Enclosed Light Scattering Instruments and Goniometer Based Light Scattering Instruments - A Comparison

When the acquisition of a new instrument for particle sizing is being considered, two options are presented: a goniometer based or an enclosed light scattering instrument. It is thus valuable to collate the strengths and weaknesses of these two technological approaches as a way of determining the most suitable instrument for the task.

or both instruments, Dynamic Light Scattering (DLS), is the fundamental technology being implemented. Among modern instruments, either a technological approach needs to be equipped with a digital correlator able to transform photon counts into the correlation function, which is the foundation for all quantifications generating size data. Additionally, the correlator must supply a broad array of time delays and yield a fairly high quantity of delay channels to generate the required resolution.

Nevertheless, correlators utilized in goniometer based and enclosed light scattering systems also diverge noticeably. In a goniometer light scattering system, the user can select the layout of the correlator and preset the distribution of the available delay channels over the desired time range. Therefore, it is possible to fine-tune them to particular needs.

Conversely, enclosed light scattering instruments typically do not permit reconfiguration of the correlator layout and, although they might allow two or more presets, this does not amount to fine-tuning. A long debate may ask “which is superior,” but this would miss the point. The answer lies in the application.

A goniometer based light scattering system is characterized by its adaptability, allowing it to generate optimized measurements for a broad array of investigations. Therefore, a correlator layout that can be fine-tuned is a necessity. An enclosed light scattering instrument is regularly marketed according to its speed of operation.

Consequently, it is fitted with few fixed layouts, which are calibrated to provide an appropriate but not optimized configuration for most regular investigations. To summarize, these represent two solutions based on DLS, with one more fixed on adaptability to offer optimized performance for almost any application, the other more focused on speed/ease of use.

2. Looking at Non-Spherical Particles

Geometric angles represent one of the more regular topics among discussions regarding light scattering. Measurements of the light scattered by a sample in solution or a particle dispersion at divergent angles of detection is a typical method of analysis. When the hydrodynamic diameter of molecules or particles is greater than around 5% of the wavelength of the used laser source, then the intensity of the scattered light will be angle-dependent; this is the foundational principle informing this method.

Larger particles are typically forward scattered. The intensity will thus increase as the angle of observation decreases (it should be noted that at an angle of zero you would be when looking directly into the laser). The Zimm Plot utilized for molecular solutions in static light scattering benefits from this fact and is utilized to calculate the gyration radius Rg of the examined molecules.

For this, it is usual to quantify at least seven divergent angles spanning 15° (or less) to 155°. A goniometer based light scattering provides the ideal basis for this category of the task, due to its inherent capability of measuring at any angle from around 8° to 155°.

All this is valid for static light scattering quantifications, but what about its application to dynamic light scattering? Particle Size calculation is actually undertaken by measuring Translational Diffusion Coefficient Dt and consequently applying the Einstein equation. However, this equation was specifically intended for spherical particles. Indeed, it does not account for divergent particle shapes, which can lead to an erroneous determination of the hydrodynamic diameter of non-spherical particles, such as rod-shaped particles. In this scenario, the calculation of particle size utilizing divergent angles of detection can assist in the determination of the morphology of the sample.

As mentioned earlier, the intensity of scattered light is a function of the detection angle. As a result, measurements at divergent angles will return values weighted in different ways, with the larger dimension more dominant at low angles and the smaller dimension more present at high angles.

Comparing results acquired at different angles might, therefore, indicate the aspect ratio of the examined particles, thus generating shape data. While this represents an exigent investigation, it offers a valuable approach to comprehending non-spherical particle samples.

It has already been explained that goniometer based light scattering systems are precisely calibrated for angle-dependent quantifications. Conversely, how do enclosed light scattering instruments handle non-spherical particle applications? The majority of enclosed DLS instruments only offer a single angle of detection, which makes this investigation impossible. A handful of enclosed light scattering instruments are now on the market offering up to three different angles of detection spanning the range from 17° to 173°.

These instruments can thus execute quantification at these set angles, offering some data on non-spherical particles, but this is not comparable to the flexible capacities of a goniometer based light scattering system.

3. Different Software Approach

All modern Dynamic Light Scattering (DLS) instruments require software for data processing and enhancing results accessibility for the user. Consequently, a system’s software component has the most immediate effect on users, yielding intuitive access to functions with an easy to use graphical user interface.  However, it is also important to focus on what to expect in regards to fundamental functionality from a software package designed for a Goniometer based system compared to software equipped with an enclosed light scattering instrument.

As a result of the flexible design of a goniometer-based system, software packages will usually offer intuitive access to all details of the actual measurement, comprising a refractive index of the liquid and particle, correlator layout and all additional relevant analytical parameters.

Analyzing and interpreting a measurement is usually undertaken post-processing once the measurement has been undertaken. In this regard, goniometer light scattering instrument software can utilize divergent analysis algorithms (comprising Contin, NNLS and cumulants) to provide users with the greatest possible control over their interpretation of the obtained raw data. Among enclosed light scattering instruments, the approach taken software designers differs somewhat.

