Differential light scattering and the measurement of molecules and nanoparticles: A review

Within the past few decades, the application of light scattering techniques to a broad range of scientific disciplines has increased significantly, especially in the field of analytical chemistry. The resulting interest in and use of light scattering methods suggests the need for an easily understood introduction and review of material for those new to the method as well as for current users in need of a refresher. In many respects, the theory and its applications may appear so overwhelming for many studying the field for the first time, that they rarely can spend the time just needed to understand the basic measurements and their interpretations. A variety of applications in analytical chemistry especially have resulted in a greater understanding of many of the macromolecular processes themselves from molar mass distributions, to the macromolecular interactions responsible for aggregation processes, to determinations of structure and function. The use of such analytical processes to obtain a better understanding of nanoparticle structure and function has become almost universal.

•

Deriving the distribution of fractionated macromolecules by light scattering.

•

Deriving the distribution of fractionated nanoparticles by light scattering.

•

Deriving the masses of fractionated macromolecules from their scattered light.

•

Deriving the size of nanoparticles from the angular variation of scattered light.

•

Deriving bacterial size and structure by light scattering.

Measuring nanoparticles in the size range to 2000 nm

Measurement of light scattered from suspensions of monodisperse nanoparticles in solution ("turbidity") long has been used to derive their size. Following some means of fractionation, the light (monochromatic) scattered by the particles into a set of distinct angles is collected and a non-linear least squares fit was made to an appropriate theory in order to extract their size. For a wide range of particle structures, where this process becomes very complex and of questionable validity, there is a far simpler interpretive means based upon measurements at extremely small, and often inaccessible, scattering angles. A method is described whereby the required small angle values are derived from measurements made over a range of larger, more readily accessible, angles. Although the basis for the analyses developed is the Rayleigh-Gans approximation, the results presented confirm that the method provides meaningful results up to a size of about 2000 nm. The larger sizes are well beyond the RG limits.

Measuring proteins with greater speed and resolution while reducing sample size

A multi-angle light scattering (MALS) system, combined with chromatographic separation, directly measures the absolute molar mass, size and concentration of the eluate species. The measurement of these crucial properties in solution is essential in basic macromolecular characterization and all research and production stages of bio-therapeutic products. We developed a new MALS methodology that has overcome the long-standing, stubborn barrier to microliter-scale peak volumes and achieved the highest resolution and signal-to-noise performance of any MALS measurement. The novel design simultaneously facilitates online dynamic light scattering (DLS) measurements. As National Institute of Standards and Technology (NIST) new protein standard reference material (SRM 8671) is becoming the benchmark molecule against which many biomolecular analytical techniques are assessed and evaluated, we present its measurement results as a demonstration of the unique capability of our system to swiftly resolve and measure sharp (20~25 µL full-width-half-maximum) chromatography peaks. Precise measurements of protein mass and size can be accomplished 10 times faster than before with improved resolution. In the meantime the sample amount required for such measurements is reduced commensurately. These abilities will have far-reaching impacts at every stage of the development and production of biologics and bio-therapeutic formulations.

Submicrometer Particle Sizing by Multiangle Light Scattering following Fractionation

The acid test for any particle sizing technique is its ability to determine the differential number fraction size distribution of a simple, well-defined sample. The very best characterized polystyrene latex sphere standards have been measured extensively using transmission electron microscope (TEM) images of a large subpopulation of such samples or by means of the electrostatic classification method as refined at the National Institute of Standards and Technology. The great success, in the past decade, of on-line multiangle light scattering (MALS) detection combined with size exclusion chromatography for the measurement of polymer mass and size distributions suggested, in the early 1990s, that a similar attack for particle characterization might prove useful as well. At that time, fractionation of particles was achievable by capillary hydrodynamic chromatography (CHDF) and field flow fractionation (FFF) methods. The latter has proven most useful when combined with MALS to provide accurate differential number fraction size distributions for a broad range of particle classes. The MALS/FFF combination provides unique advantages and precision relative to FFF, photon correlation spectroscopy, and CHDF techniques used alone. For many classes of particles, resolution of the MALS/FFF combination far exceeds that of TEM measurements. Copyright 1998 Academic Press. Copyright 1998Academic Press

Aerosol particle analyzer

A new multiangle light scattering instrument is described for the classification of single aerosol particles in a flowing gas stream. Simultaneous measurement at sixteen scattering angles over the surface of a spherical chamber permits determination of a variety of optical observables useful for the subsequent characterization of each measured particle. Particles of 0.2 - 4.0-microm diameter have been measured and data collected at rates exceeding 200 particles/s. Data from polystyrene latex particles, liquid droplets, and irregular particles are shown. Spherical particle data are compared to theoretical calculations and used to confirm the validity of the measurement as well as a powerful calibration procedure.

The taste of things to come

The differential light scattering (DLS) properties of six popular cola drinks are compared (three standard and three saccharine-sweetened). Each produces a unique DLS pattern characteristic of its unique taste. The probable demise of the subjective taste test in favor of a quantiative quality control DLS measurement is proposed.

