There are two wellestablished methods for determining the particle size of a sample: dynamic light scattering and laser diffraction. Choosing which particle size analysis method to use requires consideration of the expected size range, the sample type (liquid or solid), the amount of available sample, the chemical stability, and the application field. However, the physical principle behind each technique is also important and the choice of the most suitable one is not always straightforward.
This article describes the principal differences between dynamic light scattering and laser diffraction and also provides an overview of the options for data interpretation and possible comparison.
Particle size analysis methods: Dynamic light scattering vs. laser diffraction
Comparison of particle sizing methods: laser diffraction vs. dynamic light scattering
Characteristics  Dynamic light scattering (DLS)  Laser diffraction (LD) 

Assumption  Particles are spherical  
Size range  0.3 nm to 10 µm  40 nm to 2500 µm 
Theory of particle size measurement 
  The diffracted/scattered intensity vs angle (diffraction pattern) is a function of the particle size.  The final particle size distribution is obtained from the sum of diffraction patterns produced by the particles randomly oriented along the direction of the laser beam. 
Equivalent diameter*  Hydrodynamic diameter (HDD):
 Laser diffraction equivalent diameter: diameter of a sphere having on the crosssectional area the same diffraction pattern as the investigated particle ^{(1)}. For particles ≤0.1 µm, the definition can be extended into volumeequivalent diameter. In this case, the crosssectional area becomes nearly the same as that of a sphere with equal volume ^{(2)}. 
Initial weighting model  Intensitybased
 Volumebased

Mean particle size result  Monodisperse sample: Hydrodynamic mean diameter (HDD) Polydisperse sample: Peak analysis of particle size distribution graph 

Parameters of the particle population 


Other weighting models  Recalculation of the intensitybased distribution to a number and volumebased one is possible when the refractive index of the particles is known.  Recalculation of the volumebased distribution to a surface and numberbased one is possible. The refractive index does not have any function in this case. 
ISO norm  ISO 22412  ISO 13320 
*According to the IUPAC definition, the equivalent diameter of a nonspherical particle is equal to a diameter of a spherical particle that exhibits identical properties (e.g. aerodynamic, hydrodynamic, optical, electrical) to that of the investigated nonspherical particle.
Results and data interpretation
Is it possible to compare DLS results with laser diffraction results? Which parameters should be compared?
The DLS particle size analysis method and laser diffraction technique provide results which are based on different equivalent diameters and weighting models.
For this reason, for many samples results measured with DLS and laser diffraction are not directly comparable and a deviation of 10 % or more between the results is to be expected. To decrease the deviation, the same weighting model should be used for comparison. When using the dynamic light scattering technique it is possible to recalculate the volumeweighted distribution as well as Dvalues.
However, any kind of recalculation will introduce an error which adds to the initial one related to the assumption of spherical particles. Therefore, calculations from the raw data are always more reliable than a recalculation.
Understanding the difference between DLS and laser diffraction results
The following considerations are crucial for understanding the difference in the results and the inaccuracy of the comparison:
 In order to directly compare the technologies the results of a DLS measurement have to be recalculated to obtain a volumebased distribution.
 Mean size is based on different weightings: D[4,3] based on volume; HDD based on intensity.
 The median (D_{50}) should be considered to get a more accurate “average particle size”.
Dynamic light scattering (DLS) results
DLS measurements provide results as the hydrodynamic diameter (HDD) and peak size (see Figure 1, 2).
The HDD cannot be measured by laser diffraction while the median of the peak size may be obtained by using further calculations. Therefore, the recalculation of the particle size distribution from intensity to volumebased and then the calculation of the volumeweighted Dvalues are necessary for a reasonable comparison.
By means of the material refractive index the intensitybased result can be recalculated into the volumebased one.
Laser diffraction (LD) results
The Dvalues provided by laser diffraction results do not give information about the peak size but describe the whole particle size distribution (see Figure 3). In order to define the particle size of the investigated sample, the median or the mean size (e.g. D [4,3]) has to be considered.
Comparison of DLS and LD results
It can be observed that although there is a certain similarity (difference smaller than 10 %) between the HDD intensityweighted (DLS: 0.13 µm) and the D50 volumeweighted (LD: 0.14 µm), the comparison would not be accurate because they have two different meanings (D50 is not a mean but a median) and they are based on different weighting models.
For this reason, from the recalculated DLS volumebased distribution, the Dvalues can be determined and used for the comparison with the laser diffraction results (see Table 1). The comparison between the volumeweighted Dvalues is shown below. The difference is higher than 20 %.
Instrument  D10 [µm]  D50 [µm]  D90 [µm] 

