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Sharing is very practical [Some problems with XRF screening methods]

Sharing is very practical [Some problems with XRF screening methods]

2018/12/11
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[Abstract]:
Sharing is very practical [Some problems with XRF screening methods]

1. Method scope, application and overview

This method is used to screen and analyze lead, cadmium, mercury, chromium and bromine in basic materials of electrical and electronic products. Generally speaking, the test result of XRF is the total content of the above elements, but the different valence states and different compounds of the elements cannot be distinguished. form. Therefore, the actual content of hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ethers needs to be confirmed by detailed chemical analysis methods.

 

XRF is an instrument that is quantified by comparison, so its performance depends on the quality of the calibration method (calibration curve), ie on the selected calibration material and the selected instrument response mode. XRF analysis is susceptible to matrix interference (absorption and enhancement) and spectral interference. Not all types of XRF can be applied to samples of various sizes and shapes.

 

There is a general calibration method, the Basic Parameter Calibration (FP). The FP method refers to a method of calibrating with a pure element, a pure compound, or a very small number of calibration materials having a certain matrix composition. For all XRF calibration methods, the closer the composition of the calibration material is to the sample, the higher the accuracy of the test.

 

There is an empirical calibration method that uses a calibration material and uses an arithmetic method to correct the interference (correction factor) generated by the substrate and the spectrum. However, this method requires that the elemental composition of the calibration material be consistent with the sample. If the calibration material lacks an element that may cause interference and the sample contains the element, the test results may cause large deviations. Due to the limited number of existing calibration materials, it is a very complicated or arguable task to solve all possible matrix and spectral interferences in one method while maintaining optimum accuracy.

 

For coated materials and materials with multiple coating structures, it is difficult to obtain accurate results without knowing the structure of the coating beforehand, because the choice of calibration mode depends mainly on the structure of the coating in the sample. For a thin coating or coating, care must be taken to ensure that the XRF has sufficient sensitivity to detect very low levels of the coating.

 

XRF screening analysis can be performed in two ways: non-destructive - direct analysis of the obtained sample; destructive - using a mechanical or chemical method for sample preparation prior to analysis.

 

The purpose of this test method is to screen for the presence of restricted substances in different materials. This method provides a test method commonly referred to as semi-quantitative, that is, the relative uncertainty of the results is generally 30% or better, and the confidence at this time is 68%. According to the user's own needs, some users can also accept higher uncertainty. Through this semi-quantitative test, the user can screen out the materials that need detailed analysis. The main purpose of this test is to provide information for risk management.

 

2. sampling

2.1 For the non-destructive mode: the operator should place the sample in the appropriate position of the instrument, that is, to ensure that the position of the part to be tested is accurate, and that other non-test parts are not detected. The operator must ensure the reproducibility of the distance and geometric position between the part to be tested and the instrument. The operator must consider that the portion to be tested has as many regular shapes as possible, such as area, surface roughness, known physical structure, and the like. If it is necessary to obtain a portion to be tested from a large object, the operator needs to record the sampling step in text form.

 

2.2 For the way of destruction: The operator needs to record the entire sampling method and process in text form, because doing so is important for the subsequent correct interpretation of the test results. If the part to be tested is to be powdered, it is necessary to ensure that the particle size of the test point is known or controllable; in order to prevent the particles from having different chemical, morphological or mineral compositions, the particle size must be as small as possible to reduce the differential absorption effect. If the material is to be dissolved in a liquid matrix, record the quality and physical characteristics of the material being dissolved; the final solution should be completely uniform, and the insoluble portion must be treated to ensure a reasonable interpretation of the final test results; Reproducible introduction of the solution to be tested into the instrument for testing, for example using a liquid pool of a certain volume and structure. If the material is to be mixed with a solid matrix, the quality and physical characteristics of the material are to be recorded; the final solid (melted or compacted pellets) must be uniform; the handling of the unmixable parts must be stated to ensure reasonable Explain the final test results.

 

3. Test Procedure The test procedure consists of preparation of the instrument, preparation of the sample to be tested, and calibration. The corresponding operating instructions must be available. Also pay attention to the limitations of the instruments used. For example, some models are not able to detect or accurately quantify samples with very small or thin thickness. This requires the operator to handle it carefully and record it in detail to ensure Reasonable interpretation of the final test results. The instrument needs to be optimized before testing, and the performance of the instrument is confirmed: sensitivity, spectral resolution, detection limit, applicable area size, reproducibility of sample preparation and test, and accuracy of calibration method for each analyte. Sensitivity is an important indicator to distinguish the performance of different instruments and to ensure that the calibration used is meaningful; spectral resolution is an important indicator to ensure that analytes and interference lines can be correctly distinguished and processed during data collection and calibration; This is the standard deviation of the triple blank repeat test (no less than 7 measurements, 95% confidence interval); the applicable area size determines what the actual part to be tested is; the reproducibility of sample preparation and testing is proof An important parameter that can be controlled by the test method.

