Why different angles matter

Introduction

The high sensitivity of dynamic light scattering (DLS) to protein aggregation explains the widespread use of Zetasizer instruments within the biopharmaceutical industry. Since the amount of light scattered by a particle is proportional to the sixth power of its size, trace amounts of aggregate can be detected using DLS. Conversely, this can make the measurement of native, monomeric protein in impure samples difficult, since the signal from large impurities may obscure that of the molecule of interest.

Mie theory describes how intensity and angular distribution of scattered light depend on particle size relative to the laser wavelength, refractive index contrast, and scattering angle. Figure 1 illustrates the optical scattering principles for a fixed incident wavelength of 633 nm, where very small particles scatter weakly and nearly isotropically (orange dash lines), while larger particles scatter much more strongly and scattering becomes highly angle dependent (grey line), with a strong preference for forward scattering at 13°. 

Figure 1: Adopted illustration according Mie theory showing the optical scattering principles of angular distribution of scattered light from a small particle being nearly isotropical (orange dashed linie) and from a large particle being highly angle dependent (grey line) (left). If aggregation formation occurs in a sample over time, because of low stability, the Mie theory predicts that scattering intensity at both angles will change (right). For a large particle, collecting the scattered light in backscatter at 173° is less sensitive than in comparison to forward scatter at 13°. 

A detection angle of 173° (non-invasive back-scatter (NIBS)) reduces the obscuration of small molecules by large molecules. According to Mie theory, larger particles (specifically those with a diameter greater than 1/20 of the incident laser wavelength) scatter less light in the backward direction compared to the forward direction, allowing the signal from smaller particles to be recovered more reliably. The Mie theory helps to understand how scattering intensity will change at both angles, if the sample has low colloidal stability and forms aggregates over time (Figure 1, right graphs). It predicts that for a large particle, the back scatter measurement at 173° reduces the influence of the aggregates, making it a great tool for stability screenings (Figure 1, upper right graph).

Nevertheless, the increased sensitivity of forward scatter optics to trace amounts of aggregates is often highly useful, with forward scatter (13°) measurements used alongside back-scatter measurements for enhanced detection of large protein aggregates, which is crucial for trace analysis (Figure 1, lower right graph). 

The Zetasizer Advance range supports multi-angle measurements, allowing a more comprehensive assessment of IgG stability. In this application note, dual-angle DLS measurements are used to follow an IgG stability study and demonstrate the value of measuring at more than one angle. This is important because, according to Mie theory, the scattering from larger particles is angle-dependent and may be influenced by Mie maxima and minima, which can affect the result obtained at a single angle. 

Experimental methods

A sample consisting of IgG in 50 mM arginine was prepared and filtered using a 20 nm pore size filter. An aliquot was removed and analyzed immediately after preparation, with the remainder of the sample then stored at room temperature. Further aliquots were removed and analyzed after 1 and 3 days of storage. Dual angle DLS measurements (173° and 13°) of automatic duration were carried out using a Zetasizer Advance.

The Aggregation Index is a parameter based on the mean z-average size measured for the two angles of scattering, according to the equation:

Results

Prior to storage of the IgG sample, DLS measurements acquired using both angles were highly similar (Figure 2). Following storage, 13° detection demonstrated the presence of aggregate that was not detected at 173°. This contrast between measurements made at 2 different angles can be assessed numerically using the aggregation index calculated from the data, which increases during storage (Figure 2).

Figure 2: Dual Angle DLS measurements of IgG samples stored for varying time periods. Black, 173° detection; Red, 13° detection. a) Pre-storage, b) after 1 day at room temperature, c) after 3 days at room temperature.

Conclusion

Malvern Panalytical-patented non-invasive back-scatter (NIBS) optics widen the concentration range over which accurate DLS measurements can be made, in part by reducing the effect of large artifacts (such as dust) on DLS measurements. Nevertheless, for some applications (quality control of fully developed, pure biopharmaceutical samples, for instance), the increased sensitivity of forward scatter optics to trace amounts of aggregate is highly useful. 

This application note demonstrates the use of forward scatter and back-scatter DLS measurements together to detectaggregation processes that would not be fully characterized using one angle alone.

Back-scatter detection at 173° offers a broad concentration range and reduced sensitivity to large artifacts such as dust, making it well suited to routine characterization and formulation screening. Forward scatter detection at 13° offers higher sensitivity to trace aggregate populations, ideal for when low-level aggregate detection is the primary objective, such as in quality control of purified biopharmaceutical samples or during stability monitoring under accelerated stress conditions.

Used together, the two angles provide a more complete picture of the aggregation state of a protein sample.

For applications requiring even greater resolution – for example, resolving near-size aggregate populations or characterizing multimodal distributions – patented multi-angle dynamic light scattering (MADLS) further extends this capability. Rather than reporting separate results at each angle, MADLS collects data at three detection angles simultaneously and combines the autocorrelation functions into a single, angle-independent particle size distribution. This approach improves size resolution from the conventional 3:1 to 2:1, reduces noise, and reveals populations that may be weakly scattering at any single angle, providing a more complete picture of the aggregation state of a protein sample without the interpretive ambiguity of multi-angle comparisons (Figure 3).

Figure 3: MADLS provides an increase in resolution shown in this size measurement, where both side and back-scatter measurements (green, red) only show a broad size distribution but the MADLS result is able to resolve the components in the sample (blue).

More broadly, DLS-based aggregate detection of this kind represents one component of a growing toolkit for biotherapeutic stability characterization. Whether monitoring a candidate during formulation screening, tracking stability under stress conditions, or verifying product quality at the point of manufacture, the ability to confidently detect and quantify aggregation early is essential. 

As biologics development workflows demand greater throughput and earlier decision-making, the role of high-sensitivity, multi-parameter light scattering approaches continues to grow in importance. Combining DLS with complementary techniques such as static light scattering (SLS) or intrinsic differential scanning fluorimetry (DSF) in a single workflow can extend these capabilities further, offering the prospect of comprehensive colloidal and structural stability profiling from a single sample at scale.