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Environmental FT-IR microscopy

Without suitable protection, all solid and liquid substances on the surface of the Earth are subjected to atmospheric moisture. In the case of powders or particulates, humidity-induced changes often occur at the microscopic level. The market leader in moisture sorption technology, Surface Measurement Systems, has developed a new accessory specifically designed to allow the study of humidity induced phenomena by microscopy. The VGI 2000M provides accurate and reproducible humidity control for light, FT-IR, NIR and Raman microscopes.

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The VGI 2000M consists of 3 main components; a programmable digital controller, a heat exchange/pump unit and the sample stage. The sample is mounted in the sample stage, which is fixed to the microscope stage. This stage has double-glazed top and bottom windows, allowing both transmission and reflection illumination of the sample while minimising heat loss through the windows. This last feature allows the accessory to be used over the range 15 °C to 40 °C, at up to 95% RH. The sample area and water reservoir are kept at a constant temperature and the required humidity is maintained by mixing saturated (100% RH) and dry (0% RH) carrier gas flows using electronic mass flow controllers. A miniature temperature and humidity probe situated close to the sample provides feedback to the digital controller, allowing accurate regulation of the setpoint temperature and humidity within the sample stage.

 

 

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The VGI 2000M may be used with microscopes adapted for FT-IR spectroscopy by changing the window material used, in this case to BaF2, allowing both transmission and reflection modes of the instrument to be used. All the spectra in this article were obtained on a Bruker Optics IRscope II microscope coupled to a Bruker Optics IFS 28 spectrometer at the contract research labs of Optas Ltd.

As an example, the deliquescence of a single NaCl crystal was measured by FT-IR microscopy in transmission mode. Figure 1 shows the evolution of the FT-IR spectra as the humidity above the sample is ramped from 70% RH to 79% RH at a rate of 0.1% RH min-1. The spectra show that above 75% RH two bands grow steadily at 3000-3500 cm-1 and 1500-1600 cm-1 until 76% RH, above which they saturate. These bands can be attributed to the OH stretching and OH deformation modes of liquid water molecules on the surface of the NaCl crystal and are indicative of the deliquescence phenomenon. The literature value for NaCl at 25°C is 75.3% RH, which is in very good agreement with the experimental data.

 

 

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In a second example, the hydration behaviour of Naloxone HCl, a well-known anti-narcotic agent, has been characterised by reflection mode FT-IR microscopy. The moisture sorption of Naloxone HCl has been studied previously by dynamic vapor sorption. Essentially, the material hydrates between 10% RH and 30% RH to form a monohydrate and between 30% RH and 60% RH to form a dihydrate. Figure 2 shows the evolution of the FT-IR spectra of Naloxone HCl as the humidity is stepped up from 0% RH to 90% RH. The most important features are the growth of two bands in the OH stretching region between 3300 cm-1 and 3700 cm-1, which are due to the water of hydration within the solid. A plot of the IR peak heights for these two peaks, shows the formation of the monohydrate above 10% RH and the complete dihydrate at 60% RH.

These data demonstrate that the VGI 2000M may be used to enhance the understanding of complex interactions between humidity and solids that often occur in real life applications, including pharmaceuticals, biosciences, foods and polymers.

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