Characterization of Photothermal Desorption-Compatible Diffusive Samplers for Volatile Organic Compounds

Products and starting materials containing volatile organic compounds (VOCs) can easily be found in a variety of businesses, making them a common source of occupational exposure. To prevent negative impacts on employee health, field industrial hygienists must conduct regular sampling to ensure exposures remain below the regulatory limits set by governmental and professional associations. As such, the need for sensitive and reliable exposure assessment techniques becomes evident. Over the preceding decade, the industrial hygiene research group at the University of Alabama at Birmingham (UAB) has been working on the development of an emerging, preanalytical technique known as photothermal desorption (PTD) to improve upon the analytical sensitivity of currently employed methods. PTD's novel design uses pulses of high-energy light to desorb analytes from thermally conductive, carbonaceous sorbents, to be delivered to downstream analytical detectors. Since PTD's conception, the theoretical framework and advances in sorbent fabrication have been investigated; however, further work is needed to produce a field-ready sampling device for use with PTD. As such, objectives of the present work were to design a PTD-compatible diffusive sampler prototype and characterize the prototype's sampling efficiencies for toluene, n-hexane, trichloroethylene, and isopropyl alcohol. In pursuit of these objectives, the study empirically quantified the sampled masses of toluene, n-hexane, trichloroethylene, and isopropyl alcohol, at occupationally relevant air concentrations, to be 12.17 ± 0.06, 8.2 ± 0.1, 3.97 ± 0.06, and 8.0 ± 0.1 mg, respectively. Moreover, the analyte sampling efficiencies were found to be 2.2 ± 0.1, 1.7 ± 0.1, 1.2 ± 0.1, and 0.51 ± 0.05 (unitless) when comparing empirically (i.e., laboratory observed) sample mass values to theoretically predicted values. The sampling efficiencies and collected sample masses reported herein demonstrate the promising design of PTD-compatible diffusive samplers. When used in conjunction with the PTD method, the prototype samplers present strong evidence for improving analytical sensitivity in exposure assessments of VOCs in the workplace.

Determination of activated carbon fiber adsorption capacity for several common organic vapors: applications for respiratory protection

In the context of workplace safety, activated carbon in the fiber form (i.e., activated carbon fiber, ACF) represents an alternative adsorbent to granular activated carbon (GAC) for use in organic vapor respiratory protection devices. ACFs are high surface area carbonaceous materials that are often available in a self-supporting non-woven form. The physical form of ACF suggests the potential for a filtration medium that is capable of supporting both organic vapor adsorption and particulate filtration. To study the application of these materials in respiratory protection devices, ACFs (ACFF 1200 m²/g, ACFF 1800 m²/g, and ACFF 2000 m²/g) were challenged with representative organic vapors (toluene, hexane, and methyl ethyl ketone (MEK)) at an occupationally relevant concentration (200 ppm). Breakthrough curves were generated for at least three different bed weights of adsorbent. Pressure drop (i.e., the resistance across the filtration media) was also measured to determine maximum ACF bed depths for use in respiratory protection devices. Breakthrough experiments indicate that ACFF 2000 has the highest adsorption capacity for toluene (381 mg/g), followed by ACFF 1800 and ACFF 1200 (344 mg/g and 239 mg/g, respectively). A similar trend was observed for hexane: 221 mg/g, 196 mg/g, and 146 mg/g for ACFF 2000, ACFF 1800, and ACFF 1200, respectively. ACFF 1200 showed the highest adsorption capacity for the polar adsorbate MEK (168 mg/g), followed by ACFF 1800 and ACFF 2000 (166 mg/g and 147 mg/g, respectively). Based on the constraints of pressure drop, it seems unlikely the exclusive use of ACF in a filtering facepiece respirator can provide an adsorbent mass sufficient for full shift protection against organic vapor contaminants at or above the legally enforceable permissible exposure level (PEL). Nevertheless, the incorporation of ACF into a facepiece respirator appears promising for "nuisance odor" applications; i.e., the further reduction of organic vapor concentrations when workplace exposures are already below PEL concentrations.Implications: This research brings innovation to the field of occupational health and air pollution control technology by investigating the adsorption performance of activated carbon fiber (ACF) media in the context of worker respiratory protection. ACF properties such as high specific surface area (m²/g), high permeability to airflow, and rapid adsorption kinetics make it ideal for use in thin, N95-style respirators for organic vapors. Respiratory protection is an exciting and relevant application for ACF media. A lightweight adsorbent such as ACF, if incorporated into an N95-style respirator, could potentially provide nuisance-level VOC protection in a physical form that is accessible to workers and consistent with OSHA's voluntary use provisions for facepiece respirators. The research presented in this manuscript represents one of several steps planned in the characterization of ACF media for this particular application.

