Comparison of Filters for Particle Monitoring in Air Quality Analysis

 


Global Clean Air Initiatives

There are regions of the world with unhealthy and crisis-level air quality. Many places including the Americas, Europe, Japan, China, ASEAN (Indonesia, Malaysia, Philippines, Singapore, Thailand and Vietnam), and Africa are striving for global responsibility and improvement. Japan, for example, is taking steps to avert the crisis by creating and calling the 21st Century an “Environmental Century.” They have made air quality one of the top programs to implement in urban areas1 . Due to these worldwide efforts, air quality standards have been formulated, and have necessitated methods to accurately measure, test, and report air quality results. Air monitoring sites and test labs throughout the world must abide by national and local standards. Test results can steer local action, provide health quality information to citizens, and drive improvement of global air quality. Progress has been seen in the United States where there has been a 40% improvement in air quality between 2000 and 20172.

 

To ensure analytically-valid testing, international regulated methods, including United States EPA 40 CFR Part 50 Appendix L, are used to determine the air quality of particulate matter (PM) - small solid or liquid matter suspended in the air, including dust, ash, soot, smoke, and sea salt aerosols that have been shown to have adverse health effects when inhaled3. Generally, local country test labs acquire test filters, collect air particles, measure the amount of air particles present in specific areas as dictated by standard, and report findings. Many manufacturers supply standardized, compliant, or equivalent filter products for air particle monitoring methods. These filters must fall within strict performance and tolerance criteria to be acceptable for use including particle collection efficiency, loose surface particles, moisture pickup, temperature stability, alkalinity, pore size, and dimensional parameters (membrane disc diameter, ring width, and filter thickness). Some air filter product manufacturers have added new features to filter products to improve use, handling, and compliance. Examples include packaging improvements, serial coding, handling ring design, and certifications (see Figure 1).
Air filter membranes for the measurement of PM2.5 (particles with diameters that are generally 2.5 µm or smaller) were evaluated from different manufacturers to determine suitability for use in EPA 40 CFR Part 50 Appendix L and other similar methods:

•  PTFE filter membrane discs for PM2.5 Particle Monitoring
   (Product PM2547050)
•  PTFE filter membrane discs from Manufacturer B (Product B)
•  PES filter membrane discs from Manufacturer C (Product C)
•  PTFE filter membrane discs from Manufacturer D (Product D)


Data were generated from methods including ASTM-D2986-95 pressure drop 4, ASTM-F316-03(2011) for bubble point, and ASTM-F316-94. Additionally, X-ray fluorescence (XRF) performance was included in the evaluation due to increasing interest in elemental background profile. These results showed that our PTFE filter membrane discs consistently met criteria for compliance with regulatory guidelines for PM2.5 particle monitoring. Additionally, our packaging facilitates filter verification, sample collection, sample traceability, and measurement, exceeding compliance requirements.


Quality compliance certificates included with PTFE filter membrane discs

Figure 1. Consumers require quality, out-of-the-box solutions for air monitoring. “Quality Compliance Certificates” ensure air monitoring products meet regulations. Having in-hand information packaged with the product can help verify use of the correct product for the regulated method. We provide “Quality Compliance Certificates” directly in the box.

 

Evaluation of Technical Requirements for Air Monitoring Filters

The United States and many other countries are using the 40 CFR Appendix L to Part 50 reference method as a directive or guidance document for the determination of fine particulate matter as PM2.5 in the atmosphere. Other countries rely on similar methods. We compared pressure drop, particle collection efficiency, size, thickness, rating, and cleanliness of air filter membranes from four manufacturers.

Pressure Drop and Particle Collection Efficiency

Four PM2.5 membrane assay filters were tested with international standard ASTM-D2986-91 “Evaluation of Air Assay Media by the Monodisperse DOP (Dioctyl Phthalate) Smoke Test” using a specification of ≤ 30 cm at 16.7 L/min. A metered portion of generated 0.3 µm fine monodispersed DOP particles were aerosolized and drawn through a membrane using this highly sensitive test at a defined flow rate of 16.7 L/min. A forward light scattering, linear photometer was used to measure upstream and downstream particles to determine pressure drop and collection efficiency. A pressure drop of ≤ 30 cm must be adhered to for air monitoring. PM2547050, as well as the three other filters, each exhibited maximum pressure drop values and particle collection efficiencies that met guidelines (Figure 2).

