Evaluation of Diffuse Reflection Infrared Spectrometry for End-of-Shift Measurement of α-quartz in Coal Dust Samples.

The inhalation of toxic substances is a major threat to the health of miners, and dust containing respirable crystalline silica (α-quartz) is of particular concern, due to the recent rise in cases of coal workers' pneumoconiosis and silicosis in some U.S. mining regions. Currently, there is no field-portable instrument that can measure airborne α-quartz and give miners timely feedback on their exposure. The U.S. National Institute for Occupational Safety and Health (NIOSH) is therefore conducting studies to investigate technologies capable of end-of-shift or real-time measurement of airborne quartz. The present study focuses on the potential application of Fourier transform infrared (FT-IR) spectrometry conducted in the diffuse reflection (DR) mode as a technique for measuring α-quartz in respirable mine dust. A DR accessory was used to analyze lab-generated respirable samples of Min-U-Sil 5 (which contains more than 90% α-quartz) and coal dust, at mass loadings in the ranges of 100-600 µg and 600-5300 µg, respectively. The dust samples were deposited onto three different types of filters, borosilicate fiberglass, nylon, and polyvinyl chloride (PVC). The reflectance, R, was calculated by the ratio of a blank filter and a filter with deposited mine dust. Results suggest that for coal and pure quartz dusts deposited on 37 mm PVC filters, measurements of -log R correlate linearly with known amounts of quartz on filters, with R(2) values of approximately 0.99 and 0.94, respectively, for samples loaded up to ∼4000 µg. Additional tests were conducted to measure quartz in coal dusts deposited onto the borosilicate fiberglass and nylon filter media used in the NIOSH-developed Personal Dust Monitor (PDM). The nylon filter was shown to be amenable to DR analysis, but quantification of quartz is more accurate when the filter is "free," as opposed to being mounted in the PDM filter holder. The borosilicate fiberglass filters were shown to produce excessive interference, making quartz quantification impossible. It was concluded that, while the DR/FT-IR method is potentially useful for on-filter measurement of quartz in dust samples, the use of PVC filters produced the most accurate results.

Quantifying silica in filter-deposited mine dusts using infrared spectra and partial least squares regression.

The feasibility of measuring airborne crystalline silica (α-quartz) in noncoal mine dusts using a direct-on-filter method of analysis is demonstrated. Respirable α-quartz was quantified by applying a partial least squares (PLS) regression to the infrared transmission spectra of mine-dust samples deposited on porous polymeric filters. This direct-on-filter method deviates from the current regulatory determination of respirable α-quartz by refraining from ashing the sampling filter and redepositing the analyte prior to quantification using either infrared spectrometry for coal mines or x-ray diffraction (XRD) from noncoal mines. Since XRD is not field portable, this study evaluated the efficacy of Fourier transform infrared spectrometry for silica determination in noncoal mine dusts. PLS regressions were performed using select regions of the spectra from nonashed samples with important wavenumbers selected using a novel modification to the Monte Carlo unimportant variable elimination procedure. Wavenumber selection helped to improve PLS prediction, reduce the number of required PLS factors, and identify additional silica bands distinct from those currently used in regulatory enforcement. PLS regression appeared robust against the influence of residual filter and extraneous mineral absorptions while outperforming ordinary least squares calibration. These results support the quantification of respirable silica in noncoal mines using field-portable infrared spectrometers.

Evaluation of a non-dispersive infrared spectrometer for quantifying organic and elemental carbon in diesel particulate matter.

Diesel particulate matter (DPM) is a common and well-known health hazard in the mining environment. The regulatory method for monitoring both the organic and elemental carbon (OC, EC) portions of DPM is a laboratory-based thermal-optical method with a typical turnaround time of one week. In order to evaluate exposure levels and take corrective action prior to overexposure, a portable real-time device capable of quantifying both OC and EC is needed. To that end, researchers from the National Institute for Occupational Safety and Health (NIOSH) designed and tested the feasibility of a device based on bandpass optical filters that target key infrared wavelengths associated with DPM and its spectroscopic baseline. The resulting device, referred to here as a non-dispersive infrared (NDIR) spectrometer could serve as the basis of a cost-effective, field-portable alternative to the laboratory thermal-optical method. The limits of quantification (LOD) indicate that the NDIR spectrometer can quantify EC, OC, and TC provided they are present at 20, 37, and 46 µg/m3 or more, respectively. In the event that the NDIR spectrometer is integrated with a sampler and filter tape the LOD is estimated to be reduced to 13, 7, and 10 µg/m3 for EC, OC, and TC, respectively. These LOD estimates assume a face velocity of 59 cm/s and a sampling time of 30 min.

