Jeffrey W. Childers

Fourier Transform Infrared Photoacoustic Spectroscopy Characterization of Sulfur-Oxygen Species Resulting from the Reaction of SO2 with CaO and CaCO3:

Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has been used to investigate the reaction of SO2, in He with CaO and CaCO3 particles at temperatures between 25 and 900°C. The reaction of SO2 with CaO occurs at 25°C, while the reaction of CaCO3 with SO2 is first evident at 550°C. The initial product of both of these reactions is CaSO3. The CaSO3 then further reacts with SO2 to form CaSO4, CaS2O3, and CaS at higher temperatures. At 900°C and above, the net production of SO4= and S2O3= is decreased because of the decomposition of SO3=. FT-IR/PAS depth-profiling experiments indicate the formation of a reaction-limiting product layer of SO3= and SO4= from the above reactions on the surface of the CaO and CaCO3 particles.

FTIR transmission spectrometry for the nondestructive determination of ammonium and sulfate in ambient aerosols collected on teflon filters

Abstract Fourier transform infrared (FTIR) transmission spectrometry has been used to determine ammonium and sulfate in ambient aerosol particles collected on Teflon filters. Integrated absorbance as well as maximum absorbance values for ammonium and sulfate are linearly related to X-ray fluorescence measurements of total sulfur. Apparent detection limits of 1.4 μg m −3 for sulfate and 0.5 μg m −3 for ammonium (for sample volumes of 21.6 m 3 ) were estimated for samples from Topeka, Kansas; Portage, Wisconsin; Steubenville, Ohio; and Charlottesville, Virginia. The apparent detection limits were essentially independent of sample loading over the range of 0–20 μg m −3 of total sulfur. The FTIR transmission measurements of these samples are very precise e.g. repetitive measurements vary ± 0.2% and blank variability is 0.13 μg m −3 and 0.20 μg m −3 in terms of sulfate and ammonium, respectively. Contributions to analytical uncertainty include 5–8% uncertainties in X-ray fluorescence measurements for total sulfur and the influence of unspecified site-dependent sample characteristics. The method offers the significant advantages of nondestructive analysis, no sample preparation, molecular speciation and rapid analysis.

Gas Chromatography/Matrix Isolation-Infrared Spectrometry for the Identification of Polycyclic Aromatic Hydrocarbons in Urban Air Particulate Matter

The capabilities of gas chromatography/matrix isolation-infrared (GC/MI-IR) spectrometry for detecting and identifying polycyclic aromatic hydrocarbons (PAHs) in urban air particulate matter are demonstrated. The ability of GC/MI-IR to discriminate between PAH isomers that are difficult to distinguish by conventional electron-impact ionization gas chromatography/mass spectrometry is shown. The MI-IR spectra of PAHs obtained under GC/MI-IR conditions are comparable to those reported in the literature that were obtained by using slow-spray-on matrix deposition techniques.

Comparison of an Innovative Nonlinear Algorithm to Classical Least-Squares for Analyzing Open-Path Fourier Transform Infrared Spectra Collected at a Concentrated Swine Production Facility:

Open-path Fourier transform infrared (OP/FT-IR) spectrometry was used to measure the concentrations of ammonia, methane, and other atmospheric gases at an integrated swine production facility. The concentration-pathlength products of the target gases at this site often exceeded the linear dynamic range of the OP/FT-IR spectrometer. To minimize the effect of this nonlinearity on the accuracy of the reported concentrations, a piecewise, linear classical least-squares (CLS) analysis method, which used a calibration set containing multiple reference spectra for each target gas at concentration-pathlength products that encompassed those found in the field spectra, was used to predict the path-averaged concentrations of the target gases. The predicted concentrations reported by this piecewise, linear CLS method were compared to those predicted by a conventional linear CLS method, which used a calibration set consisting of only one reference spectrum at a single concentration-pathlength product for each target gas, and an innovative nonlinear algorithm (NLA), which performs an iterative fit of the convolved spectral line data from the high-resolution transmission molecular absorption (HITRAN) database to the single-beam field spectra. The conventional, linear CLS method generally under-reported the target gas concentrations relative to those predicted by the piecewise, linear CLS method when the field spectra exhibited concentration-pathlength products larger than those of the reference spectra in the single-level calibration set. In extreme cases, for example, during measurements of methane along the waste lagoon, the concentrations predicted by the conventional CLS methods were more than 30% lower than those predicted by the piecewise, linear CLS method. In contrast, the concentrations of methane predicted by the NLA were, on average, within 4% of those predicted by the piecewise, linear CLS method. For ammonia, however, the concentrations predicted by the NLA were slightly higher than those predicted by the piecewise, linear CLS method at the lower range of observed concentration-pathlength products and slightly lower than those predicted by the piecewise, linear CLS method at the upper range of concentration-pathlength products. The NLA also consistently predicted higher concentrations of nitrous oxide and carbon dioxide relative to that predicted by the piecewise, linear CLS method. Differences in the background spectra and spectral ranges over which the analyses were conducted apparently did not contribute to the differences in the path-averaged concentrations predicted by these two analysis methods.

