DLS larger than microscopy
Possible causes include hydration layer, surface coating, weak association, dust, intensity weighting or concentration effects.
Four diameter concepts
The word “diameter” is incomplete without the method.
| Method | Reported diameter concept | Sample state | Key weighting / assumption |
|---|---|---|---|
| DLS | Hydrodynamic diameter | Particles diffusing in liquid | Optical intensity and diffusion model |
| SEM | Image-based projected dimension | Dried or immobilized sample under electron beam | Image segmentation and sampled population |
| TEM | Image-based projected dimension at high resolution | Thin, prepared specimen under electron beam | Contrast, edge detection and sampled population |
| DMA | Electrical-mobility diameter | Charged aerosol particles in gas | Mobility, charge state and flow calibration |
Expected differences
Build a reconciliation hypothesis before rejecting the data.
DMA different from microscopy
Possible causes include mobility-equivalent geometry, aerosol residue, shape, charge-state assumptions or sample conditioning.
SEM and TEM differ
Possible causes include resolution, edge detection, preparation, magnification calibration, sampling or population selection.
Comparison protocol
Make method comparisons scientifically legible.
- Use the same lot and document subsampling.
- Record the state measured by each method.
- Preserve concentration, medium and preparation details.
- Report the method-specific measurand, not only “average size.”
- Include distribution, number of observations and uncertainty.
- Explain whether results are expected to agree and within what model.
- Link the conclusion to the receiving application.
Method-selection shortcut
Use each method for the decision it can support.
| Decision | Primary evidence | Useful companion |
|---|---|---|
| Primary-particle morphology | SEM or TEM | DLS/DMA for effective population behavior |
| Liquid-state aggregation screening | DLS | Microscopy or sedimentation observation |
| Airborne monodisperse sizing | DMA/SMPS | Microscopy and aerosol-process controls |
| Colloidal stability change | DLS + zeta + direct observations | Microscopy where needed |
| Wafer-tool response | Finished deposited artifact and tool result | Particle characterization and deposition records |
Questions engineers ask
Frequently asked questions
Why is DLS often larger than TEM?
DLS reports hydrodynamic behavior in liquid and is sensitive to larger structures; TEM reports image-based projected dimensions after sample preparation.
Is DMA only for dry particles?
DMA classifies aerosolized particles by electrical mobility. The source may begin as a liquid dispersion, but aerosolization and drying become part of the method.
Can SEM and TEM be used interchangeably?
Both are imaging methods, but resolution, sample preparation, contrast and analysis differ. Define the exact method used.
Sources and verification
Technical basis
- NIST TEM sizing — Method-specific projected-size measurement.
- NIST DMA spherical-particle measurements — Mobility-diameter characterization across nanoscale sizes.
- NIST measurement protocols — Framework for comparing method-specific measurands.
Commercial specifications should be confirmed against the current Applied Physics quotation, certificate, lot documentation and product page before purchase or protocol use.
Continue the research
Related technical guidance
Characterization techniques
Choose the full evidence package.
Distribution and CV
Compare method weighting and population width.
Aggregation
Diagnose size changes after preparation.
Applied Physics technical pathway
Need a particle specification reviewed?
Send the target size, medium, concentration, application, certificate requirements and shipping destination. Applied Physics can evaluate the appropriate silica family or custom pathway.