 

Maximum pressure drop and average particle collection efficiency for air monitoring

Figure 2. Determination of pressure drop (A) and particle collection efficiency (B). Values and performance data generated during particle penetration are shown in the above graphs. All membrane types fell well within the guidelines ≤ 30 cm pressure drop and ≥ 99.7 % efficiency at a flow rate of 16.7 L/min.

Dimensional Parameters

Defined pore size ratings of 2 µm are required for air particle testing, and the physical geometry of the test filter ensures product compliance to standardized methods and compatibility with approved monitoring equipment. This makes reporting results from region-to-region and country-to-country comparable on the same scale.

Pore size rating using the bubble point method: This method is a practical, nondestructive test used for estimating the pore size of microporous filters. This test measures the minimum pressure required to force liquid out of the membrane pores as an indirect survey of pore size. Bubble point is inversely proportional to pore size (i.e., a high bubble point is indicative of a small pore size). Importantly, this test provides information about the largest pore in a membrane, since the largest pore requires the lowest pressure to push liquid through. These ratings are determined by bubble point pressure correlations using bubble point ASTM - F316-94 and US EPA 40 CFR Part 50 Appendix. Results from bubble point testing showed PM2547050, as well as the three remaining filters, had suitable pore sizes as estimated by bubble point (Figure 3). The filter from Manufacturer C had the largest calculated pore size as derived from bubble point values, while the filter from Manufacturer D had the smallest calculated pore size.

 

Bubble point test for measuring pore size for air filters used in particle monitoring

Figure 3. Membrane pore size rating. Isopropyl alcohol (100%) was used to test for bubble point determination. The results show that the manufacturers’ disc bubble points are similar and suitable, though some are more variable for this application. The lowest pressures were observed with the filter from Manufacturer C, indicating that this membrane had the most open pore size. The filter from Manufacturer D had the tightest pore size.


Geometry and dimensional tolerances:

Air monitoring discs are circular with a specified diameter of the filter disc (outer 46.2 ± 0.25 mm) and ring width (3.68 + 0 mm, - 0.51 mm). These specifications are important to ensure appropriate fit in defined collection devices. These dimensions are also used to calculate the air quality using a uniform-sized surface area, enabling consistent reporting from geography-to-geography. Both PM2547050 and the filter from Manufacturer B consistently met requirements for dimensional tolerance (Figure 4). Filters from Manufacturer C and D showed variation in replicate values beyond the threshold.

Filter disc parameters for air quality monitoring

Figure 4. Geometry and dimensional tolerance for air monitoring filters.

 

Filter thickness: Filter thickness is fundamental for particle capture efficiency, as well as strength and handling. Thinner filter discs may not be able to withstand test pressures, making them harder to handle in the field or lab. Guidelines specify filter thickness tolerance at 40 ± 10 µm. All four filters met specification tolerances for filter thickness (Figure 5).

Filter thickness parameters for air quality monitoring

Figure 5. Filter thickness tolerance. All filters met specification tolerances, the filter from Manufacturer D being the thinnest.


Alkalinity

Another measured feature is alkalinity (specification ≤ 25 µeq / filter disc). Filter membrane alkalinity is a predictor of artifact formation resulting from adsorption of acid precursors, such as sulfur dioxide and nitrogen oxides, by the filter. These precursors can be subsequently converted to particulate sulfate and nitrate. This can result in overestimation of particulate in the sample 5. Alkalinity measurements showed acceptable alkalinity for PM2547050 and the three other filters tested (Figure 6).

Alkalinity specifications for filters used in air particle monitoring

Figure 6. Alkalinity testing of air quality filters.


Loose Particles and Particles After Temperature Stability

Low levels of particles are desired as the baseline measurements for any valid test. A tight tolerance is adhered to for loose particles and particles after a temperature test (≤ 20 µg / filter disc). Particles are measured by mass (µg) for these tests. Our filter and three other filters fell within specification (Figure 7).