Quantification of Ammonium in Aerosols by Near-Infrared Photothermal Spectroscopy:

The potential of near-infrared photothermal spectroscopy for the analysis of aerosols collected on filter membranes is demonstrated. Laboratory-generated aerosols consisting of ammonium sulfate and soot are collected on Teflon® filters by the use of a dichotomous sampler. The N-H overtone/combination band at 2110 nm is used to determine ammonium in these mixtures. The presence of soot is shown to have a deleterious effect on the detectability of ammonium. The extent of this limitation is examined with the employment of well-characterized samples. An empirical expression, based on Beers law, is developed which relates the photothermal signal to the relative soot and ammonium sulfate loadings.

Effects of stray light in FTIR instruments on open-path measurements

The single-beam spectra recorded with two commercially available monostatic FT-IR open-path monitors exhibited nonzero signal intensities in the wave number regions where water vapor and CO2 totally absorb. Each instrument yielded a measureable spectral response with the transmitting/receiving telescope blocked. This response is attributed to stray light within the FT-IR system. In some cases the stray light contributed up to 17% of the total return intensity measured along a 414-m path length. Although the baseline region of the single-beam spectra can be corrected for stray light, its effect on the spectral absorbance is not uniform over the range of absorbance values recorded in long-path measruements. Thus, the concentration of species measured along the path cannot be easily corrected, which has a deleterious effect on the accuracy of the spectral data.

Quality assurance/quality control issues in open-path FTIR monitoring

Although much effort has been directed toward developing quality assurance/quality control procedures in open-path Fourier transform infrared (OP/FTIR) monitoring, many issues remain unresolved. Despite efforts in this area, there are still no universally accepted procedures for determining the accuracy and precision of OP/FTIR data. When the concentrations of atmospheric species are measured over a long, open path, several factors can influence accuracy and precision. In addition, broader issues, such as setting or determining data quality objectives for various types of programs, have not been discussed at all. Some aspects of these topics are presented in this paper.

Quality assurance considerations in a long-term FTIR monitoring program

As part of a program to develop quality assurance (QA) and quality control (QC) procedures for open-path Fourier transform infrared (FT-IR) monitoring, ambient gas concentrations were measured daily over an 11-month period. Some of these data will be used to describe the seasonal diurnal variation of atmospheric gases. To validate these data, a daily protocol was established to verify the long-term performance of the FT-IR system. This protocol includes measuring the electronic noise, the intensity of the return signal, and the magnitude of the baseline noise; examining the features and profiles of the single-beam spectra; and determining the repeatability of the position and full width at half height of selected absorption bands. Several atmospheric gases, such as methane, nitrous oxide, and cabon monoxide, are present in virtually all spectra taken. The use of the ambient concentration measurements of these species for QA/QC purposes was investigated. Of these gases, the ambient concentrations of nitrous oxide are the most stable, which makes it the most promising gas for use as a surrogate standard. In addition to the tests that were used to determine the stability of the instrument, procedures were used to evaluate the validity of the analysis method.

Nonlinear FTIR open-path data reduction algorithm

A nonlinear spectral curve fitting computer algorithm for the reduction of Fourier transform spectrometer data is being developed at the Air Forces Arnold Engineering Development Center. The algorithm is an adaptation of an existing algorithm utilized for plume signature data and optical gas diagnostics data reduction. The nonlinear algorithm is not constrained by the linear absorbance assumptions required for classical least squares (CLS). Therefore, the nonlinear algorithm allows for the determination of gas concentrations from high absorption level spectra as well as determination of other nonlinear parameters such as spectra shift and spectral resolution. In addition, the currently implemented algorithm has the ability to simultaneously determine the background spectra or I degrees. These capabilities could make the nonlinear algorithm a welcome addition as a quality assurance and data reduction tool. Open path Fourier transform IR spectra from various sources have been analyzed utilizing this algorithm in an attempt to determine its capabilities and limitations. As various data sets are analyzed the algorithm is further refined to make its application easier and faster. A brief description of the algorithm and the results of a comparative study between the nonlinear algorithm and CLS is presented.

Analysis of Nitrocresols and Related Compounds by Gas Chromatography/Matrix-Isolation Infrared Spectrometry

The capabilities of gas chromatography/matrix-isolation infrared (GC/MIIR) spectrometry for characterizing nitrocresols in air sample extracts were evaluated. The IR spectra of selected nitrocresols were recorded in an argon matrix, in a xenon matrix, in the vapor phase, and in dilute CCl4 solution. The MIIR spectra of the nitrocresols that do not undergo intramolecular hydrogen bonding exhibited split OH stretching bands. Several factors that might cause band splitting, including aggregation, solute–matrix interactions, and the isolation of conformers, were investigated. The presence of aggregates was indicated by a broad band assigned to intermolecular hydrogen bonding in the MIIR spectra of 3-and 4-nitrocresol isomers deposited in an argon matrix at a matrix-to-solute ratio ≤ 283:1. The split OH absorption bands persisted, however, when there was no evidence of aggregate formation. The complexity of the split OH stretching bands was also influenced by steric factors and by the choice of matrix gas. This result suggests that isolation of rotational isomers and solute-matrix interactions also contribute to the band splitting. The presence of the split OH absorption bands did not preclude the use of the GC/MIIR technique to identify several nitrocresols produced from the photooxidation of toluene and NO x .