Volatile Organic Analytes Research Articles

Optically accessible gas exposure apparatus for testing and characterization of holographic gas sensors

Holographic gas sensors are of great interest due to their widespread applicability and potential for high sensitivity, fast response, immunity to electromagnetic interference, and compact and lightweight nature. For effective design and development of holographic gas sensors, it is essential to have a reliable and safe gas exposure system allowing for optical access for testing purposes. Here, the design and operation of a custom gas exposure apparatus for the performance evaluation of holographic grating-based gas sensors within a research laboratory setting is presented. The apparatus enables the real-time measurement of analyte-induced changes in key holographic grating parameters: grating diffraction efficiency and reconstruction wavelength. A demonstration of the capabilities of the optically accessible apparatus to evaluate sensor response time, sensitivity to different volatile organic compound analytes, and response to cyclical gas exposure is presented. The AutoCAD designs, as well as the material and equipment specifications for the custom apparatus, are provided to facilitate reproduction of the gas development and gas exposure setup.

Large modulation of the chemical and electronic sensitization of TiO2/Ag/NiO nanostructure via in situ hydrothermal-induced heterointerface engineering

Chemical and electronic sensitization in metal oxide gas sensors are severely limited by poor dimension controls of metal oxide nanostructure and their electric/electronic properties. These limitations are overcome using hydrothermal-induced heterointerface engineering approaches. This work demonstrates that forming spherical titanium dioxide nanoparticles on a substrate significantly reduce a surface energy barrier of nucleation and induces novel mesophorous hierarchical TiO2 structure during hydrothermal synthesis, consequently increasing the surface area of the structure by ∼ 3 times compared to that of control. In addition, we succeeded in tailoring the energetics of hierarchical TiO2 nanosctructure by decorating with the nickel oxide and silver nanoparticles, which results in a desirable semiconductors/metal heterointerface for fast charge transfer where silver nanoparticles bridge nickel oxide and TiO2 nanostructure, and silver nanoparticles serve as preferential sites for chemisorption and migration of oxygen anions. The resulting heterostructure sensing properties such as sensitivity, limit of detection and selectivity are studied as a function of operating temperature (30–150 °C), relative humidity (RH) and various volatile organic analytes concentrations. The TiO2/Ag/NiO heterostructure finally exhibits a high gas response of ∼ 2.1 for acetone with a limit of detection of 34 ppb at 30 °C (or 21 ppb at 90 °C), and retains an excellent selectivity of acetone even at 90 % relative humidity. It exhibited a highly stable and speedy gas response for acetone toward various gases such as formaldehyde, ethanol, hydrogen sulfide, carbon monoxide even operating at 90 °C. Our results suggest a potential of constructed TiO2/Ag/NiO heterostructure for superior sensing volatile organic acetone and will also stimulate research on hetero-structured gas sensors with high sensitivity and selectivity.

Polydiacetylene and imidazole mixed self-standing films for colorimetric detection of various volatile organic analytes

Polydiacetylens (PDAs) have attracted great attention for application in sensing field due to their unique fluorescence and colorimetric properties. Polydiacetylenes (PDAs) are important class of polymer that exists in two distinct phases namely metastable, non-fluorescent blue phase and stable, auto-fluorescent red phase under specific conditions. PDA is obtained through polymerization of diacetylene (DA) monomers, typically using UV irradiation that leads to photopolymerization of the diacetylene lipids. Herein, the diacetylene monomer 10, 12-tricosadiynoic acid (TCDA) and N-1-hexadecyl Imidazole have been used to prepare mixed films onto a filter paper surface using the self-standing technique and then polymerizes the mixed films to get blue phase by UV irradiation. The PDA and N-1-hexadecyl Imidazole mixed films in the blue phase exhibit rapid colorimetric/fluorescence response upon exposure to various volatile organic analytes. The colorimetric and fluorogenic responses of TCDA and N-1-hexadecyl Imidazole mixed films were investigated by UV-vis absorption spectroscopy, inverting fluorescence microscopy and RGB analysis. This study demonstrates that PDA and N-1-hexadecyl Imidazole mixed films can be a used for detection of various volatile organic analytes even in naked eye.

Cross-Reactive, Self-Encoded Polymer Film Arrays for Sensor Applications.

