Photoionization Detector Research Articles

Carbon nanotube sponges as an enrichment material for aromatic volatile organic compounds

Sensitive methods are required for in situ monitoring of volatile organic compounds (VOCs). Herein, carbon nanotube (CNT) sponges were investigated as a new type of adsorbent for enriching trace aromatic VOCs. A square pillar configuration (3 mm × 3 mm × 45 mm, 5 mg) of a CNT sponge was enclosed in a glass tube (4 mm i.d.). After accumulating the sample vapor, a direct current pulse (26 V, 0.5–3.0 s) through the CNT sponge allowed narrow desorption bandwidths of 0.48−0.84 s (with a photoionization detector) and 1.2 s (with a flame ionization detector) and high desorption efficiency (>96.5%). Gas chromatographic analysis of a nine-component VOC mixture (100 mL adsorption volume) gave enrichment factors of 88 (benzene) to 323 (toluene and m-xylene) with detection limits in the range of 0.9−2.6 ppb (v/v). These results demonstrate that CNT sponges are a promising preconcentrator material for trace detection of VOCs. The adsorption breakthrough experiments exhibited good correlation with the kinetic adsorption and Langmuir isotherm models. The maximum adsorption capacities of the CNT sponge increased in the order benzene (0.13 mg/g) < toluene (2.45 mg/g) < ethylbenzene (13.90 mg/g) < o-xylene (14.31 mg/g), with R2 values of >0.95. The rollup phenomena observed during multicomponent adsorption were explained by the competitive displacement or adsorption affinities of aromatic VOCs. The feasibility of the CNT sponge preconcentrator in a real environment was tested for interfering species (NO2 and NH3), laboratory air, and a human breath sample and demonstrated similar performance as in the controlled nine-component tests.

Temporal and Spatial Variability of Volatile Organic Compounds in the Forest Atmosphere.

The healing effects of the forest are increasingly being valued for their contribution to human psychological and physiological health, motivating further advances aimed at improving knowledge of relevant forest resources. Biogenic volatile organic compounds, emitted by the plants and accumulating in the forest atmosphere, are essential contributors to the healing effects of the forest, and represent the focus of this study. Using a photoionization detector, we investigated the high frequency variability, in time and space, of the concentration of total volatile organic compounds on a hilly site as well as along forest paths and long hiking trails in the Italian northern Apennines. The scale of concentration variability was found to be comparable to absolute concentration levels within time scales of less than one hour and spatial scales of several hundred meters. During daylight hours, on clear and calm days, the concentration peaked from noon to early afternoon, followed by early morning, with the lowest levels in the late afternoon. These results were related to meteorological variables including the atmospheric vertical stability profile. Moreover, preliminary evidence pointed to higher concentrations of volatile organic compounds in forests dominated by conifer trees in comparison to pure beech forests.

Compact-GC platform: A flexible system integration strategy for a completely microsystems-based gas-chromatograph

The increasing need for portable analysis systems to be used in field sensing applications with the quality and reliability of laboratory analyses, characterized by high sensitivity and specificity, calls for miniaturization and optimization of laboratory analytical techniques, as enabled by the use of micro-electro-mechanical-systems (MEMS) technology. This paper reports on an innovative all-on-MEMS platform for the implementation of miniaturized or portable multi-purpose gas-chromatographic (GC) systems. The use of a directly interconnected MEMS technology for all three main GC components, namely injector, purge&trap pre-concentrator and GC separation column, allows for a very efficient system integration and for a zero dead-volume and chemically inert analytical chain, which can be heated up to 200 °C to avoid sample condensation. In this work, the analytical chain and the integration strategy will be reported, and several applications and analyses will be demonstrated using the compact-GC platform with both photoionization (PID) and thermal conductivity detectors (TCD).

Variability of Total Volatile Organic Compounds (TVOC) in the Indoor Air of Retail Stores.

