A new approach to calibrate electron probe microanalysis (EPMA) for enhanced precision has been developed, particularly focusing on the analysis of indium concentrations in sphalerite (ZnS) crystals. This multi-point calibration method promises significant improvements over traditional one-point calibration techniques, which have long been hindered by limitations related to analytical precision and detection limits.
Commonly used for non-destructive testing, EPMA provides high spatial resolution measurements to analyze major and minor elements. Yet, measuring trace elements, especially those present at concentrations below 0.1 wt%, presents substantial challenges. The study, conducted by T. Schirmer and T. Ulrich, introduces methodologies aimed at optimizing the detection of indium within sphalerite—a mineral of strategic interest for its metallurgical applications.
To establish the multi-point calibration curve, synthesized ZnS crystals were doped with indium and cadmium, with concentrations systematically varied from 0 up to approximately 1500 µg/g. Utilizing two distinct analytical settings (25 kV and 100 nA, as well as 7 kV and 200 nA), researchers sought to assess various figures of merit, including beam stability and the limits of detection.
According to the findings, "The multi-point calibration approach results in improved analytical precision compared to the classical calibration approach using only one calibration sample," stated the authors of the article. The results showed detection limits of indium as low as 20 µg/g, representing a significant advancement over conventional practices.
A notable characteristic of the study is the long measurement times required for effective trace element analysis—often exceeding ten minutes per measurement. These extended periods necessitate exceptional stability from both the electron beam and the sample itself, underscoring the importance of the multi-point calibration method. "The measurement times for carrying out trace element analyses are very long, requiring exact and stable beam positioning," the authors emphasized.
Traditional methods of quantifying trace elements often utilize high-concentration reference materials, which can introduce complications relating to matrix mismatches and peak overlap phenomena. This study highlights how the classical reliance on these singular reference samples can lead to systematic inaccuracies, particularly when the composition between reference and target materials diverges significantly.
By employing artificially doped reference crystals, the research effectively addressed these calibration challenges. The combination of advanced instrumentation—a Cameca SXFIVE FE electron probe—along with the innovative sample preparation methods, allowed for enhanced reproducibility and accuracy of indium measurements within sphalerite samples.
Analysis showed the lower limit of quantification (LOQ) for indium was determined to be around 83 µg/g at 25 kV and 367 µg/g at 7 kV, indicative of the method's capacity to execute reliable quantifications within targeted concentration ranges. The overall analytical precision benefited from lowered deviations of measured indium concentrations, now observable below ±10%, cementing the method's potential for high-stakes mineral analysis.
The enhancements to trace element analysis methodologies such as these are pivotal, especially for the mining and materials sectors, which rely on accurate mineral composition data for strategic resource management. The integration of multi-point calibration techniques sets the stage for more reliable and precise measurements of trace elements, paving the way for advancements not only within EPMA but across various spectrometer applications.
Conclusions drawn from the research suggest the necessity for creating additional calibration materials with concentrations below the detection limit, as this would enable even greater sensitivity and accuracy. Ongoing work may involve refining methods of producing well-homogenized samples suitable for high-precision microanalysis.
Future investigation will surely explore the resilience of the multi-point calibration method as it applies to various geological materials beyond sphalerite, enhancing the field of geochemical analytes and their accuracy.