The development of chemical sensors continues to be an active area of research, especially the development of a practical electronic nose. Here, we present a spectroscopic chemical sensor based on an array of 64 self-encoded polymer films deposited on a microfabricated silicon substrate. The polymer arrays were analyzed by FTIR and Raman spectroscopy before and after exposure to a series of organic volatiles to monitor changes in their vibrational fingerprints. We show here that the spectroscopic changes of self-encoded polymer films can be used to distinguish between volatile organic analytes. Changes induced in the sensor arrays by the analyte vapor were denoted by a spectroscopic response of the self-encoded polymer sensors and transformed into a response pattern by multivariate data analysis using partial least squares regression. The results indicated that the polymer sensors provide a unique and reproducible pattern for each analyte vapor and can potentially be used in the fabrication of a novel electronic nose device.

Development of second-generation sample enrichment probe for improved sorptive analysis of volatile organic compounds

The sample enrichment probe (SEP) has recently been introduced as a user-friendly and cost-effective method for the sorptive extraction of volatile organic analytes from gaseous and aqueous samples for GC and GC–MS analyses. In a further development of the SEP technique, thinner polydimethylsiloxane (PDMS) tubing on polyimide-coated fused silica, instead of stainless steel rods or stalks, were used to produce the second-generation SEPs. The new SEP does not require widening of the needle-guiding orifice of the septum cap and analytes are desorbed at a faster rate from the thinner sleeve, which reduces the risk of carry-over. The flowless period that was previously recommended for analyses of highly volatile analytes is made redundant by the faster desorption from the thinner sorptive medium. It was found that differences in the thermal histories of SEPs are not the cause of the high relative standard deviations (RSDs) reported in our first paper on the technique. Excellent reproducibility can be attained by careful handling and storing of loaded SEPs and by rigorously following a standardised analytical protocol.

Tuning free base tetraphenylporphyrins as optical sensing elements for volatile organic analytes

Changes in the visible absorption spectra of ten free base porphyrin compounds in solution and thin film form upon exposure to a range of volatile organic compounds have been investigated. The functional groups attached to the phenyl rings varied in their electron donating/withdrawing strength. These porphyrins were studied with a view to determining their suitability for use in the fabrication of an optical based gas sensor. It has been found that the sensitivity of these sensor materials depends upon the electron donating/withdrawing strength of the functional groups attached to the phenyl rings.

Differentiating a Diverse Range of Volatile Organic Compounds with Polyfluorophore Sensors Built on a DNA Scaffold

Oligodeoxyfluorosides (ODFs) are short DNA-like oligomers in which DNA bases are replaced with fluorophores. A preliminary study reported that some sequences of ODFs were able to respond to a few organic small molecules in the vapor phase, giving a change in fluorescence. Here, we follow up on this finding by investigating a larger range of volatile organic analytes, and a considerably larger set of sensors. A library of tetramer ODFs of 2401 different sequences was prepared by using combinatorial methods, and was screened in air for fluorescence responses to a set of ten different volatile organics, including multiple aromatic and aliphatic compounds, acids and bases, varied functional groups, and closely related structures. Nineteen responding sensors were selected and characterized. These sensors were cross-screened against all ten analytes, and responses were measured qualitatively (by changes in color and intensity) and quantitatively (by measuring ΔR, ΔG, and ΔB values averaged over five to six sensor beads; R=red, G=green, B=blue). The results show that sensor responses were diverse, with a single sensor responding differently to as many as eight of the ten analytes; multiple classes of responses were seen, including quenching, lighting-up, and varied shifts in wavelength. Responses were strong, with raw ΔR, ΔG, and ΔB values of as high as >200 on a 256-unit scale and unamplified changes in many cases apparent to the naked eye. Sensors were identified that could distinguish clearly between even very closely related compounds such as acrolein and acrylonitrile. Statistical methods were applied to select a small set of four sensors that, as a pattern response, could distinguish between all ten analytes with high confidence. Sequence analysis of the full set of sensors suggested that sequence/order of the monomer components, and not merely composition, was highly important in the responses.

