimum count rate of 10kcps in excess of the scattering from the dispersant
21
The maximum concentration will be set by the properties of the sample itself
Examples: Gelation or similar: A gel is unsuitable for
measurement by DLS Particle/particle interactions: These will affect the
diffusion rate of the particles leading to an incorrect result
21
SAMPLE CONCENTRATION:Maximum Concentration For Small Particles
SAMPLE CONCENTRATION:Minimum Concentration For Large Particles
Even for larger particles, the minimum concentration is effectively still a function of the amount of scattered light, though the additional effect of “number fluctuations” must be taken into account
Such fluctuations must be avoided and this determines the lower limit for the required concentration and for a lower limit in the number of particles
The number of particles in the scattering volume should be at least 1000 (any number in the range 500 to 1000 is acceptable)
21
SAMPLE CONCENTRATION:Maximum Concentration For Large Particles
Determined by multiple scattering
The Nano S/Nano ZS and HPPS is not sensitive to multiple scattering due to NIBS
However, with increasing concentration, the effect of multiple scattering becomes more and more dominant, until the results will be distorted
22
SAMPLE CONCENTRATION:The Dilution Medium The diluent should be the same as the continuous
phase of the original sample
The equilibrium of absorbed species between the surface and bulk solution will be perfectly preserved if the diluting liquid is obtained form the original sample
This could be done by filtering or centrifuging
If this is not possible, the continuous phase should be made up to be as close as possible to that of the sample
22
SAMPLE PREPARATION:Filtration
The dilution medium should be carefully filtered
before use to avoid contaminating the dilute sample
Aqueous dispersants are normally filtered at 0.2
microns, non polar dispersants can be filtered down
to 10 or 20nm
22
SAMPLE PREPARATION:Ultrasonication
Very useful preparation technique if used properly.
Minerals (e.g. TiO2) are ideal candidates for
dispersion by ultrasound
The particle size of some materials (e.g. carbon black) may depend purely on the power and length of sonication
Emulsions should not be ultrasonicated, this is a good technique for preparing them in the first place
23
CHECKING CORRECT INSTRUMENT OPERATION
DLS instruments cannot be calibrated because they
use first principles in their measurement protocol
They can be verified that they are working correctly
by measuring sizing standards
23
CHECKING CORRECT INSTRUMENT OPERATION:Latex Standards
Polymer latex spheres are very commonly used to verify
the operation of DLS instruments.
Standards suitable for DLS range from 20nm to 900nm
The result quoted on Duke Scientific latex standards is
the certified transmission electron microscopy result
The DLS result is quoted in the specification sheet and is
not a certified value
23
SAMPLE PREPARATION OF LATEX STANDARDS
All latex standards are supplied at a concentration that is too high for DLS measurements
Latex standards should be diluted in filtered 10mM NaCl to suppress the electrical double layer (ISO 13321)
23
DATA INTERPRETATION
The quality of the data obtained from the measurement is essential in determining how well the distribution algorithm is going to perform
The better the quality of the data, the more repeatable the answers obtained will be
In order to aid the interpretation of data, it is advised that various report pages and record view parameters are added to the default selection in the DTS software
25
DATA INTERPRETATION
REPORTS
Correlogram Report Cumulants Fit Report Fit Report
28
DATA INTERPRETATION
REPORTS
Correlogram Report Cumulants Fit Report Fit Report
PARAMETERS
Mean Count Rate Measured Intercept Measurement position Cumulants fit error Multimodal fit error Attenuator
28
DATA INTERPRETATION:CORRELOGRAMS
The correlogram shows the correlation data displayed as the correlation coefficients displayed in each channel and provides information about the sample
The shape of the curve will show some obvious problems that may be present
The correlogram ought to be checked for noise contained within the data
Noisy data can result for various reasons; count rate too low, instability of the sample or external effects such as vibration or interference from another source
29
DATA INTERPRETATION:CORRELOGRAMS
29
DATA INTERPRETATION:CORRELOGRAMS
0
0.1000
0.2000
0.3000
0.4000
0.5
0.6000
0.7000
0.8000
0.9000
0.1000 10. 1000. 1.e+5 1.e+7 1.e+9
Cor
rela
tion
Coe
ffici
ent
Time (us)
Raw Correlation Data Large particles
Medium range polydispersity
Presence of very large particles/ aggregates (baseline not flat)
30
DATA INTERPRETATION:CORRELOGRAMS
0
0.1000
0.2000
0.3000
0.4000
0.5
0.6000
0.7000
0.1000 10. 1000. 1.e+5 1.e+7 1.e+9
Cor
rela
tion
Coe
ffici
ent
Time (us)
Raw Correlation Data Very small particles
Medium range polydispersity
No large particles/ aggregates present (flat baseline)
30
DATA INTERPRETATION:CORRELOGRAMS
0
0.1000
0.2000
0.3000
0.4000
0.5
0.6000
0.7000
0.8000
0.1000 10. 1000. 1.e+5 1.e+7 1.e+9
Cor
rela
tion
Coe
ffici
ent
Time (us)
Raw Correlation Data Very large particles
High polydispersity
Presence of very large particles/ aggregates present (noisy baseline)
30
CUMULANTS/DISTRIBUTION ALGORITHM FITS
The cumulants fit report shows the quality of the cumulants fit to the measured data indicating whether the z-average diameter and polydispersity obtained for a particular measurement are reliable
A fit error of less than 0.005 is considered good
31
CUMULANTS/DISTRIBUTION ALGORITHM FITS
0.1000
0.2000
0.3000
0.4000
0.5
0.6000
0.7000
0.8000
0.9000
1. 10. 100. 1000.
