David C. Tilotta

Fiber-Optic Sensor for Aromatic Compounds in Water Based on Solid-Phase Microextraction and Ultraviolet Evanescent Wave Absorption Spectroscopy

A method is described for the determination of aromatic contaminants in water samples that combines solid-phase microextraction and ultraviolet evanescent wave absorption (UV-EWA) spectroscopy. In this method, the aromatic compounds are partitioned from a 10 mL sample of water into the polymer cladding of a 10 cm length of UV-transmitting fiber. The aromatics are then detected directly in the cladding by EWA spectroscopy. The time required for four BTEX compounds to reach concentration equilibrium between the aqueous phase and the cladding ranges from 12 to 80 min, although we show that quantitative determinations can be accomplished in 30 min. Detection limits for these compounds range from 1 to 18 ppm, with relative standard deviations from 3 to 17%. Finally, preliminary work demonstrates that natural water matrices pose no significant matrix effects and that this method shows promise in determining the total aromatic concentrations of water samples contaminated with gasoline.

Poly(dimethylsiloxane) films as sorbents for solid-phase microextraction coupled with infrared spectroscopy

Poly(dimethylsiloxane) (PDMS) film was investigated as a stationary phase for solid-phase microextraction coupled with infrared (IR) spectroscopy. Five organic compounds of environmental concern (trichloroethylene, perchloroethylene, o-xylene, p-xylene, and trifluralin) were selected as test compounds for this study. Spiked solutions were extracted from 250-ml water samples into small squares (3.2×3.2 cm) of commercially available, 127-μm thick, PDMS film. The equilibration times for the test analytes in PDMS ranged from 60 to 85 min, and were about three times faster than those in Parafilm M. However, it was found that PDMS had poorer detection limits (in the range of 0.2–4.4 ppm) than those obtained with Parafilm M (in the range of 0.066–1.8 ppm). The relative standard deviations of the measurements were from 6.7–12%, and were governed primarily by volatility losses. Finally, preliminary work with PDMS demonstrates that real water matrices do not adversely affect the detection of organic compounds.

Determination of total petroleum hydrocarbons in soil by on-line supercritical fluid extraction-infrared spectroscopy using a fiber-optic transmission cell and a simple filter spectrometer

Abstract An on-line supercritical fluid extraction-infrared (SFE-IR) instrument is described for determining total petroleum hydrocarbons (TPHs) in soil samples. This instrument is constructed from a commercially-available SFE system and filter IR spectrometer, and an easily-constructed fiber optic IR cell. All SFEs are performed statically (i.e., no fluid flow once the SFE vessel is filled) for 30 min at 80°C and 340 atm. Detection limits for TPH were determined to be ca. 8 ppm for an IR cell with a 1-mm optical path, and are shown to degrade with increasing pathlength. The results from the application of this on-line SFE-IR instrument to the determination of TPHs in real-world samples show good agreement with those obtained from a standard Soxhlet extraction-IR method. Relative standard deviations of the on-line determinations are in the range of 7–10%.

Determination of total petroleum hydrocarbons in soil by dynamic on-line supercritical fluid extraction with infrared photometric detection

Total petroleum hydrocarbons (TPHs) in soil are determined by on-line dynamic supercritical fluid extraction (SFE) using infrared filter photometry detection. The filter photometer was constructed in the laboratory using a tungsten lamp, an optical notch filter that selects the C-H stretching vibration of the extracted organics, an optical chopper with demodulation electronics, and a PbSe detector. A modified high-pressure fiber optic flow cell was used to couple the SFE system to the photometer. Quantitation of TPHs was accomplished through the construction of calibration curves of integrated absorbance of C-H stretching (over time) versus concentration. Our studies show that the sensitivity of this system is affected by both the optical path length in the high-pressure cell and the SFE fluid flow-rate, and detection limits for TPHs are in the mid part-per-million range. The results of the application of this on-line SFE-IR instrument to the determination of TPHs in real-world samples show good agreement with those obtained from standard Soxhlet extraction-IR methods.