Volatile organic compounds (VOCs) are released to the indoor air of retail stores from numerous products and activities, but available literature lacks a systematic understanding of the variability of VOC concentrations. In this study, we measured concentrations of total VOCs (TVOC) in 32 retail stores using a high-sensitivity photoionization detector (PID). Indoor thermal comfort parameters, including temperature, relative humidity, and air velocity, were simultaneously measured using an anemometer. The store-level TVOC concentrations ranged from 30 to 869 ppb and exceeded the LEED guideline in 31 stores. TVOC levels were notably high in hardware stores (median = 536 ppb, p = 0.0002) and paints, household, and home accessories sections within stores (p < 0.05). TVOC levels were elevated in mornings and evenings, possibly due to low ventilation and cleaning activities at the beginning and end of business hours. The between-store, within-store, and temporal variations accounted for 85%, 0.5%, and 14% of the total variance, respectively. The variance structure suggested that in-store VOC concentrations were predominantly driven by their source location, and representative monitoring should first consider covering various store types. Current store VOC levels present health concerns, but further studies are needed to evaluate risks among customers.

VOC emissions from asphalt pavement and health risks to construction workers

Abstract Hot Mix Asphalt (HMA) is used in the construction of highway roads, parking lots and other pavement repairs in the US and worldwide. During asphalt pavement installation, a large amount of asphalt fume containing volatile organic compounds (VOCs) is emitted, causing potential health risks to construction workers. The field data investigation in this paper reports the concentration of VOCs around the workers on site using the Photo Ionization Detection (PID) device. Additionally, this paper presents a health risk evaluation model based on the Monte Carlo simulation to assess the carcinogenic and non-carcinogenic risks of workers during pavement construction. More specifically, distribution and sensitivity analyses illustrate the factors that pose the greatest health risks caused by certain VOCs. The study calls for better health risk controls by targeting the emission sources, propagating pathways, and individual receptors of the VOCs to protect workers’ health during pavement construction. This paper contributes to the knowledge of VOCs generated from HMA and the potential health risks to construction workers, as well as suggests the implementation of new requirements for pavement construction codes and safety regulations.

Spatio-temporal variations of indoor air quality in a university library

ABSTRACT The present study aims to assess the air quality status in the central library of Indian Institute of Technology Roorkee, India. Pollutants concentrations (i.e. PM10, PM2.5, PM1 and TVOC) and comfort parameters i.e. CO2, temperature and relative humidity were monitored across all floors of the library. Air quality was found to vary significantly (P < 0.05) among the different floors of the library. The average concentration of PM10, PM2.5 and PM1 was found to be highest at the first floor. On the other hand, the highest concentration of TVOC (51.7 ± 30 ppb) and CO2 (838.4 ± 99 ppm) was observed at the ground floor. Pollutant concentration was higher in the morning hours. The indoor pollutants were found positively correlated with each other except relative humidity. Indoor to outdoor ratio for PM1, TVOC and CO2 was found to be greater than 1, which indicate a substantial contribution from indoor sources. Exceedance of WHO guidelines was observed for the daily average PM2.5 concentration. Abbreviation: IAQ: indoor air quality; ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers; WHO: World Health Organization; PM: particulate matter; VOC: volatile organic carbon; CO2: carbon dioxide; TVOC: Total volatile organic compound; RH: relative humidity; HVAC: heating ventilation and air-conditioning; PID: Photo Ionization Detector; PTFE: Polytetrafluoroethylene; NDIR: Non-dispersive infra-red.

Fast Measurements with MOX Sensors: A Least-Squares Approach to Blind Deconvolution.

Metal oxide (MOX) sensors are widely used for chemical sensing due to their low cost, miniaturization, low power consumption and durability. Yet, getting instantaneous measurements of fluctuating gas concentration in turbulent plumes is not possible due to their slow response time. In this paper, we show that the slow response of MOX sensors can be compensated by deconvolution, provided that an invertible, parametrized, sensor model is available. We consider a nonlinear, first-order dynamic model that is mathematically tractable for MOX identification and deconvolution. By transforming the sensor signal in the log-domain, the system becomes linear in the parameters and these can be estimated by the least-squares techniques. Moreover, we use the MOX diversity in a sensor array to avoid training with a supervised signal. The information provided by two (or more) sensors, exposed to the same flow but responding with different dynamics, is exploited to recover the ground truth signal (gas input). This approach is known as blind deconvolution. We demonstrate its efficiency on MOX sensors recorded in turbulent plumes. The reconstructed signal is similar to the one obtained with a fast photo-ionization detector (PID). The technique is thus relevant to track a fast-changing gas concentration with MOX sensors, resulting in a compensated response time comparable to that of a PID.

