Open-path Fourier Transform Infrared Spectroscopy Research Articles

Stable gas plume composition measured by OP‐FTIR spectroscopy at Masaya Volcano, Nicaragua, 1998–1999

We report non‐eruptive plume gas composition data collected by open‐path Fourier transform infrared (OP‐FTIR) spectroscopy at the summit of Masaya Volcano, Nicaragua, in 1998 and 1999. Masaya's plume is characterised by SO2/HCl and HCl/HF mass ratios of 2.8–3.0 and 8.1–8.2 respectively. When combined with COSPEC SO2 emission rate data, these ratios indicate average emission rates of HCl and HF of 7.3 kgs−1 and 0.9 kgs−1. The gas composition is remarkably consistent, showing little temporal variation on the scale of days, weeks or even between the two observation periods, one year apart. Such striking uniformity is best explained by steady‐state open system degassing.

On the Application of Open-Path Fourier Transform Infra-Red Spectroscopy To Measure Aerosols: Observations of Water Droplets

This paper proposes the application of Open-Path Fourier Transform Infra-Red (OP−FTIR) spectroscopy to measure aerosols. A preliminary experiment conducted in a standard shower chamber generated a condensed water aerosol cloud. The OP−FTIR beam acquired spectra through the cloud of water droplets. We matched calculated extinction spectra to measured extinction in the spectral range between 500 and 5000 wavenumbers by using Mie theory for spherical particles. The results indicate that size distribution parameters may be retrieved from OP−FTIR spectra acquired over a 1 km optical path with reasonable detection limits on the order of 10 μg·m-3 for aerosols with optical properties equivalent to water.

Spectral matching quantitative open-path fourier-transform infrared spectroscopy

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 OPFTIR are the speed of data collection and the absence of sample handling. The quantitative difficulties with OPFTIR 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.

Quantitative open-path FTIR with the use of isotopic standards

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

Workplace and Environmental Air Contaminant Concentrations Measured by Open Path Fourier Transform Infrared Spectroscopy: A Statistical Process Control Technique to Detect Changes from Normal Operating Conditions

Open path Fourier transform infrared (OP-FTIR) spectroscopy is a new air monitoring technique that can be used to measure concentrations of air contaminants in real or near-real time. OP-FTIR spectroscopy has been used to monitor workplace gas and vapor exposures, emissions from hazardous waste sites, and to track emissions along fence lines. This paper discusses a statistical process control technique that can be used with air monitoring data collected with an OP-FTIR spectrometer to detect departures from normal operating conditions in the workplace or along a fence line. Time series data, produced by plotting consecutive air sample concentrations in time, were analyzed. Autocorrelation in the time series data was removed by fitting dynamic models. Control charts were used with the residuals of the model fit data to determine if departures from defined normal operating conditions could be rapidly detected. Shewhart and exponentially weighted moving average (EWMA) control charts were evaluated for use with data collected under different room air flow and mixing conditions. Under rapidly changing conditions the Shewhart control chart was able to detect a leak in a simulated process area. The EWMA control chart was found to be more sensitive to drifts and slowly changing concentrations in air monitoring data. The time series and statistical process control techniques were also applied to data obtained during a field study at a chemical plant. A production area of an acrylonitrile, 1,3-butadiene, and styrene (ABS) polymer process was monitored in near-real time. Decision logics based on the time series and statistical process control technique introduced suggest several applications in workplace and environmental monitoring. These applications might include signaling of an alarm or warning, increasing levels of worker respiratory protection, or evacuation of a community, when gas and vapor concentrations are determined to be out-of-control.