The possible sources of contamination during sample preparation include contamination from the sample preparation device, sample to sample cross contamination, and issues related to the preparation of calibration standards and routine samples. Each of these sources can significantly affect the accuracy and reliability of the analysis.
Contamination from the Sample Preparation Device: Sample pulverizers, which are commonly used to grind samples into fine powders, can introduce contamination. The grinding bowls, typically made of steel, tungsten carbide, or ceramics like alumina and zirconia, can leach elements into the sample. For instance, steel can add iron, nickel, and chromium; tungsten carbide can add tungsten; and alumina and zirconia can add aluminum and zirconium, respectively. The choice of grinding medium should be carefully considered based on the elements being analyzed to avoid unwanted contamination. Tungsten carbide is often preferred due to its hardness and the relative unimportance of tungsten in most analyses, despite its higher cost.
Sample to Sample Cross Contamination: This is a significant source of contamination, especially in environments where a variety of sample types are processed. Cross contamination occurs when materials from one sample inadvertently mix with another, compromising the integrity of both samples. This can happen during grinding, weighing, mixing, or any other step where samples are handled. Strict protocols and careful cleaning of equipment between samples are essential to minimize this risk.
Preparation of Calibration Standards and Routine Samples: The accuracy of XRF analysis depends heavily on the preparation of calibration standards and routine samples. Any deviation in preparation methods can lead to inaccurate results. Factors such as precise weighing techniques, thorough mixing, and the use of high-quality fusion means are crucial. Fusion means, which are often diluted with elements like sodium or lithium tetraborates or carbonates, must be chosen carefully to avoid introducing contaminants. The drying of samples and fusion means at 105-110°C is recommended to remove surface humidity, and heating may be necessary to remove organic components. Dilution rates typically range from 1:3 to 1:20 to minimize interelement effects and self-absorption, ensuring linear calibration curves.
In summary, contamination during sample preparation can arise from multiple sources, each requiring specific preventive measures. Careful selection of sample preparation equipment, strict handling protocols to prevent cross contamination, and meticulous preparation of calibration standards are essential to maintain the integrity of analytical results.
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