Field Analytical Chemistry & Technology最新文献

Fourier-transform infrared spectrometry furnishes a viable method of assessing a variety of airborne pollutants with passive remote monitoring. Ensuring accurate and precise open-air monitoring requires evaluation of FTIR spectrometer radiometric performance. The noise equivalent radiance per root Hertz is a radiometric figure of merit that is independent of integration time, interferometer mirror velocity, and spectral resolution. This study demonstrates the utility of the NER per root Hertz metric by examining FTIR spectrometer performance as a function of internal thermal stability, spectral resolution, and integration time. The effect of internal stability on spectrometer radiometric performance is evaluated as a function of external spectrometer temperature. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 81–94, 1999

Emphasis on air quality has increased the need for a rapid method of measuring air pollutants. Remote Fourier-transform infrared spectroscopy (FTIR) offers rapid, on-site analysis of large areas and requires no sample preparation. Simple quantitative methods that reflect conditions present at the time of the measurement are other advantages of remote FTIR. This is especially important when one is attempting to use ambient background sources where the intensity of the source alters absorption intensities and calibration curves. Quantitative remote FTIR is based on Beer's law and matching the analyte spectrum with reference spectra. The reference spectra are measured separately and do not account for variation in source intensity or temperature, or for unknown interferences. Substituting deuterium for one or more hydrogens in a volatile organic compound (VOC) shifts the vibrational frequency, allowing the deuterated compound to be measured simultaneously with the VOC. The reference spectra for the Beer's law plot can be measured at the same time as the analyte and a slope and y intercept conversion allows determination of the analyte concentration. We have used isotopic references for quantitative open-path FTIR measurements of acetone and methanol and have achieved better than 10% accuracy in the parts-per-million concentration range. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 105–110, 1999

The performance of CLS, PLS-2, and PLS-1 on calibration/validation sets of mixtures of five short-chain alcohols was examined as a function of resolution, spectral window, and type of background single beam to determine the feasibility of achieving automated calibrations for open-path Fourier-transform infrared (OP-FTIR) spectrometry. The backgrounds used in the study were created by ratioing actual single-beam OP-FTIR spectra. Some backgrounds were created by ratioing spectra measured at the same path length, and others were created by ratioing a long-path sample spectrum against a short-path reference. After conversion to absorbance, these background spectra provided a noise term that is representative of what is seen in actual field deployment. The absorbance spectra of mixtures of the spectra of up to five short-chain alcohols (chosen because of their spectral similarity) were added to these spectral baselines. CLS was shown to perform well only when using equidistant backgrounds and TO-16 spectral window selection, with average errors of 2–20%. CLS calibrations under other conditions tested typically produced average errors of greater than 100%. Under most conditions the measurement of OP-FTIR spectra at low (8 cm⁻¹) resolution produced an increase in the accuracy of the determinations over higher-resolution spectra. Both PLS-1 and PLS-2 were able to predict the concentrations of the alcohols under any conditions, including short-path backgrounds, with an error of less than 5%; errors of less than 2% were achieved in most cases. The results indicate that an automated calibration system is feasible for OP-FTIR spectra measured at low resolution, even when a short-path background is used. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 117–130, 1999

Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a relatively new measurement technique offering advantages over traditional point samples for monitoring air contaminants and validating dispersion models. In addition, this technology has the potential to produce temporally and spatially resolved chemical concentration maps. In this study, OP-FTIR spectrometer measurements and point samplers were used to evaluate two short-term refined Gaussian dispersion models for predicting the fate of volume source emissions. Additionally, these data were used for a pilot Environmental CAT scanning system using two scanning OP-FTIR spectrometers and eight retroreflectors. An environmental CAT scanning system processes a network of intersecting OP-FTIR spectrometers using a tomographic reconstruction algorithm. When the algorithm is applied to these real-time measurements, two-dimensional chemical concentration maps are created of the area. This technique is noninvasive and is capable of providing real-time spatially and temporally resolved concentration maps of multiple chemicals simultaneously and at low limits of detection (ppb–low ppm). Near real-time chemical concentration maps were generated for the entire field site, and the concentration maps were compared with model maps with respect to plume location and plume shape. We evaluated the Industrial Source Complex–Short Term (Version 3) (ISCST) model and the American Meteorological Society/Environmental Protection Agency Regulatory Model Improvement Committee (AERMOD) model. Sulfur hexafluoride was released at known rates from a simulated volume source and was measured at various points in the study field with the use of a network of Tedlar® bag point samples and OP-FTIR spectrometer measurements. Both ISCST and AERMOD underpredicted SF₆ concentrations when compared to the 5-min averaged OP-FTIR spectrometer measurements (1.3x) and the 1-h integrated Tedlar® bag samples (1.4x). For most time periods, the tomographic concentration maps compared fairly well with the model predictions in terms of plume shape and location. The tomographic maps tracked the impact of changing wind direction better than the model predictions. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 69–79, 1999

