Factors affecting the accuracy of X-ray fluorescence spectrometer

X-ray fluorescence spectra are simple to prepare, non-destructive for rapid multi-element analysis, and can quickly screen a variety of sample matrices such as solids, slurries, powders, pastes, films, air filters, and many other matrix samples. The unknown component has become a commonly used detection method for the preliminary screening of harmful substances in the electronics industry. Analyze the factors affecting the measurement results of the X-ray fluorescence spectrometer and correctly use the X-ray fluorescence spectrometer. There is a direct impact on the accurate control of hazardous substances in electronic products.

The factors affecting the measurement accuracy of the X-ray fluorescence spectrometer include the performance of the instrument itself, the working curve, the test sample and the test method. The performance of the instrument itself is determined by the hardware of the instrument purchased. It has been decided when purchasing the instrument. The test method is related to the working principle of the instrument, and the relative measurement method is generally used at present. The working curve and test sample have a great influence on the measurement results. And the measurement accuracy can be greatly improved by simple processing. Therefore, this paper will focus on the analysis of the production curve, the deviation of the working curve, the applicable range of the working curve, the shape of the sample, the sample size, the heterogeneous sample, the sample surface and the interference elements. And introduce simple and common treatment methods.

1 , the impact of the work curve

The working curve is simply a curve of the X-ray intensity of an element and the mass percentage of the element contained in the sample, and the measured characteristic X-ray intensity is converted into a concentration by a working curve. Therefore, the working curve has a great influence on the measurement results. In addition to the concentration of the element to be tested, the element to be tested, the instrument correction factor and the correction value of the absorption enhancement effect between the elements, it is also related to the standard sample of the working curve, the deviation of the working curve, the applicable range of the working curve, etc. .

(1), making a sample of the working curve

X-ray fluorescence analysis is basically a relative measurement, and a standard sample is required as a measurement reference, so the geometric conditions of the standard sample and the sample to be tested need to be consistent. The standard sample should have sufficient uniformity and stability. The control sample is different from the smelting process or analysis method of the analytical sample. The magnitude of the sample cannot be traced, and the uniformity and stability cannot be guaranteed. Therefore, a standard sample with similar chemical and physical properties to the analytical sample should be selected to produce the corresponding working curve. Including the analysis of the element content range. And maintain the appropriate gradient, the content of the analysis elements must be determined in an accurate and reliable way. Many instrument manufacturers are now trying to improve their market competitiveness. Some common calibration curves are often pre-drawn according to the type of material the user is to analyze before the instrument leaves the factory to reduce the need for standard samples during on-site analysis. However, it goes without saying that since it is a general curve, it is very versatile, and it is very difficult to achieve "fine" at the same time. Therefore, in order to ensure the accuracy of the analysis, it is better to have a substrate corresponding to a set of standard samples.

(2), the deviation of the working curve

The general curve is made at the beginning of the instrument or at the beginning of the work, but it is known at the job site whether it is consistent with the original state. It is unlikely that the work curve will be redrawn every time it is analyzed, which requires periodic inspections with traceable standard samples. To verify if the working curve is offset. If an offset occurs and the offset is within the specified allowable range, the operating curve needs to be corrected. If the offset is outside the specified allowable range, you will need to recreate the working curve.

(3), the scope of application of the work curve

When selecting the working curve, pay attention to the applicable range of the working curve. The general use range should fall within the concentration range of the standard sample used for drawing the curve. If the concentration of the standard used to draw the curve is 500 ~ 1000μg / g, the content of the test element in the test sample should be 500 ~ 1000μg / g, the test point falls on the extension of the working curve, which will also give measurement The result is an error.

2 , sample factors

As the non-destructive test instrument, the X-ray fluorescence spectrometer has the greatest advantage that the test sample can be measured without special pre-treatment. Measurement results can be obtained quickly. However, whether the shape of the test sample, the sample size meet the requirements, and whether the surface of the sample is oxidized. Whether the material is homogeneous and whether there are interference elements will affect the accuracy of the measurement results.

