Mass Concentration - Gravimetric Analysis of Teflon Filters

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Standard Operating Procedure Manual
Gravimetric Determination of Aerosol Mass on Filters

Recommended for Teflon Filter-Based Gravimetric Aerosol Mass Measurements at the Global Atmospheric Watch (GAW) Stations

Version 3.0

October 20, 2007

  1. Preamble

The mass concentration of atmospheric aerosols is clearly a fundamental parameter in the GAW measurement program. This standard operating procedure (SOP) builds on the assumption that the aerosol mass measurements use a filter sampling method of collecting aerosol particles at a given GAW monitoring station. Specifically, 47 mm diameter Teflon filters are assumed to be used for the aerosol collection as per the recommendations by the WMO/GAW program as contained in the WMO/GAW Report 153, the AEROSOL MEASUREMENT GUIDELINES AND RECOMMENDATIONS. Furthermore, the filter samples are assumed to have been collected from the station and properly conditioned and stored until they are ready for the mass measurements in a weighing facility as described below.

In WMO/GAW Report 153, it was recommended that aerosol mass measurements at the GAW stations be made gravimetrically on 47 mm diameter Teflon filters. This SOP builds upon the operational guidelines of the US EPA<ref name="ftn1">US EPA, Quality Assurance Guidance Document 2.12: Monitoring PM in 2.5 Ambient Air Using Designated Reference or Class I Equivalent Methods, November 1998.</ref> and the Canadian Air and Precipitation Monitoring Network (CAPMoN)<ref name="ftn2">CAPMoN, CAPMoN LABORATORY STANDARD OPERATING PROCEDURE MANUAL, December 2005.</ref> for gravimetric aerosol mass measurements, and tailors it to make a simplified approach.

For this SOP, specific references are made to certain trade marks and products. It is neither an endorsement of the product nor a requirement at the GAW measurements. Technically equivalent products can be used for the same purposes.

  1. Application
    1. The mass measurement is applicable to TEFLO W/RING 2 m pore size - 47MM 50/PK R2PJ047 Teflon membranes.
  1. Method Summary
    1. A microbalance is calibrated using NIST referenced weights under controlled conditions. The mass of the aerosol is based on weighing the filters pre and post sampling under specified temperature and relative humidity conditions. The mass of the aerosol is the difference of pre and post weights.
  1. Instrumentation and Apparatus
    1. Micro Balance – Mettler Toledo UMT2 (weighing capacity 2100 mg, readability 0.1 μg), or an equivalent micro balance.
    2. Computer – either a notebook computer or a desktop computer, that runs a data entry software, such as a spreadsheet program, customized for recording data from the mass measurements. The computer is intended for recording weights of calibration weights, quality control filters, and actual sample filters.
    3. There are specialized software programs that can be used on the computer to link to the Mettler Toledo micro balance, for the purpose of data capture. The use of such programs needs to be covered in supplemental standard operating procedures (supplemental SOPs).

A simpler approach is to use spreadsheet software to record the filter weights. For example, CAPMoN has an MS Excel-based spreadsheet template for recording the weighing data, with embedded criteria for accepting or rejecting a weighing result. Quality assurance and quality control plots are contained in the spreadsheet template. Manual input of sample identification is required for the spreadsheet template. This spreadsheet is available for anyone interested. (;

Manual input needs some forms of quality assurance, as transcription errors can be a big problem if all data need to be recorded into the data recording software. Double checking upon entry is one way to reduce such errors.

