Measuring Air Quality with Energy Dispersive X-Ray Fluorescence (EDXRF)

Increasing demand for the monitoring of specific elements on air filters has resulted from the commitment of the US EPA to enhance air quality across the nation in residential, industrial and recreational areas. X-Ray fluorescence is the ideal method to analyze samples related to air monitoring, thanks to its non-destructive nature and superior sensitivity.

When it comes to determining concentrations of multiple elements on ambient air filters, the ideal instrumentation is the Thermo Scientific ARL QUANT’X Energy Dispersive X-Ray Fluorescence (EDXRF) .

Elements from sodium to uranium can be studied with minimal sample preparation using the bench top instrument, which can accommodate filter sizes ranging from 25 mm to 47 mm in diameter using a sample changer or up to 220 mm if manually loaded.

The ARL QUANT’X provides an excellent heavy element response, thanks to its Si(Li) solid state detector, which also reduces interference for light element analysis. Sensitivity for a wide range of elements is increased. A high-flux rhodium anode tube is also positioned to allow direct excitation from the X-Ray tube or tailored excitation through a choice of eight different filters.

Instrument

There are a range of key benefits of the ARL QUANT’X:

• Outstanding detection of heavy elements due to extra-thick Si(Li) crystal
• Peltier cooled Si(Li) detector with <155 eV resolution at Mn K-alpha
• Elemental range sodium (Z=11) through uranium (Z=92)
• Excellent overall sensitivity thanks to superior area of detector crystal
• High-flux 50 W rhodium anode X-Ray tube with excitation voltage of 4 to 50 kV adjustable by steps of 1 kV
• 10-position automated sample changer (20-position version for 32 mm diameter and smaller samples)
• 8-position X-Ray tube filter wheel to maximize sensitivity for a wide range of elements
• Multiple element analysis under one analytical condition
• Capable of analyzing various sample types and sizes
• Wide dynamic range: ppm to percent
• Choice of 5 primary tube collimators (1.0 mm to 8.8 mm)
• Digital Pulse Processing for optimal throughput
• UniQuant standardless software enables elemental analysis of samples with or without the use of standards
• Ethernet connectivity through remote access

In addition to the above benefits, the technical features below support an instrument that will provide years of reliable service:

• WinTrace software with fundamental parameters, integrated empirical, and thin film modules
• Field transportable and rugged for mobile use
• Mechanically simple
• Low maintenance cost
• Long term stability - individual calibrations remain in QC specifications for months.

Excitation Conditions

In order to optimally excite the sample and encourage its constituent elements to fluoresce, the ARL QUANT’X uses filtered radiation. The ARL QUANT’X has an eight-position filter wheel to reduce spectrum background and tailor excitation energy. The factors influencing various acquisition parameters include the optimal excitation of the elements of concern and best limits of detection.

Sample Preparation & Presentation

Ambient air filters do 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 in order to remove the ambient atmosphere between the sample and detector thus maximizing light element sensitivity.

If greater than 31 mm in diameter, the filters are loaded into a 10-position autosampler tray and if sample is 31 mm diameters or less, a 20-position autosampler tray. To accommodate various size filters, the autosampler trays feature removable sample holders.

Quantitative Analysis

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 ensures analysis of ambient air filters. Commercial, single and dual element, thin film standards from Micromatter in addition to blank films are performed using individual empirical calibrations.

The calibrations are verified by analyzing a National Institute of Standards and Technology (NIST) thin film standard (SRM 2783).

As shown in Table 1, method precision is established through numerous analyses of a multi-element filter from Micromatter. Over a 1-year period, the standard was analyzed with each sample tray of air filters. An excellent relative precision of less than 0.5% for all elements present is achievable with the ARL QUANT’X, as can be seen from the data.

Table 1. Repeatability of analysis over 1 year using Micromatter Multi-Element QC sample. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

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)

The MDLs achievable with the ARL QUANT’X are equal to or better than those reported by the U.S. EPA in Compendium Method IO-3.3 for a majority of the elements listed. The range is between 1 ng/cm² to 11 ng/cm² for the detection levels calculated from the ARL QUANT’X data.

Below is 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.

Limits of Detection

Given that minimum detection limits (MDLs) define the use limitations of the acquired data, they are an essential analytical component. Through the repeated analysis of multiple laboratory blanks, the determination of MDLs for ambient air filter analysis is achieved.

The average instrument uncertainty from ten laboratory blanks is used to calculate the ARL QUANT’X MDLs displayed in Table 2 and the accompanying graph. The comparison is made between the ARL QUANT’X data and those specified in the U.S. EPA Compendium Method IO-3.3. As outlined in the U.S. EPA Compendium Method, the MDLs are reported as 1 σ (68% confidence level).

Figure 1. Spectrum of Micromatter Multi-Element QC Standard. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

A spectrum is displayed below, showing the elements of interest identified by the ARL QUANT’X acquired from the Micromatter Multi-Element QC standard. As can be seen in the results, the excellent spectral resolution obtained generated superior repeatability, count rates, and sensitivity.

Figure 2. Comparison of ARL QUANT’X limits of detection and U.S EPA method. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Table 2. Comparison of ARL QUANT’X limits of detection and U.S EPA method. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Conclusion

The extraordinary air filter analysis performance that can be achieved using the ARL QUANT’X is reflected in these results. The detection levels range from 0.3 nanogram/cm² to 11 nanogram/cm².

Furthermore, there is an extremely good performance for heavy and light elements alike. Moreover, the excellent stability of the ARL QUANT’X instrument for such analysis is proven through the company’s one-year-long term test.

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

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