The reference method (EPA 1997) requires that the presample filter weighing be conducted within 30 days of the sampling period. The microbalance must be located in the same controlled environment in which the filters are conditioned. The filters must be weighed without
intermediate or transient exposure to other conditions or environments.
This section presents procedures specific to a common commercially available microbalance. Calibration, QC checks (and acceptance tolerances), and operational procedures may have to be adapted for use with other microbalance models.
Researchers have found that the precision of mass measurements for unexposed filters based on replicate weighings is typically 3 to 5 µg/filter (Desert Research Institute 1994). The precision on exposed filters is typically 5 to 8 µg/filter. Precision for exposed filters with loadings heavier than 1 mg/cm of filter surface area may approach ±2 percent of the loading. The precision for net2
mass filter loadings (typically 6 to 9 µg/filter) is defined as the square root of the sum of the squares of the pre- and postsampling precision.
Follow these steps during the presampling filter weighing:
1. Record the RH and temperature of the conditioning chamber on the laboratory data form
and in the laboratory QC notebook or database.
2. Clean the microbalance’s weighing chamber with a fine brush, if necessary. Antistatic brushes for cleaning the microbalance and the surrounding area are available. Avoid using pressurized gas, which may blow damaging debris and oils into the microbalance’s
mechanism. Recoat the interior and external nonmetallic surfaces of the chamber with an antistatic solution, if necessary. Clean the surfaces near the microbalance with antistatic solution- or methyl alcohol-moistened disposable laboratory wipes. Clean the standard forceps with a lint-free cloth and the filter forceps with the moistened wipes. Allow the forceps to air-dry. Make sure the forceps are thoroughly dry before use. Even a small amount of moisture can cause a significant measurement bias.
3. To ensure maximum stability, it is recommended that the microbalance be turned on at all
times. This procedure enables the microbalance to be operational at any time and eliminates the need for a warmup period before analyses are performed. Newer microbalances are always turned on (except for their displays) when they are plugged in.
4. Zero (i.e., tare) and calibrate the microbalance according to the manufacturer’s directions. Many newer microbalances calibrate themselves automatically when the operator presses a
2.12.7.0 Filter Preparation and Analysis November 1998 Page 14 of 19
key.
5. Using smooth, nonserrated, nonmetallic forceps, weigh two working mass reference
standards (for example, a 100-mg standard and a 200-mg standard) as a QC check. Handle the working standards carefully to avoid damage that may alter their masses. Verify the working standards’ masses every 3 to 6 months or after any incident of rough handling against the laboratory’s primary standard weights (ASTM Class 1 or 1.1 standards). The working standards should bracket the mass of a blank or a loaded filter. Wait until the microbalance’s display indicates that a stable reading has been obtained. Consult the microbalance's operating manual for more information about obtaining stable readings. Record the certified and measured values of these standards on the laboratory data form and in the laboratory QC notebook or database.
If the verified and measured values of a working standard disagree by more than 3 µg (i.e., three times the microbalance’s repeatability), reweigh the working standard. If the two values still disagree, troubleshoot and take appropriate corrective action, which may include (1) recertifying the working standards against the laboratory primary standards and/or (2) having a service technician repair the microbalance. The analyst should not attempt to repair the microbalance.
Weigh enough laboratory blanks during a presampling weighing session to provide at least 10 percent or one single-use laboratory blank during each subsequent postsampling
weighing session. Weigh enough field blanks during a presampling weighing session to provide at least 10 percent or one single-use field blank during each subsequent
postsampling weighing session. Record the presampling weights on the laboratory data form and in the laboratory QC notebook or database.
6. Weigh the filters. Operate the balance according to the manufacturer’s directions. Newer microbalances are so easily rezeroed that analysts should rezero the balance before each weighing. Take the filter from its filter-handling container (petri dish or equivalent) by gently slipping the filter-handling forceps under the outer polyolefin support ring. Hold the filter only by the ring, not by the filter material. Pass the filter, support ring side up, near a
Po antistatic strip for 30 to 60 sec immediately prior to weighing. The antistatic strip
210
should be inside the weighing chamber or as close to the chamber door as is practical. Immediately transfer the filter to the microbalance’s pan and close the weighing chamber door. After the microbalance’s display indicates that a stable reading has been obtained, record the balance number, the filter number, the filter lot number, and the filter’s tare weight (presampling mass) on the laboratory data form.
7. After approximately every 10th filter weighing, the analyst should reweigh at least one of the working standards. Record the measurement on the laboratory data form and in the laboratory QC notebook or database. If this measurement disagrees from the verified value by more than 3 µg (i.e., three times the microbalance’s reproducibility), reweigh the
standard. If the two measurements still disagree, troubleshoot and take appropriate
corrective action, which may include (1) reweighing some or all of the previously weighed filters, (2) recertifying the working standards against the laboratory primary standards, and/or (3) having a service technician repair the microbalance. The analyst should not attempt to repair the microbalance.
2.12.7.0 Filter Preparation and Analysis November 1998 Page 15 of 19
At the end of the weighing session, reweigh both working standards. Record the
measurements on the laboratory data form and in the laboratory QC notebook or database.
8. Any unused filter whose weight is outside the normal range (i.e., 110 to 160 mg) should be
investigated. If there is a consistent negative replication (>15 µg) for laboratory blank filters, it is usually a sign that the filters have not equilibrated long enough. In this case,
troubleshoot and take appropriate corrective action.
9. One routine filter should be reweighed at the end of the weighing session. Record the replicate measurement on the laboratory data form. If the replicate measurement disagrees from the original measurement by more that 15 µg, reweigh the filter. If the measurements still disagree, troubleshoot and take appropriate corrective action, which may include (1) reweighing all or some of the previously weighed filters, (2) reweighing the working
standards, or (3) having a service technician repair the microbalance. The analyst should not attempt to repair the microbalance.
10. Return the filter to the filter-handling container, replace the lid to cover the filter, and return the container to the conditioning chamber to protect it from contamination prior to
sampling.
11. Check the filter cassettes and the backing screens for fractures, cracks, evidence of wear, or contamination. Clean or replace as necessary. The cassettes can be washed in a dishwasher and then rinsed with deionized water.
12. When the time comes for the filters to be used at the sites (use must occur within 30 days of the initial weighing), install each filter in a filter cassette, and put the filter/cassette assembly into a protective container for transport to the sampler (see Figure 7-3). Attach a label with the filter identification number to the outside of the protective container. Double-check the entries in the laboratory data form (Figure 7-1). Prepare several extra filters in case a filter is invalidated during the installation process.
13. If the filters will be mailed, the field operator should be supplied with reinforcing envelopes or some other means (in addition to the protective container) to protect exposed filters during their shipment back to the analytical laboratory.