Being able to measure the levels, and predict the movement of an anthropogenic trace metal released into a marine or estuarine environment is a major focus in coastal pollution studies. One major source of trace metal contamination in these environments is from ionic-copper-containing antifouling paints on ship hulls. Because ionic copper [Cu (I) + Cu (II)] complexes with organic ligands in the aquatic environment, there is a need for a quick and reliable measurement method to determine its levels. The organic dye, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine = BCP), embedded in the polymer Nafion 117, is such a reliable method. This procedure provides for the development of a probe capable of directly measuring Cu (I) in seawater. Measurement numbers are within 17% of the values obtained with the aqueous BCS standard method procedure. With this Nafion probe, it is possible to test for Cu (I), and, with a method modification, for Cu (II). The probe also allows the study of the movement of the ionic copper relative to the water movements in a basin. Results from this study indicate that ionic copper circulates, mixes, and releases from the basins or harbors with the tide. The circulation and mixing are functions of the basin flushing rate and exchange ratio. Nafion impregnated with bathocuproine (BCP) is a rapid method useful for detecting Cu (I) releases into the marine environment. ©1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3:3–18, 1999

Short-range and mid-range (grid size) spatial heterogeneity in explosives concentrations within surface soils was studied at an active antitank firing range. Intensive sampling was conducted adjacent to two target tanks by establishing sixteen 6-m-square grids. Each grid was subdivided into four quadrants, and in each quadrant an area-integrated surface sample was formed into a pile that included about 10% of the top 5 cm of soil in the quadrant. After in situ homogenization, random aliquots were combined to form replicate representative samples. Grid composites were also prepared by combining equal portions of soil from the four quadrants for each grid. In nine of the quadrants, a second area-integrated sample was prepared. On-site analysis showed concentrations of HMX ranging from as high as 2160 mg/kg near one target to ≤1 mg/kg at a distance of 20 m from the target. TNT concentrations, ranging from ≤1 to 23 mg/kg, were much lower than would be expected based on the 70 : 30 composition ratio of HMX to TNT in the melt-cast explosive used on site. On-site concentration estimates for HMX and TNT were in excellent agreement with laboratory HPLC results; correlation coefficients were 0.992 and 0.975, respectively. Spatial heterogeneity of HMX concentrations was large on both short- and midrange scales, and this factor dominated the overall uncertainty associated with site characterization. Greater emphasis on sampling is urgently needed to improve the representativeness of explosives residue determinations in soil. ©1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3:19–28, 1999

A sensitive visual colorimetric method has been developed for the semiquantitative field determination of trace molybdenum. The molybdenum-catalyzed oxidation of ascorbic acid at pH 3.2 in the presence of o-phenylenediamine was used as the indicator reaction, which produces yellow quinoxaline derivatives. After a fixed reaction time, the reaction is stopped by readjusting pH to 1 with hydrochloric acid. For the visual determination, the color intensity of the final solution (10 ml) for a water sample is compared to that of a color standard solution containing 0.04 μg (4 μg l⁻¹) of Mo^VI prepared by the same procedure. Two handmade cells of the same size with 10-, 20-, 30- and 40-mm light paths are used in the color comparison for a wide determination range (0.005–0.2 μg). The intensity of the color standard is held constant by the adjustment of the reaction time, for example, 10 min at 25 °C, with the use of a simple relationship between the reaction time and the field temperature. Molybdenum down to 1 μg l⁻¹ in a 4-ml river-water sample was determined without any special instrument. Analytical performances were evaluated and compared with those obtained by the spectrophotometric measurements. The application to field survey has revealed the distribution of molybdenum concentration along the river streams and a polluted point. ©1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3:29–35, 1999

Open-path Fourier-transform infrared spectroscopy (OP-FTIR) provides a rapid method of measuring concentrations of volatile organic compounds (VOCs) in environmental settings. The advantages of OP-FTIR are the speed of data collection and the absence of sample handling. The quantitative difficulties with OP-FTIR include variation in source intensity caused by moving the source or instrument, rapid change in temperature, low intensity of passive sources, and changes in path conditions such as humidity and the presence of interfering compounds. These difficulties are the major obstacles in using OP-FTIR quantitatively. We are developing a method that has shown promise in combating source variability, the difficulty in obtaining reference spectra, and the problems with passive sources. Reducing the resolution and graphing the intensity of the analyte peaks against a convenient reference system produces quantitative curves without subtracting background spectra. These curves can be used to determine unknown concentrations to an accuracy of 20% as long as the intensity and the general shape of the unknown sample spectrum matches a representative spectrum of the quantitative curve. We have determined sample unknowns with the use of running car engines and hot car hoods as sources. The main problem still to be studied is that accurate concentration determination requires a good match between the temperature profiles of the quantitative curve spectra and the sample spectrum. Future studies will involve the relationship between source temperature and spectral shape, source temperature and absorption peak magnitude, and the effect of distance from the source to the instrument on spectral shape and peak magnitude. ©1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 37–43, 1999