The experimental realization of a neutral homoaromatic carbocycle.

Infrared spectra recorded for 1,5-dimethyl-2,8:4,6-semibullvalenetetracarboxylic acid dianhydride (12) in the condensed and vapor phase clearly prove that in the vapor phase the dianhydride 12 is a homoaromatic ground state semibullvalene.

Quantifying elemental and organic carbon in diesel particulate matter by mid-infrared spectrometry.

A method for the quantification of airborne organic carbon (OC) and elemental carbon (EC) within aerosolized diesel particulate matter (DPM) is described in this article. DPM is a known carcinogen encountered in many industrial workplaces (notably mining) and in the ambient atmosphere. The method described here collects DPM particles onto a quartz fiber filter, after which reflection-mode infrared spectra are measured on a mid-infrared Fourier transform (FT-IR) spectrometer. Several infrared absorption bands are investigated for their efficacy in quantifying OC and EC. The thermo-optical (T-O) method is used to calibrate a linear regression model to predict OC and EC from the infrared spectra. The calibrated model, generated from laboratory DPM samples, is then utilized to quantify OC and EC in mine samples obtained from two metal mine locations under a variety of operating conditions. The feasibility of further improving these results by partial least squares (PLS) regression was investigated. A single calibration that is broadly applicable would be considered an improvement over currently available portable instruments, which require aerosol-specific calibration.

Information extraction from a complex multicomponent system by target factor analysis.

A theoretical investigation into the mechanism of information extraction by target factor analysis (TFA) is presented from experimental data in the form of a matrix, and the results were validated using composite spectra obtained by open-path Fourier transform-infrared (FT-IR) spectrometry. The composite spectra were generated by adding the spectral information of a target molecule with known path-integrated concentrations to the raw open-path FT-IR spectra obtained in a pristine atmosphere. Target molecules are deemed to be detected when the weighted correlation coefficient between the calculated spectrum of the analyte and its reference spectrum exceeds 0.90. The effective detection by TFA is shown to depend on the variation of their concentrations over the period of the measurement and not necessarily on the magnitude of concentration. When TFA fails to detect an analyte at high, but relatively constant, concentration that varies so little as to have low variance, blank spectra, i.e., spectra in which the analyte is known to be absent, are included in the data matrix. This procedure effectively increases the variance of the concentrations in the whole data set, and TFA detects the analyte.

Advances in data processing for open-path Fourier transform infrared spectrometry of greenhouse gases.

The automated quantification of three greenhouse gases, ammonia, methane, and nitrous oxide, in the vicinity of a large dairy farm by open-path Fourier transform infrared (OP/FT-IR) spectrometry at intervals of 5 min is demonstrated. Spectral pretreatment, including the automated detection and correction of the effect of interrupting the infrared beam, is by a moving object, and the automated correction for the nonlinear detector response is applied to the measured interferograms. Two ways of obtaining quantitative data from OP/FT-IR data are described. The first, which is installed in a recently acquired commercial OP/FT-IR spectrometer, is based on classical least-squares (CLS) regression, and the second is based on partial least-squares (PLS) regression. It is shown that CLS regression only gives accurate results if the absorption features of the analytes are located in very short spectral intervals where lines due to atmospheric water vapor are absent or very weak; of the three analytes examined, only ammonia fell into this category. On the other hand, PLS regression works allowed what appeared to be accurate results to be obtained for all three analytes.

Detection of chemical agents in the atmosphere by open-path FT-IR spectroscopy under conditions of background interference: II. Fog and rain.

Open-path FT-IR spectra of low-concentration releases of diethyl ether were measured both when a glycol fog was passed into the infrared beam and when large water droplets from a lawn sprinkler were sprayed into the beam. It was shown that the glycol fog, for which the droplet size was much less than the wavelength of the infrared radiation, gave rise to a significant interference such that partial least squares (PLS) regression would only yield reasonable values for the ether concentration if background spectra in which the glycol fog was present were included in the calibration set. On the other hand, target factor analysis (TFA) allowed the presence of the ether to be recognized without precalibration. When large water droplets were present in the beam, any infrared radiation entering the droplet was completely absorbed, so that both PLS and TFA would yield accurate results.

