The quantitative elemental characterization of particulate matter (PM) offers significant insights for controlling air quality and information about the origin, primary sources, and influence on health hazards.
What are Particulate Matters?
Particulate matters are microscopic suspended particles emitted into the atmosphere by human and natural causes. Their features directly result from the many manufacturing sources and are a complex combination of chemical compounds with varying quantities of their constituents.
Methods Used for Elemental Quantification
Ion beam analytical methods such as particle-induced gamma-ray emission (PIGE) and particle-induced X-ray emission (PIXE) can help carry out elemental quantification and effectively probe some light elements (Al, Mg, Na).
Using these methods, it is possible to identify more than 20 crustal and anthropogenic elements with only a few minutes of measurement.
Several other methods are also used, such as the energy dispersive X-ray fluorescence (ED-XRF), graphite furnace atomic absorption (GF-AAS), and inductively coupled plasma mass spectrometry (ICP-MS). However, GF-ASS and ICP-MS need a lot of time and work for sample preparation, making them very expensive.
A well-established spectroscopic method is a quantitative elemental assessment, screening, and the non-destructive qualitative evaluation of materials using XRF analysis. However, in response to advancements in X-ray technology, XRF spectrometers' analytical performance and features are continually increasing. In particular, handheld XRF (HHXRF) spectrometers have become popular as standard and analytical research instruments in various scientific fields, such as geology or cultural heritage.
What are Handheld XRF Analyzers?
Handheld XRF analyzers have excellent features, including a broad analytical range, autonomous exciting beam filters change, targeted quantification of bulk samples, user-friendly onboard software, short acquisition times, versatility in laboratory use, and minimum overall size and weight.
The mobility and adaptability of HHXRF analyzers have undoubtedly found direct use in various environmental applications, such as analyzing metalloids in food ingredients, marine plastic litter, paints, sediments, and soil.
Aim of the Study
This study demonstrated the suitability of commercial HHXRF analyzers for the elemental quantification of unidentified particulate matter samples.
The main objective was to identify the ideal operating circumstances (limit detection, sensitivity) for each set of elements. Using a least-squares model, calibration curves for 24 elements (Pb, Sr, Rb, Br, Se, As, Zn, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, Ca, K, Cl, S, P, Si, Al, Mg, Na) were generated.
Several compounds, single- and multi-element reference materials were employed to accomplish the objectives above, and limits of detection, sensitivities, and elemental intensities were investigated. A known benchtop energy dispersive XRF spectrometer was used to compare the two analyzers.
Using Handheld ED-XRF Spectrometer Air Particulate Matter Analysis
The researchers used a handheld energy dispersive X-ray fluorescence (ED-XRF) spectrometer. A 20 mm2 silicon drift detector (SDD) with an energy resolution of 140 eV FWHM at Mn-Kα (5.89 keV) helped detect characteristic X-ray radiation emitted by the sample. The analyzer could identify elements from sodium to uranium.
The current and voltage were limited by the instrument's predetermined ranges of 4.5 to 195 A and 6 to 50 kV, respectively, with the high voltage (kV)/maximum current (A) values: 50/35, 40/100, 35/113, 30/130, 25/135, 20/142, and 15/155.
The Benchtop ED-XRF Spectrometer
The benchtop analyzer comprises a side-window X-ray tube with a W/Sc anode with an 18 mm beam spot size.
Five targets can be selected using a spectrometer, including Al2O3, Mo, KBr, Ge, and CaF2, that polarize the X-ray beam with low background.
All measurements were made in a vacuum, and each sample's collection duration was around 40 minutes.
Researchers used 28 unknown particulate matter samples to be examined by benchtop ad handheld spectrometers.
Significant Findings of the Study
This study demonstrates the capability of modern HHXRF spectrometers to analyze the elements present in air particulate matter samples, distinguished by their multi-element composition and low concentrations.
Although an analysis of the analyzer's response revealed certain analytical limits, it also effectively determined a wide range of elements with adequate analytical accuracy and sensitivity.
Under different excitation circumstances (acquisition time, current, voltage, filter, atmosphere), the instrument's analytical response was investigated using several multi-element reference materials.
This approach established an experimental methodology for quantifying element concentrations. Five optimal operating conditions were produced, each aimed at a specific elemental range.
Comparing the limit of detections between a benchtop and handheld analyzer has shown equivalent results, with the handheld XRF obtaining lower results in certain circumstances (e.g. Ti, Ca, Si, Al, etc.). There was excellent agreement between the elemental contents of real particulate matter samples obtained by the handheld XRF and those found by the benchtop XRF analyzer.
A total of 14 of the 19 detected elements had percentage differences that were less than 40 percent, and the elemental concentrations beyond the limit of quantification were significantly correlated.
Reference
Sofia Eirini Chatoutsidou, Stefanos Papagiannis, Dimitrios F.Anagnostopoulos, Konstantinos Eleftheriadis, Mihalis Lazaridis, Andreas G.Karydas (2022) Application of a handheld X-ray fluorescence analyzer for the quantification of air particulate matter on Teflon filters. Spectrochimica Acta Part B: Atomic Spectroscopy. https://www.sciencedirect.com/science/article/pii/S0584854722001616
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