Porous Silicon‐Based Optical Microsensors for Volatile Organic Analytes: Effect of Surface Chemistry on Stability and Specificity

AbstractSensing of the volatile organic compounds (VOCs) isopropyl alcohol (IPA) and heptane in air using sub‐millimeter porous silicon‐based sensor elements is demonstrated in the concentration range 50–800 ppm. The sensor elements are prepared as one‐dimensional photonic crystals (rugate filters) by programmed electrochemical etch of p++ silicon, and analyte sensing is achieved by measurement of the wavelength shift of the photonic resonance. The sensors are studied as a function of surface chemistry: ozone oxidation, thermal oxidation, hydrosilylation (1‐dodecene), electrochemical methylation, reaction with dicholorodimethylsilane and thermal carbonization with acetylene. The thermally oxidized and the dichlorodimethylsilane‐modified materials show the greatest stability under atmospheric conditions. Optical microsensors are prepared by attachment of the porous Si layer to the distal end of optical fibers. The acetylated porous Si microsensor displays a greater response to heptane than to IPA, whereas the other chemical modifications display a greater response to IPA than to heptane. The thermal oxide sensor displays a strong response to water vapor, while the acetylated material shows a relatively weak response. The results suggest that a combination of optical fiber sensors with different surface chemistries can be used to classify VOC analytes. Application of the miniature sensors to the detection of VOC breakthrough in a full‐scale activated carbon respirator cartridge simulator is demonstrated.

Application of sensor arrays based on thin films of conducting polymers for chemical recognition of volatile organic solvents

The use of thin conducting polymer films (polyaniline, polypyrrole and poly-3-methylthiophene doped with ordinary mineral acids or monovalent anions, as well as heteropolyacids of Keggin type or corresponding heteropolyanions) as sensitive layers in the sensor arrays for chemical recognition of volatile organic analytes is investigated. The discrimination ability of various sensor subsets is evaluated by statistical analysis of the data retrieved from responses of 20 different polymer sensors to the vapors of nine volatile organic solvents. An approach for constructing optimal sensor array is developed and the method for systematic selection of separate sensors from the given set to achieve the required analyte recognition reliability is proposed.

Investigation of stability of aqueous solutions containing trace amounts of volatile organic analytes

When determining volatile organohalogen compounds in an aqueous matrix, specific properties of these compounds should be taken into consideration, when preparing aqueous standard solutions used for calibration and during model investigations, namely: • low boiling points; • high vapor pressure; and, • relatively low solubility in water. Because of the significance of this problem and the lack of practical information available in the literature, we carried out model investigations of stability of selected volatile analytes in aqueous solutions. The experimental results were used to estimate the time after which the result of determination may not reflect the true concentration of the analyte. The investigations provided information on: • the method of preparation of model solutions (in appropriate containers); • the method of storage and the storage time of these solutions; and, • number of repetitions needed in the final determination (repeated sampling of the solution from a volumetric flask, which requires the flask to be opened for each sampling). This parameter is especially important when determining the repeatability and the reproducibility in validating the procedure for TLHS-DAI-GC-ECD.

Comparative marine toxicity testing: A cold-water species and standard warm-water test species exposed to crude oil and dispersant

There are many methods in current use for testing the toxicity of discharges to marine waters. For practical purposes, standard tests and species are commonly used. None of these standard test procedures or species are done with the cold seawater and species typical of northern latitudes. This paper reports the toxicity testing of oil and dispersed oil to a cold-water species, the Tanner crab ( Chionocetes bairdi) larvae, and compares the result to two standard warm-water test species, the saltwater mysid ( Mysidopsis bahia) and fish ( Menidia beryllina) larvae. The method of reporting the exposure dose: loading rate, volatile organic analytes (VOA, C6–C9), total petroleum hydrocarbons (TPH C10–C36), or their summation, total hydrocarbon concentrations (THC C6–C36) would result in different conclusions. These differences are especially important with the water-accommodated fraction (WAF) in cold water, but may as well be significant when reporting the chemically enhanced-water accommodated fraction (CE-WAF), i.e., dispersed oil. The differences are chiefly due to the greater accommodation of VOA in the colder water.

Screening processed milk for volatile organic compounds using vacuum distillation/gas chromatography/mass spectrometry.

