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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.

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

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.

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.

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.

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.

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.

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.

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

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.

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