Personal Monitoring Using Helium Diffusion Sampling to Collect Whole Air Samples near The Breathing Zone without Using Pumps

Dan Cardin , Entech Instruments, CA, USA

Helium Diffusion Sampling (HDS) offers a means for collecting Volatile Organic Compounds (VOC’s) in air using canisters that are initially filled with helium rather than using vacuum to draw in the sample.  HDS Personal Monitoring can be performed using 50mL helium filled canisters that are placed near the breathing zone to collect a representative air sample over a desired time period.  When the containers are opened, helium leaves the canister through a diffusion zone creating a vacuum that pulls in surround air at the same rate that helium leaves.  The much higher diffusion rate of helium maintains an active flow of air into the canister that prevents collected VOCs and air from back-diffusing out the canister during sampling.  The use of different size diffusion zone allows for either 15-minute sample collection for Short Term Exposure Limit (STEL) monitoring, or longer Time Weighted Averaging over 4-8 hours when evaluating Permissible Exposure Limits (PEL’s).   All VOCs are collected without discrimination, so HDS can be used to monitor for hundreds of compounds without having to select different media, because no adsorbents are involved with the collection process.  Laboratory analysis is performed simply by weighing the samplers to determine the amount of air collected, followed by injecting an appropriate volume of gas phase sample into a Gas Chromatograph (GC).  Varying the GC injection volume allows a very wide range of concentrations to be accurately determined, from high PPM to low PPB levels.  HDS Personal Monitors can also be used for very rapid vacuum sampling over a 1-2 second period to make ceiling measurements of hundreds of compounds for which no field portable chemical sensors exist.  Data will be presented showing the use of HDS Personal Monitors for collecting common solvents found in the workplace.