An adaptation is presented of method 8261--from the Office of Solid Waste and Emergency Response Test Methods for Evaluating Solid Waste Physical/Chemical Methods (SW-846)-to analyze milk for an expanded list of volatile organic compounds is presented. The milk matrix exhibits a strong affinity for organic compounds and the surrogate based matrix normalization described in method 8261 provided accurate results. This method had the sensitivity necessary to detect volatile organic analytes at or below maximum contaminant levels (MCLs) set by EPA for drinking water. In a survey of milk samples available in Las Vegas, Nevada, 32 of 88 targeted volatile organic compounds (VOCs) were detected. Many of the detected VOCs have not previously been reported and a rationale for their presence in milk is presented.

Toxicity of dispersed and undispersed, fresh and weathered oil to larvae of a cold-water species, Tanner crab ( C. bairdi), and standard warm-water test species

There are many methods in current use for testing the toxicity of discharges to marine waters. Standard tests and species established by the Environmental Protection Agency and other regulators are commonly used. None of these commonly used test procedures are conducted with cold seawater or species typical of northern latitudes. This paper reports on the toxicity testing of oil and dispersed oil, both fresh and weathered, to larvae of a cold-water species, the Tanner crab ( Chionocetes bairdi), and compares these results to those observed with a standard warm-water test species, the saltwater mysid ( Mysidopsis bahia). The method of reporting the exposure dose is based on: loading rate (LR), concentrations of volatile organic analytes (VOA, C6–C9), total petroleum hydrocarbon (TPH, C10–C36), or on total hydrocarbon content (THC, C6–C36). Different conclusions result, depending on the reporting method. These differences are chiefly due to the greater accommodation of VOA in the colder water and the paucity of TPH in undispersed cold-water solutions.

Traditional sampling with laboratory analysis and solid phase microextraction sampling with field gas chromatography/mass spectrometry by military industrial hygienists.

The opinions or assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views of the United States Department of Defense or the Uniformed Services University of the Health Sciences. Rapid on-site detection and identification of environmental contaminants to which personnel may be exposed is often needed during military deployment situations. The availability of military industrial hygienists with capabilities for "complete" on-site exposure assessment of chemical species should allow detection and identification of a number of important stressors almost immediately following sample collection. Portable gas chromatography/mass spectrometry (GC/MS) provides a rapid and efficient separation of volatile and semivolatile organic analytes, accompanied by sensitive electron impact ionization-mass spectrometry (EI-MS) detection. The use of GC/MS in the field is limited, however, by equipment cost, complexity of the equipment, and the analytical process. Additionally, a skilled operator is needed to obtain useful separations and to interpret mass spectral data. To demonstrate benefits and limitations of "complete" exposure assessment capabilities, a previously unidentified complex mixture, produced by thermal dispersion of riot control agents, was examined. Established active sampling methods were used with laboratory analyses. Solid phase microextraction, a passive sampling method that simplifies preparation for GC/MS analysis, also was used with a field-portable GC/MS system. Both sampling/analysis methods were used to detect CS riot control agent-derived air contaminants dispersed from riot control type canisters through oxidizer-supported combustion of a chemical fuel.

EN CORE® Sampler Performance: Storing Soil for VOC Analysis

The En Core® sampler is designed to collect and store soil samples in a manner that minimizes loss of contaminants due to volatilization and/or biodegradation. Use of this device is described in American Society for Testing and Materials (ASTM) Practice D 6418, Standard Practice for Using the Disposable En Core Sampler for Sampling and Storing Soil for Volatile Organic Analysis. Two studies were performed to evaluate the performance of the En Core sampler to store VOC-contaminated soil for analysis. One study involved spiking soil samples with low concentrations of VOCs (less than 200 µg/kg) and storing them under various conditions prior to analysis. Results from this study show that the En Core sampler gives 80% or greater recoveries of low concentrations of VOCs from soil samples stored under a number of conditions. The second study was performed to generate data on the performance of the 25-g En Core sampler to store soil samples spiked with EPA Method 1311, Toxicity Characteristic Leaching Procedure (TCLP), volatile organic analytes. Results show that the En Core device gives 80% or greater recoveries of most of the Method 1311 volatile organic analytes from soil stored under various conditions for 14 days.

