High-purity aluminum is used in countless applications, and testing its purity is critical in several manufacturing processes – incoming goods control, metal sorting, process quality control (QC), final product QC, certification or simply investigation.
Any bulk material is likely to contain some contaminants. The key for materials engineers and quality control personnel is knowing exactly what contaminants and how much are present in each batch of aluminum.
Optical Emission Spectrometry is an excellent method for the rapid analysis of high-purity aluminum and also aluminum alloys. This technique results in the detection of over thirty elemental contaminants, in quantities in the sub parts per million (ppm) range.
As for any characterization technique, sample preparation impacts the quality of the results of the analytical method. For optical emission spectrometry, the sample surface is generally prepared by using a lathe or milling machine. Grinding is not used because of the risk of contamination. Homogeneity of the elements depends on the metallurgical structure obtained through the sampling procedure and on the metallurgical history including mechanical deformation by rolling. These values apply when homogeneously distributed elements are present in samples that are prepared by recommended sample preparation methods. Once a sample is prepared, the time required to analyze a sample is, on average, about 30 seconds.
In less than a minute’s time, the optical emission spectrometer can analyze the contamination from over 34 elemental contaminants in aluminum, including elements as light as lithium and boron to heavier elements of lead, titanium and mercury. Accuracy is determined by appropriate calibration in the factory, as well as occasional use of calibration standards to confirm continued accuracy. Best practices in sample analysis should also be used, including completing several decontamination runs with a pure sample after higher-concentration samples have been analyzed. This will eliminate the so-called “memory effect”, which is due to contamination of the analytical stand aftr analysis of high-concentration contaminant samples.
Other features that impact the quality of the spectrometry analysis include the type of source used for excitation (for example, the Thermo Scientific intelliSource is a double current controlled source). Additionally, acquisition methods also impact quality results; for example, time gated acquisition is a high precision version of the time resolved spectroscopy, and reduces the need for interference correction.
Signal processing algorithms also can limit results; in the Thermo Scientific instruments, two high performance specialized algorithms enhance the signal processing.
The accuracy of that answer is determined by the accuracy of the tool used for measurement. The Thermo Scientific™ ARL iSpark 8860 Optical Emission Spectrometer’s guaranteed detection limits and precision values for pure and ultra-pure aluminum are listed in this application note, as well as features that enable extreme benefits when used to assess the purity of aluminum.