    1. 200 mg NIST traceable standard calibration weight
    2. U-Ionizer – Haug, with power pack. Equivalent ionizer can be used.What are the specifications of the “U-Ionizer”? Please include manufacturer’s contact info and model number.
    3. Polonium strips and holder apparatusWhat isotope? What activity?
    4. Teflon-tipped forceps (two pairs)
    5. Polystyrene 50mm Petri dishes
    6. Tyvek shoe covers, gowns and caps or equivalent
    7. Powder free, antistatic vinyl or polyethylene gloves
    8. Clean plastic bags equipped with a closure system such as ziplock or wire (e.g., Whirlpak bags)
    9. Teflon-tipped forceps (two pairs)
  1. Safety Considerations
    1. Polonium ionizing strips are radioactive materials that pose minimal hazard to the analyst.
    2. Radioactive licensing is likely required in most countries. Check national/local regulations on radioactive material handling.
    3. For most countries radioactive materials must be returned to the originating manufacturer for disposal. Check national/local regulations on radioactive material handling.
  1. Reagents
    1. Not required.
  1. Preparation
    1. Use deionized water (18.2MÔÅó) for rinsing forceps and cleaning surfaces.
    2. Deionized water may be produced in-house from tap water using a Millipore system, a Barnestead system, or other equivalent deionizing system. Deionized water is typically stored in a large reservoir, from which the water is dispensed into smaller squirt bottles for rinsing use in the clean room. All rinse water used in this method must be taken from the reservoir on a per run basis (minimum daily).
  1. Facilities
    1. NOTE: A clean roomWhat constitutes a “clean room”? Do you mean Class 100, or some other criteria? is assumed to have been setup and dedicated to the purpose of aerosol mass measurements. All the instruments and apparatus are assumed to be inside the clean room for the mass measurements by weighing.
    2. The conditioning and weighing of the filters are carried out in a climate-controlled clean room. The clean room must meet the following standard conditions for 24 hours prior to weighing:
      1. The temperature must be controlled at 21C with a maximum variation of 1C.
      2. The relative humidity must be controlled within the range of 45% The mass determination should be harmonized with the nephelometer sample RH. I believe that we discussed this in GAW#153.with a maximum variation of 3%
    3. The temperature, atmospheric pressure, and relative humidity of the clean room should be continuously recorded. This should be done on a regular basis. Additional recording should be done for every session of weighing. See Sections 9, 10, and 11.
    4. Personnel entering this room must wear clean room grade shoe covers, cap, gown, and gloves to minimize dust contamination.
    5. The clean room should not be used for storage of any item.
    6. Wipe all benches in the clean room at least once a week using kimwipes and deionized water. Do not do this when filters are exposed for equilibrium purposes in the room.
    7. HEPA and carbon filters for the clean room must be changed on a scheduled basis, nominally annually. The frequency depends on the local environmental conditions of the clean room; where gas and particle pollutants are high in concentrations, more frequent changes must be made to maintain a good microenvironment for the clean room. During the change, the clean room needs to be closed until the room is restored to its original condition.
    8. Access shall be restricted to authorized personnel only.
  1. Calibration
    1. Calibrate using the NIST referenced weight, of 200 mg, once per 50 sample filters.
    2. Before calibration starts, the operator must have clean vinyl or polyethylene gloves on. The operator must wear Tyvek (or equivalent) shoe covers, a gown and a cap.
    3. Turn on the computer. If no special software is used, a spreadsheet program needs to be used to record the NIST reference weight. The reference weight data should be recorded in a dedicated spreadsheet file that is used for Quality Control, such as time series QC plots.
    4. Clean the ionizing bar with the special brush provided.
    5. Using forceps provided with the NIST referenced weights, grasp the 200 mg weight and pass it slowly over the ionizing bar horizontally three times. Pass the weight about 2 to 3 cm from the top and side of the ionizing bar.
    6. Slowly pass the weight over the polonium strip three times.
    7. Open the Mettler balance draft shield by pressing either the right or left-hand prompt on the front panel of the balance. On other equivalent microbalances, operate the balance according to its manuals.
    8. Place the weight on the pan forks of the balance (pan forks recommended for better stability) and close the draft shield using the keypad. On other balances, operate according to its manuals. Allow the reading to stabilize.
    9. Record the weight reading in the dedicated spreadsheet file.
    10. Wait for the draft shield to open. Using forceps to remove the weight from the weighing cell and place it back in its designated box. Replace the forceps.
  1. Quality Control Filters
    1. Two 47mm Teflo membranes (part number R2PJ047) are kept in the weighing room exposed to room air to be used as reference filters. These are identified as QC1 and QC2. The QC filters are placed in Petri dishes equipped with covers.
    2. Before reference weighing starts, the operator must have clean vinyl or polyethylene gloves on. The operator should wear Tyvek (or equivalent) shoe covers, a gown and a cap.
    3. At the beginning and end of each weighing session for sample filters (before and after field samples), and after every 10th sample filter, weigh one of the two control filters. Alternate between the two control filters.