Evaluating the Effects of Modified Windscreens on Organic Vapor Monitor Performance

Passive sampling using diffusive samplers has become popular as a convenient means of occupational compliance sampling for volatile organic compounds (VOCs). However, diffusive samplers possess sensitivity limitations when sampling low concentrations and for short durations. To reduce these limitations, our research team has been developing a novel method of sample recovery called photothermal desorption (PTD), which uses high energy visible light pulses to desorb analytes from sampling media. Newly designed passive samplers that will use PTD will be equipped with windscreens in a similar design with the 3M OVM. In a preliminary design effort, the present work sought to find a suitable, windscreen for future use in a PTD-compatible diffusive sampler prototype that would be similar to those found in commercially available diffusive samplers. To do so, 2 stainless steel windscreens (wire diameters 0.015″ and 0.0055″ respectively) were compared to a standard windscreen by exposing modified (ie, steel mesh installed) and non-modified 3M OVM samplers to 3 analytes. To mimic in-field conditions, each sampler was exposed to analyte concentrations at their short-term and personal exposure limits (STELs and PELs). From these comparisons, it was determined that the 0.0055″ mesh was most similar to the standard windscreen in contributing to sample collection based on the uptake and concentration determinations for each analyte and concentration.

Determining the Thermal Properties of Buckypapers Used in Photothermal Desorption

Volatile organic compounds (VOCs) pose an occupational exposure risk due to their commonplace usage across industrial and vocational sectors. With millions of workers annually exposed, monitoring personal VOC exposures becomes an important task. As such, there is a need to improve current monitoring techniques by increasing sensitivity and reducing analysis costs. Recently, our lab developed a novel, preanalytical technique known as photothermal desorption (PTD). PTD uses pulses of high-energy, visible light to thermally desorb analytes from carbonaceous sorbents, with single-walled carbon nanotube buckypapers (BPs) having the best overall performance. To apply this new technology most effectively for chemical analysis, a better understanding of the theoretical framework of the thermal phenomena behind PTD must be gained. The objectives of the present work were 3-fold: measure the thermal response of BPs during irradiation with light; determine the best method for conducting such measurements; and determine the thermal conductivity of BPs. BPs were exposed to four energy densities, produced by light pulses, ranging from 0.28 to 1.33 J/cm2, produced by a xenon flash lamp. The resulting temperature measurements were obtained via fast response thermocouple (T/C) mounted to BPs by three techniques (pressing, adhering, and embedding). Temperature increase measured by T/C using the adhering and pressing techniques resulted in similar values, 29.2 ± 0.8 to 56 ± 3 °C and 29.1 ± 0.9 to 50 ± 5 °C, respectively, while temperature increase measured by embedding the T/C into the BP showed statistically larger increases ranging from 35.2 ± 0.9 to 76 ± 4 °C. Peak BP temperatures for each mounting technique were also compared with the temperatures generated by the light source, which resulted in embedded BPs demonstrating the most temperature conversion among the techniques (74-86%). Based on these results, embedding T/Cs into the BP was concluded to be the best way to measure BP thermal response during PTD. Additionally, the present work modeled BP thermal conductivity using a steady-state comparative technique and found the material's conductivity to be 10.6 ± 0.6 W/m2. The present work's findings will help pave the way for future developments of the PTD method by allowing calculation of the energy density necessary to attain a desired sorbent temperature and providing a means for comparing BP fabrication techniques and evaluating BP suitability for PTD before conducting PTD trials with analytes of interest. Sorbents with greater thermal conductivity are expected to desorb more evenly and withstand higher energy density exposures.

