Pacific Nanotechnology Inc. - Printable version

Given the wide variety of applications that use particles, it makes sense that there are many different ways to analyze and characterize particles. The following is a partial list of commercially available techniques employed in particle measurement:

Acoustic Attenuation Spectroscopy
Aerosol Mass Spectroscopy (Aerosol MS)
Cascade Impaction
Condensation Nucleus Counter (CNC)
Differential Mobility Analysis (DMA)
Dynamic Light Scattering (DLS) or Photon Correlation Spectroscopy (PCS)
Quasi-elastic Light Scattering (QELS)
Electrical Zone Sensing (Coulter Counting)
Electroacoustic Spectroscopy
Electrokinetic Sonic Amplitude
Gas Adsorption Surface Area Analysis (e.g. BET)
Laser Doppler Velocimetry (LDV)
Laser Light Diffraction or Static Light Scattering
Light Microscopy or Optical Imaging
Microelectrophoresis
Scanning Electron Microscopy (SEM)
Sedimentation (Gravitational & Centrifugal)
Sieving
Tapered Element Oscillating Microbalance (TEOM)
Transmission Electron Microscopy (TEM)
X-ray Diffraction (XRD)

Ensemble vs. Single-Particle Techniques
Particle analysis techniques can generally be classified as ensemble or single-particle techniques.

With ensemble techniques, a signal from an individual particle cannot be isolated. Instead, ensemble techniques receive signals from multiple particles simultaneously. Laser light diffraction is a commonly employed ensemble technique.

In contrast with ensemble techniques, single-particle techniques isolate and identify signals from individual particles. Statistical information for groups of particles can be obtained by processing data from many different individual particles. A common example of a single-particle technique is optical imaging combined with image processing to measure and analyze particles.

In general, morphological information, such as shape and aspect ratio, as well as surface information, such as texture and roughness parameters, cannot be obtained using ensemble techniques. Only single-particle techniques that look at individual particles can supply such information. Physical parameters for each particle in a set of particles are recorded to generate a statistical distribution for the entire set of particles.

Which Technique is "The Best"?
Obviously, there is not one single "best technique" for all situations. Determining the best technique for a particular situation requires knowledge of the particles being analyzed, the ultimate application of the particles, and the limitations of techniques being considered.

Depending on the application of interest, a number of techniques can be used to analyze and characterize nanoparticles. In industries where aerosols play an important role, tools such as the Differential Mobility Analyzer (DMA) are commonplace. With fine powders, light scattering techniques are common.

The table on page 6 describes some common particle analysis techniques and their benefits and drawbacks in comparison with the AFM.

Technique

Common
Applications

Characteristics

AFM Comparison

Laser Light Diffraction

Powders

.	Ensemble technique
.	Commonly used in chemical and pharmaceutical industries
.	Fraunhofer and Mie light scattering are the basic principles of operation
.	Typical range: 1µm to 1000µm

.	Morphological information limited to aspect ratio
.	No surface information
.	Imaging of individual particles impossible
.	Range excludes particles <1mm, but, unlike AFM, can measure particles with much larger diameters (>10µm)

Dynamic Light Scattering

Powders

.	Ensemble technique
.	Commonly used in chemical and pharmaceutical industries
.	Relies on Brownian motion of particles in a liquid medium to determine particle size
.	Typical range: 50nm to 1µm

.	Morphological information limited to aspect ratio
.	No surface information
.	Sample must be dispersed in liquid, which can alter particle characteristics
.	Range is comparable to AFM, but fails to span the gap to measure in the 1 µm to 10µm range

Sedimentation

Powders

.	Ensemble technique
.	Level of obscuration of visual light or X-ray signal determines particle size distribution
.	Typical range: >0.1µm

.	No morphological information
.	No surface information
.	Imaging of individual particles impossible
.	Range excludes particles <100nm
.	Sample must be dispersed in liquid

Coulter Counting

Powders

	Single particle technique
.	Established technique for particle counting
.	Provides measurement based on volume displacement
.	Typical range: >0.5µm

.	No morphological information
.	No surface information
.	Imaging of individual particles impossible
.	Range excludes particles <0.5µm
.	Sample must be dispersed in electrolytic liquid

DMA + CNC

Aerosols

.	Ensemble technique
.	DMA creates monodisperse stream of particle; relies on mass-based charge to isolate particles within a specified size range
.	NC grows small particles to a size large enough to detect with other techniques, such as light scattering
.	Typical range: >10nm

.	No morphological information
.	No surface information
.	Imaging of individual particles impossible
.	CNC alters particles before they are measured

Light Microscopy

Powders Aerosols

.	Single particle technique
.	Typical range: 1µm

.	Resolution limited by light wavelength; range excludes particles <1mm

SEM

Powders Aerosols

.	Single particle technique
.	Compositional information can be obtained with EDS
.	Typical range: 50nm to 1cm

.	Sample preparation can be complex
.	Generally must be performed at vacuum
.	Costly

TEM

Powders Aerosols

.	Single particle technique
.	Compositional and crystallographic information can also be obtained.
.	Typical range: 5nm to 500µm

.	Since e-beam is transmitted through sample, image is 2D projection of sample
.	Sample preparation can be very complex
.	Must be performed in vacuum
.	Costly

Atomic Force Microscopy for Nanoparticles

Common Particle Analysis Techniques