The IVIS spectroscopy laboratory at the University of Pittsburgh houses instrumentation used to characterize the major mineralogy of a range of samples, from bulk rock and man-made substances to fine powders and soils. Analysis can be performed in the visible-near infrared (VNIR), the short-wave infrared (SWIR), and mid infrared (MIR) regions (0.4 to 5.0 microns) using reflectance spectroscopy and an external light source. In addition, thermal infrared (TIR) analysis (5.0 to 25.0 microns) is performed using emission spectroscopy where the sample serves as the source. Single samples from 1 mm to as large as 10 cm, and particulate samples from 0.10 to 10’s of grams can be accommodated.
The primary analysis tool is a Nexus Nicolet 670 FTIR spectrometer fitted with two external ports to measure both reflected and emitted wavelengths. The spectrometer is constantly purged using air scrubbed of water vapor (H2O) and carbon dioxide (CO2) to minimize spectral interference by these gases and reduce degradation of the hydroscopic components in the spectrometer.
VSWIR Measurements: The hemispherical reflectance analysis takes place using external equipment mounted to the left spectrometer exit port. An internal quartz beam splitter (0.37 – 3.57 microns) is paired either with a silicon detector (0.37 – 1.16 microns) or thermo-electrically cooled deuterated triglycine sulfate (TE-DTGS) detector (0.8 – 28.6 microns) mounted to a Labsphere six inch integrating sphere coated with Spectralon. The sphere, detector and sample holder sit outside of the spectrometer body and within a purged glovebox. Samples placed in the sphere are illuminated by reflected light from a Tungsten-Halogen white light source within the spectrometer. Calibration is performed by measuring the reflected energy within the sphere with the sample removed and the port covered. Both powder and whole rock samples (up to 2.5 cm) can be measured. Reflectance spectroscopy is ideal for identifying a range of chemical constituents and minerals include vegetation, iron and transition element-bearing minerals, hydrated clay minerals and ices.
A VNIR (0.4 – 1.1 microns) field spectrometer (ASD-1143) is also set up for quick acquisitions of samples. It is paired with a full-solar spectrum lamp and a twelve-inch Spectralon plate for calibration. Calibrated measurements can be made in a few seconds and either used for analysis and/or compared to the spectrometer hemispherical reflectance measurements. This bench-top setup of the field spectrometer is ideal for students completing class or senior thesis projects because of the ease of use.
TIR Measurements: The thermal emission analysis also takes place using several pieces of external equipment fabricated in-house. An internal liquid nitrogen cooled mercury cadmium telluride (MCT-B) detector (range: 0.8 – 25 microns) is paired with an extended range potassium bromide (XT-KBr) beam splitter (0.9 – 26.7 microns). The external equipment required for holding the sample and performing the calibration is mounted to the right spectrometer port and housed in a large purged glovebox. It includes two temperature-stabilized precise blackbody calibration sources, various sample holders, a motorized moveable mounting stage, and a temperature controlled shroud into which the samples are placed prior to measurement. Samples are typically heated for 24 hours at 80˚C in an oven prior to the analysis. Particulate samples can be continuously heated during spectral acquisition using a heated sample stage.
We have two approaches of TIR emission analysis: (1) our standard measurement where samples are removed from the oven and analyzed immediately, and (2) a unique micro-furnace apparatus, capable of heating several grams of material from 500 – 1600˚C in order to study the structural and radiative property changes as samples undergo phase transitions and melting (Lee et al., 2013). Both experiments are located on a sliding platform within the glovebox allowing easy access to either.
The standard measurement approach follows the methodology of Ruff et al. (1997). Calibration measurements from the two blackbody sources (at 70 and 100˚C), required to calculate the instrument response function, are taken. Once obtained, samples are then raised into the temperature-stabilized shroud. Emitted energy from the sample passes through a port in the shroud and reflected by a collimating mirror into the spectrometer. Typically, 200-500 scans are acquired and average (taking several minutes) for improved signal to noise. Data are acquired using the OMNIC software, and later processed into calibrated emissivity using the DaVinci software.
The high-temperature furnace measurements are markedly different. The furnace is slid under the collimating mirror replacing the shroud. Several grams of sample are placed in a platinum crucible within the furnace hot-zone, which is several-centimeter-wide. The sample is then covered and heating to the desired temperature takes place. Measurements of this hot zone without the sample serve as the high-temperature blackbody calibration. Once the sample is heated to the desired temperature, the cover is removed and spectra acquired. Thus far, measurements have been made of both melted rock samples to simulate the emission spectra of natural lavas, as well as of pure minerals as they undergo temperature dependent phase transitions. The furnace requires a much shorter scan time (several seconds) due to the large increase in radiance.
The spectroscopy lab contains a binocular stereoscopic and a light polarizing microscope for petrological and mineralogical analysis of rock/soil samples and thin sections. The microscope is equipped with an attached digital camera and monitor screen to allow for sample imaging.