    4. If the reference filter does not fall within 3 standard deviations of its average weightOver what period of time is the average /standard deviation calculated?, all filters weighed since the last reference filter reading must be re-weighed.How much drift in the weight of the reference filter is allowable?
    5. Record the reference filter weight measurements in a dedicated spreadsheet, preferably in the same file as the NIST calibration weight measurements. Plot the reference filter weights as a time series. Plotting should be automatic as the new reference filter weight is entered. Separate the weight data for the QC1 and QC2 reference filters into different spreadsheets.
    6. Refer to Steps 9.2 to 9.9 to complete weighing procedure.
  1. Sample Preparation
    1. Filters must be removed from their respective packages for one month prior to weighing. Remove the Teflo filters from the outer wrap and inner tray. Remove the spacers. Place a stack of 10-20 filters in a single Petri dish. These are then left in the clean room for equilibration.
    2. One week prior to weighing, separate the above filters into one filter per Petri dish and leave uncovered on a clean room bench.
    3. Before sample preparation, the operator must have clean vinyl or polyethylene gloves on. The operator should wear Tyvek shoe covers, gowns and caps.
    4. Avoid contaminationThere is no discussion of avoiding contamination by NH3 in the operator’s breath. and handle filters with Teflon-tipped forceps. Grasp the filters by the plastic ring of the filter edge.
    5. Remove the filter from the packaging and inspect each filter visually for integrity before placing in a clean Petri dish. Check for dirt, fibers, holes or tears and discard filters with any imperfections.
    6. Place a Petri dish on a clean laboratory bench. Using Teflon tipped forceps place the inspected a filter face up in the dish. Place the Petri dish cover face down on the bench next to the dish.
    7. After all filters have been placed in Petri dishes, record the time to the nearest minute in a properly identified worksheet. Record the temperature and the humidity of the room in the Calibration sheet, the QC sheets, and the sample sheet.
  1. Instrument Setup, Maintenance & Troubleshooting
    1. Before these activities take place, the operator must have clean vinyl or polyethylene gloves on. The operator should wear Tyvek (or equivalent) shoe covers, a gown and a cap.
    2. The Mettler Toledo UMT2 (or equivalent) must be turned on prior to weighing for a minimum two-hour warm-up period.
    3. Check that the balance is not tilted. See that that the balance bubble is in the centre of the inner circle of the viewing window.
    4. Check the weighing chamber for dust and sweep using a specialWhat’s so special about it? soft brush.
    5. Clean the exterior of the balance with isopropanol or deionized water, whichever is more appropriate for the balance.
  1. Operational Requirements
    1. Before filter weighing measurement starts, the operator must have clean vinyl or polyethylene gloves on. The operator should wear Tyvek (or equivalent) shoe covers, a gown and a cap.
    1. First Pre-Weighing of Filters
      1. Affix an identifying label to the exterior of each Petri cover. Each label will show a unique ID number that will be used to identify the filter. Put on the Petri dish cover.
      2. Turn on the computer.
      3. Clean the ionizing bar with the special brush provided.
      4. On the computer, open a spreadsheet file, preferably the template that has all the necessary headings, formats, and calculation logic built in. Enter the ID number on the Petri dish into the spreadsheet.
      5. Check the ID numbers. Does the sample ID number match the Petri dish label? Change the number if necessary.
      6. Using Teflon coated tweezers to grasp the Teflo filter by the plastic ring and pass it slowly through the ionizing bar horizontally three times. Pass the weight about 2-3 cm from the bottom and side-bars.
      7. Slowly pass the filter under the polonium strip three times.
      8. Open the draft shield of the Mettler balance by pressing either the right or left-hand prompt on the front panel of the balance. On other balances, follow their operational manuals for operating the balances.
      9. Place the filter on the pan forks of the balance and close the draft shield using the keypad. On other balances, follow their operational manuals for operating the balances. Allow the reading to stabilize
      10. Record the weightsPlural? in the spreadsheet.
      11. Remove the filter from the weighing cell and place it back in the Petri dish. Put on the Petri dish cover.
      12. Repeat steps 13.2.6-13.2.11. Tare the balance if the readings drift between weights.Steps 13.2.12 and 13.2.13 seem to be equivalent.
      13. Weigh all of the filters once, then weigh them all again starting at the filter at the top of the spreadsheet. Record both the first and second weighing results in two separate columns of the spreadsheet.
      14. Save the spreadsheet file.
      15. For every 50 filters, the NIST calibration weight must be measured. See Section 9.
      16. For every 10 filters, the reference filters must be weighed once. Alternating between the QC1 and QC2 reference filters. See Section 10.
      17. The filters are weighed again, 24 hours after the initial weighing. See following section for details. In total, four weights will have been recorded for each filter.
    1. Second Pre-Weighings of same filters
      1. A second pre-weighing after the first pre-weighing is necessary to ensure accuracy and I would suggest that the second pre-weighing does nothing to ensure accuracy, but a lot to ensure precision.precision of the weight measurements.
      2. Open the same spreadsheet file containing the first pre-weighing results.