Loose particle measurement for filters used in air particle monitoring

Figure 7. Filter types were tested (0.1% of lot); all fall well within specification.


Elemental Composition

There is increasing interest in the use of X-ray fluorescence (XRF) for analysis of particulate matter. For samples analyzed by XRF, the elemental background profile of the filter membrane disc is relevant. Low levels of metals are desired to minimize baseline interference. Our filter was analyzed by XRF to measure and categorize concentrations of metal for the membrane disc (ng/cm2) 6. Data showed that metal concentrations for our filter membrane were quite low, with the majority significantly under 3 ng/cm2(Figure 8), demonstrating suitability for use with XRF analysis.

Metal profile for PM2.5 filters used in XRF analysis

Figure 8. Elemental composition analysis by XRF. Low levels are desired to minimize baseline interference.

Comparison of Handling Features of Air Monitoring Filters

Packaging

Filters used in air quality monitoring have strict performance guidance requirements; however, packaging is not standardized. Features such as low particle-creating packaging material are being adopted. Detailed, easy-to-read labels denoting filter type are important. Packaging should also be easy to open and close in critical test areas. Packaging was compared across the four different manufacturers. PM2547050 was easy to open, with the filter type clearly labeled to avoid errors. A quality certificate was included directly in the box, providing immediate verification of filter compliance. We observed that when filter membranes from Manufacturer C were removed from the box, they curled slightly. This made the filter hard to open or place into filter holders, and potentially reduced the surface area for collecting particles. Filters from Manufacturer C were also packed in groups of five with five pieces of blue relief plastic between each filter, generating more waste and making it difficult to remove a single filter for use.

Sample Traceability

Filters can be provided with serial numbers, bar-coding, or smart codes to enable better sample tracking. Representative images of each type filter disc with support rings are shown in Figure 9. Upon observation, differences were seen in serial marking location, general font legibility, and overall print quality. The serial number on PM2547050 was printed in dark font with good resolution and conveniently located on the handling disc, making the text easily legible. It was difficult to read the serial number on filters from Manufacturer C due to poor printing. The serial number on the filter from Manufacturer D was not placed on the ring, but rather on the filter itself – potentially interfering with measurement.

 

Filter disc comparison for air particle monitoring

Figure 9. All filter discs are white with support rings on the outer diameter. Visual differences can be seen in serialization marking location, general font readability and overall print quality by manufacturer type.


Conclusions

Reliable air quality testing performance is critical. Sampling performed on filters necessitates high standards for filter quality, performance, and compliance. Worldwide governing bodies have created strict filter performance and tolerances for adherence. It is the responsibility of labs and agencies worldwide to use validated or equivalent filters meeting requirements.

Our PTFE filter membrane discs consistently met criteria for compliance with regulatory guidelines for PM2.5 particle monitoring, including US EPA 40 CFR Part 50 Appendix L. Additionally, our PM2.5 filter was designed with packaging features that facilitated sample collection, traceability, and measurement, exceeding compliance requirements.

 

 

 References

  1. Japan, M. o.-G. (2012). The Environmental Century (Japan) (MOE) is Tackling Challenges. Japan, Ministry of the Environment.
  2. EPA, U. Air Quality Designations for Particle Pollution. 2018. Retrieved from United States Environmental Protection Agency: https://www.epa.gov/particle-pollution-designations
  3. US EPA, OAR. Health and Environmental Effects of Particulate Matter (PM). US EPA. 26 April 2016. Retrieved 5 October 2019.
  4. Taylor, J. K. Quality Assurance of Chemical Measurements. Chelsea, MI: Lewis Publishers, Inc. 1987.
  5. Witz, S. Effect of Environmental Factors on Filter Alkalinity and Artifact Formation. Environ Sci Technol. 1985;19(9):831-835.
  6. Brouwer, P. Theory of XRF. PANalytical, ISBN 90-9016758-7. 2010. Retrieved from PANalytical B.V.