The presence of toxic pollutants in the air has been a subject of research for many years in countries around the world. In the United States, air quality standards are governed by the ‘Clean Air Act’ and administered by the US Environmental Protection
Agency (EPA). One of the key areas of concern for the US EPA is the Suspended Particulate Matter (SPM) of air. Research on the health effects of SPM in ambient air has focused increasingly on particles that can be inhaled into the respiratory system, i.e. particles of aerodynamic diameter of < 10 µm. These particles are referred to as PM10 (2.5 – 10 µm) and PM2.5 (<2.5 µm). As well as chemical toxicity, these particles are a significant threat to health.
The elemental analysis of the SPM on these air filters is traditionally performed by energy dispersive X-ray fluorescence spectroscopy (EDXRF) using EPA method IO-3.3, which outlines the protocol of the analysis of 44 elements on air filters. This application notes shows the capability of the Air Quality edition of Epsilon 4, a bench top EDXRF spectrometer, as an analytical tool for the analysis of air filters according to the US EPA method IO-3.3.
Measurements were performed using an Air Quality edition of Epsilon 4, equipped with a 15 W, 50 kV silver anode X-ray tube, 6 beam filters, a helium purge facility, a high-resolution Silicon Drift Detector, a sample spinner and a 10-position removable sample changer.
In total, 71 single-element air filter standards from Micromatter Co (Eastsound, WA) were used to set up the calibrations; series 17833 to 17887 with Mylar supporting film backing, series 41360 to 41387 with polycarbonate aerosol membrane backing, and a few blank standards including NIST SRM 2783 blank.
Five measurement conditions were used to optimize the excitation of groups of elements (Table 1). The total measurement time per sample was 25 minutes. The measurement time for each condition can be optimized according to specific needs.
The Epsilon software features a powerful deconvolution algorithm, which analyzes the sample spectrum and determines the net intensities of element peaks, even when they overlap one another. This is essential to perform accurate trace element analysis.
Table 1. Five optimized measurement conditions
To evaluate the accuracy of the method, two physically different NIST SRM 2783 air filter standards (# 405 and # 431) were measured 10 consecutive times each. For each element, the average concentration and standard deviation (SD, 1 sigma) of the measurements were compared with the certified value reported on the certificate (see Table 2).
The results show a good agreement between the certified and measured concentrations of both air filter standards. For some of the elements the concentration in the filter standard is lower than the detection limit of the method. For those elements, a “<” is reported.
Table 2. Accuracy results of NIST SRM 2783 air filter standard
Detection limits (LLD)
Detection limits are an important measure of an instrument’s performance. The detection limits (LLD) for this application were calculated from 20 replicate measurements of a blank sample and are based on 1 sigma (as specified in method IO-3.3). Calculations are based on the application time (25 minutes) and are shown in Table 3. As a comparison, the LLD values reported in the EPA method are also shown. With a measurement time of 45 minutes per sample, all LLD values are smaller than the EPA limits (see Table 3.) and therefore the Air Quality edition of Epsilon 4 fully complies with the EPA IO-3.3 method.
Table 3. Detection limits (LLD, 1 sigma) for particulate matters on air filters
The Air Quality edition of Epsilon 4 is fully capable of analyzing particulate matter on air filters according to US EPA IO-3.3 with a high degree of accuracy for a wide range of elements across the periodic table. The non-destructive nature of the method makes it possible to increase measurement times in case an even lower detection limit is needed. Furthermore, samples can be measured repeatedly without damage, ensuring the longevity of standards.