The Story of MALS

The concept of measuring light scattered from a solution at each of a plurality of angles simultaneously began in late 1976 in a very unlikely application. At that time, scientists from Science Spectrum (the predecessor upon which Wyatt Technology Corporation [Santa Barbara, CA] was founded) were developing instrumentation1 for the Food and Drug Administration to detect antibiotic residues in meat. Fluid exudates were collected from suspect meat samples and then combined with exponentially growing bacterial strains chosen because of their high susceptibility to a broad range of antimicrobial agents. If antibiotics were present in the combined sample, then, following incubation, the morphology of the susceptible bacteria would change—an effect easily observed by monitoring changes in the recorded angular variation of the light2 scattered from the solution following incubation for about 60 minutes.

The measurements in themselves were remarkable for a number of reasons. By the time the specialized instrument was being developed for the FDA, Science Spectrum had already begun to sell its new light scattering photometers incorporating lasers3 (argon ion) as their light sources (λ~514 and 488 nm).

Historically, such photometers incorporated Hg-arc lamps collimated to produce a fine beam (~5 mm) of light (λ~546 and 436 nm) that would pass through samples contained in glass cuvettes. Used primarily for the determination of molar masses, such measurements were needed following World War II for measuring high-molar-mass polymers, including natural and synthetic rubbers.

The birth of multiangle light scattering

Figure 1 – Schematic representation of multiangle light scattering measurement. ©Wyatt Technology Corporation.

The FDA-developed instrument made its measurements of solutions collected by the early autosampler-type device (described in Ref. 1) and flowing through a fine HeNe laser (633 nm) beam constrained within a glass capillary tube. Scanning the sample as it passed through the beam was not possible, so Phillips proposed attaching small photodetectors around the capillary following some earlier ideas4 to measure aerosol particles as they passed through a laser beam. The concept worked well and formed the basis for the first array-based systems introduced by the newly formed Wyatt Technology in 1982. Multiangle light scattering (MALS) was born!

It is important to emphasize that the formal MALS measurement geometry refers specifically to measurements made in a plane that includes the incident laser light source and the discrete detector array surrounding the illuminated sample, as shown schematically in Figure 1 and as implemented in the modern MALS read-head shown in Figure 2. The incident light is usually a collimated beam from a laser source polarized perpendicular to the scattering plane shown.

Figure 2 – Photograph of the read-head of a DAWN HELEOS II (Wyatt Technology) multiangle light scattering photometer. ©Wyatt Technology Corporation.

The first actual MALS instrument was sold to S.C. Johnson & Son (the 130-year-old family household products business [Racine, WI]) at the request of their most astute scientist, Dr. Bob Warner, who wanted to measure particles separated by means of hydrodynamic chromatography. This first system built by Wyatt Technology Corp. (WTC), incorporating a capillary surrounded by seven detectors, was the first commercial MALS system. Within a few months of the delivery of Warner’s system, Joe Parli of Amoco Production Company (now Amoco Research Center, Tulsa, OK) ordered a system to make the first MALS measurements of gel permeation chromatography (GPC) (size exclusion chromatography [SEC])-separated polymers.

Rather than use the capillary-based flow system of S.C. Johnson, WTC delivered a system incorporating its new flow cell5 with 15 detectors spaced about it. By that time in 1974, Beckman Instruments (now Beckman Coulter, Brea, CA) had sold its low angle laser light scattering (LALLS) device (introduced unsuccessfully in 1972) to Chromatix, which began further developments of such low angle measurements on GPC-separated polymer samples. The KMX-6 system was introduced in 1976. The extra “L” was added to the LALLS acronym (“low angle laser light scattering”) to symbolize “laser” reminding the user, thereby, of the very narrow beam diameter facilitating small angle light scattering measurements.

The introduction of MALS (and the new KMX-6 LALLS instrument from Chromatix) caused a major shift for polymer (and later protein) scientists. By combining SEC separation with light scattering, a far greater understanding of the samples so measured was obtained. The separation of samples using GPC columns before flowing into the MALS detector permitted the study of the samples’ associated mass and size distributions. What was once limited to a determination of the weight average molar mass determination was now replaced with the ability to obtain explicit mass and size distributions and their number, weight, and Z-moments.

Dr. Wallace W. Yau, the du Pont (Wilmington, DE) scientist, was another early adopter of the MALS device and helped accelerate its applicability. In an exchange surely filled with irony, Yau, a chemist, pointed out to Phillips, a physicist, that the photodetectors then incorporated in the early DAWN units must be able to produce far larger signals than were being generated. Phillips had overlooked this obvious fact, and within weeks had increased the electronic gain of the detectors by a factor in excess of 100! Yau’s understanding of basic electronics was exceptional.

