Evaluation of flow controllers used with evacuated canisters to assess VOC exposures in occupational and non-occupational environments.

Ideally, measuring exposures to volatile organic compounds should allow for modifying sampling duration without loss in sensitivity. Traditional sorbent-based sampling can vary sampling duration, but sensitivity may be affected when capturing shorter tasks. Diaphragm and capillary flow controllers allow for a range of flow rates and sampling durations for air sampling with evacuated canisters. The goal of this study was to evaluate the extent to which commercialized capillary flow controllers satisfy the bias (±10%) and accuracy (±25%) criteria for air sampling methods as established by the National Institute for Occupational Safety and Health (NIOSH) using the framework of ASTM D6246 Standard Practice for Evaluating the Performance of Diffusive Samplers to compare their performance with diaphragm flow controllers in a long-term field study. Phase 1 consisted of a series of laboratory tests to evaluate capillary flow controller flow rates with respect to variations in temperature (-15-24 °C). The results demonstrated a slight increase in flow rate with lower temperatures. In Phase 2, the capillary flow controller was evaluated utilizing a matrix of parameters, including time-weighted average concentration, peak concentration (50-100× base concentration), air velocity across the sampler inlet (0.41-0.5 m/s), relative humidity (20-80%), and temperature (10-32 °C). Comparison of challenge concentrations with reference concentrations revealed the aggregate bias and overall accuracy for four tested compounds to be within the range of criteria for both NIOSH and ASTM standards. Additionally, capillary flow controllers displayed lower variability in flow rate and measured concentration (RSD: 2.4% and 4.3%, respectively) when compared with diaphragm flow controllers (RSD: 6.9% and 7.2%, respectively) for 24-hr laboratory tests. Phase 3 involved further testing of flow rate variability for both diaphragm and capillary flow controllers in a field study. The capillary flow controller displayed a lower level of variability (RSD: 5.2%) than the diaphragm flow controller (RSD: 8.0%) with respect to flow rate, while allowing for longer durations of sampling.

Evaluation of Long-Term Flow Controller for Monitoring Gases and Vapors in Buildings Impacted by Vapor Intrusion.

This study evaluated the use of a long-term capillary flow controller paired with an evacuated canister for indoor air exposure monitoring in a vapor intrusion (VI) environment with trichloroethylene in comparison to the traditional method utilizing a diaphragm flow controller. Traditionally, air sampling with 6 L evacuated canisters equipped with diaphragm flow controllers has been best suited for 8 to 24 h samples. New advances in capillary flow controllers can extend sampling to up to 3 weeks by reducing flow rates to 0.1 milliliters min-1. During six 2 wk sampling events, conventional diaphragm flow controller canisters were used to collect 24 h samples simultaneously with capillary flow controllers collecting 2 wk samples. Testing was performed at four indoor locations in buildings impacted by VI with co-located samples for each method at each location. All samples were analyzed using GC/MS, and the results were statistically analyzed to produce a direct comparison of the two sampling systems. Ninety-two percent of the 14 d capillary samples were within the 95% levels of agreement of the average concentration of the diaphragm flow controllers. The ability to collect 14 days of data, with less occupant disturbance, allows for improved exposure assessments and thus improved risk management decisions.

A field study to assess the long-term sampling feasibility of evacuated canisters and the development of a mathematical model to analyze potential sampling bias.

Small, evacuated canisters (300 mL) equipped with a unique capillary flow controller were used to evaluate airborne concentrations of Stoddard solvent. The physical characteristics of the flow controller permitted the collection of air samples for a time period of 40 hours (5 consecutive work days). Long-term sampling (greater than 8 hours) is rarely performed in industrial hygiene due to limitations in current air sampling technology but may provide valuable information in characterizing worker cumulative exposures for some processes. A field study was performed to evaluate the feasibility of collecting a 40-hour area sample using the small canisters. Six canister samplers were used as area monitors to evaluate a cleaning operation for an entire workweek. For comparison, 30 diffusive badges (6 per day) were simultaneously used to monitor the same process. No statistical difference was found between the time-weighted average for the two sampling methods (p > 0.05). In addition, the canister samples integrate airborne concentrations for an entire workweek and therefore peak concentrations are not explicitly observed. Thus, an examination of peak exposures using simulated concentrations was conducted. A mathematical model was developed to determine whether a significant sampling bias was associated with long-term canister sampling when peak concentrations are present. The maximum possible bias was determined to be less than 9% for peak amplitudes having 10 times the background concentration and well below that for smaller amplitudes. Long-term sampling with the small, evacuated canisters was found to provide results comparable to sorbent sampling methods but with the added benefit of a significantly increased sampling time.

Development of a flow controller for long-term sampling of gases and vapors using evacuated canisters.

Anthropogenic activities contribute to the release of a wide variety of volatile organic compounds (VOC) into microenvironments. Developing and implementing new air sampling technologies that allow for the characterization of exposures to VOC can be useful for evaluating environmental and health concerns arising from such occurrences. A novel air sampler based on the use of a capillary flow controller connected to evacuated canisters (300 mL, 1 and 6 L) was designed and tested. The capillary tube, used to control the flow of air, is a variation on a sharp-edge orifice flow controller. It essentially controls the velocity of the fluid (air) as a function of the properties of the fluid, tube diameter and length. A model to predict flow rate in this dynamic system was developed. The mathematical model presented here was developed using the Hagen-Poiseuille equation and the ideal gas law to predict flow into the canisters used to sample for long periods of time. The Hagen-Poiseuille equation shows the relationship between flow rate, pressure gradient, capillary resistance, fluid viscosity, capillary length and diameter. The flow rates evaluated were extremely low, ranging from 0.05 to 1 mL min(-1). The model was compared with experimental results and was shown to overestimate the flow rate. Empirical equations were developed to more accurately predict flow for the 300 mL, 1 and 6 L canisters used for sampling periods ranging from several hours to one month. The theoretical and observed flow rates for different capillary geometries were evaluated. Each capillary flow controller geometry that was tested was found to generate very reproducible results, RSD < 2%. Also, the empirical formulas developed to predict flow rate given a specified diameter and capillary length were found to predict flow rate within 6% of the experimental data. The samplers were exposed to a variety of airborne vapors that allowed for comparison of the effectiveness of capillary flow controllers to sorbent samplers and to an online gas chromatograph. The capillary flow controller was found to exceed the performance of the sorbent samplers in this comparison.