Using Polarization Intensity Differential Scattering (PIDS) for the Sizing of Non-Spherical Sub-Micron Particles

Oct 30 2014

Beckman Coulter offers the LS™ Series multi-wavelength particle size analyzers (Figure 1), which employs a complementary scattering technology for sub-micron particle sizing. This article discusses the application of the PIDS system for the sizing of non-spherical sub-micron particles.

Beckman Coulter LS 13 320 Particle Size Analyzer with the Aqueous Liquid Module (ALM) and the Auto Prep Station.

Figure 1. Beckman Coulter LS 13 320 Particle Size Analyzer with the Aqueous Liquid Module (ALM) and the Auto Prep Station. Image credit: Beckman Coulter

Since the signal scattered from smaller particles of below 1µm size is weak, the maxima and minima in the scattering pattern can only be measured at very high angles (Figure 2).

However, it is a challenging task. The PIDS developed by Beckman Coulter addresses the challenges of sub-micron sizing by providing high-resolution sub-micron sizing.

The weak scattering signal from small particles below 1µm in size present difficulties in measuring the maxima or minima at high angles.

Figure 2. The weak scattering signal from small particles below 1µm in size present difficulties in measuring the maxima or minima at high angles. Image credit: Beckman Coulter

PIDS™ Technology

The PIDS™ technology is based on the Mie theory of light scattering. PIDS takes advantage of the light’s transverse nature, involving an electric vector and a magnetic vector at right angle to it. For instance, the light is vertically polarized if the electric vector is "up and down".

When a sample is illuminated by whena light of a specific polarized wavelength, a dipole, or oscillation, of the electrons in the material is established by the oscillating electric field. The plane of polarization of these oscillations will be the same as the propagated light source. The light scattered by the oscillating dipoles in the particles in all direction other than the direction of the irradiating light source. PIDS leverages this phenomenon.

A sample is sequentially illuminated by three wavelengths of light (450 nm, 600 nm, and 900 nm), first with vertically polarized light and followed by horizontally polarized light. The light scattered from the samples is then measured over a range of angles. The particle size distribution can be determined by measuring the variations between the horizontally and the vertically scattered light for each wavelength.

The integration of the intensity vs. scattering angle data obtained from the PIDS signals and the standard algorithm from the intensity vs. scattering angle information obtained from the laser light scattering provides a continuous size distribution. The presence of smaller particles can be rapidly confirmed with the PIDS data.

Sub-Micron Sizing

The suitability of the PIDS™ technology for sizing spherical particles is first demonstrated by testing polystyrene spheres using an LS Series Analyzer. The results, presented in Figure 3 and Table 1, clearly demonstrate the effectiveness of the PIDS™ technology for spherical particle sizing.

Overlay distributions from five assayed polystyrene samples using an LS™ Series Analyzer.

Figure 3. Overlay distributions from five assayed polystyrene samples using an LS™ Series Analyzer. (Source: Duke Scientific, Inc.) Image credit: Beckman Coulter

Table 1. Attributed mean size values.

Standard (nominal size)	Assayed Values	LS Series Reported Values
100 nm	102 nm ± 3 nm	108 nm
150 nm	155 nm ± 4 nm	151 nm
220 nm	220 nm ± 6 nm	211 nm
300 nm	300 nm ± 5 nm	306 nm
404 nm	404 nm ± 6 nm	400 nm

For analysis of non-spherical particles, the University of Utrecht’s colloid chemistry department creates a range of mono-dispersed, non-spherical materials. The SEM analysis results of the sub-micron, hematite spindles (oblate spheroids) are presented in Figure 4, revealing the presence of mono-sized particles that are readily dispersed.

The SEM photomicrograph shown is a representative sample of the hematite spindles analyzed.

Figure 4. The SEM photomicrograph shown is a representative sample of the hematite spindles analyzed. Image credit: Beckman Coulter

UV/Vis spectroscopy data and UV/Vis spectroscopic ellipsometry data are used to determine the hematite spindles’ optical properties for the refractive index’ imaginary component and real component, respectively. The sizing results of sample using the LS Series Analyzer are presented in Figure 5, showing good agreement with the expected value considering the random motion of the particles in the sample cell.

The reported mean value of 78 nm using an LS™ Series Analyzer is expected, assuming random motion of particles in sample cell.

Figure 5. The reported mean value of 78 nm using an LS™ Series Analyzer is expected, assuming random motion of particles in sample cell. Image credit: Beckman Coulter

Advantages of the PIDS™ Technology

The raw data of the PIDS™ technology quickly confirms the presence of small particles in a distribution, making the technology superior to other laser-based particle size analyzers by eliminating most of the ‘guess work’ regarding the presence of a sub-micron population.

The PIDS signals are free from the interference of large particles present in the sample, thus avoiding artifacts. The signal scattered from the large particles is not differential in nature and can be detected by the PIDS system. The variations between the polarization scattering signals from the small particles are only measured by the PIDS system.

The raw data in PIDS is viewed for comparison of two samples (Figure 6). From the data, the mean size of the first and second sample was to be 102nm and 13µm, respectively. In total, 42 discreet measurements are carried out for each sample, representing a concentration detector, six detectors, and six sublets with a given wavelength and polarization.

Overlay raw PIDS data from a typical analysis of small and large particles.

Figure 6. Overlay raw PIDS data from a typical analysis of small and large particles. Image credit: Beckman Coulter

For the sub-micron test material, the PIDS intensities vary significantly for each polarization at each wavelength. The angular dependency in the horizontal light scattering intensity data obtained for each wavelength is substantial, in particular, the parabolic (curved) shape of the horizontal data sets. The scattered signal (Detector Flux, Y-axis) is large and easily detectable.

Conclusion

The results show that the PIDS technology provides an accurate and practical solution for the sizing of spherical, sub-micron materials. Furthermore, meaningful data can be obtained for non-spherical, sub-micron particles using the PIDS technology despite the limitations faced by all scattering methods.

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