Detection of chemical agents in the atmosphere by open-path FT-IR spectroscopy under conditions of background interference: I. High-frequency flashes.

Open-path FT-IR spectra were measured while fireworks were emitting smoke and incandescent particles into the infrared beam. These conditions were designed to simulate the appearance of smoke and explosions in a battlefield. Diethyl ether was used to simulate the vapor-phase spectra of G agents such as sarin. The measured interferograms were corrected by a high-pass filter and were rejected when interfering features were of such high frequency that they could not be removed by application of this filter. The concentration of diethyl ether was calculated correctly by partial least squares regression in the absence of fireworks but significant errors were encountered when the spectra of the oxide particles were not included in the calibration set. Target factor analysis allowed the presence of the analyte to be detected even when the incandescent particles were present in the beam.

Completely automated open-path FT-IR spectrometry.

Atmospheric analysis by open-path Fourier-transform infrared (OP/FT-IR) spectrometry has been possible for over two decades but has not been widely used because of the limitations of the software of commercial instruments. In this paper, we describe the current state-of-the-art of the hardware and software that constitutes a contemporary OP/FT-IR spectrometer. We then describe advances that have been made in our laboratory that have enabled many of the limitations of this type of instrument to be overcome. These include not having to acquire a single-beam background spectrum that compensates for absorption features in the spectra of atmospheric water vapor and carbon dioxide. Instead, an easily measured "short path-length" background spectrum is used for calculation of each absorbance spectrum that is measured over a long path-length. To accomplish this goal, the algorithm used to calculate the concentrations of trace atmospheric molecules was changed from classical least-squares regression (CLS) to partial least-squares regression (PLS). For calibration, OP/FT-IR spectra are measured in pristine air over a wide variety of path-lengths, temperatures, and humidities, ratioed against a short-path background, and converted to absorbance; the reference spectrum of each analyte is then multiplied by randomly selected coefficients and added to these background spectra. Automatic baseline correction for small molecules with resolved rotational fine structure, such as ammonia and methane, is effected using wavelet transforms. A novel method of correcting for the effect of the nonlinear response of mercury cadmium telluride detectors is also incorporated. Finally, target factor analysis may be used to detect the onset of a given pollutant when its concentration exceeds a certain threshold. In this way, the concentration of atmospheric species has been obtained from OP/FT-IR spectra measured at intervals of 1 min over a period of many hours with no operator intervention.

Nanostructural silver and gold substrates for surface-enhanced Raman spectroscopy measurements prepared by galvanic displacement on germanium disks.

A germanium disk on which silver nanoparticles have been deposited by galvanic displacement is shown to be an inexpensive substrate for surface-enhanced Raman spectroscopy (SERS). The preparation, which is based on spontaneous reduction of silver cations at the surface of a germanium disk, is quick and requires nothing more than a Petri dish. The SERS enhancement of silver and gold substrates prepared in this way was measured using benzenethiol and was compared to enhancement of Klarite, a commercially available gold-coated nanoengineered SERS substrate. The enhancement provided by electrolessly deposited metals was found to be superior over Klarite and the reproducibility was generally below 15%. Furthermore, unlike the case for nanoengineered substrates, germanium disks can be polished and reused.

Correcting nonlinear response of mercury cadmium telluride detectors in open path Fourier transform infrared spectrometry.

The effect of a nonlinear response of mercury cadmium telluride (MCT) detectors to photon flux is to cause a large offset and a slow variation in the zero-line of single-beam Fourier transform infrared (FT-IR) spectra, which dramatically reduce the accuracy to which strongly absorbing bands or lines can be measured. We describe a noniterative numerical technique by which the baseline offset can be corrected by adjusting the values of the maximum point in the interferogram (the "centerburst") and the points on either side. The technique relies on the presence of three spectral regions at which the signal is known to be zero. Two of these are found in all spectra, namely, the region below the detector cutoff and the high-wavenumber region just below the Nyquist wavenumber where the interferogram has been electronically filtered. In open path FT-IR measurements there are several regions where atmospheric water vapor and CO2 are totally opaque. We have selected the region around 3750 cm(-1). This algorithm is even shown to work well when the interferogram is clipped, i.e., the value at the centerburst exceeds the dynamic range of the analog-to-digital converter.

