Expanding Applications of Raman for Particle Analysis

Chemical analysis of particles is normally performed using analytical methods that are optimized for small samples. Micro-FTIR spectroscopy is a powerful tool for the identification of particles, but there the lower particle size limit is diffraction limited. The lower size limit varies somewhat between instrument detectors, but for particles less than 25 µm, the signal to noise ratio may be too low to obtain an interpretable spectrum. SEM/EDS can be used to obtain the elemental composition of particles much smaller than 25 µm, but it is not specific for organic compounds. Raman microscopy is a vibrational spectroscopy technique that is complementary to FTIR. It can be used to identify particles that are not readily characterized by IR less than 25 µm and can be used to perform particle size analysis as well as particle characterization.

Sulfur crystals have been found in certain biological therapy products. Sulfur particles have multiple morphologies in solution including needle-like (Figure 1), flake-like, dendritic crystals, blade-like and rounded. The elongated particles sometimes have serrated edges. Sulfur crystal particles are often observed during the prefiltration examination. Sulfur is inactive in the mid-infrared (no IR peaks), but it has a strong Raman spectrum (Figure 2). The Raman small laser spot size (514 nm) allows for analysis of small particles and thin needle-like crystals such as these. Small sulfur particles also tend to sublimate and disappear under ambient conditions, so it is crucial to use the appropriate analysis method as soon as possible to avoid loss of the particle.

McCrone Associates has utilized Raman microscopy as part of its particle identification toolbox for some time, but innovations in software now allow automated chemical analysis of large numbers of particles. Particles such as micro plastic particles can be analyzed with minimal sample preparation in a reasonable timeframe. Figure 3 below shows Raman analysis of particles filtered from bottled water onto a gold coated polycarbonate filter membrane.

One of the particles was suspected to be a polyethylene-based polymer. After the automated analysis, a particle that was approximately 25 µm was re-located (Figure 4) and analyzed using an extended spectral range. The particle was confirmed to be a polyethylene-based polymer (Figure 5).

Work continues to document and expand the applicability of this new approach for particles one to five micrometers in size.

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