      3. Continue with steps 13.2.5 to 13.2.16.
      4. If the average duplicate weight of a filter does not fall within 0.01Your previous criteria was based on standard deviations, now you’ve switched to a maximum difference. Is the a reason for using two different criteria? mg average of the initial weight, the filter is weighed a fifth and sixth time 24 hours later, in a third pre-weighing of the same filter and repeating the steps from 13.3.1 to 13.3.3.What is the criterion for acceptance/rejection of the third pre-weighing? Should a fourth weighing ever be done?
      5. Place weighed and conditioned filters in their own Petri dishes, and place the Petri dishes in small Whirlpak (or equivalent) plastic bags, 20 Petri dishes per bag. Close the bag tightly.
  1. Filter Handling
    1. NOTE: Filter handling, after pre-weighing, needs to be harmonized with the field sampling operation of any GAW station. The following SOP is generic and needs to be tailored to the needs and operations of a particular GAW station. Where an SOP exists for the field sampling using filters, they should be followed, but with a special effort to record the connection between the ID of the pre-weighed filters with the actual sample ID and start and end date/times for sampling.
    1. Filter Loading
      1. After shipping to the field, the Petri dishes containing the pre-weighed filters are ready to be loaded into filterpacks for sampling at a given GAW station. Make sure that a printout of the ID of the Petri dishes, together with the weights as obtained in Section 13, accompanies the Petri dishes/filters. A copy of the spreadsheet containing this information should also be forwarded to the GAW station operators together with the Petri dish/filters.
      2. At the station, the spreadsheet file should be opened up using a local computer. Enter the station ID and record the station sample ID for the lab-generated filter ID. Record the sampling start date/time and end date/time for each filter. Save the file.
      3. Load the filters onto filterpacks according to the SOP for the filter sampling program at the station. Load in a clean area. Wipe down the bench top with deionized water and a Kimwipe before starting.
      4. Put on a pair of polyethylene gloves.
      5. Using Teflon-coasted forceps and grasping the plastic ring of the filter, transfer the filter from its Petri dish into the filterpack. Assemble the filterpack as per the station SOP for the filter sampling program at the station.
      6. Close the empty Petri dish and put it aside. Store the labeled Petri dishes in a clean Whirlpack (or equivalent) plastic bag in the order of the pre-weighings in the spreadsheet file.
      7. If multiple filterpacks are loaded at one time, repeat steps 14.2.3-14.2.6 for each successive filterpack.
    1. Filter Unloading
      1. The following steps are to be conducted in the field at the GAW station as per the station SOP for filterpack handling.
      2. After sampling is completed at the station, unload the filters in a clean area at the GAW station. Wipe down a bench top with deionized water and a Kimwipe before starting.Do your recommend using a glove box, with ammonia scrubber, for sample handling in the field?
      3. Put on a pair of polyethylene gloves.
      4. Gently remove the filter from the filterpack using forceps, grasping the plastic ring and taking care not to damage the filter.
      5. Without turning the filter upside down, examine the filter for defects that may have occurred during sampling, and for evidence of leaks in the filterpack.
      6. Record the start and end date/time in the spreadsheet for each filterpack. Add comments specific to the filter and site conditions to the spreadsheet as appropriate. Save the file.
      7. Referring to the spreadsheet containing the filter ID number and weights, locate the filter ID number. Select the Petri dish with the accompanying filter number on the lid.
      8. Place the filter in its original labeled Petri dish. Close the Petri dish with its lid.
      9. Place Petri dish with filter in a Whirlpak (or equivalent) plastic bag for transport to the clean room for weighing.
      10. Field blank filters must be subjected the same procedures as the true sampled filters.
      11. Clean all filterpack stages three times with deionized water.
    1. Post-Weight Procedure
      1. Once the Petri dishes are sent back to the clean room for weight measurements, the following procedures need to be followed.
      2. The conditioning and weighing of the sampled filters are carried out in the same climate controlled weighing room as the unexposed filters.
      3. Place the Petri dishes containing the sampled filters on the bench.
      4. Remove the lids of the Petri dishes and place them beside the dishes.
      5. Record the filter number, temperature, pressure, relative humidity, date and time. The temperature, pressure, and relative humidity of the weighing room should be continuously recorded separately.
      6. Allow the filters to condition for 24 hours.
      7. Prepare the Mettler (or equivalent) balance and the computer as for the pre-weight procedure; but the appropriate spreadsheet file needs to be opened for recording the post-weight measurements.
      8. Proceed as in Section 9.4 to 9.10.
      9. Perform the required quality control following instructions 10.3 to 10.6 with the reference filters; but record the results as post-weighing.
      10. Relate the sampled filter ID to station ID information with the Petri dish label and record both IDs for the same sample.
      11. Follow the steps in 13.2 to proceed with the post-weighing for the sampled filters.
      12. Record the results in the spreadsheet.
      13. Post-weights must be more than the pre-weights and duplicate values must be within + 0.025 mg of each other. Post-weighing of sampled filters is done in duplicate once only.