Effective path length in attenuated total reflection spectroscopy.

Attenuated total reflection (ATR) spectroscopy is now the most popular sampling technique for the measurement of infrared spectra of condensed phase samples. Most practitioners of ATR spectroscopy use the equation for depth of penetration, d(p), to estimate the path length of the evanescent wave through the sample. However, the effective path length, d(e), of the evanescent wave in an ATR measurement, i.e., the equivalent path length in a transmission measurement that would lead to an absorption band of the same intensity, is a more accurate metric than d(p). In measurements designed to obtain the absorptivity of bands in the spectrum of a strongly absorbing viscous liquid, we have shown that the refractive index used in the expressions for d(e) must be modified to take into account the effect of anomalous dispersion before accurate effective path lengths and band absorptivities can be measured.

Mid-infrared diffuse reflection of a strongly absorbing analyte on non-absorbing and absorbing matrices. Part I: homogeneous powders.

KBr disks of caffeine were prepared quantitatively so that the absorptivity of caffeine in this environment could be measured. The disks were then ground up finely and their diffuse reflection spectrum was measured. The average path length traveled by remitted photons was then calculated by the simple application of the Beer-Lambert law. The path length that an average photon travels within a finely powdered matrix of KBr is approximately 4 mm when the concentration of a uniformly dispersed, strongly absorbing analyte such as caffeine is 0.01% or less. This path length then decreases as the concentration of the analyte increases as more photons are absorbed by the analyte. When carbon black is added to the mixture of caffeine and KBr and the measurements are repeated, the effective path length drops even further. The average photon that is remitted from a fine infrared-transparent powder containing a very low concentration of an analyte has been shown to encounter at least 400 particles, indicating a highly random path. The more strongly the matrix absorbs, the shorter the path length. When 0.1% of carbon black is added to the disk, the path length drops to about 100 microm.

Mid-infrared diffuse reflection of a strongly absorbing analyte on non-absorbing and absorbing matrices. Part II: thin liquid layers on powdered substrates.

The average total path length traveled by photons in mid-infrared diffuse reflection spectrometry of powdered potassium chloride matrices coated with a thin layer of silicone oil has been estimated. The average path length of the photons that had passed through the silicone oil layer was first calculated by the application of the Beer-Lambert law. The total path length through the coated particles was then estimated by dividing the path length of the silicone oil by its concentration. The average path length of the photons that had been diffusely reflected from KCl particles with an average diameter of approximately 5 microm that had been coated with a very thin layer of the silicone oil (corresponding to a concentration of 0.01% w:w) was estimated to be about 8 mm. This path length was significantly reduced either as the silicone oil concentration or the absorption of the matrix was increased. The result helps to explain why diffuse reflection spectrometry is a far poorer sampling technique for trace analysis when the matrix has strong absorption and why diffuse reflection is not a very promising technique for the standoff detection of nonvolatile chemical warfare agents.

Long and Short-Term Impact of Papers in Applied Spectroscopy.

In 1955, Eugene Garfield introduced the concept of a journal impact factor as a metric for measuring the importance or influence of scholarly journals. These days a journal's fate is often tied strongly to the impact factor. It is a topic that comes up regularly and a source of concern for the journal because of the slavish focus on metrics in the publishing world and in the academic community. From our perspective, the impact factor is shown to be a poor metric for illustrating the long-term significance of papers published in Applied Spectroscopy. The five-year impact factor is a better indicator for the short-term impact of the papers published in this journal, while the cited half-life and the citing half-life both provide a better measure of the long-term impact of papers published in Applied Spectroscopy. Of the most highly cited papers published in this journal, those that describe innovative data processing techniques have been cited more than papers that describe specific applications of a given technique such as infrared (IR), Raman, or laser-induced breakdown spectroscopy (LIBS).

On the Widths of Bands in the Infrared Spectra of Oxyanions.