Peaks, Means, and Determinants of Real-Time TVOC Exposures Associated with Cleaning and Disinfecting Tasks in Healthcare Settings.

Cleaning and disinfecting tasks and product use are associated with elevated prevalence of asthma and respiratory symptoms among healthcare workers; however, the levels of exposure that pose a health risk remain unclear. The objective of this study was to estimate the peak, average, and determinants of real-time total volatile organic compound (TVOC) exposure associated with cleaning tasks and product-use. TVOC exposures were measured using monitors equipped with a photoionization detector (PID). A simple correction factor was applied to the real-time measurements, calculated as a ratio of the full-shift average TVOC concentrations from a time-integrated canister and the PID sample, for each sample pair. During sampling, auxiliary information, e.g. tasks, products used, engineering controls, was recorded on standardized data collection forms at 5-min intervals. Five-minute averaged air measurements (n = 10 276) from 129 time-series comprising 92 workers and four hospitals were used to model the determinants of exposures. The statistical model simultaneously accounted for censored data and non-stationary autocorrelation and was fit using Markov-Chain Monte Carlo within a Bayesian context. Log-transformed corrected concentrations (cTVOC) were modeled, with the fixed-effects of tasks and covariates, that were systematically gathered during sampling, and random effect of person-day. The model-predicted geometric mean (GM) cTVOC concentrations ranged from 387 parts per billion (ppb) for the task of using a product containing formaldehyde in laboratories to 2091 ppb for the task of using skin wipes containing quaternary ammonium compounds, with a GM of 925 ppb when no products were used. Peak exposures quantified as the 95th percentile of 15-min averages for these tasks ranged from 3172 to 17 360 ppb. Peak and GM task exposures varied by occupation and hospital unit. In the multiple regression model, use of sprays was associated with increasing exposures, while presence of local exhaust ventilation, large room volume, and automatic sterilizer use were associated with decreasing exposures. A detailed understanding of factors affecting TVOC exposure can inform targeted interventions to reduce exposures and can be used in epidemiologic studies as metrics of short-duration peak exposures.

Micro Milled Microfluidic Photoionization Detector for Volatile Organic Compounds.

Government regulations and environmental conditions are pushing the development of improved miniaturized gas analyzers for volatile organic compounds. One of the many detectors used for gas analysis is the photoionization detector (PID). This paper presents the design and characterization of a microfluidic photoionization detector (or µPID) fabricated using micro milling and electrical discharge machining techniques. This device has no glue and facilitates easy replacement of components. Two materials and fabrication techniques are proposed to produce a layer on the electrodes to protect from ultraviolet (UV) light and possible signal noise generation. Three different microchannels are tested experimentally and their results are compared. The channel with highest electrode area (31.17 mm²) and higher volume (6.47 µL) produces the highest raw signal and the corresponding estimated detection limit is 0.6 ppm for toluene without any amplification unit.

Autonomous multi-species environmental gas sensing using drone-based Fourier-transform infrared spectroscopy.

Unmanned aerial vehicles (UAVs)-or drones-present compelling new opportunities for airborne gas sensing in applications such as environmental monitoring, hazardous scene assessment, and facilities' inspection. Instrumenting a UAV for this purpose encounters trade-offs between sensor size, weight, power, and performance, which drives the adoption of lightweight electrochemical and photo-ionisation detectors. However, this occurs at the expense of speed, selectivity, sensitivity, accuracy, resolution, and traceability. Here, we report on the design and integration of a broadband Fourier-transform infrared spectrometer with an autonomous UAV, providing ro-vibrational spectroscopy throughout the molecular fingerprint region from 3 - 11 µm (3333 - 909 cm-1) and enabling rapid, quantitative aerial surveys of multiple species simultaneously with an estimated noise-limited performance of 18 ppm (propane). Bayesian interpolation of the acquired gas concentrations is shown to provide both localization of a point source with approximately one meter accuracy, and distribution mapping of a gas cloud, with accompanying uncertainty quantification.