      14. Replace filters in corresponding Petri dishes and place in small Whirlpack bags. Freeze in filter archive freezer at –18oC for later use.
  1. Aerosol Mass Concentration Calculation
    1. Data manipulation is to be done after the post-weighing is completed. Two files will be at hand for the data manipulation and combination, one for the field data, the other one for the weighing data.
    1. The field data file, generated as a result of procedures in Section 14.2 at the GAW Station, needs to be combined with the post weighing results to give final aerosol mass concentrations. This file contains detailed information on the sample station name, the ID for the samples that have gone through the weighing procedures, their start and end date and times, and their air volumes sampled. All other metadata must be included in this file to assist subsequent quality controls of the data generated from the weighing room procedures as outlined above.
    1. It must be emphasized that all necessary quality assurance and quality control checks on the field data must have been performed and that the air volumes Are volumes reported at STP? If not, need to report average T&P during filter exposure.have been properly calibrated with known standards. This is expected from the SOP for filter sampling at the station.
    1. In the mass weighing data file, quality control the weighing data. After the pre-weighing procedures in Sections 13.2 and 13.3 and the post weighing procedures in Section 14.4 are completed, inspect the weighing data for the two Quality Control filter results (QC1 and QC2) for any large outliers to identify measurement artifacts. Inspect the QC filter results for baseline drifts in the balance (e.g., Section 16.1.4).
    1. Correction to the weighing data for baseline drift over time, usually quite small compared to the total weights, may be necessary. This is accomplished by carrying out a box-car running mean of the QC data time series; the length of the box-car must ensure a smooth running mean. The time series differentials, of the running means subtracting the long term mean value, are the time-dependent baseline drift corrections, and thus must be added to the pre-weighing and post-weighing results. Since the pre- and post-weighings are conducted at different times, the corrections can be different for both weighings.
    1. In the same file, calculate the mean weights for each filter from both pre-weighing and post weighing after the baseline drift correction is completed (if such correction is necessary, e.g., for situation shown in Section 16.2). Calculate the differences from the corrected pre- and post-weighings to obtain the net aerosol masses.
    1. Create a new spreadsheet to handle the weighing data for the field blank filters. Field filter blank masses are handled with the same procedures as the true aerosol samples. For each batch of filters, the mean blank mass is calculated. The standard deviation on the mean is calculated for these field filter blanks. A method detection limit for the mass determination can be defined as the mean blank plus 3 standard deviation on the mean.
    1. The mean field filter blank mass is subtracted from the net aerosol mass of each sample filter. The resulting blank-subtracted net aerosol mass is the true aerosol mass loading for each filter sample.
    1. In cases where the blank-subtracted net aerosol mass becomes negative, the sampled aerosol mass on the filters is below the method detection limit for mass measurements. In these cases, the mass value for the filters is set to a specified flag for Below-Detection-Limit (BDL).
    1. Merge the air volume data from the field data file with the blank-subtracted net aerosol mass data. This is done within the field data file by creating a new spreadsheet. Copy the station sample ID, start and end date and time, and air volume for each filter from the existing sheet and paste them into the new spreadsheet. Copy the blank-subtracted net aerosol mass data, together with sample ID, from the weighing spreadsheet file, and paste into this new spreadsheet. Align the mass data with the air volume data to make sure that the IDs from the field file and from the weighing file match each other. This alignment is a critical part of the quality assurance procedure and thus must be done carefully to assure that no mistakes are made.
    1. In a new column of the spreadsheet, calculate the aerosol mass concentration in air by dividing the blank-subtracted net aerosol mass with the corresponding air volume for each filter sample. In cases where the mass is BDL, use specific flags.
    1. The field data file now contains the air concentrations of aerosol mass. Time series plotting of the aerosol mass concentration is made to inspect for unusual temporal variations, such as unusually high concentrations. As part of further quality control, inspect to see if these unusual variations are related to potential artifacts during sampling and/or sample handling. If yes, flag them with appropriate flag designations.
    1. At this point, the data will have gone through parts of the most important quality assurance and quality control procedures for the data.
  1. Attachments:
    1. Spreadsheet template for the NIST calibration weight and reference filter QC1 and QC2 weighings
      1. Example screen shot of the NIST calibration weight measurement results over time.


      1. Screen shot of the NIST calibration weight quality control plot.


      1. Example screen shot of the QC reference weight results.


      1. Screen shot of the QC1 reference filter quality control plot.


    1. Spreadsheet template for the pre-weighing and post-weighing of filters
      1. Example screenshot of the template.



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