The assessment of a method for measurements and lead quantification in air particulate matter using total reflection X-ray fluorescence spectrometers☆
Graphical abstract
Introduction
Particulate matter (PM) is a prominent pollutant in the air, produced by human activity and natural phenomena [1]. PM morphology and chemical composition are strongly dependent on their origin [2]. Common elements in PM are aluminum, calcium, iron, magnesium, potassium, and silicon, usually present in their oxidized states. Other potentially toxic elements, such as cadmium, arsenic, chromium, lead, and nickel may also be present [3,4].
PM causes respiratory, cardiovascular [5,6], and other diseases, mainly depending on their particle size: PM with aerodynamic diameters less than 10 μm (PM10) is eliminated by the inhalation system while PM with aerodynamic diameters less than 2.5 μm (PM2.5) enters the lung alveoli and can get into the bloodstream [5,7,8]. In view of the strong interaction of PM with human health, it is important to determine the chemical composition and identify their origins [9].
The reliability and accuracy of air sampling methods depend on many factors including the concentration levels and particle size of interest. Many methods and instruments are proposed, based on specific PM physical properties, collection medium type, airflow rates, and sampling efficiency [[10], [11], [12], [13]]. The filtering media, more commonly called filter, is the most important element in PM sampling, and gravimetric filter analysis remains the reference method since the filters have proven to be a reliable medium for trapping the PM [2,[14], [15], [16], [17], [18]]. Three main kinds of commercial filters are used to evaluate specific characteristics of the particles: fibrous, porous, and capillary pore membrane filters. The collection efficiency, the pressure drop during sampling, and the analytical method employed after sampling determine the proper filter to be used [19]. The European norm EN 12341 “Air quality – Determination of the PM10 fraction of suspended particular matter – Reference method and field test procedure to demonstrate reference equivalence of measurement methods” (1999) [20], defines the principle method for PM10 analysis as sampling particulate matter on a filter. When elemental analysis must be performed, the suitable filters are: Acetate Cellulose Filters, polytetrafluoroethylene (PTFE), Polycarbonate, Borosilicate and quartz filters. PTFE filters are usually preferred since they have lower impurities than quartz and a smoother surface.
The chemical analysis of PM can be performed using sensitive wet chemistry based methods such as Atomic Absorption Spectroscopy (AAS) [21], inductively coupled plasma-optical emission spectroscopy (ICP-OES), and mass spectrometry (ICP-MS), ion chromatography and voltammetry. Reference analytical methods for heavy metals analysis of PM filter samples, established by European Directive [22,23], are Graphite furnace-AAS and ICP-MS. Today, ICP-Atomic Emission Spectroscopy (AES) and ICP-MS have become the principal analytical tools to determine trace elements in PM [24]. The main drawback of these techniques is the preparation procedure, which is time consuming, expensive, and environmentally unsustainable, since it is based on acid digestion. Problems associated with the determination of trace elements in airborne PM samples using microwave digestion method have already been discussed [25].
X-ray based techniques overcome these disadvantages since sample digestion is not necessary and are considered suitable for PM analysis on PTFE filters [26,27]. Recently, XRF spectrometry techniques have become more common in multi elemental analysis [28]. Indeed, ED-XRF is recognized as a proper technique for the determination of metals and metalloids for ambient atmospheric PM filter samples by the United Stated Environmental Protection Agency (EPA) [29].
Quantitative chemical analysis by XRF is achieved using standard samples with known concentrations: the calibration procedure is based on the relationship between elemental mass and the intensity of the element fluorescence lines assuming a constant matrix effect [30]. The volume/spatial homogeneity of the sample and the stability of the signal are fundamental to obtain reliable analytical results. Additional methods have been proposed to correct for instrumental drift inter-element interferences and matrix effects [30]. A simplified calibration procedure, known as “empirical”, directly compares the net intensity of the element fluorescence line with the concentration of the standard sample, assuming a constant matrix calibration factor. This “empirical” approach can be used when the sample in analysis and the standard sample matrix compositions are similar. Typically, direct analysis of aerosol samples has been performed by EDXRF, which establishes a relationship between X-ray intensities and a number of calibration standard samples. However, there are still some disadvantages mainly due to the lack of appropriate reference standards for quantitative analysis calibration method [[31], [32], [33]].