The interference of tetrachloromethane in the measurement of benzene in the air by a gas chromatography–photoionisation detector (GC-PID)

Abstract. The European Union requires that benzene in the air is continuously measured due to its toxicity and widespread presence in the population nuclei, mainly motivated by vehicle emissions. The reference measuring technique is gas chromatography (GC). Automatic chromatographs used in monitoring stations must verify the operating conditions established in Standard EN 14662 part 3, which includes a type approval section with a number of tests that analysers must pass. Among these tests, the potential interference of a number of compounds is evaluated. The 2005 version of the mentioned standard requires the evaluation of the potential interference of tetrachloromethane (TCM). The 2015 version eliminates TCM as a potential interferent. Although most consumer uses of TCM have been banned, recent studies have measured significant concentrations of TCM in the air. In this paper, the potential interference of TCM in benzene measurements obtained with gas chromatography coupled to a photoionisation detector (GC-PID) has been investigated. Our study shows that the simultaneous presence of benzene and TCM causes a significant decrease in benzene readings. For TCM concentrations of 0.7 µg m−3 (typical of urban areas) and 4.5 µg m−3 (detected in the vicinity of landfills), the relative errors in benzene measurements were 34 % and 70 %, respectively, which are far too high compared to the maximum overall uncertainty allowed for benzene measurements (25 %). Possible mechanisms to qualitatively and quantitatively explain the behaviour of the PID when measuring benzene with and without TCM have been proposed.

Sub-ppb Level Detection of BTEX Gaseous Mixtures with a Compact Prototype GC Equipped with a Preconcentration Unit.

In this work, a compact gas chromatograph prototype for near real-time benzene, toluene, ethylbenzene and xylenes (BTEX) detection at sub-ppb levels has been developed. The system is composed of an aluminium preconcentrator (PC) filled with Basolite C300, a 20 m long Rxi-624 capillary column and a photoionization detector. The performance of the device has been evaluated in terms of adsorption capacity, linearity and sensitivity. Initially, PC breakthrough time for an equimolar 1 ppm BTEX mixture has been determined showing a remarkable capacity of the adsorbent to quantitatively trap BTEX even at high concentrations. Then, a highly linear relationship between sample volume and peak area has been obtained for all compounds by injecting 100-ppb samples with volumes ranging from 5–80 mL. Linear plots were also observed when calibration was conducted in the range 0–100 ppb using a 20 mL sampling volume implying a total analysis time of 19 min. Corresponding detection limits of 0.20, 0.26, 0.49, 0.80 and 1.70 ppb have been determined for benzene, toluene, ethylbenzene, m/p-xylenes and o-xylene, respectively. These experimental results highlight the potential applications of our device to monitor indoor or outdoor air quality.

Sensitivity enhancement in photoionization detector using microelectrodes with integrated 1D nanostructures

Photoionization detectors (PIDs) that use high-energy photons to ionize gas molecules have attracted considerable attention as volatile organic compound (VOC) sensors owing to their high sensitivity, selectivity, and reliability. Recently, miniaturized PIDs have been developed to further improve the sensing performance. However, most studies have focused solely on the miniaturization of the ionization chamber, although the electrode is an important factor for determining the sensitivity and operation voltage. This work demonstrates a PID composed of Si microelectrodes fabricated via microfabrication with one-dimensional (1D) nanostructures coated on them. The interdigitated microelectrodes offer a larger surface area capable of enhanced capturing of positively charged ions and electrons compared to typical mm-scale electrodes, resulting in the successful detection of 500 ppb toluene at an applied voltage of 0.2 V. Furthermore, taking advantage of the outstanding surface-to-volume ratio of 1D nanostructures, Ag nanowire-coated microelectrodes and carbon nanotube (CNT)-coated microelectrodes exhibit 38% and 76% improvement in sensitivity, respectively, than the bare Si microelectrodes. The CNT-coated microelectrodes show superior linearity and repeatability at toluene concentrations from 0.1 to 1.0 ppm. The proposed PID will be a promising candidate for portable VOC sensors.

Considerations for emission monitoring and liner analysis of thermally manufactured sewer cured-in-place-pipes (CIPP).

