BUTTON SAMPLE HOLDER APPLICATIONS FOR MID-INFRARED SPECTROSCOPY STUDIES OF MINERALS

Abstract

Infrared (IR) spectroscopy is a rapid and non-destructive molecular analysis method derived from the infrared radiation and matter. Its use is prevalent throughout industrial, academic, and government laboratories for materials characterization. Despite its ubiquity, major advancements in methods or sampling techniques have been largely stagnant since the 1960’s. This dissertation presents mid-IR spectroscopy mineralogical applications of a novel button sample holder and describes various analytical features of this new technique. Buttons are manufactured by spot welding various sizes of stainless-steel meshes to polished stainless-steel backings. The thin wire mesh and shallow well of a button sample holder allow for easy preparation and analysis of thin layers of neat solid, liquid, paste, and grease samples. Additionally, radiation reflected off the steel wires and backing allow for diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis of non-scattering samples like liquids. Button sample holders eliminate the necessity of time-consuming, expensive, and potentially sample altering predilution and handling. The sieving ability of the mesh selects for particle sizes smaller than the void sizes of the mesh. Well depth and wire diameter limit the maximum allowable sample layer thickness. Furthermore, the robust button design permits abrasive sampling which removes small amounts of sample by simply rubbing the rough button mesh over a sample surface. This method is applicable to in-situ sampling, storage, and analysis. It is especially ideal for samples where minimal damage is desired and sample homogenization is not possible, such as archaeological and historical artifacts. Traditional DRIFTS and variable temperature DRIFTS (VT-DRIFTS) were employed for this body of work. Diffuse reflectance from the samples and button is collected at all angles except the incident radiation angle. Total pathlengths on the order of a few microns are possible, thus permitting analyses of high absorptivity neat samples, such as coals and carbon black. With attention to particle size homogenization, mesh choice, and sample packing, layer thickness could be reproducibly adjusted. Versatility of sample type and variable total pathlengths offer an advantage over conventional methods like attenuated total reflectance infrared spectroscopy (ATR-IR). The sensitivity and reproducibility of VT-DRIFTS measurements with button sample holders was demonstrated through various mineralogical applications. The first of these studies, performed on clay minerals such as kaolinite, was done with a modified traditional VT-DRIFTS setup after replacing the sample cup with a button sample holder and removing the sealed environmental chamber. This increased optical throughput, eliminated diluent interactions, and mitigated temperature-dependent alignment issues. It also enhanced measurement sensitivity and minimized spectral artifacts. The utility of Button VT-DRIFTS was further enhanced through the construction of a heating and cooling block made of stacked thermoelectric chips. Using this device, infrared spectra at temperatures between -40 °C and 150 °C could be reached and maintained within seconds. This apparatus was combined with a programmable temperature controller to implement unique heating and cooling profiles. Continuous and step temperature profiles could be applied to probe different sample properties and discriminate between reversible and irreversible changes. This allowed for a quick pre-analysis to determine if further exploration of temperature-dependent changes in particular samples is warranted. The innovative thermoelectric heating and cooling apparatus was combined with button sample holders to investigate molecular level dehydration and rehydration processes and low temperature induced structural changes in additional mineral samples. Reversible and irreversible temperature-dependent changes were observed via spectral subtractions using this method. These differences are not readily apparent when simply overlaying spectra of samples before and after cooling. Furthermore, conventional variable temperature studies often require probing large temperature differences or using multiple samples. This methodology uniquely allows for variable temperature studies of temperature-dependent structural changes on a single sample. The selectivity and reproducibility of Button VT-DRIFTS studies permitted monitoring and discrimination of changes in different hydroxyl group types in solid state mineral samples.