Jenni L. Briggs

For analysis of non-particulate solids, the diffuse reflection sampling technique may offer an easy, non-destructive method for mid-infrared measurements. Spectral results of a polypropylene face mask collected via diffuse reflection and attenuated total reflection (ATR) were compared.

The concentration of ethanol in hand sanitizer was determined using a calibration developed with an automated ATR sampling accessory.

Mixtures of methanol in ethanol were investigated using an automated ATR accessory. The spectral bands of methanol and ethanol were evaluated showing the expected absorbance changes.

Protein secondary structure during thermal unfolding and aggregation is readily acquired using IR spectroscopy and a temperature-controlled mid-IR transmission accessory. Myoglobin was used as a model system to illustrate the method.

Protein secondary structure during thermal unfolding and aggregation is readily acquired using IR spectroscopy and a temperature-controlled Mid-IR transmission accessory.

As the temperature increases, significant changes in the myoglobin secondary structure are observed. Between 25 °C and 70 °C, the 1649 cm-1 peak height decreases and linewidth increases. These changes in the amide I band reflect increased protein structural disorder. A temperature increase from 70 °C to 75 °C causes a drop in the 1649 cm-1 band intensity. Simultaneously, two peaks grow in at 1683 and 1637 cm-1. These changes indicate conversion of random coil/α-helix to intermolecular β-sheet. Further conversion to β-sheet is observed as the temperature rises to 90 °C. The final β-sheet structure is an insoluble aggregate. This aggregation process is not reversed when the temperature is driven back to 25 °C.

Measuring the transmission of diffuse samples such as paper or filled polymer films requires an integrating sphere to obtain accurate transmission values and useful chemical information, because the IR light is scattered as it passes through the sample. An integrating sphere serves as a collection device for the scattered light.

Experimental data show the crystal dimensions of an ATR-FT-IR sampling accessory influence the usable spectral range. An expanded range germanium crystal is introduced for the measurement of high refractive index samples.

The approach to infrared measurements of diffusely scattering materials often is dictated by the objective of the analysis. Spectral data from three different mid-infrared reflectance sampling accessories are contrasted.

In ATR/FT-IR, magnified visual monitoring of a sample benefits many applications. For micro-sampling and defect analysis, viewing capabilities decrease overall measurement time by allowing the user to locate the desired sampling area quickly and enhance confidence in the collected data by assuring the sampling point. Visual changes in the sample also may be easily monitored during testing. As an example application, the ease of micro-sampling is shown through fiber analysis and the results are discussed.