It is well known that the antisymmetric stretching (ν3) band in the mid-infrared spectra of oxyanion salts is usually very broad, whereas all the other fundamental bands are narrow. In this paper, we propose that the underlying cause of the increased width is the effect of the very high absorption index of this band for samples prepared with a range of particle sizes. When oxyanion salts are ground, the diameter of the resulting particles usually varies from less than 100 nm to about 2 µm. While the peak absorbance of the ν3 band of the smaller particles (diameter < 200 nm) is less than 1, that of the larger particles can be as high as 6. We show that the average transmittance of these particles leads to a significant band broadening, especially when there are small voids in the resulting sample. Although the effect is always seen in the spectra of alkali halide disks and mineral oil mulls, it is also seen in diffuse reflection and attenuated total reflection (ATR) spectra. Because the depth of penetration of infrared radiation below 1500 cm-1 is less than 1 µm for ATR spectra measured with a germanium internal reflection element (IRE), the width of the ν3 band is lower than that of ATR spectra measured with an IRE of lower refractive index such as diamond on zinc selenide.

Investigation of the Christiansen effect in the mid-infrared region for airborne particles.

During measurements of open-path Fourier transform infrared spectra, airborne dust may be present in the infrared beam. We have investigated the feasibility of identifying and quantifying the airborne particulate matter from spectra measured in this way. Although the results showed that analysis of the particulate matter was not able to be performed from these spectra, insight into the size and wavelength dependence of the Christiansen effect at wavelengths where the particles absorb strongly was obtained. Airborne particles larger than or equal to the wavelength of the incident radiation give rise to asymmetrical features in the spectrum caused by the Christiansen effect. However, the transmittance at wavelengths where the refractive index of the particles equals that of the atmosphere never reaches 1.0 because of absorption by the particles. As the particle size becomes much smaller than the wavelength of the incident radiation, the Christiansen effect becomes less pronounced and eventually is not exhibited.

The Early Days of Commercial FT-IR Spectrometry: A Personal Perspective.

The development of Fourier transform infrared (FT-IR) spectrometers in the mid-1960s followed along three lines. Interferometers for far-infrared FT spectrometry typically had a slow scan speed and the beam of radiation was modulated by a rotating chopper. Several instruments based on this system were developed commercially. Very high-resolution near-infrared FT spectrometers were based on cats-eye retroreflectors mounted in a step-scan interferometer; the beam of radiation was usually modulated by dithering one of the cats-eyes (phase modulation). No commercial instruments based on this principle were developed. In the third type of FT spectrometer, the beam was modulated by rapidly scanning one of the mirrors of a Michelson interferometer. While the early instruments based on this principle only gave rise to low-resolution spectra, the incorporation of laser fringe referencing at the end of the decade led to instruments that were the fore-runners of contemporary FT-IR spectrometers. In this article, the author's experiences with instruments of the first and third category are described.

Direct-on-Filter α-Quartz Estimation in Respirable Coal Mine Dust Using Transmission Fourier Transform Infrared Spectrometry and Partial Least Squares Regression.

In order to help reduce silicosis in miners, the National Institute for Occupational Health and Safety (NIOSH) is developing field-portable methods for measuring airborne respirable crystalline silica (RCS), specifically the polymorph α-quartz, in mine dusts. In this study we demonstrate the feasibility of end-of-shift measurement of α-quartz using a direct-on-filter (DoF) method to analyze coal mine dust samples deposited onto polyvinyl chloride filters. The DoF method is potentially amenable for on-site analyses, but deviates from the current regulatory determination of RCS for coal mines by eliminating two sample preparation steps: ashing the sampling filter and redepositing the ash prior to quantification by Fourier transform infrared (FT-IR) spectrometry. In this study, the FT-IR spectra of 66 coal dust samples from active mines were used, and the RCS was quantified by using: (1) an ordinary least squares (OLS) calibration approach that utilizes standard silica material as done in the Mine Safety and Health Administration's P7 method; and (2) a partial least squares (PLS) regression approach. Both were capable of accounting for kaolinite, which can confound the IR analysis of silica. The OLS method utilized analytical standards for silica calibration and kaolin correction, resulting in a good linear correlation with P7 results and minimal bias but with the accuracy limited by the presence of kaolinite. The PLS approach also produced predictions well-correlated to the P7 method, as well as better accuracy in RCS prediction, and no bias due to variable kaolinite mass. Besides decreased sensitivity to mineral or substrate confounders, PLS has the advantage that the analyst is not required to correct for the presence of kaolinite or background interferences related to the substrate, making the method potentially viable for automated RCS prediction in the field. This study demonstrated the efficacy of FT-IR transmission spectrometry for silica determination in coal mine dusts, using both OLS and PLS analyses, when kaolinite was present.