Laboratory Estimation of Occupational Exposures to Volatile Organic Compounds During Nail Polish Application

In the United States, there are more than 120,000 nail salons in which workers could be potentially exposed to a number of volatile organic compounds (VOCs) used in various procedures. Measuring workers exposure in the field is time-consuming and could be very expensive. The purpose of this study was to estimate the VOC levels in the proximity of workers in nail salons through simulating the application process of some popular nail polishes in a laboratory chamber. The worst-case scenario was defined as a worker's exposure during nail polish application to one set of fingernails every 15 minutes for an 8-hour shift (total nail sets = 32). Nail polish was applied on paper plates in a flow-controlled test chamber. Air was sampled during the application of five different nail polishes for 8 hours using passive air samplers and the experiment was triplicated. Passive samplers were used for VOCs and formaldehyde. In this worst-case scenario setting, a total of 17 VOCs were detected, with eight that were found in all the samples. The mean concentration of butyl acetate (161-330 ppm, parts per million) and ethyl acetate (440 ppm) exceeded the threshold limit value (TLV) of 150 ppm and 400 ppm, respectively. Formaldehyde was analyzed separately and the mean concentrations exceeded the TLV of 0.10 ppm in all types of nail polish, ranging from 0.12 ppm to 0.22 ppm. Occupational safety and health professionals could use these data to increase awareness of workers' potential exposure to high levels of VOCs in nail salons and recommend practical measures to reduce potential exposures.

Preparation and characterization of flax, hemp and sisal fiber-derived mesoporous activated carbon adsorbents

The first aim of this study was to investigate mesoporous activated carbon adsorbents from sisal, hemp, and flax fibers by cost-effective methods. Fibers were impregnated with low concentration (20 wt.%) phosphoric acid. Carbonization temperatures were defined by thermal analysis. Bast fibers (hemp, flax) decompose at lower temperatures (419.36℃, 434.96℃) than leaf fibers (sisal, 512.92℃). The second aim was to compare bast and leaf fibers-derived activated carbon adsorbents by determining physical adsorption properties, chemical compositions, scanning electron microscope, and Fourier transform infrared spectroscopy. Results showed that natural fibers have good candidates to prepare mesoporous activated carbon adsorbents with high surface area (1186–1359 m2/g), high mesopore percentage (60–72%), and high C content (80–86%). Even though leaf-derived activated carbon developed more mesoporous structure (72%), bast-derived activated carbons provided higher surface areas (Shemp = 1359 m2/g; Sflax = 1257 m2/g) and C content. Fourier transform infrared spectra for bast fibers-derived activated carbon adsorbents were quite similar while leaf fiber-derived activated carbon adsorbent had a different spectrum.

Determination of pressure drop across activated carbon fiber respirator cartridges

Activated carbon fiber (ACF) is considered as an alternative adsorbent to granular activated carbon (GAC) for the development of thinner, lighter, and efficient respirators because of their larger surface area and adsorption capacities, thinner critical bed depth, lighter weight, and fabric form. This study aims to measure the pressure drop across different types of commercially available ACFs in respirator cartridges to determine the ACF composition and density that will result in acceptably breathable respirators. Seven ACF types in cloth (ACFC) and felt (ACFF) forms were tested. ACFs in cartridges were challenged with pre-conditioned constant air flow (43 LPM, 23°C, 50% RH) at different compositions (single- or combination-ACF type) in a test chamber. Pressure drop across ACF cartridges were obtained using a micromanometer, and compared among different cartridge configurations, to those of the GAC cartridge, and to the NIOSH breathing resistance requirements for respirator cartridges. Single-ACF type cartridges filled with any ACFF had pressure drop measurements (23.71-39.93 mmH2O) within the NIOSH inhalation resistance requirement of 40 mmH2O, while those of the ACFC cartridges (85.47±3.67 mmH2O) exceeded twice the limit due possibly to the denser weaving of ACFC fibers. All single ACFF-type cartridges had higher pressure drop compared to the GAC cartridge (23.13±1.14 mmH2O). Certain ACF combinations (2 ACFF or ACFC/ACFF types) resulted to pressure drop (26.39-32.81 mmH2O) below the NIOSH limit. All single-ACFF type and all combination-ACF type cartridges with acceptable pressure drop had much lower adsorbent weights than GAC (≤15.2% of GAC weight), showing potential for light-weight respirator cartridges. 100% ACFC in cartridges may result to respirators with high breathing resistance and, thus, is not recommended. The more dense ACFF and ACFC types may still be possibly used in respirators by combining them with less dense ACFF materials and/or by reducing cartridge bed depth to reduce pressure drop to acceptable levels. ACFF by itself may be more appropriate as adsorbent materials in ACF respirator cartridges in terms of acceptable breathing resistance.