Some chemical, physical, and optical properties of fly ash particles

Fly ash samples from two different generating plants have been examined by laser light scattering techniques. Individual particles below approximately 3-microm diam, found to be primarily spherical, were electrostatically levitated in a single-particle light scattering photometer while their differential light scattering (DLS) patterns were recorded. During this measurement period, the relative humidity within the scattering cell could be varied to study the water accretion properties of the suspended particle. The recorded DLS patterns are used to derive the complex refractive index, size, and accreted layer thickness of the particle. Each particle appears to have a different refractive index, probably indicative of the varied microscopic conditions of formation. Even at very high relative humidities, the fly ash particle surface does not appear to have a natural affinity for water, contrary to popular expectations.

Light scattering by polydisperse suspensions of inhomogeneous nonspherical particles

The extended boundary condition method is used to make angular scattering calculations for a polydisperse suspension of prolate spheroidal particles. To aid in interpretation of the nonspherical results, calculations are also made for spherical particle suspensions. The basic particle models are two-layered approximations of common bacteria. Using a quasi-Gaussian size distribution, changes in the average particle size are found to have the dominant effect on the scattering of a polydisperse suspension. The spherical and prolate spheroidal two-layered results are compared with similar calculations using the Rayleigh-Gans-Debye approximation and to results based on volume-averaged homogeneous models.

Laser differential light scattering bioassays for selected antitumor agents

Bioassay methods for actinomycin D, 6-thioguanine, and adriamycin which utilize laser light scattering from suspensions of drug-sensitive bacteria have been developed. In 3 hours, serum and urine levels of the 3 drugs can be reproducibily measured. A sample volume of 0.1 ml may be assayed with a precision of +/- 10%. The linear response range for actinomycin D was 0.03 to 3.0 microgram/ml. For 6-thioguanine and adriamycin the linear responses range was 0.03 to 10.0 and 0.25 to 10.0 microgram/ml, respectively. Thetively. The present studies further confirm the general utility of differential light scattering as a rapid and inexpensive assay procedure of broad application.

Laser light scattering bioassay for 1-beta-D-arabinofuranosylcytosine (ARA-C, NSC-63878)

A bioassay method for the estimation of ara-C in biological samples which utilizes laser light scattering from suspensions of drug-sensitive bacteria has been developed. The species employed in the assay was Streptococcus faecium var. durans resistant to methotrexate and 6-mercaptopurine. In less than four hours, serum and urine levels of ara-C can be reproducibly measured using the method. A sample volume of 0.1 ml containing 30 ng/ml may be assayed with a precision of +/-10 percent. The present studies further confirm the utility of the method as a rapid and relatively inexpensive assay methodology of broad application. Time variations of drug serum levels and urinary excretion rates in dogs are compared via the differential light scattering (DLS) assay, standard disc diffusion assay, and radiosotopic assay. The results obtained by the various methods are in excellent agreement.

Single-particle light-scattering measurement: photochemical aerosols and atmospheric particulates

The use of single-particle light-scattering measurements to determine the origin of atmospheric hazes has been explored by measurement of laboratory aerosols, field samples, and computer analysis of the light-scattering data. The refractive index of measured spherical particles 800 nm to 1000 nm in diameter was determined within 2%. For particles of diameter less than 500 nm the measurement of absolute scattering intensity is required for complete analysis. Distinctive nonspherical and absorbing particles were observed both in automotive exhaust and atmospheric samples. Electrostatic suspension of atmospheric particulates is demonstrated to provide a practical approach to optical measurement of single particles. The technique may be used to calibrate optical particle counters or identify particles with unique shape or refractive index.

Osmotic sensitivity in Staphylococcus aureux induced by streptomycin

Differential light-scattering measurements of Staphylococcus aureus cultures were made before and after treatment with streptomycin. Changes were observed in the light-scattering characteristics of streptomycin-treated sensitive cells within 5 min after suspension in a hypotonic solution. No changes were observed with a resistant strain of cells nor with either strain in an isotonic solution. The observed effects occur more slowly when the cells are growing slowly. The physical effects consonant with the changes in the light-scattering curves are a broadening of the cell size distribution, a slight reduction in mean size, and the appearance of clumps or debris. We conclude that streptomycin rapidly alters the selective permeability of the cell membrane and makes the cells susceptible to increased osmotic stresses.

Differential light scattering measurements of heat-treated bacteria

Effects of heat on diameter, size distribution, and refractive index of Staphylococcus epidermidis suspensions were determined accurately by computer analysis of differential light scattering data.

Differential light scattering: a physical method for identifying living bacterial cells

Differential light scattering techniques appear to represent an attractive physical method for the rapid identification of various microorganisms. Certain general results of inverse scattering theory suggest strongly that characteristic of each distinct microorganism that scatters light is an essentially unique scattering pattern, i.e., unique differential scattered intensity and polarization. Although a mathematically rigorous inversion procedure seems impractical at this time, the use of detailed differential scattered intensity data as an identification fingerprint shows considerable. promise. Published measurements on nonbiological scatterers confirm this possibility. A variety of calculations are presented that contrast the expected scattering characteristics of certain microorganisms such as Bacillus subtilis, B. anthracis, Staphylococcus epidermidis, S. aureus, Escherichia coli, and the spores of B. mcgaterium and B. cereus. Experimental and instrumentation difficulties and possible procedures are discussed. A review and laboration of some applicable features of Rayleigh-Gans scattering are included as an appendix.