Cured-in-place-pipes (CIPP) are plastic liners chemically manufactured inside existing damaged sewer pipes. They are gaining popularity in North America, Africa, Asia, Europe, and Oceania. Volatile and semi-volatile organic compound (VOC/SVOC) emissions from storm sewer CIPP installations were investigated at a dedicated outdoor research site. Tedlar bag, sorbent tube, and photoionization detector (PID) air sampling was conducted for five steam-CIPP installations and was coupled with composite characterizations. New CIPPs contained up to 2.21 wt% volatile material and only 6-31% chemical mass extracted per CIPP was identified. Each 6.1 m [20 ft] liner contained an estimated 5-10 kg [11-22 lbs] of residual chemical. Extracted chemicals included hazardous air pollutants and suspected and known carcinogens that were not reported by others. These included monomers, monomer oxidation products, antioxidants, initiator degradation products, and a plasticizer. PID signals did not accurately represent styrene air concentration differing sometimes by 10s- to 1000s-fold. Multiple VOCs found in air samples likely affected PID responses. Styrene (>86.4 ppmv) and methylene chloride (>1.56 ppmv) air concentrations were likely greater onsite and phenol was also detected. Additional studies are needed to examine pollutant emissions so process monitoring can be improved, and environment impacts and associated human exposures can be minimized.

Low-cost photoionization sensors as detectors in GC × GC systems designed for ambient VOC measurements

Conventional volatile organic compound (VOC) monitoring based on thermal desorption - gas chromatography–mass spectrometry (TD-GC–MS) or gas chromatography-flame ionization detector (TD-GC-FID) is relatively cumbersome and expensive. In this study commercial off the shelf low-cost and low-power photo-ionization detector (PID) sensors are used as simple detectors in VOC analysis systems based on GC, including a miniaturised GC × GC device with portable, low-cost, and low-energy-consumption features. PID sensors produce a voltage signal positively proportional to VOC concentration, which when incorporated into a TD-GC system gave limit of detection of 0.02 ppbV for isoprene. To test PID performance in real-world applications, PID sensors were deployed as (i) a second alternative detector in a GC-Quadruple Time Of Flight Mass spectrometry (GC-Q-TOF-MS), and (ii) the main detector in a compact two-dimensional gas chromatograph (GC × GC). PID sensors with 10.6 eV and 11.7 eV lamps were used to measure eight toxic chemicals including organic sulfide and organic phosphonates via GC; two species were ionized by a 10.6 eV lamp and four species by the 11.7 eV lamp. Commercially available low-cost PIDs designed for standalone could be straightforwardly and effectively re-used as detectors in compact GC × GC systems, in this work showing excellent VOC sensitivity, fast response and low operational demands compared to comparable field instruments based on GC-FID or MS.

Compact prototype GC-PID system integrated with micro PC and micro GC column

In this work, a portable gas chromatography (GC) photo ionization detector (PID) system integrated with a micro column and a micro pre-concentrator (PC) was developed for GC application. The micro pre-concentrator filled with single walled-carbon-nanotubes as adsorption material was able to concentrate trace gas. Different from conventional GC column, the micro GC column was able to completely separate the gas mixtures, dramatically reducing volume of GC system. The experimental results indicate that the microfabricated compact prototype GC PID system demonstrated high repeatability of 0.42% and low detection limit of less than 1 ppb to rapidly detect volatile organic compounds (VOCs).

Assessment of a passive sampling method and two on-line gas chromatographs for the measurement of benzene, toluene, ethylbenzene and xylenes in ambient air at a highway site

Accurate, continuous long-term measurements of aromatic volatile organic compounds with known health impacts are needed for comprehensive assessments of air quality in the vicinity of industrial and vehicular emission sources. Historically, evacuated canister sampling and on-line gas chromatography (GC) have been the most popular approaches applied by government institutions in North America to measure benzene, toluene, ethylbenzene and xylenes (BTEX) in ambient air. Canister sampling is labour-intensive and typically does not provide the continuous data needed for epidemiological studies, while on-line GCs are costly and require shelter and power. In this work, the accuracy and suitability of three techniques for generating long-term ambient BTEX datasets are assessed. The first is a passive sampling method involving sorbent-packed thermal desorption tubes, the second is an on-line miniature gas chromatograph (miniGC) coupled with a photoionization detector (PID) and the third is a more traditional on-line gas chromatograph coupled with a flame ionization detector (GC-FID). These three techniques were deployed at an air quality monitoring station located adjacent to a busy highway for six weeks in summer 2018 to compare method performance and accuracy. Ambient mean concentrations of BTEX determined from the three methods for the entire study period were found to agree within 7% for benzene and within 30% for the other species. Interestingly, a temperature dependent positive bias previously identified for 14-day passive tube sampling for ethylbenzene and xylenes under cold wintertime conditions was found to be negligible under summertime conditions.