These instruments often yield limited options for not only parameters for the measurement, but also analysis techniques. Supporting ease of use for the occasional user, enclosed light scattering instruments will typically offer automation capabilities, comprising time or temperature dependent measurements, while also supporting external autotitrator devices.

This automation approach requires little to no post-processing operations, permitting the rapid but non-optimized generation of final results. The two software approaches differ radically. Among goniometer-based systems, the software is developed to offer the greatest possible degree of control and freedom for the user, while among enclosed light scattering instruments, the software is developed to generate automated results in accordance with a limited set of parameters and calculation options.

4. Comparing System Components and Consumables

The sections above have examined the similarities and dissimilarities between an enclosed light scattering instrument and a goniometer-based system in terms of the most regularly assessed parameters.

Nevertheless, a number of scientists intending to acquire a light scattering system frequently want to undertake evaluation and comparison with other instrument components comprising lasers, detectors, and measurement cell assemblies. Dissimilarities between the enclosed instruments and the goniometer based system and are here far less considerable than might be expected.

Starting with the measurement cell assembly. In each case, inexpensive measurement cells are utilized, usually square cells in many enclosed light scattering instruments and round cells in goniometer-based systems. In each case, glass or plastic disposable cells are utilized as a benchmark to ensure a low cost of ownership, relating to consumable cells. It is possible to recycle used cells, but this necessitates thorough cleaning, the costs of which help to justify using a new measurement cell.

Both goniometer-based and enclosed light scattering instruments regularly utilize Avalanche Photo Diodes (APD). These detectors have replaced Photomultiplier Tubes (PMT), the most common light scattering detector in the past. APD’s provide outstanding sensitivity across the whole wavelength spectrum and, most importantly, manifest little to no propensity towards “Afterpulse,” a phenomenon identified with PMTs and a cause of significant distortion of light scattering measurements.

Nevertheless, there are some applications (e.g., measuring very small particles at very low concentration) for which Afterpulse still poses a problem. For these exigent applications, goniometer-based light scattering systems provide a unique cross-correlation solution. Cross-correlation simultaneously utilizes two detectors, with the signals combining into a cross-correlation which completely eradicates any Afterpulse effect.

A laser is the most elementary constituent of any Dynamic Light Scattering (DLS) instrument. At present, manufacturers typically implement diode lasers rather than gas lasers, which were the favored option among previous generations of light scattering instruments. Diode lasers provide numerous advantages over gas lasers, such as longer lifetime, a smaller footprint, and lower power consumption. They can also be switched on and off regularly, without being damaged. Diode lasers can be obtained in a broad array of wavelengths and output power.

Among common DLS applications, red laser sources are typically used, at embodying a wavelength of around 640 nm (traditionally close to the 632 nm of the HeNe Lasers) and output power of 40 to 140 mW. This is the case for both enclosed light scattering instruments and goniometer-based systems.

Nonetheless, the superior design flexibility of goniometer based light scattering systems means that the laser source is typically far more replaceable, and can be easily adapted to different needs in terms of wavelength and power. Consequently, they are the preferred system among applications requiring a non-standard laser.

5. Static Light Scattering Applications – There is Only One Choice

Static Light Scattering (SLS) is the original light scattering technology, but it is still frequently used. This technique is executed by measuring the integral intensity of scattered light photons at one or more angles of observation. Applying Static Light Scattering is unrelated t the calculation of the size of particles in a dispersion.

However, it is the tested and favored solution for the determination of the gyration radius and molecular weight of polymer molecules dissolved in a solvent. This indicates that, by interpreting the signal deriving from the detector differently, a user’s field of application is shifted from particle suspensions to molecular solutions.

It would be legitimate to query the appropriateness of either a goniometer-based system or an enclosed light scattering instrument for this application. As previously explained, SLS necessitates measuring at multiple angles of detection, with at least seven angles (the more the better) required to yield precise data.

Due to the fact that even the most cutting-edge enclosed light scattering instruments only allow three angles of detection, SLS is offered on an optional basis, and the data they provide must be treated with care and skepticism. As it enables numerous angles of light scattering quantification, a goniometer-based system is a perfect fit for any SLS requirement and application, generating absolute molecular weights of any polymer.

In Summary

On an initial glance, goniometer based and enclosed light scattering instruments appear highly divergent in terms of the bench space they occupy. This brief comparison has sought to underscore various features of these two different approaches to DLS which may not be immediately clear to a beginner to the subject.

This comparison explicitly demonstrates how different needs, such as flexibility or ease of use, engender equally divergent approaches to instrumentally realizing identical technology and physical principles.

It can be concluded that the DLS solutions described above yield divergent optimized solutions. The excellent flexibility and performance of goniometer-based light scattering systems render them the ideal R&D solution, whereas the ease of use of the enclosed light scattering instrument is of great benefit to a QC context.

Therefore, the optimal decision regarding the selection of a DLS instrument should not just be made according to specifications, price or popularity, but on the user’s intended applications for this potent technology.

This information has been sourced, reviewed and adapted from materials provided by Brookhaven Instrument Corporation.

For more information on this source, please visit Brookhaven Instrument Corporation.