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1、Accepted for platform presentation at the Global Plastics Environmental Conference, GPEC-2004February 18 19, 2004, Detroit, MichiganFEASIBILITY OF ANALYSIS AND SCREENING OF PLASTICS FOR HEAVY METALS WITHPORTBALE X-RAY FLUORESCENCE ANALYZER WITH MINIATURE X-RAY TUBEStanislaw PiorekNiton LLC, 900 Midd
2、lesex Turnpike, Billerica, MA 01821AbstractMetals and metal compounds have been used for many years in the manufacture of plasticproducts. The metallic compounds added to plastics although encapsulated in polymer matrix, areusually not chemically bound to polymer molecules and consequently can be gr
3、adually released into theenvironment over the service life of a plastic made object.Similarly, when disposing of plastic waste, either by incineration or by placing it in a landfill, toxicmetals released from plastics can enter the atmosphere or leach into soil. Environmentally responsiblehandling o
4、f plastics requires monitoring of potentially toxic elements in plastics during their production,recycling and disposal operations.In this paper we report on the application of a small, lightweight (1.5 kg), battery operatedportable X-ray fluorescence analyzer for in-situ analysis and screening of p
5、lastic for toxic metals.IntroductionElements such as lead, cadmium, chromium, mercury, bromine, tin and antimony are, or havebeen, added to polymers as pigments, fillers, UV stabilizers, and flame retardants. Typically theseelements are added as compounds which often do not chemically bond with mole
6、cules of plastic, butrather create a suspension in the solid plastic polymer. Therefore, in time they may potentially dislodgefrom the plastics matrix. The finer the particles of added compound, the easier it is for them to beremoved. A visible symptom of this process is a hazing on the surface of s
7、ome plastics caused bymigration of bromine from the bulk of the material to its surface. PVC based plastics contain considerableamounts of chlorine which, when released, facilitates the leaching of metals into the environment. Thiscan create serious health and environmental problems, as most of thes
8、e elements have been identifiedas toxic to humans. Specifically, because many toys and objects of common use are made of plastics,they pose a particular danger to small children. The initiatives undertaken to correct this growingproblem target the maximum allowable concentrations of toxic metals in
9、plastics, and ultimately aim tocompletely eliminate them from production.The first regulations to specifically target heavy metals in plastics were introduced in the midnineties by the European Community. The European Community Packaging Directive - EC-Directive94/62/EEC, 1 regulates the total amoun
10、t of metals such as Cd, Cr, Hg and Pb in plastic packagingmaterials to less than 100 mg/kg. Another EU Directive, 91/338/EC 2, sets the maximum allowableconcentration of cadmium in plastics used for consumer goods at 100 mg/kg. In the US, Proposition65 introduced in California banned cadmium from us
11、e. A separate effort is directed at proper handlingof plastic waste. Specifically, European Council Directive 2002/96/EC on waste electrical and electronicequipment (WEEE) 3, mandates the removal of all plastic containing brominated flame retardants, allmercury containing components, batteries, etc.