Compact, Automated, Inexpensive, and Field-Deployable Vacuum-Outlet Gas Chromatograph for Trace-Concentration Gas-Phase Organic Compounds.

The identification and quantification of gas-phase organic compounds, such as volatile organic compounds (VOCs), frequently use gas chromatography (GC), which typically requires high-purity compressed gases. We have developed a new instrument for trace-concentration measurements of VOCs and intermediate-volatility compounds of up to 14 carbon atoms in a fully automated (computer-free), independent, low-cost, compact GC-based system for the quantitative analysis of complex mixtures without the need for compressed, high-purity gases or expensive detectors. Through adsorptive analyte preconcentration, vacuum GC, photoionization detectors, and need-based water-vapor control, we enable sensitive and selective measurements with picogram-level limits of detection (i.e., under 15 ppt in a 4 L sample for most compounds). We validate performance against a commercial pressurized GC, including resolving challenging isomers of similar volatility, such as ethylbenzene and m/ p-xylene. We employ vacuum GC across the whole column with filtered air as a carrier gas, producing long-term system stability and performance over a wide range of analytes. Through theory and experiments, we present variations in analyte diffusivities in the mobile phase, analyte elution temperatures, optimal linear velocities, and separation-plate heights with vacuum GC in air at different pressures, and we optimize our instrument to exploit these differences. At 2-6 psia, the molecular diffusion coefficients are 6.4-2.1 times larger and the elution temperatures are 39-92 °C lower than with pressurized GC with helium (at 30 psig) depending on the molecular structure, and we find a wide range of optimal linear velocities (up to 60 cms-1) that are faster with broader tolerances than with pressurized-N2 GC.

Real time monitoring of soil contamination with diesel fuel using photoionization detectors

Soil contamination is a very relevant environmental problem, which is directly connected with both groundwater and air contamination. Petroleum products represent often a major source of soil contamination. In this sense, leakage of diesel fuel can cause important environmental problems due to the persistence of it in contaminated soils and to its toxicity at low concentrations. Photoionization detectors (PID) have the ability to evaluate the ampleness of contamination by detection and quantification in real time of diesel fuel vapours. Finally, this achievement can lead to a rapid evaluation (in minutes) of soil contamination and will facilitate the application of most suitable remediation technologies. The purpose of this study was to simulate an accidental spill of diesel on the ground, with the aim to monitor the degree of volatilization of chemical compounds during six weeks. Thus, the reasons behind the low volatility of diesel fuel in the investigated soil samples, based on the trend of volatilization and on soil characteristics as well.

Correlations between on-site screening methods and laboratory analyses of oil-contaminated fuelling station sites

In order to assess the potential of soil contamination from petroleum-based products and determine the soil samples required for laboratory analysis, investigators generally use portable instruments such as photo ionisation detectors (PIDs) or flame ionisation detectors (FIDs) for on-site screening. In this study, PIDs, FIDs, and turbidimetric test kits were integratively used to screen 47 soil samples from 28 fuelling station sites suspected of total petroleum hydrocarbon (TPH) contamination. The PIDs and FIDs achieved high accuracy in screening sites contaminated by gasoline (TPHC6-C9); the test kits yielded high accuracy in screening those contaminated by diesel fuel (TPHC10-C40), as well as high correlation with laboratory analysis. Regression analyses showed high coefficients of determination (R2), reaching up to 0.819 (PIDs versus TPHC6-C9), 0.788 (FIDs versus TPHC6-C9), and 0.653 (test kits versus TPHC10-C40). The integration of PIDs, FIDs, and test kits with laboratory analysis can significantly increase the efficiency and accuracy of soil contamination investigations. We suggest that the application of turbidimetric test kits to screen potential oil-contaminated fuelling station sites can produce accurate assessment results.