In more recent years, total reflection XRF (TXRF) spectrometers have also been used to analyse PM filter samples using various sample preparation procedures [28]. TXRF is a technique for the surface chemical analysis exploited in many application fields to measure low sample volumes on reflective surfaces [[34], [35], [36]]. This technique has multiple advantages, such as the simple calibration and low detection limits for many elements [37]. The assumption of the method is that the sample must be a thin film on a flat substrate. Development and commercialization of benchtop TXRF instrumentation have promoted its application in many environmental fields including the analysis of water [34,[38], [39], [40]], biomonitors [[41], [42], [43]], plants [[44], [45], [46]], biological samples [13,47,48], cosmetic [49,50], pharmaceutical and drug [[51], [52], [53]] and food [[54], [55], [56]]. Attempts have been made to achieve direct sampling of PM on quartz reflectors [57], but the possibility of PM bounce effects and the lack of specific standards for such sampling lead to low representativeness of this collection method with respect to filters.
Previous studies have shown how TXRF instrumentation can be employed to evaluate the elemental composition of PM on PTFE filters [6,58]and tree leaves [41], successfully addressing contamination and sample thickness issues by sandwiching the sample between two thin polymeric sheets, cutting the plastic ring stretching the PTFE, and placing the sample on a TXRF carrier for the analysis. This sample preparation procedure, called SMART STORE®, was developed in 2008 to measure PM filter samples by a commercial TXRF instrument to avoid possible detector damages. The benefits of sample protection and storage, and the suitability of SMART STORE® for the qualitative analysis based on X-Ray techniques have previously been discussed [59]. However, the quantitative analysis has some difficulties, related to the experimental setup and analysis.
The PM filter samples are complex samples that can be described as a smooth surface covered by particles with different sizes, shapes, and chemical compositions (see Fig. 1c). The PM deposition on the filter surface is assumed to be spatially homogeneous, and that assumption mainly depends on the sampling device. Previous works showed that the behavior of filters prepared by the SMART STORE ® procedure may be modeled with a high degree of accuracy as thin-film-like [58].
The main aim of this work is to assess the SMART STORE ® procedure to perform quantitative elemental analysis of the Pb content in PM filter samples using a TXRF spectrometer with variable incidence angle, the empirical calibration approach, and a set of novel reference standards. To demonstrate the reliability of this method, it has been applied to a set of model samples created by Atomic Layer Deposition of TiO2 thin films with a thickness of about 1 nm on silicon wafer and PTFE membrane substrates. An in-depth evaluation is discussed in this paper.
Section snippets
Samples
The samples analysed in this study are summarized in Table 1. The calibration curve was built using Pb loaded mono- and multi-element reference samples of PM on 2 μm pore size 47 mm diameter PTFE membranes, provided by Air Quality Research Center at University of California, Davis (AQRC-UCD) [26,33]. The mono-Pb samples were generated using lead acetate trihydrate salt (99.999% purity, Sigma-Aldrich, St. Louis, MO, USA) solution prepared in ultrapure water (Type 1 water, Milli-Q, Billercia, MA,
Results and discussion
A sandwiched filter having a total thickness about 350 μm doesn't fulfill the ideal conditions of total reflection of X-Rays on the quartz reflector surface [6,41,58]. Thus, the proposed method is more properly named Grazing Incidence XRF (GIXRF), even though TXRF spectrometers are used to [59] provide an enhancement of the signal compared to the intensity obtained by the conventional XRF geometry (incidence angle about 45°). However, for a quantitative analysis, issues arise related to both
Conclusions
The assessment of a method for measurement and analysis of PM filter samples using three commercial TXRF spectrometers is presented. The most suitable experimental conditions to analyse PTFE filters prepared with the SMART STORE® are determined by applying the empirical calibration approach. It is shown that sample repositioning in the commercial TXRF spectrometer (S2 Picofox) with a fixed incidence angle configuration is the most critical parameter to achieve a reproducible and accurate
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The corresponding author personally likes to thank Angelo Borgese for his crucial contribution during data analysis; Diane Eichert, Thomas Hase and Giacomo Siviero for the insightful discussions on this paper topic.
This article/publication is based upon work from COST Action CA18130 European Network for Chemical Elemental Analysis by Total Reflection X-Ray Fluorescence ENFORCE TXRF, supported by COST (European Cooperation in Science and Technology).
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Selected Paper from the 18th International Conference on Total Reflection X-Ray Fluorescence Analysis and Related Methods (TXRF 2019) held in Girona (Spain), June 25-28, 2019