Breakthrough curves for toluene adsorption on different types of activated carbon fibers: application in respiratory protection

Activated carbon fibers (ACF) are considered viable alternative adsorbent materials in respirators because of their larger surface area, lighter weight, and fabric form. The purpose of this study was to characterize the breakthrough curves of toluene for different types of commercially available ACFs to understand their potential service lives in respirators. Two forms of ACF, cloth (AC) and felt (AF), with three surface areas each were tested. ACFs were challenged with six toluene concentrations (50-500 p.p.m.) at constant air temperature (23°C), relative humidity (50%), and air flow (16 l min-1) at different bed depths. Breakthrough data were obtained using continuous monitoring by gas chromatography using a gas sampling valve. The ACF specific surface areas were measured by an automatic physisorption analyzer. Results showed unique shapes of breakthrough curves for each ACF form: AC demonstrated a gradual increase in breakthrough concentration, whereas AF showed abrupt increase in concentration from the breakpoint, which was attributed to the difference in fiber density between the forms. AF has steeper breakthrough curves compared with AC with similar specific surface area. AC exhibits higher 10% breakthrough times for a given bed depth due to higher mass per bed depth compared with AF, indicating more adsorption per bed depth with AC. ACF in respirators may be appropriate for use as protection in environments with toluene concentration at the Occupational Safety and Health Administration Permissible Exposure Limit, or during emergency escape for higher toluene concentrations. ACF has shown great potential for application in respiratory protection against toluene and in the development of thinner, lighter, and more efficient respirators.

Photothermal desorption of single-walled carbon nanotubes and coconut shell-activated carbons using a continuous light source for application in air sampling

Many techniques exist to measure airborne volatile organic compounds (VOCs), each with differing advantages; sorbent sampling is compact, versatile, has good sample stability, and is the preferred technique for collecting VOCs for hygienists. Development of a desorption technique that allows multiple analyses per sample (similar to chemical desorption) with enhanced sensitivity (similar to thermal desorption) would be helpful to field hygienists. In this study, activated carbon (AC) and single-walled carbon nanotubes (SWNT) were preloaded with toluene vapor and partially desorbed with light using a common 12-V DC, 50-W incandescent/halogen lamp. A series of experimental chamber configurations were explored starting with a 500-ml chamber under static conditions, then with low ventilation and high ventilation, finally a 75-ml high ventilation chamber was evaluated. When preloaded with toluene and irradiated at the highest lamp setting for 4min, AC desorbed 13.9, 18.5, 23.8, and 45.9% of the loaded VOC mass, in each chamber configuration, respectively; SWNT desorbed 25.2, 24.3, 37.4, and 70.5% of the loaded VOC mass, respectively. SWNT desorption was significantly greater than AC in all test conditions (P = 0.02-<0.0001) demonstrating a substantial difference in sorbent performance. When loaded with 0.435mg toluene and desorbed at the highest lamp setting for 4min in the final chamber design, the mean desorption for AC was 45.8% (39.7, 52.0) and SWNT was 72.6% (68.8, 76.4) (mean represented in terms of 95% confidence interval). All desorption measurements were obtained using a field grade photoionization detector; this demonstrates the potential of using this technique to perform infield prescreening of VOC samples for immediate exposure feedback and in the analytical lab to introduce sample to a gas chromatograph for detailed analysis of the sample.

