Search

As feedstocks transition from conventional oil to unconventional petroleum sources and biomass, it will be necessary to determine whether a particular fuel or fuel blend is suitable for use in engines. Certifying a fuel as safe for use is time-consuming and expensive and must be performed for each new fuel. In principle, suitability of a fuel should be completely determined by its chemical composition. This composition can be probed through use of detailed analytical techniques such as gas chromatography–mass spectroscopy (GC–MS). In traditional analysis, chromatograms would be used to determine the details of the composition. In the approach taken in this paper, the chromatogram is assumed to be entirely representative of the composition of a fuel, and is used directly as the input to an algorithm in order to develop a model that is predictive of a fuel’s suitability. When a new fuel is proposed for service, its suitability for any application could then be ascertained by using this model to compare its chromatogram with those of the fuels already known to be suitable for that application. In this paper, we lay the mathematical and informatics groundwork for a predictive model of hydrocarbon properties. The objective of this work was to develop a reliable model for unsupervised classification of the hydrocarbons as a prelude to developing a predictive model of their engine-relevant physical and chemical properties. A set of hydrocarbons including biodiesel fuels, gasoline, highway and marine diesel fuels, and crude oils was collected and GC–MS profiles obtained. These profiles were then analyzed using multi-way principal components analysis (MPCA), principal factors analysis (PARAFAC), and a self-organizing map (SOM), which is a kind of artificial neural network. It was found that, while MPCA and PARAFAC were able to recover descriptive models of the fuels, their linear nature obscured some of the finer physical details due to the widely varying composition of the fuels. The SOM was able to find a descriptive classification model which has the potential for practical recognition and perhaps prediction of fuel properties.

In this work, a Fourier transform infrared spectroscopy-based method is developed to measure the gas-phase dynamics occurring during atomic layer deposition. This new technique is demonstrated during the deposition of hafnium oxide using tetrakis(ethylmethylamido)hafnium and water vapor. The repeatability of the deposition process is utilized to signal average across multiple cycles. This approach required synchronizing the precursor injection pulses with the moving mirror of the spectrometer. The system as implemented in this work achieves spectra with a time resolution of ≈150 ms, but better resolution can be easily obtained. Using this technique, the authors are able to optically measure transients in the molecular number densities of the precursors and product that are the effects of mass transport and surface reactions.

Optical fiber based supercontinuum light sources combine the brightness of lasers with the broad bandwidth of incandescent lamps and thus are promising candidates for sources in spectroscopic applications requiring high brightness and broad bandwidth. Herein, near-infrared (IR) Fourier transform (FT) spectrometry with a supercontinuum (SC) light source is investigated. The efficient, collimated propagation of broad bandwidth SC light through an 18 m path length multipass cell is demonstrated. A normalized spectral difference is calculated for the SC spectrum on consecutive FT mirror scans and is found to vary by less than 0.5%, indicating excellent spectral stability. The rms noise on zero absorbance lines is obtained as a function of the number of mirror scans at 0.125, 2, and 16 cm−1 resolution for both the SC and conventional tungsten lamp source. The SC source has approximately a factor of ten times more noise than the lamp under comparable conditions for each resolution and data acquisition time. This clearly indicates that spectral acquisition with the SC source is not detector noise limited. NIR-FT spectra of methane and methyl salicylate, acquired with both the SC and lamp source, are reported. These spectra illustrate the advantage the SC source has over the incandescent source in that it can efficiently traverse long path lengths, thus providing a sensitivity advantage. The spectra also demonstrate the disadvantage of the SC source with respect to the lamp in the increased level of amplitude noise. Prospects for the future use of SC sources in absorption spectroscopy, including possible noise mitigation strategies, are briefly discussed.