From measurement of the so-called excess Rayleigh ratio (proportional to the light scattered from the solution in excess of light scattered from the pure solvent) Ri(Θj, ci) at scattering angles Θj and concentration (using an in-line differential refractive index detector) ci, the associated mass Mi for that particular eluting fraction could be measured. As will be recalled from the Zimm6,7 theory, this relation at the elution slice i is:

(1)

where  is the wavelength of the incident light in vacuum, A2 is the 2nd virial coefficient, NA is Avogadro’s number, P(Θj) is the scattering form factor, (dn/dc) is the change of solution refractive index dn for a change of the solute concentration dc, and n0is the solvent refractive index. The scattering form factor may be shown to be:

(2)

where <rg2> is the molecular mean square radius. For MALS measurements, of course, the concentrations at a particular elution slice are usually so low that Eq. (1) simplifies immediately to:

(3)

It is with Eq. (3) that the power of the MALS technique became evident. Historically, from the earlier days of unfractionated samples, M, <rg2>, and A2 were derived from manually constructed Zimm plots7,8 based on measurements of scattering from unfractionated samples extrapolated to Θ = 0° to obtain M. <rg2> was obtained from the initial slope of the variation of R(Θ,c) with angle and A2 from measurement of its variation with concentration. Surprisingly, these techniques and concepts continued for many years after the combination of light scattering with separation by SEC became more popular. Indeed, one of the great attractions of the LALLS devices up to this very day is their purported ability to read off the molar mass directly since the low angle measured is “almost” zero. The single angle measurement cannot yield molecule size and, in addition, is highly sensitive to column shedding and other mobile phase contaminants, thereby limiting its range of applicability. In addition, size characterizations of larger molecules as well as virus-like particles, lipids, particles, etc., are not possible with a single angle instrument.

For many years, MALS measurements followed the general Zimm formalism often based on extending the range of applicability by working with the reciprocal Rayleigh ratio, 1/Ri(Θj, ci), and variations thereof. Basically, the graphical techniques developed by Zimm were now reduced to computer-based calculations involving extrapolations to zero angles, etc.

Many years ago, a young scientist working briefly at Wyatt Technology pointed out that Eq. (3) and its relationships were valid at all measured angles and separated concentrations, so why abandon all these data just to obtain extrapolated values? Bruno Zimm, a member of the WTC Scientific Advisory Board for almost 20 years, agreed; yet it was not until Trainoff9 implemented this so-called “Global fit” to extract far more accurate results that the exceptional capabilities of MALS were overwhelmingly confirmed.

Conclusion

The WTC instrumentation and the companion ASTRA® software remain unique in their versatility. Almost 9000 journal-reviewed articles based upon Wyatt Technology instruments have appeared in the literature.

MALS detection has been expanded significantly to include the characterization and measurement of particles whose refractive index and size place them far beyond the limitations of the formalism of Eq. (1). These include carbon nanotubes, gold and silver particles, emulsions, colloids, and a variety of polystyrene latexes with sizes in excess of 2000 nm. With the addition of quasi-elastic light scattering (QELS of DLS [dynamic light scattering]) measurable at a range of different scattering angles, unique structural properties of many new classes of particles are attainable.

On-line DLS, originally developed at WTC, continues to be a powerful tool with the MALS capability, allowing measurement of an extremely broad range of particle sizes. The incorporation of narrow bandpass filters at each MALS angle permits the measurement of molecules previously inaccessible due to fluorescence. An extraordinarily broad range of polarization characterizations including confirmation of Cabannes’ factors have been added in recent years.

References

  1. Maldarelli, L.V.; Phillips, D.T. et al. Programmable Action Sampler System. U.S. Patent 4,140,018; 1979.
  2. Berkman, R.M.; Wyatt, P.J. et al. Rapid detection of penicillin sensitivity in Staphylococcus aureus. Nature  1969, 221, 1257–8.
  3. Phillips, D.T. Evolution of a light scattering photometer. BioScience  1971, 21, 865–7.
  4. Wyatt, P.J. Microparticle Analyzer Employing a Spherical Detector Array. U.S. Patent 3,624,835; 1971.
  5. Phillips, S.D.; Reece, J.M. et al. Sample Cell for Light Scattering Measurements. U.S. Patent 4,616,927; 1986.
  6. Zimm, B.H. The scattering of light and the radial distribution function of high polymer solutions. J. Chem. Phys.  1948, 16, 1093–9.
  7. Zimm, B.H. Apparatus and methods for measurement and interpretation of the angular variation of light scattering; preliminary results on polystyrene solutions. J. Chem. Phys1948, 16, 1099–1116.
  8. Wyatt, P.J. Light scattering and the absolute characterization of macromolecules. Anal. Chim. Acta  1993, 272, 1–40.
  9. Trainoff, S.P.; Wyatt, P.J. Method for Determining Average Solution Properties of Macromolecules by the Injection Method. U.S. Patent 6,651,009; 2003.

Philip J. Wyatt, Ph.D., is Founder and Chief Executive Officer, Wyatt Technology Corp. (WTC), 6300 Hollister Ave., Santa Barbara, CA 93117, U.S.A.; tel.: 805-681-9009; fax: 805-681-0123; e-mail: pwyatt@wyatt.com; www.wyatt.com.

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