Laura E. Martin

New Hybrid Algorithm for Maintaining Multivariate Quantitative Calibrations of a Near-Infrared Spectrometer

Our newly developed prediction-augmented classical least-squares/partial least-squares (PACLS/PLS) hybrid algorithm can correct for the presence of unmodeled sources of spectral variation such as instrument drift by explicitly incorporating known or empirically derived information about the unmodeled spectral variation. We have tested the ability of the new hybrid algorithm to maintain a multivariate calibration in the presence of instrument drift using a near-infrared (NIR) spectrometer (7500–11 000 cm−1) to quantitate dilute aqueous solutions containing glucose, ethanol, and urea. The spectral variations required to update the multivariate models for both short- and long-term drift were obtained using a single representative midpoint sample whose spectrum was repeatedly measured during collection of calibration data and during collection of separate validation sample spectra on three subsequent days. The performance of the PACLS/PLS model for maintaining a calibration was compared to PLS with subset recalibration, a method that has previously been applied to maintenance and transfer of calibration. Without drift corrections, both PACLS/PLS and PLS had poor predictive ability on sample spectra collected on subsequent days. Unlike previous maintenance of calibration studies that corrected for long-term drift only, the PACLS/PLS and PLS models demonstrated the best predictive abilities when short-term drift was also corrected. The PACLS/PLS hybrid model outperformed PLS with subset recalibration for near real-time predictions when instrument drift was determined from the repeat samples closest in time to the measurement of the unknown. Near real-time standard errors of prediction (SEPs) for the hybrid model were comparable to the cross-validated SEPs obtained with the original calibration model.

Assay for lignin breakdown based on lignin films: insights into the Fenton reaction with insoluble lignin

We report a new assay for breakdown of high molecular weight, insoluble lignin based on lignin films. In this method, decrease in film thickness is detected upon solubilization of mass through either chemical alteration of the lignin or molecular weight reduction. The assay was performed with organosolv lignin, the only chemical modification being an oxidative pretreatment to provide film stability with respect to dissolution. The assay is sensitive to release of as little as 20 A of material from the film. A multiplexed format was developed using a silicone block in the form of a standard 96-well plate, allowing simultaneous assaying of a large number of reaction conditions. The assay was demonstrated using the Fenton reaction, revealing new insights into the physicochemical aspects of this reaction system with insoluble lignin. In particular, mass solubilized from the film was found to pass through a maximum as a function of the initial concentration of FeCl2 ([FeCl2]o), with the maximum occurring at [FeCl2]o = 1 mM for [H2O2]o = 5%. At that condition, solubilization of mass occurs in two stages. The reaction produces mostly ring-opened products of mass greater than 700 g mol−1, along with a minority of low molecular weight aromatics. The new insight from this work is an important step toward optimizing this complex reaction system for effective lignin breakdown.

Crystallization behavior of vapor-deposited hexanitroazobenzene (HNAB) films

Vapor-deposited hexanitroazobenzene (HNAB) has been shown to form an amorphous structure as-deposited that crystallizes over a period ranging from several hours to several weeks, depending on the ambient temperature. Raman spectroscopy and x-ray diffraction were used to identify three distinct phases during the crystallization process: the as-deposited amorphous structure, the HNAB-II crystal structure, and an as-yet unidentified crystal structure. Significant qualitative differences in the nucleation and growth of the crystalline phases were observed between 65°C and 75°C. While the same two polymorphs form in all cases, significant variation in the quantities of each phase was observed as a function of temperature.

Pressure dependent Raman spectra used to validate DFT EOS of hexanitrostilbene (HNS)

Due to its thermal stability and low vapor pressure, Hexanitrostilbene (HNS) is often used in high-temperature or vacuum applications as a detonator explosive or in mild detonating fuse. Toward improving the accuracy of the equation of state used in hydrodynamic simulations of the performance of HNS, we have measured the spe ctra of this material under static pressure in a diamond anvil cell. Density functional theory calculations were used to simulate the pressure dependence of the Raman spectra along the Hugoniot and 300K isotherm for comparison and to aid in interpreting the data. We discuss changes in vibrational signatures of HNS under pressure, comparison with simulated spectra, and using this data as a basis for understanding future pulsed Raman measurements on dynamically compressed HNS samples.

Waveguide sensor for detection of HNS degradation

Hexanitrostilbene (HNS) is a secondary explosive widely used in a variety of commercial and military applications, due in part to its high heat resistivity. Degradation of HNS is known to occur through exposure to a variety of sources including heat, UV radiation, and certain chemical compounds, all of which may lead to reduced performance. Detecting the degradation of HNS within a device, however, has required destructive analyses of the entire device while probing the HNS in only an indirect fashion. Specifically, the common methods of investigating this degradation include wet chemical, surface area and performance testing of the devices incorporating HNS rather than a direct interrogation of the material itself. For example, chemical tests frequently utilized, such as volatility, conductivity, and contaminant trapping, provide information on contaminants present in the system rather than the chemical stability of the HNS. To instead probe the material directly, we have pursued the use of optical methods, in particular infrared (IR) spectroscopy, in order to assess changes within the HNS itself. In addition, by successfully implementing miniature silicon (Si) waveguides fabricated at Sandia National Laboratories to facilitate this spectroscopic approach, we have demonstrated that HNS degradation monitoring may take place in a non-destructive, in-situ fashion. Furthermore, as these waveguides may be manufactured in a variety of configurations, this direct, non-destructive, approach holds promise for incorporation into a variety of devices.

Vibrational Spectroscopy of HNS Degradation

Hexanitrostilbene (HNS) is a widely used explosive, due in part to its high thermal stability. Degradation of HNS is known to occur through UV, chemical exposure, and heat exposure, which can lead to reduced performance of the material. Common methods of testing for HNS degradation include wet chemical and surface area testing of the material itself, and performance testing of devices that use HNS. The commonly used chemical tests, such as volatility, conductivity and contaminant trapping provide information on contaminants rather than the chemical stability of the HNS itself. Additionally, these tests are destructive in nature. As an alternative to these methods, we have been exploring the use of vibrational spectroscopy as a means of monitoring HNS degradation non-destructively. In particular, infrared (IR) spectroscopy lends itself well to non-destructive analysis. Molecular variations in the material can be identified and compared to pure samples. The utility of IR spectroscopy was evaluated using pressed pellets of HNS exposed to DETA (diethylaminetriamine). Amines are known to degrade HNS, with the proposed product being a σ-adduct. We have followed these changes as a function of time using various IR sampling techniques including photoacoustic and attenuated total reflectance (ATR).

Infrared ATR: a probe for cellular activation

We employ infrared spectroscopy (IR) with attenuated total reflectance (ATR) as a sampling technique to monitor live and dried RAW cells (a murine macrophage cell line) during activation with g-interferon and lipopolysaccharide. By comparing the spectra of activated cells at various time points to the spectra of healthy control cells, we identify spectral bands associated with nucleic acids that are markers for the cell activation process. These spectral changes are slight and can be complicated with the normal metabolic changes that occur within cells. We will discuss the use of data pretreatment strategies to accurately correct for the contribution of the buffer to the live cell spectra. We find the standard background correction method inadequate for concentrated solutions of cells. Data presented shows the severe effect incorrect background subtraction has on the cell spectra. We report a more accurate correction for phosphate buffer spectral contribution using an interactive subtraction of the buffer spectrum. We will show classification of dried control and activated macrophage cell spectra using partial-least squares analysis with multiplicative scatter correction.