Reference spectra of ammonia from four sources are compared. Low-resolution spectra (i.e., spectra for which the spectrometer resolution is numerically greater than the full-width at half-height of the rotational lines) were obtained from the Environmental Protection Agency (EPA) web site and from Infrared Analysis, Inc. High-resolution (0.12 cm−1) spectra were obtained from the National Institute for Standards and Technology (NIST) and Pacific Northwest National Laboratory (PNNL). Two protocols were used to validate the EPA and Infrared Analysis spectra, with one requiring that Beer's law be obeyed by the low-resolution spectrum and the other that Beer's law be obeyed by the high-resolution spectrum. In all cases, the second protocol gave a significantly better spectral match. It is shown that the path-integrated concentrations for the low-resolution reference spectra were in error by as little as 4% to as much as an order of magnitude, presumably because of the effect of adsorption on the cell walls. Measured absorptivities of the NIST and PNNL spectra were different by ≈2.6% and it is believed that the effect of adsorption on these spectra is small. When the same protocols were used to test the reference spectrum of methane, the calculated path-integrated concentration was only ∼2% different from the one that was calculated from the NIST reference spectrum, suggesting that the data processing protocols provide accurate data.

The absolute accuracy and long-term precision of atmospheric measurements hinge on the quality of the instrumentation and calibration standards. To assess the consistency of the ozone (O3) and nitrogen oxides (NO(x)) standards maintained at the National Institute of Standards and Technology (NIST), these standards were compared through the gas-phase titration of O3 with nitric oxide (NO). NO and O3 were monitored using chemiluminescence and UV absorption, respectively. Nitrogen dioxide (NO2) was monitored directly by laser-induced fluorescence and indirectly by catalytic conversion to NO, followed by chemiluminescence. The observed equivalent loss of both NO and O3 and the formation of NO2 in these experiments was within 1% on average over the range of 40-200 nmol mol(-1) of NO in excess O3, indicating that these instruments, when calibrated with the NIST O3 and NO standards and the NO2 permeation calibration system, are consistent to within 1% at tropospherically relevant mixing ratios of O3. Experiments conducted at higher initial NO mixing ratios or in excess NO are not in as good agreement. The largest discrepancies are associated with the chemiluminescence measurements. These results indicate the presence of systematic biases under these specific conditions. Prospects for improving these experiments are discussed.

Recent developments in microwave spectroscopy have encouraged researchers to develop this technique for analytical applications such as environmental monitoring, industrial process control, and homeland defense. This paper presents a general overview of microwave spectroscopy with a focus on aspects relevant for detecting chemical warfare agents (CWAs) and their surrogates. In particular, the high spectral resolution of microwave methods provides exceptional selectivity which is critical for detecting and identifying CWAs given the complex environments and numerous interferents that may obscure measurements by instruments with poor specificity. Ongoing efforts to develop a microwave spectral database of CWAs and improve the quantitative capabilities of Fourier transform microwave spectrometers are discussed. Additionally, future improvements to achieve a field-deployable sensor are presented.

The National Institute of Standards and Technology (NIST) and the Pacific Northwest National Laboratory (PNNL) are each creating quantitative databases containing the vapor-phase infrared spectra of pure chemicals. The digital databases have been created with both laboratory and remote-sensing applications in mind. A spectral resolution of ≈0.1 cm−1 was selected to avoid degrading sharp spectral features, while also realizing that atmospheric broadening typically limits line widths to 0.1 cm−1. Calculated positional (wavenumber, cm−1) uncertainty is ≤0.005 cm−1, while the 1σ statistical uncertainty in absorbance values is

Background: Nitrogen dioxide (NO2) is a toxic by-product of inhalation therapy with nitric oxide (NO). The rate of NO2 formation during NO therapy is controversial. Methods: The formation of NO2 was studied under dynamic flows emulating a base case NO ventilator mixture containing 80 ppm NO in a 90% oxygen matrix. The difficulty in measuring NO2 concentrations below 2 ppm accurately was overcome by the use of tunable diode laser absorption spectroscopy. Results: Using a second-order model, the rate constant, k, for NO2 formation was determined to be (1.19 ± 0.11) × 10−11 ppm−2s−1, which is in basic agreement with evaluated data from atmospheric literature. Conclusions: Inhaled NO can be delivered safely in a well-designed, continuous flow neonatal ventilatory circuit, and NO2 formation can be calculated reliably using the rate constant and circuit dwell time.

