Analysis of Air Filters Using EDXRF to Measure Air Quality

The commitment of the US EPA to enhance air quality across the nation has resulted in an increasing demand for the monitoring of specific elements on air filters collected in residential, industrial and recreational areas. The choice method to analyze samples related to air monitoring because of the non-destructive nature of the technique and superior sensitivity achievable is X-ray fluorescence.

The Thermo Scientific ARL QUANT’X Energy Dispersive X-Ray Fluorescence (EDXRF) instrumentation is ideal for determining concentrations of multiple elements on ambient air filters. The bench top instrument can study the elements sodium to uranium with minimal sample preparation and can accommodate filter sizes from 25mm to 47mm in diameter using a sample changer or up to 220mm if loaded manually. The ARL QUANT’X has a Si(Li) solid state detector which reduces interference for light element analysis while providing excellent heavy element response. Sensitivity for a wide range of elements is increased, a high-flux rhodium anode tube positioned to allow direct excitation from the X-ray tube or tailored excitation through a choice of eight different filters.

Instrument

The key benefits that make the ARL QUANT’X are highlighted below:

  • Peltier cooled Si(Li) detector with <155eV resolution at Mn K-alpha
  • Outstanding detection of heavy elements thanks to extra-thick Si(Li) crystal
  • Elemental range sodium (Z=11) through uranium (Z=92)
  • Excellent overall sensitivity thanks to superior area of detector crystal
  • High-flux 50W rhodium anode X-ray tube with excitation voltage of 4 to 50kV adjustable by steps of 1kV
  • 8-position x-ray tube filter wheel to maximize sensitivity for a wide range of elements
  • 10-position automated sample changer (20-position version for 32mm diameter and smaller samples)
  • Multiple element analysis under a single analytical condition
  • Wide dynamic range: ppm to percent
  • Capable of analyzing various sample types and sizes
  • Digital Pulse Processing for optimal throughput
  • Choice of 5 primary tube collimators (1.0mm to 8.8mm)
  • Ethernet connectivity for remote access
  • UniQuant standardless software enabling elemental analysis of samples with or without the use of standards
  • WinTrace software with integrated empirical, fundamental parameter and thin film modules
  • In addition to the above benefits, the technical features below support an instrument that will provide years of reliable service
  • Field transportable and rugged for mobile use
  • Low maintenance cost
  • Mechanically simple
  • Long term stability - individual calibrations remain within QC specifications for months.

Excitation Conditions

The ARL QUANT’X uses filtered radiation to optimally excite the sample and cause its constituent elements to fluoresce. The ARL QUANT’X has an eight position filter wheel to reduce spectrum background and tailor excitation energy. The various acquisition parameters are chosen for optimal excitation of the elements of concern and best limits of detection.

Sample Preparation and Presentation

Ambient air filters does not need any sample preparation prior to analysis, just an inspection of the filter for defects that may affect the analytical results. The filters are studied under vacuum conditions to eliminate the ambient atmosphere between the sample and detector thus maximizing light element sensitivity. The filters are loaded into a 10 position autosampler tray if greater than 31mm in diameter and a 20-position autosampler tray if sample is 31mm diameters or less. The autosampler trays feature removable sample holders to accommodate various size filters.

Quantitative Analysis

Analysis of ambient air filters is accomplished following the U.S. EPA Method IO-3.3 titled Compendium of Methods for the Determination of Metals in Ambient Particulate Matter as modified for use with the ARL QUANT’X. Individual empirical calibrations are done using commercial, single and dual element, thin film standards from Micromatter in addition to blank films.

Verification of the calibrations is accomplished by analyzing a National Institute of Standards and Technology (NIST) thin film standard (SRM 2783). Method precision is established through numerous analyses of a multi-element filter from Micromatter as shown in Table 1. The standard was analyzed with each sample tray of air filters over a one year period. As can be seen from the data, excellent relative precision of less than 0.5% for all elements present is achievable with the ARL QUANT’X.

Table 1. Repeatability of analysis over 1 year using Micromatter Multi-Element QC sample.