Improved method for the storage of groundwater samples containing volatile organic analytes

The sorption of volatile organic analytes from water samples by the Teflon septum surface used with standard glass 40-ml sample collection vials was investigated. Analytes tested included alkanes, isoalkanes, olefins, cycloalkanes, a cycloalkene, monoaromatics, a polynuclear aromatic, and two chloroethenes. Both laboratory prepared test mix solutions and petroleum contaminated groundwater from three field sites were tested. A rapid loss of n-alkane and isoalkane concentrations (>10%) was observed within 24 h when stored at room temperature. Aliphatic losses were also observed (>10%) over a 21-day holding period when samples were held at 4 degrees C. Loss of the less sorptive analytes was demonstrated by exposing analyte solutions to greater Teflon surface areas. The demonstrated sorption of aliphatics from water samples by Teflon-lined septa indicates that the accuracy of volatile petroleum hydrocarbon determinations may be reduced by the traditional storage method. An alternative storage protocol is reported combining a lead foil septum surface and 1% (w/w) tribasic sodium phosphate dodecahydrate (Na3PO4. 12H2O) preservative. This method prevented loss of the test analytes, including alkanes and isoalkanes for at least 21 days at room temperature.

Use of a presolvent to include volatile organic analytes in the application range of on-line solid-phase extraction–gas chromatography–mass spectrometry

The application range of the on-line solid-phase extraction–gas chromatographic (SPE–GC) analysis of aqueous samples has been extended to volatile analytes. In the new set-up, after conventional aqueous-sample loading and drying of the SPE cartridge with nitrogen gas, 30–50 μl of an organic solvent, the so-called presolvent, such as methyl acetate or ethyl acetate are introduced into the retention gap prior to the actual desorption to ensure that a solvent film is already present in the retention gap when the introduction of the analyte-containing desorption solvent starts. This procedure allows the recovery of analytes as volatile as monochlorobenzene and xylene. Aspects such as the type of retaining precolumn, and the type and amount of presolvent have been studied systematically to explain the performance of the novel set-up. Actually, when using 50 μl of presolvent, the use of a retaining precolumn did not have any significant influence on the recovery of the volatile analytes. The modified SPE–GC procedure was tested by analysing 10 ml of river Rhine water spiked at the 0.5 μg/l level with about 80 microcontaminants covering a wide range of volatility. The test compounds included chlorobenzenes, substituted and nonsubstituted aromatic compounds, anilines and phenolic compounds and organonitrogen and organophosphorus pesticides. The system performance in terms of recovery (typically 70–115% at the 0.5 μg/l level) and repeatability (R.S.D. values typically 1–9%; n=7) was satisfactory, even for monochlorobenzene, the most volatile analyte of the test mixture. Low recoveries due to early breakthrough (polar analytes) or adsorption to the tubing (apolar analytes) were observed for a few analytes only. The detection limits in SPE–GC–MS using full-scan acquisition generally were 20–50 ng/l.

Volatile organic analysis by direct aqueous injection

Gas chromatographic environmental analysis by direct aqueous injection (DAI) was studied for 24 volatile organic analytes (VOAs). Internal standardization was used to determine the precision of analysing these compounds by DAI. Aqueous samples were directly introduced to a gas chromatograph using fused-silica, mega-bore capillary column separation with subsequent full-scan ion trap mass spectral detection. Triplicate injections at seven levels of VOA standard solutions over a 10 3 concentration range were performed using an autosampler set up for on-column injection of 0.2 μl. Comparison of single-ion response curves to triple-ion response curves showed that triple-ion quantitation was more sensitive and precise than single-ion quantitation. Of the 24 VOAs determined at the 20 parts per billion (ppb) level, 19 and 20 were detected by the single-ion calibration and triple-ion calibration, respectively. The weighted and non-weighted regression correlation coefficients, r 2, for the 24 responses curves by the two methods, ranged from 0.910 to 0.998, with 76 of 96 being greater than 0.990. Precision, as measured by per cent relative standard deviation, was shown to be best for later eluting compounds and for higher concentrations. Analysis of an environmental sample by DAI was accomplished in 12 min and indicated the presence of benzene at 80 ppb and chlorobenzene at 2 ppm. This demonstrated the feasibility of applying this technique for screening. Several chlorinated benzenes were also detected, establishing the potential for expanding the method to include higher boiling compounds.