The feasibility of using catalytic conversion of methanol to formaldehyde to produce standard amount of substance fractions of formaldehyde was examined. The conversion efficiencies of several catalysts were measured as a function of temperature, balance gas, catalyst bed length, and methanol amount of substance fraction in an effort to identify conditions which yield high and consistent conversion of methanol to formaldehyde. The highest observed conversion rate was (95 {+-} 4)% using a molybdenum catalyst, where the error is the 2{sigma} uncertainty. The conversion efficiency was found to be consistent over repeated cycles and over a long lifetime test, suggesting that a molybdenum catalyst is a viable candidate for a standard formaldehyde generator, particularly for low formaldehyde amount of substance fractions (< 15{micro}mol/mol).

Performance standard specifications for point chemical vapor detectors are established in ASTM E 2885-13 and ASTM E 2933-13. The performance evaluation of the detectors requires the accurate delivery of known concentrations of the chemical target to the system under test. Referee methods enable the analyte test concentration and associated uncertainties in the analyte test concentration to be validated by independent analysis, which is especially important for reactive analytes. This work extends the capability of a previously demonstrated method for using Fourier transform infrared (FT-IR) absorption spectroscopy for quantitatively evaluating the composition of vapor streams containing hazardous materials at Acute Exposure Guideline Levels (AEGL) to include test conditions colder than laboratory ambient temperatures. The described method covers the use of primary reference spectra to establish analyte concentrations, the generation of secondary reference spectra suitable for measuring analyte concentrations under specified testing environments, and the use of additional reference spectra and spectral profile strategies to mitigate the uncertainties due to impurities and water condensation within the low-temperature (7 °C, −5 °C) test cell. Important benefits of this approach include verification of the test analyte concentration with characterized uncertainties by in situ measurements co-located with the detector under test, near-real-time feedback, and broad applicability to toxic industrial chemicals.

The photodissociation dynamics of methyl radical have been investigated at 193.3 nm using photofragment translational spectroscopy. The formation of CH2 and H(2S) was the only dissociation pathway observed. Although it is not possible to assign the spin state of the methylene unambiguously, we believe that methylene is produced predominately in the a 1A1 excited state. The translational energy distribution of the products is peaked at ∼13 kcal/mole which is consistent with the magnitude of the exit barrier on the excited state potential energy surface. The breadth of the distribution suggests that the methyl radicals dissociate from a wide range of geometries. From the photofragment angular distribution an anisotropy parameter of β=−0.9±0.1 was determined.

Laser-induced desorption of CO from an ion beam etched and annealed Si (111) surface is reported. Stateresolved measurements of the desorbed CO reveal very high translational and vibrational energy contents, with the rotational excitation being quite low. The results suggest that the CO photodesorption is derived from CO bound to ion beam etch-induced active “defect” site(s) on the Si(111)7 × 7 surface that are only minimally influenced by either the ion beam etching conditions or the anneal. The very high translational energy in the CO implies desorption from active “defect” site(s) either in the very-near-surface region or on the surface. Comparison of the photoyields at 266 and 355 nm suggests that the desorption mechanism cannot be described using existing models for thermal- or carrier-mediated processes.

State‐resolved detection techniques have been used to characterize the ultraviolet photodecomposition dynamics of Mo(CO)6 on Si(111) 7×7 at 100 K. Details of the excitation/fragmentation mechanism including adsorbate energy transfer were examined by measuring the cross sections and the internal and translational energies of the photoejected CO from submonolayer through multilayer coverage regimes. The CO energy distributions are found to be independent of Mo(CO)6 coverage, and can be characterized by two components with markedly different mean energies. In contrast to the coverage independence of the measured energy disposal, the cross section was found to decrease by a factor of 3 from multilayer coverages to submonolayer coverages.

The adsorption and photodecomposition of Mo(CO)6 adsorbed on Si(111) 7×7 surfaces has been studied with Auger electron spectroscopy, temperature programmed desorption,low energy electron diffraction and infrared reflection absorption spectroscopy in a single external reflection configuration. The external‐reflection technique is demonstrated to have adequate sensitivity to characterize submonolayer coverages of photogenerated Mo(CO) x fragments. It is proposed that the first layer of Mo(CO)6 adsorbs in ordered islands with a Mo(CO)6 atop each adatom of the 7×7 reconstructed Si surface.UV irradiation of these islands produces a carbonyl fragment, identified as chemisorbed Mo(CO)5. The Mo(CO)5 thermally decarbonylates via two subcarbonyl intermediates with little CO dissociation. Photolysis of thicker layers results in the formation of Mo x (CO) y dimers/polymers, as evidenced by the appearance of bridging CO, which is attributed to a facile association reaction. The dimer/polymer species correlate with deposition of C and O on the surface.