12、 from such wasteThe effective enforcement of such regulations requires that sensitive and reliable methods andinstruments be readily available for responsible parties. For example, given the amount of importedgoods crossing borders every day, the task of customs inspectors to effectively inspect the
13、 goods ispossible only with a small, portable, easy to operate analytical tool(s) that can quickly identify andquantify prohibited elements in plastic products.Over the last decade, small, lightweight Field-Portable XRF analyzers (or FPXRF) have becomeindispensable tools for accurate and rapid in-si
14、tu analysis and identification of alloys in metals production,fabrication and recycling. A second major use of these instruments has been in soil screening andanalysis for heavy metals 4. The problems encountered when attempting XRF analysis of plastics arein many ways similar to those that had to b
15、e addressed in analysis of soil. Therefore, the use of portableXRF analyzer to measure the concentration levels of metals in plastics seems to be a natural extension ofits already tested capabilities for soil analysis.The Principle of XRF AnalysisX-ray fluorescence analysis is based on the phenomeno
16、n of the emission of x-rays by the atomsof a sample when excited by an external source of radiation. If a gamma- or sufficiently energetic x-rayfrom an isotope or x-ray tube impinges on an atom of the sample material, it may eject one of the innershell electrons of the atom. The vacancy created is a
17、lmost instantaneously (in less than 10-8 sec.) filledby one of the electrons from the higher energy shell. The energy difference between the two energyshells involved in the process is emitted in the form of x-ray radiation. We call this radiation acharacteristic x-ray because its energy is specific
18、 and unique to each atom. By being able to measurethe energy and intensity of the characteristic x-rays, we realize qualitative and quantitative aspects ofXRF analysis.X-ray fluorescence spectrometry has long been recognized as a major analytical tool, originally inthe wavelength dispersive (WDXRF),
19、 and later also in the energy dispersive (EDXRF) version. Thereprobably is no metallurgical facility without at least one WDXRF spectrometer. The speed, reliability andtruly nondestructive character of the XRF method make it suitable not only for laboratory applications, butespecially for field and
20、plant use. However, the successful expansion of x-ray fluorescence analysis fromlaboratory to plant and field environments was made possible only by the recent availability of portableXRF analyzers. Several critical factors have contributed to this, namely:-the availability of small, sealed radioiso
21、tope sources to excite the characteristic x-rays of thesample;-the availability of small, room temperature, high energy resolution detectors;-the availability of powerful microprocessors; and-the availability of compact, high capacity, rechargeable batteries to make the instrumentindependent of the
22、AC power.In particular, developments in microprocessor technology have made it possible for portable,battery-operated x-ray analyzers to perform in real time complex analyzes of the x-ray spectra from thesample, followed by sophisticated data processing; a task previously assigned only to off-line c
23、omputers.The portable, microprocessor based x-ray analyzers have been a real breakthrough by combining speedand truly nondestructive character of analysis (the instrument being brought to the sample and nototherwise) with an expert identification/sorting software. Most recently, another technologica
24、lbreakthrough - the development of a miniature, low power x-ray tube - improved the performance ofportable XRF analyzers and made them even easier to use. Replacing the isotope with an x-ray tubenot only improves the instruments sensitivity and analytical range, but also makes it easier to transport
25、and overall safer to operate because no x-rays are emitted from the x-ray tube once the power to theinstrument is turned off.ExperimentalThe AnalyzerThe portable analyzer used in this work is shown in Figure 1. It is an ergonomically designedhand-held, lightweight (1.5 kg), environmentally sealed, b
26、attery operated unit. It employs a miniature, lowpower x-ray tube as the source of sample excitation. The end-window, transmission anode tube operates2at 38 kV of High Voltage and 20 uA of current. A high resolutionsilicon p-i-n diode detector (energy resolution better than 230 eV )is used to detect
27、 and register characteristic x-rays from thesample. The instrument is powered by a rechargeable Li-ionbattery which sustains its operation for up to about 8 to 10 hours.A proprietary operating system controls all the functional blocks ofthe analyzer. The operator communicates with the analyzer via a
28、built-in touch screen display with an intuitive user interface. Thefront end (nose) of the analyzer is a flat plate with a small 10 by20 mm rectangular window sealed with polyimide foil. This plateprovides a repeatable means of sample presentation for analysis.The measurement of solid, extended samp
29、les is performed bypressing the nose of the instrument against the analyzed objectand pulling the trigger. Alternatively, to measure bulk samplessuch as powders or pellets, the instrument may be put into aspecial stand which accepts sample cups filled with bulk materialas shown in Figure 2.The prima
