• Jun 20, 2024
  • Particle Size
  • By Emerson Bella

Laser Diffraction or Single Particle Optical Sensing: Which Is Right for You?

Laser diffraction and Single Particle Optical Sensing (SPOS) are some of the most commonly used techniques in the determination of particle size. PTL offers both of these services, but how do you know which technique better suits your sample?

What Is laser diffraction?

Laser diffraction is an ensemble technique, meaning it measures multiple particles at once as opposed to measuring individual particles, and utilizes patterns of diffracted light and mathematical model in order to obtain a size distribution on a volume-weighted basis of equivalent sphere diameter.

For all laser diffractors, a beam of monochromatic light is shined through a measurement cell in which particles constantly pass through. When the laser hits these particles, light is scattered. The pattern at which the light interacts is collected via detectors that are positioned all around the measurement cell. These detectors record the patterns of light scatter and, in conjunction with user input, use the angle of light scatter to compare to the scatter of a spherical particle of a known size, resulting in a volume distribution of particulate. These detectors are able to detect particles from approximately 0.02 µm to 3000 µm.

Laser diffraction utilizes either Mie theory or Fraunhofer theory to calculate a particle’s size. Mie theory relies on knowledge of optical model properties such as refractive index and absorption value of the sample, as well as the refractive index of the liquid dispersant if analyzed on a liquid basis. If this information is not known, default values may be used for refractive index of the sample. Fraunhofer theory on the other hand does not require a refractive index or absorption for a sample, but should only be used for opaque particles over approximately 50 microns.

Is laser diffraction a fit for my needs?

As mentioned previously, laser diffraction is an ensemble technique, and therefore measures particle size from a cloud or group of particles passing through the measurement cell at any given time. In order to get a representative distribution, it is important that samples be concentrated enough to hit a proper obscuration; a range at which enough sample obscures the laser to give a precise measurement. If too many particles pass through a measurement cell at once, individual particles may be detected as one large particle.

On the other hand, if not enough sample is added, artifact peaks which are unrepresentative peaks caused by background noise, may be present in the distribution. Since laser diffraction is not a resolute technique and does not measure individual particles, any calculated number distribution from a volume distribution is not recommended. ISO 13320 Section 7 Reporting of Results states that “characteristic size values below D5,3 or x5,3 and above D95,3 or x95,3 are likely to be vulnerable to additional uncertainty and systematic error, as a result of sampling problems as well as by limitation of laser diffraction. Quotation of a D100,3 or x100,3 value or a number distribution by laser diffraction shall not be permitted.”

Samples that tend to be ideal for laser diffraction are those in which a general particle size distribution is required. This technique is also useful for quality control checks. Laser diffraction is not recommended to be used to determine outlier populations.

When it comes to the dispersion technique of analyzing a sample, laser diffraction has two options: liquid or dry analysis. A liquid analysis can be done using a dry material or a liquid suspension. A dry material would be made into a suspension using a carrier fluid in which it does not dissolve and a liquid suspension would ideally be diluted into a preparation using carrier fluid. A dry analysis on the other hand, is only available for dry materials. An ideal material for dry analysis would be one that is free-flowing such as sand.

For laser diffraction, PTL complies with cGMP guidelines as well as ASTM, ISO, and USP chapters such as USP <429>.

What is SPOS?

Single Particle Optical Sensing, or SPOS, is similar to laser diffraction in that size distributions are obtained based on how a particle interacts with light. During analysis, particles pass one-by-one through a measuring zone where a laser is directed at particles. The light from the interaction between the laser and the particle is measured by one of two detectors. One detector uses the principle of light scattering, known as summation, and one uses the principle of obscuration, known as extinction. The summation detector is able to detect particles from 0.5 µm to 400 µm, while the extinction detector is able to detect particles 1.5 µm to 400 µm.

Optical models are not necessary for SPOS analysis, unlike laser diffraction. The mathematical models used to calculate particle size are based on a detected pulse that is produced from the way the laser interacts with particles when passing through the measurement zone. The magnitude of the pulse is directly correlated to particle size, as the increase in particle diameter causes an increase in pulse. The pulses are then compared to a standard calibration curve, made with spherical reference materials, to obtain a particle size distribution. Similar to laser diffraction, particle size is calculated by assuming particles are spherical.

Is SPOS a fit for my needs?

During SPOS analysis, particles pass through a measurement zone one at a time. As such, the data is calculated one particle at a time. This makes SPOS a more resolute technique and therefore is able to give accurate number and volume distributions as well as a concentration value.

However, since particles are counted one at a time, it is important not to overwhelm the sensor. If multiple particles pass through the measurement zone at once, they may not all be counted, or they may be counted as one large particle and therefore provide inaccurate results. Because of this, it is important to ensure that samples are relatively dilute, or can be diluted enough before analysis to avoid these issues.

Common goals that can be achieved through SPOS are ones in which outliers need to be identified as well as verifying proper functioning of filtration systems. SPOS, unlike laser diffraction, can only be performed as a liquid analysis. It is possible to suspend powders into a liquid suspension or directly add a liquid sample to the instrument.

For SPOS, PTL complies with cGMP guidelines as well as ISO, USP, and ASTM chapters such as USP <788> and <789>.

Conclusion

Laser diffraction and Single Particle Optical Sensing both use a similar principle of light’s interaction with particles to measure a particle’s size, however differences such as resolution, method of data calculation, sample properties, and reason for testing are what sets them apart from each other.

Particle Technology Labs offers both Laser diffraction and SPOS options. We are equipped with the Malvern Mastersizer 3000, Beckman Coulter LS13 320, and Sympatec HELOS/BR for laser diffraction options, and the AccuSizer 7000AD and Accusizer 780SIS for SPOS. PTL offers routine analysis of materials as well as method developments, evaluations, and validations.

Whether it be water or a pharmaceutical, PTL can cater to your testing needs. Contact us today to get started!

By Emerson Bella, Particle Characterization Chemist II.

Laser Diffraction

Laser Diffraction (also known as Static Light Scattering) is one of the most widely used particle sizing distribution techniques. Samples are passed through a laser beam, scattering the light and detectors measure the intensity of light scattered at fixed positions. Output is a particle size distribution.

Learn More About this Technique

Single Particle Optical Sensing (SPOS)/Light Obscuration

Single Particle Optical Sensing (SPOS) utilizes two physical principles of detection; light extinction (also known as light obscuration) for particles larger than approximately 1.5 µm and light scattering for particles smaller than 1.5 µm. During analysis, a dilute suspension of particles is passed through a region of uniform illumination produced by a laser diode. Particles greater than approximately 1.5 µm are detected by the amount of light they obscure to...

Learn More About this Technique