Sampling Using Filters

Filters include glass fiber filters, Teflon filters, mixed cellulose ester membranes, polyvinyl

chloride filters, and treated filters and pads.

Sampling with filters can be done open or closed faced. Open-faced sampling is done with three (3) piece cassettes with just the top of the cassette removed. Closed-faced sampling is performed with only the buttons removed from the cassette. Open-faced sampling is used in situations where even dispersal of the analyte over the filter is desired, such as in asbestos sampling or on certain coated filters such as for isocyanate sampling. Closed-faced sampling is performed in most cases of particulate sampling, which deposits the analyte near the center of the cassette.

Sampling should be performed in the following manner:

  1. Calibrate the sampling pump at the recommended flow rate with a representative filter in line using a bubble meter or equivalent flow measuring device.
  2. Remove the cassette buttons (closed faced sampling) or cassette top (open faced sampling). Air being sampled should not pass through any hose or tubing before entering the cassette.
  3. Attach the cassette to the sampling pump using flexible tubing or a cassette holder, making sure the filter pad end of the cassette is closest to the pump.
  4. Prepare field blanks using cassettes from the same lot by briefly removing and replacing cassette buttons and handling these blanks in the same manner as the samples, without drawing air through the blanks.
  5. Sample at a known flow rate for the length of time necessary to obtain the specified air volume, or until there is evidence of excessive filter loading. Check the pump during sampling to detect any changes in flow rate. If the accurate measurement of volume cannot be determined due to inconsistent flow rates, discard the sample.
  6. Disconnect the filter after sampling. Replace the buttons (or top) and properly label the sample and blanks. Be sure to keep an accurate record of sample data including sample location, beginning and ending times or air sampling, temperature, relative humidity, atmospheric pressure or elevation above sea level, and name of employee or area.
  7. Ship the samples to the laboratory as soon as possible in a container designed to prevent damage in shipment. Enclose a completed chain of custody with each batch.