30、ry result of the measurement is an x-rayspectrum of the sample. It is the information extracted from thespectrum that is then converted into qualitative and quantitativedata of the elemental concentrations in the sample. Examples Figure 1. Portable X-Ray Analyzer, Niton Model XLt-794of x-ray spectra
31、 from a sample of plastic measured with the analyzer described above are shown inFigure 3. In this case, the sample has been a handle of a screwdriver. The two spectra shown,marked with black and yellow color, represent the black and yellow areas of the handle, respectively.The yellow spectrum revea
32、ls the presence of lead (two major peaks at 10.5 and 12.6 keV) in the yellowsections of the handle. The integral of any lead peak is a measure of the lead content.Figure 2. Portable X-Ray Analyzer measuring bulk samples in specialstand. Also shown are sample preparation accessories.376543210TiCrPbCr
33、aftsman Screwdriver - yellowsectionCraftsman Screwdriver - blacksectionSb23456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34Energy of X-rays, keVFigure 3. An example of an x-ray spectrum of plastic object, here the HDPE handle of a screwdriver. Notethat lead, titanium
34、 and chromium are present in the yellow sections of the handle. It appears that yellowsections are painted with paint containing titania and lead chromate pigment. Antimony is present inplastics. A relatively thin layer of yellow paint does not absorb antimony x-rays. This example illustratesexcelle
35、nt identification capabilities of the XRF Method.Analytical ApproachIn order to test the capabilities of the analyzer, a number of polymer-based samples of knowncompositions were procured. These samples were prepared from PVC and PE resins by adding to themantimony, bromine, titanium, chromium, merc
36、ury, cadmium and lead. The concentrations of additiveswere varied to imitate the concentration ranges encountered in real polymers. While concentrations ofheavy metals were maintained between 0 to 1000 mg/kg, the other elements were allowed to reachseveral percent levels. It is worth noting that the
37、re are very few polymer based certified materialsavailable, and those that do exist are prepared in polyethylene medium only 5, 6.The XRF analysis is known for the so called matrix effects which are manifestation of interactionof x-rays with the medium they pass through. In any analytical method, on
38、e expects the measuredsignal from a given element to be proportional, preferably linearly, to the elements concentration in thesample. In XRF analysis this relation is generally nonlinear for two reasons. First, the element itselfabsorbs its own characteristic x-rays when its concentration in the sa
39、mple increases. Second, thepresence of another element in the sample also increases the absorption of x-rays from the analyte inquestion. For example, two samples, each containing 5% iron in a polyethylene matrix will producedifferent intensities of iron if one of the samples will additionally conta
40、in, say, 10% titanium. Should wenot be aware of this fact, we might infer a false conclusion about the iron concentration in samples fromthe iron intensities alone. In the past a number of methods (empirical, semi-empirical and mathematical)have been proposed to correct for these effects.Based on ea
41、rlier experience it has been determined that the best analytical approach to plasticsanalysis using XRF is the method of fundamental parameters (FP). This method utilizes the fact thatthe measured x-ray intensities of the elements in a sample can be fully described by a complete set ofmathematical e
42、quations. The equations tie together the physics of the interaction of x-rays with matter4 Coun t Rate, cp sand sample composition. These equations can only be solved by iterative methods which requireconsiderable speed and computing power. The alternatives to the FP-based method are a classicalempi
43、rical calibration, or the so called Compton normalization (CN) method. The empirical calibration hasthe advantage of being the most accurate of all methods. However, its serious disadvantage is therequirement of availability of an extensive set of analyzed samples with compositions as close as possi
44、bleto those of unknown samples. Given the number of various plastic materials, this requirement ispractically impossible to satisfy. The CN method, while being less demanding on the availability ofstandards, is not capable of handling wide concentration ranges encountered in the analysis of plastics
45、.An excellent, in depth discussion of the calibration methods mentioned here can be found in reference 7.ResultsAccuracy, Precision and Minimum Detection LimitsAll samples used in this study were measured for 200 sec each. Samples prepared from PE orPVC resin were made into solid discs of 31 mm diam
46、eter, and at least 6 mm thick for PVC and 13 mmthick for PE. Additional samples collected from various sources were all at least 4 mm thick and coveredthe rectangular (10 by 20 mm) measuring window of the probe. Some samples were available only as 3mm pellets. These were placed in 31 mm diameter by 20 mm high sample cups (Chemplex Industries).Figure 4 shows measured data plotted against certified values for lead in PE and PVC samples.As can be seen, the Compton Normalization (CN) method cannot handle high levels of bromine, whichoften is present