Element n Min Mg/cm2 Max Mg/cm2 Avg. Mg/cm2 Std.Dev. Mg/cm2 % RSD
Si 631 5.19 5.29 5.24 0.022 0.41
Ti 631 6.59 6.69 6.64 0.022 0.33
Fe 631 6.86 6.95 6.90 0.015 0.22
Cd 631 5.85 6.00 5.91 0.023 0.39
Se 631 3.94 4.05 4.00 0.019 0.47
Pb 631 9.23 9.39 9.30 0.020 0.21

n = Number of Observations
Min = Minimum Value Observed
Max = Maximum Value Observed
Avg. = Average Value Observed
Std. Dev. = Standard Deviation
%RSD = % Relative Standard Deviation ((Std Dev/Average)*100)

For a majority of the elements listed, the MDLs achievable with the ARL QUANT’X are as good as or better than those reported by the U.S. EPA in Compendium Method IO-3.3. The detection levels calculated from the ARL QUANT’X data range from less than 1ng/cm2 to 11ng/cm2. A graphical representation of the 1-sigma MDLs achievable with the ARL QUANT’X versus those reported by the U.S. EPA in Compendium Method IO-3.3 is presented below.

Limits of Detection

Minimum detection limits (MDLs) are an essential 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. The ARL QUANT’X MDLs displayed in Table 2 and the accompanying graph are calculated using the average instrument uncertainty from ten laboratory blanks. The ARL QUANT’X data is compared to those specified in the U.S. EPA Compendium Method IO-3.3. The MDLs are reported as 1σ (68% confidence level) as outlined in the U.S. EPA Compendium Method.

Figure 1. Spectrum of Micromatter Multi-Element QC Standard.

Below is a spectrum displayed by the ARL QUANT’X acquired from the Micromatter Multi-Element QC standard with the elements of interest identified. Note the excellent spectral resolution obtained leading to superior count rates, repeatability, and sensitivity.

Figure 2. Comparison of ARL QUANT’X limits of detection and U.S EPA method.

Table 2. Comparison of ARL QUANT’X limits of detection and U.S EPA method.

Analyte ARL QUANT’X MDLS (ng/cm2) – 1 sigma Method IO-3.3 MDLS (ng/cm2) – 1 sigma
Na 8.9 5.3
Mg 3.4 3.2
Al 3.8 17.6
Si 2.7 8.0
P 2.7 2.6
S 2.0 2.6
Cl 2.2 4.8
K 2.1 6.3
Ca 2.2 9.0
Sc 3.5 1.5
Ti 1.5 16.9
V 1.1 5.3
Cr 0.7 3.0
Mn 0.5 0.8
Fe 0.5 0.7
Co 0.4 0.4
Ni 0.3 0.6
Cu 0.5 0.7
Zn 0.5 1.0
Ga 0.6 1.6
As 0.3 0.8
Se 0.4 0.7
Br 0.4 0.6
Rb 0.6 0.7
Sr 0.7 1.1
Y 0.8 1.2
Zr 1.0 1.2
Mo 0.9 1.6
Pd 3.1 22.9
Ag 3.7 20.2
Cd 4.9 22
Sn 5.8 30.5
Sb 11.1 31.4
Cs 3.2 48.9
Ba 3.1 51.8
La 2.6 70.6
W 1.8 3.4
Au 1.2 1.7
Hg 1.5 1.5
Pb 0.8 1.5

Conclusion

These results show the extraordinary performance that the ARL QUANT’X can achieve for air filter analysis. The detection levels range from 0.3 nanogram/cm2 to 11 nanogram/cm2. Performance for heavy and light elements alike is extremely good. Furthermore our one year long term test proves the excellent stability of the ARL QUANT’X instrument for such analysis.

About Thermo Fisher Scientific - Elemental Analyzers

Thermo Fisher Scientific Inc. is the world leader in serving science. Their mission is to enable their customers to make the world healthier, cleaner and safer. With revenues of $17 billion, they have approximately 50,000 employees and serve customers within pharmaceutical and biotech companies, hospitals and clinical diagnostic labs, universities, research institutions and government agencies, as well as in environmental and process control industries.

They create value for their key stakeholders through four premier brands, Thermo Scientific, Life Technologies, Fisher Scientific and Unity™ Lab Services, which offer a unique combination of innovative technologies, convenient purchasing options and a single solution for laboratory operations management. their products and services help our customers solve complex analytical challenges, improve patient diagnostics and increase laboratory productivity.

For over 80 years, their Ecublens site in Switzerland has been a worldwide supplier of spectrochemical instrumentation to such major industries as steel, transportation, cement, construction, food, pharmaceuticals, chemicals, academic research, petroleum and electronics. They offer unsurpassed capabilities in the areas of optical emission (OE), X-ray fluorescence (XRF), X-ray diffraction (XRD) and automation of spectrometers.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

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