filter. Fine particles (#0.5 µm) will increase the pressure drop much faster than coarse particles ($10 µm). Heavy loading ($ about 1 mg) may result in poor adhesion of collected particles to the filter surface.
- *NOTE: It is important to distinguish between equilibrium (saturation) adsorptive capacity and kinetic (breakthrough) adsorptive capacity of the solid sorbent. Breakthrough capacity is the important characteristic in actual sampling situations; it may be affected significantly by sampling flow rate and relative humidity of the air being sampled and may be significantly less than saturation capacity, which is not dependent on sampling flow rate or relative humidity.
2. FIGURING SAMPLING PARAMETERS
Once the sampling media and measurement method are chosen, then the specific sampling parameters need to be determined [12]. For most methods, this will not pose a problem as the flow rate recommended in the method can be used for the desired sampling period, e.g., 1 to 3 L/min for 8 h for most aerosols or 10 to 200 mL/min for 8 h for most sorbent tube samples. Generally, the parameters which must be considered are flow rate, total sample volume, sampling time (tied into the two previous parameters), and limit of quantitation (LOQ) (see Glossary of Abbreviations, Definitions and Symbols). Some of these variables will be fixed by sampling needs, e.g., sampling time or by the measurement method of choice (LOQ or maximum sampling volumes). The choice of these variables can best be explained through the use of the following examples.
Examples:
| Given parameters: | Method 1501 for Styrene |
| Recommended Sample Volume: | 5L |
| Useful Range of the Method: | 85 to 2560 mg/m³ (20 to 600 ppm) |
| OSHA PEL: | 850 mg/m³ (200 ppm) - Ceiling |
| 425 mg/m³ (100 ppm) - TWA | |
| Recommended Flow Rate: | 0.2 L/min |
| Breakthrough Time: | 111 min @ 0.2 L/min and 1710 mg/m³ |
| Breakthrough Capacity | 38 mg |
Suppose it is desired to determine both ceiling and TWA exposures of workers exposed to styrene and the concentrations are unknown.
Ceiling Determination: If sampling were done at 0.2 L/min for 30 min and a total sample volume of 6 L collected which is above the 5 L recommended sample volume, would this a problem? Probably not. For instance, in the breakthrough test, a concentration of 2 times the OSHA Ceiling Standard (1710 mg/m³) was sampled at 0.2 L/min for 111 min (22.2 L) before breakthrough occurred, collecting a total weight of 38 mg of styrene. Of course, this test was conducted in a dry environment with only styrene present. A safety factor of 50% should be allowed to account for humidity effects. Thus, if sampling is done for about 55 min at 0.2 L/min, levels of styrene up to 400 ppm could still be collected without sample breakthrough.
Also to be considered are the other organics present. If a concentration of 200 ppm acetone exists in this environment, then an additional safety factor should be added. An arbitrary 50%
