Light Transmission Spectroscopy

The analysis of nanoparticles in liquid suspension has been practiced for many decades by using a wide variety of techniques. The need to analyze nanoparticles arises from their considerable practical and scientific importance. It is difficult to overestimate the abundance, diversity, and importance of nanoparticles - both natural and man-made. Nanoparticles (from a few to thousands of nanometers in diameter) are everywhere in our Earthly environment and throughout space. They are comprised of both organic and inorganic materials and are of course vital components in living organisms. Their importance is prominent in many areas of industry, medicine (and pharmacology), and basic science.

While many types of devices are used to measure particle-size distributions, they are for the most part based on a few fundamental measurement principles. Mechanical centrifugation is one such early- and still-used method of segregating and measuring nanoparticles. Another early methodology involves measuring the resistance of a cylinder of conducting fluid (salt water) through which particles are made to flow one-by-one. Assuming the particles are weakly electrically conducting, the volumetric displacement of the particle causes a corresponding increase in the resistance of the fluid. In this way, the diameter of individual particles can be ascertained.

This is the principle upon which the Coulter counter is based, a technique that remains in broad use. A related technique is resistive-pulse sensing, where resistive pulses are measured as particles in a conducting fluid move through micropores. Another widely used technique involves photon correlation spectroscopy or so-called dynamic light scattering (DLS). This technique is based on the scattering of light by particles subjected to Brownian motion. By ascertaining the velocity of the particles, and with knowledge of the fluid temperature and viscosity, the hydrodynamic diameter of particles can be found from the Stokes-Einstein equation. A final class of techniques involves the scattering of waves by particles in suspension.

This can involve the scattering (perhaps most prominently diffraction) of sound waves, visible or near-visible light, or X-rays. Particles can also be directly imaged using optical or scanning electron microscopy (SEM). A number of works have explored the origins, technical implementations, and applications of particle-size analysis.

In this article, we introduce light transmission spectroscopy (LTS), a new technique for determining the size distribution of particles in liquid suspension. LTS is based on the principle of measuring the absolute optical extinction (absorption) of light over a broad wavelength range (UV to infrared) at high-resolution using two balanced optical channels. This allows for a special feature of LTS: The ability to perform “optical subtraction" whereby objects of a given size and concentration can be eliminated from the analysis by inserting these objects in the reference channel.

The original implementation of LTS employed a laser-driven optical source with 1 nm optical resolution, which led to the development of an LTS instrument employing a broadband light source and matched, broad-wavelength, high-resolution (1nm) optical spectrometers. A single-channel implementation of the approach has also been reported. The principal advantages of (dual-channel) LTS over other optical-based methodologies are that it provides absolute particle concentration, and it is capable of simultaneously determining the physical, as opposed to hydrodynamic, diameter, and concentration of particles in the 1-3000 nm range. Optimally, LTS can distinguish between two particle sizes differing by ~100 nm for particles with diameters in the range of 100-1000 nm; ~10 nm for particles in the range of 10-100 nm; and ~1-3nm for particles in the range of 1-10 nm.