David Tuschel

Deep Ultraviolet Resonance Raman Excitation Enables Explosives Detection.

We measured the 229 nm absolute ultraviolet (UV) Raman cross-sections of the explosives trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), cyclotrimethylene-trinitramine (RDX), the chemically related nitroamine explosive HMX, and ammonium nitrate in solution. The 229 nm Raman cross-sections are 1000-fold greater than those excited in the near-infrared and visible spectral regions. Deep UV resonance Raman spectroscopy enables detection of explosives at parts-per-billion (ppb) concentrations and may prove useful for stand-off spectroscopic detection of explosives.

Potential of Raman Spectroscopy and Imaging Methods for Rapid and Routine Screening of the Presence of Melamine in Animal Feed and Foods

The impact of melamine-contaminated animal feed ingredients on food safety has become a major public concern ever since melamine was identified as the organic compound responsible for the deaths of a significant number of cats and dogs in 2007 by way of adulterated pet food. Melamine, a common industrial chemical often added to resins to improve flame resistance and proposed as an alternative form of fertilizer-N for plant growth, was found to be intentionally added to animal feed in amounts ranging from 0.2% to 8% of total mass as a way to boost the products’ apparent protein content. It was also used as a binder in the production of pelleted feed for animals. In addition to melamine, a small amount of cyanuric acid, ammeline, and ammelide were also detected in pet feed and in the tissues and urine of dead pets that had consumed the contaminated food. Even though it is possible that cyanuric acid, ammeline, and ammelide were added, their presence more likely resulted from the degraded derivatives of melamine. There is a great concern that melamine will again enter the food chain and be consumed by humans and animals. As part of the Food Protection Plan, US federal agencies such as the FDA and FSIS and other organizations have established GCMS and LC-MS/MS procedures for the analysis of melamine in food/feed commodities. Although they can detect melamine contaminants in trace amount, these time-consuming laboratory procedures require chemical solvents for the extraction steps and depend on expensive mass spectrometry. Rapid, nondestructive, and routine methods for the specific detection of melamine in raw feed materials are increasingly important, not only for public health concerns but also for melamine screening to prevent protein fraud. Undoubtedly, the well-defined mass spectroscopic technique is preferred due to its low detection limit and capability for structural elucidation; however, since adulteration of raw materials by melamine usually occurs in higher concentrations in order to affect protein content, the high sensitivity of the mass spectroscopic technique may not be necessary. In addition, mass spectrometry might not be sufficiently rapid to screen for the presence of melamine in a large number of food/feed materials from very different sources, because the identification process includes sample-specific extraction procedures, which are labor-intensive and time-consuming. Fast melamine screening requires minimal sample preparation (e.g., no extraction or centrifugation), routine analysis of a number of samples without reagents, minimal procedural steps, and easy operation and interpretation of results. The Raman technique, which has been used to obtain structural information on melamine, is an alternative approach that can be applied to solid materials with no sample pretreatment. In addition, the use of the Fourier transform (FT) methodology and a 1064 nm excitation laser in the near-infrared (NIR) region provides precise wavenumber measurement and good-quality Raman spectra by reducing the interference from fluorescence and photodecomposition of colorants present in food and feed. Raman studies of melamine and melamine-modified resins have been reported in the literature, and the results have revealed the feasibility of the Raman technique for the structural characterization of melamine state in resins. However, there have been few reports on Raman investigation and identification of melamine in complex food and feed systems. The objectives of this study were (1) to identify the characteristic Raman bands in melamine-contaminated wheat flour, corn gluten, and soybean meal mixtures; and (2) to develop simple and universal ratio algorithms for qualitative and quantitative analysis of melamine in mixtures. The ultimate goal is to develop both Raman spectroscopy and Raman chemical imaging methods for rapid, accurate, nondestructive, specific, and routine screening of the presence of melamine in food and feed for public/animal safety and security.

Raman detected differential scanning calorimetry of polymorphic transformations in acetaminophen.

Acetaminophen is known to crystallize in three polymorphic forms. Thermally induced transformations between the crystalline forms and the super-cooled liquid have been observed by differential scanning calorimetry (DSC), but the assignment of calorimetric transitions to specific polymorphic transformations remains challenging, because the transition temperatures for several transformations are close to one another, and the characteristics of the observed transitions depend on experimental variables that are often poorly controlled. This paper demonstrates the simultaneous application of DSC and Raman microscopy for the observation of thermally driven transitions between polymorphs of pharmaceutical materials. Raman detected differential scanning calorimetry (RD-DSC) has been used to monitor the DSC thermograms of super-cooled liquid acetaminophen and confirms the assignment of two exothermic transitions to specific polymorphic transformations. Principal component analysis of the Raman spectra have been used to determine the number of independent components that participate in the phase transformations, and multivariate regression has been used to determine transition temperatures from the spectral data. The influence of the laser excitation source on measured DSC thermograms has also been investigated, and it has been demonstrated that a baseline shift occurs in RD-DSC when a polymorphic transformation occurs between crystalline and amorphous forms. RD-DSC has been used to examine the influence of sample aging and sample pan configuration on the observed polymorphic transformations, and both of these variables were found to influence the thermal behavior of the sample. The results demonstrate the advantage of simultaneous Raman spectroscopy and differential scanning calorimetry for the unambiguous assignment of thermally driven polymorphic transformations.

Raman Chemical Imaging of Microcrystallinity in Silicon Semiconductor Devices

Silicon integrated circuits are fabricated by the creation of complex layered structures. The complexity of these structures provides many opportunities for impurities, improperly annealed dopants, and stress effects to cause device contamination and failure. Nondestructive metrology techniques that rapidly and noninvasively screen for defects and relate silicon device structure to device performance are of value. We describe the first use of a liquid crystal tunable filter (LCTF) Raman chemical imaging microscope to assess the crystallinity of silicon semiconductor integrated circuits in a rapid and nondestructive manner without the need for sample preparation. The instrument has demonstrated lateral spatial resolving power of better than 250 nm and is equipped with a tunable imaging spectrometer having a spectral bandpass of 7.6 cm−1. The instrument rapidly produces high-definition Raman images where each image pixel contains a high-quality Raman spectrum. When combined with powerful processing strategies, the Raman chemical imaging system has demonstrated spectral resolving power of 0.03 cm−1 in a test silicon semiconductor wafer fabricated by using ion implantation. In addition, we have applied Raman chemical imaging for volumetric Raman imaging by analyzing the surface distribution of polycrystalline thin film structures. The approaches described here for the first time are generally applicable to the nondestructive metrology of silicon and compound semiconductor devices.

CRYSTALLINE COLLOIDAL ARRAY DEEP UV NARROW BAND RADIATION FILTER

The present invention provides a method of making highly charged, monodisperse particles which do not absorb deep ultraviolet (UV) light and a method of making crystalline colloidal array (CCA) deep UV narrow band radiation filters by using these highly charged monodisperse particles. The CCA filter rejects and/or selects particular regions of the electromagnetic spectrum while transmitting adjacent spectral regions. The filtering devices of the present invention are wavelength tunable over significant spectral intervals by changing the incident angle of the CCA filter relative to the light. Larger wavelength changes can be obtained by changing the concentrations of particles in the CCAs. The present invention also includes applications of the CCA filter to hyperspectral imaging and Raman imaging devices.