Mass reference standards should be in the range of 100 to 200 mg, given that the mass range of a typical 46.2-mm diameter filter is from 110 to 160 mg. They should be certified as being
traceable to NIST mass standards (see ASTM 1993b; Harris 1993; Kupper 1990). Additionally, they should have an individual tolerance of no more than 0.025 mg. Examples of mass reference standards that meet these specifications are American National Standards Institute/American Society for Testing and Materials (ANSI/ASTM) Classes 1 and 1.1. The mass reference
standards should be recalibrated on a regular basis (e.g., yearly) at a State weights and measures laboratory holding a NIST certificate of traceability or at a calibration laboratory accredited by the National Voluntary Laboratory Accreditation Program (NVLAP), which is administered by NIST (Harris 1994; White 1997). The recalibration frequency should be determined from records of previous recalibrations of these standards.
Note: The microbalance’s resolution and repeatability are smaller than the tolerance of the most
accurate classes of mass reference standards. However, the reported uncertainty of a mass reference standard recalibrated at NIST-traceable or NVLAP-accredited laboratories will be smaller than the individual tolerance of the standard. The accuracy of the gravimetric analysis may be limited by the uncertainty of the standards rather than by the microbalance’s character- istics. Because the loading mass is determined by difference, the loading error associated with the calibration standard’s tolerance can be estimated as
loading error in µg = (standard uncertainty in mg/standard mass in mg) × (loading mass in µg). Two separate sets of mass reference standards are recommended. Working calibration standards should be used for routine filter weighing and kept next to the microbalance in a protective container. Laboratory primary standards should be handled very carefully and should be kept in a locked compartment. The working standards’ masses should be verified against the laboratory primary standards every 3 to 6 months to check for mass shifts associated with handling or contamination. The verified values of the working standards as measured relative to the
2.12.7.0 Filter Preparation and Analysis November 1998 Page 3 of 19
Table 7-1. Filter Preparation and Analysis Checks
Activity Method and frequency Requirements
Action if the requirements are not met
Microbalance Resolution of 1 µg, repeatability of 1 µg. Obtain proper microbalance. Microbalance environment Climate-controlled, draft- free room or chamber or equivalent; clean area around microbalance.
Modify the environment.
Mass reference standards
Working standards verified every 3 to 6 months against National Institute of
Standards and Technology (NIST)-traceable laboratory primary standards.
Standards bracket weight of filter; individual
standard’s tolerance less than 25 µg; handle with smooth, nonmetallic, clean forceps.
Obtain proper standards or forceps.
Filter handling
Observe handling procedure.
Use powder-free gloves and smooth, clean forceps. Replace polonium-210( 210Po) antistatic strips every 6 months.
Discard mishandled filter or old antistatic strip.
Filter integrity check
Visually inspect each filter. No pinholes, separation, chaff, loose material, dis- coloration, or filter non- uniformity.
Discard defective filter.
Filter identification
Write filter number on filter handling container and on laboratory data form in permanent ink.
Make sure the numbers are written legibly.
Replace label or correct form.
Presampling filter
conditioning
Determine the correct conditioning period (at least 24 hours) for each new lot of filters. Observe and record the conditioning chamber's RH and temperature; enter laboratory data form.
Check for stability of lot blank filter weights. Weight changes should be <15 µg per week before and after equilibration. Mean RH between 30 and 40 percent, with a variability of not more than ±5 percent over 24 hours. Mean temperature should be held between 20 and 23 EC, with a variability of not more than ±2 EC over 24 hours.
Revise conditioning period; repeat conditioning.
2.12.7.0 Filter Preparation and Analysis November 1998 Page 4 of 19
Table 7-1. (continued)
Activity Method and frequency Requirements
Action if the require- ments are not met Pre- and post-
sampling filter weighing
Observe all weighing procedures. Perform all quality control (QC) checks.
Neutralize the electrostatic charge on filters. Wait until balance indicates a stable reading to record value.
Repeat weighing. Many laboratories routinely repeat the weighing.
Internal QC After approximately every 10th filter, rezero the micro- balance and reweigh at least one working standard. Weigh approx.10 percent laboratory blanks/weighing session. Reweigh one replicate filter at the end of the weighing session. Weigh approx. 10% field blanks.
The working standard measurements should agree within 3 µg of the verified values. Laboratory blank and replicate measurements should agree within 15 µg. Field blank measurements should agree within 30 µg.
Troubleshoot and take appropriate corrective action as specified in the quality assurance project plan (QAPP) to attain acceptable levels. Do not correct or invalidate PM2.5
measurements based on high blank levels. Postsampling
filter storage
Monitor the time between sampling and weighing.
Weighing should be completed within 240 hours (10 days) after the end of sampling, unless the filter is maintained at 4 EC or below during the entire time between retrieval from the sampler and start of the condition- ing, in which case the period shall not exceed 30 days.
Troubleshoot and take appropriate corrective action. Postsampling inspection, documentation, and verification
Examine the filter and field data sheet for correct and complete entries. If sample was shipped in a cooled container, verify that low temperature was maintained.
No damage to filter. Field data sheet complete. Sampler worked OK.
Save filter for inspection. Flag sample. Troubleshoot and take appropriate corrective action.
Postsampling filter
equilibration
Equilibrate filters for at least 24 hours. Observe and record the equilibration chamber's RH and temperature; enter on laboratory data sheet.
Mean RH between 30 and 40 percent, with a
variability of not more than ±5 percent over 24 hours. Mean temperature should be held between 20 and 23 EC, with a variability of not more than ±2 EC over 24 hours.
2.12.7.0 Filter Preparation and Analysis November 1998 Page 5 of 19
the calibration of the microbalance. If multiple microbalances are being used, all working standards should be verified at the same time to ensure that all gravimetric measurements are intercomparable.
Always use smooth, nonmetallic forceps for handling mass reference standards. The standards are handled only with these forceps, which are not used for any other purpose. Mark these forceps to distinguish them from the forceps used to handle filters. Forceps should be cleaned with alcohol and lint-free wipes before handling standards and then should be allowed to air-dry. Handle the standards carefully to avoid damage that may alter their masses.