Adsorption characteristics of activated carbon fibers (ACFs) for toluene: application in respiratory protection

Granular activated carbon (GAC) is currently the standard adsorbent in respirators against several gases and vapors because of its efficiency, low cost, and available technology. However, a drawback of GAC due to its granular form is its need for containment, adding weight and bulkiness to respirators. This makes respirators uncomfortable to wear, resulting in poor compliance in their use. Activated carbon fibers (ACF) are considered viable alternative adsorbent materials for developing thinner, light-weight, and efficient respirators because of their larger surface area, lighter weight, and fabric form. This study aims to determine the critical bed depth and adsorption capacity of different types of commercially available ACFs for toluene to understand how thin a respirator can be and the service life of the adsorbents, respectively. ACF in cloth (ACFC) and felt (ACFF) forms with three different surface areas per form were tested. Each ACF type was challenged with six concentrations of toluene (50, 100, 200, 300, 400, 500 ppm) at constant air temperature (23°C), relative humidity (50%), and airflow (16 LPM) at different adsorbent weights and bed depths. Breakthrough data were obtained for each adsorbent using gas chromatography with flame ionization detector. The ACFs' surface areas were measured by an automatic physisorption analyzer. The results showed that ACFC has a lower critical bed depth and higher adsorption capacity compared to ACFF with similar surface area for each toluene concentration. Among the ACF types, ACFC2000 (cloth with the highest measured surface area of 1614 ± 5 m(2)/g) has one of the lowest critical bed depths (ranging from 0.11-0.22 cm) and has the highest adsorption capacity (ranging from 595-878 mg/g). Based on these studied adsorption characteristics, it is concluded that ACF has great potential for application in respiratory protection against toluene, particularly the ACFC2000, which is the best candidate for developing thinner and efficient respirators.

pH-responsive hydrogel cubes for release of doxorubicin in cancer cells

Doxorubicin (DOX)-loaded poly(methacrylic acid) hydrogel cubes release the drug at pH <5. These hydrogels are developed for shape-directed cellular uptake for drug delivery.

A novel method for designing and fabricating low-cost facepiece prototypes

In 2010, the National Institute for Occupational Safety and Health (NIOSH) published new digital head form models based on their recently updated fit-test panel. The new panel, based on the 2000 census to better represent the modern work force, created two additional sizes: Short/Wide and Long/Narrow. While collecting the anthropometric data that comprised the panel, additional three-dimensional data were collected on a subset of the subjects. Within each sizing category, five individuals' three-dimensional data were used to create the new head form models. While NIOSH has recommended a switch to a five-size system for designing respirators, little has been done in assessing the potential benefits of this change. With commercially available elastomeric facepieces available in only three or four size systems, it was necessary to develop the facepieces to enable testing. This study aims to develop a method for designing and fabricating elastomeric facepieces tailored to the new head form designs for use in fit-testing studies. This novel method used computed tomography of a solid silicone facepiece and a number of computer-aided design programs (VolView, ParaView, MEGG3D, and RapidForm XOR) to develop a facepiece model to accommodate the Short/Wide head form. The generated model was given a physical form by means of three-dimensional printing using stereolithography (SLA). The printed model was then used to create a silicone mold from which elastomeric prototypes can be cast. The prototype facepieces were cast in two types of silicone for use in future fit-testing.

Comparison of a novel surface laser scanning anthropometric technique to traditional methods for facial parameter measurements

This study was designed to determine if three-dimensional (3D) laser scanning techniques could be used to collect accurate anthropometric measurements, compared with traditional methods. The use of an alternative 3D method would allow for quick collection of data that could be used to change the parameters used for facepiece design, improving fit and protection for a wider variety of faces. In our study, 10 facial dimensions were collected using both the traditional calipers and tape method and a Konica-Minolta Vivid9i laser scanner. Scans were combined using RapidForm XOR software to create a single complete facial geometry of the subject as a triangulated surface with an associated texture image from which to obtain measurements. A paired t-test was performed on subject means in each measurement by method. Nine subjects were used in this study: five males (one African-American and four Caucasian females) and four females displaying a range of facial dimensions. Five measurements showed significant differences (p<0.05), with most accounted for by subject movements or amended by scanning technique modifications. Laser scanning measurements showed high precision and accuracy when compared with traditional methods. Significant differences found can be very small changes in measurements and are unlikely to present a practical difference. The laser scanning technique demonstrated reliable and quick anthropometric data collection for use in future projects in redesigning respirators.

