Dynamic Light Scattering (DLS) Measurement Principles
📌 Accuracy in DLS is primarily confirmed using polystyrene latex (PSL) standards, as few other referee techniques exist in the submicron domain.
📌 Repeatability requires measurements to be within at least 10% agreement, while reproducibility demands repeating the entire process (sample prep and measurement) to yield the same result.
📌 Method validation tests on a pharmaceutical emulsion showed expected intensity mean results between systems around 200 and 207 (units unspecified, but close), demonstrating real-world variability.

System Verification and Accuracy Testing
🔬 When testing DLS accuracy, use traceable standards (e.g., nominal 90 nm standards from Thermo Fisher) and dilute the sample using a 10 mMolar sodium chloride (NaCl) solution instead of DI water to prevent particle agglomeration.
🔬 Pass/Fail criteria for instrument verification should be ±15% for the intensity mean (accuracy) and ±5% for repeatability.
🔬 Using DI water for dilution resulted in a less accurate and broader measurement (higher Polydispersity Index) compared to dilution with the NaCl solution.

Sample Preparation: Key Determinants of Quality
💧 If a sample fails verification, the issue is almost always improper sample preparation or system setup; there is no way to "tweak" the instrument to force an answer.
💧 Concentration effects must be checked by diluting the sample (e.g., 2:1 dilution) and confirming the result remains constant; the more dilute the measurement, the more accurate the result.
💧 For highly concentrated or opaque samples, start with a 10:1 dilution and continue diluting until successive dilutions yield the same result, indicating the measurement is independent of concentration.

Powder Dispersion and Advanced Techniques
🧪 For powders, which inherently aggregate, use a combination of tools: choosing the correct solvent (water preferred, followed by alcohols), surfactants, stabilizers (like sodium hexametaphosphate), and ultrasound.
🧪 When selecting a surfactant, a non-ionic option like PEG Pal CA 630 is often a good starting point if the particle surface charge is unknown; surfactants must be used in very dilute solutions (e.g., 1 volume percent) to avoid artifacts.
🔊 To break up severe agglomerates, use an ultrasonic probe, performing a short study (e.g., 10, 30, 60 seconds) to find the optimal energy level; excessive ultrasound can cause re-agglomeration.

Measurement Timing and Quality Control
🚫 Do not be in a hurry with DLS; always ensure measurement parameters stabilize over time, typically requiring at least five minutes for most samples.
🚫 If the mean size is consistently growing over the measurement time, it indicates aggregation occurring in the sample, necessitating a re-check of sample preparation (potentially adding a stabilizer like sodium hexametaphosphate).
🔬 When using round glass cells, centrifuging them briefly can help force large, unwanted particles to the bottom, allowing measurements to be taken from the middle of the cell for better individual particle representation.

Key Points & Insights
➡️ Accuracy relies on calibration using known standards (PSLs) and validating that the system yields the expected results within ±15%.
➡️ Avoid high concentrations; if results change upon dilution, the initial reading is inaccurate due to multiple scattering or restricted diffusion errors.
➡️ Sample cleanliness matters profoundly: Contaminated glassware (e.g., residual soap from external washing) can alter surface chemistry and ruin DLS accuracy.
➡️ Always test repeatability; if results vary, lengthen the measurement time (aim for stabilization beyond 5-10 minutes) or re-evaluate sample preparation techniques.

📸 Video summarized with SummaryTube.com on Jan 12, 2026, 10:28 UTC