 

4. Matrix effect and spectral line interference Due to the diversity of electronic and electrical products and the complexity of materials and components, X-ray fluorescence spectrometry for rapid screening of restricted substances in electrical and electronic products, lead, mercury, chromium, cadmium and bromine are inevitably received. The influence of various substrates in the material and the spectral interference of the overlapping of other elements of the material including the analytical elements. The interference of the characteristic line of the analysis element mainly comes from the absorption effect and the enhancement effect of the line.

 

4.1 Influence of various material matrix The matrix influence of the characteristic line of the analysis element mainly comes from the absorption effect and the enhancement effect of the spectral line.

 

4.1.1 Analysis of polymer materials The characteristic line of the elements is seriously affected by the background of the spectral line generated by the organic polymer matrix; the Cl element in the PVC material, the elements such as Ca, Ti, Sn in the additive, Br and Sb in the flame retardant The elements have an absorption effect on the fluorescence intensity of the characteristic line of the analysis element; elements such as Sb, Sn and Br in the sample also have a secondary enhancement effect on the fluorescence intensity of the characteristic line of the analysis element; for high power (>500W) wavelength dispersion X-ray fluorescence spectrometer (WDXRF), the surface of the polymer material may be blackened by the use of high-power X-ray source for a long time, which affects the accuracy of the test results. It is recommended to use the newly prepared test for each measurement. sample.

 

4.1.2 Metallic materials Different metal matrixes have different absorption effects and secondary excitation effects on the characteristic lines of the analytical elements, such as:

 

Ferroalloy: Fe, Cr, Ni, Nb, Mo, W, etc.;

 

Aluminum alloy: Al, Mg, Si, Cu, Zn, etc.;

 

Copper alloy: Cu, Zn, Sn, Pb, Mn, NiCo, etc.;

 

Lead-tin alloy: Pb, Cu, Zn, Sn, Sb, Bi, Ag, etc.;

 

Zinc alloy: Zn, Al, etc.;

 

Precious metal alloys: Rh, Pd, Ag, Ir, Pt, Au, Cu, Zn, etc.;

 

Other metal substrates: Ti, Mg, and the like.

 

The electronic components and printed circuit board materials can refer to the influence factors of metal materials and polymer materials.

 

4.2 Spectral Interference of Overlapping Lines The characteristic line of the element is characterized by spectral line overlap interference and spectral line overlap interference from other elements in the sample.

 

The interfering elements of Cd may have Br, Pb, Sn, Ag and Sb;

 

The interfering elements of Pb may have Br, As, Bi;

 

The interference elements of Hg may have Br, Pb, Bi, Au, high content of Ca and Fe;

 

The interfering element of Cr may have Cl;

 

The interfering elements of Br may have Fe and Pb.

 

4.3 The effect of matrix effect on the detection limit of the analytical element Take the elements Pb and Cd to be analyzed in the polymer material as an example: If the detection limit of Cd in the pure polymer material is A, due to the influence of the matrix effect, when the polymer material Containing ≥2% of Sb, but without Br, the detection limit of Cd is between A and 2A; when the polymer material contains ≥2% Br, but does not contain Sb, then Cd The detection limit is ≥ 2A. If the detection limit of Pb in pure polymer material is B, due to the effect of matrix, when the polymer material contains ≥2% Sb, but does not contain Br, the detection limit of Pb is ~2B; When the polymer material contains ≥ 2% Br, but does not contain Sb, the detection limit of Cd at this time is ≥ 3B.

 

5. Interpretation of results According to the requirements of the EU RoHS Directive, the limit requirements for lead, mercury, chromium, cadmium and bromine in electronic and electrical products are lead, mercury, hexavalent chromium, polybrominated diphenyl ether (PBDE) or polybrominated biphenyl (PBB). ) is less than or equal to 1000 mg/kg, and cadmium is less than or equal to 100 mg/kg. XRF is suitable for rapid screening of lead, mercury, chromium, cadmium and bromine in restricted substances in electrical and electronic products, ie semi-quantitative analysis. The analytical method requires that the result of the test is 30% confidence. %, for complex samples of the matrix (such as small electronic components, etc.), the error range is required to be wider, up to 50%.

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