Comparison of toluene adsorption among granular activated carbon and different types of activated carbon fibers (ACFs)

Activated carbon fiber (ACF) has been demonstrated to be a good adsorbent for the removal of organic vapors in air. Some ACF has a comparable or larger surface area and higher adsorption capacity when compared with granular activated carbon (GAC) commonly used in respiratory protection devices. ACF is an attractive alternative adsorbent to GAC because of its ease of handling, light weight, and decreasing cost. ACF may offer the potential for short-term respiratory protection for first responders and emergency personnel. This study compares the critical bed depths and adsorption capacities for toluene among GAC and ACF of different forms and surface areas. GAC and ACF in cloth (ACFC) and felt (ACFF) forms were challenged in stainless steel chambers with a constant concentration of 500 ppm toluene via conditioned air at 25°C, 50% RH, and constant airflow (7 L/min). Breakthrough data were obtained for each adsorbent using gas chromatography with flame ionization detector. Surface areas of each adsorbent were determined using a physisorption analyzer. Results showed that the critical bed depth of GAC is 275% higher than the average of ACFC but is 55% lower than the average of ACFF. Adsorption capacity of GAC (with a nominal surface area of 1800 m(2)/g) at 50% breakthrough is 25% higher than the average of ACF with surface area of 1000 m(2)/g, while the rest of ACF with surface area of 1500 m(2)/g and higher have 40% higher adsorption capacities than GAC. ACFC with higher surface area has the smallest critical bed depth and highest adsorption capacity, which makes it a good adsorbent for thinner and lighter respirators. We concluded that ACF has great potential for application in respiratory protection considering its higher adsorption capacity and lower critical bed depth in addition to its advantages over GAC, particularly for ACF with higher surface area.

Influence of temperature on styrene emission from a vinyl ester resin thermoset composite material

Composite materials made with vinyl ester resins are lighter, stronger and corrosion resistant compared to most metals, and are increasingly being used as building materials and in public transportation. Styrene monomer is used as both a diluent and strengthener in the production of vinyl ester resin (VER) composites. Some researchers contend that free styrene in VER composites is available to diffuse out of the material into air, perhaps leading to adverse health effects via inhalation exposures in humans, yet there is no known data on styrene emissions from these materials in the literature. In this study, a typical VER composite made with resin containing 38% by weight styrene, reinforced with E-glass fiber and formed using a vacuum assisted resin transfer method was characterized for styrene emissions by environmental test chamber (ETC) methodology. Styrene concentrations in the ETC were measured over a temperature range of 10 to 50 °C. Initial evaporative styrene emissions increase with increasing temperature. There is a nearly linear relationship in the total mass of styrene emitted and emission factor as emissions increase with increasing temperature. Styrene emission factors appear to vary for different materials, which could indicate more complex processes or the influence of material physical properties on emission rates. These results can be used to validate and improve mass transfer emission models for the prediction of volatile organic compound concentrations in indoor environments.

Exposure of jeepney drivers in Manila, Philippines, to selected volatile organic compounds (VOCs)

The objective of this study was to assess the occupational exposure of jeepney drivers to selected volatile organic compounds (VOCs) in Manila, Philippines. Personal sampling was conducted on 15 jeepney drivers. Area sampling was conducted to determine the background VOC concentration in Manila as compared to that in a rural area. Both personal and area samples were collected for 5 working days. Samples were obtained using diffusive samplers and were analyzed for 6 VOCs (benzene, toluene, ethylbenzene, m,p-xylene and o-xylene) using gas chromatography. Results showed that the average personal exposure concentration of jeepney drivers was 55.6 (+/-9.3), 196.6 (+/-75.0), 17.9 (+/-9.0), 72.5 (+/-21.1) and 88.5 (+/-26.5) microg/m(3) for benzene, toluene, ethylbenzene, m,p-xylene and o-xylene, respectively. The urban ambient concentration was 11.8 (+/-2.2), 83.7 (+/-40.5) and 38.0 (+/-12.1) microg/m(3) for benzene, toluene and o-xylene, respectively. The rural ambient concentration was 14.0 (+/-6.0) and 24.7 (+/-11.9) microg/m(3) for toluene and o-xylene, respectively. The personal samples had significantly higher (p<0.05) concentrations for all selected VOCs than the urban area samples. Among the area samples, the urban concentrations of benzene and toluene were significantly higher (p<0.05) than the rural concentrations. The personal exposures for all the target VOCs were not significantly different among the jeepney drivers.