MICROSCOPY AND IMAGING
The alpha300 R Raman microscope
APPLYING TOPOGRAPHIC RAMAN MICROSCOPY
Damon Strom & Eleni Kallis discuss 3D chemical characterisation on curved and rough surfaces for nutritional science applications
I
n nutritional science and the food industry, it is necessary to examine products in their commercial state. To ensure compliance with standards of quality and to refine manufacturing processes, dietary supplements in tablet form, and food as it is served, must be investigated at high resolution over large areas. For conventional microscopes this can be impractical as roughly textured or curved samples will present a surface that is only partially in focus. When confocal Raman imaging is employed for its precision and chemical sensitivity, this challenge is particularly acute as the technique strongly rejects all light from outside the focal plane. WITec solved this problem by
developing TrueSurface microscopy, a module for its alpha300 series that integrates an optical profilometer with Raman imaging microscopy for active focus stabilisation. Te module traces the sample surface and simultaneously compensates for height differences during the Raman measurement in one-pass operation. Tus, the surface stays in focus while the sample topography is acquired along with the chemical information. In the two following examples,
TrueSurface microscopy is demonstrated with a measurement of the components of an abraded nutritional supplement tablet, and the chemical characterisation
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of two different substances used to print on candy.
TOPOGRAPHIC RAMAN IMAGING OF A NUTRITIONAL SUPPLEMENT TABLET Te tablet shown in Fig. 1 is coated with a layer of titanium dioxide (TiO2
) and
has a smooth but curved surface with a pronounced groove in the middle. A large- area topography image of the untreated tablet shows the surface curvature and the more than 300µm-deep cleft in the tablet (Fig. 1A). To investigate the tablet’s chemical components, the TiO2
Recently it’s been the subject of increased scrutiny due to concerns that it might be carcinogenic. In May 2021, the European Food Safety Authority (EFSA) announced that the substance can no longer be considered a safe food additive. Tis led European food manufacturers to replace TiO2
white pigments.
effective method for analysing food additives and TiO2
coating was removed.
Te surface of the abraded tablet no longer has the cleft, but is still very roughly textured (Fig. 1B). TrueSurface kept the laser in focus during the entire Raman measurement and the topographic Raman image visualises the distribution of the ingredients on the rough surface (Fig. 1C). Te nutritional supplement provides a source for magnesium (red) and calcium (green). A binder (blue) and lubricant (yellow) were also identified.
TOPOGRAPHIC RAMAN IMAGING OF CANDY COATINGS TiO2
has long been used as a white pigment (E 171) in the food industry.
in their products with alternative Raman imaging microscopy is a very is especially easy to
detect as it exhibits a strong Raman signal. Tis study is a comparison of the same kind of candy produced for two different markets: the EU and the USA. All of the pieces have a white “S” printed on their colourful coatings (Fig. 2A) and Raman analysis reveals the chemical identities of the pigments used. Te measurements were performed with an alpha300 R equipped with a 488nm excitation laser and TrueSurface technology for topographic Raman imaging and active focus stabilisation.
Raman imaging clearly distinguished the print from the surrounding sugar coating in both candies and the topographic Raman images show their curved surfaces (Figs. 2B and 2C). Te white pigments forming the S were
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