The United States Environmental Protection Agency’s (U.S. EPA) commitment to improve air quality across the USA has created an increasing demand for the monitoring of specific elements in ambient particulate matter collected on filters in residential, industrial and recreational areas.
“research now strongly suggests that atmospheric particles commonly occur in two distinct modes, the fine (<2.5 µm) mode and the coarse (2.5 to 10.0 µm) mode. The fine or accumulation mode (also termed the respirable particles) is attributed to growth of particles from the gas phase and subsequent agglomerization, whereas the coarse mode is made up of mechanically abraded or ground particles. Because of their initially gaseous origin, the fine range of particle sizes includes inorganic ions such as sulfate, nitrate, and ammonium as well as combustion-form carbon, organic aerosols, metals, and other combustion products. Coarse particles, on the other hand, normally consist of finely divided minerals such as oxides of aluminum, silicon, iron, calcium, and potassium. Samplers which separate PM into two size fractions of 0-2.5 µm and 2.5-10 µm are called dichotomous samplers. In 1997, the EPA promulgated a new standard with fine particles. The new PM2.5 standard replaced the previously NAAQS for PM10.”
X-ray fluorescence is the preferred method to analyze samples related to air monitoring due to the non-destructive nature of the technique and the superior sensitivity achievable. XRF analyzers determine the chemistry of a sample by measuring the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays (“a fingerprint”) that is unique for that specific element, which is why XRF spectroscopy is an excellent technology for qualitative and quantitative analysis of material composition.
Energy Dispersive X-Ray Fluorescence (EDXRF) instrumentation is used for determining concentrations of multiple elements on air filters. EDXRF is designed to analyze groups of elements simultaneously in order to rapidly determine those elements present in the sample and their relative concentrations—in other words, the elemental chemistry of the sample.
EDXRF Spectrometers provides major, minor and trace element quantification across the broadest range of samples, including bulk solids, granules, powders, thin films and liquids – thus meeting the elemental analysis needs of central and contract laboratories, as well as the environmental monitoring industry, when it comes to air filter analysis.
Analysis of air filters should be accomplished following the U.S. EPA Compendium Method IO-3.3 titled Determination of Metals in Ambient Particulate Matter as modified for use with specific spectrometers. Individual empirical calibrations can be achieved using commercial, single and dual element, thin film standards in addition to blank films. Verification of the calibrations can be accomplished by analyzing a National Institute of Standards and Technology (NIST) standard reference material (SRM) 2783 Air particulate on filter media.
Air filters require no sample preparation prior to analysis except for an inspection of the filter to identify any defects that may affect the analytical results. However, the filters should be analyzed under vacuum conditions to eliminate the ambient atmosphere between the sample and detector thus maximizing light element sensitivity.
When considering air filter analysis equipment, look for instruments that can accommodate filter sizes from 25 mm to 47 mm in diameter using a sample changer or up to 220 mm if loaded manually, as well as state-of-the-art silicon drift detectors (SDD) which minimize spectral interference while providing excellent response. A large active area of 30 mm2 ensures a large solid angle for efficient X-ray collection. A high-flux rhodium anode tube, positioned to allow direct excitation from the X-ray tube or tailored excitation through a choice of many different filters, maximizes sensitivity for a wide range of elements.
Minimum detection limits (MDLs) are an important analytical component as they define use limitations of the acquired data. The determination of MDLs for ambient air filter analysis is achieved through the repeated analysis of multiple laboratory blanks.
We recently published an application note that outlined why Energy Dispersive X-Ray Fluorescence (EDXRF) instrumentation is ideal for determining concentrations of multiple elements on air filters. It showed how a bench top instrument is capable of analyzing the elements sodium to americium with minimal sample preparation. Read the application note, which discusses the instrument used, excitation conditions, sample preparation and presentation, the spectrum, excitation conditions, limits of detection, comparison charts, and results. View the application note, Analysis of air filters using the ARL QUANT’X EDXRF Spectrometer.
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