(1), the form of the sample

Solid samples, such as PE, PVC plastic, steel, copper alloys and aluminum alloys, can smooth the test surface. Do not touch the polished surface with your hands before use to avoid oil stain on the surface and affect the measurement accuracy. If it is stained with oil, wipe it off with a clean flannel. For powder samples. Including mineral powder, dust, ash, cement and lime. The powder sample can be placed directly in the sample cup. A sample of about 7 g is enough. Cover the bottom of the sample cup approximately 10 mm thick. Measurements can be made. It is also possible to use a tableting method to obtain more accurate results. For liquid samples, the liquid sample is typically poured into a specific sample cup, sealed with a special sealing material, and then placed in the test chamber for measurement.

(2), the size of the sample

According to the spot size of the X-ray fluorescence spectrometer, the spot can be completely illuminated on the sample and the sample thickness can meet the requirements. It can be placed directly into the test room for measurement. The spot cannot be completely illuminated on the sample. That is, the sample is smaller than the spot (for example, a granular part). It needs to be placed in a sample cup. A certain amount is reached, and then it is pressed tightly, leaving no gaps, and then analyzed. Thin samples (samples that X-rays can penetrate), similarly, similar small samples should be stacked together to achieve the minimum sample thickness limit. Thereby an effective analysis is performed. Usually all samples must be completely within the measurement range of the spectrum. For polymer samples and some light metals, such as aluminum, magnesium, titanium, etc., at least 5 mm thick, for liquid samples, at least 15 mm thick. For other alloy samples, at least 1 mm thick.

(3), heterogeneous samples

Heterogeneous samples are often characterized by oily or heavy metal contamination, coated or electroplated surfaces, oil stains, or heavy metal contamination. These oils or heavy metals should be removed before measurement. If the metal surface is plated, to test the harmful element of the internal metal, the surface coating should be scraped off as much as possible before testing. For example, you need to measure the content of elements in the coating, because the coating is very thin. X-rays can penetrate the sample, the fluorescence intensity of its elements is a function of the thickness of the sample, so in theory, it is possible to analyze the element content in the coating. However, the strength of the matrix element will be very strong, so it is not easy to say whether the analysis of the element content in the coating can be carried out. It depends on whether the thin layer correction and the deduction of the matrix interference can be performed, as well as the analysis of the sample. The complexity of the plating elements and the change in the elemental content of the substrate.

3 , the influence of the surface of the sample,

The surface of the sample is exposed to air and oxidized, while the X-ray fluorescence spectrometer is a surface analysis method, which may lead to an increasing trend of sample analysis results over time. Before the measurement, the oxide film should be ground off. The degree of gloss on the surface of the sample has a great influence on the analysis result. The surface of the sample is not smooth, and the unevenness of the sample will affect the measurement result. Therefore, the surface should be smoothed as much as possible.

4 , the impact of interference elements

Although the X-ray fluorescence spectrum is relatively simple, most of them are separate lines, but in a complex sample, line interference is still not negligible. Sometimes it can cause serious interference. Due to the presence of interfering elements, the spectral line of the interfering element overlaps with the spectral line of the element to be tested when the sample is analyzed, resulting in a large measured intensity, which leads to deviations in the analysis result. Generally speaking, it is not too difficult to look at the interference of elemental lines. First, we must understand the position of some common and easily disturbing elements and the interference. When judging the test pattern of the sample, it is very important that If there is some kind of element. Then it should have multiple locations and multiple lines at the same time. To see more than two lines, such as Pb, you need to look at both PbLa and PbLbl. If the Pb element really contains, then both lines will have obvious peaks. If the Pb element really contains and is disturbed by other elements, the peaks will appear on both lines, but the results of the two lines will be quite different. The methods to overcome are: avoiding interference lines, and selecting interference-free lines for analysis lines. Appropriately select the instrument measurement conditions, improve the resolution of the instrument, reduce the tube voltage of the X-ray tube to below the excitation element excitation voltage, prevent the spectral line of the interference element, and perform mathematical correction. There are mathematical correction procedures on modern instruments.

This article is reproduced from: http://