Near-infrared Fourier transform absorption spectrometry with a supercontinuum (SC) source is demonstrated for trace methane detection. The SC source allows for long distance propagation while exhibiting amplitude noise ten times greater than comparable incandescent sources.

Chemical detectors are crucial tools for first responders during emergency-response scenarios and for continuous monitoring of public spaces for general safety. For those who depend upon chemical detectors for safety and security, ensuring that detectors alarm at specified levels is critical. During detector performance evaluation, the accurate delivery of known concentrations of the chemical target to the detector is a key aspect of the test. Referee methods enable the analyte test concentration and associated uncertainties in the analyte test concentration to be validated by independent analysis, which is especially important for reactive analytes. This work demonstrates a method to use Fourier transform infrared (FT-IR) absorption spectroscopy for quantitatively evaluating the composition of vapor streams containing hazardous materials at acute exposure guideline levels (AEGL) under test conditions defined in recently published standard specifications for chemical vapor detectors. The described method covers the use of primary reference spectra to establish analyte concentrations, the generation of secondary reference spectra suitable for measuring analyte concentrations under specified testing environments, and the use of referee feedback to compensate for depletion of the test analyte. Important benefits of this approach included verification of the test analyte concentration with characterized uncertainties by in situ measurements co-located with the detector under test, near-real-time feedback, and broad applicability to toxic industrial chemicals.

Quantitative, high resolution (0.1 cm -1 ) infrared spectra have been acquired for a number of pressure broadened (101.3 KPa N 2 ), vapor phase chemicals including: Sarin (GB), Soman (GD), Tabun (GA), Cyclosarin (GF), VX, nitrogen mustard (HN3), sulfur mustard (HD) and Lewisite (L). The spectra are acquired using a heated, flow-through White cell of 5.6 m optical path length. Each reported spectrum represents a statistical fit to Beer's law, which allows for a rigorous calculation of uncertainty in the absorption coefficients. As part of an ongoing collaboration with the National Institute of Standards and Technology (NIST), cross-laboratory validation is a critical aspect of this work. In order to identify possible errors in the Dugway flow-through system, quantitative spectra of isopropyl alcohol from both NIST and Pacific Northwest National Laboratory (PNNL) are compared to similar data taken at the Dugway Proving Ground (DPG).

Two Fourier transform infrared intensity artifacts have been observed at moderately high (0.1 cm(-1)) spectral resolution: Light reflected off the aperture was double modulated by the interferometer, producing a 2f alias, and the warm (approximately 310 K) annulus of the aperture seen by a cooled detector resulted in distorted line shapes and anomalous intensities in the fingerprint region. Although the second artifact has been alluded to previously, we report corrections to remove both of these anomalies and to demonstrate the efficacy of these corrections. Prior to correction, integrated-band intensities were found to be in error by up to 12%.

There is a continuing need for improved analytical techniques to measure the concentration of trace gases for monitoring hazardous air pollutants, industrial emissions, chemical-warfare agent release, etc. Methods of analysis that can conclusively identify several analytes in a mixture are particularly desired. Towards this end, the use of Fourier-transform microwave (FTMW) spectroscopy as a quantitative analytical technique has been proposed. The high spectral resolution of FTMW provides a quick and unambiguous method for identifying multiple analytes in the gas phase. A small-scale FTMW spectrometer has recently been constructed for use in quantitative analysis. Prior to the present investigation, however, the use of this spectrometer in quantitative work has not been rigorously evaluated. This work summarizes efforts to identify and categorize sources of signal instability in the FTMW spectrometer. Methods employed to minimize these effects will also be discussed.

Two FTIR intensity artifacts are observed: 1) the warm annulus of the aperture seen by the detector resulted in distorted line shapes and intensities in the fingerprint region., and 2) light reflected off the aperture was double modulated by the interferometer producing a 2f alias. Corrections to remove these anomalies are discussed.