G1
Cor
rela
tion
Func
tion
Time (us)
Cumulants Fit
GOOD FIT ERROR 0.00037
31
CUMULANTS/DISTRIBUTION ALGORITHM FITS
0.11
0.12
0.13
0.1400
0.15
0.16
0.1700
0.18
10. 100. 1000. 10000.
G1
Cor
rela
tion
Func
tion
Time (us)
Cumulants Fit
GOOD FIT ERROR 0.008
31
COUNT RATE REPEATABILITY
Perform at least 3 repeat measurements on the same sample
The count rates should be all within a few percent of one another
32
COUNT RATE REPEATABILITY
Count ratedecreases withsuccessive measurements
Particle sedimentation is occurring
Remove sedimenting particles and measure only particles suitable for technique
COUNT RATESYMPTOM
POSSIBLE EXLANATION
ACTION
32
COUNT RATE REPEATABILITY
Count ratedecreases withsuccessive measurements
Particle creaming is occurring
Prepare a better stabilised dispersion to prevent creaming
COUNT RATESYMPTOM
POSSIBLE EXLANATION
ACTION
Try and disperse sample in a more suitable dispersant to prevent particles dissolving
Particles are dissolving or breaking up
32
COUNT RATESYMPTOM
COUNT RATE REPEATABILITY
Formation of bubbles during the duration of the measurement
Remove bubbles, de-gas dispersant
POSSIBLE EXLANATION
ACTION
Count ratedecreases withsuccessive measurements
32
COUNT RATE REPEATABILITY
Count rateincreases withsuccessive measurements
Dispersion instability present. Aggregation, flocculation etc. occurring
Prepare a better stabilised dispersion
COUNT RATESYMPTOM
POSSIBLE EXLANATION
ACTION
32
COUNT RATE REPEATABILITY
Count rateincreases withsuccessive measurements
Measurements taken too soon after instrument was switched on - laser still warming up
Allow instrument 30 minutes to warm up after switching on before beginning measurements
COUNT RATESYMPTOM
POSSIBLE EXLANATION
ACTION
32
COUNT RATESYMPTOM
COUNT RATE REPEATABILITY
Formation of bubbles during the duration of the measurement
Remove bubbles, de-gas dispersant
POSSIBLE EXLANATION
ACTION
Count rateincreases withsuccessive measurements
32
COUNT RATE REPEATABILITY
COUNT RATESYMPTOM
POSSIBLE EXLANATION
ACTION
Count rate is random between successive measurements
Dispersion instability present. Sample is changing with time – aggregating, breaking up etc.
Prepare a better stabilised dispersion
Sample not suitable for PCS
32
Count rate is random between successive measurements
COUNT RATE REPEATABILITY
Sample contains large particles
Remove by filtration or centrifugation or allow to settle before commencing measurement
POSSIBLE EXLANATION
ACTIONCOUNT RATESYMPTOM
32
COUNT RATESYMPTOM
Count rate is random between successive measurements
COUNT RATE REPEATABILITY
Formation of bubbles during the duration of the measurement
Remove bubbles, de-gas dispersant
POSSIBLE EXLANATION
ACTION
32
COUNT RATESYMPTOM
Count rate is random between successive measurements
COUNT RATE REPEATABILITY
Sample not properly mixed
Ensure thorough mixing
POSSIBLE EXLANATION
ACTION
32
z-AVERAGE DIAMETER REPEATABILITY
The z-average diameter is calculated from the cumulants (monomodal) analysis
Repeat measurements should be within 1 or 2 percent of one another
33
z-AVERAGE DIAMETER REPEATABILITY
SIZE SYMPTOM POSSIBLE EXLANATION
ACTION
Size decreaseswith time
Temperature notstable
Allow plenty of time for temperatureequilibration (1 min per degreeplus 5 mins)
Sample not stable - solubilisation of break up ofparticles
Prepare a betterstabiliseddispersion
33
z-AVERAGE DIAMETER REPEATABILITY
SIZE SYMPTOM POSSIBLE EXLANATION
ACTION
Size increaseswith time
Temperature notstable
Allow plenty of time for temperatureequilibration (1 min per degreeplus 5 mins)
Sample not stable - aggregation ofparticles
Prepare a betterstabiliseddispersion
33
REPEATABILITY OF SIZE DISTRIBUTIONS
The size distributions displayed are derived from a NNLS analysis and these should be checked for repeatability in terms of the peak positions and % areas obtained
If the distributions are not repeatable then it is suggested that the measurements are re-taken using longer measurement duration
33
EXTRACTING THE MEASUREMENT SOP
The ability to highlight any measurement record and to extract the SOP is a very powerful tool in looking at data quality
The exact setup which was used to take that measurement can be accessed and any obvious errors in the measurement procedure can be highlighted (for example, incorrect measurement position for sample type, manual override of the measurement duration etc)
33