Measurement of petroleum fuel contamination in water by solid-phase microextraction with direct Raman spectroscopic detection

A method is described for determining petroleum fuel contamination in water based on solid-phase microextraction and Raman spectroscopy (SPME/Raman). In this method, contaminants are extracted from aqueous solutions into a solid phase and then detected directly by using spontaneous Raman spectroscopy. The solid phase consists of a small volume (∼ 20 μL) of poly(dimethylsiloxane) that has an optical window in the 830–1600 cm−1 Raman shift region. This region is suitable for determining both aromatic and aliphatic fuel components. Four “real world” fuels—aviation gasoline, unleaded gasoline, jet fuel A, and #1 fuel oil—were used to test this new method. As shown, the SPME/Raman determinations performed in the visible spectral region were not adversely affected from the fluorescence of the marker dyes in these fuels. The linear dynamic ranges for the determinations exceeded the water solubility limits and spanned 1–3 orders of magnitude. The limits of detection with the use of the most sensitive Raman bands in each fuel were in the 8–12 ppm range with the exception of #1 fuel oil (25 ppm), and relative standard deviations typically were 2–10%. Finally, the SPME/Raman method was applied to the determination of total petroleum hydrocarbons in natural water matrices. With the use of hexane and undecane as standards, the results of the determinations compared favorably to the standard methods of purge-and-trap gas chromatography and liquid-liquid extraction followed by infrared detection.

Near-Infrared Molecular Emission from a Gas Fountain

An electrothermal fountain is used to heat gas-phase samples in the range of 300 to 400°C in order to observe their near-infrared (NIR) emissions. In conjunction with the fountain, a 1/8-m Ebert monochromator and an uncooled PbS detector are shown to be sufficiently sensitive for recording the NIR fingerprints of CH4, CO2, N2O, and C2H6. At fountain temperatures of approximately 400°C, the molecular emission is confined to the long wavelength NIR region (1600–2500 nm) and yields limits of detection in the range of 4 to 10% v/v (3–10 mg/s). Reproducibilities have relative standard deviations of 3.0%. The calibration curves for the gases examined in this study have small linear dynamic ranges (factors of 2 to 10) and exhibit some degree of upward curvature. Ramifications of the application of NIR molecular emission spectroscopy to the qualitative and quantitative analyses of mixtures are discussed.

Design and Performance of a Hadamard Transform Infrared Spectrometer with No Moving Parts

The development of an improved Hadamard transform infrared HT-IR) spectrometer with no moving optical parts is described. This spectrometer is based on a thermo-optic array for the stationary encoding of radiation (TOASTER) fabricated from Nichrome resistance wire. The TOASTER HT-IR instrument utilizes a reversed Czerny-Turner optical mount and relies on entrance focal plane modulation for operation. This design allows the dedispersion optics to be eliminated while still providing for a multiplex advantage. In addition, a conventional mechanical chopper is not needed in this instrument because the TOASTER is used as a natural modulator. The TOASTER HT-IR spectrometer can acquire an infrared spectrum in the range of 2.5 to 11 μm (4000 to 910 cm−1). Its spectral resolution is 0.07 μm (∼7 cm−1 at 1000 cm−1), and, for fifteen spectral resolution elements, the observation window is 0.5 μm (∼48 cm−1 at 1000 cm−1). In addition to the design of the instrument, this communication also examines the thermo-optic switching properties of the Nichrome wire used in the prototype TOASTER. Although the maximum switching speed of the Nichrome wire is governed by its thermal decay time, we show that the TOASTER can be operated faster than this limit by employing a preheating step.

Determination of chlorinated hydrocarbons introduced into air/acetylene flames by Fourier transform infrared emission spectroscopy.

A Fourier transform spectrometer is used to record the infrared emission from chlorinated hydrocarbons combusted in an air/acetylene flame. In this manner, the chlorinated hydrocarbons are determined by monitoring the infrared emission of hydrogen chloride at 2653 cm(-1). Discussion is presented of the air/acetylene flame background, and the potential spectral interference from the emission of deuterated species. Practical detection limits for chloroform, carbon tetrachloride and methylene chloride in acetone, methanol, and ethanol are solvent independent and are found to be 1.1, 0.80, and 1.0%, respectively. Calibration curves for these three analytes are linear from their detection limits to approximately 55% (v/v). In addition, evidence is presented that flame flicker-noise does not lead to a multiplex disadvantage when the Fourier transform instrument is used for data acquisition.

Detection of alcohols in gas chromatographic effluent by laser-light scattering

Abstract A laser-light-scattering detector that is sensitive to alcohols has been developed for gas chromatography. The detector consists of a miniature concentric nebulizer that uses a cold atomization gas and an Ar + laser. Calibration curves for the alcohols exhibit characteristic sigmoidal shapes. Signal-to-noise ratios were optimized by examining the photomultiplier tube temperature, collection wavelength and detection scheme ( i.e. , photon counting vs . direct current detection). Limits of detection for five test alcohols were in the 2–8 μg/s range.