LD  0.06  0.15  0.34 
DLS  0.08  0.10  0.14 
Table 1: Comparison of volumeweighted Dvalues derived from DLS and LD for the polystyrene latex
The difference in results obtained from the two different techniques might be confusing at first glance, thus making the decision of which technique to choose even more challenging. In this case, however, it is useful to consider the sample type.
Which technique for which application?
The following points should be taken into account:
Conditions  DLS  LD 

Sample type  Liquid dispersions (liquid/liquid, powder/liquid)  Liquid dispersions, dry powders 
Sample amount available  Liquid sample: µL to mLDry samples: mg to g  Liquid sample: mLDry samples: g 
Sample recovery  Always possible  
Temperature control  Always possible, typically from 0 °C up to 90 °C  Possible to measure at different temperatures with the suitable accessory 
Quality parameters 


Method development  No method optimization necessary  Method optimization necessary for new samples 
Measurements of highly concentrated liquid samples  Possibility of measuring undiluted samples up to 70 % (m/v), otherwise predilution is possible before filling the cuvette  The sample is directly diluted in the liquid dispersion unit. The stirrer, ultrasound, and the circulation help to adjust the particle concentration (obscuration) in the instrument. 
Lower size limit  Primary particles in the lower nanometer range (typical lower size range 0.3 nm)  LD is less sensitive to the weak signals coming from small particles in the nanometer range (<100 nm) and the accuracy can be limited. 
Measurement time  Both techniques are very fast (a few minutes). In some cases, laser diffraction measurements can be faster than DLS measurements.  
Detection of impurities  Small contaminants in a very low concentration will be observed by dynamic light scattering more readily than by laser diffraction.  
Dispersion stability  Laser diffraction is more suitable for analyzing samples that have large particles (>10 µm). The intensity signal is stronger and results are related to the particle volume; dispersion via ultrasound, stirring, and circulation ensure that particles remain in suspension. There is no sedimentation risk like with the DLS technique.  
Statistics  The accuracy of results and the repeatability (relative standard deviation %) have to be considered, e.g. RSD <5 %.  
Typical applications 


Conclusion
Laser diffraction and dynamic light scattering are particle sizing methods which are wellestablished in both research and industry. Deciding on which technique to use for the particle analysis of a specific substance requires consideration of a number of factors, including the goal of the measurement (e.g. measurement of primary particles or agglomerates), the available sample amount, ease of sample preparation, measurement duration, and the nature of expected impurities. Results measured by laser diffraction can be compared with results measured by DLS and vice versa but this comparison must always be assumed to include a significant deviation. Therefore, comparison is best carried on results determined using the same technique.
Learn more about dynamic light scattering which is used in particle analyzers such as the Litesizer series
Learn more about laser diffraction which is used to determine particle distribution by the PSA series
Read more about laser diffraction for particle sizing
Look up the principles of dynamic light scattering
Read about particle size analysis of food, pharmaceuticals, paints and coatings, and building materials
References
1. Gregorová, W., Pabst, E. (2007). Characterization of particles and particle systems. old.vscht.cz/sil/keramika/Characterization_of_particles/CPPS%20_English%20version_.pdf. [Online].
2. Jonasz, M. Nonsphericity of suspended marine particles and its influence on light scattering. s.l. : Limnol. Oceanogr. 32:1059–1065, 1987.